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1.
The method of caustics, as developed and applied to fracture mechanics over the last twenty years, contains several approximations which limit its applicability. In this paper the development of caustics is reviewed and the implicit assumptions made in the past are clarified and discussed. The exact equations are derived for caustics formed by the reflection of light from a general surface. In addition, the conditions for the formation of a caustic curve are derived and explained in detail. Numerically generated shadow spots are given for the case of light reflected from a surface deformed due to the presence of a plane stress, mode-I, elastic crack. Attention is focused on the near tip region where severe deformation gradients violate the assumptions made by previous approximate analyses. The results demonstrate significant deviation from the approximate analyses resulting in errors as large as 15% in the determination of the stress intensity factor from shadow spot measurements. 相似文献
2.
Based on the first invariant of stress singular field in the vicinity of running tip of an interface crack, mapping equations
of the caustic curve on the reference plane and the initial curve on the specimen plane are developed. The dynamic caustics
are analyzed for the crack propagating along the interface between two bonded dissimilar materials. The variation of the caustic
configurations is shown with the velocity change of the running crack and the ratio change of the stress intensity factors.
Two characteristic dimensions are proposed that are not only practically measurable from optical caustic contours but also
suitable to represent the behavior of transient caustics.
The project supported by the National Natural Science Foundation of China and the Scientific Commission of Yunnan Province
of China 相似文献
3.
A simple model was proposed for the interpretation of the non-circular form of the Rayleigh wavefronts emitted by a fast running crack in a plate. The surface deformation around the crack tip, due to the high stress concentration there, propagated as a surface wave after fracture of this zone. On the other hand, the moving singularity of the crack tip created a dynamic stress field of varying intensity with time all over the specimen. This dynamic stress field resulted in a significant change of the mechanical properties of a strain-rate dependent material and therefore it influenced the velocity of propagation of fracture-Rayleigh wavefronts. An analysis of this varying dynamic strain field explained the non-circular form of Rayleigh waves, accompanying the propagating crack. For the experimental evaluation of the K1-factor the method of dynamic caustics was used in conjunction with the high-speed photography technique. 相似文献
4.
三点弯曲试样动态应力强度因子计算研究 总被引:2,自引:0,他引:2
利用Hopkinson压杆对三点弯曲试样进行冲击加载,采集了垂直裂纹面距裂尖2mm和与裂纹面成60°距裂尖5mm处的应变信号。根据裂尖附近测试的应变信号计算试样的动态应力强度因子,并与有限元计算结果进行比较,结果表明由于裂尖有一段疲劳裂纹区,通过裂尖附近应变信号来计算动态应力强度因子时,如果裂尖位置确定不准及粘贴应变片位置不够准确对计算结果将带来很大影响。因此利用应变片法计算动态应力强度因子时,为了获得更准确的计算结果,在实验后应对试件裂纹面进行分析测量,重新确定裂尖位置,必要时需对应变片至裂尖距离进行修正后再计算动态应力强度因子值。 相似文献
5.
Edge-cracked stiffened panels analyzed by caustics 总被引:3,自引:0,他引:3
Externally bonded composite patches have been proven to be an effective method for repairing damaged aircraft structural components. They are ease in application and provide excellent in-service performance. The major function of a repair is to reduce the stress intensity factor at the crack tip. Calculation of stress intensity factor of a repaired crack has been performed by analytical and numerical methods. However, these methods are based on simplifying assumptions regarding material behavior and repair conditions. In the present paper an experimental determination of mode-I stress intensity factor (SIF), KI, at the tip of an edge-crack or a V-notch reinforced with double bonded strips or with compression pre-stresses applied along the crack surfaces is undertaken by using the optical method of caustics. This method is simple in its application and has successfully been used for the solution of a host of crack problems of engineering importance. 相似文献
6.
The equation of static and dynamic caustics, and the formulae determining the position of crack tip and stress intensity factor
are given. It is proven that for the case of low speed of crack propagation the static formula is applicable in calculation.
