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The Debonding and Pull-Out of Ductile Wires from a Brittle Matrix
Abstract
An experimental investigation into the debonding and pull-out of nickel wires from epoxy resin and cement paste matrices has been carried out. Above a critical embedded length both the debonding and pull-out stresses attain limiting values. A theory based on the model of a yielded zone travelling up the wire behind a debonding front was shown to describe the observed dependence of the limiting debonding stress on the yield stress, diameter and surface roughness of the wire. Pull-out behaviour subsequent to debonding was explained using this model in terms of an unyielded plug at the end of the wire. Orientation of the wire to the loading direction was found to raise the limiting debonding and pull-out stresses due to enhanced friction at the wire exit point.
... The important role of interfacial asperities (i.e. roughness) on the fiber sliding behavior has been identified (Bowling and Groves, 1979; Jero et al., 1991; Carter et al., 1991; Mackin et al., 1992; Sorensen, 1993; Parthasarathy et al., 1994; Liu et al., 1994). Experimental evidence of the interfacial roughness effect has been obtained from the fiber push-back test (Jero et al., 1991; Mackin et al., 1992; Parathasarathy et al., 1994; Liu et al., 1994; Singh and Reddy, 1995). ...
... The intrinsic frictional stress Z'int, which is the product of/a by -trc (i.e. Z'in t = -¢ta¢), can be derived from eqn (8), such that _ --aOpo [ 3. THE ROUGHNESS EFFECT In the presence of interfacial asperities, stresses are induced at the interface as the fiber slides past the surrounding matrix (Bowling and Groves, 1979). Depending upon the length of the relative sliding displacement between the fiber and the matrix during fiber push-out, different approaches have been adopted to analyze the roughness effect. ...
... Depending upon the length of the relative sliding displacement between the fiber and the matrix during fiber push-out, different approaches have been adopted to analyze the roughness effect. When the sliding displacement is larger than half of the wavelength of the roughness such that the asperities slide over their nearest neighbors, a constant radial mismatch strain between the fiber and the matrix has been assumed which, in turn, results in a uniform interfacial compressive radial stress (Bowling and Groves, 1979; Jero et al., 1991; Mackin et al., 1992; Sorensen, 1993). When the sliding displacement is smaller than half of the wavelength of the roughness, the roughness effect influences fiber sliding on a local scale (Carter et aL, 1991; Parthasarathy et al., 1994; Liu et al., 1994). ...
The interfacial properties of Nicalon fiber-reinforced SiC composites with tailored (i.e. weak or strong) interfaces are characterized using single-fiber push-out tests. A simple analysis is proposed to include the surface roughness of the fiber in the push-out model. Specifically, the roughness effect is characterized by the difference between the fiber push-out stress and the fiber reseating stress, which is obtained from fiber push-back tests. A method is developed to analyze the loading stress vs fiber-end displacement relation (i.e. the push-out curve) during the push-out process to evaluate the interfacial properties. The push-out curve for composites with weak fiber bonding can readily be analyzed by the existing push-out model incorporated with the roughness effect. However, premature catastrophic debonding at the interface is required to interpret the push-out curves for composites with strong fiber bonding.
... The problem of pullout of fibers or bars from the surrounding matrix has received considerable attention in recent years and many important results have been achieved; see, e.g., Lawrence (1972); Freund (1992) ;Fuller et al. (1990); Gao et al. (1988); Leung and Li (1990a, b); Li et al. (1991); Shah and Ouyang (1991); Stang et al. (1990); Steif and Hoysan (1986); Wang et al. (1988); Beaumond and Aleszka (1978); Bowling and Groves (1979); and Gray (1984aGray ( , 1984b). An excellent review of the pullout test analysis has recently been presented by Shah and Ouyang (1991). ...
... But if this pressure is unknown, a more general solution that takes into account the interface dilatancy, i.e., the normal relative displacement across the crack, is required. In the simplified one-dimensional analysis, the interface dilatancy can be approximately taken into account by adjusting the values of the parameters in the functional relationship ,r(v) linking the interface shear stress ~ to the relative slip displacement v; see, e.g., Lawrence et al. (1972), Bowling and Groves (1979), and Hutchinson and Jensen (1990). Stang et al. (1990) considered the stress-slip relation to consist of an elastic part followed by a sudden stress drop and a residual constant friction [Fig. ...
The paper analyzes the size effect, which is an inevitable consequence of softening in the relation of interface shear stress and slip displacement between a fiber or reinforcing bar and the surrounding matrix. The problem is simplified as one-dimensional. Closed-form solutions of pull-pull and push-pull failures are obtained for a linear softening stress-slip law with residual strength, and for an exponential law without residual strength. The postpeak softening is shown to lead to localization of slip and interface shear fracture with a process zone of finite length. This zone propagates along the interface during the loading process, causing the distribution of interface shear stress to become strongly nonuniform. The larger the bar or fiber size, the stronger the nonuniformity. The size effect in geometrically similar pullout tests of different sizes is found to represent a smooth transition between two simple asymptotic cases: (1) The case of no size effect, which occurs for very small sizes and is characteristic of plastic failure; and (2) the case of a size effect of the same type as in linear elastic fracture mechanics, in which the difference of the pullout stress from its residual value is proportional to the inverse square root of the fiber or bar diameter. An analytical expression for the transitional size effect is obtained. This expression is found to approximately agree with the generalized form of the size effect law proposed by Bažant. The shape of the size effect curve is shown to be related to the shape of the softening stress-slip law for the interface. Finally, it is shown how measurements of the size effect can be used for identifying the interface properties, and a numerical example is given.
... Testing and modelling of the textile-to-mortar bond behaviour in cementitious matrices have been studied for decades, see e.g. [19,20,32,[46][47][48][49][50][51][52][53]. Besides the categorisation given in Fig. 4 and followed throughout Sect. ...
