聚乙烯自增强复合材料损伤过程的声发射特征

复合材料在承受外载时, 声发射可产生于基体破裂、纤维-基体界面脱粘和纤维断裂等。测定了U HMWPE/ HDPE 复合材料在拉伸载荷作用下的声发射(AE) 振幅信号。对特殊试样, 即预测到断裂有明确方式, 如纤维-基体界面脱粘、基体破裂、纤维断裂和分层等的试样, 实施加载直至破坏。用扫描电子显微镜(SEM) 观测试样的断裂表面, 对产生于若干特殊损伤类型的AE 信号进行了鉴别。在相同加载条件下, 完成了不同种类的U HMWPE/ HDPE 准各向同性层合板声发射检测。结果在特殊试样损伤类型与声发射信号事件振幅之间建立了对应关系, 揭示了上述各种准各向同性层合板损伤扩展过程的AE 特征与损伤破坏机制。各种准各向同性层合板试样的声发射事件累计数对拉伸应力关系曲线相异, 其相同损伤类型发生时所对应的拉伸载荷水平不等, 表明它们的铺设角度和铺设顺序对损伤演变过程有显著的影响。结果证实了它们的最终破坏由严重层间分层造成。      

A computational continuum damage mechanics model for predicting transverse cracking and splitting evolution in open hole cross‐ply composite laminates

The present study focuses on a computational constitutive model which predicts the matrix cracking evolution and fibre breakage in cross‐ply composite laminates with open hole under in‐plane loading. To consider the effects of matrix cracking on the nonlinear response of laminates, a simplified crack density based model is applied which evaluates the representative damage parameters of matrix cracking. Furthermore, a developed subroutine based on continuum damage mechanics concepts is applied in ANSYS code which is capable to consider the transverse cracking/splitting evolution and predict the final failure load of mentioned laminate under monotonic loading in a progressive damage analyses. It is shown that the obtained stress–strain behaviours and the damage evaluation of considered laminates are in good agreement with the available experimental results.     

APMOC/环氧复合材料层板破坏过程声发射特征

本文利用声发射测试技术,对APMOC/环氧复合材料各种铺层的单层板、层合板的损伤机理和破坏过程进行了详细的研究。结果表明,在不同的声发射信号参量特征与不同的损伤机理之间存在对应关系。声发射信号参量的变化过程能够描述层板的动态损伤过程,不同的声发射参量表征层板的不同损伤过程与机理。     

GLARE层板挤压渐进损伤分析

对GLARE36/5层板进行挤压性能试验研究,采用超声C扫描、断口微距拍摄和扫描电子显微镜等方法观测GLARE层板挤压渐进损伤过程和最终破坏模式。结果表明:GLARE层板挤压起始损伤为铝合金塑性变形;损伤扩展阶段,0°纤维主要承受挤压正应力,铝合金塑性变形增大,铺层间分层起始并扩展;0°纤维屈曲折断后层内纤维基体损伤和分层损伤急剧扩展,层板最终发生挤压破坏。将GLARE层板挤压失效分为层内失效和层间失效,采用应变描述的Hashin准则和界面单元方法并引入金属塑性建立GLARE层板挤压渐进损伤数值模型,数值模型对层板损伤起始位置、分层产生位置、损伤演化过程、最终破坏模式及破坏载荷进行了预测,计算结果与试验结果吻合较好,说明该计算方法能够有效模拟GLARE层板挤压渐进损伤性能。      

低速冲击后复合材料层合板的压缩破坏行为

对缝纫层合板和无缝纫层合板进行低速冲击后压缩破坏实验,以研究低速冲击后层合板的压缩破坏机理。采用C扫描、X射线、热揭层等技术对层合板内的损伤进行测量和对比。结果表明,界面不是很强的碳纤维增强复合材料层合板低速冲击后受压时,层合板非冲击面的子层屈曲及其扩展是导致层合板冲击后压缩强度下降的重要因素,而且子层屈曲主要是沿垂直载荷的方向(90°)扩展;对于准各向同性板,屈曲子层中与母层相邻的铺层的方向一般为90°。层合板的剩余压缩强度与板的冲击损伤面积无直接关系。      

