Experimental and numerical investigation of concrete properties effects on fracture toughness under complex loading

Abstract

This article is presenting the common experimental specimen for determining the fracture toughness of the first pure mode and second pure mode. The Notched beam is chosen from a presented common specimen in the form of three-point flexure beam and four-point flexure beam that were built in the concrete laboratory. For prevention of cracks growth, a critical load of first pure mode and the second pure mode of each specimen computed. Obtained results are used for computing the fracture toughness. For the purpose of investigating the effective fracture parameters in the suggested specimen, finite element analysis on the mentioned geometry is performed. Obtained results show that different parameters are effective on the fracture toughness including crack length, cement percentage, water and the thickness of biggest used aggregate in the sand. Also with changing each of these parameters, the fracture mechanic properties are changed. Each of these effects is examined separately in this article and the conclusions presented in tables and figures.

Communicated by Dumitru Caruntu.     

割口单向叠层板的拉伸应力集中与强度

基于剪滞理论,建立一种分层剪滞模型,分析了含割口的单向叠层板在拉伸载荷作用下的应力重新分布问题,获得了割口前缘完整纤维的应力集中因子.在此基础上,采用细观统计破坏理论,研究了割口单向叠层板的拉伸破坏强度,得到了与现有实验较吻合的结果.     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

镁合金MB2在高应力三轴度下的拉伸破坏行为研究

通过光滑试件及不同曲率半径缺口圆柱试件的拉伸试验,实现对镁合金MB2的单向及多向应力状态加载。结合数值模拟分析,研究了不同试件在拉伸加载过程中应力状态的变化。以应力三轴度为参数,给出了镁合金MB2等效破坏应变的变化规律,在应力三轴度-等效破坏应变空间建立了镁合金MB2的失效破坏准则。利用扫描电镜对试件断口形貌进行观察,分析了导致材料宏观延性变化的微观损伤机理,对不同应力状态下镁合金MB2的失效破坏行为做出了合理解释。     

Refined Dugdale plastic zones of an external circular crack

The refined Dugdale-type plastic zones ahead of an external circular crack, subjected to a uniform displacement at infinity, are evaluated both analytically and numerically. The analytical method utilizes potential theory in classical linear elasticity with emphasis on the contrast from the internal crack problem. A closed-form solution to the mixed boundary problem is obtained to predict the length of the plastic zone for a Tresca yield condition. The analytical solution is also used to benchmark the results obtained from the numerical method, which show good agreement. Through an iterative scheme, the numerical technique is able to estimate the size of crack tip plasticity zone, which is governed by the non-linear von Mises criterion. The relationships between the applied displacement and the length of the plastic zone are compared for the different yielding conditions. Computational modeling has demonstrated that the plastic constraint effect based on the true yield condition can significantly influence the load-bearing capacity. It is also discovered from the comparative study that the stress components predicted by the three different yield conditions may differ notably; however, the stress triaxiality in the ligament region has only small deviations. The proposed study may find applications in predicting the plastic flow in a circumferentially notched round bars under tension.     

A fibre microbuckling model for predicting the notched compressive strength of composite sandwich panels

Cohesive-zone models have been successfully applied to predicting the damage from notches in engineering materials loaded intension. They have also been used to determine the growth of fibre microbuckling from a hole in a composite laminate under compression. The usual strategy is to replace the in elastic deformation associated with plasticity or microbuckling with a line crack and to assume some form of stress-displacement bridging law across the crack faces. This paper examines recent published experimental data for notched glass-fibre epoxy/honey comb sand wich panels loaded in uniaxial compression. A plastic fibre kinking analysis and a linear softening cohesive-zone model are used for the prediction of the unnotched and open-hole compressive strength and the theoretical results are found to be in a good agreement with experimental data. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 1, pp. 73–84, January–February, 2007.     