Textile-reinforced concrete (TRC) has gained a lot of attraction in recent years. Adequate bond between the phases in this system allows to transfer high loadings, thus enabling high performance. The terminus textile reinforcement, however, comprises many different types of fabrics, which differ in their chemical composition, geometry, surface properties etc., and thus exhibit substantially different bond properties. In the course of RILEM’s Technical Committee 292 work on TRC it was found that a comprehensive understanding of the complex interactions between individual parameters is still lacking. This is amplified by the fact that different types of textile reinforcement are preferably used in different regions of the world. This paper therefore attempts to compile findings from literature on the bond in TRC. The database used was created in the course of the TC work. Additional papers of relevance were identified by scanning scientific web databases. The different influencing parameters are given in this paper in a hierarchical order, starting from the level of the individual constituents (filament and matrix) to impregnated fabrics and the influence of textile manufacturing and architecture on the bond. Finally, by mapping all the cited literature used in this paper based on grouped keywords the complex intercorrelations are visualised.
... Although testing and modelling of the textile-to-mortar bond behaviour in cementitious matrices have been the subject of several studies for decades, see e.g. [22][23][24][25][26][27], the existing literature on the bond behaviour of TRM composites that are of interest for strengthening of masonry structures is very limited, (see e.g. [1,20,21] for a steel and a glass-based TRM). ...
Application of lime-based textile-reinforced mortars (TRMs) for strengthening of masonry structures have received a growing attention in recent years. An extensive effort has been devoted to understanding of the performance of these composites and their effectiveness in improving the seismic safety of existing masonry structures. Nevertheless, several aspects regarding the durability and mechanics of these composites still remain unknown. This letter is an effort on highlighting those aspects considering both experimental and numerical modelling approaches.
... (2) Propagation of debonding accompanied by small load drops until catastrophic interfacial failure. The small load drops in the curve represent debonding fronts propagating intermittently rather than continuously [47]. (3) Final pull-out of the cord via a frictional sliding mechanism. ...
... In contrast, the LLDPE cores display a completely different pullout behavior which is analogous to the debonding mechanism reported by Bowling and Groves for the pull-out of ductile wires [33]. The presence of MA in the pulled-out LLDPE þ MAPE cores has been confirmed by means of ATR-FTIR, where the spectroscopic analysis indicates the existence of typical MA bands such as ring structure carbonyl asymmetric and symmetric stretches around 1830 cm À1 and 1780 cm À1 and a C¼O carboxyl stretching band at 1720 cm À1 , respectively [34,35]. ...
The core – sheath interfacial interactions in a bicomponent thermoplastic filament have been analyzed by means of an adapted micro-mechanical approach for interfacial pull-out. The method relies on a specimen preparation procedure that selectively removes a segment of the sheath component while leaving the core intact. This adaptation of the classical pull-out method enables the controlled extraction of the exposed core from the embedded bicomponent segment. Using linear low density polyethylene (LLDPE) – polyamide 6 (PA6) filaments with a diameter of approximately 100 μm, it has been shown that the addition of 2–5 weight % maleic anhydride grafted polyethylene (MAPE) significantly increases the surface energy of LLDPE, which results in substantial improvements in the mean interfacial shear strength and work to debond. Use of 10 % wt. MAPE leads to the formation of voids which are detrimental to the mechanical properties of the core – sheath interface, despite a further enhancement of surface energy.
... Second, failure may occur upon yielding at the interface at some value of stress Tjy, in which case a constant shear st ress distribution can be assumed along the embedded fiber length, as long as work-hardening effects are negligible, and thus: (Eq 8) Finally, failure may occur if the interface fractures with work of fracture (G) per unit area of interface . The source of the required fracture surface energy is the strain energy stored within the specimen components (Ref [26][27][28][29][30][31][32][33][34][35][36][37]. ...
... As a piezoelectric semiconductor, ZnO has been widely used in microelectromechanical systems as sensors and actuators 8 and in communication field as surface acoustic wave devices. 9,10 However, the device performances are restricted by its relatively low piezoresponse to some extent. As an important parameter evaluating piezoelectric performance, the d 33 coefficient of ZnO is ϳ9.9 pC/ N for a bulk 11 and ϳ12.4 pC/ N for an oriented film, 12 which is about one order of magnitude lower comparing with prevalent piezoelectric perovskite ceramics. ...
(0001) oriented polycrystalline Cr-doped ZnO films have been prepared on n-Si(111) single-crystal substrates by nonequilibrium reactive magnetron cosputtering. The c-axis texture of the films weakens and a transformation of doping mechanism from CrZn to CrZn+Cri is indicated as the doping concentration increases. The Cr dopants are demonstrated to exist as Cr3+ ions in the films. Ferroelectric measurements show that the Ag/Zn0.94Cr0.06O/n-Si heterostructure displays well-defined hysteresis loop with a remanent polarization ~0.2 muC/cm2 and a coercive field ~50 kV/cm at room temperature. The capacitance-voltage curves with clockwise traces show typical memory windows, which symmetrically widen as the sweep amplitude increases. Ferroelectricity in Cr-doped ZnO was also established by a displacement-voltage ``butterfly'' loop. The observed ferroelectric behavior is attributed to the partial replacement of host Zn2+ ions by smaller Cr3+ ions, which occupy off-center positions and thereby induce permanent electric dipoles. Moreover, electrical transport studies reveal that the conduction mechanism in Cr-doped ZnO is a combination of field-assisted ionic conduction and trap-controlled space-charge-limited conduction, which prevail in lower and higher voltage regions, respectively. A higher leakage occurs as the doping concentration increases, which may originate from a higher density of defects. Besides, a high piezoelectric d33 coefficient ~120 pm/V is also achieved by Cr substitutions, which could make Cr-doped ZnO a promising material in piezoelectric devices.
... However, the whole picture of the fiber pull-out process is not yet well understood. This is especially true for the stick-slip responses observed in many fiber pull-out tests (Bowling and Groves, 1979;Cook et al., 1989). In this paper we present the results of fiber pull-out tests in which optical fibers with various coatings were pulled out of an epoxy matrix. ...
