Low-velocity impact and residual tensile strength analysis to carbon fiber composite laminates

In this paper, low-velocity impact characteristics and residual tensile strength of carbon fiber composite laminates are investigated by experimentally and numerically. Low-velocity impact tests and residual tensile strength tests are performed using an instrumented drop-weight machine (Instron 9250HV) and static test machine (Instron 5569), respectively. The finite element (FE) software, ABAQUS/Explicit is employed to simulate low-velocity impact characteristics and predict residual tensile strength of carbon fiber composites laminates. These numerical investigations create a user-defined material subroutine (VUMAT) to enhance the damage simulation which includes Hashin and Yeh failure criteria. The impact contact force and the tensile strength are accurately estimated using the present method. Two different tensile damage modes after different impact energies are observed. The degradation of residual tensile strengths can be divided to three stages for different impact energies, and amplitudes of degradation are affected by stacking sequences.     

Neural network approach for estimating the residual tensile strength after drilling in uni-directional glass fiber reinforced plastic laminates

The drilling of fiber reinforced plastics (FRP) often results in damage around the drilled hole. The drilling induced damage often serves to impair the long-term performance of the composite products with drilled holes. The present research investigation focuses on developing a predictive model for the residual tensile strength of uni-directional glass fiber reinforced plastic (UD-GFRP) laminates with drilled hole which has not been developed worldwide till now. Artificial neural network (ANN) predictive approach has been used. The drill point geometry, the feed rate and the spindle speed have been used as the input variables and the residual tensile strength as the output. The results of the predictive model are in close agreement with the training and the testing data.     

Lightning ablation suppression of aircraft carbon/epoxy composite laminates by metal mesh

Three-dimensional finite element (FE) models of carbon/epoxy composite laminates with copper mesh and aluminum mesh protection were established subjected to lightning strike, in which different mesh spacing was selected. Effectiveness of numerical method was verified and impulse current waveforms with different current peaks were applied according to aircraft lightning zones. Thermal-electrical material parameters varying with temperature were added into numerical models. Element deletion method was used to deal with lightning ablation elements of composite structures. The results show that ablation area and depth of composite laminates with metal mesh protection are significantly smaller, which proves good protection effectiveness of metal meshes on anti-lightning strike. The denser the mesh spacing, the better the anti-lightning strike will be. Protection of composite laminates with copper mesh has better effects than that of aluminum mesh. Considering the effect of mesh spacing variation on composite structural weight and anti-lightning strike, the ideal mesh spacing was obtained.     

Fracture strength of various lay-ups of carbon/epoxy laminates using the modified inherent flaw model

Modifications are made in the inherent flaw model of Waddoups, Eisenman and Kaminski (known as the WEK model) for accurate prediction of notched tensile strength of composite laminates containing a sharp notch. To examine the adequacy of the model, fracture data of center cracked carbon/epoxy composite laminates with various lay-ups are considered. The notched strength estimates are found to be close to the test results.     

Failure assessment on central-sharp notched carbon/epoxy laminates

Fracture models to predict the strength of laminated composites having sharp notches demand the un-notched strength and the critical damage size ahead of the notch. The critical damage size, in general, depends on the material, geometry of the specimen and size of the sharp notch. The extraordinary success of a fracture model lies in its ability to combine a theoretical framework with experimentally measured quantities. Modifications are made in one of the stress-fracture criteria known as the point stress criterion for accurate prediction of notched tensile strength of composite laminates containing sharp notches. To examine the adequacy of these modifications, fracture data of central-sharp notched carbon/epoxy composite laminates with various lay-ups are considered. The notched strength estimates are found to be close to the test results. The modified point stress criterion is very simple and accurate in predicting the notched tensile strength of laminated composites.     

The influence of hygrothermal conditions on the damage processes in quasi-isotropic carbon/epoxy laminates

The effects of hygrothermal conditions on damage development in quasi-isotropic carbon-fiber/epoxy laminates are described. First, monotonic and loading/unloading tensile tests were conducted on dry and wet specimens at ambient and high temperatures to compare the stress/strain response and damage development. The changes in the Young's modulus and Poisson's ratio were obtained experimentally from the monotonic tensile tests. The critical stresses for transverse cracking and delamination for the above three conditions are compared. The delamination area is measured by using scanning acoustic microscopy (SAM) at various loads to discuss the effects of delamination on the nonlinear stress/strain behavior. Next, the stress distributions under tensile load including hygrothermal residual stresses are computed by a finite-element code and their effects on damage initiation are discussed. Finally, a simple model for the prediction of the Young's modulus of a delaminated specimen is proposed. It is found that moisture increases the critical stresses for transverse cracking and delamination by reducing the residual stresses while high temperature decreases the critical stresses in spite of relaxation of the residual stresses. The results of the finite-element analysis provide some explanations for the onset of transverse cracking and delamination. The Young's modulus predicted by the present model agrees with experimental results better than that predicted by conventional models.     

