The model for fracture toughness

It is pointed out that the distribution introduced by Neville [1] (for modeling fracture toughness to failures) is contained by at least two families of distributions known since the 1940s. Some elementary statistical properties of these families are discussed. Six data sets on fracture toughness are used to demonstrate that these families are much better models for fracture toughness than the one introduced by Neville [1].     

Effects of superimposed hydrostatic pressure on fracture in sheet metals under tension

The effect of superimposed hydrostatic pressure on fracture in sheet metals under plane strain tension is studied numerically using the finite element method based on the Gurson damage model. It is demonstrated that the superimposed hydrostatic pressure p has no noticeable effect on necking but significantly increases the fracture strain due to the fact that a superimposed pressure delays or completely eliminates the nucleation, growth and coalescence of microvoids or microcracks. The experimentally observed transition of the fracture surface, from P-type mode under atmospheric pressure to C-type mode under high pressure, is numerically reproduced.     

EFFECT OF DYNAMIC LOADING ON FRACTURE BEHAVIOR OF WELDED JOINTS OF STRUCTURAL STEEL

To investigate the effect of dynamic loading on fracture behavior of welded joints of structural steel Q23SB and 16Mn in common use and compare the earthquake resistances of the two kinds of materials, dynamic tension and fracture toughness tests are carried out at room temperature. On the basis of the tests, the stress-strain fields near the crack tip of the compact specimens are analyzed by three-dimensional finite element model. The test results and finite element analysis results show that, the fracture toughness of welds and base metal of 16Mn steel increases with the increment of loading rate. Compared with 16Mn steel, Q235B steel is more sensitive to dynamic loading. The fracture toughness of welds of Q235B is comparatively low under static loading and dynamic loading at room temperature. Compared with the static loading, the fracture toughness of Q235B parent metal under dynamic loading decreases by about four times. Therefore, it can be concluded that compared with 16Mn steel, the earthquake re     

基于ANSYS的铰链型膨胀节结构强度分析

为提高波纹管膨胀节的使用性能及其可靠性,根据膨胀节的结构特点和使用状况,运用Solidworks建立其实体模型,并对载荷加以简化和等效处理,通过ANSYS软件分析其承力构件的静力学特性。结果表明,膨胀节的最大位移发生在受力侧端管两端,最大位移为3.339 mm;承力构件的最大应力发生在万向环,其应力为99.8 MPa,小于其材料的许用应力。该型铰链膨胀节满足设计载荷下的强度要求,为其强度校核及结构优化设计提供了理论依据。     

The fracture toughness behaviour of interpenetrating phase composites

The fracture toughness properties of two interpenetrating phase composites (IPCs)—a bronze-infiltrated porous 420 stainless steel and a polymer resin-impregnated porous 316L stainless steel—have been measured using ASTM Standard E 813-89. Both IPCs exhibited stable crack growth at all volume fractions, resulting from an increase in toughness with crack growth (R-curve behaviour). Initiation toughness, J_Ic, increased and R-curve behaviour became more pronounced with increasing volume fraction of the more ductile constituent phase. R-curve behaviour is attributed to the mechanisms of crack bridging and unloading in the wake of a process zone, which is characterized by secondary cracking and plasticity. The importance of an interpenetrating phase morphology is dependent upon the combination of materials, but it appears that interconnecting the more ductile phase will result in increased toughness, particularly if this is the stronger and stiffer phase. The application of ASTM Standard E 813-89 to the IPCs investigated was found to result in a large number of validity criterion failures. The implications of these failures are discussed.     

Stress concentration analyses of bi-material bonded joints without in-plane stress singularities

The problem of stress concentration in bi-material bonded joint is investigated under the condition of without stress singularities. Disappearance conditions of stress singularity near interface corners and edges are determined based on analyses of eigenvalue equations. Straight-side and curved interface of materials are designed for the bi-material models to avoid singular stress fields around the interface corner and edge. Assuming that one stress component or combined stresses are responsible for failure at or near the interface, the stress concentration becomes critical for the design of bi-material joints with higher interfacial strength. Numerical results show that the stress state near the interface depends strongly on both the interface geometry and the combination of materials, and stress concentration may always occurs at or near the interface. Emphasis is placed on the necessity for geometric optimization of an interface in order to design singularity-free junction with higher interfacial strength.     

Finite element modeling of adhesive contact using molecular potential

A finite element technique for analysis of adhesive contact is developed in which the adhesive force is modeled as a body force derived from Lennard–Jones 12–6 potential. Adhesive contact of an elastic hemispherical asperity with the plane surface of a semi-infinite rigid body is analyzed. Variations of the interaction force and contact radius during approach and withdrawal, and the dependence of pull-off force on the asperity radius are shown to be in good agreement with those of Maugis–Dugdale model. Analysis results reveal that smaller asperity is superior for preventing stiction and for reducing adhesive friction, but is subject to more severe adhesive wear. It is anticipated that this technique can be utilized in designing a low-adhesion surface profile for MEMS applications since the effect of various surface geometries can be examined.     