Results of experimental and modeling studies on the micromechanics of fiber pull-out are reported. For the experiment an optical glass fiber coated with a layer of acrylate or gold-palladium alloy is embedded in an epoxy matrix and then pulled out at various speeds. The fiber has a diameter of 125 or 200 μm. While the fiber is pulled out, the coating is left embedded in the epoxy matrix, producing frictional sliding between the contact surfaces of the glass fiber and the coating. As the thin and long fiber is pulled out, the trace of pull-force versus displacement shows several distinct stages corresponding to different pull-out processes. In the debonding process of the glass-acrylate interface, stable crack growth was observed prior to unstable sliding. The stable crack growth behavior is believed mainly to be caused by the fact that the interface fracture toughness is strongly mode dependent and the mode mixity of the debonding crack varies towards tougher mode as the crack advances. After the interface is completely debonded, the trace of pull-force versus displacement shows stick-slip oscillations about a constant mean full force. Through the use of photoelasticity it is found that the unstable stick-slip sliding of the glass-acrylate interface is caused by the propagation of a highly concentrated active sliding zone, a dislocation, along the interface. When a thin gold-palladium coating is introduced at the interface to produce debonding and sliding along the glass-gold-palladium interface, the initial stable crack growth is not observed and the interface dislocation emission is suppressed. The interface fracture toughness and the frictional sliding resistance are found to depend on the thickness of the coating; the interface fracture toughness is higher for thicker gold-palladium coatings, while the frictional sliding resistance is higher for thinner gold-palladium coatings. The sliding at the glass-gold-palladium interface also shows a stick-slip behavior under certain conditions. However, unlike the stick-slip process accompanying dislocation emission observed in the sliding process of glass-acrylate interface, the stick-slip is generated, while the entire contact interface slides simultaneously, by rate-dependent softening and hardening of the frictional interface. It is demonstrated that significant features of this type of stick-slip process can be predicted using a phenomenological friction law with an internal state variable.
Interphase regions that form in heterogeneous materials through various underlying mechanisms such as poor mechanical or chemical adherence, roughness, and coating, play a crucial role in the response of the medium. A well- established strategy to capture a finite-thickness interphase behavior is to replace it with a zero-thickness interface model characterized by its own displacement and/or traction jumps, resulting in different interface models. The contributions to date dealing with interfaces commonly assume that the interface is located in the middle of its corresponding interphase. We revisit this assumption and introduce a universal interface model, wherein a unifying approach to the homogenization of heterogeneous materials embedding interfaces between their constituents is developed. The proposed novel interface model is universal in the sense that it can recover any of the classical interface models. Next, via incorporating this universal interface model into homogenization, we develop bounds and estimates for the overall moduli of fiber-reinforced and particle-reinforced composites as functions of the interface position and properties. Furthermore, we elaborate on the computational implications of this interface model. Finally, we carry out a comprehensive numerical study to highlight the influence of interface position, stiffness ratio and interface parameters on the overall properties of composites, where an excellent agreement between the analytical and computational results is observed. The developed interface-enhanced homogenization framework also successfully captures size effects, which are immediately relevant to emerging applications of nano-composites due their pronounced interface effects at small scales.
Joints that connect thermoplastic polymer matrices (TPMs) and metals, which are obtained by comolding, are of growing importance in numerous applications. The overall performance of these constructs is strongly impacted by the TPM-metal interfacial strength, which can be tuned by tailoring the surface chemistry of the metal prior to the comolding process. In the present work, a model TPM-metal system consisting of poly(methyl methacrylate) (PMMA) and titanium is used to prepare comolded joints. The interfacial adhesion is quantified by wire pullout experiments. Pullout tests prior to and following surface modification are performed and analyzed. Unmodified wires show poor interfacial strength, with a work of adhesion (Ga) value of 3.8 J m-2. To enhance interfacial adhesion, a biomimetic polydopamine (PDA) layer is first deposited on titanium followed by a second layer of a poly(methyl methacrylate-co-methacrylic acid) (P(MMA-co-MAA)) copolymer prior to comolding. During processing, the MAA moieties of the copolymer thermally react with PDA, forming amide bonds, while MMA promotes the formation of secondary bonds and molecular interdigitation with the PMMA matrix. Control testing reveals that neither PDA nor the copolymer provides a substantial increase in adhesion. However, when used in combination, a significant increase in adhesion is detected. This observation indicates a pronounced synergistic effect between the two layers that strengthens the PMMA-titanium bonding. Enhanced adhesion is optimized by tuning the MMA-to-MAA ratio of the copolymer, which shows a maximum at a 24% MAA content and a greatly increased Ga value of 155 J m-2; this value corresponds to a 40-fold increase. Further growth in the Ga values at higher MAA contents is hindered by the thermal cross-linking of MAA; MAA contents above 24% restrict the formation of secondary bonds and molecular interdigitation with the PMMA chains. Our results provide new design principles to produce thermoplastic-metal comolded joints with strong interfaces.
Alumina (Al2O3) ceramics have today become a multibillion dollar global industry which has changed the world in the last few decades. Alumina uniquely combines low cost with extreme hardness, extreme electrical resistivity, extreme corrosion resistance, and high refractoriness, and it is the most biocompatible material in current clinical use. Of the 1.3 million hip replacements implanted annually in the global $7 Billion hip replacement industry, 55% now use alumina or zirconia-toughened alumina (ZTA) bearings. A decade ago, alumina or ZTA bearings were rare and still considered experimental. This meteoric rise has led to strong interest in ceramic hip resurfacing and a ceramic knee, a field currently solely serviced by metal bearings. This is not possible with alumina, and is right at the limit of what is possible with ZTA. A quantum leap in toughness could bring alumina ceramic hip resurfacing and an alumina ceramic knee to the mass market. As discussed in Chapter 2, metal microfiber reinforcement can provide this quantum leap in toughness as it can produce up a 600-fold toughness enhancement, and commonly 100-fold or more (two orders of magnitude) increase in work of fracture over the parent ceramic. Zirconia toughening of alumina can only give a threefold work of fracture enhancement of alumina. Alumina is the number one wear-resistant ceramic used in wear-resistant linings in the global $500 Billion mining industry. For high impact applications, such as jaw crushers and heavy-duty ore-chute liners, the tough cermet tungsten carbide (Chapter 8) is the material of choice, but it is more than four times more expensive than alumina. Metal microfiber-reinforced alumina could potentially compete with tungsten carbide in this role. However metal fiber-reinforced SiC composite technology developed by author Ruys, and discussed in Chapter 4, is a much lower cost high-impact wear-resistant ceramic technology, much better suited to competing with tungsten carbide. This chapter outlines the background to alumina bearings in orthopedics and alumina in the mining industry. This is followed by a comprehensive literature review of alumina ceramic matrix composites. The chapter then concludes with an overview of research done by the authors on metal microfiber-reinforced alumina as a biomaterial, and a synopsis of the proposed manufacturing process for such a material.
