A theoretical approach for the assessment of intralaminar fracture mode and toughness of multilayered composites

Analytical evaluation of fracture toughness of a multilayered composite laminate was well-established using modified crack closure integral (MCCI) approach based on test data on the failure load. For this purpose the crack initiation direction, which is treated as a branch crack direction for the theoretical prediction, is required. The crack initiation direction in a multilayered composite laminate depends on mode of failure. In the present work, a fracture parameter n ^* is introduced to predict the mode of failure in multilayered composite having a crack and is validated. Analytical relationship for the prediction of fracture toughness of multilayered composite between a base laminate and its constituent sublaminates is also arrived at. With available test data on the toughness of a set of sub-laminates, toughness of base laminate is determined and validated. The present approach is useful in evaluating the load carrying capability of composite structures with defects in the form of cracks and this information is valuable for design.     

基于断裂韧性预报复合材料层合板的开孔拉伸强度

开孔层合板的强度预报往往取决于孔边的临界长度,它不仅与材料性能,而且与铺层、孔径都有关。本文基于线弹性断裂力学,提出了一种预报对称铺层层合板开孔拉伸强度的新方法,只需提供正交层合板的断裂韧性和无缺口层合板的拉伸强度,显著降低对实验数据的依赖性。首先,将临界长度表作为层合板断裂韧性和无缺口拉伸强度的函数,再通过正交层合板[90/0]_8s的紧凑拉伸试验和虚拟裂纹闭合技术,确定出0°层断裂韧性,进而计算得到任意对称铺层层合板的断裂韧性。本文测试了T300/7901层合板[0/±45/90]_2s和[0/±30/±60/90]_s的开孔拉伸强度,孔径分别为3 mm、6 mm和9 mm。理论预报结果与试验值吻合较好,最大误差为15.2%,满足工程应用需求。      

各向异性复合材料开孔板拉伸强度预测及模型验证

基于有限断裂力学方法建立了一种预测多向复合材料开孔板拉伸强度的通用和半经验模型。该模型同时采用基于应力形式的失效准则和基于能量形式的失效准则预测失效。模型仅需铺层弹性常数、无缺口层合板的强度以及0°铺层的断裂韧性等参数。基于线弹性断裂力学建立了多向复合材料层合板的断裂韧性与0°铺层断裂韧性之间的关系, 进而预测了任意铺层复合材料开孔板发生纤维主导拉伸失效时的强度。将模型预测结果与开孔板拉伸强度的试验数据进行了对比验证, 预测误差最大为9.7%, 与点应力和平均应力等方法的对比表明, 该模型的预测精度高于传统的特征长度方法。      

Novel test method for accurate characterization of intralaminar fracture toughness in CFRP laminates

A novel initial crack insertion method, “intralaminar film insertion method”, was proposed to investigate the fracture toughness of unidirectional carbon fiber reinforced plastic (CFRP) laminates when the crack propagates inside the ply and not in the interlayer resin-rich area. Here, a release film was inserted inside a single lamina during the resin impregnation process of prepreg manufacturing. Mode I intralaminar fracture toughness tests were carried out for conventional CFRP laminates and interlayer toughened CFRP laminates. For comparison, two conventional methods were used to introduce initial cracks. One is the “interlaminar film method”, where a release film is inserted between two prepreg plies during the lay-up process. The other is the “machined slit method”, where a slit notch is machined in parallel to the layer of CFRP laminates. It was demonstrated that the proposed “intralaminar film method” can correctly evaluate the intralaminar fracture toughness of both conventional CFRP laminate and interlayer toughened CFRP laminate from the initial value to the propagation value. For this range, it was also found that the intralaminar fracture toughness of interlayer toughened CFRP laminate was the same as that of conventional CFRP laminate. Thus, the intralaminar fracture toughness was not influenced by interlayer toughening.     

A finite fracture mechanics model for the prediction of the open-hole strength of composite laminates

A new model based on finite fracture mechanics is proposed to predict the open-hole tensile strength of composite laminates. Failure is predicted when both stress-based and energy-based criteria are satisfied. The material properties required by the model are the ply elastic properties, and the laminate unnotched strength and fracture toughness. No empirical adjusting parameters are required. Using experimental data obtained in quasi-isotropic carbon–epoxy laminates it is concluded that the model predictions are very accurate, resulting in improvements over the traditional strength prediction methods. It also is shown that the proposed finite fracture mechanics model can be used to predict the brittleness of different combinations of materials and geometries.     

