EFFECT OF OVERSTRESSING ON FATIGUE BEHAVIOUR OF AUSTEMPERED DUCTILE IRON

Abstract Studies have been conducted on the effect of overstressing in rotary bending fatigue on the fatigue properties of an annealed and austempered ductile iron containing 1.5 Ni–0.3 Mo. For various R ratios S–N curves were determined and the fatigue limit estimated. It was found that the fatigue limit was a function of the level of overstressing and cycle ratio. In the case of austempered samples a beneficial effect of overstressing was observed at a certain level of overstressing. This was related to the work hardening behaviour of the austenite phase. In annealed samples, a reduction in the fatigue limit was observed at all levels of overstressing.     

A DUCTILE FRACTURE MODEL IN MIXED MODES 1 AND 2

Abstract— Fracture due to crack growth under mixed Modes 1 and 2 loading is considered. The cracking mechanism addressed is that of microvoid growth and linkage under small-scale-yielding conditions. Stress analyses are described which take into account the blunting of the crack tip as the load increases. A damage function is defined, and is evaluated in the intensely-straining region near the blunted notch tip. As the load increases, the size of this damaged region increases until a critical level of damage has been reached over a material-dependent length scale and fracture ensues. The local fracture criterion is then related to the applied loads. The predictions of the model are tested against experiments under Mode 1 loading and also against some mixed Modes 1 and 2 data on a range of materials. The results of the model are conservative and reproduce the experimentally observed Mode 1 failures, in terms of AT, at fracture, to within 25%. In the case of mixed mode loading the model conservatively predicts the fracture load for most of the data.     

MICROMECHANICAL MODELLING OF DAMAGE AND FRACTURE OF DUCTILE MATERIALS

A micromechanical model of ductile damage by void nucleation, growth and coalescence is widely and successfully applied to describe phenomena of ductile tearing. The model's fundamental principles, and especially the constitutive equations of Gurson, Tvegaard and Needleman (GTN-model), are briefly described. Some of the material parameters of the GTN-model are calibrated by performing cell model calculations, which is a method of determining the structural behaviour of a single void in a plastic material. The approach is used to study the dependence of material strength and toughness on microstructural features of nodular cast iron. The numerical simulations were realized within the FE-program ABAQUS by a user-supplied material model.     

气门疲劳断头失效分析

采用宏、微观检验、化学成分分析和硬度测试等方法对断头的排气门进行了分析.结果表明,气门断裂方式属于疲劳失效.结合气门服役受力情况,指出气门失效主要由于气门间隙调整过大使气门盘部与座圈的落座力过大,气门与座圈经长时间撞击,在气门根部产生金属疲劳.气门杆端面在运行中不断与摇臂撞击磨损使该现象进一步加剧,导致气门颈部撞击疲劳断裂失效.     

面外约束对韧性材料的断裂韧度的影响

与脆性断裂的断裂韧度随厚度的增加而逐渐趋近于一个常数的变化趋势不同,韧性较好材料的弹塑性断裂韧度的特征为:在一定范围内随着厚度的增加,弹塑性断裂韧度逐渐增加。主要研究在保持试件的面内约束(a/w)保持不变的条件下,面外约束(厚度的变化)对同种材料的试件的断裂韧度的影响。首先采用韧性较好的材料进行断裂韧性实验,通过实验得到结果:试件的弹塑性断裂韧度J积分及临界裂纹尖端张开位移CTOD随着试件厚度的增加而线性增加。然后从能量平衡的角度出发,考虑在裂纹扩展过程中的所有的宏观能量耗散机制,根据通过实验验证的假设,化简后,得到最终的结果:一定程度上表征能量耗散的断裂韧度随着厚度的增加而增加,与实验结果吻合得较好。     

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

This paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high-pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions. It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low-temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature. An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.     