A functionally graded material (FGM) is a composite material whose properties change over a gradient across one or more axes of the composite material. This is most commonly a compositional gradient, but it can also be a microstructural gradient, such as a gradation in porosity or fiber reinforcement. It can also be graded atom order, or other property gradient. The most common type of FGM is the compositionally graded FGM, and of these, the most relevant to the present book is the metal–ceramic continuous bulk FGM. The concept of the FGM as an engineering material was first proposed in 1972, but it did not rise to prominence until it was proposed in 1984 at the National Aerospace Laboratory in Japan in response to a demand for a new material for the hypersonic space plane. The engineering objective was to develop a thermal barrier spaceplane skin capable of withstanding a surface temperature of 2000 K and a temperature gradient of 1000 K across a cross-sectional thickness of less than 10 mm. The material was also required to have corrosion and high temperature resistance on the outer face. The only material capable of this would be a metal–ceramic continuous bulk FGM. Metal–ceramic FGM thin-film coatings and thin-film interface layers are now a well-established technology. Metal–ceramic continuous bulk FGMs remain an experimental technology. They show significant potential for extreme applications for which few alternatives exist, such as spaceplane heat shields, plasma facings for nuclear reactors, ballistic armour, and load-bearing implantable medical devices. While many methods have been published for manufacturing continuous bulk metal–ceramic FGMs (mm to cm thick), few are capable of producing broad regular continuous gradients. A case study in this chapter demonstrated that thixotropic casting successfully produced regular gradients in hydroxyapatite-316L stainless steel biomaterial FGMs. An impeller dry blending (IDB) case study in this chapter showed the potential of IDB for producing excellent linear gradients in metal–ceramic FGMs. A hydrostatic shock forming case study showed its potential as a densification method for continuous bulk FGMs for which the metal and ceramic components have greatly differing melting points. Metal infiltration in combination with IDB forming of pore-graded ceramics was shown to be the most viable densification for FGMs for which the metal and ceramic components have greatly differing melting points. This chapter overviews the FGM concept, the manufacturing methods, and has three research case studies: continuous bulk FGM forming by thixotropic casing, continuous bulk FGM forming by impeller dry blending, and densification by hydrostatic shock forming.
This chapter represents the first detailed review of metal fiber-reinforced ceramics since the 1970s. In the 1960s, the space race began. This era of rapidly advancing aerospace technology brought in a new materials revolution. Lightweight, tough, heat-resistant materials suddenly became an imperative. Cemented carbides, the outstanding metal-reinforced cermet technology reviewed in Chapter 8, are extremely heavy and unsuitable for this application. As a result, in the 1960s, there was a sudden burgeoning of interest in metal fiber-reinforced aerospace ceramics. In the early 1970s, two ground-breaking papers were published on metal fiber-reinforced ceramics. The first one, from Canada in 1971, reported a 225-fold enhancement in work of fracture for molybdenum microfiber-reinforced alumina ceramics. The second one, from the United Kingdom in 1972, showed a 600-fold enhancement in work of fracture for nickel microfiber-reinforced magnesia ceramics. These achievements were extraordinary at the time, and even today they are comparable to the best achievements in metal microfiber-reinforced ceramics that followed later. They are well ahead of the best achievements in ceramic fiber-reinforced ceramics, and transformation-toughened ceramics, to date. This chapter reviews the field of metal fiber-reinforced ceramics, from their genesis in the 1960s, to the present day. Three of the chapters in this book concern metal fiber-reinforced ceramics (Chapters 4, 5, and 6456), but each has a different focus. Therefore this review is not just a standalone review on the topic, but it also lays the groundwork for Chapters 4, 5, and 6456. Cermet technologies are reviewed in their respective chapters (Chapters 7 and 878). Functionally graded materials are reviewed in Chapter 9. There are a number of other interesting metal-reinforced ceramic technologies whose scope is not large enough to justify an entire chapter. These are briefly overviewed at the end of this chapter.
A proper characterization of the fiber/matrix bond in fiber reinforced cementitious — FRC — composites is of great importance not only in the evaluation of the quality of a given fiber/matrix system. Different kinds of fiber/matrix debonding mechanisms are included in many models for the macroscopic mechanical behavior of FRC-materials and the applicability of such models naturally depends on the availability of experimentally determined fiber/matrix bond parameters. The present paper consist of a unified treatment of the fiber/matrix bond models proposed in the literature along with an evaluation of these models from a experimental and theoretical point of view. When dealing with perfectly bonded interfaces, basically two approaches are identified: the stress criterion and the fracture mechanical criterion approach. The results of these two approaches are examined and discussed. Finally some new research directions are proposed.
3-point bending fracture toughness tests have been carried out on the FRP specimens with low fiber volume fraction in 4 different fiber orientations, together with the direct observation of the local fracture event at the growing crack front. It is shown that a macro crack growth becomes stable when the micro unstable crack growth due to micro pop-in in the matrix resin is restrained by the pinning effect of fibers and the anti-crack opening effect of unbroken fiber behind the crack tip. Dependence of crack growth resistance on fiber orientation is evaluated by the R - curve method with special attention to the intermittent crack growth behavior.
In Part I of this book we have considered the several elements required in a description of adhesion. Now let us use these ideas to describe adhesion observations on nanoparticles, viruses and cells in different circumstances.
At scales above 10 μm, where Brownian movement can be largely neglected, adhesion appears macroscopic, steady and static when viewed with the optical microscope. The range of van der Waals forces is negligible and we can use a single parameterWto describe the adhesion. In reflected light, an area of adhesion appears black, in contrast to the non-contacting and non-adhering areas which look brighter. Thus we can identify the‘black contact spot’which represents a true molecular contact where the van der Waals adhesion between the surface molecules is found. Identifying the way in which this black spot changes in size is critical to understanding the adhesion process
This chapter presents the measurement methods for fiber-matrix adhesion in composite materials. It is clear that each of these methods for measuring fiber-matrix adhesion in composite materials requires one to make some assumptions about the material properties in the interphase and also requires that the system studied conform to the boundary conditions established for the analysis of the results. None of these techniques offers a complete and unambiguous method for measuring the interfacial shear strength between fiber and matrix. However, each method has proven to be sensitive to slight changes in adhesion in a given fiber-matrix system. The chapter reviews the three most common direct methods for measuring fiber-matrix adhesion, focusing on the sample preparation and fabrication, the experimental protocols and the underlying theoretical analyses upon which evaluation of these methods are based. In addition, finite-element nonlinear analyses and photoelastic analyses are used to identify differences in the state of stress that is induced in each specimen model of the three different techniques. To provide an objective comparison among the three different techniques to measure the interfacial shear strength for the prospective user, data and a carbon fiber-epoxy resin system is used as a baseline system throughout the chapter. However, these methods and procedures can be applied for adhesion measurements to any fiber-matrix combination.