A simple method to measure the static stress-optical constants is proposed. An Optical system which is suitable for the experiments
of dynamic caustics was set-up and used to study the fracture in beam and rings with initial crack under impact loading. A
series of dynamic caustics' photographs and curves showing the variations of corresponding crack lengths and dynamic stress
intensity factors with time, are presented. 相似文献
7.
基于数字散斑相关方法测定Ⅰ型裂纹应力强度因子 总被引:1,自引:0,他引:1
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. 相似文献
8.
爆炸应力波作用下裂纹与孔洞的动态焦散线分析 总被引:4,自引:1,他引:4
将高速摄影技术与动态焦散线方法相结合,研究了爆炸应力波对裂纹、空孔的作用历史,记录了环绕它们的动态焦散斑图,给出了爆炸应力场中裂纹尖端复合应力强度因子的时间依赖关系以及空孔周围应力场分布的瞬态变化过程,为固体介质的爆破机理研究提供了新方法。 相似文献
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10.
动态焦散线实验方法及其在断裂力学中的初步应用 总被引:8,自引:0,他引:8
本文给出了静、动态的焦散线方程,和确定裂纹尖端位置、动应力强度因子K_1~d的计算公式,并证明了在裂纹扩张速度较低时,可用静态公式计算,还研究了带预裂纹的三点弯曲梁和圆环在冲击载荷下的断裂问题,得到了系列动态焦散线照片、裂尖位置和动应力强度因子随时间的变化曲线。 相似文献
11.
Determination of stress intensity factors using shadow optical method of caustics attracts increasing attention. However, the reliability and ranges of applicability of this method are seldom discussed. This paper presents such an analysis regarding the reliability and predictive power of caustic method in fracture mechanics. This analysis is performed according to the accepted methodology of testing any analytical, numerical, or experimental procedure, namely by testing its theoretical bases. The following four basic assumptions of the shadow optical method of caustics in fracture mechanics, and their consequences, are analyzed: assumption of a generalized plane stress state near crack tip; assumption that the radiant energy used to produce shadow images propagates rectilinearly inside a stressed body; assumption that the light velocity inside a stressed body depends only on stress components in planes normal to wave normals; assumption that there is no alteration in state of polarization of radiation impinging upon a stressed plate at oblique incidence.The first two basic assumptions are tested using results obtained by means of three analytical-experimental procedures, namely isodynes, gradient index method, and classical strain gages. It is known that the inaccuracies of the fourth assumption are within the noise level only when the angles of incidence are small.It is shown that the magnitudes of the effects caused by the geometric lens effect and the gradient index lens effect are comparable, and that the stress states near crack tips and bottoms of notches, which produce the geometric lens effect, are clearly three-dimensional.It is also indicated that the gradient index lens effect is caused jointly by the stress/strain-induced alteration of the optical path and by the bending of the light path caused by strain gradient.Obtained empirical evidence shows the existence of a particular relationship between the observed gradient index lens effect and the order of singularity in a particular singular solution of linear fracture mechanics, with the exception of the vicinity of the crack tip where singularities are inadmissible. 相似文献
12.
The S-theory is applied to determine crack initiation and direction for beams with edge crack. A simple method for obtaining approximate stress intensity factors of straight cracked beams is also proposed. It takes into account the elastic crack tip stress singularity while using the elementary beam theory. The results are in reasonable agreement with the more accurate calculations. 相似文献
13.
混凝土的断裂韧度是极为重要的断裂力学材料参数,本文利用临界距离理论(TCD)中的点法和混凝土标准断裂梁试样裂缝尖端应力场的近似解,提出了一种对混凝土断裂问题按临界距离的点法进行分析的方法.基于已有文献的试验材料及其实验结果,分析了这种断裂研究分析方法的可行性.将临界距离点法与双K断裂准则应用于几何相似的混凝土梁的断裂韧性和断裂过程区分析,验证了TCD点法的可靠性.实验表明:临界距离理论的点法能够获得比双K断裂准则相对安全的失稳断裂韧度,利用临界距离法还可得到相应断裂过程区长度的极限估算值. 相似文献
14.