Toughening and self-healing of epoxy matrix laminates using mendable polymer stitching

This paper presents an experimental study into a new type of stitched fibre–polymer laminate that combines high interlaminar toughness with self-healing repair of delamination damage. Poly(ethylene-co-methacrylic acid) (EMAA) filaments were stitched into carbon fibre/epoxy laminate to create a three-dimensional self-healing fibre system that also provides high fracture toughness. Double cantilever beam testing revealed that the stitched EMAA fibres increased the mode I interlaminar fracture toughness (by ∼120%) of the laminate, and this reduced the amount of delamination damage that must subsequently be repaired by the self-healing stitches. The 3D stitched network was effective in delivering self-healing EMAA material extracted from the stitches into the damaged region, and this resulted in high recovery in the delamination fracture toughness (∼150% compared to the original material). The new self-healing stitching method provides high toughness which resists delamination growth while also having the functionality to repeatedly repair multiple layers of damage in epoxy matrix laminates.     

铺层方向和裂纹混合比对复合材料层压板混合型断裂韧性影响的试验研究

试验研究了复合材料层压板的铺层方向以及裂纹混合比对层间裂纹分层扩展的影响规律。试验结果显示: 在非0°单向板的 Ⅰ 型层间裂纹分层扩展过程中, 会出现层间裂纹穿过分层开裂面的铺层而偏离到相邻铺层间扩展的现象, 而0°铺层具有阻止该裂纹偏离扩展的作用; 在不同裂纹混合比的层压板分层开裂试验中, 相应的0°单向板的断裂韧性均可以作为下限值而偏安全; 混合断裂韧性( Ⅰ 型断裂韧性+ Ⅱ 型断裂韧性)随着裂纹混合比的变化呈现类似正弦曲线的变化规律。      

Influence of adsorption behaviour of a silane coupling agent on interlaminar fracture in glass fibre fabric-reinforced unsaturated polyester laminates

The relationship between the interphase consisting of physisorbed and chemisorbed silane on glass fibres and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate, was studied. The Mode I interlaminar fracture toughness of the laminate specimen was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of the laminate specimen finished with two silane concentrations and washed in methanol solvent, is discussed on the basis of the interlaminar fracture toughness. In order to determine the amount of physisorbed and chemisorbed silane on the glass fibre, the amount of total carbon was determined using an analysis instrument. The physisorbed silane migrated into the resin matrix and influenced the mechanical properties and interlaminar fracture of the laminate specimen. The amount of unsaturated polyester resin blended with a silane coupling agent was measured using dynamic mechanical spectroscopy, and a DCB specimen for mechanical properties and fracture toughness.     

Dynamic delamination growth in laminated composite structures

The dynamic growth of a thin strip delamination in a thick base laminate under in-plane loadings has been analysed. A variational principle, coupled with a Griffith-type fracture criterion, is used to formulate the delamination growth problem. Two approximate solutions, including one mode and two modes, respectively, are calculated in this paper. The resulting equations of motion and the dynamic local growth condition at the crack tip turn out to be two and three coupled ordinary differential equations for one-mode and two-mode solutions. A fourth-order Runge-Kutta method is then used to obtain the numerical solutions. The results show that delamination growth will approach a state of arrest for materials with high fracture toughness, and continue all the way without a limit for low fracture toughness materials. The inertial effect is important and should not be ignored in calculation of the arrested delamination length for high fracture toughness materials. For materials with low fracture toughness, the inertial effect is significant and high admissible modes are noticeable. A comparison between the present results and the previously known quasi-dynamic solution is also given.     

The influence of silicate-based nano-filler on the fracture toughness of epoxy resin

Dispersion of nano-sized, silicate-based filler in epoxy resin is expected to yield improved materials properties in several areas. Various mechanical properties, specifically improved fracture toughness, as well as improved flame-retardant effects are of interest. The final objective of the research is investigating whether a nano-modified epoxy matrix yields improved delamination resistance in a fiber-reinforced laminate compared to a laminate with neat epoxy as matrix material. As a first step towards this goal, the fracture toughness of nano-modified epoxy resin is compared with that of the neat resin. Fracture toughness improvement up to about 50% and energy release rates increased by about 20% are observed for addition of 10 wt.% of organosilicate clay.     

