脆性材料复合型裂纹的断裂准则

实际工程中,脆性材料中的裂纹多处于复合型受力状态,因此,确定脆性材料中的复合型裂纹起裂角和临界荷载有着重要的理论意义和实用价值。以复合型裂纹为研究对象,将裂纹尖端的最小无量纲塑性区尺度ρmin和广义合成偏应力强度理论相结合,建立脆性材料复合型裂纹的断裂准则,预测裂纹起裂角及临界荷载,将其结果与最大周向应力准则和应变能密度因子准则相比较发现,基于该文方法得到的临界荷载曲线大于最大周向应力准则得到的临界荷载曲线,与应变能密度因子准则得到的临界荷载曲线比较接近。因而,表明了用该文的方法来预测脆性材料复合型裂纹起裂角和临界荷载是行之有效的。     

相互贯通裂纹动态断裂的试验研究

为了研究动荷载作用下裂纹角度对相互贯通裂纹动态断裂特性的影响,采用动焦散试验系统进行了含预制裂纹的三点弯冲击试验。试验结果表明:两分支裂纹相互贯通,在动荷载作用下翼裂纹只会从与竖直方向夹角较小的分支裂纹尖端起裂扩展,当有一支分支裂纹起裂后,另一分支裂纹动态应力强度因子快速下降,试件内应变能重新分布;起裂的翼裂纹首先沿准竖直方向扩展,当扩展到试件约2/3高度后,翼裂纹扩展速度下降,扩展轨迹弯曲程度加大,I-II复合型断裂明显;分支裂纹起裂瞬间的KdI和KdII随着该裂纹与竖直方向夹角θ的增加而增大,其扩展的平均速度随着夹角θ的增加而减小。     

巷道内Ⅰ型及Ⅰ/Ⅱ复合型裂纹在冲击载荷作用下的断裂行为分析

研究了巷道内纯Ⅰ型及Ⅰ/Ⅱ复合型裂纹在冲击载荷作用下的动态扩展特征,预制裂纹位于巷道拱顶圆弧区域与巷道对称轴线平行,并与巷道对称轴线的间距不同。以中低速落锤冲击试验机对砂岩巷道模型试样进行动态冲击加载,巷道内裂纹的起裂时间与扩展特性采用粘贴在裂纹尖端的应变片进行相应的测试分析。采用AUTODYN模拟巷道内裂纹的扩展路径,总体上数值结果与试验结果比较吻合,采用ABAQUS计算裂纹的应力强度因子,结合试验-数值法确定巷道模型内预制裂纹的起裂韧度。研究结果表明:Ⅰ/Ⅱ复合型裂纹起裂过程中,既有Ⅰ型起裂韧度又有Ⅱ型起裂韧度,其比例大小与裂纹距离巷道对称轴的距离有关;纯Ⅰ型与Ⅰ/Ⅱ复合型裂纹的扩展行为有很大差别,纯Ⅰ型裂纹起裂沿着原裂纹方向竖直向上扩展,而Ⅰ/Ⅱ复合型裂纹与原裂纹成一定角度起裂,形成翼型裂纹,最后沿着最大主应力方向进行扩展,并且复合型裂纹在扩展过程中,存在明显的拐点特征。     

混凝土直切槽平台巴西圆盘冲击劈裂拉伸断裂特性试验和数值模拟研究

采用变截面霍普金森压杆(SHPB),对高速冲击荷载下混凝土的断裂特性和裂纹演化进程进行了研究。对含复合型裂纹直切槽平台巴西圆盘(CSTFBD)进行劈裂拉伸试验,并结合理论研究了不同水灰比、不同应变率、不同预设裂纹长度和倾角对裂纹分布及断裂韧性的影响;采用扩展有限单元法(XFEM)模拟了具有不同预设裂纹倾角试件的开裂进程,得出开裂过程中试件内部的应力分布情况。结果表明,复合断裂韧性比对预制裂纹倾角的变化敏感,与裂纹长度和倾角呈负相关,而与应变率无关;次生裂纹的开裂并非发生在预设裂尖处,且裂纹倾角越大,裂尖应力集中越小、主裂纹发展越缓慢。     

