Compressive behaviour of thin-skin stiffened composite panels with a stress raiser

The in-plane compressive behaviour of thin-skin stiffened composite panels with a stress concentrator in the form of an open hole or low velocity impact damage is examined analytically. Drop weight impact in laminated polymer composites causes matrix cracking, delaminations and fibre breakage, which together can seriously degrade the laminate compressive strength. Experimental studies, using ultrasonic C-scan images and X-ray shadow radiography, indicated that the overall damage resembles a hole. Under uniaxial compression loading, 0° fibre microbuckling surrounded by delamination grows laterally (like a crack) from the impact site as the applied load is increased. These local buckled regions continued to propagate, first in discrete increments and then rapidly at failure load. The damage pattern is very similar to that observed in laminated plates with open holes loaded in compression. Because of this resemblance, a fracture mechanics model, developed initially to predict notched compressive strength, was applied to estimate the compression-after-impact (CAI) strength of a stiffened panel; in the analysis the impact damage is replaced with an equivalent open hole. Also, the maximum stress failure criterion is employed to estimate the residual compressive strength of the panel. The unnotched compressive strength of the composite laminate required in the analysis is obtained from a three-dimensional stability theory of deformable bodies. The influence of the stiffener on the compressive strength of the thin-skin panel is examined and included in the analysis. A good agreement between experimental measurements and predicted values for the critical failure load is obtained.     

Quantitative evaluation of the adhesion of metallic coatings using cylindrical substrate

This paper introduces an effective interfacial fracture toughness test based on interface fracture mechanics theory. This testing method uses a circumferentially notched tensile (CNT) specimen, which is ideally suited for determining the interfacial fracture resistance of coatings. Unlike other interfacial fracture tests, this test is simple to prepare, requires minimum test setup and is easy to model. An interfacial pre-crack was generated between a nickel coating and mild steel cylindrical substrate to evaluate adhesion strength. In situ acoustic and SEM analyses were used to determine the crack initiation or the critical load of failure. The critical energy release rate, critical stress intensity factors and phase angle were determined using the J integral which was determined by applying the critical load to the finite element model. A detailed finite element analysis was carried out to study the effect of different interface pre-crack positions and mode mixity on energy release rate for different notch angles and elastic modulus ratios. The cracking resistance of the interface was characterised by the notch angle of CNT specimens. The analysis showed an increase in interfacial fracture toughness as phase angle increases and was significant when the phase angle was large. The combined results of computational and experimental analysis showed that any defect or stress concentration at the interface could significantly weaken the adhesion of coating.     

A finite fracture mechanics approach to structures with sharp V-notches

Criteria assuming that failure of quasi-brittle materials is affected by the stresses acting over a finite distance from the crack tip are widely used inside the scientific community. For instance, they have been applied to predict the failure load of specimens containing sharp V-notches, assuming as a critical parameter the average stress ahead the notch tip. However, this kind of approaches disregards energy balance considerations, which, as well known, are the basis of linear elastic fracture mechanics (LEFM). In order to overcome these drawbacks, the present paper uses a recently introduced finite fracture mechanics (FFM) criterion, i.e. a fracture criterion assuming that crack grows by finite steps. The length of this finite extension is determined by a condition of consistency of both energy and stress requirements; as a consequence, the crack advancement is not a material constant but a structural parameter. The criterion is applied to structures with sharp V-notches. The expression of the generalized fracture toughness, which is a function of material tensile strength, fracture toughness and notch opening angle, is given analytically. Finally, we provide comparisons with: (i) the experimental data we obtained from testing Polystyrene specimens under three point bending; (ii) some experimental data available in the literature. The agreement between theoretical predictions and experimental results is generally satisfactory and, for most of the cases analyzed, the FFM predictions are better than the ones provided by the simple average stress approach.     

Intraply fracture of fiber-reinforced composites: Microscopic mechanisms and modeling

The fracture behavior parallel to the fibers of an E-glass/epoxy unidirectional laminate was studied by means of three-point tests on notched beams. Selected tests were carried out within a scanning electron microscope to ascertain the damage and fracture micromechanisms upon loading. The mechanical behavior of the notched beam was simulated within the framework of the embedded cell model, in which the actual composite microstructure was resolved in front of the notch tip. In addition, matrix and interface properties were independently measured in situ using a nanoindentor. The numerical simulations very accurately predicted the macroscopic response of the composite as well as the damage development and crack growth in front of the notch tip, demonstrating the ability of the embedded cell approach to simulate the fracture behavior of heterogeneous materials. Finally, this methodology was exploited to ascertain the influence of matrix and interface properties on the intraply toughness.     