ARALL层板剥离特性的研究（一）──胶粘剂对层板层间性能的影响

本文采用了Ｂｅｌｌ─剥离法研究了胶粘剂性质及其含量对ＡＲＡＬＬ层间剥离特性的影响。结果表明，层板的剥离过程为多重混合型破坏。由于芳纶／胶粘剂界面结合较差，其剥离强度值较低。使该界面脱粘而破坏，胶粘剂发生内聚破坏以及胶粘剂的韧性增大或含胶量提高均使剥离强度值提高。改善胶粘剂与芳纶的界面粘合状况、提高胶粘剂的韧性是提高层板层间性能的有效途径。     

缝合层合板的I型层间断裂韧性研究

通过缝合的方法改善织物增强复合材料层合板的层间断裂韧性.采用双悬臂梁(DCB)试验测试和研究了缝合层合板的层间断裂韧性与断裂行为.为了评价缝合工艺参数(缝合密度)对层间断裂韧性的影响, 用改进的插入型夹具在实测不同缝合工艺层合板的I型层间断裂韧性值(GIC)的基础上, 分析和阐明了缝合工艺参数(缝合密度)与GIC间的关系; 以提高层合板的平均层间断裂韧性值为目标, 以拉伸和弯曲强度为约束条件优化了缝合工艺; 采用摄影显微镜对分层断裂面进行了观察, 分析和考察了缝合对其它性能的影响.结果表明 改进的插入型夹具可方便地完成缝合层合板的I型层间断裂韧性测试; 缝合后裂纹不连续扩展, 缝合密度对裂纹扩展行为有较大影响; 随着缝合密度的增大, 层间断裂韧性值增大, 但拉伸和弯曲强度降低, 缝合密度存在最佳值.     

缝合层合板的Ⅰ型层间断裂韧性研究

通过缝合的方法改善织物增强复合材料层合板的层间断裂韧性.采用双悬臂梁(DCB)试验测试和研究了缝合层合板的层间断裂韧性与断裂行为.为了评价缝合工艺参数(缝合密度)对层间断裂韧性的影响,用改进的插入型夹具在实测不同缝合工艺层合板的Ⅰ型层间断裂韧性值(G_IC)的基础上,分析和阐明了缝合工艺参数(缝合密度)与G_IC间的关系;以提高层合板的平均层间断裂韧性值为目标,以拉伸和弯曲强度为约束条件优化了缝合工艺;采用摄影显微镜对分层断裂面进行了观察,分析和考察了缝合对其它性能的影响.结果表明:改进的插入型夹具可方便地完成缝合层合板的Ⅰ型层间断裂韧性测试;缝合后裂纹不连续扩展,缝合密度对裂纹扩展行为有较大影响;随着缝合密度的增大,层间断裂韧性值增大,但拉伸和弯曲强度降低,缝合密度存在最佳值.     

Z-pin增强复合材料Ⅰ型断裂韧性数值分析

采用细观力学方法以及虚拟裂纹闭合法(VCCT)对含有Z-pin增强复合材料双悬臂梁(DCB)结构Ⅰ型断裂韧性进行了研究。利用有限元法建立了结构模型,采用实体单元模拟复合材料层压板结构和非线性弹簧元模拟Z-pin。通过计算应变能释放率对含有不同体积分数Z-pin的复合材料层压板Ⅰ型断裂韧性与不含Z-pin的复合材料层压板Ⅰ型断裂韧性进行了对比分析。研究表明,含有Z-pin增强复合材料双悬臂梁(DCB)结构Ⅰ型断裂韧性在裂纹扩展过程中受到Z-pin桥联作用的影响而显著增强,且其增强效果与Z-pin的体积分数、处在桥联区的Z-pin数目均相关,这表明Z-pin增强方法能够有效提高复合材料层压板的分层扩展阻力。     

Analysis of the effect of biaxial loading on the fracture toughness of reactor pressure-vessel steels

The stress-strain state at the crack tip and its relation to the crack opening displacement and the J-integral under biaxial loading have been studied by solving elastoplastic problems in a geometrically nonlinear formulation by the finite-element method. Numerical investigations have been performed for various cracks and two modes of biaxial loading (tension and bending) under conditions of both small-scale and large-scale yielding. For prediction of the influence of biaxial loading on fracture toughness (at brittle fracture) a procedure has been developed that is based on established laws of stress-strain state formation at the crack tip under biaxial loading and a criterion of brittle fracture proposed earlier. The effect of biaxial loading on fracture toughness is predicted as applied to reactor pressure-vessel steels. Calculated results are compared with avilable experimental data. Alternative approaches to prediction of the effect of biaxial loading on fracture toughness are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 5, pp. 5–26, September–October, 1999.     

