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Non-destructive inspection of good quality panels via pulse-echo ultrasound (left) and pulsed thermography (right).
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The elastic behavior and failure response of discontinuous carbon fiber/epoxy laminates produced by compression molding of randomly-oriented preimpregnated unidirectional tape is characterized. Commercial applications for this type of material form already exist, such as Hexcel HexMC®. Complex relationships between unnotched and notched tensile str...
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... to machining of the individual specimens, each panel is in- spected via pulse-echo ultrasound using a 5 MHz sensor. An exam- ple of a good quality panel is shown in Fig. 1. The signal is particularly noisy due to the non-homogeneous nature of the material form, and overlapping chips act as hard points for the sig- nal, which therefore loses its strength. This is not unlike the phe- nomenon observed for woven or braided fabrics. As an additional method of inspection, a limited set of pulsed thermography ...
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... shown in Fig. 10 refer to five families with varying widths and hole diameters, which are grouped into two sets having the same d/w. It is interesting to observe that for the same d/w ra- tio, the specimens with smaller holes or narrower widths tend to exhibit higher strength. Fig. 11 on the other hand contains seven families with varying widths and ...
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... shown in Fig. 10 refer to five families with varying widths and hole diameters, which are grouped into two sets having the same d/w. It is interesting to observe that for the same d/w ra- tio, the specimens with smaller holes or narrower widths tend to exhibit higher strength. Fig. 11 on the other hand contains seven families with varying widths and d/w ratios, which are grouped into three sets by hole diameter. It appears that for the same hole size, the specimens tend to exhibit contradicting trends with vary- ing d/w ratio, with either greater or lower strength for higher or lower d/w. It appears therefore that ...
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... size, the specimens tend to exhibit contradicting trends with vary- ing d/w ratio, with either greater or lower strength for higher or lower d/w. It appears therefore that there are non-trivial interac- tions between specimen geometry and strength, of the same or greater complexity then for continuous fiber composites. It should be noted that in Figs. 10 and 11 all strength values are calculated as gross section strengths, regardless of their failure location, and that finite width effects have been ...
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... specimens also fail in a brittle fashion, in a combination of chip disbonding and wedging, as well as chip/fiber kinking and fracture (Fig. 12). The load-displacement curve exhibits an initial nonlinear region typical of most compression test (Fig. 13, left), however the stress-strain curve is perfectly linear up to cata- strophic failure (Fig. 13, ...
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... specimens also fail in a brittle fashion, in a combination of chip disbonding and wedging, as well as chip/fiber kinking and fracture (Fig. 12). The load-displacement curve exhibits an initial nonlinear region typical of most compression test (Fig. 13, left), however the stress-strain curve is perfectly linear up to cata- strophic failure (Fig. 13, ...
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... specimens also fail in a brittle fashion, in a combination of chip disbonding and wedging, as well as chip/fiber kinking and fracture (Fig. 12). The load-displacement curve exhibits an initial nonlinear region typical of most compression test (Fig. 13, left), however the stress-strain curve is perfectly linear up to cata- strophic failure (Fig. 13, ...
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... focus of the study on UNC strength is also to isolate geomet- ric relationships between test specimen size and measured strength. Three specimen widths are tested, and results show that there is a clear trend of decreasing strength with decreasing speci- men width (Fig. 14), with the 0.5-in. specimen (12.7 mm) over 25% less strong than the 1.5-in. specimens (38.1 mm). UNC results are therefore more marked and in direct antithesis with those observed for UNT (Fig. 5), and suggest that the stability considerations have a predominant effect over the measured strength than the length- scale effects. With ...
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... suggest that the stability considerations have a predominant effect over the measured strength than the length- scale effects. With regards to thickness, the trend is consistent with that observed for UNT (Fig. 6), and there still appears to be a 17% de- crease in strength between the 0.220-in. (5.6 mm) and the 0.076-in. (1.9 mm) thick laminates (Fig. 15). For Families P and Q it can be seen that there is still noticeable variation in the measured modu- lus, as observed for Family O, but the average remains constant around 6.5 Msi (44.8 GPa) regardless of specimen ...