Fracture measurements on cement paste

This paper describes an investigation into the fracture behaviour of hardened cement paste. Notched specimens of the material were tested to failure in flexure and tension. In the initial flexural tests on beams of fixed overall depth, the stress intensity factor at failure as calculated from linear-elastic fracture mechanics appeared to be a material constant. However, further investigation showed that this factor varied with specimen size, and suggested that linear-elastic fracture mechanics and the concept of fracture toughness are not readily applicable to hardened cement paste, which would appear to be a relatively notch insensitive material whose strength is not greatly reduced by the presence of flaws. A “tied crack” model explains semi-quantiatively the observed behaviour.     

Strength prediction in CFRP woven laminate bolted double-lap joints under quasi-static loading using XFEM

The current paper is concerned with modelling damage and fracture in woven fabric composite double-lap bolted joints that fail by net-tension. A 3-D finite element model is used, which incorporates bolt clamp-up, to model a range of CFRP bolted joints, which were also tested experimentally. The effects of laminate lay-up, joint geometry, hole size and bolt clamp-up torque were considered. An Extended Finite Element (XFEM) approach is used to simulate damage growth, with traction–separation parameters that are based on previously reported, independent experimental measurements for the strength and toughness of the woven fabric materials under investigation. Good agreement between the predicted and measured bearing stress at failure was obtained.     

Hybrid nanoreinforced carbon/epoxy composites for enhanced damage tolerance and fatigue life

Hybrid nano/microcomposites with a nanoparticle reinforced matrix were developed, manufactured, and tested showing significant enhancements in damage tolerance properties. A woven carbon fiber reinforced polymer composite, with the polymer (epoxy) matrix reinforced with well dispersed carbon nanotubes, was produced using dispersant-and-sonication based methods and a wet lay-up process. Various interlaminar damage tolerance properties of this composite, including static strength, fracture toughness, fatigue life, and crack growth rates were examined experimentally and compared with similarly-processed reference material produced without nanoreinforcement. Significant improvements were obtained in interlaminar shear strength (20%), fracture toughness (180%), shear fatigue life (order of magnitude), and fatigue crack growth rate (factor of 2). Observations by scanning electron microscopy of failed specimens showed significant differences in fracture surface morphology between the two materials, related to the differences in properties and providing context for understanding of the enhancement mechanisms.     

Combination of biological mechanisms for a concept study of a fracture-tolerant bio-inspired ceramic composite material

The biological materials nacre and wood are renowned for their impressive combination of toughness and strength. The key mechanisms of these highly complex structures are crack deflection at weak interfaces, crack bridging, functional gradients and reinforcing elements. These principles were applied to a more fracture-tolerant model material which combined porous stiff ceramic layers, manufactured by freeze casting, infiltrated and bonded by a polymer phase reinforced with fabric layers. In the hybrid composites, crack deflection occurred at the ceramic–fabric interface and the intact fabric layers served as crack-bridging elements. Fabric-reinforced epoxy layers stabilized the fracture behaviour and delayed catastrophic failure of the material. The influence of the different components was analysed by varying the ceramic, fabric and interface properties. More ductile fabrics lead to larger strain to failure and more crack bridging but reduced the composite strength and stiffness after initial cracking. Higher elastic mismatch between the components improved crack deflection and bridging but resulted in deterred load transfer and a lower strength. The stiffness and strength of the ceramic layers influenced the elastic properties of the laminar composite and the initial crack resistance. Flaw tolerance was increased with polymer infiltration. We show with our hybrid ceramic–fabric composite as a bio-inspired concept study how fracture toughness, work of fracture and tolerance for cracking can be tailored when the contributing factors, i.e. the ceramic, the fabric and their interface, are modified.     