Characterization of damage onset in glass-ceramic matrix composite angle-ply laminates

The tensile mechanical properties of a series of model [0/θ/0] glass-ceramic matrix composite angle-ply laminates have been measured at room temperature in an attempt to characterize the onset of damage in this class of material as a function of fibre orientation. The material selected for this study was a silicon carbide fibre-reinforced calcium aluminosilicate composite. The experimentally determined values of composite elastic modulus, strength and first ply failure stress have been compared with those predicted from classical laminate theory. Work carried out to date has shown that at high angles of θ, the damage onset of [0/θ/0] laminates is accurately predicted by the Tsai-Hill first ply failure stress whilst at low angles of θ the onset of damage is in reasonable agreement with that predicted by the Aveston-Cooper-Kelly model for multiple fracture (with the Tsai-Hill failure criterion predicting the ultimate strength of the composite). However, these models give no account of the mechanisms by which failure occurs and assume well-defined and single-valued failure strengths. In reality, glass-ceramic matrices have a distribution of strengths as a result of the inherent flaws within them and an attempt to quantify this has been made by mechanical and structural evaluation of the monolithic matrix material.     

Numerical investigation of matrix cracking propagation in cross‐ply laminated composites subjected to three‐point bending load using concurrent multiscale model

The purpose of the present study is to analyze fiber‐matrix debonding and induced matrix cracking formation as two major micromechanical damage modes in cross‐ply composite laminates using a two‐dimensional numerical approach. To this aim, the cross‐ply laminates containing 90‐degree layers are modeled, where the fibers are arranged randomly in transverse plies. Damage modes in this numerical model are simulated by the cohesive surface method. The performed analyses reveal that in the laminates with 90‐degree layers located in the outer positions, the primary micro damage mode is micro matrix cracking which is initiated from the fiber‐matrix debonding damage mode and will be followed by matrix cracking. The main benefit of the present study in comparison to other numerical methods is proposing a virtual test method for damage analysis of different cross‐ply laminates in which, the matrix cracking formation will emerge physically in a random and antisymmetric pattern similar to the experimental observations.     

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.     

Investigation of damage mechanisms in self-reinforced polyethylene composites by acoustic emission

The objective of this study is to investigate the damage mechanisms in self-reinforced polyethylene composite laminates (UHMWPE/HDPE) under monotonic tensile loading by the acoustic emission (AE) technique. Fracture surface examinations were conducted using a scanning electron microscope (SEM). Using model specimens exhibiting a dominant failure mechanism, correlations were established between the observed damage growth mechanisms and the AE results in terms of the events amplitude. These correlations can be used to monitor the damage growth process in the UHMWPE/HDPE composite laminates exhibiting multiple modes of damage. Results from this study revealed that the AE technique is a viable and effective tool for identifying damage mechanisms such as fiber–matrix debonding, matrix cracking, fiber pull-out, fiber breakage and delamination in the UHMWPE/HDPE composite materials.     

3D constitutive model of anisotropic damage for unidirectional ply based on physical failure mechanisms

A 3D anisotropic continuum damage model is developed for the computational analysis of the elastic–brittle behaviour of fibre-reinforced composite. The damage model is based on a set of phenomenological failure criteria for fibre-reinforced composite, which can distinguish the matrix and fibre failure under tensile and compressive loading. The homogenized continuum theory is adopted for the anisotropic elastic damage constitutive model. The damage modes occurring in the longitudinal and transverse directions of a ply are represented by a damage vector. The elastic damage model is implemented in a computational finite element framework, which is capable of predicting initial failure, subsequent progressive damage up to final collapse. Crack band model and viscous regularization are applied to depress the convergence difficulties associated with strain softening behaviours. To verify the accuracy of the damage model, numerical analyses of open-hole laminates with different lay-up configurations under tension and compression were performed. The numerical predictions were compared with the experimental results, and satisfactory agreement was obtained.     

