Effects of camber height and boundary condition on energy absorption of arched composite laminates

The scope of this study on arched composite laminates subjected to low-velocity impact was two-fold. One was to investigate the effect of camber height on energy absorption and the other the effect of boundary condition on energy absorption. All specimens were made of the same prepreg tape material and a cross-ply stacking sequence of [0/90]_3s such that a comparison among flat panels and arched specimens with various camber heights can be made. Analysis on the load–deflection relation, the energy profile and the damage process were of primary interest as they provided the insight into the impact behavior of arched composites. Experimental results showed that the maximum deflection and the energy absorption increased significantly as the camber height increased while the peak load decreased slightly. It was also found that the boundary condition played an important role in the energy absorption process. Frame-clamped specimens experienced higher slippage, and hence higher energy absorption, than bolted specimens. This slippage was observed after impact because the specimen actually would pull out of the frame. Video footage also verified this phenomenon.     

冲击作用下斜接修补CFRP层间分层和胶层的损伤机制及参数分析

首先,针对斜接修补CFRP抗冲击性能差的问题,分别使用基于接触的内聚力模型（SCZM）和基于单元的内聚力模型（ECZM）描述层间分层和斜接胶层破坏,研究CFRP层板的冲击响应和两种失效的演化规律。然后,分析了冲击能量、斜接角度和预拉伸作用对两种失效的影响。结果表明：层间分层起始时间早于胶层破坏,与冲击能量无关;分层和胶层破坏面积随冲击能量增加而增大,胶层破坏面积增加的更明显;斜接角度主要影响胶层破坏,对分层面积几乎无影响;预拉伸作用对两种失效均具有负面作用。最后,进一步讨论分层对胶层破坏的影响,通过与只考虑胶层破坏的情况进行对比,发现层间分层使胶层破坏的面积降低,延缓了胶层的最终失效。     

基于应变信号的复合材料层合板低速冲击损伤识别研究

为了进行复合材料层合板的冲击损伤识别研究, 设计了由小到大的14个等级的冲击能量, 对应由小到大的冲击损伤, 其中最小能量的冲击没有造成损伤, 最大能量的冲击造成穿透复合材料板的损伤。应用冲击过程中测试得到的应变响应进行损伤识别。基于冲击过程中获得的脉冲应变信号, 提出了10个新的冲击损伤识别指标：脉冲上升时间、下降时间、总时间、上升时间比下降时间、下降时间比上升时间;脉冲上升面积、下降面积、总面积、上升面积与下降面积之比、下降面积与上升面积之比。将提出的10个新冲击损伤识别指标与冲击能量的关系进行了对比研究。研究表明：脉冲下降时间、脉冲总时间、脉冲下降面积、脉冲总面积与冲击能量之间的关系大体上都是单调的, 这4个指标适合0.8J/mm到6.67J/mm的大范围的冲击损伤程度识别, 其余6个指标在一定的冲击能量范围内可以识别冲击损伤程度。应用脉冲下降面积、脉冲总面积指标进行了冲击定位识别研究, 研究表明, 这两个指标也可以识别冲击损伤的位置。     

Effect of basalt fiber hybridization on the impact behavior under low impact velocity of glass/basalt woven fabric/epoxy resin composites

The low velocity impact behavior of E-glass/basalt reinforced hybrid laminates, manufactured by resin transfer moulding technique, was investigated. Specimens prepared with different stacking sequences were tested at three different impact energies, namely 5 J, 12.5 J and 25 J. Residual post-impact mechanical properties of the different configurations were characterized by quasi static four point bending tests. Post-impact flexural tests have been also monitored using acoustic emission in order to get further information on failure mechanisms. Results showed that basalt and hybrid laminates with an intercalated configuration exhibited higher impact energy absorption capacity than glass laminates, and enhanced damage tolerance capability. Conversely, the most favorable flexural behavior was shown by laminates with symmetrical sandwich-like configuration (E-glass fiber fabrics as core and basalt fiber fabrics as skins).     

A Study on the Charpy Impact Response of the Cracked Aluminum Plates Repaired with FML Composite Patches

Fiber metal laminates (FMLs) are widely used in aerospace industries nowadays. Reparation of the cracks in these advanced materials was first done by some aeronautical laboratories in the early 1970s. In this study, experimental investigations were done on the effects of repairing the edge-cracked aluminum plates using the FML patches. The repairing processes were conducted to characterize the response of the repaired structures to the Charpy impact tests. The composite patches were made of one aluminum layer and two woven glass–epoxy composite layers. Three different crack lengths, crack angles, and patch lay-ups were examined. It was indicated that for the lengthen cracks, the effect of increasing the crack angle on energy absorption in the structure was more. When the ratio of crack length to the specimen width, i.e., a/w, is 0.5, the energy absorption per unit area of the specimens having different crack angles but the same patch lay-ups was so different. It was also observed that the percentage of the absorbed energy of 45° cracked angle specimens was about 25% higher than the 0° ones. Also it was observed that the lay-up of the patches and the place where the metal layer was embedded in the FML patches had an important effect on the impact response of the tested specimens. The more the metal layer of the patches is far from the interfacial surface of the aluminum plate and the FML patches, the less the energy absorbs in the structure.     

