爆炸载荷下边界锐V型切口尖端裂纹扩展行为

为了研究爆炸应力波作用下板条边界锐V型切口端部的动态扩展行为,采用动态焦散线实验方法,进行了爆炸载荷下板条边界锐V型切口端部裂纹扩展规律的实验研究。研究结果表明,爆炸应力波作用下,板条试件边界锐V型切口端部的扩展过程中,裂纹扩展速度、加速度和切口端部动态应力强度因子随时间是波动变化的。扩展速度最大值达到210m/s,裂纹扩展加速度最大值为9.47Mm/s2,切口端部动态应力强度因子KdⅠ最大值为0.6MN/m3/2。     

定向断裂双孔爆破含缺陷介质裂纹扩展的动焦散试验

利用爆炸加载数字激光动态焦散线试验系统,进行双孔爆破爆炸应力波作用下缺陷介质裂纹扩展试验。研究了含水平预制裂纹和竖直预制裂纹的介质裂纹扩展路径、速度、加速度和裂尖动态应力强度因子变化规律。试验结果表明:在爆炸应力波作用下,预制裂纹尖端起裂,并扩展。炸药爆炸后,主裂纹的扩展速度迅速达到峰值,之后开始振荡减小,其加速度呈现波浪起伏式的振荡变化。次裂纹起裂后速度增大至峰值,然后开始减小。主裂纹尖端的动态应力强度因子K_Ⅰ从峰值振荡减小,又振荡增加至第二个峰值,之后振荡减小。次裂纹尖端的动态应力强度因子K_Ⅰ达到最大时,次裂纹起裂,之后K_Ⅰ振荡减小。裂纹扩展的过程中K_Ⅱ基本都小于K_Ⅰ。     

束状炮孔爆破倾斜张开节理的动态断裂特性研究

采用动态焦散线实验和ABAQUS数值模拟,对束状炮孔柱部区域和端部区域节理处裂纹的动态断裂特性进行了研究。结果表明,在炮孔柱部区域,初始爆炸应力波在张开节理处产生透射波与绕射波并与其相互叠加,节理端部产生拉剪应力集中形成翼裂纹,垂直于节理面起裂扩展。且节理近端翼裂纹扩展速度、扩展长度和裂纹尖端应力强度因子较节理远端处翼裂纹的相应值大。而次生裂纹是爆炸应力波在试件边界处产生的反射拉伸波与倾斜张开节理相互作用起裂的,并沿水平方向扩展。远端次生裂纹的起裂韧度约为翼裂纹的0.5倍,且由于反射波较弱,次生裂纹的扩展长度远小于翼裂纹。炮孔端部区域翼裂纹和次生裂纹是在倾斜张开节理处的反射拉伸波和绕射波与倾斜张开节理的相互作用下先后起裂的,翼裂纹偏向炮孔方向起裂,并向相反方向扩展,而次生裂纹近似沿着爆炸应力波的传播方向扩展。     

爆炸荷载下缺陷介质裂纹扩展规律数值分析研究

利用爆炸加载数字激光动态焦散线试验系统,同时借助ABAQUS有限元分析中内聚力模型数值计算方法,研究了爆炸应力波作用下缺陷介质裂纹扩展规律,并将试验结果与数值计算结果进行了对比。研究表明:在爆炸应力波作用下预制缺陷两端产生了两条翼裂纹A、B,扩展长度基本相同,方向垂直于预制缺陷。两条翼裂纹的扩展基本是对称的,只是在尾端发生轻微翘曲;翼裂纹扩展速度先增大至峰值又振荡减小,之后又增大至第二个较小的峰值,然后又减小,这种变化趋势和裂纹尖端应力强度因子KⅠ保持一致;扩展角β为85°时,计算结果较为接近试验,内聚力模型为动态裂纹扩展的研究提供了一种有效的方法。     