Durability of hygrothermally aged graphite/epoxy woven composite under combined hygrothermal conditions

The objective of this study was to examine the effects of hygrothermal aging on the durability of a graphite/epoxy woven composite material system. The study was part of a larger project in which the objective was to evaluate and model the effects of moisture, temperature, and combined hygrothermal conditions on the strength and life of a graphite/epoxy woven composite material system. The results presented here represent an extension of the work by Patel and Case (Int. J. Fatigue 22 (2000) 809).

The hygrothermal aging consisted of cyclical temperature and moisture variations which were meant to simulate mission conditions for an advanced subsonic aircraft. Durability studies were carried out on the aged material system in the form of fatigue and residual strength testing under humid and elevated temperature environments. Damage mechanisms and failure modes were determined through fatigue testing, residual strength testing, and nondestructive evaluation.

Changes in physical appearance, thermal analysis results, fracture surfaces, and moisture diffusion behavior all supported the idea that the material was affected by the aging process. However, experimental testing also showed that the initial and residual tensile properties of the aged material were virtually unaffected by the imposed environmental aging (as compared to unaged material testing results), except when tested at elevated temperature. At elevated temperature, both the dynamic stiffness and residual strength were noticeably reduced from that at room temperature.     

缝合复合材料层板低速冲击及冲击后压缩实验研究

通过对缝合复合材料层板进行低速冲击和冲击后压缩实验, 研究了不同类型的缝合复合材料层板的冲击损伤特性及冲击后压缩的剩余强度。实验研究表明: 基体损伤和分层是缝合层板与未缝合层板低速冲击的主要损伤模式, 缝合层板具有更好的抗冲击性能, 更高的冲击后压缩强度。缝合密度越大的层板其抗冲击性能越好, 冲击后压缩强度越高。缝合方向为0°的缝合层板较缝合方向为90°的缝合层板具有更好的抗冲击性能和更高的冲击后压缩强度。增加0°方向铺层, 减少45°、-45°方向铺层, 可以提高缝合层板的抗冲击性能和冲击后压缩强度。     

Impact resistance and tolerance of interleaved tape laminates

This paper presents and discusses the results of low-velocity impact and compression-after-impact (CAI) tests conducted on interleaved and non-interleaved carbon/epoxy tape laminates. Olefin film interleaves provided a strong interface bond, resulting in a reduction in projected damage area. These interleaves changed the stress distribution under impact and restricted delamination formation at the ply interface. An investigation into the compression behaviour of these laminates revealed a reduction in undamaged strength using olefin interleaves. This was attributed to the lack of lateral support for fibres at the fibre/interleaf interface, allowing fibre microbuckling to occur at a low load. Low modulus copolyamide web interleaves resulted in an increase in damage area and minor changes to CAI strength. Examination of laminate cross-sections revealed that this was due to both the open structure of the interleaf and poor resin/interleaf adhesion. High shear modulus polyethylene interleaves resulted in a significant decrease in damage area at various impact energies, with CAI strength improved compared to the non-interleaved laminates.     

碳纤维/环氧树脂复合材料高速冲击性能

采用树脂传递模塑(RTM)工艺制备碳纤维/环氧树脂复合材料,通过空气炮冲击实验研究树脂韧性和碳纤维类型对复合材料抗高速冲击性能的影响,并对高速冲击后的试样进行压缩性能测试,研究高速冲击损伤对复合材料剩余压缩性能的影响。结果表明:树脂的韧性可以降低复合材料遭受高速冲击时的内部损伤程度,大幅提高复合材料的抗高速冲击性能和冲击后剩余压缩性能;T700S碳纤维增强复合材料抗高速冲击性能优于T800H碳纤维增强复合材料;复合材料的破坏模式与冲击速率有关,冲击速率较低时,复合材料弹击面出现圆形凹坑,背弹面出现鼓包;冲击速率较高时,复合材料弹击面出现圆形通孔,背弹面出现沿纤维方向撕裂断口。     

Residual strength distribution of impacted glass/epoxy laminates with embedded shape memory alloy at low temperature

This paper evaluated the strength reduction and probabilistic behaviors of the residual flexural strength for impacted glass/epoxy laminates with embedded shape memory alloy (SMA) wires at various temperatures. A series of impact tests were performed on base (glass/epoxy laminates without SMA wires) and SMA laminates (glass/epoxy laminates with embedded SMA wires) at temperatures of 293 K, 263 K and 233 K. Three point flexural tests were then carried out so as to investigate the post-impact strength at the aforementioned temperatures. Strength reduction behavior of impacted laminates could be described by Caprino’s residual strength prediction model. A probabilistic model was developed in order to estimate the variation in residual strength of the impacted laminates with temperature. As the temperature decreased, the variation in residual strength increased due to the embrittlement of the constituent materials of the laminates at lower temperatures. When compared to the base laminates, the SMA laminates exhibited a higher variation in residual strength, especially at lower temperatures.     