Damage response of stitched cross-ply laminates under impact loadings

The impact response of stitched graphite/epoxy laminates was examined with the aim of evaluating the efficiency of stitching as a reinforcing mechanism able to improve the delamination resistance of laminates. The investigation, which focussed on two classes of cross-ply stacking sequences ([0₃/90₃]_s and [0/90]_3s), showed that the role of stitches in controlling damage progression of laminates and their capability to reduce the impact sensitivity of specimens are greatly dependent on the impact behaviour of base (unstitched) laminates. In [0₃/90₃]_s laminates, in particular, stitching is able to reduce damage area, on condition that the impact energy is higher than a threshold level and delaminations are sufficiently developed. In [0/90]_3s laminates, on the other hand, stress concentration regions generated by the stitching process appear to promote the initiation and propagation of fibre fractures, thereby inducing a decrease in the penetration resistance of the laminate.     

Damage evolution in glass/epoxy composites engineered using core–shell microparticles under impact loading

The overall objective of the investigation presented in this paper was to study the effect of dispersion of core–shell polymer (CSP) particles within the ply interfaces on damage evolution of glass/epoxy laminates under impact loading. These laminates were fabricated with the CSP particle dispersion controlled to 14 % of the total weight of the used prepreg. A series of impact experiments were done with instrumental drop tower device at all probable impact energies within a practical low velocity impact range. The damage phenomena occurring in the internal microstructure of the laminates were analysed with the help of scanning electron microscope and correlated to the structural response of the laminate. The predominant damage modes were dependent on the magnitude of the applied impact energy. The CSP particle incorporation does not change the sequence of the fracture events but it delays and mitigates the damage creation. The deformation of the CSP particles and the tearing of their outer shells absorb most of the impact energy thereby preventing initiation of matrix cracks at lower impact energies and delaying fibre damage at higher energies. The crushed particles along with their nano-size rubber cores impede crack propagation requiring the cracks to follow torturous paths consequently dissipating additional amount of energy. These particles also promote elastic energy absorption of the laminates minimizing their tendency to fracture easily under impact. The ultimate load bearing capability of the modified laminate showed 60 % improvement and the deflection characteristics indicated lower proneness to impact.     

Improving impact resistance of carbon-fibre composites through interlaminar reinforcement

This work compares the impact resistance enhancement available through modifications to the interlaminar region of carbon fibre epoxy composites. Interleaving and short fibre interlaminar reinforcement techniques were used to improve the impact resistance of [0/90]₅ laminates tested using a Dynatup 8250 instrumented impact tester. Visual observation was also employed to assess the affects of the interlaminar reinforcement whilst quantifiable assessment compared incident and restitutive energies to determine energy absorbed during the impact event. The results confirmed that when short fibres are used to interlaminarly reinforce the laminates, lower quantities of incident energy are absorbed than when laminates are interleaved or remain unreinforced. Visual inspection however showed damaged areas to be smallest in interleaved specimens, followed by short fibre reinforced and unreinforced laminates.     

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.     

A study on low velocity impact damage evaluation and repair technique of small aircraft composite structure

Composite structures are very prone to damage at fairly modest levels of impact energy due to foreign object damages. A repair technique using external patch is recognized as an effective method to recover the damaged structures during service life. This work is focusing on the impact damage evaluation and the external patch repair techniques of the aircraft composite structure. The impact damages of composite laminates of the carbon/epoxy UD laminate and the carbon/epoxy fabric face sheets-honeycomb core sandwich laminate are simulated by the drop-weight type impact test equipment. The damaged specimens are repaired using the external patch repair method after removing the damaged area. The compressive strength test and analysis results of the repaired impact damaged specimens are compared with the compressive strength test and analysis results of the undamaged specimens and the impact damaged specimens. Finally, the strength recovery capability after repairing is investigated.     

