Compressive failure of hybrid multidirectional fibre-reinforced composites

In this paper, the hybridisation of multidirectional carbon fibre-reinforced composites as a means of improving the compressive performance is studied. The aim is to thoroughly investigate how hybridisation influences the laminate behaviour under different compression conditions and thus provide an explanation of the “hybrid effect”. The chosen approach was to compare the compressive performance of two monolithic carbon fibre/epoxy systems, CYTEC HTS/MTM44-1 and IMS/MTM44-1, with that of their respective hybrids. This was done by keeping the same layup throughout ((0/90/45/−45)_2S) while replacing the angle plies in one case or the orthogonal plies in the other case with the second material, thus producing two hybrid systems. To investigate the compressive performance of these configurations, compact and plain compression test methods were employed which also allowed studying the sensitivity of compressive failure to specimen geometry and loading conditions. The experimental results and the subsequent fractographic analysis revealed that the hybridisation of selective ply interfaces influenced the location and severity of the failure mechanisms. Finally, in light of this knowledge, an update of the generic sequence of events, previously suggested by the authors, which lead to global fracture in multidirectional fibre-reinforced composites under compression is presented.     

On the transition from shear-driven fibre compressive failure to fibre kinking in notched CFRP laminates under longitudinal compression

This paper investigates longitudinal compressive failure in notched unidirectional and cross-ply carbon/epoxy specimens. Dedicated test jigs were developed to observe the failure processes at the microscale. In situ and post-mortem fractography reveals two types of failure mechanisms: (i) shear-driven fibre compressive failure and (ii) kink-band formation. The sequence of events leading to failure and the reasons for shear-driven fibre compressive failure or kink-band formation are investigated and discussed. Those findings are discussed further in a separate paper (Gutkin et al., accepted for publication) [1] where an FE micromechanical model is used to investigate numerically the failure mechanisms found in longitudinal compression.     

Micromechanical modeling to determine the compressive strength and failure mode interaction of multidirectional laminates

Computational models to predict the compressive strength of carbon fiber reinforced polymer matrix composites are proposed here, motivated by the failure mechanisms observed in compression tests. Delamination, fiber kink-banding and their interaction are seen to dominate the failure response. An upscaled semi-homogenized laminate model is developed to predict the observed compressive response of multidirectional laminates. A generalized 2-D formulation is presented to determine the interfaces most susceptible to delamination. Subsequently, cohesive elements are added along these interfaces to introduce delamination capability in the model. Predictions of the model are compared against experimental data, and are found to be in agreement with respect to compressive strength and failure modes. Further, the effect of stacking sequence on the compressive strength and failure mode is investigated.     

2.5D机织复合材料压缩性能实验与数值模拟

为了研究2.5D机织复合材料的压缩损伤和失效机制,验证双尺度渐进损伤有限元数值模拟方法的有效性,对这类复合材料分别沿经纱方向和纬纱方向进行了准静态压缩实验,获得了其相应的应力-应变曲线,并测定了材料的初始弹性模量和极限强度。在此基础上,利用双尺度渐进损伤有限元数值方法模拟分析了材料的压缩应力-应变响应和损伤演化行为,取得了与实验吻合较好的模拟结果。结果表明:2.5D机织复合材料在纬向压缩下的主要失效模式是纬纱的轴向压溃与断裂,可获得相对较高的压缩强度;但在经向压缩下,经纱因弯曲会承受附加弯矩作用,从而对周围基体造成挤压,故在经纱轴向断裂之前容易出现经纱之间基体的压溃和纱线之间的分层开裂,使强度降低,不利于发挥纤维的承载优势。     

Fractographic observations on delamination growth and the subsequent migration through the laminate

This paper describes fractographic observations from the detailed examination of delamination fracture surfaces and offers an interpretation of the key growth mechanisms. Firstly, the relationship between toughness, delamination failure criteria and fracture morphology is presented and the influence of cusp formation and deformation on toughness is discussed. Observations regarding delaminations migrating through the lamina at multidirectional ply interfaces are then discussed. It is demonstrated how this migration process can be avoided in fracture toughness coupons and consequently the toughness of multidirectional ply interfaces can be characterised. The influence of migration on delamination growth from embedded defects in laminates under compression is presented, and these results are extended to demonstrate how migration influences damage growth in structures. The paper concludes by making recommendations for realistic modelling of migration, and suggests how it can be exploited in damage tolerant structural design.     