The interfacial properties of E-glass fiber/epoxy resin have been investigated in terms of moisture absorption, fiber diameter, and fiber surface condition by using the two-fiber fragmentation technique. The interfacial shear strength decreased with increasing moisture absorption and fiber diameter, and the interfacial shear strength of sized fiber was higher than that of desized fiber in the two fiber fragmentation test. The interfacial shear strength which decreased by moisture absorption was recovered up to some extent by drying the specimen.
Ductile reinforcements can supply fracture toughness to a polymer matrix by pulling out and by plastically deforming. In the case of metal reinforcements that are not in a toughened condition, there may be more toughening to be gained when the fibers remain in the matrix and plastically deform rather than pulling out. These fibers can be said to have an unused plastic potential. When these fibers bridge a crack, their plastic deformation causes a rapid rise in the force which is trying to pull out the fiber. Because of this, the shape of the fiber must be adjusted along its length if it is to remain anchored and contribute its plastic work. The use of anchored, ductile fibers provides a new design axis that brings new possibilities not achievable by the current research focus on the fiber-matrix interface. This paper describes the experimental pullout of aligned ductile fibers from a polymer matrix, and indicates the effect of the shape and embedded length of the fiber on the toughness increase of the composite. Anchored, plastically deforming fibers are shown to provide a major improvement to the toughening. Even for unoptimized ductile fibers, the calculated toughening improvement equals or exceeds the toughening available from current short glass or graphite fibers.
Experimental results obtained from single fibre pull-out tests on specimens with different fibre embedment lengths, consisting of a brass-coated steel wire as fibre and a cementitious mortar as matrix, are analysed using the appropriate theories reviewed in the first part of this paper. The analyses indicate that both adhesional bonding and frictional resistance to slipping along the portion of the interface over which the adhesional bond has failed contribute significantly to the total resistance to completion of fibre debonding and initiation of fibre pull-out in these specimens. Estimated values of the adhesional (maximum) interfacial bond shear strength and the frictional resistance to slipping obtained from the apparent variation of maximum pull-out load with embedded fibre length are compared and found, for theories which are similar, to be generally in agreement.
The processes for debonding and pull-out in parallel-sided as well as tapered fibre composites are described. Models which can predict and account for all the reported experimental debonding and pull-out behaviour are developed. The effect of the interfacial properties on the plot of maximum pull-out force against fibre embedded length is elucidated. Knowledge of the interfacial parameters of a composite allows proper characterization and leads to better prediction of the mechanical properties.
The shear lag model has been used extensively to analyze the stress transfer in a singe fiber-reinforced composite (i.e., a microcomposite). To achieve analytical solutions, various simplifications have been adopted in the stress analysis. Questions regarding the adequacy of those simplifications are discussed in the present study for the following two cases: bonded interfaces and frictional interfaces. Specifically, simplifications regarding (1) Poisson's effect, and (2) the radial dependences of axial stresses in the fiber and the matrix are addressed. For bonded interfaces, the former can be ignored, and the latter can generally be ignored. However, when the volume fraction of the fiber is high, the radial dependence of the axial stress in the fiber should be considered. For frictional interfaces, the latter can be ignored, but the former should be considered; however, it can be considered in an average sense to simplify the analysis. Comparisons among results obtained from analyses with various simplifications are made.
In this study, reinforced dual concrete beam (RDC beam) composed of steel fiber reinforced concrete (SFRC) in the tension part and normal strength concrete (NSC) in the compression and remaining part is proposed. It is the epochal structural system that improves the overall structural performances of beam by partially superseding the steel fiber reinforced concrete in the lower tension part of conventional reinforced concrete beam (RC beam). Flexural and shear tests are performed to prove the structural excellence of RDC beam in comparison with RC beam. An analytical method is proposed to understand the flexrual behavior and is compared to experimental results. And for shear behavior, experimental results are compared to empirical equations predicting the ultimate shear strength of full-depth fiber reinforced concrete beam to examine the behavior of RDC beam under shear. From this studies, it is proved that RDC beam has more superior structural performance than RC beam, and the analytical method for flexural behavior agrees well with experimental results, and the partial-depth fiber reinforcements have no noticeable effect on ultimate shear strength but it is considerably effective to control and prevent evolutions of crack.
The effects of fiber orientation on acoustic emission(AE) characteristics have been studied for the unidirectional and satin-weave, continuous glass-fiber reinforced plastic(UD-GFRP and SW-GFRP) tensile specimens. Reflection and transmission optical microscopy was used for investigation of the damage zone of specimens. AE signals were classified as different types by using short time fourier transform(STFT) : AE signals with high intensity and high frequency band were due to fiber fracture, while weak AE signals with low frequency band were due to matrix and interfacial cracking. The feature in the rate of hit-events having high amplitudes showed a process of fiber breakages, which expressed the characteristic fracture processes of individual fiber-reinforced plastics with different fiber orientations and with different notching directions. As a consequence, the fracture behavior of the continuous GFRP could be monitored as nondestructive evaluation(NDE) through the AE technique.