Characterisation of impact response of metallic foam/ceramic laminates

Abstract

The impact response of laminated composites consisting of alternate layers of Al alloy foam and Al₂O₃ was studied experimentally in low and intermediate velocity regimes. Low velocity impacts (1.2–2.8 m s^-1) were conducted using an instrumented falling weight apparatus and were compared with static indentation tests (0.2×10^-4 m s^-1). Intermediate velocity impacts were carried out by means of both Hopkinson bar (60 m s -1)and gas gun (200 m s^-1) tests. Post­impact damage was assessed using X-ray radiography and microscopy. It was found that there is good correlation between low velocity impact and quasi­static responses. In both cases, penetration of the layered targets resulted in the formation of a distinctive plug. Increasing impact velocity (intermediate velocity range) switched the penetration mode from plugging to fragmentation, giving rise to an increase in the absorbed energy. In this range, impacts led to localisation of damage in the region under the projectile. Furthermore, a comparison has been made between the penetration response of foam laminates and dense metal laminates of equivalent areal density. Preliminary results suggest that the dense metal laminates are superseded by the foam laminates on an energy absorption basis.     

环氧玻璃钢层合板冲击破坏及其断口微观特征的研究

S-GF/Ep648复合材料层合板[0₉°/θ]_s冲击抗力试验的结果表明其冲击抗力值与铺层角θ无关,只受浸水时间的影响;而另一种铺层形式[+θ₈/-θ₈]的复合材料层合板,在落重冲击试验中却发现:在相同的冲击能量下,θ越大,分层破坏越严重,损伤区面积也越大。利用SEM-505扫描电子显微镜对发生穿透性断裂、层间分层断裂或仅出现冲击损伤等破坏形式的断口表面进行了观察和分析,讨论了影响该复合材料冲击断裂和冲击损伤性能的因素。     

Delaminations of barely visible impact damage in CFRP laminates

CFRP laminates were impacted by projectiles of low masses, accelerated in an air gun, to have barely visible impact damage (BVID) to simulate damage to aircraft by runway debris. The delamination damage on individual interfaces was revealed by the destructive method of thin strips. In sub-BVID and BVID specimens, the damage was confined mostly to the front 30% of the laminate thickness. Delamination areas in the BVID specimens were found to be considerable — the largest dimension exceeding 12 mm on several interfaces. Nucleation of delamination damage was observed in interfaces adjacent to the mid plane in BVID specimens. At higher impact energies, about 110 to 150% more, the delamination damage was observed on almost all the interfaces with no sign of spalling at the rear surfaces. In comparison with a lightweight projectile of aluminium (4·4 g), a higher density steel projectile (11·8 g) caused more delamination damage for the same impact energy and an identical geometry of projectiles.     

缝合复合材料层板低速冲击损伤数值模拟

建立了缝合复合材料层板在低速冲击载荷下的渐进损伤分析模型。模型中采用空间杆单元模拟缝线的作用;采用三维实体单元模拟缝合层板,通过基于应变描述的Hashin准则,结合相应的材料性能退化方案模拟层板的损伤和演化;采用界面单元模拟层间界面,结合传统的应力失效判据和断裂力学中的应变能释放率准则判断分层的起始和扩展规律。通过对碳800环氧树脂复合材料（T800/5228）层板的数值仿真结果和试验结果相比较,验证了模型的正确性,同时讨论了不同冲击能量下缝合层板的损伤规律。研究结果表明：缝线能够有效地抑制层板的分层损伤扩展;相同冲击能量下缝合与未缝合层板的基体损伤和纤维损伤在厚度分布上相似,缝合层板的损伤都要小于未缝合层板。     

Macroscale assessment of low-velocity impact on hybrid composite laminates

Composites are usually brittle materials and have low impact properties. Structural dimensions, stacking sequence, ply materials, ply thicknesses and ply angles are standard variables that influence composite‘s performance against impact loads. Stacking sequence in hybrid laminates affects the failure and impact resistance. Failure mechanisms at the low-velocity impact of a rigid object in hybrid laminates are complex, and the subsurface damage in a composite laminate cannot be detected directly. However, various simulation platforms make it easy to see the impact damage between the plies of laminate. This paper numerically investigated the effect of stack sequence and hybridization of two fiber types against low-velocity impact. The current study adopted four-layer composite laminates of carbon and glass fiber layers with a stacking plan [C/C/C/C], [C/G/C/G] and [G/C/G/C], having lay-up angles as [0°/45°/−45°/90°]. Keeping the impactor mass and the incident velocity constant, the laminates were subjected to low-velocity impact. The damage contours for a failure mode were recorded and compared at the ply level. The numerical study resulted in impact imitations showing comparisons of the damage contours using Hashin failure criteria. Hybrid laminates display better performance in absorbing impact energies; however, hybrid laminates experienced more subsurface damage due to more impact energy absorption.     