基于微孔贯通细观损伤模型的金属韧性断裂分析

韧性材料断裂过程通常可看作是材料内部微孔洞的形核、扩展及相互贯通的积累。经典的Gurson- Tvergaard (GT)模型能够很好地模拟具有变形均匀、各向同性的孔洞的萌生及扩展过程;但无法模拟由孔洞贯通而引起的局部变形过程,因此需要对其修正,引入相应的孔洞贯通准则。该文采用两种贯通准则对经典GT模型进行修正,即Thomason的塑性极限载荷准则和临界等效塑性应变准则。借助用户自定义程序UMAT将采用这两种贯通准则修正的GT本构关系嵌入至商用有限元软件ABAQUS中,从而可通过对金属材料应力状态和断裂机理的分析控制孔洞的贯通。以一组含有不同缺口根半径的圆棒拉伸试验件为例,分析了该类金属构件自孔洞萌生至最终断裂的整个损伤演化过程,并与试验数据进行了对比,验证了该模型的有效性。该文还讨论了金属断裂过程中应力三轴度对微裂纹萌生与扩展的影响。     

Ni在球墨铸铁中的行为及作用

采用电子探针、图像处理和差热分析仪等测定观察了Ｎｉ在球铁中的偏析分布特性，研究分析了Ｎｉ对铸态态球铁组成比例，Ｆｅ－Ｃ－Ｓｉ平衡相图Ｍｓ点和贝氏体转变动力学的影响及其机制，结果表明：Ｎｉ在球铁中呈连续负偏析，其加入可以改变铸态球铁中各组成相的比例，使得Ｍｓ点降低，奥氏体稳定性增加，等温转变过程中，贝氏体转变速度减慢。     

FRACTURE TOUGHNESS OF BAINITIC NODULAR CAST IRON

Abstract— The fracture toughness of bainitic ductile iron transformed at various austempering temperatures and austempering times was evaluated by using compact tension specimens and compared with the fracture toughness of bulleye casting structure. Using Scanning Electron Microscopy, the mechanism of the fracture mode can be understood by observing the fracture surface. An X-ray diffractometer was used to determine the volume fraction of retained austenite. From the results of fracture toughness properties, it can be concluded that the most suitable austempering temperature of the material used in the present study is from 300 to 350°C.     

超高韧半晶聚合物PP多相体系的结构及脆韧转变的研究

通过严格控制工艺条件，得到了不同分散相含量和不同粒径的ＰＰ／ＥＰＤＭ／ＨＤＰＥ和ＰＰ／ＥＰＤＭ共混体。利用ＳＥＭ分析了ＰＰ／ＥＰＤＭ／ＨＤＰＥ的结构特点，通过测量Ｉｚｏｄ缺口冲击强度，得到了ＰＰ／ＥＰＤＭ／ＨＤＰＥ的脆韧转变主曲线，证明其符合脆韧性转变规律；同时利用ＳＥＭ照片，分析了主曲线不同区域的增韧机理。     

THE APPLICATION OF FRACTURE MECHANICS TO FRETTING FATIGUE

Abstract— A fracture mechanics model of fretting fatigue has been developed. This enables the calculation of the critical size of defect which can grow under fretting conditions. The model has been applied to the fretting of aluminium alloy 2014A fatigue specimens in contact with 3.5NiCrMoV steel fretting contact pads. The input parameters to the model include externally applied stresses, contact pressure between pad and specimen and frictional forces between the two contacting surfaces. The latter have been measured using a strain gauge technique. Small fretting defects in the fatigue specimens have been investigated metallographically and fractographically. There is good agreement between their size and critical defect sizes calculated using the fretting model.     

AN EVALUATION OF THE FRACTURE RESISTANCE OF ENGINEERING MATERIALS FROM THE FRACTURE SURFACE MORPHOLOGY

Abstract— The following work examines the fracture morphology in relation to the COD R -curves of three stainless steels. Additional considerations are noted concerning crack growth behaviour based on simple, optical microscopic observations. Procedures for the determination of R -curves and initiation values are outlined when a multi-specimen technique is used and the crack growth is measured by a light microscope. The COD R -curves are obtained for different conditions which reduce fracture ductility in two 310S and one 304H stainless steels. These conditions include a sigma-phase in one 310S, and sensitization of the large grain size microstructure in the other 310S stainless steel.     