Characterization of high-velocity impact damage in CFRP laminates: Part I – Experiment

This study investigated mechanisms of the extension of high-velocity impact damage in CFRP laminates. To this end, damage states due to near-perforation impact were studied in detail. This study consists of two parts. Part I presents the experiment results of high-velocity impact tests for CFRPs with specified stacking sequences. A crater and splits were observed on the impacted surface, while multiple splits with fiber breaks extended on the back surface. The cross-section beneath the impact point included catastrophic ply failure with extensive fiber breaks. Impacted specimens also exhibited a particular delamination pattern consisting of pairs of symmetric fan-shaped delaminations emanating from to the impact point and elongated delamination along the cracks in the bottom ply. These damage patterns were common to all of the stacking sequences. Part II of this study presents a numerical analysis of high-velocity impact based on smoothed-particle hydrodynamics and discusses damage extension mechanisms.     

In‐plane progressive matrix cracking analysis of symmetric cross‐ply laminates with holes

Most of the previously performed damage analyses in composite laminates have been restricted to model the plain laminates without geometry discontinuities. In this study, a micromechanical damage model is combined with the finite element formulation and is implemented in the integration points to perform progressive damage analyses of composite laminates. A micromechanical damage model based on the stress transfer method is used to find the degradation of mechanical properties of composite laminates. Crack density is also used as an only state variable representing the damage in each Gauss point of every layer of the laminate. The strain energy and critical energy release rate criterion is also used to predict the damage initiation and evolution in each layer. A finite element discretization is used in conjunction with the user element definition capability of ABAQUS commercial software. To verify the developed procedure, a single element is analysed, and the obtained results are compared with available results in the literature. Progressive damage analyses are also performed for several symmetric cross‐ply laminates with and without geometry discontinuity subjected to matrix cracking damage mechanism under in‐plane loading conditions. The obtained mechanical response and variations of matrix crack density versus the applied load are also discussed.     

Prediction of plastic strain accumulation in continuous fiber reinforced laminates by a constitutive ply model

The present paper is concerned with the prediction of plastic ply strains which accumulate in continuous fiber reinforced laminates with polymeric matrix materials. The study is based on a constitutive model which is implemented within the Finite Element Method. Plastic strains are expected to evolve when the ply is subjected to a pronounced shear load and/or to pronounced transverse compression. Two plasticity mechanisms are modeled at ply level under plane stress assumption (i.e. for thin shells and plates). They either concern the evolution of plastic shear strains or the evolution of plastic normal strains. The proposed plasticity model is combined with an existing ply level continuum damage model. The capabilities of the proposed model are assessed by comparing its predictions to experimental data from literature. Emphasis is placed on loading conditions which drive the evolution of plastic strains. Excellent correlation with experimental results is shown for proportional as well as for non-proportional ply loadings.     

Numerical analysis of intralaminar failure mechanisms in composite structures. Part I: FE implementation

This paper presents a three-dimensional continuum damage mechanics-based material model which was implemented in an implicit finite element code to simulate the progressive intralaminar degradation of fibre reinforced laminates. The damage model is based on ply failure mechanisms and uses seven damage variables assigned to tensile, compressive and shear damage at a ply level. Non-linear behaviour and irreversibility were taken into account and modelled. Some issues on the numerical implementation of the damage model are discussed and solutions proposed. Applications of the methodology are presented in Part II [1].     

