三维五向编织复合材料低速冲击及冲击后压缩性能实验研究

通过实验研究三维五向碳纤维/环氧树脂编织复合材料低速冲击及其冲击后压缩(CAI)性能。测试试件虽然有不同的编织角度,但承受相同的冲击能力。采用冲击后压缩测试表征不同编织结构的冲击后剩余力学性能。结果表明:编织角较大的试件由于其更紧密的空间结构,能承受更高的冲击载荷且冲击损伤区域更小。CAI强度和损伤机理主要取决于编织纤维束的轴向支撑。随着编织角的增加,CAI强度降低,材料的破坏模式也由横向断裂转变为剪切破坏。     

Cooling rate influences in carbon fibre/PEEK composites. Part III: impact damage performance

The effect of cooling rate on impact damage performance of carbon fibre/polyether ether ketone (PEEK) matrix composite is characterised based on the instrumented drop-weight impact test, scanning acoustic microscopy (SAM) damage evaluation and compression-after-impact (CAI) test. Judging from the incipient impact load, incipient impact energy and total damage area, the ability to resist damage initiation upon impact was higher in the order of fast-cooled carbon/PEEK, slow-cooled carbon/PEEK and carbon/epoxy laminates. Furthermore, the threshold impact energy was higher and the CAI strength reduction rate was lower for the fast-cooled specimen than the slow-cooled counterpart, strongly indicating higher impact damage tolerance of the former system. The present study demonstrates that the impact damage performance and other important properties of carbon/PEEK composites can be optimised, if not maximised, by proper control of processing conditions, especially the cooling rate.     

ESTM-fabric/3266复合材料低速冲击响应及冲击后压缩行为研究

基于“离位”技术,分别开发两种新型聚醚砜(PES)点阵附载型(ES-L)和PES无规附载U3160织物型(ES-R)ES^TM-fabric织物,采用RTM工艺制备ES^TM-fabric织物增强3266中温环氧树脂基复合材料(ES^TM-fabric/3266),对其进行冲击阻抗及冲击后压缩测试,并利用荧光显微镜、SEM结果分析离位增韧机理,还对比研究未增韧U3160织物增强3266中温环氧树脂基复合材料的性能。低速冲击测试结果表明:相比未增韧U3160/3266(ES-U),ES^TM-fabric/3266的起始损伤阈值载荷显著提高,冲击损伤面积明显减少,裂纹扩展更加平缓,且以层内基体裂纹、纤维束内的纤维-基体脱粘和局部铺层断裂为主。ES-L的CAI值比ES-U增大了37%。ES-R层间出现均布式相反转结构,ES-L层间存在硬相区(富BMI连续相)和软相区(富PES连续相/3266相反转结构);ES-L的相结构能够更加有效地缓解应力集中、耗散冲击能量,从而使其表现出最佳的损伤阻抗和损伤容限性能。     

Investigation of knitting architecture on the impact behavior of glass/epoxy composites

In this study, the effect of impact and post-impact behavior of E-glass/epoxy composite plates having different knitted fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5–25 J) were subjected to the plates consisting of eight layers of Plain [P]₈, Milano [M]₈, and Rib [R]₈ knitted fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression after Impact (CAI) strength of layer fabric samples. The results of these impact and post-impact tests showed that the maximum contact force was observed in the [R]₈ fabric and the minimum contact force was observed in the [P]₈ fabric and the CAI strength reduced by increasing the impact energy.     

