Effective simulation of the mechanics of longitudinal tensile failure of unidirectional polymer composites

An efficient computational model to simulate tensile failure of both hybrid and non-hybrid composite materials is proposed. This model is based on the spring element model, which is extended to a random 2D fibre packing. The proposed model is used to study the local stress fields around a broken fibre as well as the failure process in composite materials. The influence of fibre strength distributions and matrix properties on this process is also analysed. A detailed analysis of the fracture process and cluster development is performed and the results are compared with experimental results from the literature.     

Kinking and tensile,compressive and interlaminar shear failure in carbon-fibre-reinforced plastic beams tested in flexure

The first stage of the failure process in pultruded, 60% volume fraction, type III carbon fibre-epoxide beam specimens with span-to-depth ratios of 5, 15 and 40 deformed in flexure at atmospheric pressure was the initiation of kinking by the compression roller. Kink growth during the non-linear part of the load-deflection curve was followed by kink propagation at peak load. Acoustic emission and load-unload tests to detect irrecoverable deflection supported direct microscopic observations of damage. Kink growth with decreasing load, increasing deflection and accompanying redistribution of stresses led to two types of failure, commonly referred to as flexural and interlaminar. In the former, tensile failure was concurrently initiated to give the characteristic tensile and compressive zones on the failure surfaces. In the latter, the growing kink initiated interlaminar cracks in resin-rich zones as it propagated (with decreasing load) towards the convex surface. Kinking was associated with triaxial compressive stresses in the contact zone of the compressive roller or rollers (in the case of four-point bend specimens). When hydrostatic pressure was superposed on flexure, at pressures between 150 and 300 MNm^–2 depending on the type of specimen, kinking was inhibited and eventually suppressed to give tensile failures, even in the so-called interlaminar shear strength type of specimen. When non-linear deflections were not large, the maximum principal tensile stress in the beams was close to the tensile strength of the carbon-fibre-reinforced plastic (1.8 GN m^–2).     

From virgin to recycled bitumen: A microstructural view

In the present work, soft and hard bitumens recovered from unaged, aged and recycled asphalt concrete (AC) mixtures, which in laboratory tests performed mechanically as well as an AC mixture produced with virgin materials, were investigated regarding rheological, thermal and surface microstructural aspects. For comparison purposes, bitumen containing 50 wt% of virgin bitumen and 50 wt% of bitumen recovered from reclaimed asphalt pavement (RAP) was studied. Some properties of the bitumens remained unchanged throughout the preparation of the AC mixture, aging and recycling: Soft and hard bitumens retained their general rheological properties significantly, and their thermal and surface microstructural properties partially. Soft bitumens presented larger “bee” structures and, therefore, higher surface roughness, while hard bitumens presented smaller “bee” structures and, thus, lower surface roughness. Furthermore, soft bitumens seemed to contain higher crystalline-like content than hard bitumens. For the soft cases, the unaged recovered bitumen did not show the same characteristics (rheological and surface microstructure) as the virgin bitumen. Similarly the recovered recycled bitumen did not show the same characteristics (surface microstructure) as the bitumen prepared from the mixture of virgin bitumen and RAP bitumen. Aging of the AC mixture changed the rheological properties of the soft bitumen by increasing the complex modulus and decreasing the phase angle. Similarly, recycling changed the rheological properties by increasing the complex modulus and decreasing the phase angle. Compositional changes occurred during AC mixture preparation (possibly also aging and recycling) for both soft and hard bitumens. Consequently, more “phases” were observed on the surface microstructure for the recovered bitumens as compared with the virgin bitumens. However, no significant trend was found for the surface microstructure characteristics between the unaged, aged and recycled recovered bitumens. Moreover, the nature of the virgin bitumen influenced the properties of the recycled recovered bitumen, e.g. the glass transition temperature.     

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.     

单向纤维增强聚合物复合材料压缩渐进破坏单向纤维增强聚合物复合材料压缩渐进破坏

复合材料结构在承压时破坏如何演化,是其强度破坏分析的基础和核心任务。本文提出了基于连续介质损伤力学（CDM）的单向纤维增强聚合物复合材料压缩破坏渐进损伤分析（PDA）模型。建模中考虑了材料非线性行为、失效判断及损伤演化中材料性能退化等基本问题,分别对应于拉压不对称弹塑性本构关系、Puck准则、LaRC05准则及考虑破坏面方向的刚度退化方法。该模型通过用户材料子程序接口VUMAT引入到有限元软件ABAQUS中实现了有限元求解。对文献中提供的纵向、横向及偏轴压缩案例进行了数值计算并与试验数据对比。数值分析结果与试验数据吻合较好,证明了该方法的合理性和有效性,对开展多向层合板压缩破坏分析富有参考价值。      

