Structural degradation and mechanical fracture of hybrid fabric reinforced composites

This investigation involves the study of accelerated environmental aging in two polymer composite laminates reinforced by hybrid fabrics based on carbon, Kevlar and glass fibers. Composite laminate configurations are defined as a laminate reinforced with E‐glass fiber and Kevlar 49 fiber hybrid fabric (GK) and another laminate reinforced with E‐glass fiber and AS4 carbon fiber hybrid fabric (GC). Both laminates were impregnated with epoxy vinyl ester thermosetting resin (Derakane 470‐300) consisting of four layers. Morphological studies (photo‐oxidation process and structural degradation) of environmental aging were conducted, in addition to comparative studies of the mechanical properties and fracture characteristics under the action of uniaxial tensile and three‐point bending tests in specimens in the original and aged conditions. With respect to uniaxial tensile tests for both laminates, good mechanical performance and little final damage (small loss of properties) was caused by the aging effect. However, for the three‐point bending tests, for both laminates, the influence of aging was slightly higher for all parameters studied. The low structural deterioration in the laminates is attributed to the high performance with the heat of the matrix (Derakane 470‐300) and the characteristics of the hybrid fabric, exhibiting fiber/matrix interface quality. POLYM. ENG. SCI., 56:657–668, 2016. © 2016 Society of Plastics Engineers     

The impact resistance of fiber–metal laminates based on glass fiber reinforced polypropylene

The high velocity impact response of a range of fiber–metal laminates (FMLs) based on a woven glass fiber reinforced polypropylene and an aluminum alloy has been investigated. Tests on FMLs, based on 2024‐O and 2024‐T3 aluminum alloys, were undertaken using a nitrogen gas gun at velocities up to 150 m/s. The failure processes in the FMLs were investigated by examining the samples after impact and by sectioning a number of specimens through the point of impact. The impact response of these multilayered samples was also characterized by measuring the residual out‐of‐plane displacement of the targets after testing. Energy absorption in the FMLs occurred through gross plastic deformation, membrane stretching and tearing in the aluminum plies, as well as delamination, fiber fracture, and matrix cracking in the composite layers. In the multilayered FMLs, the permanent displacement at the perforation threshold remained roughly constant over a range of target configurations, suggesting that the aluminum layers deform almost independently through a membrane stretching mechanism during the perforation process. The impact resistances of the laminates investigated were compared by determining their specific perforation energies (s.p.e.), where it was shown that s.p.e. of several of laminates was almost three times that of the corresponding aluminum alloy. The perforation resistances of the FMLs as well as those of the plain composite were predicted using the Reid–Wen perforation model. Here good agreement was noted between the model and the experimental data for the range of laminates investigated here. POLYM. COMPOS. 27:700–708, 2006. © 2006 Society of Plastics Engineers     

汽车复合材料层合板准静态力学性能的试验测定

以应用于某新能源电动汽车的复合材料层合板为研究对象,利用万能试验机和静态应变测试分析系统等提出了可靠的复合材料层合板准静态拉伸和压缩力学性能试验测定方法,从而为复合材料结构在汽车轻量化中的设计和应用提供了试验依据。该层合板结构采用±45°交叉铺层方法,由2层碳纤维、1层芳纶纤维和2层玻璃纤维层叠构成。试验结果表明,该复合材料层合板在准静态拉伸时呈现沿±45°方向和层间分离挤压的断裂失效模式,这与其内部纤维铺层方向是一致的。同时,由于在复合材料板材中加入了增韧和板材失效时起连接作用的芳纶纤维和玻璃纤维铺层,该复合材料层合板的整体力学性能较常见碳纤维增强复合材料板材,其弹性模量和强度性能均有所降低。     

Influence of the fiber surface properties on the mechanical strength of unidirectional fiber composites

The influence of the thermodynamic adhesion between fibers and matrix on the mechanical properties of a continuous fiber reinforced composite is studied for two systems: carbon fiber reinforced poly(ether ether ketone) and glass fiber reinforced poly(ether imide). The fibers are modified chemically and characterized by measuring the contact angle formed by molten resin on the fibers. Various fiber treatments yield a wide range of contact angles, which are determined optically. Unidirectional fiber reinforced laminates are manufactured and transverse flexural strength is measured with the values reported as a function of the specific work of adhesion. It is shown that adhesion at the fiber-resin interface correlates with both the composite strength and the void morphology within the laminate after consolidation.     