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... OHC specimens failed at the hole as in Fig. 16, which shows the failure mode to be of the same kind of chip shearing, wedging and fracturing seen in the UNC tests. As the hole diameter in- creases, the gross section strength decreases in a linear fashion (Fig. ...
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... OHC specimens failed at the hole as in Fig. 16, which shows the failure mode to be of the same kind of chip shearing, wedging and fracturing seen in the UNC tests. As the hole diameter in- creases, the gross section strength decreases in a linear fashion (Fig. ...
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... shown in Fig. 18 refer to two families with different widths and hole diameters, but having the same d/w ratio. It is interesting to observe that the family with smaller hole or nar- rower width tends to exhibit lower strength. Fig. 19 on the other hand contains four families with varying widths and d/w ratios, which are grouped into two sets by hole ...
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... shown in Fig. 18 refer to two families with different widths and hole diameters, but having the same d/w ratio. It is interesting to observe that the family with smaller hole or nar- rower width tends to exhibit lower strength. Fig. 19 on the other hand contains four families with varying widths and d/w ratios, which are grouped into two sets by hole diameter. It appears that for the same hole size, the OHC specimens with higher d/w appear to have lower strength, as evidenced in the case of OHT for D = 0.25 in. (6.3 mm) and D = 0.375 in. (9.5 mm). Once again it ap- ...
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... D = 0.25 in. (6.3 mm) and D = 0.375 in. (9.5 mm). Once again it ap- pears that there are non-trivial interactions between specimen geometry and strength, where the influence of the hole and its size may have contrasting or reinforcing effects with respects to the ef- fects caused by other length scales (such as width). It should be noted that in Figs. 18 and 19 all strength values are calculated as gross section strengths, regardless of their failure ...
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... with the presence of the hole, and hence it could be classified as exhibiting notch-insen- sitive behavior. A truly notch-insensitive material is shown in the dark solid straight line, where the reduction in load-carrying capa- bility is linearly proportional to the reduction in available cross- section area. To better clarify this concept, Fig. 21 [16] gives a sum- mary of the traditional behaviors, and it represents the ratio of notched (OHT) to unnotched (UNT) strength as a function of d/w ratio. The three behaviors observed include purely notch-sensitive (1/K t curve), purely notch-insensitive (1:1 straight line), and typi- cal continuous fiber composite laminates. These ...
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... 17 It results in the presence of inherent weakness, for example, clusters of tow-ends, resin pockets, or voids that are randomly distributed during fabrication. 18,19 Consequently, the composites present dispersed local hotspots and insensitivity to notches, 20 leading to a noticeable randomness in the mechanical response and energy absorption sensitive to microstructural features. Under quasi-static conditions, the coefficient of variation of strength in randomly oriented SMCs can reach up to 19%, 20 which is even more pronounced in highly oriented materials. ...
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... Driven by the need to reduce operating costs and increase performance and safety, the transport industry is looking for advanced, high-specific strength materials to meet cost, quality, and volume targets. Discontinued fiber-reinforced composite materials are appreciated for their high processability, high stiffness [1], notch insensitivity [2], and ability to withstand harsh environmental conditions [3]. However, they exhibit relatively low strength due to their limited reinforcement length. ...
This work proposes a novel manufacturing solution to locally reinforce a Sheet Molding Compound (SMC) with continuous fiber prepreg in a single-step compression molding operation. An adapted SMC process allowing for the manufacturing of continuous carbon fiber-reinforced SMC was successfully implemented, and a layer of fabric was added to mitigate the waviness induced during processing. Plates with different stacking sequences were manufactured and used for mechanical characterization to understand the effect of hybridization over tensile and flexural properties. It emerges that hybridization led to a 137% increase in tensile strength and up to 147% rise in flexural modulus compared to the standard SMC baseline. Additionally, the hybridization process helped reduce the variability in elastic and strength properties.