Essais d'arrachement des fibres metalliques dans 3 systemes: fil de cuivre-matrice d'aluminium, fil de kanthal-matrice d'aluminium et fil de kanthal-matrice en alliage Al-11%Si. Differences des courbes obtenues. Conclusions sur une liaison chimique entre Cu et Al, et sur la formation d'une zone intermediaire dans les systemes comportant du fil de kanthal
The single-fibre pull-out test was employed to determine the effect of thermal history on a single-fibre composite. This was done by characterizing the fibre/coating/resin system with respect to the physical parameters of the polymer, the normal pressure exerted on the fibre by the polymer and the failure mechanism of the composite as a function of cure temperature. The experimental fracture data were quantified by determining the strain energy release rate for crack initiation and, with the consideration of friction, its propagation. Also determined were the interfacial shear stress of the bond (τbond) and the interfacial shear stress associated with debonding (τdebond) and sliding (τpull-out). The model material system was a polyimide-coated optical fibre embedded in uncatalysed tetraglycidyl-4,4′-diaminodiphenylmethane with 4,4′-diaminodiphenylsulfone. Cure temperatures (Tcure) of 150, 177, 230 and 250°C were employed. The average critical strain energy release rates increased from the 150 to 177 to 230°C sample sets, then decreased for the 250°C sample set. Since the glass transition temperature (Tg) of the fully cured resin is 260°C, these results support the hypothesis of increasing residual stress as a function of Tcure for cure in the vitreous state. With regard to the 250°C cure data set, since Tcure = Tg ± 15°C, it is hypothesized that the internal pressures due to crosslinking were minimized due to cure in a rubber-like state. The residual pressure, independently determined from both the resin characterization and fracture data, increased by a factor of 2.4 with a temperature increase from 150 to 230°C for the 2 h cure period. The strain energy release rate and τpull-out increased by a factor of 2.64 and 2.1, respectively. The coefficient of friction remained statistically constant at 0.6.
Fiber pull-out resistance is an important mechanism of energy absorption during the failure of fiber-reinforced composite materials. This paper deals with axial stress distribution in the fiber during a pull-out The frictional constraint between the fiber and the matrix is modeled with a perturbed Lagrangian approach and Coulomb's law of friction. Stress distribution has been determined for three cases, using the finite element method. The first case deals with the pull out of a fully embedded fiber. The second determines the stress distribution during fiber pull-out in the presence of a broken-embedded fiber. The third model attempts to solve the pull out of a coated fiber. The results for the first case compares favorably with those in existing literature. A local ''pinching'' effect, due to the matrix collapse behind the pulled fiber, is brought out clearly by this model. The second study indicates that the ''plug'' effect may not be significant in affecting the stress distribution, Lastly, the effects of coating stiffness and thickness are investigated.
The properties of the fiber–matrix interface are of great importance for the macroscopic mechanical properties of composite materials. The two-dimensional interphases or finite-thickness interphases are considered when analyzing the interaction between fibers and matrix in composites. On the molecular level, an interphase can be envisaged, with a chemical or physical bond to the residual groups on the fiber surface, and an effective functional gradient in the radial direction with properties different from those of the bulk matrix—for example, by interdiffusion of polymer networks of the size and the matrix. A simple relationship between the fiber strength and the interface strength is described in the chapter. This relationship makes it possible to establish that the strength properties of the interface should be to obtain an overall composite failure instead of a fiber–matrix interface failure. A mathematical expression is described in the chapter that relates the interlaminar shear stress, interfacial shear stress, fracture toughness, and physical and mechanical properties of the composites.
In this chapter, the modelling of constitutive relationship of steel-concrete interface is discussed. Studies of the interfacial zone between steel and bulk cementitious matrix demonstrated that the properties of this special region could be much different from the behavior of the bulk matrix and thus should be used as a physical background for the interface modelling. Various characterizations of steel-concrete interface are reviewed. In general, the constitutive relationship of steel-concrete interface can be divided into two categories according to the consideration of adhesion. If one assumes that adhesion exists on a part of the interface, then the constitutive relationship can be established by treating the interface as a bonded-debonded case. Otherwise, so called slip-based interface model, in which the constitutive equation is based on the bond stress-slip relationship, can be applied. For the bonded-debonded interface model, the constitutive equations are established by dividing the interface, corresponding to the bonded and the debonded regions. For the debonded case, three different methods of describing frictional force, i.e. constant frictional force distribution, Coulomb frictional force distribution and cohesive frictional force distribution, are examined. In this chapter, the constitutive relationship for bonded-debonded interface is emphasized and two criteria for interface debonding, stress based criterion and fracture parameter based criterion, are inspected. The various constitutive relationships are further evaluated by comparing them with the recently conducted Moire Interferometry test results. The comparison supports the bonded-debonded characterization for steel-concrete interface and shows the limitation of the modelling of frictional force in a debonded region.
Rubber-modified epoxy resins are used as a matrix material for glass and carbon-fiber composites. Mechanical properties of fiber reinforced composites depend on the interfacial shear strength between the reinforced fiber and the matrix resin. This study is focused on the interfacial shear strength in the reinforced carbon fiber and rubber-modified epoxy resin system. To evaluate interfacial shear strength between the fiber and the resin, pull-out test is performed using a microdroplet method. Based on experimental results, numerical analysis was also simulated. It is concluded that the interfacial shear strength of carbon fiber/unmodified epoxy resin system was higher than that of carbon fiber/modified epoxy resin system. The reason for decreased the interfacial shear strength of rubber-modified system is that contractive forces in neat epoxy resin acting on carbon fiber were less than those in rubber-modified epoxy resin system.
Fiber pull-out is simulated through a quasi-static analysis of a circular elastic cylinder with a rigid cylindrical fiber embedded in its center. The interface between the fiber and the matrix is characterized in terms of a rate dependent internal variable friction constitutive relation. The analysis is carried out in two steps; one simulating the residual stresses that develop while cooling the cylinder from its processing temperature and the other simulating the mechanical response during fiber pull-out. Depending on parameter values, fiber pull-out can occur smoothly or a stick-slip instability can occur. Numerical simulations of fiber pull-out are presented that explore the effects of loading device stiffness, loading rate, and friction law parameters on the predicted behavior. For example, the amplitude of the load fluctuations during stick-slip was found to decrease as the rate of pull-out increased.