Post-impact behaviour of aerospace composites for high-temperature applications: experiments and simulations

The post-impact performance of different carbon-fabric-reinforced composite materials were studied experimentally and analytically. Three types of thermosetting matrix were considered: conventional epoxy, high-temperature curing epoxy and epoxy-isocyanate. Experimental testing consisted of impacting rectangular specimens at different energy levels by using a spring-driven impact apparatus that was able to impart velocities of up to 5 m s⁻¹ to masses of 0.5, 1.0, 2.5 and 5.0 kg travelling horizontally. After impact, coupons were non-destructively inspected by means of opaque-enhanced dye-penetrant X-radiography and tested in static compression to correlate impact energy, damage extent and residual strength. Epoxy composites contain damage within a narrow region, while epoxy-isocyanate materials propagate the damage far away from impact point. Epoxy composites show an asymptotically decreasing failure strength with impact energy up to a lower threshold (0.3–0.4 times that of the undamaged material), while epoxy-isocyanate material shows a trend of ever decreasing residual strength. An analytical study was performed by means of the finite element code PAM-FISS, used to simulate the compression-after-impact (CAI) tests. Type, size and location of damage, as well as the mechanisms leading to final failure, were reproduced quite well by the finite element analysis (FEA), while some discrepancies between FEA and experimental CAI residual strength tests were found (7% for undamaged specimens and 10% for blister-delaminated specimens); higher errors were found in the case of completely delaminated specimens, mainly owing to the inability of the present software and hardware to conveniently model the complete state of damage.     

三维编织玻璃/环氧复合材料梁的低速冲击和冲击后压缩性能研究

试验制备了三维编织四向结构、五向结构和六向结构的玻璃纤维预制件增强环氧树脂梁的复合材料试样,每种试样包含20°、30°和40°三个编织角度.研究了编织结构和编织角参数对复合材料低速冲击及冲击后压缩性能的影响,分析了损伤后的试样形貌及破坏情况.试验结果表明:编织参数对复合材料的损伤容限影响较显著;编织角相同时,五向结构具有较高的CAI强度,而六向结构则表现出较好的冲击韧性;编织结构相同时,30°编织角试样的抗冲击性能较好;同时,冲击后压缩试样表现出脆性断裂特征.     

Impact and post impact behavior of layer fabric composites

In this study, the effect of impact and post impact behavior of E-glass/epoxy composite plates having different layer fabrics were investigated by considering energy profile diagram and the related load–deflection curves. Different impact energies (5 J–60 J)were subjected to the plates consisting of eight layers of plain weave (1D), double (2D) and triple (3D) layer fabrics. The impact tests were continued until complete perforation of layer fabrics. The damage modes and damage processes of layer fabrics under varied impact energies were also discussed. At the end of the impact tests, the damaged samples were mounted into a compression apparatus to determine the Compression After Impact (CAI) strength of layer fabric samples. The results of these impact and post impact tests showed that contact force occurring between the impactor and the composite specimen increased and the CAI strength reduced by increasing the impact energy. The objective of this study was to determine the perforation threshold of E-glass/epoxy composite plates having different layer fabrics as plain weave (1D), double (2D), and triple (3D) layer fabrics.     

Tribological and mechanical behaviour of dual-particle (nanoclay and CaSiO<Subscript>3</Subscript>)-reinforced E-glass-reinforced epoxy nanocomposites

An E-glass-reinforced epoxy-based nanocomposite containing organomodified nanoclay (15–20 nm) and calcium silicate particles (75–149 μm) was developed through mechanical shearing mixing and hand layup techniques. Three weight fractions (2, 3 and 4%) of nanoclay were selected to study the effects of nanoclay on mechanical and wear behaviour of nanocomposites. Tensile and flexural properties of nanocomposites were evaluated and compared. The wear properties were evaluated for three speed (3.14, 4.19 and 5.24 m s^?1) and load (20, 50, and 80 N) conditions based on a design of experiment (L16 matrix) concept. The wear loss results were statistically analysed to study the significance of load, speed and nanoclay content. The morphologies of wear surface and fracture surface were examined with the aid of a scanning electron microscope (SEM) to identify the wear and fracture mechanisms. It was found that the wear loss increases with increasing nanoclay amount due to the particle agglomeration effects. Statistical analysis determines that the load is the most significant parameter affecting the wear resistance of nanocomposites. The mean and S/N ratio analyses rank the parameters significance in affecting wear resistance as follows: load > nanoclay content > speed. The wear mechanisms of nanocomposites are complex due to the observation of multiple features such as fibre thinning, matrix wear and fibre/matrix debonding as against abrasive wear in the pure epoxy. Tensile and flexural test results show that a good dispersion of nanoclay is achieved with 2 wt% amount in epoxy-based nanocomposites. The mechanical properties degrade above 2 wt% due to the excessive reinforcement, uneven distribution and the particle agglomeration effects. Fractography studies of tension-failed samples show that pure epoxy resin fails by multimode gauge explosive mode, whereas nanocomposites fail mainly by the matrix/fibre interface failure and fibre breakages.     