单向静压下柱状炮孔端部爆生裂纹的扩展规律

高地应力对岩层地下工程爆破动态断裂过程有重要影响。采用数字激光动态焦散线测试系统,研究了不同单向静压下柱状炮孔端部爆生裂纹动态断裂行为,明确了柱状炮孔端部爆生裂纹的扩展规律。结果表明:单向静压越大,端部裂纹平均扩展长度越短,但单向静压下端部裂纹尖端积聚能量的快速释放会导致裂纹初始扩展速度提升;裂纹尖端应力强度因子基本随单向静压增加而递减,单向静压越大,应力强度因子随时间下降越剧烈,裂纹的止裂韧度越高,止裂时间越早;单向静压作用下的爆生裂纹在整个扩展阶段基本表现为I型裂纹,无静压作用下爆生裂纹在扩展初期表现为I型裂纹,中后期表现为复合型裂纹。研究结果对认识静压作用下的柱状炮孔端部破坏机理具有一定意义。     

含裂纹悬臂梁的动态弯曲断裂行为研究

本文采用动态焦散线方法对含裂纹悬臂梁承受横向冲击的弯曲断裂行为进行了动态断裂力学实验研究，分析了无量ａ／ｈ（ａ：初始裂纹长度；ｈ：梁高度）对于裂纹动态扩展行为（裂纹起始状态、裂纹尖端的应力强度因子、裂纹扩展速度、裂纹扩展轨迹）的影响，并借助动态光弹性应力分析，对应力波与扩展裂纹的相互作用、以及应力波传播规律进行探讨。     

爆炸荷载下反向弧形裂纹扩展规律的实验研究

为了研究爆炸荷载作用下预制反向弧形裂纹的动态断裂特性,采用数字激光动态焦散线实像-虚像综合实验系统分析爆炸应力波作用下和斜入射爆生运动裂纹作用下预制弧形裂纹尖端实像和虚像焦散斑变化特征、预制弧形裂纹尖端应力强度因子变化特征以及爆生裂纹与翼裂纹运动扩展特征。结果表明：通过实像光路和虚像光路可以定量分析裂纹尖端拉剪应力和压剪应力的动态变化过程,从而获得爆炸荷载与预制弧形裂纹作用的全过程;爆生主、次裂纹能量具有大小之分,且其动态应力强度因子变化趋势具有一致性;定向爆生运动裂纹与预制弧形裂纹贯通位置是造成预制弧形裂纹上下端点应力强度因子差异性的主要原因。     

含缺陷结构的冲击断裂试验研究

利用新型数字激光动态焦散线试验系统和落锤冲击加载平台,以缺陷端部曲率为单一变量,将孔状缺陷和裂纹缺陷纳入同一个试验研究体系。研究了冲击荷载下,含不同端部曲率中央缺陷的PMMA条形试件的三点弯曲动态断裂过程,断裂破坏经历三个阶段:前期为冲击应力波作用下,试件下边界裂纹的起裂与扩展;中期为裂纹在缺陷处的应力释放和停滞;后期为落锤自重作用下,缺陷端部的起裂与试件贯穿。在相同的试验条件下,不同端部曲率的缺陷对前期裂纹起裂与扩展基本没有影响;缺陷端部曲率越大,中期缺陷处停滞时间越短,并得出二者之间的近似函数关系;后期缺陷端部起裂时的应力强度因子随着缺陷端部曲率的增大呈现出先减小后增大的变化趋势。     

基于分形理论不同装药量的爆破动焦散线实验研究

为研究装药量对爆生裂纹扩展行为的影响。采用透射式数字激光动态焦散线实验系统,分析了不同装药量的爆生裂纹扩展规律,并基于计盒维数的计算原理,编写MATLAB程序计算爆生裂纹的分形维数。结果表明:①起爆后裂纹扩展分2阶段,Ⅰ阶段(0～114.3μs)为爆炸应力波与爆生气体对裂纹尖端的作用,在裂纹的起裂时刻扩展速度达到峰值,随即迅速降低;Ⅱ阶段(114.3μs～裂纹止裂)在反射应力波对裂纹尖端的作用下,裂纹扩展速度继续提升;②裂纹扩展速度峰值、动态应力强度因子峰值、粉碎区面积、爆生裂纹分形维数与装药量正相关;③采用回归分析与线性拟合的方法,得到了裂纹扩展速度与裂纹扩展轨迹分形维数的线性关系,同一裂纹扩展速度的变化符合分形规律。     

