Stellite12钴基合金的疲劳性能及其断裂机理研究

采用三点弯曲疲劳法测得光滑试样和直缺口试样的S-N曲线以研究Stellite12钴基合金的疲劳性能,并通过断口形貌观察进一步探究该钴基合金的断裂过程。结果表明:光滑试样的疲劳极限为545 MPa,约为原始抗弯强度1552 MPa的25.4%;直缺口试样的疲劳极限约为101MPa,约为静态抗弯强度517.6MPa的19.1%。对于疲劳敏感性,光滑试样与直缺口试样的疲劳敏感性分别为397和31。此外发现疲劳裂纹多萌生于近表层聚集的碳化物处,同时表面缺陷也可诱发疲劳裂纹的萌生。疲劳裂纹的扩展主要表现为碳化物的穿晶断裂,钴基体在应力比R=0.1的疲劳加载条件下虽表现出一定的韧性且呈现出较多的撕裂脊,但也呈现出一定的脆性断裂模式,因此疲劳裂纹扩展模式为真疲劳与静态疲劳的混合模式。     

回火处理对一种新型钴基合金表面结构和压缩力学性能的影响

本文对钴基合金试样进行了不同时间的回火处理,对压缩应力-应变曲线进行了分析,通过对回火前后钴基合金的微观组织和断口形貌的扫描电镜观察,解释了该钴基合金回火处理前后的压缩断裂过程变化和压缩性能差异。研究发现,回火处理前后的压缩试样断裂过程有较大差异:未经回火处理的试样,其压缩断裂过程表现为典型的脆性断裂,断裂面与压缩方向呈45°夹角;而经过回火处理的压缩试样,首先在试样表面产生平行轴线方向的裂纹,同时在试样顶端形成一个45°锥形断裂面,最后试样断裂。回火过程中生成的氧化层是回火试样压缩断裂过程变化的主要原因,但是对整个压缩曲线的变化起主导作用的仍然是材料的基体组织。回火后的钴基合金组织中fcc-Co相比例显著增加,Cr_(23)C_6碳化物颗粒尖角消失,形状变得圆滑,显著改善了该钴基合金的韧性。     

缺口对不同塑性金属材料拉伸断裂行为的影响

以10CrNi3MoV钢、5083铝合金、500-7球墨铸铁3种不同塑性的金属材料为研究对象,分别加工光滑和缺口试样,通过对比试样的拉伸性能及断口形貌,研究了缺口对不同塑性金属断裂行为的影响。结果表明:缺口改变了材料的应力状态,减少了剪切方向的变形,提高了试样正拉断裂的比例。对于塑性材料,缺口根部可以通过塑性变形来缓解应力集中,且缺口深度越大,材料塑性越好,抗拉强度越高;对于脆性材料,缺口根部产生的应力集中会导致拉伸过程中试样过早断裂,降低材料的抗拉强度。     

带缺口塑性材料的声发射特性研究

以带线切割加工缺口的碳钢在拉伸试验时的断裂过程为研究对象,通过对其产生的声发射信号进行检测和分析,研究其开裂时的声发射特性。试验结果表明:带缺口塑性材料在拉伸断裂过程中会产生幅值在47dB￣90dB范围内的声发射信号;信号的振铃累计数在材料的塑性变形段与裂纹稳态扩展段均与应变成线性关系,但后者斜率大于前者;试样缺口深度越大,材料开裂时间越早,振铃计数率越高,且信号能量累计数与应变成线性关系。     

铸钢表面镍基合金渗层的热疲劳行为

采用铸渗技术在铸钢ZGCr5Mo试样表面制备了镍基合金渗层,渗层的厚度为0.6—1.2 mm,考察了镍基合金渗层的热疲劳行为,用扫描电镜(SEM)、X射线衍射(XRD)对热疲劳循环后的渗层表面进行了形貌观察与成分分析。结果表明:在热循环次数低于20次时,表面仅发生了氧化现象,当热循环次数超过90次时,渗层表面出现微裂纹,随着循环次数的增加,在渗层与基体的侧表面上出现贯穿渗层与基体的微裂纹以及在渗层与基体的界面处的表面氧化膜层出现了平行于渗层表面以及发散的微裂纹;随着镍基合金渗层厚度的增加,出现微裂纹的热循环次数略有降低。表面的氧化膜层主要为镍、铬的氧化物以及镍铬的复杂氧化物。     

金属材料力学性能试验 第五讲 冲击试验

1 概述 1.11 材料的韧性和冲击韧性 众所周知,强度和塑性是金属材料的两个基本的力学性能。在零部件的实际工作中,除了对材料的强度和塑性提出要求外,还会对材料的韧性提出一定的要求。 韧性的定义是材料在变形和断裂过程中吸收能量(以外力作功来衡量)的一种性能。按照这一定义,韧性可分为: 1)静韧性 即在静载荷(拉伸、压缩、弯曲和扭转)下材料经历弹性变形、塑性变形和断裂时所吸收的能量就叫做材料的静韧性。 2)冲击韧性 即在冲击载荷下材料吸收的能量A_k除以试样缺口根部的面积所得的商,称为材料的冲击韧性,记为a_k。 3)断裂韧性 在试样上预制一条裂纹,然后在静载下(拉伸或弯曲)使其变形直至断裂,再按一定的公式计算出材料的断裂韧性。     