Structurally stitched NCF CFRP laminates. Part 1: Experimental characterization of in-plane and out-of-plane properties

The insertion of local through-thickness reinforcements into dry fiber preforms by stitching provides a possibility to improve the mechanical performance of polymer-matrix composites perpendicular to the laminate plane (out-of-plane). Three-dimensional stress states can be sustained by stitching yarns, leading to increased out-of-plane properties, such as impact resistance and damage tolerance. On the other hand, 3D reinforcements induce dislocations of the in-plane fibers causing fiber waviness and the formation of resin pockets in the stitch vicinity after resin infusion which may reduce the in-plane stiffness and strength properties of the laminate.In the present paper an experimental study on the influence of varying stitching parameters on in-plane and out-of-plane properties of non-crimp fabric (NCF) carbon fiber/epoxy laminates is presented, namely, shear modulus and strength as well as compression after impact (CAI) strength and mode I energy release rate. The direction of stitching, thread diameter, spacing and pitch length as well as the direction of loading (which is to be interpreted as the direction of the three rail shear loading or the direction of crack propagation in case of mode 1 energy release rate testing) were varied, and their effect on the mechanical properties was evaluated statistically.The stitching parameters were found to have ambivalent effect on the mechanical properties. Larger thread diameters and increased stitch densities result in enhanced CAI strengths and energy release rates but deteriorate the in-plane properties of the laminate. On the other hand, a good compromise between both effects can be found with a proper selection of the stitching configurations.     

Mechanical,thermal and microstructural characteristics of cellulose fibre reinforced epoxy/organoclay nanocomposites

Epoxy nanocomposites reinforced with recycled cellulose fibres (RCFs) and organoclay platelets (30B) have been fabricated and investigated in terms of WAXS, TEM, mechanical properties and TGA. Results indicated that mechanical properties generally increased as a result of the addition of nanoclay into the epoxy matrix. The presence of RCF significantly enhanced flexural strength, fracture toughness, impact strength and impact toughness of the composites. However, the inclusion of 1 wt.% clay into RCF/epoxy composites considerably increased the impact strength and toughness. The presence of either nanoclay or RCF accelerated the thermal degradation of neat epoxy, but at high temperature, thermal stability was enhanced with increased char residue over neat resin. The failure micromechanisms and energy dissipative processes in these nanocomposites were discussed in terms of microstructural observations.     

Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites.     

Damping characteristics of nanoclay filled hybrid laminates during medium velocity impact

The objective of this paper is to study the vibrational damping characteristics during medium velocity impact of nanoclay filled glass fiber reinforced epoxy hybrid laminates. A series of laminates with varying degree of nanoclay concentration (0–5 wt.%) and fiber weight fraction (25–75 wt.%) were prepared by vacuum assisted resin infusion molding (VARIM) method. The laminates were subjected to medium velocity projectile impact using in-house built gas gun set-up and the ballistic limit of laminates series was determined. The result indicated that during impact, the laminate undergoes vibrational damping. This damping property is a function of fiber weight fraction and orientation, nanoclay concentration and nanocomposite structure. A 42% increase of ballistic limit was observed for 5 wt.% nanoclay filled hybrid (50 wt.% fiber) when compared with unfilled composite. Structural and modal analysis of hybrids showed that the increased ballistic limit of nanoclay filled hybrids is due to the nanocomposite structure and improved damping and fracture properties.     