... The results of the measurements are also designed for comparison with FEM analysis in different SW applications. Comparison of calculations and measurements are important for the development of new products [1,2,7,9], and parts thereof from composite materials [3,8,14]. Problem of these profile is in their joining together and also to join other parts to them which aren't made of carbon. ...
... The differences between specimens was in their post process of hardening the epoxy [10,13,14]. First specimen has epoxy hardened in shape form and the form had special shape that turn the planar profile to groove squared profile with 2 slots in opposite side along the whole length of profile (see Fig. 2). ...
... The cross-section shows that the typical defects such as macroscopic voids, swirling, and fibre kinking (and resin rich areas) are common defects at these WLDs. It appears that the failure in tow-based SMC is insensitive not only to macroscale notches [4,32], but also in smaller voids between tows near the WLD. The failure seems to run around the tow ends at the WLD and circumvents the intra tow cracks and voids, suggesting that the failure is matrix dominated. ...
... It was shown earlier, in Figures 7 and 9, that the EC uniformity map in "forced defect free" material has an associated strain distribution map with corresponding minima and maxima throughout the sample, showing that the observed variation in local modulus is "material inherent" to DFC. It was previously shown that these materials are virtually notch insensitive at the macro-scale [4,32]. ...
Discontinuous fibre reinforced composites enable the manufacture of integrated structural components via the complex flow process of compression moulding. However, such processes can lead to the formation of detrimental weld-lines. Here, the meso-structure of carbon fibre sheet moulding compounds (C-SMC) was analysed using conventional non-destructive techniques and automated eddy current (EC) scanning, as well as destructive methods, in an attempt to identify defects such as weld-lines in this class of materials. Compression-moulded plaques with forced weld-lines in two different configurations (adjacent and opposing flow joints) were analysed, showing up to 80% strength reduction versus a defect-free plaque. The EC-determined local fibre orientation and elucidated local microstructure matched those obtained using conventional techniques, showing a dramatic fibre tow alignment parallel to the weld-lines. It was found that failure occurred in proximity to the “non-uniformity” defect regions identified by EC analyses, demonstrating the use of robot-guided EC for successful defect detection in C-SMC structures.
... Therefore, OHT LDFC coupons may fail either in the vicinity or away from the notches depending on the dominant stress concentrations, which can reside either in the local morphology or in the geometrical discontinuity. Feraboli et al. [30] showed that the notch sensitivity of a stochastic PPMC is less pronounced compared with the continuous fiber laminates due to the internal stress concentration arising from the heterogeneous mesostructure. ...
... where σ 0 is the unnotched strength; K ∞ is the stress concentration factor of infinite plate with circular hole, which can be calculated with the effective engineering constants of material in principle directions (Eq. (4)) [30,32]. To calculate the K ∞ of organosheet material, the engineering constants were adopted from [7] and listed in Table 1. ...
In this study, the hole size effect in the open hole tension (OHT) of discontinuous glass fiber-polyamide 6 organosheet composite material was studied. Organosheet used in this study is a novel long discontinuous fiber-reinforced polymer composite with long glass fiber and in-situ polymerized polyamide 6 (nylon) matrix. A progressive failure analysis (PFA) using finite-element model was proposed to predict OHT strength using measured fiber orientation distribution. The generated virtual ogranosheet samples were used to conduct OHT Monte Carlo simulations to predict variability in strength. The notch sensitivity was observed experimentally and analyzed using PFA model, which captured interactions between various failure modes. The experimental and PFA results of OHT strength were used for calibrating analytical models, Point Stress Criterion (PSC) and Modified Point Stress Criterion (MPSC). Both, experimental and modeling approaches, yielded nearly identical analytical curves, indicating that virtual testing of organosheet can be used for OHT characterization of organosheet. The results show that the notched strength of organosheet with the diameter of 2 mm was at the same level as strength of unnotched organosheet. The notch insensitivity was attributed to the increased plasticity in the matrix due to ductility in nylon.