40Cr材料动态起裂韧性KId()的实验测试 总被引:4,自引:0,他引:4
描述了利用Hopkinson压杆技术加载三点弯曲试样测试40Cr,材料动态起裂韧性KId()的试验方法。试样上的动态载荷历程由Hopkinson杆直接测得,并分别代入动态有限元程序及近似公式求得动态应力强度因子历史;由贴在试样裂尖附近的应变片确定起裂时间,最终确定起裂时的动态应力强度因子值,即动态起裂韧性KId()。试验结果表明:利用Hopkinson压杆技术加载三点弯曲试样测试材料动态起裂韧性的方法是可行的,起裂时,动态有限元的位移法、应力法及近似公式法求得的动态应力强度因子值比较吻合;在本文的载荷速率下,40Cr材料动态起裂韧性KId()与准静态裂韧性KId()相比,降低了约28%。 相似文献
15.
A.J. Rosakis J. Duffy L.B. Freund 《Journal of the mechanics and physics of solids》1984,32(6):443-460
Dynamic crack propagation experiments have been performed using wedge loaded double cantilever beam specimens of an austenitized, quenched and tempered 4340 steel. Measurements of the dynamic stress intensity factor have been made by means of the optical method of caustics. The interpretation of experimental data, obtained from the shadow spot patterns photographed with a Cranz-Schardin high speed camera, is based on an elastodynamic analysis. The instantaneous value of the dynamic stress intensity factor KdI is obtained as a function of crack tip velocity. Finally, the interaction of reflected shear and Rayleigh waves with the moving crack tip stress field is considered. 相似文献
16.
The critical stress intensity factor KIc is determined by a simple and accurate method, using small test specimens and a simple procedure in this paper.Single edge V-notched tension specimens made of PMMA are subjected to a load which is slowly increased until the crack begins to move from the notch tip. During the crack propagation event shadow patterns at the tip of the crack are recorded in a video recorder. Under these loading conditions, the creating real crack propagate slowly until the crack propagation velocity take an abrupt increase and the entire fracture of the specimen takes place. The stress intensity factor which correspond to the transition from the slow to fast crack speed, is the critical stress intensity factor KIc and it can be the fracture toughness of the material.The results are accurate and in good agreement with those values of KIc which are calculated by approximate theoretical expressions.The purpose of this paper is to introduce an improved, simple and accurate experimental method for the determination of fracture toughness of brittle materials. 相似文献
17.
Y. Z. Chen 《Theoretical and Applied Fracture Mechanics》1999,31(3):563
A singular integral equation containing the crack opening displacement (COD) is developed for solving plane elasticity problems. The crack may contain any number of kinks at different intervals and orientations, such as a saw-tooth shape. Cracks in the form of a sine wave can also be treated. The crack tip stress intensity factors are evaluated for a variety of crack shapes and the results are displayed graphically. The distance between the crack tips is found to be a dominant factor on the crack tip stress intensity while the angle between the tangent to the crack tip and load direction determines the proportion of Mode I and II stress intensity factors. 相似文献
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19.
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 总被引:1,自引:0,他引:1
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。 相似文献
20.
冲击载荷下扩展裂纹尖端动态能量释放率分布的焦散线分析 总被引:5,自引:2,他引:3
借助高速摄影捕捉裂纹瞬态扩展过程,利用动态焦散线研究了含有裂纹的三点弯曲梁在冲击载荷作用下扩展裂纹尖端的动态能量释放率分布规律;综合分析了裂纹扩展时间、长度、速度,以及扩展裂纹尖端动态应力强度因子与它的变化关系,表明了动态能量释放率在裂纹扩展过程中的驱动作用。 相似文献