[±θ/902]碳/环氧层板横向裂缝与分层的测试及分析

本工作对[±θ/90₂]_S和[0_n/90₂]_S两系列碳/环氧层板在拉仲载荷下的横向裂缝与分层损伤,进行了实验研究和有限元分析.采用声发射技术跟踪配合显微观测多向层板损伤过程,分析了θ角变化与力学性能、初始损伤、累积等的关系.表明实测横向开裂与分层结果和采用能量判据有限元计算预测比较,符合良好.同时在扫描电镜内进行各类层板压缩试验,动态观测破坏形貌,讨论了不同θ铺层角的微观破坏机理.     

Acoustic structural health monitoring of composite materials : Damage identification and evaluation in cross ply laminates using acoustic emission and ultrasonics

The characterisation of the damage state of composite structures is often performed using the acoustic behaviour of the composite system. This behaviour is expected to change significantly as the damage is accumulating in the composite. It is indisputable that different damage mechanisms are activated within the composite laminate during loading scenario. These “damage entities” are acting in different space and time scales within the service life of the structure and may be interdependent. It has been argued that different damage mechanisms attribute distinct acoustic behaviour to the composite system. Loading of cross-ply laminates in particular leads to the accumulation of distinct damage mechanisms, such as matrix cracking, delamination between successive plies and fibre rupture at the final stage of loading. As highlighted in this work, the acoustic emission activity is directly linked to the structural health state of the laminate. At the same time, significant changes on the wave propagation characteristics are reported and correlated to damage accumulation in the composite laminate. In the case of cross ply laminates, experimental tests and numerical simulations indicate that, typical to the presence of transverse cracking and/or delamination, is the increase of the pulse velocity and the transmission efficiency of a propagated ultrasonic wave, an indication that the intact longitudinal plies act as wave guides, as the transverse ply deteriorates. Further to transverse cracking and delamination, the accumulation of longitudinal fibre breaks becomes dominant causing the catastrophic failure of the composite and is expected to be directly linked to the acoustic behaviour of the composite, as the stiffness loss results to the velocity decrease of the propagated wave. In view of the above, the scope of the current work is to assess the efficiency of acoustic emission and ultrasonic transmission as a combined methodology for the assessment of the introduced damage and furthermore as a structural health monitoring tool.     

Numerical analysis of intralaminar failure mechanisms in composite structures. Part II: Applications

A three-dimensional continuum damage mechanics-based material model was implemented in an implicit Finite Element code to simulate the progressive intralaminar degradation of fibre reinforced laminates based on ply failure mechanisms. This paper presents some structural applications of the progressive failure model implemented. The focus is on the non-linear response of the shear failure mode and its interaction with other failure modes. Structural applications of the damage model show that the proposed model is able to reproduce failure loads and patterns observed experimentally.     

Indentation of polyethylene laminates by a flat-bottomed cylindrical punch

Cross-ply polymer laminates reinforced by ultra-high molecular weight polyethylene (UHWMPE) fibers and tapes have been subjected to quasi-static indentation by a flat-bottomed, circular cross section punch and their penetration resistance and failure mechanisms investigated. Three fiber- and two tape-reinforced grades progressively failed during indentation via a series of unstable failure events accompanied by substantial load drops. This resulted in a ‘saw-tooth’ load versus indentation depth profile as the load increased with indentation depth after each failure event. The penetration behavior scaled with the ratio of the thickness of the remaining laminate to the diameter of the punch, and the indentation pressure scaled with the through thickness compressive strength. Failure occurred by ply rupture. The results are consistent with penetration governed by an indirect tension failure mechanism, and with experimental reports that tape-reinforced materials have a similar ballistic resistance to the higher tensile strength fiber-reinforced grades in rear-supported test conditions.     

分析复合材料螺栓连接接头静强度的一种方法

根据复合材料机械连接区逐点逐层破坏的物理本质,采用了每层破坏单元刚度退化和应力空间二阶张量破坏准则,用有限元素法,计算了T300/648碳纤维复合材料21种不同铺层情况的接头强度,分析了它们的破坏模式和破坏过程并与试验结果进行了比较.      

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]_2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking.