使用焦散线法与光弹法测定三维裂纹混合型应力强度因子

本文介绍了将焦散线法与应力冻结，“解冻”技术相结合，使焦散线法用于测量三维体内部裂纹前缘应力强度因子的实验方法，并针对复杂应力状态下三维裂纹前缘的不同应变奇异场，合理地综合运用焦散线法与三维光弹法，实际测量与分离了三维裂纹前缘混合型应力强度因子。     

确定应力强度因子的光弹性法与焦散线法

以光弹性法及焦散线法的基本原理为基础,对两种方法在确定应力强度因子方面进行了比较。发现对于纯I型裂纹问题,光弹性法的精度低于焦散线法的精度;对于I－Ⅱ混合型裂纹问题,就张开型应力强度因子而言,光弹性法的精度低于焦散线法的精度,而就滑移型应力强度因子而言,光弹性法的精度高于焦散线法的精度。     

泊松系数对用光弹性法确定应力强度因子的影响

光弹性法确定应力强度因子的概念是 G.RIrwin 在1950年提出的,之后,特别是七十年代以来,许多人进行了这方面的研究,使这一方法由最初提出的求解二维静态 K_1的方法发展成为能解决三维静态 K_Ⅰ,K_Ⅱ,K_Ⅲ和二维动态 K_1,K_Ⅱ的方法。由于工程上常见的复杂三维裂纹体的应力强度因子用计算方法求解的困难,三维光弹性确定应力强度因子的方法就更具有吸引力了。然而,在用三维光弹模型确定工程结构的应力强度因子时还存在一个不符合相似准则的     

用全息光弹性法解三维应力分析问题

本文介绍了进行三维应力分析的全息光弹性方法及有关技术,分析了全息光弹性方法的主要困难在于冻结材料的应力光学常数比B/A接近-0.5,并提供了一种新的光弹性材料制作和冻结工艺。这种材料冻结时的应力光学常数比可以远离-0.5。用这种材料制成的三维模型,可以通过全息光弹性方法获得三维应力的全应力实验解答。用这种方法测定了对径受压圆球和高压压头对称断面上的应力分布。实验结果和理论解进行了比较。     

三维光弹性中预应力结构的应力冻结研究

本文提出用三维光弹性应力冻结法模拟分析预应力钢筋混凝土结构中的应力。通过模拟体与实体结构的相似性研究,给出了模型载荷比例系数Cq参数,通过参数合理的选取,有效的解决了应力冻结水平预测这一技术关键;用光弹性应力二次冻结法,将结构预应力和负载应力冻结在同一试件中;应分析精度对模型尺寸的要求,采用了精密浇铸的方法制作模型。此外,还研制了受力清晰明确的加载系统。对这些涉及应力冻结的若干关键问题结合工程实例进行探讨。实验应力分析结果与有限元数值法一致性较好。     

基于有限元方法的航空发动机叶片应力强度因子计算

为研究叶片裂纹尖端的应力奇异性,以某型航空发动机压气机叶片为例,利用有限元方法研究了叶片裂纹尖端应力强度因子的计算方法,并研究了旋转叶片振动状态下裂尖应力强度因子随裂纹长度的变化规律。建立计算模型时,在裂纹尖端划分了三维奇异单元,在裂尖外围划分了过渡单元。计算结果表明：研究旋转叶片振动状态下的裂尖应力奇异性,仅利用I型应力强度因子就具有足够的精度;对于同一裂纹,绝大多数情况下叶盆面应力强度因子大于叶背面应力强度因子,故研究叶片应力强度因子时只需研究叶盆应力强度因子即可;随着裂纹扩展,叶盆面I型应力强度因子不断增大。本文的研究方法及结论为进一步研究叶片的裂纹扩展规律及损伤容限奠定了基础。     

带凸缘弯曲梁应力集中区内表面裂纹应力强度因子的研究

用三维光弹性冻结应力实验技术与修正的多点超定法相结合研究了带凸缘弯曲梁应力集中区内表面裂纹的应力强度因子。分析了不同过渡圆弧的应力集中对两种表面裂纹(半圆形表面裂纹与前缘直线表面裂纹)的影响。用实验方法得到了应力强度因子放大系数的数值.结果表明,应力集中对浅裂纹的影响是更大的,是不可忽视的,但放大系数随表面裂纹的几何形状变化很小。这些对管节点的断裂力学评估提供了有价值的实验依据.     

拉伸螺杆半椭圆表面裂纹应力强度因子

将拉伸螺杆简化为理论应力集中系数Kt不同的带“V”形缺口圆杆,采用裂纹尖端为20节点奇异单元的三维有限元模型,对螺杆半椭圆表面裂纹的应力强度因子进行了计算.给出了具有普遍性意义的螺杆表面裂纹应力强度因子公式.为验证本文计算结果的有效性,还将本文M22×1.5螺杆的应力强度因子计算结果与试验结果进行了对比.     

滚动接触下的轨道表面三维裂纹研究方法

针对已有的求解三维裂纹的应力强度因子方法工作量大、难以实现复杂的三维模型建立的问题,提出了一种求解广义移动荷载下三维裂纹尖端应力强度因子的普遍方法。采用包络式三维实体建模方法,有效避免了三维裂纹有限元模型网格划分时因受限于拓扑误差容忍而出现网格划分失败的现象;通过MATLAB三维插值施加节点荷载,解决了不规则三维裂纹有限元模型施加复杂空间荷载的难题。计算结果表明:荷载中心距离裂纹中心约4mm时裂纹尖端应力强度因子达到最大值7.41×107Pa·m1/2;轮轨接触疲劳以张开型为主。此外,给出了最大应力强度因子条件下裂纹尖端塑性区的分布及裂纹扩展情况,可为轮轨滚动接触疲劳的研究及设计提供一定的参考。     

在轧机机架产生双表面贯穿裂纹下的断裂强度分析

某钢厂的2030平整轧机机架上发现有R=60mm的双表面张开型圆裂纹，为了安全生产，必须给出断裂力学强度评价。但是，要获得这种裂纹的应力强度因子，还没有理论解，文章应用弹性论并结合三维光弹性法和有限元法得到了K2值，二个方法所取得的值不仅吻合而且获得了机架在设计载荷下工作裂纹是稳定和不扩展的结论，此结论对生产现场有极大指导意义。     

Stress intensities for nozzle cracks in reactor vessels

A series of six frozen stress photoelastic tests was conducted to investigate the distribution of stress-intensity factor (SIF) along a crack which occurred at the juncture of a pipe (nozzle) with a cylindrical pressure vessel. Typical photoelastic-fringe patterns are shown for slices which were taken mutually orthogonal to the flaw border and the flaw surface. A typical plot of normalized apparent SIF vs. square root of normalized distance from the crack tip is presented. The variation in SIF along the flaw border is given for all six different crack geometries and, also, the variation of SIF with varyinga/T is presented.     

Stress-intensity factors in plates with a partially patched central crack

The fatigue and fracture performance of a cracked plate can be substantially improved by providing patches as reinforcements. The effectiveness of patches is related to the reduction they cause in the stress-intensity factor of the crack. Hence, an accurate evaluation of SIF in terms of various parameters is required for reliable patch design.In this paper, the influence of patch parameters on the opening-mode stress-intensity factor for a plate with a central crack is studied by employing transmission photoelasticity. Cracked plates made of photoelastic material are patched on one side as well as both sides by epoxy, phenolic and E glass-epoxy composite materials. The patch is located on the crack in such a way that the crack tip is not covered. Magnified isochromatic fringes are obtained by using a projection microscope of 50 magnification, converted into a polariscope. Irwin's method with extrapolation is employed to compute the stress-intensity factor from photoelastic data. The reduction in the stress-intensity factor is presented in graphic form as a function of pathch parameters, namely stiffness, width and length.     

An investigation of propagating cracks by dynamic photoelasticity

A 16-spark gap, modified schardin-type camera was constructed for use in dynamic photoelastic analysis of fracturing plastic plates. Using this camera system, dynamic photoelastic patterns in fracturing Homalite-100 plates, 3/8 in. × 10 in. × 15 in. with an effective test area of 10 in. × 10 in., loaded under fixed grip condition were recorded. The loading conditions were adjusted such that crack acceleration, branching, constant velocity, deceleration and arrest were achieved. The Homalite-100 material was calibrated for static and dynamic properties of modulus of elasticity, Poisson's ratio, and stress-optical coefficient. For dynamic calibration, a Hopkinson bar setup was used to record the material response under constant-strain-rate loading conditions. The precise location of the dynamic isochromatic patterns in relation to the crack tip was determined by a scanning microdensitometer. This information was then used to determine dynamic stress-intensity factors which were compared with corresponding static stress-intensity factors determined by the numerical method of direct stiffness. Although the response of the dynamic stress-intensity factor to increasing crack length was similar to the static stress-intensity-factor response, the dynamic values were approximately 40 percent higher than the static values for constant-velocity cracks. for decelerating cracks, the peak values of dynamic stress-intensity factors were 40 percent higher than the corresponding static values.     

Determination of mixed-mode stress-intensity factors K I and K II for a subsurface crack near a concentrated force

Two-dimensional photoelastic experiments were conducted to obtain isochromatic fringe fields in the region between a crack tip and a concentrated load on the boundary of a half plane. An analysis method is developed to determine the mixed-mode stress-intensity factors due to two interacting stress singularities. The results obtained showed the dominance of the opening mode in extending a delamination crack by sliding surface loads. Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 1.     

