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Compressive load as a function of the measured and computed strains in the loading direction: toughened material (a) and material sensitive to fibre bridging (b).
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Increasing the Mode I inter-laminar fracture toughness of composite laminates can contribute to slowing down delamination growth phenomena, which can be considered one of the most critical damage mechanisms in composite structures. Actually, the Mode I interlaminar fracture toughness (GIc) in fibre-reinforced composite materials has been found to c...
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Interference-fit is attracting increasing attention in the aerospace field because of its excellent enhancement of the sealing and fatigue life of carbon fiber reinforced plastic (CFRP). However, it also induces imbalanced pre-tightening forces on the interface, and affects its mechanical behavior. A series of experiments were conducted to assess t...
Citations
... The fiber bridging could greatly improve the load-bearing capacity of the structure and appeared to be an effective toughening mechanism that could delay the delamination growth. Riccio et al. 37 found that the specimens that were easy to produce the fiber bridging were characterized by a lower value of the fracture toughness at the beginning of the delamination growth and exhibited an earlier and more stable delamination growth compared with toughened specimens. Sakai et al. 38 reported that the rising stage of the curves in fiber composites resulted from the fiber bridging process. ...
The composite interlamination is the key part of load transfer and plays a decisive role in the overall mechanical performance of composite structures. The interlaminar property is predominantly influenced by the matrix resin which is easily affected by the hygrothermal environment. This paper delved into the effect of hygrothermal aging duration on the interlayer debonding behavior of unidirectional (UD) laminates. The pre‐cracked specimens were treated by hygrothermal aging with a period of 0, 15, 30, 60, 90, and 120 days. The double cantilever beam (DCB) tests were employed to evaluate the interlaminar property. The effect of hygrothermal aging on load–displacement responses, R ‐curves, crack growth speed, and fracture surface topography was analyzed. The results revealed that the load‐bearing capacity of aged specimens presented a reduction of 18.6%–29.0% compared with unaged ones. The interlaminar fracture toughness of specimens subjected to 15 days of treatment decreased significantly compared with that of unaged specimens due to the increase of water absorption caused by hygrothermal aging. With the increase in hygrothermal aging time, the interlaminar fracture toughness peaked at 60 days because of the post‐curing effect, followed by a declining trend influenced by the dominant hygrothermal aging effect. The fiber bridging of aged samples was increasingly obvious, became the most prominent at 60 days, and then gradually reduced. The fiber bridging diminished the crack propagation speed, hindered interlaminar debonding, and contributed to the interlaminar toughening.
Highlights
The samples underwent hygrothermal aging for 0, 15, 30, 60, 90, and 120 days.
Load ability of aged samples was at least 18.6% lower than that of unaged ones.
R ‐curves rose before the crack length of 79 mm and then tended to stabilize.
Fracture toughness first decreased, then increased and reduced with aging time.
Fiber bridging could hinder crack growth and improve the interlayer property.
... Jacobsen [51] and other researchers [52,53,54,55,56,57,58]. Other works also include extensive experimental and numerical efforts to accurately predict and model the fiber bridging mechanism [59,60,61,62]. ...
