Relationships between multiaxial stress states and internal fracture patterns in sphere-impacted silicon carbide

Internal fracture patterns developed in silicon carbide cylindrical targets as a result of dynamic indentation (63–500 m/s) by tungsten carbide spheres are defined. Microscopy of recovered and sectioned targets delineate into three regions, each associated with distinct cracking modes, i.e., shallow cone macrocracking at and near the impact surface, steep interior cone macrocracks that radiate into the target from the impact region and local grain-scale microcracking directly underneath the impact region. The observed fracture patterns are found to maintain a noticeable degree of self-similarity upto the impact velocity of 500 m/s. Linear elastic analysis of the full (surface and interior) stress field developed under static (Hertz) contact loading delineate the target into four regions, based on the number of principal stresses that are tensile (none, 1, 2 or all 3). A strong correlation is found between the principal stress conditions within each region and the forms of cracking, their locations and orientations present therein. This correlation has a number of implications, including non-interaction of crack systems, which are discussed. Illustrative linear elastic fracture mechanics analyses are performed for three regions, and calculated and observed macrocrack lengths are found to be in reasonable agreement.     

砼坝裂缝稳定性分析

通过比较线弹性断裂力学中的两种计算应力强度因子的方法，外推法和奇异单元法，得到相应的结论；并以某砼坝下游部位出现裂缝为例，分析在不利荷载作用下裂缝的稳定性。     

大跨悬索桥索梁锚固区疲劳养护概率优化方法

合理的桥梁养护规划应在养护费用与失效风险之间找到一个合理的平衡.将养护费用分为检测费用,维修费用与失效费用,将线弹性断裂力学、结构可靠度理论、全寿命成本分析法结合起来,提出了基于全寿命周期成本的焊接构件疲劳养护概率优化的方法.考虑了检测概率、养护决策者的维修意愿、维修措施概率和失效概率等多种因素,把桥梁养护优化问题变为在焊接结构或构件满足在生命周期内最小允许可靠度的基础下,养护总费用最小的问题,并提出了简化计算方法.以青草背长江大桥索梁锚固区的关键疲劳细节养护为例,研究了主要参数对最优检测时刻的影响,研究结果表明,考虑了失效成本后,最优疲劳养护时刻提前,折现率对养护费用影响明显.当车辆荷载逐年增加时,索梁锚固区在设计基准期内疲劳破坏风险增大,应引起足够的重视.     

Three‐dimensional integration strategies of singular functions introduced by the XFEM in the LEFM

The use of the extended finite element method in the scope of linear elastic fracture mechanics induces the integration of singular functions in terms of the stiffness matrix and in the computation of stress intensity factors using the interaction integral method. An adapted method is proposed in this paper to treat efficiently the three‐dimensional case. The improvement is demonstrated by comparison with standard and other methods found in the literature. Copyright © 2012 John Wiley & Sons, Ltd.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

Failure analysis of a PVC sewer pipeline by fractography and materials characterization

Sewer rising mains are an integral part of wastewater network and convey sewage under pressure between gravity fed systems or from gravity systems to treatment plants. The failure of a rising main can result in significant volumes of sewage being discharged to sensitive environments, significant clean-up costs and severe indirect consequences being incurred.This paper describes the analysis of an Unplasticised Polyvinyl Chloride (PVC-U) pressure sewer (sewer rising main), nominal diameter 200 mm, pressure class 12, that failed prematurely after 34 years of service considering the design life of >100 years. It was hypothesised that the root cause of pipe failure was either material degradation from exposure to an aggressive environment (the conveyed sewage) or an inherent defect in the pipe. To test this hypothesis, a thorough visual examination of the exhumed failed section and the fracture surface was undertaken, followed by Fourier Transform Infrared Spectroscopy, micro-scale examination of the fracture surface using Scanning Electron Microscopy and Energy Dispersive X-ray Spectrometry. Fourier Transform Infrared Spectroscopy showed no evidence of material degradation due to contact with sewage. Visual and micro-scale examination revealed that failure occurred in two major stages initiated at an inherent defect (foreign inclusion) in the pipe wall that was attributed to the original manufacture. In addition to the initiating defect in the pipe wall, numerous other potential stress risers were also identified. The application of an established crack growth model predicted pipe failure close to the observed lifetime. Application of the staged methodology presented is recommended to establish relationships between manufacturing eras of PVC pipes and severity of inherent defects.     

Extended FEM modeling of crack paths near inclusions

The extended FEM is applied to model crack growth near inclusions. A procedure to handle different propagation rates at different crack tips is presented. The examples considered investigate uniform tension as well as equibiaxial tension under plane strain conditions. A parameter study analyzes the effects on the crack path when changing the relative stiffness between inclusion and matrix material, the relative distance between initial crack and inclusion, and the size of the inclusion. Both edge cracks and internal cracks are studied. An example with an internal crack near an inclusion is presented, where both crack tips propagate at different growth rates until one crack tip eventually stops growing, as the related energy release rate drops below the critical value. In another example, only one crack tip propagates initially, but eventually, the energy release rate of the second crack tip becomes critical, and both crack tips propagate. Finally, an example of two cracks near an inclusion is presented in which up to four crack tips propagate simultaneously. Copyright © 2011 John Wiley & Sons, Ltd.     

Integration of LEFM and Strength Criteria for the Prediction of Notched Strength of Polymeric Composites

The prediction of the notched strength of polymeric composites has been the object of much interest in recent years. Theories based on both linear elastic fracture mechanics (LEFM) and strength criteria have been proposed by numerous investigators. In the present paper a model that integrates the concepts of a general strength criterion with the energy concept of LEFM is presented. This enables the transition between the extremes of both theories. Predictions made by the model are shown to compare well with experimental data for center-slit specimens of a variety of material systems. The use of the proposed model in ranking materials on the basis of notch sensitivity is also shown.     

Analysis of thermoelastic interaction of manufacturing stresses along the interface on interlaminar delamination progression in multi-ply composite laminates

This paper deals with the study of interaction of manufacturing thermal residual stresses and mechanical loading in penny-shaped delaminations embedded between dissimilar, anisotropic fiber composite layers by conducting two sets of three-dimensional thermoelastic finite element analyses with and without residual stress effects. Modified crack closure integral (MCCI) techniques based on the concepts of linear elastic fracture mechanics (LEFM) have been used to calculate the distribution of individual modes of strain energy release rates (SERR) to investigate the interlaminar delamination initiation and propagation characteristics. Asymmetric variations of strain energy release rates obtained along the delamination front are caused by the overlapping stress fields due to the coupling effect of thermal and mechanical loadings. It is found that parameters such as ply sequence and orientation, thermoelastic anisotropy and material heterogeneity, and ply properties of the delaminated interface dictate the interlaminar fracture behavior of multi-ply laminated FRP composites.