平面应变断裂韧度的试验原理与过程控制

阐述了平面应变断裂韧度试验的理论模型和GB/T 4161-2007中平面应变断裂韧度KIC有效性判定依据的构建原理,在此基础上分析了预置裂纹应力强度因子和拉伸速率两个影响试验结果的因素,明确说明增大试样厚度是有效提高试验获得KIC值的方法,并以TC4和TA15两种钛合金材料的平面应变断裂韧度试验数据加以证明。     

An experimental cum numerical technique to determine dynamic interlaminar fracture toughness

A method combining experimental and finite element analysis is developed to determine interlaminar dynamic fracture toughness. An interlaminar crack is propagated at very high speed in a double cantilever beam (DCB) specimen made of two steel strips which are bonded together by epoxy with a precrack of about 40 mm length. The face of the front cantilever is bonded to a large solid block and a special fixture is designed to apply impact load to the rear cantilever through a load bar. In the load bar, a compressive square shaped elastic stress pulse is generated by impacting it with a striker bar which is accelerated in an air gun. The rear cantilever is screwed to the load bar; when the incident compressive pulse reaches the specimen, a part of the energy is reflected into the load bar and the rest of it passes to the specimen. By monitoring the incident and the reflected pulses in the load bar through strain gauges, deflection of cantilever-end is determined. The crack velocity is determined by three strain gauges of 0.2 mm gauge length bonded to the side face of the rear cantilever. Further, the first strain gauge, bonded very close to the tip of the precrack, and the crack velocity determine the initiation time of crack propagation.

The experimental results are used as input data in a finite element (FE) code to calculate J-integral by the gradual release of nodal forces to model the propagation of the interlaminar crack. The initiation fracture toughness and propagation fracture toughness are evaluated for an interlaminar crack propagating with a velocity in the range of 850 to 1785 m/s. The initiation toughness and propagation toughness were found to vary between 90–200 J/m² and 2–13 J/m², respectively.     

脆性材料断裂韧性测定的山形缺口试件和方法

本文扼要地介绍了测定新型结构陶瓷、高强度金属、以及其他脆性材料断裂韧性的山形缺口试件与方法。概述了这个方法的发展史、优缺点、目前研究概况、主要理论分析与实验研究成果,以及存在的问题和发展动向。     

用密栅云纹法研究高温新材料的断裂力学行为

介绍了高温云纹技术,并对密栅云法用于高温新材料的断裂力学行为测试进行了讨论,包括高温材料的零厚度光栅制作技术,Ｕ、Ｖ场分离装置,以及高温断裂的理论依据及图像的后处理,最后作了国内外情况对比分析和应用介绍,阐述了密栅云纹法研究高温新材料力学行为的重要性。     

A direct estimate of JIc from the load versus load-line displacement record

The purpose of this study is to examine the current standard fracture toughness test procedure to determine if there could be an easier method to get a J_Ic value from the test record. The current method for determining J_Ic involves a detailed computational and construction procedure. The objective in this study is to simplify the analysis for the determination of J_Ic. The results of this study show that the load and displacement record for a fracture toughness test can be used to directly estimate a J_Q value, a provisional value for fracture toughness, J_Ic. The J value taken at the maximum load point can be used along with an adjustment factor to estimate a J_Q value. This J_Q estimate is close to the one obtained from the construction procedure of ASTM Standard E 1820. When a unit‐sized specimen is tested, that is, a specimen with a width of 50 mm and a thickness of 25 mm the maximum load point provides a direct estimate of J_Q. Other sizes require a size adjustment factor, which is simply a square root relationship between the width of the test specimen and a unit width. The proposed new method of estimating J_Q is simple in concept and requires a minimum number of calculations. It appears to produce values of J_Q which are comparable to those obtained from the ASTM E1820 construction procedure and may produce less scatter.     