Over the last two decades, composites have become a key material in aircraft design and construction. Thermoplastic composites are steadily gaining ground over thermosetting composites due to their high impact load resistance, weldability and recyclability. In order to maximise the benefits of thermoplastic materials and reduce production costs, it is necessary to manufacture large and uniform structural sections and to replace the mechanical fastening methods with alternative techniques of joining such as adhesive bonding, co-consolidation and thermoplastic welding. However, the above-mentioned alternative joining methods have not yet been certified for use in primary structural elements. One means of compliance, suggested by EASA, is the incorporation into the structure of crack arrest features that will decelerate the progression of interfacial damage beyond a critical size between the pre-defined inspection intervals. In this effort, an understanding of the mechanical behavior of the thermoplastic composite layers and the fracture mechanics of the thermoplastic interface is of major importance, as is the availability of reliable numerical simulation models. In the present thesis, numerical models were developed for the simulation of interfacial failure of thermoplastic co-consolidated joints subjected to quasi-static and fatigue loading conditions, which, after being validated upon experimental data, were used for the design and evaluation of joints with two different crack arrest features: Refill Friction Stir Spot Welds (RFSSW), and Induction Low Shear Friction Stir Rivets. The selection of the specific features was made after an extensive literature study which generally concerned crack arrest features in composite joints. The numerical models were based on the finite element method and were developed using the commercial code LS-DYNA. The cohesive zone method was used to simulate failure at the thermoplastic co-consolidated interface. For the study, the thermoplastic material LM-PAEK with T700 carbon fiber reinforcement was used. An extensive experimental campaign was designed and implemented to extract input data and validate the numerical models. Specifically, mechanical tests were conducted on Double Cantilever Beam (DCB), End-Notch Flexure (ENF), Single Lap Shear (SLS) and Crack Lap Shear (CLS) specimens, which were subjected to quasi-static and fatigue loading. The experiments were conducted on reference specimens and specimens with crack arrest features. The length of the interfacial crack during the tests was measured either by visual means or by acoustic ultrasound monitoring carried out at pre-defined loading intervals during the mechanical tests. Since most failure criteria and degradation laws for composite materials’ properties have been developed for thermosetting composites, an applicability study of the available criteria and laws for thermoplastic materials was initially carried out. Four different damage models were evaluated in terms of their ability to simulate the mechanical response, the failure evolution and the ease of application/required data. Optimal performance was achieved through the combination of Hashin-type failure criteria and degradation through a progressive damage model. In addition, this material model has the ability to simulate interlaminar failure, which is not the case for the other models. The results from the mechanical testing of SLS specimens revealed the phenomenon of fiber bridging at the failure surfaces of the thermoplastic joints. This type of failure is very common in thermoplastic interfaces. Since the available traction-separation laws in the cohesive zone method are only suitable for simulating cohesive failure, a modified tri-linear traction-separation law, resulting from the superposition of the bi-linear behaviors of the thermoplastic matrix and fibers, was developed in this thesis to simulate the occurrence of fiber bridging. Initially, the data from the mode I (DCB) and mode II (ENF) tests were used to construct the crack propagation resistance curves (R-curves) of the interface. The curves were incorporated into the numerical models using a user-defined material model developed in the LS-Dyna finite element code. An algorithm was then developed to derive the fiber bridging law directly from the simulation results, thus eliminating the need for continuous crack monitoring during the analysis. The final model predicted with significantly improved accuracy the failure of the SLS specimens. A new fatigue crack growth model based on the cohesive zone method was developed to simulate the crack evolution at the interface of co-consolidated thermoplastic joints due to mixed-mode loading conditions. The model was specifically designed to account for any loading mode-mixity, requiring input data from corresponding pure mode I and mode II loadings. The interfacial crack growth rate is continuously updated using a function of the energy release rate and the mode-mixity ratio of the loads which is calculated via a linear law. The model can be applied both for force-controlled and displacement controlled loadings, and for structures from coupon-scale to larger structural elements, such as stiffened panels. A user-defined subroutine was developed to implement the model in LS-Dyna FE software. Numerical results revealed that the model accurately predicts the fatigue crack propagation for mode I, mode II, and mixed-mode loadings in DCB and ENF, SLS and CLS specimens, respectively. After the validation of the numerical models of the fatigue crack-growth and fiber bridging, they were then used to evaluate the effectiveness and parametrically design the crack arrest features in CLS specimens and in a mono-stiffener element with initial interfacial damage subjected to quasi-static loading and fatigue. The parameters studied were the features’ diameter, the number of features and their distance from the pre-crack. The effect of both crack arrest features on the CLS specimens for quasi-static loading was negligible. However, in the case of fatigue loading both mechanisms achieved significant retardation of interfacial crack evolution. Refill Friction Stir Spot Welding performs better than Induction Low Shear Friction Stir Spot Riveting since it has been more lab-studied, thus more technologically mature and appears to perform well as a joining method as well. In the mono-stiffener element, the RFSSW feature achieved deceleration of the interfacial failure for fatigue loading, while the mechanism of ILSFSR had no effect. All the geometric parameters investigated were found to have a significant effect on the efficiency of the crack arrest features, however the most significant effect comes from their diameter. The larger the diameter the more significant the deceleration in the crack growth evolution. It is noted that similar results regarding the efficiency of the crack arrest features are also expected for thermoplastic welded interfaces due to the similar fracture mechanics behavior of the co-consolidated interface and the weld seam. In conclusion, both mechanisms show high potential for use in thermoplastic material structures, especially the RFSSW feature. In summary, in this thesis the interfacial failure of co-consolidated thermoplastic composites was experimentally studied, and numerical methodologies were developed for the analysis and design of thermoplastic joints with crack arrest features. The following key innovations emerge from the methodologies and results of the thesis: a complete experimental characterization of the co-consolidated joints was carried out for the LM-PAEK/T700 material; a new tri-linear traction-separation law for simulating fiber bridging at thermoplastic interfaces was developed, numerically implemented and experimentally validated; a new model for the simulation of mixed-mode fatigue crack growth along thermoplastic interfaces was developed, numerically implemented and experimentally validated; the efficiency of two new crack arrest features in co-consolidated thermoplastic joints in coupon-scale and mono-stiffener element level was evaluated, and a parametric study on the influence on the features’ efficiency of some key geometric parameters was performed.