Mechanical characterization of melt spun fibers from recycled and virgin PET blends

In this study two Poly (Ethylene Terephthalate) (PET) polymers obtained from mineral water bottles and a virgin fiber grade PET polymer were investigated. In order to improve their properties when reprocessed at high temperatures, recycled polymers were blended with virgin one. Thermal and rheological properties of extruded recycled/virgin (PET-V/R) blends showed a good microstructural stability compared to extruded pure recycled polymers. Mechanical behaviour of melt spun fibers obtained from recycled/virgin blends were investigated in static (tensile) and dynamic (DMA) modes and gave interesting properties. Fatigue failure of fibers was also studied and resulting fracture morphologies were analysed by Scanning Electron microscopy (SEM).     

Fracture behaviour of virgin and recycled isotactic polypropylene

Fracture behaviour of virgin and six times recycled isotactic polypropylene (PP) has been studied. Instrumented Charpy impact tests have been carried out to characterise the fracture parameters and scanning electron microscopy was used to study the fracture surfaces. Recycled PP presents slightly smaller spherulites and lower fracture toughness than virgin one. Fractography analysis reveals that crazing is the dominant fracture micromechanism for both materials, and that the difference in fracture toughness is a consequence of the smaller plastically deformed volume at the notch tip of the recycled polypropylene.     

The bending properties of integrally woven and unidirectional prepreg T-sections

The relative bending properties of T-sections produced from three-dimensional (3D) integrally woven preforms and unidirectional prepreg have been investigated with respect to their stress–strain behaviour, skin-stiffener separation loads and fracture behaviour. These tests were carried out to simulate localised skin-stiffener separation that is normally responsible for the failure of postbuckling blade-stiffened composite structures. It has been found that the woven T-sections performed better than their prepreg counterpart in terms of initial and peak failure loads, while at the same time also sustaining less damage which is more contained. Finite element analysis has been carried out to ascertain the suitability of the modified three-point bend test, which also successfully modelled the deformed shape of the T-sections and their critical skin-stiffener separation forces.     

On the elastic stress transfer and longitudinal modulus of unidirectional multi-short-fiber composites

An analysis is presented on the elastic stress transfer and longitudinal modulus of unidirectional multi-short-fiber composites. The analysis involves a three-cylinder model consisting of fiber, matrix and composite medium. The fiber axial stress and the interface shear stress are derived as functions of the fiber axial position. The effects of fiber-aspect ratio, fiber-volume fraction, fiber-to-matrix modulus ratio and inter-fiber separation (or fiber end gap) on stress transfer are studied in detail. The influence of neighbouring fibers on stress transfer is considered in a global manner by including the effect of the composite medium. The significance of the influence is clearly shown by comparing the stress transfer in multi-short-fiber composites with that in single-short-fiber composites. The composite modulus can be expressed by a modified rule of mixtures equation by introducing a fiber-length factor. Then, the effects of fiber-aspect ratio, fiber-volume fraction, fiber-to-matrix modulus ratio and inter-fiber separation (or fiber end gap) on the fiber-length factor are investigated. Some interesting findings are obtained. Finally, the present theory is compared with several existing theories.     

On compressive failure of multidirectional fibre-reinforced composites: A fractographic study

The compressive failure of multidirectional carbon fibre-reinforced composites is investigated in this paper. Cross-ply and multidirectional compact compression IM7/8552 specimens were tested to deduce the failure mechanisms that occurred during compressive loading. The experimental results and the subsequent fractographic analysis revealed that the stacking sequence had a significant effect on the performance of multidirectional composites under compression. Delamination and in-plane shear fracture were the dominant failure mechanisms both in cross-ply and multidirectional configurations. While multidirectional configurations exhibited a stiffer response and higher failure load compared to cross-ply configurations, they were also more prone to delaminations and post-failure damage. Multidirectional laminates also exhibited significantly more complex fracture morphologies, which made the failure process interpretation more difficult. The sequence of events that lead to global fracture in multidirectional fibre-reinforced composites is presented.     