爆炸冲击荷载下相邻巷道裂隙扩展机理模拟试验

为探究邻近巷道裂纹缺陷受爆炸荷载作用的扩展机理,采用透射式动态焦散线实验系统进行模拟实验。结果表明,对迎爆侧隐性裂纹,当裂纹与巷道左壁相对距离10 mm时,裂纹右尖端扩展因巷道的存在使预制裂纹导向作用更显著,引导爆生主裂纹与预制裂纹左尖端贯穿,衍射后作用于右尖端反射拉伸引起;对背爆侧隐性裂纹,当裂纹与巷道右壁相对距离4 mm时,裂纹扩展源于薄弱的巷道临空面将绕射的应力波能传递并导向裂纹缺陷,裂纹受拉扩展贯穿。巷道存在会增加其围岩裂纹缺陷处动应力集中,在爆炸应力波作用下会诱发裂纹扩展。     

Asymptotic thermal and residual stress distributions due to transient thermal loads

The paper deals with the development of thermal and residual stress distributions arising from the solidification of a fusion zone near a V-notch tip. A set of numerical solutions of the problem was carried out under the hypothesis of generalized plane strain conditions by means of SYSWELD code. The intensity of the thermal and residual asymptotic stress fields at the sharp V-notch tip was studied for a given V-notch specimen geometry and a predefined fusion zone dimension after simulations on materials with different thermal, mechanical and phase transformation properties and after changing the clamping conditions at the specimen's boundary. The results were compared in terms of the elastic or elastic-plastic notch stress intensity factors giving a contribution to the interpretation of the experimental behaviour of welded joint.     

Transient response of a surface crack subjected to dynamic anti-plane concentrated loadings

In this study, the transient response of a surface crack in an elastic solid subjected to dynamic anti-plane concentrated loadings is investigated. The angles of the surface crack and the half-plane are 60° and 90°. In analyzing this problem, an infinite number of diffracted and reflected waves generated by the crack tip and edge boundaries must be taken into account and it will make the analysis extremely difficult. The solutions are determined by superposition of the proposed fundamental solution in the Laplace transform domain and by using the method of image. The fundamental solution to be used is the problem for applying exponentially distributed traction on the crack faces. The exact transient solutions of dynamic stress intensity factor are obtained and expressed in formulations of series form. The solutions are valid for an infinite length of time and have accounted for the contribution of an infinite number of diffracted waves. The explicit value of the dynamic overshot for the perpendicular surface crack is obtained from the analysis. Numerical results are evaluated which indicate that the dynamic stress intensity factors will oscillate near the correspondent static values after the first three or six waves have passed the crack tip.     

Analysis of the stress intensity factor dependence with the crack velocity using a lattice model

In this work, the influence of crack propagation velocity in the stress intensity factor has been studied. The analysis is performed with a lattice method and a linear elastic constitutive model. Numerous researchers determined the relationship between the dynamic stress intensity factor and crack propagation velocity with experimental and analytical results. They showed that toughness increases asymptotically when the crack tip velocity is near to a critical. However, these methods are very complex and computationally expensive; furthermore, the model requires the use of several parameters that are not easily obtained. Moreover, its practical implementation is not always feasible. Hence, it is usually omitted. This paper aims to capture the physics of this complex problem with a simple fracture criterion. The selected criterion is based on the maximum principal strain implemented in a lattice model. The method used to calculate the stress intensity factor is validated with other numerical methods. The selected example is a finite 2D notched under mode I fracture and different loads rates. Results show that the proposed model captures the asymptotic behaviour of the SIF in function of crack speed, as reported in the aforementioned models.     