Evaluating the linear normalization technique for deriving J-resistance curves

In this paper, results from the linear normalization (LN) technique of Reese and Schwalbe for deriving J‐crack resistance (J–R) curves have been compared, related to J–Δa (J‐integral–ductile crack growth) data points, to those obtained from traditional elastic compliance technique. Research results regarding a nuclear grade steel exhibiting a wide range of elastic–plastic fracture resistance agree quite well for both techniques until a certain level of toughness of the material. Below this critical level, LN produces inconsistent results for the sub‐sized compact tension specimens (0.4T C[T]). The evidence suggests that the loss of applicability of the LN technique can be determined on the basis of the η plastic factor (η_pl) for the best linear correlation achieved for ΔP_N–Δa (normalised load gradient–ductile crack growth) data.     

铁素体钢断裂韧度尺寸效应的小冲孔试验

对六种不同厚度的试样进行了小冲孔试验,并进行了有限元模拟,此外还通过扫描电镜观察并分析了断裂面的微观结构。试验结果显示,试样的SP断裂变形能和断裂韧度随试样厚度的增大而增大。提出了裂纹起始遵循断裂应变准则及裂纹扩展遵循断裂能密度准则的观点。有限元模拟结果与试验结果基本吻合。断裂面呈典型的韧性断裂特征,试样变形后的半球状外表面布满微小的褶皱状突起和微裂纹。     

A simplified improved beam analysis of the DCB specimen

Large deviations from the simple cantilever beam model have been observed when analysing double cantilever beam tests in composite materials. In the present paper a simplified improved beam model is proposed. The cracked part of the specimen is analysed by means of shear-corrected classical beam theory. The uncracked part is analysed by considering Saint Venant effects and deformation of a beam on an elastic foundation. Superposition of compliances results in a simple closed form expression which can be used to isolate the effects of different material parameters and analytical simplifications. Crack length corrections are discussed as well as the corrections that have to be made when calculating the strain energy release rate. An extensive numerical comparison is made with the more elaborate solutions of Whitney and Williams. Finally, a discussion is made on the relevance of one-dimensional analysis as compared with two- and three-dimensional analyses. The discussion is supported by numerical comparisons.     

微胶囊自修复智能复合材料断裂特性的模拟实验研究

微胶囊型自修复复合材料设计的关键是基体材料和胶囊壳体材料的断裂韧性要在一定的范围内,使得基体材料的断裂特性和胶囊壳体材料的断裂特性达到最佳的匹配,这样才能保证自修复过程的实现.为得到这种匹配关系,首先对环氧树脂、不饱和聚酯和尼龙等基体材料试件进行了断裂韧性测定.然后从上述基体材料中以不同的材料组合制备出9种组配的夹层试件,每种组配所选用的两种材料(基体材料和夹层材料)的KIC值有差值ΔK IC.实验表明,在同样界面强度条件下,当基体材料和夹层材料的断裂韧性差值ΔKIC＜89669Pa·m1/2时,裂纹绕过夹层;而当ΔKIC＞89669Pa·m1/2时,裂纹穿过夹层.对模拟胶囊试件的实验验证了上述结论的正确性.     

50th Anniversary Article: Review on Interface Fracture and Delamination of Composites

In this review paper, a methodology for measuring the interface fracture toughness of a crack between two isotropic, homogeneous materials and a delamination between two laminae of unidirectional composite materials of differing directions is presented. Four cases are considered. Two isotropic material pairs are described: glass/epoxy and two ceramic clays. Similar studies are presented for two cross‐ply laminates: 0°/90° and +45°/?45. The Brazilian disk specimen was used to carry out mixed mode fracture tests. The load and crack or delamination length at fracture were measured and used in a finite element analysis to determine the displacement field. An interaction energy or M‐integral was used to obtain the stress intensity factors at failure. These in turn were employed to calculate the critical interface energy release rate and two phase angles ψ and φ, which measure the mode mixity. For the M‐integral and for each interface crack or delamination, the first term of the asymptotic solution of the fields is required. For two isotropic materials, these solutions are well known. For the laminates described here, they were determined by the Stroh and Lekhnittski formalisms. A failure criterion determined from first principles is presented. The values of , ψ and φ are used to specify the criterion for each material pair. A statistical analysis is presented, which predicts a 5% probability of failure.     