On the mechanical properties,deformation and fracture of a natural fibre/recycled polymer composite

A composite laminate based on natural flax fibre and recycled high density polyethylene was manufactured by a hand lay-up and compression moulding technique. The mechanical properties of the composite were assessed under tensile and impact loading. Changes in the stress–strain characteristics, of yield stress, tensile strength, and tensile (Young's) modulus, of ductility and toughness, all as a function of fibre content were determined experimentally. A significant enhancement of toughness of the composite can be qualitatively explained in terms of the principal deformation and failure mechanisms identified by optical microscopy and scanning electron microscopy. These mechanisms were dominated by delamination cracking, by crack bridging processes, and by extensive plastic flow of polymer-rich layers and matrix deformation around fibres. Improvements in strength and stiffness combined with high toughness can be achieved by varying the fibre volume fraction and controlling the bonding between layers of the composite.     

连续纤维增强树脂复合材料纵向压缩强度预测模型的发展及其影响因素

基于高强、高韧、高模和压拉平衡为特征的第三代先进复合材料的需求,综述了连续纤维增强树脂复合材料纵向压缩强度预测模型的发展历程。基于纤维微屈曲、纤维扭结带、联合预测模型及渐进损伤失效模型,分别讨论了连续纤维增强树脂复合材料压缩失效机制,并在联合预测模型基础上,探究了碳纤维（直径、模量、体积分数、初始偏角）、树脂基体（弹性模量、剪切模量）及纤维/树脂界面三要素对连续纤维增强树脂复合材料纵向压缩强度和压缩失效形式的影响。      

Analytical modelling of the compressive and tensile response of woven composites

An analytical model to predict the compressive and tensile response of woven composites up to failure is proposed. The model is based on a beam supported by an elastic foundation. The elastic foundation provides both normal and shear support. Its characteristics are derived from kinematic models for the deformation of the weave and account for: (i) weave effect, (ii) support provided by the adjacent layers, and (iii) properties of matrix and transverse tows.     

Dynamic compressive response and failure behavior of fiber polymer composites embedded with tetra-needle-like ZnO nanowhiskers

Glass fiber polymer composites embedded with tetra-needle-like zinc oxide (ZnO) nanowhiskers were prepared. The prepared composites exhibited excellent mechanical properties after the effective dispersion of ZnO nanowhiskers in the resin. The static and dynamic compressive properties of the composites were tested in the thickness and in-plane directions. The macro- and microfracture morphologies of the damaged specimens were obtained by using a scanning electron microscope. The results show that the compressive properties of the composites could be significantly affected by strain rates. As the strain rate increases, the composites have a higher strength. The compressive properties of the composites are affected by the content of ZnO nanowhiskers in the resin. The high strength of the composites can be attributed to the three-dimensional structures of ZnO nanowhiskers and the corresponding stress transfer.     

Factorial design used to model the compressive strength of mortars containing recycled rubber

This paper presents results from an investigation on the potential use of waste vulcanized rubber scrap (WRS) particles as aggregate in construction mortars. The investigation was carried out using a 3² factorial design of experiments and the response surface methodology. Mortar mixtures were prepared using WRS as fine aggregate (10, 20 and 30 vol.%) with water/cement ratios of 0.52, 0.55 and 0.60. Fresh mortar consistency index and hardened mortar 28-day compressive strength were evaluated. The influence of the WRS content on the physical and mechanical properties was established, and the suitability of WRS use in a mortar application was demonstrated.     

持压荷载与干湿循环作用下再生混凝土氯盐侵蚀行为

采用干湿比为3∶1和质量分数为5wt%的NaCl溶液,开展了持压荷载与干湿循环共同作用下不同再生粗骨料取代率(r=0%、30%、50%、100%)混凝土的氯离子传输试验,分析了持压应力水平(λ_c=0.1、0.3、0.5)对氯盐侵蚀性能的影响。基于非饱和混凝土的氯离子对流-扩散模型,提出了考虑应力水平和再生骨料取代率影响的水分和氯离子扩散系数模型,并验证了该模型的有效性。结果表明：相同再生粗骨料取代率的混凝土内自由氯离子含量、氯离子扩散系数和表面氯离子浓度均随应力水平的增加呈先减小后增大的趋势,同一应力水平下与再生粗骨料取代率呈正相关,再生粗骨料取代率为100%的试件承受0.1f_c、0.3f_c、0.5f_c(f_c为再生混凝土(RAC)立方体抗压强度值)应力作用的氯离子扩散系数分别是无应力状态的0.97、0.88和1.48倍;所建立的持压荷载与干湿循环作用下RAC氯离子传输模型,为再生混凝土耐久性分析提供理论依据。     

Measurement of the fracture toughness associated with the longitudinal fibre compressive failure mode of laminated composites

The fracture toughness associated with the fibre compressive failure was obtained from testing notched unidirectional carbon/epoxy four-point-bend specimens. Microscopy of failed specimens revealed that onset of damage was characterised by the formation of a single line of fibre breaks at approximately 45° to the plane of the initial notch. A micromechanical finite element model was used to investigate this failure scenario and it was concluded that the most probable cause of the damage morphology was compression-induced shear failure of the composite. An intrinsic material property in this case was deemed to be the mode II critical strain energy release rate associated with the initiation of the 45° crack. For IM7/8552, this was measured to be G_IIc = 4.5 ± 0.8 kJ/m².     