SS400钢焊接接头抗断性能的数值评估

SS400钢是一种超细晶粒、超纯净、具有优良综合力学性能的新一代钢铁材料，其焊接结构性能的研究对其推广应用具有重要的意义。文中通过数值分析方法，以SS400母材性能为参照，对其实际焊接接头的抗断裂性能进行评估。采用MARC商用软件，对不同中心贯穿裂纹尺寸的SS400母材及其实际焊接接头拉伸板模型进行三维有限元计算和拉伸过程模拟。焊接方法为脉冲熔化极混合气体保护焊，焊接板的中心裂纹处于焊缝与热影响区之间的熔合区。用作比较的母材板的裂纹尺寸和位置、网格划分、边界条件等与焊接板完全相同。根据计算结果，分析焊接接头力学不均匀性对应力应变场的影响，并依据全面屈服理论对断裂参量CTOD进行分析比较。结果表明，焊接接头的CTOD值均低于同裂纹尺寸的纯母材。因此，该焊接接头拉伸板的抗断性能应优于纯母材。     

Measurement of dynamic fracture toughness and failure behavior for explosive mock materials

In this work, a pre-cracked semi-circular shaped explosive simulant was loaded using a split Hopkinson pressure bar (SHPB). A high-speed camera was used to capture the deformation and fracture process of the specimen in situ. The digital images were processed using the digital image correlation (DIC) method. Next, full displacement and strain fields were obtained. The displacement vector field shows that the specimen fractured under tensile stress action. The strain field can be used to predict the crack propagation. Results show that the method of combined DIC and SHPB is effective to study the dynamic deformation and fracture behavior of explosive simulants. In addition, the specimen was loaded using a drop weight. The fracture toughness of the specimen was preliminary measured.     

Correction of constraint loss in fracture toughness measurement of PCVN specimens based on fracture toughness diagram

The aim of this paper is to suggest an approach to generate master curves by using miniature specimens, especially pre-cracked Charpy V-notched (PCVN) specimen, made of SA508 carbon steel. Firstly, fracture toughness diagram is derived from comparing finite element analyses results with the fixed mesh size at crack tip between standard compact tension and PCVN specimens. To compensate the constraint effects from different geometry, further examination based on the fracture toughness diagram was performed. In this context, a scale factor to deal with specimen size effects is proposed by statistically manipulating the numerical analysis data. Finally, the proposed scale factor is applied to calculate reference temperature which affects on the master curve. We expect that the approach can be applicable to compensate the geometrical constraint effects on fracture toughness of SA508 carbon steel when the PCVN specimen is used.     

A versatile stereo photogrammetry based technique for measuring fracture mode displacements in structures

The measurement of fracture mode displacements in structures which are susceptible to cracking such as adhesive joints in composite components – is becoming increasingly important. Such measurements are essential for the understanding of the root causes for specific fracture damage types. Furthermore, they can be used to assess the remaining life span of a structure for its safe operation. An improved version of a previously devised small displacement measurement system (SDMS) is used to measure local relative displacements (LRDs) at the trailing edge of a wind turbine blade. A purpose-made automated image processing software (AIPS) allows a rapid and reliable evaluation of a multitude of subsequently taken measurements at a high-precision level. The SDMS is used to measure the LRDs at three different locations close to the trailing edge of a wind turbine rotor blade. In addition, complementary measurements obtained by linear transducers are compared with the associated LRD component obtained by the SDMS. The 3D LRD measurements showed to be in good agreement with the predictions of non-linear finite element analysis. The paper closes with a brief discussion of the proposed measurement approach and the nature of LRDs as they appear in close vicinity to trailing edge joints.     

31Si2MnCrMoVE钢薄板试样表面裂纹断裂韧度测试

31Si2MnCrMoVE钢是为符合固体火箭发动机壳体设计需要而专门研制的超高强度钢。随着冶炼技术的进步,31Si2MnCrMoVE钢断裂韧度不断提高,构件采用的板厚也越来越薄。由于较高的断裂韧度和较小的板厚,给钢板表面裂纹断裂韧度测试带来困难,韧带尺寸偏小,难以满足有效性判据。这种情况下,不应该用线弹性断裂力学方法评价材料的断裂韧度,而应采用弹塑性断裂力学测试材料的延性断裂韧度JIC。基于以上原因,在条件断裂韧度不满足有效性判据的情况下,采用试验与有限元分析相结合的方法,通过试验测出裂纹启裂时的条件载荷,用有限元法计算出在条件载荷作用下的延性断裂韧度JIC,再用断裂力学理论转换成表面裂纹断裂韧度KIe。用JIC作为断裂参量,就必须分析J积分的有效性,因此讨论超高强度钢表面裂纹前缘的J守恒和J主导的有效性,从而为固体火箭发动机设计提供依据。     