HIGH-CYCLE FATIGUE BEHAVIOUR OF SPHEROIDAL GRAPHITE CAST IRON

An expression for the cumulative failure probability of a structure is proposed for cyclic loading conditions. This expression is dependent on an initial flaw distribution and a microcrack propagation law. Two sets of experiments were carried out on specimens made of spheroidal graphite cast iron. These specimens are tested under cyclic tension with two different load ratios. The initial flaw distribution is experimentally identified from microscopic observations. The crack propagation law parameters are identified from experimental results obtained with a load ratio R = 0.1. The expression for the failure probability is then used to predict experimental data obtained with a load ratio R = −1.     

LIFE PREDICTION FOR ADVANCED FERRITIC STEELS SUBJECT TO THERMAL FATIGUE

Abstract— Thermal fatigue is a well recognised source of damage in headers and steam piping of thermoelectric power plant. This topic has been extensively examined in the past for low alloy ferritic steels typically used in such applications. Experimental evidence obtained in low cycle fatigue testing with tensile hold times on Modified 9Cr1Mo and E911 steels suggests that the Linear Damage Summation rule conventionally used in engineering codes for high temperature damage analysis may not be particulary appropriate for the advanced 9Cr steel family. For this reason two alternatives have been examined: (a) a strain based creep damage evaluation using the R5 ductility exhaustion approach and (b) a creep-fatigue continuum damage mechanics method. The potential advantages and disadvantages of both are discussed. In addition, results from low cycle fatigue and thermomechanical fatigue tests on crossweld specimens machined from welded joints in the Mod.9Cr1Mo alloy are evaluated. Even if the usual cyclic life reduction factor of 2 with respect to base material behaviour appears adequate to account for the mean trend of cross-weld results, the large variability observed risks making the use of such a factor non-conservative for accurate life prediction.     

COMBINED MODE FRACTURE OF A DUCTILE MATERIAL UNDER PLANE STRESS CONDITIONS

Abstract— On the basis of our experimental results on Ly12-CZ (similar to 2024-T3) aluminium alloy tensile sheet specimens and Tresca's theory, a fracture criterion for a combined mode crack in ductile materials under plane stress conditions is proposed.     

ANALYSIS AND MODELISATION OF SHORT CRACK GROWTH BY DUCTILE FRACTURE MICROMECHANISMS

Abstract— A ductile medium strength steel has been modelled by means of the Gurson model, and been used to investigate the effect of crack tip constraint in several fracture mechanics specimens. Both numerical and experimental results have been obtained, in the course of the crack extension process, for single edge notch bending specimens with different crack length-to-width ratios. The geometries with the shorter cracks always exhibited higher J values at initiation and steeper J crack growth resistance curves, and these results have been explained in terms of the stress and strain fields and damage development in the region ahead of the crack tip.     

高应力弹簧端圈疲劳断裂机理研究

利用电阻应变仪测定了高应力弹筑各有效圈上所受的力，利用扫描电镜观察其断口形貌，得出结论，造成弹簧端圈断裂原因有二，其一是弹簧端圈受力较大，其二是端圈接触部位的摩擦磨损引起的。     

前轮毂旋转弯曲疲劳断裂分析

采用化学分析，金相检验，力学性能测试及扫描电镜分析方法对轿车前轮毂早期断裂原因进行了分析。结果表明，冷却不足使组织中出现网状及小块状铁素体，导致硬度偏低，加之磨削加工痕迹较粗是引起前轮毂早期断裂的原因。     

PC/ABS共混合金冲击断口形态和强度

研究了不同原料和配比的PC/ABS共混合金冲击破坏断面形态和缺口冲击强度之间的关系。用注射成型方法制备缺口冲击样条,用扫描电镜观察断面形态,用简支梁测定冲击强度,结果表明:冲击强度得到提高的合金断面是PC和ABS部分断面形态的加合和增强。其特征形态是微坑、抛物线和裂纹带分枝。冲击强度较PC和ABS低的合金断面,其形态既不同于PC也不同于ABS。原料和配比对合金断面形态和强度有影响,其中配比的影响最大。