用双悬臂夹层梁试样研究复合材料与橡胶粘接界面的断裂韧性

用Ⅰ 型加载下的双悬臂夹层粱试样, 借助于线弹性断裂力学和声发射技术研究橡胶/复 合材料的粘接界面的断裂韧性GIC与裂纹扩展阻力R 并探讨加载速率、试验温度及橡胶片厚度对 它们的影响。结果表明, 在本试验范围内, 低速加载或高温将会降低界面的断裂韧性, 增大橡胶片 厚度会使断裂韧性稍有提高。      

Mechanism of effects of warm prestressing(WPS) on apparent toughness of notched steel specimens: Part I: Experimental

In this investigation, a series of experiments are carried out in blunt notched specimens to explore the various factors controlling the warm prestressing (WPS) effect on apparent toughness of a HSLA steel. A great number of specimens were tested using Cool-Fracture (CF) without WPS, Load-Cool-Fracture (LCF) and Load-Unload-Cool-Fracture (LUCF) cycles. More complex cycles have also been used to produce residual stress distributions and notch deformations different in quantities and signs. All fracture surfaces of the specimens were observed. Some specimens were unloaded after WPS and details of microscopic features in front of notch roots were investigated. Experimental results show that warm prestress cycles raising the residual compressive stress and opening the notch root improve notch toughness at low temperatures. Oppositely, WPS cycles raising the residual tensile stress and closing the notch root deteriorate notch toughness. One distinct effect of WPS involves deactivating inclusions and second phases particles. With increasing the preload of WPS, more and more particles being potential cleavage nuclei are decohered and blunted to cavities. This effect is proposed to be involved in improvement of notch toughness.     

Strength Assessment for Butt Joints of Steel 50 by the Fracture Toughness Criteria

We propose an experimental-theoretical engineering procedure for assessing the strength of butt-welded joints using the force (K_Ic) and strain (_c) criteria of the brittle and quasibrittle fracture mechanics, respectively. For this purpose, beam specimens with square and bevel welds and an initial edge crack or notch in the weld metal are tested under three-point bending. The parameter K_Imax controlling fracture of a bent beam with an inclined (Mode I + Mode II) crack is assessed taking into account the values of the stress intensity factors K_I and K_II, and the crack inclination angle ^*. We also studied the plastic zone at the crack tip and the crack propagation kinetics depending on the weld geometry and the V-notch tip radius for butt-welded joints. The data obtained allow one to rate such joints by their strength according to the fracture toughness criteria K_Ic and _c.     

Effects of precracked specimen geometry on local cleavage fracture stress σf of low alloy steel

An elastic-plastic finite element method (FEM) is used to analyze the stress distributions ahead of crack tips for three types of COD specimens with different precracked depth a/W and height W of a low alloy steel, and the tensile and COD tests are carried out at various temperatures. By accurately measuring the distances of the cleavage initiation sites from the blunted crack tips, the local cleavage fracture stresses _f are measured. With increasing precrack depth a/W, specimen height W and test temperatures in a certain range, it was found that the _f essentially does not change. The _f is a steady inherent parameter of the material whose value is independent of the precracked specimen geometry.     

Mechanism of effects of warm prestressing (WPS) on apparent toughness of notched steel specimens Part II: Calculations and analyses

Based on the experimental results of Part I of present work, this paper describes results of FEM calculations and analyses in details which identified that the effect of tensile-warm pre-stressing (WPS) on improvement of the apparent toughness of notched specimens results from three factors i.e. the residual compressive stress, macroscopic blunting of the original notch, and prestrain-deactivating cleavage initiation. The effects of three factors are separated and is effective for each at various extents of prestressing specified with a prestress-ratio, P₀/P_gy, defining the prestressing load P₀ as a fraction of general yield load P_gy. For values of prestress-ratio lower than 1.0, the residual compressive stress acts as the main factor. Between 1.0 to 1.5 of prestress-ratio values, in addition to the residual compressive stress the macroscopic blunting plays increasing role. The effect of the prestrain-deactivating cleavage initiation presents at the prestress-ratio P₀/P_gy1.2. In the case of compressive-warm prestressing, the apparent toughness is deteriorated due to the residual tensile stress. The effects of complex cycles of WPS, with various steps of loading and unloading different in signs, are determined mainly by the loading step just before the fracturing step.     