Delamination properties of z-pinned composites in hot–wet environment

The effect of hot–wet environment (75 °C and 85% relative humidity) on the delamination fracture properties and interlaminar toughening mechanisms of z-pinned carbon fibre–epoxy composite is investigated. The absorption rate of water from the hot–wet environment into the composite is accelerated slightly by z-pins, although the pins did not change the saturation limit of the material. Absorbed water weakens the pin/composite interface and this lowers the ultimate elastic traction load generated by z-pins under mode I interlaminar loading. However, once the pin/composite interface has failed, the traction load and energy required to pull-out the z-pins is not affected by absorbed water. The mode I interlaminar fracture toughness and low-energy impact damage resistance of z-pinned composites is not degraded significantly by exposure to hot–wet environment, and this is because absorbed water does not affect the pull-out traction properties of z-pins.     

Effect of resin and fibre properties on impact and compression after impact performance of CFRP

The effect of resin and fibre properties on composite impact, compression after impact (CAI) and mode II energy release rate (G_IIC) performance has been studied. Impact events were instrumented to record values of P_c, the critical load for initiation of impact damage. Impact response of the laminates was strongly influenced by the fracture toughness of the resin. In contrast, use of high strength and high stiffness fibres did not improve the resistance to impact. The differences in impact and CAI response of the laminates were largely a consequence of the impact damage created at the damage threshold, P_c, rather than of the differences in delamination growth. As a strong correlation was found between G_IIC values measured by delamination tests, and those calculated from measurements of P_c, it is suggested that instrumented impact testing may be a more convenient way of determining G_IIC in CFRP laminates than delamination tests.     

Mechanical properties and water uptake of epoxy–clay nanocomposites containing different clay loadings

Nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) epoxy reinforced with 1–10 wt% I.30E nanoclay were fabricated using high shear mixing technique and characterized to determine the effects of clay loading on their mechanical, thermal, and water uptake properties. The XRD and TEM analyses revealed that the structures of the resultant nanocomposites were a combination of disordered intercalated and exfoliated morphologies. Tensile strength increased for nanocomposite containing 1 % clay loading and decreased for higher nanoclay loading. Unlike strength, the stiffness increased almost linearly with clay loading, showing 46 % improvement in modulus of elasticity for nanocomposites containing 5 % of nanoclay. Water uptake measurements indicated enhancement in the barrier properties of epoxy matrix as nanoclay loading increased from 1 up to 5 wt%.     

Nano-structured sandwich composites response to low-velocity impact

This paper investigates the influence of exfoliated nano-structures on sandwich composites under impact loadings. A set of sandwich composites plates made of fiberglass/nano-modified epoxy face sheets and polystyrene foams was prepared. The core was 25 mm thick and the face sheets were made of eight layers of woven fabric glass fibers and nano-modified epoxy (≈0.8 mm of thickness). The epoxy system was bisphenol A resin and an amine hardener. The fiber volume fraction used was around 65%, while the nanoclay content varied from 0 wt.% to 10 wt.%. The nanoclay used was Cloisite 30B from Southern Clay. The sandwich panels were submitted to low-velocity impact tests with energies from 5 J to 75 J. Two sets of experiments were performed, i.e. high velocity + low mass and low velocity + high mass. Damage caused by the two groups of experiments and peak forces measured were dissimilar. The results show that the addition of 5 wt.% of nanoclay lead to a more efficient energy absorption. The failure modes were also analyzed, and they seems to be affected by the nanoclay addition to face sheets.     

Tribological behavior of nanoclay/epoxy composites

This paper aims to discuss the mechanical performance of nanoclay/epoxy composites (NCs) through micro-hardness and abrasive tests. It was found that the hardness and wear resistance of NCs increased with increasing nanoclay content of up to 4 wt.%. The improvement of mechanical properties of the NCs by increasing nanoclay content is explained by the degree of agglomeration of nanoclay clusters inside the NCs through SEM and XRD investigations. A mathematical interpretation for the determination of hardness and wear resistance of the NCs at different nanoclay contents in relation to the diameter of nanoclay clusters and their inter-particle distance is given.     