正交铺设陶瓷基复合材料单轴拉伸行为

采用细观力学方法对正交铺设陶瓷基复合材料单轴拉伸应力-应变行为进行了研究。采用剪滞模型分析了复合材料出现损伤时的细观应力场。采用断裂力学方法、 临界基体应变能准则、 应变能释放率准则及Curtin统计模型4种单一失效模型确定了90°铺层横向裂纹间距、 0°铺层基体裂纹间距、 纤维/基体界面脱粘长度和纤维失效体积分数。将剪滞模型与4种单一损伤模型结合, 对各损伤阶段应力-应变曲线进行了模拟, 建立了复合材料强韧性预测模型。与室温下正交铺设陶瓷基复合材料单轴拉伸应力-应变曲线进行了对比, 各个损伤阶段的应力-应变、 失效强度及应变与试验数据吻合较好。分析了90°铺层横向断裂能、 0°铺层纤维/基体界面剪应力、 界面脱粘能、 纤维Weibull模量对复合材料损伤及拉伸应力-应变曲线的影响。      

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.     

Numerical investigation to prevent crack jumping in Double Cantilever Beam tests of multidirectional composite laminates

During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) or to grow asymmetrically, invalidating the tests.The aim of this study is to check the feasibility of defining the stacking sequence of Double Cantilever Beam (DCB) specimens so that these undesired effects do not occur, leading to meaningful onset and propagation data from the tests. Accordingly, a finite element model using cohesive elements for interlaminar delamination and an intralaminar ply failure criterion are exploited here to thoroughly investigate the effect of specimen stiffness and thermal residual stresses on crack jumping and asymmetric crack growth occurring in multidirectional DCB specimens.The results show that the higher the arm bending stiffness, the lower the tendency to crack jumping and the better the crack front symmetry. This analysis raises the prospect of defining a test campaign leading to meaningful fracture toughness results (onset and propagation data) in multidirectional laminates.     

考虑骨料体积含量影响的混凝土准脆性断裂预测模型及应用

基于对准脆性断裂边界影响模型参数的分析,该文将平均骨料粒径d_ave引入模型中,得到了考虑骨料体积含量及尺寸影响的混凝土准脆性断裂预测模型。模型中的有效裂缝与特征裂纹的比值,明确表征了三分点加载单边切口梁(SENB)试件的尺寸及初始缝长度变化时服从的断裂失效准则;模型中d_ave及分散系数β_ave将影响最大荷载P_max作用下临界微裂纹扩展区的平均虚拟裂纹长度Δa_fic。通过SENB试件在P_max时的受力分析,得到了临界正应力σ_n、有效裂缝长度a_e、拉伸强度f_t及断裂韧度K_IC之间的关系式。通过Amparano的试验结果,当a_fic为0.8~1.4倍d_ave时,采用混凝土准脆性断裂模型能较好预测混凝土拉伸强度及断裂韧度。通过对Δa_fic=1.2d_ave时模型得到的预测曲线与试验结果的对比,证明了模型计算结果的可靠性。考虑骨料体积含量影响的混凝土准脆性断裂模型能基于RILEM规范中三分点加载SENB试验预测混凝土断裂韧度与拉伸强度。     

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。      

Fatigue crack propagation and in-situ observations in three tool steel alloys manufactured using a rapid solidification technique

By utilizing special manufacturing conditions, e.g., using only pure elements and applying a rapid cooling rate, tool materials with high quasi-static fracture toughness can be produced. However, tool materials are often subjected to cyclic loading and, hence, their lifetime is dominated by fatigue failure. This study is focused on fracture mechanics and in-situ experiments to characterize the fatigue crack propagation behavior of three newly developed tool steels at a stress ratio R of 0.05. Microstructural examinations revealed that the materials consist of the phases α′-martensite, retained austenite, and complex carbides in different amounts. Results of preliminary tests are presented, in which it was attempted to grow the crack in a plane parallel to the plane of the starter notch. The determined ?K threshold values ranged between 4 and 5 MPa√m with Paris–Erdogan exponents of 3.3–4.6. In-situ observations were performed to understand the inherent damage mechanisms and microstructural effects during fatigue loading. These observations showed that fatigue crack growth is mainly dominated by the ductility of the martensitic–austenitic matrix. Only in cases in which the primary carbides are oriented favorably (with respect to the direction of crack propagation) does the crack follow the coherent carbide network to a certain extent. Furthermore, for the first time, a phase transformation from retained austenite to α′-martensite was detected at the crack tip during fatigue crack propagation for the material group of tool steels.     