Photoelastic determination of stress-intensity factors

The increasing number of analytical and numerical solutions for the crack-tip stress-intensity factor has greatly widened the scope of application of linear elastic fracture-mechanics technology. Experimental verification of a particular solution by elastic stress analysis is often a necessary supplement to provide the criteria for proper application to actual design problems. In this paper, it is shown that the photoelastic technique can be used to obtain rather good estimates of the stress-intensity factor for various specimen geometries and loading conditions. Treated are the following cases: wedge-opening load specimen, several notched rotating-disk configurations, and a notched pressure vessel. A sharp crack is simulated by a relatively narrow notch terminating in a root radius of 0.010 in or less. Stress distributions along the section of symmetry ahead of the notch tip are obtained using three-dimensional frozen-stress photoelasticity. The results are used to determine the stress-intensity factor, cK _I , by three methods. Two of these are based on Irwin's expressions for the elastic stress field at the tip cf a crack, and the other is a result of Neuber's hyperbolic-notch analysis. Agreement, with available analytical solutions is good.     

Dynamic fracture toughness determined from load-point displacement

The paper presents a method to determine dynamic fracture toughness using a notched three-point bend specimen. With dynamic loading of a specimen there is a complex relation between the stress-intensity factor and the force applied to the specimen. This is due to effects of inertia, which have to be accounted for to evaluate a correct value of the stress-intensity factor. However, the stress-intensity factor is proportional to the load-point displacement if the fundamental mode of vibration is predominant in the specimen. The proportionality constant depends only on the geometry and stiffness of the specimen. In the present method we have measured the applied force and load-point displacement by a modified Hopkinson pressure bar, where two-point strain measurement has been used to evaluate force and displacement for times greater than the transit time for elastic waves in the Hopkinson bar. We have compared the method with the stress-intensity factor derived from strain measurement near the notch tip and good agreement was obtained. The method is well suited for high-temperature testing and results from fracture toughness tests of brittle materials at ambient and elevated temperatures are presented.     

Observation of Stress Field Around an Oscillating Crack Tip in a Quenched Thin Glass Plate

The variation of stress field around an oscillating crack tip in a quenched thin glass plate is observed using instantaneous phase-stepping photoelasticity. The successive images around the propagating crack are recorded by a CCD camera that is equipped with a pixelated micro-retarder array. Then, the phase maps of the principal stress difference and the principal direction are easily obtained even though the photoelastic fringes cannot be visualized. The path of the crack growth as well as the stress intensity factors and the crack tip constraint are obtained from these phase distributions. Results show that the mode I stress intensity factor and the crack tip constraint vary remarkably with the crack growth. In addition, the results show that the mode-II stress intensity factor exists even though the crack propagates smoothly.     

Dynamic stress-intensity-factor determination from isopachics

The governing equations for determination of dynamic stress-intensity factor at the tip of a running crack are developed from a dynamic analysis of dynamic isopachicfringe patterns. The equations are applied to investigate dynamic crack propagation in Homalite 100 by means of dynamic holographic interferometry. A simple method based on simultaneous measurement of the widths of two isopachics allows determination of Irwin's additional stress field, and a dynamic correction function for the stress-intensity factor is derived. It was found that dynamicK-values obtained from dynamic isopachic-fringe-pattern analysis are lower than their corresponding static values. This implies a modification of the crack velocity vs. stress-intensity-factor relationship towards lower values ofK for dynamic crack propagation.     

Stress-intensity distributions for corner cracks emanating from open holes in plates of finite width

A series of frozen stress photoelastic tests is carried out in the present investigation. Linear Elastic Fracture Mechanics is used to analyze the problem of a part-through corner crack at an open hole in a plate of finite width loaded in tension. The photoelastic modeling capability of three-dimensional subcritical crack growth problems is assessed. This is achieved by comparing monotonically grown flaw shapes in epoxy models with crack profiles relative to fatigue crack propagation tests in a different material. Photoelastic stress-intensity factor distributions are checked against numerical results obtained for quarter elliptical geometries. The dependence of stress intensity factors on flaw shape is also discussed.     

基于数字散斑相关方法测定Ⅰ型裂纹应力强度因子

提出了一种通过数字散斑相关方法测定金属材料Ⅰ型裂纹尖端位置和应力强度因子的实验方法.实验采用疲劳试验机对含Ⅰ型缺口的Cr12MoV钢试件预制裂纹,通过数字散斑相关方法测试试件在三点弯曲加载条件下裂纹的扩展过程及裂尖区域的位移场.将位移场数据代入裂尖位移场方程组,采用牛顿-拉普森方法求解含未知参量的裂尖非线性位移场方程组,计算裂尖位置和应力强度因子.实验结果表明,采用该方法可以准确地测定金属材料Ⅰ型裂纹应力强度因子、裂尖位置及裂纹扩展长度,解决了以往研究中因不能准确测定裂纹尖端位置,而无法准确计算Ⅰ型裂纹裂尖断裂参数的难题,揭示了金属材料裂纹扩展过程中应力强度因子演化特征.