... Experimental studies of delamination are devoted to interesting articles by Chermoshentseva A et al [9] and Riccio A et al [10]. The results of numerical studies of the reinforced panels damage tolerance are given in Riccio A et al [11][12]. ...
... To calculate the stresses, it is first of all necessary to find critical parameters of wave formation using equations (9) -(10), then to calculate the deflection amplitude f from equation (6) and further to determine longitudinal stresses in every point of the plate using formula (11). Let us also give an expression for transverse and tangential stresses ...
When creating modern thin-walled aircraft composite structures, it is mandatory to analyze the behavior of possible delamination-type defects and determine allowable defect parameters. In this paper, we consider surface defects close to one of the surfaces of a multilayer panel. The purpose of the work is to determine the local stress-strain state (SSS) under geometrically nonlinear behavior of free-edge surface defects of delamination type, considering the anisotropic structure. The objects of the research are compressed rectangular thin plates for delamination modeling. The approximation of the anisotropic plates deflection used in this work corresponds to the following boundary conditions: one long side is rigidly pinched and the other long side is free. The geometrically nonlinear problem is solved analytically by the Bubnov-Galerkin method. The anisotropic delamination structure is considered and the initial relations of the layered composite structures technical theory are used. The solution of the problem is reduced to the analytical expression concerning the compressive force depending on the characteristics of the composite package of anisotropic structure and the deflection amplitude with a possible geometrically nonlinear behavior. Analytical dependences for determining membrane stresses are also presented. The practical significance of the work lies in the possibility of expert analytical evaluation of the SSS of local compression delamination type surface defects.
... Rarani [18] studied the finite element model strategies of DCB test and discussed the advantages and limitations of VCCT, CZM and XFEM. Ricco [19,20] considered the effect of fiber bridging behavior of delaminated laminates and numerically simulated the experiments. ...
In this paper, the influence of the bonding materials on the failure modes and the critical energy release rate (CERR) is studied through the double cantilever beam (DCB) test. The test results show that the failure mode and CERR of the bonded structure are closely related to the bonding materials, and three failure modes, i.e., the cohesive failure, the interface failure and the mixed-mode failure are identified on the bonding surface. The finite element method is used to simulate the interface debonding behavior of the DCB test specimens, and the influence of material randomness on the interface failure is introduced. A XFEM/CZM coupled approach is proposed to model the crack migration phenomena. The predicted results have a good agreement with the experimental results.
... Other efforts in the development of geometry independent bridging laws have been conducted by Sørensen and Jacobsen [7] and other researchers [8][9][10][11][12][13][14]. Other works also include extensive experimental and numerical efforts to accurately predict and model the fiber bridging mechanism [15][16][17][18]. ...
