Predicting fatigue S-N (stress-number of cycles to fail) behavior of reinforced plastics using fracture mechanics theory

The present study describes a model to predict fatigue S-N behavior, and thus fatigue life, of glass fiber reinforced thermoplastics by using a fracture mechanics approach. The model assumes the presence of an inherent initial flaw in the molded plastic parts and thus ignores crack initiation contributions. In this paper we describe how fatigue crack propagation rate data were obtained for the same three glass fiber reinforced plastics whose S-N behavior was previously described in detail. Using the measured constants from the crack growth data, and corresponding S-N data for uncracked specimens, the validity of the single initial flaw hypothesis was evaluated. From the analyzed results it is concluded that accurate S-N predictions are possible using this simple fracture mechanics model for some materials. The best results are obtained for glass filled polyamide, PA (nylon 66) and polycarbonate, PC; however, with polybutylene terephthalate, PBT, predictions were poor. It is also shown that S-N data for different glass fiber orientations can be predicted by combining the single flaw model with predicted fatigue crack propagation rate measurements. The latter are calculated from a generalized crack growth rate expression utilizing the strain energy release rate fracture mechanics parameter, which was previously described.     

Fracture resistance of a glass-fibre reinforced rubber-modified thermoplastic hybrid composite

Toughening mechanisms in a hybrid amorphous thermoplastic composite containing both distributed rubber particles and rigid glass fibres have been investigated. Tensile properties were measured for a range of materials with varying rubber particle and glass-fibre contents, and different rubber particle sizes. Fracture toughness was characterized by separating the overall fracture into its initiation and propagation components. Deformation and fracture modes at crack tips were optically characterizedin situ during loading. The results indicate that both initiation and propagation toughness are enhanced by rubber particle additions to the glass-fibre reinforced composite. Synergistic effects between glass fibres and rubber particles are identified: for example, glass fibres inhibit crazing at rubber particles, and rubber particles tend to promote crazing at fibre/matrix interfaces and also void initiation at fibre ends. Toughening mechanisms are discussed in the light of available models.     

FATIGUE CRACK PROPAGATION CHARACTERISTICS OF GLASS FIBRE REINFORCED POLYCARBONATE IN HOT DISTILLED WATER

The fatigue crack propagation behaviour of polycarbonate and glass fibre reinforced polycarbonate was studied in hot distilled water. The effects of temperature, distilled water and glass fibre content on fatigue crack growth rate were determined. In distilled water at 333 K, the fatigue crack growth rate decreased with increasing glass fibre content. A melting was observed of the adhesive bond between the glass fibres and the matrix. It was evident that the fracture morphology closely relates to the fatigue crack growth rate which depends on the ΔK parameter rather than the K_max parameter when the materials are tested in hot distilled water.     

Schwingbruchstrukturen an faserverstärkten Kunststoffen

Structures of Fatigue Fractures in Fibre-Reinforced Plastics The report deals with macrofractographic and microfractographic structures in non-reinforced epoxide resins as well as in glass-fibre reinforced plastics and carbon-fibre reinforced plastics after fatigue stresses. Epoxide resins show fatigue striations and fracture lines similar to those in metals. The distance between the fatigue striations amounts to between 0.70 and 150 m?m and increases continuously in the direction of fracture propagation. The instantaneous fractures in non-reinforced resin show parabolic cusps besides Wallner-lines and lance-shaped structures. Surfaces of delamination fractures of glass-fibre reinforced plastics show fatigue striations which are characteristic for fibre-reinforced composites, but their morphology differs from the fatigue striations in non-reinforced resin. This was the first time that fatigue striations could be discovered in fibre-reinforced composites of a fibre content of 70 per cent by volume. These fatigue striations are strongly marked at the positions of the fibres and will probably arise only in case of a sufficiently high local stress intensity. Moreover, the amounts of the local slicing and shearing stress components, of which is composed the stress causing the fracture, could be partly responsible for the formation of fatigue striations. The formation of the fatigue striations allows to draw conclusions with respect to the fracture-causing local stress distribution and the local crack propagation in fibre-reinforced plastics. The surfaces of instantaneous fractures in glass-fibre reinforced plastics and carbon-fibre reinforced plastics show parabolic cusps. This is another distinguishing characteristic as compared with the surfaces of fatigue fractures.     