... The specimens that failed away from the notch or at the notch but not at the tip, showed no difference in nominal strength when compared to specimen that failed at the notch. Previous studies [13,14,19,49,50] have seen similar fracture behaviors presented here in unaged DFCs. Fig. 18 shows the load vs displacement curves for out time 1×, 2×, and 3× DFC specimens. ...
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From tension, compression, and shear experiments it is seen that the out time has no effect on the elastic moduli. However, the strength increases with increasing age of the specimens for all the loading conditions. The percent increase compared to the unaged material ranges from 15% to 33%. This is likely caused by plasticization of the matrix with age, allowing for higher energy absorption.
More complex trends are reported for the fracture tests on geometrically-scaled Single Edge Notch Tension (SENT) specimens. It is found that the fracture energy and characteristic length initially decrease with age, and then finally increase for the longest out time. This trend is owed to two factors with countering effects on the fracture behavior: 1) the increase of the number of platelets with aging due to increase in resin viscosity, and 2) the plasticization of the matrix with aging.
The results of this study suggest that the mechanical performance of Discontinuous Fiber Composites (DFCs) made from reused materials is not negatively affected by aging of the starting material. On the contrary, repurposed DFCs can exhibit equal, if not better, performance than unaged DFCs. The results of this work further corroborate the feasibility of using DFCs as a mean to mitigate the environmental impact of thermoset composite waste. The experimental data presented in this work can be used as a baseline to design DFC composite components made from repurposed prepreg scrap and waste.
... The large geometric scale of prepreg platelets as compared to the scale of the tensile specimen produces an interplay between the internal and external stress concentrations. Experimental work with open hole specimens [66] [67] [69] [70] have summarized two major findings: (i) notched strength levels were found to be indicative of notch insensitivity of the composite, and (ii) a number of the notched test specimens were found to fail away from the notch (so called gross-section failure). ...
... The notched behavior of a continuous fiber, quasi-isotropic laminate is shown in Figure 4b [0/ ± 45/90] to exhibit notch sensitivity over the complete range in values of d/w, while the PPMC material exhibits notched strength data that lies above the complete notch insensitivity line. Despite the experimental scatter, the discontinuous PPMC system appears to be notch generally notch insensitive, in agreement with previous studies using similar materials [66]. This notched behavior of a PPMC cannot be solely explained by the stress relief mechanisms confined in the regions near the notch alone, but rather suggests that there are complex stress redistribution mechanisms not present in traditional continuous fiber composite laminates. ...
... Feraboli et al. [66] and Qian et al. [67] used an open-hole specimen configuration to experimentally study the notched strength dependency on the hole diameter, while Tang et al. [69] [70] studied the fatigue behavior of open-hole specimens using both experimental and computational methods. The open-hole tension test (ASTM D5766) configuration utilized in[66] ...
The present treatise offers an investigation into the operative mechanisms of stress redistribution and notch insensitivity of the prepreg-based, long-fiber discontinuous composites with stochastic fiber orientation state manufactured via a compression molding process. Open-hole tensile behavior is examined to determine the source of notch insensitivity. Both experimental and simulation approaches reveal that local damage mechanisms result in stress redistribution and corresponding reduction in notch sensitivity as measured by the ratio of the notched to un-notched tensile strengths. The analysis showed that open-hole coupon fracture may occur in locations at or away from the hole, depending on the details of the stochastic meso-morphology and the notch size. It was also demonstrated that as the notch diameter is increased, the probability of fracture at the notch is increased. The experimentally observed notch insensitivity of the prepreg platelet composite was reproduced in the numerical simulations.