Partially stiffened elastic half-plane with an edge crack

A technique, using the Brazilian disk specimen, for measuring the fracture toughness of unidirectional fiber-reinforced composites, over the entire range of crack-tip mode mixities, was developed. The fracture toughness of a graphite/epoxy fiber-reinforced composite was measured, under both mode-I and mode-II loading conditions. We found that for certain material orientations the mode-II fracture toughness is substantially higher than the mode-I toughness. The complete dependence of the fracture toughness on the crack-tip mixity was determined for particular material orientations and the phenomenological fracture toughness curves were constructed. Using the Brazilian disk specimen, together with a hydraulic testing machine, the fracture toughness of the composite under moderate loading rates was measured. We observed that the mode-I fracture toughness was not sensitive to the loading rate at the crack tip, K, while the mode-II ‘dynamic’ fracture toughness increased approximately 50 percent over the quasi-static fracture toughness. A qualitative explanation of the dependency of fracture toughness on crack-tip loading rate is discussed. Finally, a mechanical fracture criterion, at the microscopic level, which governs the crack initiation under mixed-mode loading conditions is presented; these theoretical predictions closely follow the trend of experimental measurements. This revised version was published online in August 2006 with corrections to the Cover Date.     

Plane strain fracture toughness of MP35N in aged and unaged conditions measured using modified CT specimens

The multiphase alloy MP35N (35% Ni, 35% Co, 10% Mo, 20% Cr) is a high strength, high toughness alloy of choice for several safety-critical applications in aerospace, oil drilling, and biomedical industries. Several previous attempts in literature to measure the plane strain fracture toughness of commercially drawn MP35N did not produce reliable values since they violated one or more of the criteria stipulated by ASTM standards for a valid measurement of K_Ic. In most cases, the requirements for plane strain and small scale yielding conditions were not met, since the commercially drawn material was available only with limited cross-sectional dimensions. In this investigation specially designed specimens (modified compact tension (CT) specimens) have been used to measure the plane strain fracture toughness of MP35N in both the unaged and the aged conditions. The K_Ic of the commercially drawn (53% reduction level) MP35N was measured to be 126 MPa√m, while that of the commercially drawn and aged MP35N was measured to be 98 MPa√m. Both these measurements satisfied all the required criteria stipulated by ASTM standards for valid measurements of K_Ic. The new procedure used in this study has been verified by the measuring of fracture toughness of Al alloy, using both the modified specimen, as designed here, and the standard one. The results for plane strain fracture toughness of MP35N alloy have been verified by the standard measurement of J_Ic values for both the aged and the unaged alloys. Finally, on a suitably normalized plot, introduced in this paper, the toughness-strength envelop for MP35 is higher than most of structural alloys, but significantly lower than that of the TRIP steels.     

Effect of compressive transverse normal stress on mode II fracture toughness in polymeric composites

Effect of transverse normal stress on mode II fracture toughness of unidirectional fiber reinforced composites was studied experimentally in conjunction with finite element analyses. Mode II fracture tests were conducted on the S2/8552 glass/epoxy composite using off-axis specimens with a through thickness crack. The finite element method was employed to perform stress analyses from which mode II fracture toughness was extracted. In the analysis, crack surface contact friction effect was considered. It was found that the transverse normal compressive stress has significant effect on mode II fracture toughness of the composite. Moreover, the fracture toughness measured using the off-axis specimen was found to be quite different from that evaluated using the conventional end notched flexural (ENF) specimen in three-point bending. It was found that mode II fracture toughness cannot be characterized by the crack tip singular shear stress alone; nonsingular stresses ahead of the crack tip appear to have substantial influence on the apparent mode II fracture toughness of the composite.