Modelling off-axis ply matrix cracking in continuous fibre-reinforced polymer matrix composite laminates

The fracture process of composite laminates subjected to static or fatigue tensile loading involves sequential accumulation of intra- and interlaminar damage, in the form of transverse cracking, splitting and delamination, prior to catastrophic failure. Matrix cracking parallel to the fibres in the off-axis plies is the first damage mode observed. Since a damaged lamina within the laminate retains certain amount of its load-carrying capacity, it is important to predict accurately the stiffness properties of the laminate as a function of damage as well as progression of damage with the strain state. In this paper, theoretical modelling of matrix cracking in the off-axis plies of unbalanced symmetric composite laminates subjected to in-plane tensile loading is presented and discussed. A 2-D shear-lag analysis is used to determine ply stresses in a representative segment and the equivalent laminate concept is applied to derive expressions for Mode I, Mode II and the total strain energy release rate associated with off-axis ply cracking. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed.     

Stiffness and fracture analysis of laminated composites with off-axis ply matrix cracking

Matrix cracks parallel to the fibres in the off-axis plies is the first intralaminar damage mode observed in laminated composites subjected to static or fatigue in-plane tensile loading. They reduce laminate stiffness and strength and trigger development of other damage modes, such as delaminations. This paper is concerned with theoretical modelling of unbalanced symmetric laminates with off-axis ply cracks. Closed-form analytical expressions are derived for Mode I, Mode II and the total strain energy release rates associated with off-axis ply cracking in [0/θ]_s laminates. Stiffness reduction due to matrix cracking is also predicted analytically using the Equivalent Constraint Model (ECM) of the damaged laminate. Dependence of the degraded stiffness properties and strain energy release rates on the crack density and ply orientation angle is examined for glass/epoxy and carbon/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is also discussed.     

Fatigue fracture of nylon polymers

The purpose of this study was to determine the effect of frequency on fatigue crack propagation rates in unfilled nylon polymers. Specifically it was of interest to investigate the frequency dependence under conditions where hysteretic heat generation did not occur. For dry injection-moulded nylon the results demonstrate that a strong frequency dependence exists with higher crack propagation rates at lower frequencies. This indicates that the mechanism of fatigue crack growth at room temperature is primarily one of creep crack growth, especially at frequencies below 1.0 Hz. It is also noted that hysteretic heating causes fracture mode transitions to occur during stable fatigue crack propagation in injection-moulded nylons, even at relatively low cyclic frequencies (5.0 Hz).     

Fracture toughness of unidirectional fibre reinforced composites in MODE II

Experimental investigations have been performed on unidirectional glass fibre reinforced/epoxy composites in Mode II (Forward shear) with the presence of crack parallel to the fibres direction through the use of end-cracked beam. A concentrated load at the Centre of the beam produced bending-induced shear deformation at the crack tip. Calibration factors for Mode II have been obtained. The stress-intensity factor at instability $\text{K}$_IIR(INR) is obtained by experiments on a small end cracked beam through a compliance matching procedure. The crack growth resistance at instability and the corresponding critical strain energy release rate are independent of initial crack in the range of crack length investigated. In composite materials, fibre-matrix interfacial shear stress play an important role in load transfer mechanism: hence Mode II study may be very useful to analyse the interfacial mechanisms and to understand the fracture behaviour of unidirectional fibre reinforced composites in Mode I when load is applied in the direction of the fibres.     

Response of fibre reinforced aluminium–lithium laminates to different fatigue conditions

Under fatigue conditions fibre reinforced aluminium–lithium laminates do not respond in the same manner as monolithic aluminium alloys. The variation of fatigue crack growth rates with initial loading condition has been examined for both carbon and glass fibre reinforced laminates, and compared with the behaviour of unreinforced 8090 aluminium–lithium alloy for a range of conditions (different initial nominal stress intensity factor range, load range and reversed loading). During fatigue, cracks grow in the metal layers of these laminates whilst the fibres in the crack wake remain intact, bridging the crack faces. The fibre bridging mechanism, inherent in this laminate system, reduces the fatigue crack growth rate. The magnitude of the bridging effect appears to be inversely related to the applied load range. This relationship can account for the behaviour observed in the performed experiments. This revised version was published online in November 2006 with corrections to the Cover Date.     

TWINTEX纤维增强塑料的应力腐蚀开裂

研究了TWINTEX纤维增强塑料在酸中的应力腐蚀,并与玻璃纤维增强塑料的应力腐蚀进行了比较.结果表明,TWINTEX纤维增强塑料具有比玻璃纤维增强塑料好得多的耐应力腐蚀性能,而且这种好的耐应力腐蚀性主要表现在长的裂纹孕育期.聚丙烯纤维良好的耐蚀性是造成长的裂纹孕育期的主要原因.外加载荷的变化对TWINTEX纤维增强塑料的应力腐蚀裂纹扩展速率的影响不如其对玻璃纤维增强塑料的应力腐蚀裂纹扩展速率的影响显著.     