含倾斜裂纹三点弯动态断裂试验研究

为研究冲击荷载下巷道围岩不同角度径向裂纹的破坏机制,采用落锤冲击加载平台和数字激光动态焦散线实验系统,以有机玻璃为试验材料,设计冲击荷载下半圆孔上不同角度裂纹的三点弯动态断裂试验,记录预制裂纹的角度α的改变对裂纹动态力学行为的影响,通过分析动态应力强度因子和裂纹扩展轨迹的分形维数对实验现象进行归纳总结。研究发现:①预制裂纹角度对裂纹尖端应变能的积累和释放的快慢有较大影响,随着角度的增大,起裂时间变短,起裂更容易,裂纹尖端应变能积累的更快;②裂纹尖端应变能释放的快慢在α=45°两侧表现出不同的规律;③不同角度裂纹的Ⅰ型动态应力强度因子随时间的变化规律具有相似性,但最大值却具有差异性;④不同角度裂纹的扩展轨迹满足一定的分形规律。     

含单侧预制裂纹梁的冲击动态断裂过程试验研究

利用动焦散线试验方法研究了冲击下预制裂纹梁的动态断裂行为,对比分析了冲击荷载作用下单裂纹与双裂纹试件的应力强度因子、扩展轨迹以及速度、加速度等参数的变化规律。试验结果表明:冲击荷载作用下,含双裂纹且主裂纹在冲击点正下方的试件起裂时间最早,裂纹扩展后期朝向次裂纹方向发生较小的偏移;含Ⅰ型单裂纹的试件起裂时间次之,裂纹扩展路径呈直线;含双裂纹且两条裂纹均偏置于冲击点的试件起裂时间最晚,扩展过程中发生明显的曲裂现象。同时,裂纹扩展过程中曲裂现象越严重,裂纹扩展的最大速度就越小。在落锤冲击试件到试件断裂的整个阶段,应力强度因子一直表现出振荡变化。含双裂纹的试件,在主裂纹扩展中期,次裂纹上的应力强度因子有一个快速下降的过程。     

预制裂纹梁柱节点冲击破坏过程的动焦散线试验研究

利用透射式动焦散线研究了冲击作用下梁柱节点的动态断裂特性.由焦散斑特征尺寸获得梁柱构件的动态裂纹起裂时间、应力强度因子、扩展速度和材料的断裂韧性.试验结果表明预置裂纹梁柱结构的裂纹尖端应力集中诱发裂纹扩展,梁端节点裂纹受到正应力和剪应力的共同作用,裂纹表现为复合型断裂,而柱端节点的开裂时间明显滞后于梁端节点裂纹开裂,为I型裂纹.研究工作对于梁柱构件的抗冲击性能评估具有科学研究意义和工程应用价值.     

形状改变比能密度因子断裂准则

将形状改变比能密度因子Sd用于建立复合型裂纹的断裂准则。该准则表明了裂纹在金属材料里的扩展过程中,起决定作用的是形状改变比能,而不是整个应变能。作为应用举例,它成功地预测了Ⅰ-Ⅱ复合型裂纹的开裂角和临界荷载,其结果与现有公开文献提供的一些理论结果和实验数据取得了较好的一致。     

冲击荷载下深梁动态断裂行为的光弹性实验

采用动态光弹性实验方法,对简支深梁进行冲击实验,研究其冲击动态断裂行为。实验得到了冲击断裂过程中深梁的等差条纹变化图片,分析了冲击荷载下裂纹尖端的动态应力强度因子和裂纹扩展速度的变化规律。研究结果表明:由于应力波的作用,在试件开裂前,预制裂纹尖端应力强度因子KdI始终呈波动上升的趋势,最大值为1.995MPa·m~(1/2);试件起裂后,裂尖动态应力强度因子KdI先迅速下降,然后保持在1.222~1.677 MPa·m~(1/2)内震荡,平均值为1.458 MPa·m~(1/2);裂纹起裂后,扩展速度先迅速增大,后保持在310~380 m/s,动态裂纹基本呈匀速扩展,平均扩展速度为345.703 m/s。     

复合型裂纹起裂角厚度效应的实验研究

完成了飞机结构铝合金LC4CS的2、4、8和14mm四种不同厚度试样在I+II复合加载条件下的复合型断裂实验,系统分析了厚度和复合载荷对裂纹起裂角的影响,揭示了常用复合型断裂准则的厚度适用范围,用三维断裂理论对结果进行了讨论。结果表明:复合型裂纹起裂角具有明显的厚度效应;最大周向应力准则能够准确预测薄试样和厚试样(厚度为2 mm 和14 mm)在各种复合加载条件下的起裂方向,但是不适用于中间厚度的试样,尤其是8 mm厚度的情况。最大三轴应力准则试图考虑裂纹尖端三维约束对裂纹起裂的影响,但是结果并不理想。最小应变能密度因子理论的预测结果与最大周向应力准则的预测结果非常接近,但同样不能预测8mm厚度试样的起裂方向。非常有必要建立一个普遍适用的三维复合型断裂准则。     