Effect of loading‐unloading cycles on impact‐damaged jute/glass hybrid laminates

The production of glass/plant fiber hybrid laminates is a possibility for obtaining semistructural materials with sufficient impact properties, and a better life cycle analysis (LCA) profile than fiberglass. The simplest and possibly the most effective configuration for the production of these hybrids would involve the use of a plant fiber reinforced laminate as the core between two glass fiber reinforced laminates. A main limitation to the use of composites including plant fibers is that their properties may be significantly affected by the presence of damage, so that even the application of a low stress level can result in laminate failure. In particular, it is suggested that when loading is repeatedly applied and removed, residual properties may vary in an unpredictable way. In this work, E‐glass/jute hybrid reinforced laminates, impacted in a range of energies (10, 12.5, and 15 J), have been subjected to post‐impact cyclic flexural tests with a step loading procedure. This would allow evaluating the effect of damage dissipation offered by the plant fiber reinforced core. The tests have also been monitored by acoustic emission (AE), which has confirmed the existence of severe limitations to the use of this hybrid material when impacted at energies close to penetration. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Structure–properties relations in titanium‐based thermoplastic fiber–metal laminates

This paper investigates the interfacial, tensile, and fatigue properties of a titanium alloy fiber–metal laminate (Ti‐FML) based on woven glass‐fiber‐reinforced polyetherimide (GF/PEI). Initial tests, using the single cantilever beam (SCB) geometry have shown that it is not necessary to surface treat the titanium alloy in order to achieve a high value of metal–composite interfacial fracture toughness. Tensile tests have shown that the mechanical properties of the FML lie between those offered by its constituent materials. Tension–tension fatigue tests have shown that the fatigue lives of these laminates are superior to those offered by the plain titanium alloy. The mechanical properties of this glass fiber/PEI FML have also been compared with those offered by an FML based on a unidirectional carbon‐fiber‐reinforced polyetheretherketone (CF/PEEK) composite. Here, it has been shown that although the fatigue properties of this woven GF/PEI composite are inferior to those of the CF/PEEK FML, they do offer a higher temperature capability due to the higher glass transition temperature of the PEI matrix. Polym. Compos. 27:264–270, 2006. © 2006 Society of Plastics Engineers.     

Effect of stacking sequence on the compressive performance of impacted aramid fiber/glass fiber hybrid composite

Aramid fiber/glass fiber hybrid composites were prepared to examine the compressive performance of impacted composites. The effect of stacking sequence and surface treatment on compression after impact (CAI) performance of three‐layer hybrid composites was investigated with respect to delamination area. As the impact velocity increased, the laminates exhibited a significant reduction of compressive strength owing to larger delamination area within laminate. The surface treatment aramid fiber reduced the delamination area and enhanced the resistance to buckling. The strength reduction of laminate AAA was considerable because of wide delaminated region, whereas the residual strength of laminate GGG was not affected by impact energy because the laminate absorbed most of impact energy through formation of fiber cracks rather than delamination. Considering stacking sequence, the laminate GAG and GAA exhibited an energy threshold due to insensitivity to impact damage. As a result, the residual performance of composite was primarily dominated by delamination extent rather than fiber cracks.     

Real‐Time damage detection in thermoplastic‐based composite materials with embedded multi‐mode optical fiber sensors

In this paper we describe the use of an inexpensive standard communication‐grade optical fiber for the purpose of detecting damage in glass fiber reinforced thermoplastic composite materials and hybrid fiber metal laminates based on the same composite constituent. These multi‐mode, step‐index optical fibers were embedded at different locations within the composite structure to assess their ability for structural health monitoring. In this study, the specimens were loaded under a three‐point bend test configuration, Results from the investigation have shown that these optical fibers are capable of detecting the peak failure load in the host material when embedded at locations where structural failure initiates at peak load. The output from the optical fibers was shown to be sensitive to the applied loading conditions suggesting that they can also be used to monitor structural loading. Optical micrographs of selected specimens were obtained by sectioning the specimens at the fracture location to provide confirmation of the optical fiber results.     