Ces fibres lignocellulosiques (Crotalaria juncea ou chanvre du Bengale) sont impregnees du melange uree-amidon-acide phosphorique afin de mieux resister a l'action de l'humidite et du feu. Mais ce melange modifie leurs proprietes mecaniques et en particulier leur deformation en cas de choc, mesuree suivant la methode ASTM D 256 (essai Izod). Analyse des resultats. (CSTB)
Worldwide there is a need for the renewal of infrastructure because of age, deterioration, misuse, lack of repair, use of improper materials and techniques, and even due to changing needs. The corrosion of steel reinforcing bars is one of the major causes of degradation of concrete structures. Composites are increasingly being considered as a potential replacement for steel rebar, but there exists a critical lack of fundamental data and understanding of their behavior in relationship to traditional civil engineering materials such as concrete. This paper presents a preliminary investigation on the bond and pullout behavior of glass fiber reinforced composite reinforcement in concrete. The pullout behavior of the system was studied using a concentric pullout test and results are compared with theoretical predictions. The study is aimed at the development of a fundamental understanding of bond behavior and materials-configuration interrelationships needed to further the use of composites as reinforcements for application in areas where corrosion and electromagnetic interference make the use of steel reinforcement inefficient.
A basis for evaluating the interfacial properties of aligned ceramic matrix composites from slice compression tests is provided in the present study. Specifically, experimental results of tests performed on unidirectional Tyrrano SiC fiber-reinforced lithium aluminosilicate composites are analyzed. Precautions in experimental procedures are discussed, and the analytical solutions required for data analysis are summarized. To extract the interfacial properties, the measured relation between the residual fiber protrusion after complete unloading and the peak loading stress is fitted to the theoretical analysis.
The toughness of brittle materials can be significantly improved by the incorporation of fibers. Moreover, both theoretical and experimental investigations have shown that properly designed fiber composites can exhibit pseudoductile tensile behavior with multiple cracking rather than brittle failure with the formation of one single crack. To attain pseudoductile behavior, two criteria have to be satisfied: (1) the steady-state cracking criterion; and (2) the further cracking criterion. In the literature, criteria for steady-state cracking are usually obtained numerically or derived approximately with assumed profiles of the bridged crack. In this paper, a general analytical approach for the determination of an exact condition for steady-state cracking is presented. The steady-stare cracking criteria for several. important cases are then derived and compared with available numerical or approximate solutions. Then, an approximate but conservative criterion for further cracking is also developed. Since the two criteria involve the microparameters of the composite (such as properties of fiber, matrix, interface, fiber size, and volume fraction), they can be used as guidelines for the choice of microparameters in composite design.
The pullout of a single fiber from a brittle matrix is widely recognized as one of the basic tests to be performed to provide information about the expected behavior of a given fiber-reinforced brittle matrix composite material. Thus, it is of great importance that the pullout test be interpreted in a way that yields the true material parameters. Two approaches to the fiber/matrix debonding problem can be made: (1) The stress approach where the criterion for growth of the debonded fiber/matrix interface is expressed in terms of the interfacial stress; and (2) the fracture mechanical approach where the criterion for interfacial debonding is expressed in terms of energy equilibrium. This paper investigates these two approaches by applying both to the same model, which includes frictional stresses on the debonded interface. The debonding load-versus-crack length relationships predicted by the two approaches are compared and differences in the parametric dependency are discussed. The results predicted by the fracture mechanical approach are compared with available experimental results.
The mechanical performance of brittle and quasi-brittle materials can be significantly improved by the introduction of fibers. Fiber debonding, which results in the formation of bridging ligaments at the wake of cracks, is the major mechanism accounting for performance improvement. A strength-based two-way debonding theory, which accounts for fiber debonding at both the pulled and embedded ends, has recently been developed. In this paper, two-way debonding based on a fracture criterion is considered. With the fracture-based debonding theory, expressions for fiber stress and displacement, and thus the general debonding features, are found to be similar to those given by the strength-based theory. However, interpreting the same interfacial measurement (such as a pullout test record) with the strength and fracture approaches will lead to predictions of different debonding behavior in composites with practical volume fractions. To determine whether debonding is governed by strength or fracture, interfacial properties should be measured from a number of specimens with different fiber sizes or fiber volume fractions.
Ductile reinforcements can supply fracture toughness to a polymer matrix by pulling out and by plastically deforming. In the case of metal reinforcements that are not in a toughened condition, there may be more toughening to be gained when the fibers remain in the matrix and plastically deform rather than pulling out. These fibers can be said to have an unused plastic potential. When these fibers bridge a crack, their plastic deformation causes a rise in the force which is trying to pull out the fiber. Because of this, the shape of the fiber must be adjusted along its length if it is to remain anchored and contribute its plastic work. The use of anchored, ductile fibers provides a new design axis that brings new possibilities not achievable by the current research focus on the fiber–matrix interface. This paper describes the experimental pullout of aligned ductile fibers from a polymer matrix, and indicates the effect of the shape and embedded length of the fiber on the toughness increase of the composite. Anchored, plastically deforming fibers are shown to provide a major improvement to the toughening. Even for unoptimized ductile fibers, the calculated toughening improvement equals or exceeds the toughening available from current short glass or graphite fibers. In addition, pullout values are obtained for fibers that are embedded at an angle, simulating fiber bridging from fibers not perpendicular to the crack surface. These results further demonstrate the toughening efficiency of ductile fibers.
The interface failure observed in quasi-static fiber pushout tests performed on a model fiber-reinforced composite is simulated using a cohesive volumetric finite element scheme. The numerical analysis is conducted under axisymmetric condition. The debonding process is captured with the aid of intrinsic rate-independent cohesive elements. The augmented Lagrangian approach is used to solve the frictional contact between the crack faces. The numerical method is first applied to a model polyester/epoxy system, showing excellent agreement with the experimentally obtained load-deflection curve and with the observed evolution of the debonding length. The numerical scheme is then further applied in a parametric study of the effects of the friction coefficient, the interfacial bond strength and the process-induced residual stresses on the fiber–matrix interface failure process.