Dynamic fracture mechanics—A scientific tool for the prevention of catastrophic failure

The major area of research in dynamic fracture has been the extension of the concept of static fracture toughness to predict crack arrest for a propagating crack. In this work crack propagation due to a ductile (microvoid) mechanism and cleavage (brittle) mechanism, as well as transition from one mode to another, has been analysed theoretically. Dynamic fracture toughness as a function of crack velocity has been determined. Temperature distribution near a propagating crack tip has been predicted for plane stress condition. The effect of reflected stress wave in a single edge notch specimen under transient crack growth conditions has also been analysed.     

Analytical modelling of dynamic fracture and crack arrest in DCB specimens under fixed displacement conditions

An analytical solution via the beam theory considering shear deformation effects is developed to solve the static and dynamic fracture problem in a bounded double cantilever beam (DCB) specimen. Fixed displacement condition is prescribed at the pin location under which crack arrest occurs. In the static case, at first, the compliance function of a DCB specimen is obtained and shows good agreement with the experimental results cited in the literature. Afterward, the stress intensity factor is determined at the crack tip via the energy release rate formula. In the dynamic case, the obtained governing equations for the model are solved supposing quasi‐static treatment for unstable crack propagation. Finally, a closed form expression for the crack propagation velocity versus beam parameters and crack growth resistance of the material is found. It is shown that the reacceleration of crack growth appears as the crack tip approaches the finite boundary. Also, the predicted maximum crack propagation velocity is significantly lower than that obtained via the Euler–Bernoulli theory found in the literature.     

Time-domain BEM analysis of a rapidly growing crack in a bi-material

Rapid propagation of a matrix crack in a bi-material system is studied with emphasis on the dynamic interaction between the crack and the interface by combining the traditional time-domain displacement boundary element method (BEM) and the non-hypersingular traction BEM. The crack growth is controlled by the fracture criterion based on the maximum circumferential stress, and is modeled by adding new elements to the moving crack tip. Detailed computation is performed for an unbounded bi-material with a crack subjected to incident impact waves and a bounded rectangular bi-material plate under dynamic wedged loading. Numerical results of the crack growth path, speed, dynamic stress intensity factors (DSIFs) and dynamic interface tractions are presented for various material combinations and geometries. The effects of the interface on the crack growth are discussed.     

A semi-theoretical and experimental approach for the determination of the stress intensity factors

The stress intensity factors for plexiglass plates containing edge cracks and subjected to either pure bending or tension are determined herein. The method of investigation was based on a semi-theoretical and experimental approach, where the stress intensity factors are expressed in terms of the measured diameter of the caustic, the crack length, and the width of the specimen. First, two basic crack arrangements (single and double edge cracks) were studied and then the method was utilized for the investigation of more complicated crack arrangements which are difficult or maybe impossible to be investigated otherwise. In particular, the stress intensity factor for plates having a sharp V-notch of various angles θ, and semi-infinite plates containing equal parallel edge cracks subjected to pure bending and tension respectively, were investigated in order to verify the validity of this method.     

Dynamic stress field for torsional impact of a penny-shaped crack in a transversely isotropic functional graded strip

The torsional impact response of a penny-shaped crack in a transversely isotropic strip is considered. The shear moduli are assumed to be functionally graded such that the mathematics is tractable. Laplace and Hankel transforms are used to reduce the problem to solving a Fredholm integral equation. The crack tip stress field is obtained by considering the asymptotic behavior of Bessel function. Investigated are the effects of material nonhomogeneity and orthotropy and strip’s highness on the dynamic stress intensity factor. The peak of the dynamic stress intensity factor can be suppressed by increasing the shear moduli’s gradient and/or increasing the shear modulus in a direction perpendicular to the crack surface. The dynamic behavior varies little with the increasing of the strip’s highness.