复合材料层板低速冲击后剩余压缩强度

对两种材料体系和铺层的复合材料层合板进行低速冲击后压缩强度试验 , 以研究低速冲击后层合板的压缩破坏机理。讨论了表面凹坑深度、 背面基体裂纹长度、 损伤面积以及剩余压缩强度与冲击能量的关系。在试验研究的基础上 , 建立了复合材料低速冲击后剩余强度估算的一种椭圆形弹性核模型。该模型将冲击损伤等效为一刚度折减的椭圆形弹性核 , 采用含任意椭圆核各向异性板杂交应力有限元分析含损伤层合板的应力应变状态 ,并应用点应力判据预测层板的压缩(或压、 剪)剩余强度。理论分析与试验结果对比表明 , 该模型简单有效。      

Damage development in carbon/epoxy laminates under quasi-static and dynamic loading

The contrasting characteristics of damage evolution have been examined in a multidirectional carbon/epoxy composite laminate (IM7/8551-7) subjected to both quasi-static and dynamic loading. Our experiments were performed on bend-test bars that were loaded either in ‘supported' four-point bending or under ‘unsupported' conditions with a Hopkinson pressure bar to induce dynamic loading. We found differences in the damage that occurred in specimens loaded by the two techniques, in terms of the number of cracks and the length of the cracks. In the case of quasi-static loading, there were many matrix cracks within individual plies and only a few delamination cracks between plies; the maximum ratio of numbers of matrix to delamination cracks observed was 6:1. Despite their small number, the delamination cracks had a greater total length than the matrix cracks, and specimen failure occurred as a result of delamination crack propagation. During dynamic loading, the ratio between numbers of matrix and delamination cracks was 3:1, and in this case the ratio between the total crack lengths was unity. A quantitative assessment of damage induced during quasi-static bending was made from specimen stiffness results. Using simple beam theory and knowing the location of the damage, we correlated beam stiffness to the materials effective elastic modulus. We found that the composite's effective modulus decreased rapidly with small amounts of initial damage, but that subsequent increases in damage decreased the effective modulus at a much lower rate.     

Residual flexural strength after low-velocity impact in glass/polyester composite beams

This work focuses on an experimental study of flexural after impact behaviour of glass/polyester composite beams. The influence of impact energy, beam width, and impactor-nose geometry on the residual flexural strength was evaluated. Two widths of plain woven laminate specimens were selected. For each specimen width, the geometries of two impactor-noses (Charpy and hemispherical) were chosen to carry out impact tests using a three-point bending device, so that four different test configurations were executed. The residual flexural strength of damaged specimens, evaluated by quasi-static three-point bending tests, was found to depend on the extent of the damage, so that the residual flexural strength was lower in the specimens in which the damage reached the edges of the beam. For this reason, the residual strength was lower in specimens impacted with a Charpy-nose impactor than in the specimens impacted with a hemispherical-nose impactor. Analogously, the narrower specimens presented a lower residual flexural strength than did the wider ones.     

基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

不同湿热条件下碳纤维/环氧复合材料湿热性能实验研究

对比研究了环氧5228A树脂及碳纤维/环氧5228A树脂复合材料层合板在3种湿热环境(水煮、70℃水浸,70℃85%相对湿度)下的湿热性能,考察了湿热条件对复合材料层间剪切性能的影响规律,并从吸湿特性、物理化学特性、树脂力学性能、湿应力等方面分析了不同湿热环境下复合材料性能衰减的机制。研究表明,碳纤维/高温固化环氧树脂复合材料层间剪切性能主要是由吸湿率决定,相同吸湿率不同湿热条件下性能的下降幅度基本相同;3种湿热条件下该树脂及其复合材料未发生化学反应、微裂纹等不可逆变化,复合材料层合板湿热老化机制主要是吸入水分后基体增塑和树脂、纤维湿应变不一致导致的湿应力对复合材料性能的负面作用。     