Energo-Mechanical Evaluation of Damage Growth and Fracture Initiation in Aviation Composite Structures

The aim of the present paper is to (1) highlight the results of laboratory damage detection and monitoring in the aviation composite materials, during a mechanical testing constituted of multiple loadings, and (2) obtain a detailed understanding of damage evolution of composite specimens with regard to impact energy. Woven 12-ply glass fiber and 16-ply carbon fiber–reinforced epoxy composites (GFRP 92 125/L285/287 and CFRP 98 131/L285/287) were used as less studied subjects in research. This study explored the resistance to cracking and delamination of glass and carbon fiber laminates with the same resin system under low-load conditions.     

Stamp forming of fabric-reinforced thermoplastic composites

A stamp forming technique has been used to process a fabric woven composite made of glass fibers (GF) and polyetherimide (PEI). A hemispherical mold with a built-in hold-down arrangement was designed and used at room temperature to stamp parts from preheated flat preconsolidated laminates. Tensile properties of the material were measured under similar heating conditions as in the relevant stamp-forming process. Stretch in the fiber direction was found to be smaller than the maximum elastic extension of the glass fibers. Reduction of the angle between the crossing fibers was quite large when the satin woven fabric composite was pulled in the 45° direction. The effect of die geometries and original laminate dimensions on the “shear-buckling” were studied. The results described the correlations between processing parameters and fiber buckling. Finally, the local strain of fiber bundles was investigated in relation to different directions of fiber orientation.     

Low velocity impact behavior of glass/epoxy cross-ply laminates with different fiber treatments

This paper deals with the influence of the fiber/matrix adhesion quality on the impact behavior of cross-ply glass/epoxy laminates. Glass fibers with two different treatments (one to promote and one to prevent adhesion to the matrix) were embedded in epoxy matrix systems and subjected to low velocity impacts at energies below perforation energy. It will be shown that the laminates with good fiber/matrix adhesion are significantely more damage resistant than the plates with poor adhesion. It will be pointed out that the composites with the more brittle matrix system show the lower damage resistance. For all materials, the absorbed energy correlates well with the amount of impact induced damage. Furthermore, an experimental/mathematical model is introduced that gives the possibility of predicting the maximum deflection and maximum force for any given impact energy. Only one experiment is needed for each material to identify the model parameters. These parameters describe the real material behavior and can be used as characteristic values to classify materials with respect to their impact stiffness and damage resistance.     

Effect of matrix on the ballistic impact of aramid fabric composite laminates by armor piercing projectiles

Ballistic impact performance of aramid fiber fabric‐epoxy and aramid fiber fabric‐polypropylene (PP)‐based composite laminates has been studied against 7.62 mm armor piercing projectiles. Twaron® was used as aramid fiber fabric in the composites. Role of matrix on the damage pattern has been investigated by impacting the composites of different thickness with projectiles having different strike velocity (SV). Ballistic limit (BL) for each composite has been estimated through correlation of SV and residual velocity (RV) of the projectile by usual V₅₀ method. Ballistic limit was found to vary linearly with composite laminate thickness. Twaron®‐PP composites exhibited higher ballistic limit compared toequivalent thickness of Twaron®‐epoxy composites. Epoxy‐based composites exhibited localized damage mode compared to a global mode of failure in PP‐based composites. Scanning electron microscopy revealed that fibers in Twaron®‐epoxy composites failed largely by shear while tensile mode of failure was observed for Twaron®–PP composites. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Cryogenic microcracking of rubber toughened composites

This study investigated the influence of carboxyl‐terminated butadiene acrylonitrile (CTBN) liquid rubbers on the microcracking response of polymeric composite materials to cryogenic cycling. Matrices of carbon fiber/epoxy prepregs were modified with different concentrations of two CTBN liquid rubbers. The glass transition temperature and the interlaminar shear strength of the laminate systems were depressed as a result of the presence of CTBN in the epoxy phase. An increase in total rubber concentration with the continuous phase was found to decrease and in some cases eliminate microcracking in laminates exposed to cryogenic cycling.     