The effect of interfiber distance on the interfacial properties in three-dimensional multi-E-glass fiber/epoxy resin composites has been investigated using fragmentation test. In additions, the effect of the fiber surface treatment on the interfacial properties has been studied. The interfacial shear strength decreased with the decreasing the interfiber distance at the range of under 50 μm and the extent of the decreasing was more serious as the increasing of the number of adjacent fiber. This is probably due to the fact that the interface between the fiber and the resin was damaged by the adjacent fiber breaks and the damage increased with closing the interfiber spacing and the number of adjacent fiber. It was found that the interfacial shear strengths saturated when the interfiber distance was over 50 μm, the ones were saturated regardless of fiber surface treatment and the ones were in close agreement with those of the single fiber fragmentation test. Finally, the interfacial shear strength evaluated using three-dimensional fragmentation tests are shown as real values in-site regardless of fiber surface treatment, interfiber distance and existing of matrix cracks. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010
Both the fiber push-in and the slice compression tests have been used to evaluate the interfacial properties of fiber-reinforced ceramic composites. Mechanics of sliding at the fiber-matrix interface obtained with these two tests are compared in the present study. While the interfacial radial stress induced by Poisson's effect is always compressive for the push-in test, it is tensile for the slice compression test when the fiber is stiffer than the matrix. This difference in Poisson's effect results in different interfacial frictional resistance between these two tests. Compared to the push-in test, the slice compression test produces a lower frictional resistance along the debonded interface. The interfacial frictional stress during unloading is lower than that during loading for the push-in test, but this trend is reversed for the slice compression test.
Single fiber pullout experiments were conducted to determine the adhesion quality, debond behavior and subsequent matrix fracture behavior for a variety of end-modified copper fibers. The matrices were: two different epoxy resins, polyester and polyurethane; the end-modified copper fibers were: straight, flat end-impacted, flat end-impacted with release agent applied and straight end-oxidized. The goal was to determine how the bonding and debonding behavior as well as the pullout behavior of the various fiber-matrix combinations affected the composite fracture toughness increment (ΔG). Results indicate that the greatest improvement in the calculated ΔG occurred with a fiber-matrix combination that had a moderate interface bond strength with an interfacial bond failure, minor matrix damage during fiber pullout and moderate post-debond interface friction. Selective oxidation of the fiber end was performed to determine if chemical anchoring of the fiber end could be as effective as mechanical (end-shaping) anchoring of the fiber into the matrix. Improvement in the adhesion bond strength as a result of the chemical anchoring resulted in a significantly lower ΔG compared to the end-impacted fibers because interfacial failure was not possible. This indicates that for the materials tested, mechanical anchoring of the fiber was better than chemical anchoring in improving ΔG. To decrease the adhesion bond strength and allow the fibers to debond, a release agent was applied to the flat end-impacted fiber prior to embedment into the matrix. This resulted in a significantly lower ΔG compared to straight and flat end-impacted fibers for all matrices tested, because the resulting debonding force and friction were significantly reduced. Pullout curves showed that with release agent applied, the end-shape did not effectively anchor the fiber into the matrix. The reduction in the pullout work indicates that the friction at the fiber-matrix interface plays a crucial role in actively anchoring the end-shaped fiber into the matrix after debonding.
Improvement in composite fracture toughness can be achieved by using short ductile fibers with shaped ends which utilizes the plastic work potential of the fiber volume through anchoring of the fiber end into the matrix. Single fiber pullout tests performed on copper wire demonstrates that a 2D flat-end-impacted fiber geometry that is easier to produce improves the fracture toughness increment at least as well as a 3D “axisymmetric” end-impacted fiber. Calculations based on pull out tests and a model for predicting the fiber contribution to the fracture toughness increment “ΔG” show a 46% higher ΔG for the flat-end-impacted fiber compared to a straight fiber at 0° orientation. Results further indicate that for a given fiber geometry there is an optimum end volume; above or below this volume results in a lower ΔG. Due to the small fiber end volume, the packing density of the fibers will not be significantly affected by the end shaping of the fiber.Annealing and subsequent oxide removal of the end-impacted fiber is not necessary because there is no significant improvement in the ΔG. However, a moderate 3.5–15% decrease in the debonding force is seen in the annealed fibers, depending on the embedment depth. This indicates that annealing weakens the fiber–matrix bond. Furthermore, results indicate that a fiber that has multiple mechanical interlocks in the matrix is not as effective in fracture toughening as single anchoring of a shaped fiber end. However, the load displacement curve for the light rippled fiber shows a unique “wavy” behavior related to the geometry of the fiber end which may be useful in other applications. Round fiber ends produced with an acetylene torch had a 70% lower ΔG compared to a straight fiber because of the large fiber end volume and the poor microstructure resulting from the high torch temperature.
An experimental method of investigating rapid crack propagation and crack arrest in dynamically loaded fiber composites was developed. The reinforcement geometry and the impact energy are the major variables influencing the crack velocity v and the crack length Δa at arrest. However, there is a unique relation between v and Δa independent of the imposed variables.
Efficiency factors are proposed describing the effectiveness of utilization of fibre stiffness and strength when a random distribution of extensible fibres is incorporated in a brittle matrix.
A length efficiency factor for strength calculation is derived, which includes the effect of sliding friction during fibre pull-out; this reduces to the two previously applied efficiency factors in the limits.
The interaction of fibre length and orientation is considered, and general efficiency factors describing the effect of both length and orientation on the strength of randomly reinforced short fibre brittle matrices are derived.
The behaviour of stainless steel, work-hardened nickel and annealed nickel wires bridging a crack in a brittle-matrix has been studied as a function of the length and orientation of the wire. The pull-out stress for stainless steel wire in epoxy resin increases less than linearly with wire length, following the behaviour predicted by Takaku and Arridge [6]. Wires inclined at 20 and 40 to the tensile axis gave pull-out stresses some 30% higher than wires parallel to the tensile axis, this increase being attributed mainly to enhanced friction on the bent wire near its point of exit from the matrix. Work-hardened nickel wires fractured when their length exceeded a critical value, and the critical length was significantly shorter for inclined wires than for wires parallel to the tensile axis. In contrast, annealed nickel wires, no matter how long, did not fracture but pulled out at a limiting stress which was slightly higher for inclined wires than for wires parallel to the tensile axis. The results show that, in some cases, there does not exist a critical length above which an embedded wire will fracture rather than pull out of the matrix.
Previous theoretical work on fibre pull-out from an elastic matrix is briefly discussed and its relation to this present work is indicated. The form of the distribution of shear stress and that of load along the fibre length is determined and its dependence on the elastic properties and fibre length is shown. The theory has been developed to account for the debonding of fibres from the matrix. The maximum fibre load necessary to cause complete debonding and subsequent pull-out is determined and the dependence of the maximum shear stress on the effective embedded fibre length is shown to affect the shear strength calculated from a pull-out test.