A size effect law for notched tensile strength of woven carbon/epoxy laminates

Specimen-size effect and notch-size effect on the tensile strength of woven fabric carbon/epoxy laminates are evaluated and modeled. For two different layups of [(0/90)₁₂] and [(±45)₂/(0/90)₅]_S, respectively, static tension tests were performed on two-dimensional geometrically similar unnotched and double-edge notched specimens scaled to three different sizes. Experimental results demonstrate that the notched strength of the woven CFRP laminates depend on the size of specimen as well as the size of notch. The ratio of notched strength to unnotched strength decreases as the length of notch increases, regardless of the size of specimen. For a given size of notch, the notch strength ratio becomes larger with decreasing size of specimen. A notch-size effect law is derived by means of the Neuber interpolation method. A specimen-size effect is embedded into the notch sensitivity parameter involved by the notch-size effect law to establish a size effect law that can cope with these two kinds of size effect. The engineering size effect law proposed can adequately describe the specimen-size effect as well as notch-size effect on the tensile strength of the woven CFRP laminates. It is also demonstrated that the size effect law allows determining the size independent fracture toughness on the basis of notched strengths of small specimens that fail in a quasi-brittle manner.     

Response of notched carbon/PEEK and carbon/epoxy laminates subjected to tension fatigue loading

In this study a comparison is made between the tensile static and fatigue behaviours of quasi-isotropic carbon/PEEK and carbon/epoxy notched laminates, selected as separate representatives of both tough and brittle matrix composites. Damage progression was monitored by various non-destructive (ultrasonic scanning and x-radiography) and destructive (deply and microscopic examinations) techniques, and by continuously measuring the change in stiffness, in order to identify the effect of damage on mechanical properties.
The experimental observations indicated that fatigue damage in carbon/epoxy laminates consists of a combination of matrix cracks, longitudinal splitting and delaminations which attenuate the stress concentration and suppress fibre fracture at the notch; as a consequence, fatigue failure can be reached only after very high numbers of cycles while tensile residual strengths continuously increase over the range of lives investigated (10³–10⁶ cycles). Due to the superior matrix toughness and the high fibre-matrix adhesion, the nature of fatigue damage in carbon/PEEK laminates strongly depends on the stress level. At high stresses the absence of early splitting and delaminations promotes the propagation of fibre fracture therefore resulting in poor fatigue performances and significant strength reductions; while at low stress levels damage modes are matrix controlled and this again translates into very long fatigue lives. These results indicate a strong influence of the major damage mechanisms typical of the two material systems on the behaviour of the laminates, with the nature, more than the amount, of damage appearing as the controlling parameter of the material response up to failure.     

含低速冲击损伤复合材料层合板剩余压缩强度及疲劳性能试验研究

对T300/QY8911复合材料层板进行了低速冲击、 冲击后压缩以及冲击后疲劳试验研究。通过对冲击后的层板进行目视检测和超声C扫描获得了层板受低速冲击后的若干损伤特征; 在压-压疲劳试验中, 测量了损伤的扩展情况。讨论了冲击能量与损伤面积以及冲击后剩余压缩强度的关系, 分析了含冲击损伤层合板在压缩载荷及压-压疲劳载荷下的主要破坏机制。结果表明, 低速冲击损伤对该类层板的强度和疲劳性能影响很大, 在3.75 J/mm的冲击能量下, 层板剩余压缩强度下降了65%; 在压-压疲劳载荷作用下, 其损伤扩展大致可分为两个阶段, 占整个疲劳寿命约60%的前一阶段损伤扩展较为缓慢; 而疲劳寿命的后半阶段损伤则开始加速扩展, 并导致材料破坏。     

Characterization of fracture modes in stitched and unstitched cross-ply laminates subjected to low-velocity impact and compression after impact loading

The insertion of transverse reinforcing threads by stitching is a very promising technique to restrict impact damage growth and to improve post-impact residual strength of laminates. In order to develop general models capable of addressing the issues of impact resistance and damage tolerance of stitched laminates, detailed understanding of the nature and extent of damage, identification of the dominant fracture modes and assessment of the effect of stitches on the damage development are essential. In this study, both instrumented drop-weight tests and compression-after-impact tests were carried out to examine and compare the damage responses of stitched and unstitched graphite/epoxy laminates subjected to low-velocity impact. The progression of damage and its effect on post-impact performance was investigated in detail in two classes of cross ply laminates ([0₃/90₃]_s and [0/90]_3s) by means of an extensive series of damage observations, conducted with various complementary techniques (X-radiography, ultrasonics, optical microscopy, deply). The results of the analyses carried out during the study to characterize the key fracture modes and to clarify their relationship with the structural performance of both stitched and unstitched laminates are reported and discussed in the paper.