The influence of cooling rate on the fracture properties of a glass reiforced/nylon fiber-metal laminate

The effect of varying cooling rate on the microstructure and resulting mechanical properties of a novel fiber-metal laminate (FML) based on a glass fiber-reinforced nylon composite has been investigated. Polished thin sections removed from plain glass fiber/nylon composites and their corresponding fiber-metal laminates indicated that the prevailing microstructure was strongly dependent on the rate of cooling from the melt. Mode I and Mode II interlaminar fracture tests on the plain glass fiber reinforced nylon laminates indicated that the values of G_Ic and G_IIc averaged approximately 1100 J/m² and 3700 J/m² respectively at all cooling rates. The degree of adhesion between the aluminum alloy and composite substrates was investigated using the single cantilever beam geometry. Here, the measured values of G_c were similar in magnitude to the Mode I interlaminar fracture energy of the composite, tending to increase slightly with increasing cooling rate. The tensile and flexural fracture properties of the plain composites and the fiber metal laminates were found to increase by between 10% and 20% as the cooling rate was increased by two orders of magnitude. This effect was attributed to over-aging of the aluminum alloy plies at elevated temperature during cooling. Finally, fiber metal laminates based on glass fiber/nylon composites were shown to exhibit an excellent resistance to low velocity impact loading. Damage, in the form of delamination, fiber fracture, matrix cracking in the composite plies, and plastic deformation and fracture in the aluminum layer, was observed under localized impact loading. Here, the fast-cooled fiber metal laminates offered superior post-impact mechanical properties at low and intermediate impact energies, yet very similar results under high impact energies.     

Open hole flexural and izod impact strength of unidirectional flax yarn reinforced polypropylene composites as a function of laminate lay‐up

An open hole flexural strength and impact energy of flax yarn‐reinforced polypropylene (PP) composites were studied in this work. Highest flexural strength and strength retention were observed for axial (0₆) and cross‐ply (0/90/0)_s laminates, respectively, while also examining the influence of laminate lay‐up and open hole size on flexural strength. It was found that maleic anhydride‐grafted polypropylene (MAPP)‐treated composite laminates achieved marginal improvement on flexural strength for all kinds of laminate lay‐up. Off‐axial laminates (±45₆) showed a good strength retention for open hole laminates after MAPP treatment. The fractography study confirmed microbuckling and matrix crack propagation over the compressive and tensile side of the laminate, respectively. Furthermore, severe surface damage was detected over the tensile side of 8‐mm hole size laminates. Impact test of the flax/PP laminates showed slight improvement by MAPP treatment. High‐ and low‐impact energy was experienced for axial and off‐axial laminates. The damaged impact sample shows evidence of fiber pull‐out for untreated flax yarn reinforced laminates. POLYM. COMPOS., 34:1912–1920, 2013. © 2013 Society of Plastics Engineers     

Mechanical and thermal enhancements of benzoxazine‐based GF composite laminated by in situ reaction with carboxyl functionalized CNTs

An easy and efficient approach by using carboxyl functionalized CNTs (CNT‐COOH) as nano reinforcement was reported to develop advanced thermosetting composite laminates. Benzoxazine containing cyano groups (BA‐ph) grafted with CNTs (CNT‐g‐BA‐ph), obtained from the in situ reaction of BA‐ph and CNT‐COOH, was used as polymer matrix and processed into glass fiber (GF)‐reinforced laminates through hot‐pressed technology. FTIR study confirmed that CNT‐COOH was bonded to BA‐ph matrices. The flexural strength and modulus increased from 450 MPa and 26.4 GPa in BA‐ph laminate to 650 MPa and 28.4 GPa in CNT‐g‐BA‐ph/GF composite, leading to 44 and 7.5% increase, respectively. The SEM image observation indicated that the CNT‐COOH was distributed homogeneously in the matrix, and thus significantly eliminated the resin‐rich regions and free volumes. Besides, the obtained composite laminates showed excellent thermal and thermal‐oxidative stabilities with the onset degradation temperature up to 624°C in N₂ and 522°C in air. This study demonstrated that CNT‐COOH grafted on thermosetting matrices through in situ reaction can lead to obvious mechanical and thermal increments, which provided a new and effective way to design and improve the properties of composite laminates. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Evaluation of adhesion of continuous fiber–epoxy composite/aluminum laminates