In the present work, a numerical model based on the cohesive zone modeling (CZM) approach has been developed to simulate mixed-mode fracture of co-consolidated low melt polyaryletherketone thermoplastic laminates by considering fiber bridging. A modified traction separation law of a tri-linear form has been developed by superimposing the bi-linear behaviors of the matrix and fibers. Initially, the data from mode I (DCB) and mode II (ENF) fracture toughness tests were used to construct the R-curves of the joints in the opening and sliding directions. The constructed curves were incorporated into the numerical models employing a user-defined material subroutine developed in the LS-Dyna finite element (FE) code. A numerical method was used to extract the fiber bridging law directly from the simulation results, thus eliminating the need for the continuous monitoring of crack opening displacement during testing. The final cohesive model was implemented via two identical FE models to simulate the fracture of a Single-Lap-Shear specimen, in which a considerable amount of fiber bridging was observed on the fracture area. The numerical results showed that the developed model presented improved accuracy in comparison to the CZM with the bi-linear traction–separation law (T–SL) in terms of the predicted strength of the joint.
... Yao et al. [38][39][40][41] employed Δ√G in fatigue delamination growth in composite laminates with accounting fiber bridging effect, exhibiting some important conclusions according to energy dissipation analysis. Important conclusions provide by Yao et al [38][39][40][41] follows the energy principles associated to fiber bridging in fatigue delamination growth, which provides low contribution to permanent energy dissipation, except when occurs the fiber failure [43,44]. In most cases, the fiber bridging phenomenon in fatigue delamination just periodically stores and releases strain energy under fatigue cycles but have little contribution to permanent energy release. ...
The aim of this study was to characterize the mechanical behavior of a carbon-glass/epoxy hybrid composite under cyclic loading and following physical-based interpretation for mode I delamination modeling. The hybrid composite shows a higher surface roughness due to a micro-change in the crack direction at the carbon/epoxy and glass/epoxy interfaces, with the simultaneous presence of both reinforcements along the entire fracture surface. The organosilane bond (at the glass fiber surface) extends the interphase chain, increasing the deformation interfacial area. In conclusion, the application of the maximal carbon-glass/epoxy interfacial number in hybrid laminates is a feasible option to increase delamination resistance, since a greater amount of energy needs to be overcome to enable damage formation, which results in longer fatigue life.
... The digital image correlation technique has been used extensively to monitor the displacements and strains of test specimens [20,21]. The study conducted by Riccio et al. [20] successfully captures the buckling shape changes of delaminated laminates during compression tests. ...
... The digital image correlation technique has been used extensively to monitor the displacements and strains of test specimens [20,21]. The study conducted by Riccio et al. [20] successfully captures the buckling shape changes of delaminated laminates during compression tests. The DIC and numerical results are compared and reveal the influence of fibre bridging on the delamination evolution within the laminates. ...
The damage mechanisms of biaxial non-crimp-fabric (NCF) reinforced composites with a quasi-isotropic [(45/-45)(90/0)]4s layup subject to low-velocity impact (LVI) and compression-after-impact (CAI) loadings are investigated. Experimental results indicate that fibre compressive failure in the 0° plies controls the peak load during CAI, which is influenced by the out-of-plane transverse deformation of the impacted specimen. The study proposes a novel finite element (FE) model to predict the CAI strength of the NCF specimen. While the number of delaminations in the LVI model is less than the actual number of the multiple delaminations in the test specimen, the same out-of-plane transverse deformation measured by the experiment is still captured. The strength predicted from the subsequent CAI model is in good agreement with the experiment. The study provides a potential numerical design tool for the use of NCF materials in damage-tolerant structures.
... Considering the loading conditions found in impact events, the main cause of failure within the composite structure can be related to the shear stress [41][42][43] localised within a structure during the dynamic loading. Consequently, it is possible to assume that Mode II failure is dominant and high dependency from the ply angle is expected for the Gc value in this loading condition. ...