Influence of fiber orientation and thickness on the response of glass/epoxy composites subjected to impact loading

Composite laminates, made of glass/epoxy using compression molding technique, were subjected to impact loading. The ballistic limit and energy absorption capacity of the laminates were obtained. Experiments were carried out to study the effect of fiber orientation and thicknesses on ballistic limit and energy absorption of the laminates, by using a rigid conical bullet having 9.5 mm diameter and mass of 7.5 g in an air gun. Analytical expressions were obtained to find the ballistic limit, residual velocity and energy absorption capacity of the laminates. The expressions obtained by considering the various damage modes, which were involved in penetration, when laminates subjected to impact loading. The values obtained from analysis were compared with experimental results and good agreement was found. The strain rate sensitivity of the glass/epoxy composites was considered for analysis.     

相对冲击位置和补片层数对胶接修理CFRP复合材料层合板抗冲击性能的影响

本文针对碳纤维增强聚合物（CFRP）复合材料修补结构,基于连续损伤力学和粘结单元模型,在ABAQUS软件中对低速冲击载荷下不同冲击位置和补片层数的CFRP复合材料层合板内部和层间损伤进行了数值分析,并与试验结果进行了对比。选择相对冲击位置为0 mm、10 mm、20 mm、30 mm和40 mm时对应的五种修补结构,通过数值计算和试验,获得了修补结构在低速冲击过程中的冲击力、冲击能量等数据。在保持补片单层厚度不变的前提下,使补片层数从1层增加到5层,计算获得了修补结构的低速冲击响应。研究结果表明:冲头接触修补结构时会对补片造成较大的损伤,补片可以提高含孔损伤母板的抗冲击性能;冲击点离修补结构损伤孔越近,结构受冲击所产生的分层损伤越严重;增加补片的层数可以提高修补结构的抗冲击性能;通过对补片层数进行优化,得到优化层数为2,其对应的修补结构与无修补结构相比分层损伤面积减少了19.9%,较好地提升了母板的抗冲击性能。      

Multi-site impact response of S2-glass/epoxy composite laminates

High velocity transverse impact to laminated fiber reinforced composites is of interest in marine, military and civilian applications. Most studies in literature have addressed single point isolated impact events; while this work draws distinction in that we consider multi-site sequential and simultaneous impacts to composite structures. The overall objectives of this work were to investigate the response of laminated composites subjected to high velocity, multi-site impacts from a modeling and experimental viewpoint. Energy absorption, new surface creation, and failure mechanisms from sequential and simultaneous multi-site high velocity impacts are compared to assess additive and cumulative effects of these scenarios. Finite element modeling (LS-DYNA 3D) was used to gain insight into failure modes, energy absorption, and damage prediction. The modeling results correlated well with experimental data obtained from three layer laminates of vacuum assisted resin transfer molding (VARTM) processed S2-glass/SC-15 epoxy. The impact damage has been characterized using optical nondestructive evaluation (NDE) techniques. Specimens subjected to sequential impact exhibited average of 10% greater energy absorption and 18% increase in damage than specimens impacted simultaneously.     

Impact damage and residual strengths of woven fabric glass/polyester laminates

Thick glass/polyester laminates of four different dimensions subject to low-velocity impact have been investigated using a guided drop-weight test rig with a flat-ended impactor in ascending energy order up to 3100 J. The characteristics of impact response and energy absorption have been determined by impact force and absorbed energy histories, and impact damage incurred was examined by cross-sectioning and ultrasonic C-scanning. Residual compressive strengths were measured, and the damage tolerance of the laminates was assessed by the retaining ability of these strengths. It is found that the salient features in force-time history curves can be related to fracture processes occurring in the laminates, and that the established relationships between impact force and incident kinetic energy (IKE) can be used to identify damage initiation without examining impacted specimens, which is later confirmed by the damage force maps. The constructed damage force and energy maps have shown not only damage initiation in an unstable fashion but also increase of damage size with IKE and force until reaching their load-bearing capabilities. Residual compressive strengths are reduced very rapidly with the increase of impact damage due to extensive delamination.     

Hybrid composites based on aramid and basalt woven fabrics: Impact damage modes and residual flexural properties

The low-velocity impact behaviour of hybrid laminates reinforced with woven aramid and basalt fabrics and manufactured by resin transfer moulding was studied. Specimens with different stacking sequences were tested at three different energies, namely 5, 12.5 and 25 J. Residual post-impact properties of the different configurations of aramid/basalt hybrid laminates were characterized by quasi static four point bending tests. Post-impact flexural tests have been monitored using acoustic emission in order to get further information on failure mechanisms. Results indicate that hybrid laminates with intercalated configuration (alternating sequence of basalt and aramid fabrics) have better impact energy absorption capability and enhanced damage tolerance with respect to the all-aramid laminates, while basalt and hybrid laminates with sandwich-like configuration (seven basalt fabric layers at the centre of the laminate as core and three aramid fabric layers for each side of the composite as skins) present the most favourable flexural behaviour.