Toughness properties of a three-dimensional carbon-epoxy composite

The three-dimensional (3D) orthogonal interlocked fabric contains through-the-thickness rein-forcement in order to enhance the interlaminar fracture toughness of the composite. The interlaminar fracture toughness of a carbon-epoxy orthogonal interlocked fabric composite was experimentally determined by use of the recently developed tabbed double cantilever beam specimen. The data reduction methods applicable to these tests and materials and the interpretation of the results were discussed. The results of critical strain energy release rate,G _Ic, were compared to those of a two-dimensional (2D) laminate having the same in-plane structure. The energy-dissipating crack propagation processes were described. The in-plane fracture toughness of the 3D fabric was experimentally measured and compared to that of the 2D laminate. The through-the-thickness fibres were found to create a ten-fold increase in interlaminar toughness, and a 25% improvement in the in-plane fracture toughness.     

Fracture toughness of transverse cracks in graphite/epoxy laminates at cryogenic conditions

Stress singularity of a transverse crack normal to ply-interface in a composite laminate is investigated using analytical and finite element methods. Four-point bending tests were performed on single-notch bend specimens of graphite/epoxy laminates containing a transverse crack perpendicular to the ply-interface. The experimentally determined fracture loads were applied to the finite element model to estimate the fracture toughness. The procedures were repeated for specimens under cryogenic conditions. Although the fracture loads varied with specimen thickness, the critical stress intensity factor was constant for all the specimens indicating that the measured fracture toughness can be used to predict delamination initiation from transverse cracks. For a given crack length and laminate configuration, the fracture load at cryogenic temperature was significantly lower. The results indicate that fracture toughness does not change significantly at cryogenic temperatures, but the thermal stresses play a major role in fracture and initiation of delaminations from transverse cracks.     

Stress intensity factors as the fracture parameters for delamination crack growth in composite laminates

A “mutual integral” approach is used to calculate the mixed-mode stress intensity factors for a free-edge delamination crack in a laminate under tensile loading conditions. This “mutual integral” approach, for generalized plane strain conditions, is based on the application of the path-independent J integral to a linear combination of three solutions: one, the problem of the laminate to be solved using the quasi 3-D finite element method, the second, an “auxiliary” solution with a known asymptotic singular solution, and the third, the particular solution due to the out-of-plane loading. A comparison with the exact solutions is made to determine the accuracy and efficiency of this numerical method. With this “mutual integral” approach, it was found that the calculated mixed-mode stress intensity factors of the free-edge delamination crack remain relatively constant as the crack propagates into the laminate. It was also found that the fracture criterion based on the mixed-mode stress intensity factors is more consistent with the experimental observations than the criterion based on the total energy release rate, and hence demonstrates the importance of the ability to calculate each individual component of the stress intensity factors. Furthermore, it was found that the fracture toughness measurements from double cantilever beam specimens can be used directly to predict the onset of delamination crack growth between two dissimilar laminae. Using these fracture toughness measurements from the double cantilever beam specimens, some examples are given to show that the fracture criterion based on the mixed-mode stress intensity factors can accurately predict the failure load for various laminates under tensile loading conditions.     

Effect of mendable polymer stitch density on the toughening and healing of delamination cracks in carbon–epoxy laminates

This paper presents an investigation into the effect of stitch density on the delamination toughening and self-healing properties of carbon–epoxy laminates. The stitches provide the laminate with the synergistic combination of high mode I interlaminar fracture toughness to resist delamination cracking and healing properties to repair delamination damage. The results show that the fracture toughness of the laminate increased with stitch density, due to higher traction (crack closure) loads exerted by the stitches bridging the delamination. During the healing process these bridging stitches first melt and then flow into the delamination, leading to self-healing with full restoration of the mode I fracture toughness. Furthermore, the stitches were capable of repairing delamination cracks many times larger than the original size of the stitches. The effect of stitch density on the healing process of delamination cracks and restoration of fracture toughness was found to remain approximately the same under multiple repair operations.     

Influence of the thermomechanical processing on the fracture mechanisms of high strength aluminium/pure aluminium multilayer laminate materials

The microstructure and mechanical properties, with emphasis in the impact fracture toughness behaviour, of two multilayer laminate materials have been investigated. The multilayer materials are constituted by alternated sheets of pure aluminium (Al 1200 or Al 1050) and high strength Al 7075 alloy. Stacked layers of these alloys have been successfully joined using two processing routes with different total hot rolling strains. Both laminates have been tested at room temperature under impact Charpy tests, three-point bend tests and shear tests on the interfaces. Both laminates exhibited more than eight times improvement in impact fracture toughness over the monolithic Al 7075-T6. The toughness increase in the higher rolling strained laminate is almost entirely due to crack blunting mechanism, while in the lower strained laminate, crack deflection by delamination and crack renucleation processes were active.