Continuous fiber composite/metal laminates (FMLs) offer significant improvements over currently available composite materials for aircraft structures due to their excellent fatigue endurance and low density. Glass fiber–epoxy composite laminae and aluminum foil (GLARE) are commonly used to obtain these hybrid laminates. In this work, FMLs were produced by treating the aluminum foil to promote adhesion bonding by two methods: sulphuric chromic acid etching (SCAE) and chromic acid anodization (CAA). The surface treatments were evaluated by contact angle, roughness and scanning electron microscopy techniques. In order to compare different families of fiber composite/metal laminates, carbon fiber and glass fiber fabrics were used as reinforcements for the hybrid laminates. The adhesion of the hybrid laminates was evaluated by scanning electron microscopy (SEM) and three-point bending test. CAA resulted in better wetting properties. The interlaminar shear strength results for both carbon fiber-epoxy/metal and glass fiber-epoxy metal, were close to the interlaminar shear strength results found in the literature (approx. 40.0 MPa).     

Crashworthiness of automotive composite material systems

The energy absorption capability of a composite material is important in developing improved human safety in an automotive crash. In passenger vehicles, the ability to absorb impact energy and be survivable for the occupant is called the crashworthiness of the structure. The crashworthiness in terms of the specific energy absorption (SEA) of a chopped carbon fiber (CCF) composite material system was compared with that of other fiber resin systems such as graphite/epoxy cross‐ply laminates (CP#1 and CP#2), a graphite/epoxy‐braided material system (O), and a glass‐reinforced continuous‐strand mat (CSM). The quantity of these material systems needed to ensure passenger safety in a midsize car traveling at various velocities was calculated and compared. The SEA of the chopped carbon fiber composite material was the highest compared to that of all the other composites investigated. It was calculated that only 4.27 kg of it would need to be placed at specific places in the car to ensure passenger safety in the event of a crash at 15.5 m/s (35 mph). This clearly led to an important practical conclusion that only a reasonable amount of this composite material is required to meet the necessary impact performance standard. The CCF composite tested at 5 mm/min crushing speed met both the criteria that need to be satisfied before a material is deemed highly crashworthy: A high magnitude of energy absorption and a safe allowable rate of this energy absorption. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3218–3225, 2004     

The morphing properties of a smart fiber metal laminate

This article investigates the activation characteristics of a novel fiber‐metal laminate (FML) based on a nickel–titanium (Ni–Ti) shape memory alloy. Initial attention focuses on manufacturing this smart FML in which a woven glass fiber reinforced epoxy material is sandwiched between two shape memory alloy (SMA) outer skins. Activation tests on cantilever beams using a hot air gun have shown that the FMLs exhibits a distinct actuation capability in which beam rotations of up to 11° were recorded. An examination of the edges of polished samples indicated that no damage was incurred by the FML during the activation process. The functionality of the FMLs was enhanced through the introduction of embedded electrical resistance wires located between the composite and metal plies. Here, the embedded electrical wires were heated by passing an electric current through them, thereby activating the SMA plies in a more effective and controllable manner. As before, significant beam tip rotations were recorded in the FMLs in a relatively short time period. Finally, polymer‐based optical fiber (POF) and fiber‐bragg grating (FBG) sensors were introduced into the FMLs in order to monitor their deflection during the activation process. The results of these tests showed that such sensing elements can be successfully employed to monitor the actuation response of these layered laminates. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Tensile behavior of unidirectional polyethylene fibers PMMA and glass fibers—PMMA composite laminates

Unidirectional (UD) composite laminates based on glass fibers (GF) and high‐performance polythylene fibers (PEF) were prepared with partially polymerized methyl methacrylate (MMA) at room temperature, followed by heating at 55°C (well below the softening point of PEF) for 2 h. The tensile strength, modulus of elasticity, fiber efficiency and strength efficiency of both the composite laminates, loaded parallel to the fibers, at the same volume fraction range, were investigated. All the properties were compared between the two composite laminates. It was observed that the measured tensile strength and modulus of elasticity deviated from the values calculated from the Rule of Mixture (ROM). The deviation was minimal at the lower volume fraction of fibers, and increased with the fiber volume. An interesting feature that was observed was that the efficiencies of PEF‐reinforced composite was higher than that of the GF‐reinforced composite at the same volume fraction of the fibers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1489–1493, 2000