缺口对7A85铝合金拉伸性能和疲劳性能的影响

在光滑试样表面预制深度为0.1 mm和0.2 mm的环状缺口，研究了不同缺口尺寸对7A85铝合金拉伸性能和疲劳性能的影响，并分析了7A85铝合金含缺口试样的疲劳断裂机理。结果表明，随着缺口深度由0 mm增加至0.2 mm, 7A85铝合金试样的抗拉强度、断后伸长率和疲劳强度分别下降了6%、47.5%、44.4%。缺口对7A85铝合金塑性的影响远大于拉伸强度，且其抗拉强度与疲劳强度呈线性关系。试样疲劳缺口系数随着缺口尺寸和循环次数的增加而增大。7A85铝合金试样的疲劳裂纹源通常是富铁的第二相颗粒，环状缺口根部应力集中促进了多疲劳裂纹源萌生，多个裂纹源同时扩展使得试样有效承载面积快速减少，导致疲劳寿命急剧缩短。     

返回料添加比例对K44合金热疲劳性能的影响

研究了返回料添加比例对新型抗热腐蚀高温合金K44热疲劳性能的影响.结果表明:新料和返回料合金试样V型缺口尖端主裂纹扩展长度与热循环次数之间遵循L=bNa规律.新料合金热疲劳裂纹萌生和扩展速率最低,随着合金中返回料比例的增大,热疲劳裂纹萌生速率和扩展速率也增大.热疲劳裂纹萌生于V型缺口尖端附近区域,沿枝晶间、晶界和开裂的碳化物扩展,主裂纹扩展以裂纹尖端连续开裂的形式进行.返回料合金由于氮含量增加导致共晶和夹杂物增多,碳化物聚集块化,加速了热疲劳裂纹的萌生与扩展.合金经热疲劳实验后,裂纹两侧产生氧化带和γ'相贫化带.     

冲击试样缺口投影仪标准比对样板缺口半径的校准方法及测量不确定度分析

针对金属材料冲击试验缺口加工中冲击试样缺口投影仪的计量现状,介绍了冲击试样缺口投影仪的结构和标准比对样板V形、U形缺口圆弧半径的技术要求,并提出了冲击试样缺口投影仪标准比对样板详细的测量标准设备和校准方法并做测量不确定度分析,实现了冲击试样缺口投影仪标准比对样板缺口半径的量值溯源和计量校准的要求。     

全层TiAl合金裂纹扩展阻力的研究

本文通过对直缺口两个层团厚度拉伸试样的扫描电镜原位拉伸实验、相应的断裂表面观察以及有限元计算研究了全层铸状TiAl基合金组织的裂纹扩展机理.研究表明:为了扩展主裂纹,外加载荷需要增加.外加载荷的增加一方面归功于试样表面观察到的主裂纹并没有穿透整个试样厚度,另一方面因为应力场由缺口控制,只有随着外加载荷的增加,扩展的裂纹尖端才保持一定的应力.当两个层团的位向使裂纹从沿层扩展过渡到穿层扩展时,裂纹扩展阻力才提高.     

Fracture process of thermally shocked discontinuous fibre-reinforced glass matrix composites under tensile loading

Fracture mechanisms of discontinuous carbon-fibre-reinforced glass matrix composites were experimentally studied for specimens with initial damage induced by thermal shock. First, matrix cracking due to thermal shock was observed using both optical microscopy and scanning acoustic microscopy (SAM) to reveal the damage state. Secondly, tensile stress-strain behaviour and acoustic emission during tensile tests were measured for specimens with and without thermal shock. The progress of microscopic damage during tensile loading was also investigated using both replica and in-situ SAM techniques. Finally, macroscopic transient thermal stresses during thermal shock were calculated using finite-element analysis. It is proved that the fracture process of the composite specimen with thermal-shock-induced cracks is different from that of the virgin specimen. This difference in fracture processes is attributed to the difference in the evolution of matrix cracking, which is affected by pre-existing microcracks in the matrix. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation of fracture mechanism of 6063 aluminum alloy under different stress states

Fracture mechanisms in a 6063 aluminum alloy were investigated and analyzed carefully by in-situ tensile tests in SEM with a vacuum chamber. Specimens used were designed to produce different stress states. Studies indicated that with stress triaxiality (σ _m/σ _e) decreasing, the fracture modes changed from normal fracture to shear fracture and the fracture surfaces changed from the dimples and intragranular dominated fracture mode to the shear dominated fracture mode. The grain boundaries of the 6063 aluminum alloy were the weakest positions. In the case of high stress triaxiality, the grain boundary cracks were produced by normal stress or by the incompatibility of deformation between neighboring grains, and the normal stress dominated the crack propagation. In the case of low stress triaxiality, the boundary cracks were produced by the relative slipping of grains against neighboring grains, and the shear stress dominated the crack propagation. The final fracture of the specimens occurred by connections of cracks through transgranular cracking of the ligaments among these cracks.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