Modeling hysteresis behavior of cross-ply C/SiC ceramic matrix composites

In this paper, the loading/unloading tensile behavior of cross-ply C/SiC ceramic matrix composites at room temperature has been investigated. The loading/unloading stress–strain curve exhibits obvious hysteresis behavior. An approach to model the hysteresis loops of cross-ply ceranic matrix composites including the effect of matrix cracking has been developed. Based on the damage mechanisms of fiber sliding relative to matrix during unloading and subsequent reloading, the unloading interface reverse slip length and reloading interface new slip length of different matrix cracking modes are obtained by the fracture mechanics approach. The hysteresis loops of cross-ply C/SiC ceramic matrix composites corresponding to different peak stresses have been predicted.     

Characterization of strain-softening behavior and failure mechanisms of composites under tension and compression

A methodology is presented to directly measure the damage properties and strain softening response of laminated composites by conducting over-height compact tension (OCT) and compact compression (CC) tests. Through the use of digital image correlation (DIC) technique, and analysis of the measured surface displacement/strain data, the strain-softening response of composites is constructed. This method leads to a direct determination of the Mode I translaminar fracture properties with the assumption that the shear stress is negligible around the damage zone and the crack growth occurs in the symmetric opening mode. Using this methodology, and by correlating the observed failure mechanisms with the strain-softening curves, the interaction of failure mechanisms leading to the final failure and also the distinction between the tensile and compressive failure mechanisms can be studied. The effectiveness of the method in accurate identification of the damage parameters is demonstrated through sectioning and deplying techniques. As a consistency check and further verification of the method, the obtained strain-softening curves are fed into a numerical damage mechanics model and successfully used to simulate the detailed response of the very same OCT and CC specimens from which the strain-softening curves were extracted.     

Fracture mechanisms of wood/polyester laminates under quasi-static compression and shear loading

There is increasing use of natural fiber/polymer composites as alternatives to traditional structural materials like concrete and metals and to the inorganic fibers like carbon. While the fracture mechanisms during crushing of synthetic fiber/polymer composites have been thoroughly studied, limited information is available on post-fracture investigation and identification of the dominant fracture mechanisms of wood/polyester composites. In this study laminates of Douglas-fir veneer were fabricated using a catalyzed polyester resin and their potentials as energy absorbers have been investigated and discussed. Factors for this study were (i) laminates symmetry (face layers of 0° or 90°), (ii) lay-up balance (balanced and unbalanced) and (iii) number of lamina (8, 11, and 12). Samples were tested under quasi-static Combined Loading Compression (CLC) and their compressive performances were compared to control specimens using glass fiber as reinforcement. Results indicated that the effect of symmetry on compressive properties of wood veneer/polyester laminates was significant with laminates with face layers of 90° and core layers of 0° had the highest deflection to failure. Increasing the wood/polyester laminate thickness enhanced their energy absorbing ability by bringing more fracture mechanisms into play but it noticeably reduced the laminates compressive modulus. Despite the brittle failure of glass fiber composites wood laminates exhibited a progressive fracture mechanisms with shear buckling as the dominant mode of failure in symmetric samples. This progressive failure with high energy absorbing ability make wood/polyester laminates a good candidate to be used as an energy absorber structure where high deflection to failure and longer failure time are required.     

Compressive damage mechanism of GFRP composites under off-axis loading: Experimental and numerical investigations

Experimental and computational studies of the microscale mechanisms of damage formation and evolution in unidirectional glass fiber reinforced polymer composites (GFRP) under axial and off-axis compressive loading are carried out. A series of compressive testing of the composites with different angles between the loading vector and fiber direction were carried out under scanning electron microscopy (SEM) in situ observation. The damage mechanisms as well as stress strain curves were obtained in the experiments. It was shown that the compressive strength of composites drastically reduces when the angle between the fiber direction and the loading vector goes from 0° to 45° (by 2.3–2.6 times), and then slightly increases (when the angle approaches 80–90°). At the low angles between the fiber and the loading vector, fiber buckling and kinking are the main mechanisms of fiber failure. With increasing the angle between the fiber and applied loading, failure of glass fibers is mainly controlled by shear cracking. For the computational analysis of the damage mechanisms, 3D multifiber unit cell models of GFRP composites and X-FEM approach to the fracture modeling were used. The computational results correspond well to the experimental observations.     