Applications of normal stress dominated cohesive zone models for mixed-mode crack simulation based on extended finite element methods

In conventional cohesive zone models the traction-separation law starts from zero load, so that the model cannot be applied to predict mixed-mode cracking. In the present work the cohesive zone model with a threshold is introduced and applied for simulating different mixed-mode cracks in combining with the extended finite element method. Computational results of cracked specimens show that the crack initiation and propagation under mixed-mode loading conditions can be characterized by the cohesive zone model for normal stress failure. The contribution of the shear stress is negligible. The maximum principal stress predicts crack direction accurately. Computations based on XFEM agree with known experiments very well. The shear stress becomes, however, important for uncracked specimens to catch the correct crack initiation angle. To study mixed-mode cracks one has to introduce a threshold into the cohesive law and to implement the new cohesive zone based on the fracture criterion. In monotonic loading cases it can be easily realized in the extended finite element formulation. For cyclic loading cases convergence of the inelastic computations can be critical.     

The effect of tip plastic energy on mixed-mode crack initiation

An approach to the fracture initiation prediction of a ductile crack with mixed-mode loading (mode I and II) conditions is presented. The tip plastic energy around the crack tip is applied for evaluating the crack initiation load and the plastic zone shape. It is proposed that a mixed-mode crack will initiate as the tip plastic energy reaches a critical value. Numerical results for various loading conditions are illustrated. These results indicate that the predicted crack initiation loads correlate well with the experimental data available. This revised version was published online in August 2006 with corrections to the Cover Date.     

Frictional contact induced crack initiation in incompressible substrate

Surface crack initiation in an incompressible substrate induced through frictional fretting contact is analyzed using an energy-based fracture mechanics model. A closed-form energy release rate for surface crack initiation at the contact boundary has been derived with the crack growth angle determined by the mixed mode singular stress field at the contact edge. The driving forces in the form of J_i-integral, the critical energy release rate and the critical load for crack initiation from the crack free surface have been formulated. The relations between the friction coefficient and crack initiation angle, critical load have been specified.     

Numerical Simulation of a Mixed-mode Dynamic Fracture Experiment

A time-domain boundary element method is applied to simulate mixed-mode dynamic fracture experiments performed on compact compression specimens (CCS) in a split Hopkinson bar. This numerical investigation addresses the time histories of the mode I and II stress intensity factors for the precracked CCS, the instant and angle of crack initiation as well as the resulting crack paths in dependence upon the external load. The aim of the study is two-fold: to gain deeper insight into the mixed-mode dynamic fracture process under investigation and to test the applicability of our recently developed numerical method. Numerical results are compared to experimental data reported in the literature. Furthermore, aspects of modelling the experimental set-up in the framework of the boundary element method are discussed, and advantages and shortcomings are analyzed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interface crack initiation at V-notches along adhesive bonding in weakly bonded polymers subjected to mixed-mode loading

In this paper, interface crack initiation at V-notches along adhesive in bonded Polycarbonate (PC) and Poly Methyl Methacrylate (PMMA) subjected to mixed-mode loading conditions was investigated based on a combined experimental, finite element and matched asymptotic analysis. The V-notch specimens with an adhesive interface starting from its tip made at different notch angles were tested under three-point bending conditions. The experimental observations show that the specimens mainly fail by cracks along the interface. Also, the load at the crack initiation increases when the notch angle increases. The computational results are then used to explain and to correlate with the experimental data. A two-fold criterion developed by Leguillon (Eur J Mech A/Solids 21:61?C72 2002) that requires a simultaneous satisfaction of both Griffith energy and stress conditions for the crack initiation at a notch in the specimen made of a homogeneous brittle material is first extended for V-notch specimens under mixed-mode loading conditions and then used to estimate the crack initiation load. The estimated loads appear to agree well with the experimental data. Finally, an inverse method is proposed to estimate the values of fracture toughness at different mode mixity ratios.     

Brittle Mixed-Mode (I+II) Fracture: Application of the Equivalent Notch Stress Intensity Factor to Cracks Emanating from Notches

In the present paper, crack initiation in mixed-mode (I+II) fracture has been studied using notched circular ring specimens. A new criterion of brittle mixed-mode (I+II) fracture based on the notch tangential stress and the volumetric approach has been developed. The critical value of the equivalent notch stress-intensity factor has been considered as fracture toughness in mixed-mode (I+II) fracture.     

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.