Composite laminated materials have been largely implemented in advanced applications due to the high tailorability of their mechanical performance and low weight. However, due to their low resistance against out-of-plane loading, they are prone to generate damage as a consequence of an impact event, leading to the loss of mechanical properties and eventually to the catastrophic failure of the entire structure. In order to overcome this issue, the high tailorability can be exploited to replicate complex biological structures that are naturally optimised to withstand extreme impact loading. Bioinspired helicoidal laminates have been already studied in-depth with good results; however, they have been manufactured by applying a constant pitch rotation between each consecutive ply. This is in contrast to that observed in biological structures where the pitch rotation is not constant along the thickness, but gradually increases from the outer shell to the inner core in order to optimise energy absorption and stress distribution. Based on this concept, Functionally Graded Pitch (FGP) laminated composites were designed and manufactured in order to improve the impact resistance relative to a benchmark laminate, exploiting the tough nature of helicoidal structures with variable rotation angles. To the authors’ knowledge, this is one of the first attempts to fully reproduce the helicoidal arrangement found in nature using a mathematically scaled form of the triangular sequence to define the lamination layup. Samples were subject to three-point bending and tested under Low Velocity Impact (LVI) conditions at 15 J and 25 J impact energies and ultrasonic testing was used to evaluate the damaged area. Flexural After Impact (FAI) tests were used to evaluate the post-impact residual energy to confirm the superior impact resistance offered by these bioinspired structures. Vast improvements in impact behaviour were observed in the FGP laminates over the benchmark, with an average reduction of 41% of the damaged area and an increase in post-impact residual energy of 111%. The absorbed energy was similarly reduced (−44%), and greater mechanical strength (+21%) and elastic energy capacity (+78%) were demonstrated in the three-point bending test.
... They found that the compressive modulus and ultimate stress of NFCs were inversely proportional to the reinforcement orientation angle with the highest stress and modulus respectively with a fiber orientation at 10 o . Aniello Riccio et al. [36] studied the effect of fiber bridging strengthening on the compressive properties of delamination composite plates through experiments and numerical simulation, which was helpful to improve the understanding of the interaction of failure mechanism in the studied composite samples. Anthony M. Waas et al. [37] presented the advantages and disadvantages of the many different experimental techniques to measure fiber strengths and gived many suggestions for future investigations. ...
In order to study the axial compression behavior of short columns confined with carbon fiber reinforced polymers (CFRP), 21 specimens were tested under monotonic loading. Four failure modes were classified based on the test results which were adhesive layer failure, compression failure, buckling failure and local buckling failure. With the improvement of external reinforcement effect, the failure mode changed from compression failure to buckling failure or local buckling failure. With the increase of cloth ratio, the ductility and ultimate bearing capacity of the specimens increased, but not linearly. However, the increase range gradually decreased even the cloth ratio kept increasing. When the cloth ratio reached 2.56%, the ductility reached the maximum. The lateral deformation of laminated bamboo columns was restricted by CFRP during compression, thus lateral strain and Poisson's ratio gradually decreased with the increase of cloth ratio. Stress-strain model and formula for calculating the compressive strength of reinforced columns were proposed which gave a good agreement with the test results.
... In practical engineering applications, composite structures are generally submitted to compressive loadings. Due to the sensitivity of compressive failure to the fiber misalignments and initial imperfection, composite materials may experience an intricate fracture process accompanied by multiple failure modes, such as kinking failure, delamination buckling, matrix shear deformation, fiber microbuckling and fiber/matrix interface splitting [2][3][4][5][6]. Generally, the longitudinal compression strengths of composites are rarely greater than 60% of the tension strengths [7], which greatly hinders the widespread applications of composites. ...
This paper investigates the compressive damage mechanisms of 3D woven composites through a coupled numerical-experimental approach. A comprehensive progressive damage model, capable of characterizing the fiber kinking failure, transverse inter-fiber crack, matrix damage and interfacial debonding, is developed to capture the damage accumulations by incorporating a novel fracture angle-dependent stress reduction scheme, and the numerical implementation is accomplished accordingly. According to microscopic observations, a periodic representative volume cell with sufficient fidelity, including fiber yarns, matrix pockets and yarns/matrix interface, is extracted from the whole interlaced architecture. Moreover, the influence of inhomogeneous fiber initial misalignments on the compressive performances is parametrically investigated by adopting three stochastic distributions, which are implemented through a compiled user-defined subroutine. To validate the reliability of the proposed progressive damage model, some corresponding experiments of 3D woven composites are conducted. The numerically predicted strengths and damage development of constituents exhibit good consistency with the available experimental results.