Development of rubberized syntactic foam

This study explored a novel hybrid syntactic foam for composite sandwich structures. A unique microstructure was designed and realized. The hybrid foam was fabricated by dispersing styrene–butadiene rubber latex coated glass microballoons into a nanoclay and milled glass fiber reinforced epoxy matrix. The manufacturing process for developing this unique microstructure was developed. A total of seven groups of beam specimens with varying compositions were prepared. Each group contained 12 identical specimens with dimensions 304.8 mm × 50.8 mm × 15.2 mm. The total number of specimens was 84. Among them, 42 beams were pure foam core specimens and the remaining 42 beams were sandwich specimens with each foam core wrapped by two layers of E-glass plain woven fabric reinforced epoxy skin. Both low velocity impact tests and four-point bending tests were conducted on the foam cores and sandwich beams. Compared with the control specimens, the test results showed that the rubberized syntactic foams were able to absorb a considerably higher amount of impact energy with an insignificant sacrifice in strength. This multi-phase material contained structures bridging over several length-scales. SEM pictures showed that several mechanisms were activated to collaboratively absorb impact energy, including microballoon crushing, interfacial debonding, matrix microcracking, and fiber pull-out; the rubber layer and the microfibers prevented the microcracks from propagating into macroscopic damage by means of rubber pinning and fiber bridge-over mechanisms. The micro-length scale damage insured that the sandwich beams retained the majority of their strength after the impact.     

层间颗粒增韧复合材料层压板的损伤阻抗特性

采用热塑性颗粒对HT7/ 5228 、HT3/ N Y9200G和HT3/ 5224 三种高温固化环氧基体复合材料层压板进行层间增韧。为了提高冲击后压缩强度(CAI) 和考察损伤阻抗, 测试了平均分层起始能量e_Ⅱc以及接触力-凹坑深度关系。试验结果表明, 增韧颗粒和基体树脂形成的层间区域能有效地吸收断裂能量并抑制分层的发生, e_Ⅱc显著提高。在静压痕力下, 层间增韧复合材料层压板具有较深的凹坑深度和较小的损伤面积。层间增韧的几何效应、裂纹传播路径控制、颗粒桥联以及裂尖屏蔽是主要的增韧机理, 颗粒的塑性变形和最终失效也耗散了大量断裂能量。      

A study on nanostructured laminated plates behavior under low-velocity impact loadings

The influence of montmorillonite (MMT) silicate layers on glass-fiber-epoxy laminated composites behavior has been investigated by low-velocity impact and X-ray diffraction tests. The glass-fiber-epoxy-nanoclay laminate composites have 16 layers and 65% fiber volume fraction is manufactured by vacuum-assisted wet lay-up. Fibers have a plain-weave configuration with density of 200 g/m², while the epoxy resin system is made of diglycidyl ether of bisphenol A resin with aliphatic amine as the curing agent. The nanoclay (Nanomer I30E) is dispersed into the epoxy system in a 1%, 2%, 5% and 10% ratio in weight with respect to the matrix. X-ray diffraction tests indicate that rather than exfoliated, these nanostructures are mostly in intercalated form, with a possible presence of immiscible nanosystems at 10% concentration. The methodology used for the impact test is based on the ASTM D5628-01 standard. The results have shown that for the four edges clamped condition not only the delamination phenomenon is reduced, but also the damping is increased during the rebounds. Moreover, for the 20 J impact energy condition the energy absorption by delamination increases close to 48%, while for larger energies, i.e. 60 J, the average improvement into energy absorption is around 15%. Even for larger energies close to total perforation, i.e. 80 J, the use of nanoclays leads to an average increase in energy absorption by delamination close to 4%. Finally, the failure mechanism seems to be affected by the nanoclay presence, as the interlaminar failure shifts to a mostly intralaminar failure with the increase of nanoclay content.