Mechanical Behaviour of 3D Multi-layer Braided Composites: Experimental,Numerical and Theoretical Study

Mechanical properties and failure mechanisms of a newly designed 3D multi-layer braided composites are evaluated by experimental, numerical and theoretical studies. The microstructure of the composites is introduced. The unit cell technique is employed to address the periodic arrangement of the structure. The volume averaging method is used in theoretical solutions while FEM with reasonable periodic boundary conditions and meshing technique in numerical simulations. Experimental studies are also conducted to verify the feasibility of the proposed models. Predicted elastic properties agree well with the experimental data, indicating the feasibility of the proposed models. Numerical evaluation is more accurate than theoretical assessment. Deformations and stress distributions of the unit cell under tension shows displacement and traction continuity, guaranteeing the rationality of the applied periodic boundary conditions. Although compression and tension modulus are close, the compressive strength only reaches 70% of the tension strength. This indicates that the composites can be weakened in compressive loading. Additionally, by analysing the micrograph of fracture faces and strain-stress curves, a brittle failure mechanism is observed both in composites under tension and compression.     

三维编织碳/环氧复合材料力学性能测试及破坏机制

通过宏观拉压试验, 研究了三维正交编织碳/环氧复合材料的拉伸和压缩力学性能。对试验过程进行了声发射分析, 对断口进行了扫描电镜观察分析, 给出了该类材料的拉伸和压缩破坏机制。结果表明: 三维正交编织碳/环氧复合材料有良好的拉伸和压缩力学性能; 三维正交编织复合材料在拉伸和压缩载荷作用下的断裂均为脆性断裂, 拉伸试验的主要破坏现象是纤维断裂拔出, 而压缩试验则是纤维剪切破坏; 通过声发射参数分析可以基本判定该类材料损伤过程中的损伤类型。     

Matrix failure in composite laminates under compressive loading

The failure envelope of the matrix in composite laminates under compressive loads has not received much attention in literature. There are very little to no experimental results to show a suitable failure envelope for this constituent found in composites. With increasing popularity in the use of micromechanical analysis to predict progressive damage of composite structures which requires the use of individual failure criteria for the fibre and matrix, it is important that matrix behaviour under compression is modelled correctly.In this study, off-axis compression tests under uniaxial compression loading are used to promote matrix failure. Through the use of micromechanical analysis involving Representative Volume Elements, the authors were able to extract the principal stresses on the matrix at failure. The results indicated that hydrostatic stresses play an important role in the failure of the matrix. Thus, Drucker–Prager failure criterion is recommended when modelling compressive matrix failure in composite structures.     

Effect of off-axis ply orientation on 0°-fibre microbuckling

The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0°-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0°-plies, catastrophic failure also initiates by kinking of 0°-plies at the free-edges or manufacturing defects, followed by delamination. When 0°-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0°-plies. T800/924C carbon-fibre–epoxy laminates with a [(±θ/0₂)₂]_s lay-up are used here to study the effect of the supporting ply angle θ on the stress initiation of 0°-fibre microbuckling. Experimental data on the compressive strength of laminates with θ equal to 30, 45, 60 or 75° are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/θ) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis.     

Manufacturing and ultimate mechanical performance of carbon fibre-reinforced epoxy composite suspension push-rods for a Formula 1 racing car

The contemporary Formula 1 racing car makes extensive use of advanced composite materials in its construction. The design, manufacture and ultimate performance under compression of composite suspension push-rods, that typically could be used in a Grand Prix racing car, are described in this present paper. An aerofoil cross-section has been used based on different lay-ups of carbon/epoxy composite. One push-rod was manufactured using a uniform layup of unidirectional and woven cross-ply prepreg, whilst a further three push-rods were manufactured with a tapered layup of unidirectional and woven cross-ply prepreg. Failure mechanisms including fibre microbuckling, fibre kinking and fibre fracture were observed, whilst comparisons have been made between the experimentally observed failure strains and those that were predicted using simple buckling theory. The ultimate compressive strength of the structural component was significantly less than that of the carbon/epoxy composite.     

Split Hopkinson pressure bar testing of 3D multi-axial warp knitted carbon/epoxy composites

High-strain-rate compression experiments were performed on 3D MWK carbon/epoxy composites with different fiber architectures at room and elevated temperature using an SHPB apparatus. Macro-fracture and SEM micrographs were examined to understand the failure mechanism. The results show the dynamic properties increase with the strain rate and show a high-strain-rate sensitivity. Meanwhile, composites with [0°/0°/0°/0°] have higher properties. Moreover, composites show temperature sensitivity and the properties decrease significantly, especially for composites with [0°/90°/+45°/−45°]. The results also indicate composites take on more serious damage and failure with the strain rate. The failure of composites with [0°/0°/0°/0°] behaves in multiple delaminating, overall expansion and 0° fibers tearing. While that of composites with [0°/90°/+45°/−45°] is mainly interlaminar delaminating, local fibers tearing and fracture on different fiber layers. In addition, with increasing the temperature, the composite shows less fracture and becomes more plastic. The damage of matrix yielding, interface debonding and twisting of fibers increase significantly.     