... Furthermore, CF-SMCs have been reported to be damage tolerant with higher fracture toughness values compared to other carbon fiber composites and typical metal alloys used in lightweight applications [35][36][37][38] with relative insensitivity to notches and manufacturing defects [36,39,40]. Failure is considered matrix-dominated and rather initiates at the least favorable local fiber orientations and the presence of strain concentrations such as at platelet or bundle ends and resin pockets [14,19,29,[40][41][42]. ...
Discontinuous carbon fiber composites have been reported to be damage tolerant materials with insensitivity to notches and defects. Meso-scale damage mechanisms and crack arresting are responsible for high apparent fracture toughness and pseudo-ductility. This work studies the effect of manufacturing and structure formation on the mechanical behavior of stochastic carbon fiber sheet molding compounds based on thermosetting prepreg platelets with a fiber length of 50 mm. The process-structure–property-performance relationship is analyzed by non-destructive characterization and mechanical bending tests on the component level. Model components of various preform and stack placement scenarios were compression molded to investigate different flow conditions, weld lines and artificially introduced defects. The experimental findings revealed a process-induced failure mode transition from pseudo-ductile to brittle failure behavior. Low-flow scenarios showed a highly non-linear response with large deflection values and high energy absorption. Higher in-mold material flow and weld lines caused brittle failure at significantly lower deflections with a considerable reduction of energy absorbing capacity. Increased material flow also reduced component stiffness and maximum load values. Weld lines exhibited major strength reduction compared to the pristine material configuration. Full-scale and high-resolution X-ray computed tomography scans revealed differences in local fiber orientation state, increased levels of fiber waviness and porosity, as well as asymmetric distribution of defects.
... The random nature of the material is what allows the enhanced moldability versus continuous fiber composites [3]. Furthermore, the random nature of the material makes it insensitive to small notches [4][5]. However, the random nature of the material makes it difficult to characterize, creating difficulties for engineers trying to size STDBC parts. ...
Stochastic tow based discontinuous composites (STBDC) are fabricated from the compression molding of chips made from chopped and slit pre-impregnated uni-directional carbon fiber reinforced polymer (CFRP) tape. The discontinuous mesostructure gives the material system increased moldability versus continuous fiber composites allowing complex three-dimensional parts to be manufactured. However, the discontinuous mesostructure creates challenges for engineers designing parts as the effective properties are variable. Furthermore, the properties have been shown to be a function of the consolidation method of the chips. This study uses finite element analysis simulations of mesostructural representative volume elements to compare the elastic response and characteristics. The results are compared to the available literature regarding the elastic response STBDCs.
... The chip size allows for high complexity near net-shape moulding, where parts can have varying wall thicknesses, tight radii, rib-like features, and holes. [11][12] Therefore, a DLF composite can be a weight reducing replacement for a metal in subcomponents such as brackets. 13 The high v f enhances the mechanical performance of DLF composites when compared to short fibre composites made from injection moulding. ...
Discontinuous long-fibre (DLF) composites can be made with randomly oriented unidirectional pre-impregnated composite chips. High fibre volume fraction unidirectional fibre chips provide good mechanical properties to the DLF composite architecture, which enables this material to contribute to bridging the gap between continuous-fibre and randomly-oriented short-fibre composites. However, it is well known that unidirectional fibres have highly anisotropic in-plane behaviour, which causes weak points in the parts when chips are oriented at unfavourable angles. This can be problematic since chips are randomly oriented in DLF composites. To overcome this problem, this research utilizes woven fibre chips instead of unidirectional fibre chips to fabricate DLF composites. Woven fibres diminish the potential for weak points due to their more homogenized in-plane mechanical properties. For comparison purposes, compression moulded carbon/PEI samples were made from both unidirectional chips and 5HS woven chips. Bending and tensile tests following ASTM guidelines were performed to compare both types of fibre arrangement. The results show that woven fibre chips increase the mechanical properties of the DLF composites and reduce their variability.