硫化锌热冲击试验与裂纹间距预报

重点研究了硫化锌热冲击开裂机理和热冲击裂纹间距、深度的预报。10mm厚硫化锌试块的燃气急热试验表明:裂纹间距随热冲击能量的增大而减小,热冲击过程中加热面最先出现非贯穿裂纹,停止加热后,裂纹贯穿试件。结合传热和热强度仿真分析,获得了热冲击过程中试件的瞬态温度场和应力场。基于材料性能的损伤演化理论,以裂纹间距和深度为变量,利用最小能量原理,获得了热冲击裂纹间距的理论预报方法,预测结果与试验吻合较好,进而分析了断裂韧性、热胀系数、材料初始模量对裂纹间距、裂纹深度的影响。该文的研究对深入理解硫化锌的热冲击失效机制,对其改性和研制具有重要意义。     

Intergranular and transgranular cracking of Cu–30Zn brass during fatigue loading

Abstract

Fatigue tests have been performed on annealed α–brass to examine the dependence of fracture morphology on ?K. The cracking was predominantly intergranular at low values of ?K_Q (the stress intensity factor range when the specimen does not comply with plane strain conditions) and changed progressively to transgranular cracking at high ?K_Q values. Detailed scanning electron microscopy has been performed on the fracture surfaces of the specimens, especially from matching areas on opposite faces. It has been shown that matching extrusion and intrusion pairs as well as one–to–one matching of fine slip lines occurred on the intergranular facets indicating that plastic deformation causes the intergranular cracking. Intergranular cracking persists at low ?K_Q values even though the crack growth rate is smaller than for transgranular cracking because the latter is difficult to initiate. Transgranular cracks form only at regions of localized strain, e.g. coarse slip bands, or at cold–worked surfaces but such transgranular cracking cannot be maintained at low ?KQ values.

MST/209     

Mechanism of rock fatigue damage in terms of fracturing modes

A novel outcome of the research described is the observation of the effect of indirect tensile cyclic loading on the fracture toughness (K_IC) of rocks. A maximum reduction of the static K_IC of 46% was obtained for the highest amplitude dynamic cyclic loading test. A second series of cyclic tests showed that the 45° and 70° inclined chevron notch cracks opened from the beginning of the cyclic loading test, whereas under monotonic loading, the chevron notch cracks closed up to failure. When compared with static rupture, the detailed scanning electron microscope (SEM) examinations showed that fatigue damage in Brisbane tuff is strongly influenced by the failure of the matrix, due to both intergranular fracturing and transgranular fracturing, whereas smooth and bright cracks were formed along cleavage planes under static loading.     

Hydrogen induced cracking in high strength steel

Initiation of quasicleavage (QC) cracks in delayed fracture of hydrogen charged, quenched and tempered AISI 4340 steel was studied by means of a fractographic technique. The fatal crack leading to the delayed fracture of an unnotched specimen is found not to be caused by intergranular cracking as is commonly found in notched specimens, but by QC cracking. An SEM fractographic examination has revealed that QC cracks, which initiate around non-metallic inclusions and under the effect of hydrogen alone, grow mainly along the rolling direction of the specimen. Furthermore, the tensile component of stress causes a QC crack to grow in the plane of maximum tensile stress. The condition under which brittle fracture finally occurs in delayed fracture can be evaluated by the classical Griffith criterion from two parameters: the QC crack length and the nominal applied stress.

MST/3264     

Mechanical behaviour of a cross-weave ceramic matrix composite

The deformation and fracture processes of a cross-weave carbon fibre/SiC composite prepared by a chemical vapour deposition process has been explored by interrupted-loading tests and SEM examination of cracking and fracture processes. The tensile stress-strain curves show non-linear behaviour associated with progressive matrix cracking and spalling, and the occasional fracture of a fibre. Re-loading curves and compressive stress-strain curves show linear behaviour. The fracture process does not involve cracking by a single dominant crack but occurs by the development of multiple damage sites operating around the transverse fractures of groups of four to eight fibres followed by longitudinal cracking at their fibre-matrix interfaces and temporary arrest of the cracks, until specimen failure occurs and there is massive fibre debonding and pull-out.     

Rectangular Tensile Sheet with Double Edge Notch Cracks

This paper deals with the rectangular tensile sheet with symmetric double edge notch cracks. Such a crack problem is called an edge notch crack problem for short. By using a hybrid displacement discontinuity method (a boundary element method), two edge notch models are analyzed in detail. By changing the geometrical forms and parameters of the edge notch, and by comparing the stress intensity factors (SIFs) of the edge notch crack problem with those of the double edge cracked plate tension specimen (DECT), which is a model frequently used in fracture mechanics, the effect of the geometrical forms and parameters of the edge notch on the SIFs of the DECT specimen, is revealed. Some geometric characterestic parameters are introduced here, which are used to formulate the notch length and the branch crack length, which are to be determined in mechanical machining of the DECT specimen So we can say that the geometric characterestic parameters and the formulae used to determine the notch length and the branch crack length presented in this paper perhaps have some guidance role for mechanical machining of the DECT specimen.