织物结构对复合材料力学性能影响的试验研究

为探讨不同结构形式织物对复合材料力学性能的影响及其损伤破坏机制之间的差异,通过宏观拉压试验,研究了经编及平纹碳纤维织物增强树脂基复合材料的拉伸及压缩力学性能,并利用声发射对试验过程进行实时监测,对破坏后的断口进行显微镜观察分析,分别给出了两种材料的拉伸和压缩破坏机制.研究结果表明:织物结构形式对复合材料的力学性能有较大影响,与经编织物复合材料相比,平纹织物复合材料的拉伸、压缩强度均较低,且其拉伸、压缩破坏试样的断口相对齐平,分层现象不明显;根据声发射监测结果可以判定两种复合材料损伤过程中的损伤类型,与经编织物相比,平纹织物复合材料拉/压过程中的声发射电压信号相对稳定且整体强度较低.     

Measurement of resistance curves in the longitudinal failure of composites using digital image correlation

This paper presents a new methodology to measure the crack resistance curves associated with fiber-dominated failure modes in polymer–matrix composites. The crack resistance curves not only characterize the fracture toughness of the material, but are also the basis for the identification of the parameters of the softening laws used in the numerical simulation of fracture in composite materials. The proposed method is based on the identification of the crack tip location using Digital Image Correlation and the calculation of the J-integral directly from the test data using a simple expression derived for cross-ply composite laminates. It is shown that the results obtained using the proposed methodology yield crack resistance curves similar to those obtained using Finite Element based methods for compact tension carbon–epoxy specimens. However, it is also shown that, while the Digital Image Correlation based technique mitigates the problems resulting from Finite Element based data reduction schemes applied to compact compression tests, the delamination that accompanies the propagation of a kink-band renders compact compression test specimens unsuitable to measure resistance curves associated with fiber kinking.     

Experimental and theoretical investigation on the compressive behavior of aluminum borate whisker reinforced 2024Al composites

The compressive behavior of Al₁₈B₄O₃₃w/2024Al composites fabricated by squeeze casting was investigated under low and elevated temperature. Microstructure shows that the compression exerts a significant effect on whisker fracture and rotation. The theory of synergistic effects caused by different strengthening mechanisms is used to predict the yield strength. Experiments show that compressive yield strength of composites improves by 47% compared with those of 2024Al at 623 K and agrees relatively well theoretical value. The compressive deformation depends on matrix mainly at lower temperature and the main failure mode is shear fracture. Additionally, fracture mechanisms are investigated further through fracture surface analysis. During hot compression, the predominated softening mechanisms also include dynamic recrystallization and strain softening except for dynamic recovery, which corresponds well with the shape of flow curves, microstructural observation and change of activation energy. Lastly, the optimum process parameters are determined to be about 0.1 s^− 1 and 723 K based on Dynamic Material Model and validated by microstructure evolution. Experiments show that the strain rate has a mixed effect on whisker fracture.     

复合材料微细杆增强复合材料研究进展

纤维增强树脂基复合材料具有轻质高强的特点,但复合材料层合板层间韧性和抗冲击性能差,复合材料微细杆(Z-pin)增强技术极大地改善了这一不足,被广泛应用于各工业制造领域.近年来,Z-pin增强复合材料的制备工艺不断发展,目前主要有热压罐法和超声植入法(UAZ).Z-pin增强复合材料的层间增韧和抗冲击性能提高效果显著,但...     

瓦楞型竹束单板复合材料的制备及力学性能表征

探索了一种新型竹质工程构件--瓦楞型竹束单板复合材料(CBLC)的制备工艺, 研究了三种典型铺装类型(Ⅰ型: (0°)₆; Ⅱ型: (0°/90°)₃; Ⅲ型: (90°)₆)对其拉伸、 双向弯曲、 三维压缩性能的影响, 同时利用数字散斑相关方法(DSCM)对其弯曲应变场信息进行了表征。结果表明: 铺装类型对各项力学性能有明显影响, 对于拉伸、 纵向弯曲性能, Ⅰ型> Ⅱ型> Ⅲ型, 对于横向弯曲, Ⅱ型> Ⅲ型> Ⅰ型。不同铺装类型下CBLC的拉伸断裂机制亦各不相同: Ⅰ型为延性断裂, Ⅱ型为逐渐分层断裂, Ⅲ型为脆性断裂。 x和y方向应变场集中分布在试样底端最外层瓦楞波形连接处, 且E_xx< E_yy。多重比较分析表明: 抗压缩性能在三维方向上存在明显差异, 且y>x>z; 铺装类型对CBLC的抗压载荷、 抗压强度有很大影响, 而抗压模量差异不明显。