Local failure behavior of a dissimilar metal interface region with mechanical heterogeneity

Failure analyses and assessments for dissimilar metal welded joints are usually conducted by simplified interface regions without considering actual mechanical heterogeneity and limited crack positions. In this work, the finite element simulations based on Gurson-Tvergaard-Needleman damage mechanics model are used to investigate local failure behavior of a dissimilar metal interface region with mechanical heterogeneity and a series of initial crack positions. The results show that the fracture resistance and crack growth paths of the cracks in different locations of the interface region are significantly affected by mechanical heterogeneity. The interactions of different local mechanical (strength and work hardening properties) mismatches and intrinsic toughness around cracks determine the distributions and evolution of plastic strain, stress triaxiality and damage ahead of crack tips, which cause larger variations in fracture resistance and crack growth paths. For accurate and reliable failure analyses and assessments for the cracks in dissimilar metal interface regions with mechanical heterogeneity, it is recommended to obtain and use local fracture resistance properties related to crack locations.     

Modelling the residual strength of fatigue damage at a single semicircular edge notch: Semielliptical crack and through‐the‐thickness crack

In the present paper, computational framework for fatigue performance analysis of a semicircular edge notch with a through‐the‐thickness crack or a semielliptical crack is discussed. The failure behaviour of such configurations is theoretically examined through the stress‐intensity analysis and residual life estimation. The stress field of a damaged notch configuration is herein investigated by employing analytical and numerical approaches. Further, a fracture mechanics–based methodology, developed for fatigue life assessment, is taking into account the crack growth model proposed by Huang and Moan in which the stress ratio is involved. The efficiency of the obtained fatigue damage assessments, related to the edge notch configurations, is verified through appropriate experimental observations.     

Effect of compressive transverse normal stress on mode II fracture toughness in polymeric composites

Effect of transverse normal stress on mode II fracture toughness of unidirectional fiber reinforced composites was studied experimentally in conjunction with finite element analyses. Mode II fracture tests were conducted on the S2/8552 glass/epoxy composite using off-axis specimens with a through thickness crack. The finite element method was employed to perform stress analyses from which mode II fracture toughness was extracted. In the analysis, crack surface contact friction effect was considered. It was found that the transverse normal compressive stress has significant effect on mode II fracture toughness of the composite. Moreover, the fracture toughness measured using the off-axis specimen was found to be quite different from that evaluated using the conventional end notched flexural (ENF) specimen in three-point bending. It was found that mode II fracture toughness cannot be characterized by the crack tip singular shear stress alone; nonsingular stresses ahead of the crack tip appear to have substantial influence on the apparent mode II fracture toughness of the composite.     

Interpreting the stress ratio effect on delamination growth in composite laminates using the concept of fatigue fracture toughness

This paper provides a study on fatigue delamination growth in composite laminates using energy principles. Experimental data has been obtained from fatigue tests conducted on Double Cantilever Beam (DCB) specimens at various stress ratios. A concept of fatigue fracture toughness is proposed to interpret the stress ratio effect in crack growth. The fatigue fracture toughness is demonstrated to be interface configuration independent but significantly stress ratio dependent. An explanation for this phenomenon is given using SEM fractography. Fracture surface roughness is observed to be similar in different interfaces at the same stress ratio. But it is obviously more rough for high stress ratio in comparison with that for low stress ratio, causing the fatigue resistance increase. Therefore, the stress ratio effect in fatigue crack growth can be physically explained by a difference in resistance to crack growth.     

Strength analysis of particulate polymers

Impregnating polymeric matrix with stiff particles may significantly improve structural response of a composite material. Such improvements have to be weighed against the effects of the stress concentration at the particle–matrix interface that influence local strength and toughness. In the present paper we elaborate on the issue of local stresses and strength in particulate polymer matrix composites considering polyurethane matrix impregnated with alumina particles in numerical examples. The parametric analysis presented in the paper is concerned with the effects of the particle volume fraction and the particle-to-matrix stiffness ratio on the local stresses and initial damage. We also discuss the resilience of the impregnated polyurethane, i.e. the density of energy necessary to produce initial damage. The approach to the analysis of fracture in the composite with initial damage is discussed accounting for available experimental observations. Three scales of fracture corresponding to different phases of the development and propagation of the crack are identified, including microfracture at the particle–matrix interface and mesofracture limited to the matrix surrounding the particle. While these scales of fracture should be analyzed by numerical methods, macrofracture that occurs after the crack “emerged” from the representative unit cell where it originated can be considered using available analytical techniques. The methodology of the stress analysis of a particulate material consisting of an incompressible hyperelastic matrix and much stiffer elastic particles is also proposed in the paper.