Stiffness based technique to probe cyclic damage accumulation in micro-structurally graded bond coats via micro-beam bending tests

This paper highlights a novel technique to delineate the fatigue response of different regions within thin microstructurally graded platinum nickel aluminide bond coats. Notched clamped beam structures fabricated from distinct microstructural zones of these coatings are subjected to programmed cyclic bending using the nano-indentation system. A methodical approach is established herein to quantify the cyclic damage preceding crack pop-in by using the cyclic stiffness of the beam as an indicator to mark failure. Preliminary results from these tests show that there is a characteristic change in the stiffness of the beam before a crack pop-in event occurs and different regions within the coating show different stiffening characteristics. Factors affecting the measured stiffness such as offsets in the loading position and blunting of the notch tip have been estimated using the finite element method. A graded flow stress model has been proposed and implemented in FEM to quantify the local flow stress changes accompanying the measured rise in stiffness. Electron transparent foils lifted off from the notched region of the beam post-testing suggests that the cyclic stiffening of the beams occurs due to dislocation hardening in the plastically deformed region close to the notch tip. Toughening mechanisms active in the crack wake have thus been investigated and correlated to the measured cyclic stiffness.     

切槽爆破中V形切槽产生的力学效应研究

本文详细研究了切槽爆破中V形切槽在爆炸冲击波的动态压力和爆生气体的准静态压力作用下产生的力学效应。在此基础上,进一步阐明了V形切槽对岩石定向破裂的控制机理,提出了切槽爆破中裂缝扩展的力学判据。最后,对所得结论用实验结果进行了验证。     

增容剂对煤直接液化残渣改性沥青低温性能的影响

为了评价不同增容剂对煤直接液化残渣改性沥青低温性能的影响,首先,通过正交实验确定出三种增容剂（硅烷偶联剂、苯甲醛、二甲苯）各自的最佳掺量及掺入方式;其次,采用双边缺口拉伸（DENT）试验评价加入三种增容剂后沥青的低温抗延性断裂性能;最后,结合SEM照片并利用Image Pro plus图像处理软件计算加入三种增容剂后沥青中煤直接液化残渣的分散面积比,以定量地表征三种增容剂对煤直接液化残渣改性沥青低温性能的改善效果。结果表明,加入适量增容剂在一定程度上有助于煤直接液化残渣在沥青中的分散,提高两者之间的相容性,保持煤直接液化残渣改性沥青体系的长期稳定状态,避免因煤直接液化残渣的沉淀聚集而在相界面产生应力集中,增强煤直接液化残渣改性沥青的低温抗延性断裂性能。三种增容剂对煤直接液化残渣改性沥青低温性能改善效果不同,硅烷偶联剂最优,次之为苯甲醛,最差为二甲苯。     

16MnR钢缺口试样解理断裂行为研究

研究了低合金热轧钢16MnR缺口试样在$-196\,{^\circ}$C和$-130\,{^\circ}$C的解理断裂机 理. 拉伸试验、单、双缺口四点弯曲实验、断口形貌观察以及有限元分析结果表明, 缺口试 样发生解理断裂时均起裂于夹杂物粒子, 一种位于缺口根部前端(IC型), 另一种位于距缺口 根部较远的条形裂纹前端(SIC型); 且随温度升高, 起裂源的类型从$-196\,{^\circ}$C下的IC 型转变为$-130\,{^\circ}$C下的SIC型. 微裂纹均形核于夹杂物, 最终的断裂由铁素体晶粒尺 寸的微裂纹扩展控制. 缺口试样IC型解理断裂遵循裂纹形核条 件$\varepsilon_{\rm p} \ge \varepsilon_{\rm pc}$和裂纹扩展条件$\sigma_{yy} \ge \sigma_{f}$, 而SIC型解理断裂条件则演化为$\varepsilon_{\rm p}+\varepsilon_{\rm ps} \ge \varepsilon_{\rm pc}$和$\sigma_{yy} +\sigma_{yy{\rm s}} \ge \sigma_{f}$.