Multi-scale modeling, stress and failure analyses of 3-D woven composites

The very complex, multi-level hierarchical construction of textile composites and their structural components commonly manifests via significant property variation even at the macro-level. The concept of a “meso-volume” (introduced by this author in early 1990s) is consistently applied in this work to 3-D stress/strain and failure analyses of 3-D woven composites at several levels of structural hierarchy. The meso-volume is defined as homogeneous, anisotropic block of composite material with effective elastic properties determined through volumetrically averaged 3-D stress and strain fields computed at a lower (“finer”) level of structural hierarchy and application of generalized Hooke’s law to the averaged fields. The meso-volume can represent a relatively large, homogenized section of a composite structural component, a lamina in laminated composite structure, a homogenized assembly of several textile composite unit cells, a single homogenized unit cell, a resin-impregnated yarn, a single carbon fiber, even a carbon nanotube assembly. When composed together, distinct meso-volumes constitute a 3-D Mosaic model at the respective hierarchy level. A multi-scale methodology presented in this paper first illustrates 3-D stress/strain analysis of the Mosaic unidirectional composite, computation of its effective elastic properties and their further use in 3-D stress/strain analysis of the Mosaic model of 3-D woven composite Unit Cell. The obtained 3-D stress/strain fields are then volumetrically averaged within the Unit Cell, and its effective elastic properties are computed. The predicted effective elastic properties of 3-D woven composite are compared with experimental data and show very good agreement. Further, those effective elastic properties are used in 3-D simulations of three-point bending tests of 3-D woven composite; theoretical predictions for central deflection show excellent agreement with experimental data. Finally, a 3-D progressive failure analysis of generic 3-D Mosaic structure is developed using ultimate strain criterion and illustrated on the 3-D woven composite Unit Cell. The predicted strength values are compared to experimental results. The presented comparisons of theoretical and experimental results validate the adequacy and accuracy of the developed material models, mathematical algorithms, and computational tools.     

Evaluation of a screening method for classifying virgin and recycled paper and board samples

This paper deals with the study of volatile compounds released by recycled paper and board. The aim of the study was to demonstrate the feasibility of headspace procedure coupled to gas chromatography/mass spectrometry (GC/MS) applied to complex paper‐based samples together with a chemometric procedure as a powerful method for screening potential volatile contaminants released by the recycled and virgin paper samples. Using this procedure, the identification of virgin or recycled paper could be achieved based on the identification on specific markers of the recycled pulp. Fifteen different samples within virgin and recycled paper were studied. After equilibration, the vapour phase of the samples was analysed by automatic headspace coupled online to GC/MS. The analytical approach for volatile compounds, their identification and the selection of some compounds as markers for recycled pulp are shown and discussed. A discriminate analysis applied to the set of results obtained allows classification of the samples into four different groups according to the content of recycled pulp (0, 10–30 and > 80% of recycled pulp), the surface treatment of the paper (no surface treatment, clay coating and plastic coating), the grammage (from < 100 to > 300 g/m²) and the sample thickness (from < 300 to > 600 µm). The matrix effect on the volatilization of some compounds from the paper samples and the analytical behaviour are also discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

Influence of fibre architecture on the tensile,compressive and flexural behaviour of 3D woven composites

This paper presents a comprehensive study on the tensile, compressive, and flexural performance of six types of 3D woven carbon-fibre/epoxy composites which were manufactured using a traditional narrow fabric weaving loom and resin transfer moulding. Four orthogonal and two angle-interlock weaves were tested with the primary loading direction parallel to the warp direction. The mechanical performance was found to be affected by the distribution of resin rich regions and the waviness of the load-carrying fibres, which were determined by the fibre architectures. The binding points within the resin rich regions were found to be the damage initiation sites in all weave types under all loading conditions, which were confirmed with both visual observation and digital image correlation strain maps. Among all weave types, the angle interlock weave W-3 exhibited the highest properties under all loading conditions.