城轨列车制动盘SiC_(p)/A356复合材料热疲劳裂纹扩展机理EI北大核心CSCD

为研究制动盘服役温度载荷及材料微结构对SiC_(p)/A356复合材料热疲劳裂纹扩展行为的影响,明确其热疲劳裂纹扩展微观机理,开展SiC_(p)/A356复合材料热疲劳裂纹扩展实验。结果表明:裂纹扩展过程包括由SiC颗粒偏转作用和二次裂纹释放扩展驱动力导致的缓慢扩展阶段和主裂纹与裂纹扩展前端微损伤连接的快速扩展阶段;加热温度较低时,裂纹扩展的“台阶状”特征明显,整体扩展速率较低,裂纹宽度较小,裂纹扩展方式为颗粒断裂、轻量基体撕裂和沿界面开裂;加热温度较高时,“斜直线跃升”阶段更为明显,裂纹宽度较大且扩展速率较高,裂纹扩展以颗粒脱落以及大幅度基体撕裂为主;主裂纹总是通过选择沿SiC颗粒群或者直接穿过α-Al基体以阻力较小的方式向前扩展,Si相承载时极易发生断裂,成为裂纹扩展源,同时裂纹扩展前端的微损伤对其扩展具有引导作用。     

FATIGUE CRACK PROPAGATION AND CRACK CLOSURE BEHAVIOR IN POLYCARBONATE AND FIBRE REINFORCED POLYCARBONATE

Abstract— Fatigue crack propagation was investigated in polycarbonate and glass fibre reinforced polycarbonate and the effect of stress ratio and glass fibre content determined. The addition of glass fibre increases the tensile strength, but does not always contribute to an increase in fatigue crack propagation resistance. For polycarbonate the effect of stress ratio can be partly explained by using crack closure concepts as other researchers have suggested, but for glass fibre reinforced polycarbonate this was not possible. Fractography revealed a void growth process, which occurred by decohesion at the interface of the glass fibres and the base material, which was dependent on the maximum stress intensity factor. The process of linking the voids and the main crack growth behavior depended on the stress intensity factor range, Δ K. A proposed crack propagation model can explain the effect of stress ratio on crack propagation in fibre reinforced polycarbonate.     

A non-local stress and strain energy release rate mixed mode fracture initiation and propagation criteria

A non-local stress condition for crack initiation and propagation in brittle materials is presented. This condition is expressed in terms of normal and tangential traction components acting on a physical plane segment (damage zone) of specified length. Next, a non-local strain energy release rate criterion is proposed. This condition is based on the assumption that initiation or propagation of cracking occurs when the maximal value of the function of opening and sliding energy release rates reaches a critical value. The value of energy release rates is determined for finite elementary crack growth. Mixed mode conditions are considered, for which both the critical load value and the crack orientation are predicted from the non-local stress and energy criteria, which are applicable to both regular and singular stress concentrations. The effect of non-singular second order term (T_σ-stress) on the crack propagation is discussed.     

超高韧性水泥基复合材料疲劳裂缝扩展公式的理论与试验研究

该文基于Paris公式,对具有应变硬化和多缝开裂特征的超高韧性水泥基复合材料(UHTCC)的疲劳裂缝扩展理论进行了理论研究,并通过试验进行验证。研究表明,Paris公式适用于UHTCC。其中:裂缝扩展参数由裂缝面积扩展A表示;与之相应,使用复合断裂能增长量#x00394;J代替应力强度因子变化量#x00394;K。基于试验,该文求出UHTCC的疲劳裂缝扩展门槛值,即当疲劳过程中的断裂能幅值#x00394;J小于某一临界值#x00394;J_th时,疲劳裂缝不扩展。在疲劳过程中,UHTCC的裂缝覆盖面积随疲劳过程呈三阶段线性发展,与疲劳变形的发展趋势一致。UHTCC的疲劳裂缝扩展随疲劳最大荷载与荷载幅值的增大而加速发展。     

Toughness of glass fibres reinforced glass-ionomer cements

Two fracture toughness parameters, the critical stress intensity factor, K _c and the work of fracture, W _f have been used to characterise the toughness of conventional and resin-modified glass-ionomer cements reinforced with glass fibres. The critical stress intensity factor was determined from the peak load, and the work of fracture was determined as the energy required to extend an introduced crack through the respective glass ionomers. For both materials, crack propagation became more stable as the weight fraction of glass fibres was increased. Additionally, when the weight percent of glass fibres was increased the work of fracture increased. Fibre bridging at the crack tip resulted in the increase in the work of fracture. As the percentage weight of fibres was increased, the critical stress intensity factor decreased proportionally to the increase in porosity.     