一种考虑胶瘤的胶接简化有限元模型:应力与刚度分析

胶接接头中总存在胶瘤,由于建模复杂,胶接接头有限元分析中胶瘤常被忽略.但胶瘤能减少峰值应力,提高结构强度和刚度.为此,提出一种简化的胶接有限元模型,即用壳单元代表胶瘤,体单元代表被粘体和胶层,并用弹性理论建立壳单元等效厚度公式.以体单元精细模型结果作为对比的真实解,考察五种载荷工况下,单搭接头简化有限元模型的胶层应力和刚度.分析表明,壳单元等效厚度公式正确,胶接简化有限元模型精度高,可用于诸如汽车等大型结构中;用壳单元简单模型可定量分析胶瘤大小和形状对接头应力和总体刚度性能的影响.     

断裂韧性的两端截尾分布概率法设计

利用两端截尾分布理论及应力－ 强度干涉模型建立了断裂韧性的概率设计方法,消除了目前使用的、利用理论分布进行断裂韧性计算时可靠度永远小于１ 的情况,使可靠度计算更符合实际情况。通过大量实例计算表明该方法具有重要的现实应用意义。     

冷锻厚壁加强三通有限元分析

本文介绍了对冷锻厚壁加强三通限元分析的方法和结果。分析结果表明三通满足强度设计要求,但是复杂工况条件下三通的母管与支管相贯肩部的应力值很大。所得结论为进一步改进三通设计提供了理论依据。     

A probabilistic model for the scale effect on fracture toughness of structural ceramics

The fracture toughness of polycrystalline ceramics used to present the scale effect as well as statistical distribution. It is believed that both (scale effect and scatter) must be associated with the heterogeneity of materials. However, no generally accepted theory has been established so far. Using statistical approach, a probabilistic modelling for the fracture toughness which describes the scale effect was attempted in this paper. Weibull distribution of specific fracture energy (SFE) at local area and Griffith criterion are jointly applied to the model. Finally, the fracture toughness scale dependence of ASTM E399 standard specimens was investigated by the newly developed model.     

复合材料板与金属板榫槽粘接结合有限元分析

榫槽连接是航空结构中常用的一种连接形式。通过对金属板和复合材料板的榫槽粘接部位进行有限元计算，分析简单拉力作用下连接部位的应力分布以及金属板和复合材料板尺寸对最大等效应力的影响。通过比较发现，在所考察范围内榫槽长度、粘接层厚度和板宽度等因素对模型等效应力的影响并不十分显著，而两种材料板之间的相对厚度对最大等效应力影响较大，随着金属板厚度的增大，整个榫槽连接结构的最大等效应力有所减小。     

GH4169合金电子束焊接接头CTOD断裂韧度

根据GB/T2358—94标准,分别在20℃和650℃下,对GH4169合金电子束焊接接头的裂纹尖端张开位移(CTOD)进行测试。取SE(B)试样进行三点弯曲试验,然后由所得到的20℃和650℃下母材和电子束焊缝的P-V曲线来计算CTOD值,比较两个温度下的试验结果,对试验结果进行了分析讨论。     

基于材料断裂韧度测试的COD换算方法研究

考虑到断裂韧度J积分塑性分量的塑性功由载荷与加载线位移关系得到,通过测试CT(compact tension)和SEB(single edge bending)试样的裂纹嘴张开位移实现J积分计算,提出适用于两种试样的裂纹嘴张开位移与加载线张开位移转换的换算公式,并采用45钢CT试样和30CrMo钢SEB试样对新公式进行有限元数值比对和试验比对。结果表明,新公式结果与规范演算结果之间的相对误差不超过0.4%,与有限元分析结果之间的相对误差不超过0.6%,与CT试样试验结果之间的相对误差小于4.5%,与SEB试样试验结果之间的相对误差小于6%。     

Recent development in low-constraint fracture toughness testing for structural integrity assessment of pipelines

Fracture toughness measurement is an integral part of structural integrity assessment of pipelines. Traditionally, a single-edge-notched bend (SE(B)) specimen with a deep crack is recommended in many existing pipeline structural integrity assessment procedures. Such a test provides high constraint and therefore conservative fracture toughness results. However, for girth welds in service, defects are usually subjected to primarily tensile loading where the constraint is usually much lower than in the three-point bend case. Moreover, there is increasing use of strain-based design of pipelines that allows applied strains above yield. Low-constraint toughness tests represent more realistic loading conditions for girth weld defects, and the corresponding increased toughness can minimize unnecessary conservatism in assessments. In this review, we present recent developments in low-constraint fracture toughness testing, specifically using single-edge-notched tension specimens, SENT or SE(T). We focus our review on the test procedure development and automation, round-robin test results and some common concerns such as the effect of crack tip, crack size monitoring techniques, and testing at low temperatures. Examples are also given of the integration of fracture toughness data from SE(T) tests into structural integrity assessment.