Mixed-mode crack growth in bonded composite joints under standard and impact-fatigue loading

Carbon fibre reinforced polymers (CFRPs) are now well established in many high-performance applications and look set to see increased usage in the future, especially if lower cost manufacturing and solutions to certain technical issues, such as poor out-of-plane strength, can be achieved. A significant question when manufacturing with CFRP is the best joining technique to use, with adhesive bonding and mechanical fastening currently the two most popular methods. It is a common view that mechanical fastening is preferred for thicker sections and adhesive bonding for thinner ones; however, advances in the technology and better understanding of ways to design joints have lead to increasing consideration of adhesive bonding for traditionally mechanically fastened joints. In high-performance applications fatigue loading is likely and in some cases repetitive low-energy impacts, or impact fatigue, can appear in the load spectrum. This article looks at mixed-mode crack growth in epoxy bonded CFRP joints in standard and impact fatigue. It is shown that the back-face strain technique can be used to monitor cracking in lap-strap joints (LSJs) and piezo strain gauges can be used to measure the strain response of impacted samples. It is seen that there is significant variation in the failure modes seen in the samples and that the crack propagation rate is highly dependent on the fracture mode. Furthermore, it is found that the crack propagation rate is higher in impact fatigue than in standard fatigue even when the maximum load is significantly lower.     

Fracture and fatigue of discontinuously reinforced copper/tungsten composites

The strength, toughness and resistance to cyclic crack propagation of composites consisting of copper reinforced with short tungsten wires of various lengths have been studied and the results compared with the behaviour of continuously reinforced composites manufactured by the same method, i.e. by vacuum hot-pressing. It has been found that whereas the resistance to fatigue crack growth of continuously reinforced composites is very similar to that of continuous Al/stainless steel composites reported elsewhere, the addition of short fibres completely changes the mode of fracture, and no direct comparisons are possible. In effect, short fibres inhibit single crack growth by causing plastic flow to be distributed rather than localized, and although these composites are much less strong than continuous fibre composites, they nevertheless have much greater fatigue resistance. The fracture toughness of the composites is thought to be derived simply from the separate contributions of matrix and fibre plastic flow and, in composites containing fibres near to the critical length, from the very substantial work of fibre pull-out.     

基于XFEM的RC梁静态破碎研究

建筑基坑内支撑的拆除是影响施工进度的重要因素之一。为提高施工效率,提出一种在钢筋混凝土支撑梁内部沿着梁轴线预埋大直径孔道进行静态破碎的拆除方案。在此基础上,通过扩展有限元方法(Extended Finite Element Method,XFEM)建立含预埋静态破碎孔(孔径为90 mm)的钢筋混凝土梁(截面尺寸为500 mm×500 mm)模型,并对其在静态膨胀压力作用下的破碎及裂缝扩展过程进行了模拟分析。模拟结果表明:内支撑梁的静态破碎过程可分为弹性变形、裂缝稳定扩展和裂缝失稳扩展3个阶段;基于虚拟闭合技术,进一步计算得到了复合开裂模式下的应变能释放率,计算结果显示:裂缝扩展以Ⅰ型裂缝为主,当膨胀压力达到19.4 MPa时,可实现破碎钢筋混凝土内支撑梁的目的。     

Acoustic emission analysis for characterisation of damage mechanisms in fibre reinforced thermosetting polyurethane and epoxy

Acoustic emission analysis is used to investigate microscopic damage mechanisms and damage progress in unidirectional glass and carbon fibre reinforced composites. Under static loading the influence of fibre orientation on damage initiation and propagation is determined. A novel polyurethane matrix system significantly enhances material performance in terms of crack initiation load levels, crack growth, damage tolerance and off-axis tensile strength. Hysteresis measurements during stepwise increasing dynamic load tests highlight the effect of fibre–matrix-adhesion and resin fracture toughness in unidirectional 0° fibre reinforced composites. Acoustic detection of beginning fibre breakage correlates with a significant increase of loss work per cycle.