小冲杆试验摩擦力计算及抗拉强度公式改进

小冲杆测试技术可在不影响设备运行的情况下,对在役设备进行材料劣化程度评定及寿命评估。小冲杆试验所测得的载荷分为正压载荷和摩擦力两部分。利用有限元分析法得到了钢珠与试样之间正压载荷与摩擦力的大小,并将正压载荷与常规抗拉强度进行经验关联,改进了传统的抗拉强度经验关联式,提升了关联系数。     

小冲杆技术及其在材料与铅铋脆化效应研究中的应用

第四代反应堆中铅冷快堆是一种非常有发展前景的堆型。在铅冷快堆包壳材料设计时,需要重点考虑包壳材料与高温铅铋的相容性,关注其抗液态金属腐蚀能力,尤其是腐蚀介质与应力耦合时材料性能的变化。小冲杆实验(SPT)是一种使用小尺寸样品评估材料服役条件下力学性能变化的实验方法,这种方法可以直接在正在使用的材料中提取样品而不损害其完整性,因此小冲杆测试非常适用于所研究材料数量有限或研究在役材料的情况。本文简要介绍了小冲杆测试技术,归纳了小冲杆测试与标准尺寸测试实验的相关性,阐述了使用小冲杆技术进行的关键材料力学性能研究,特别阐述了小冲杆测试在研究结构材料液态金属脆化效应(LME)方面的应用。结果可为小冲杆技术应用于研究材料液态铅铋脆化效应提供技术方法及理论支持。     

利用小冲杆试验方法确定材料的Weibull参数的可行性

对3种具有应力集中特征的不同结构小冲杆试样在-170℃温度下进行试验,研究使用小冲杆试样确定材料Weibull参数的可行性。结果表明,小冲杆试样能够用于确定材料的Weibull参数;在使用直线槽试样确定材料的Weibull参数时,需选用薄、缺口曲率半径小的试样;中心开孔的试样非常适用对Weibull参数的确定。     

基于Beremin模型的小冲杆试验技术估算Weibull参数的试样选取选取

小冲杆试验技术可以利用小体积材料获取宏观材料的力学性能。提出基于Beremin模型估算铁素体钢在韧脆转变区的Weibull参数(m,σu)的方法。为了研究选取此方法的试样类型,在韧脆转变区对4种不同试样类型进行试验。根据试验曲线对比、SEM(扫描电镜)分析和拟合曲线结果,得出0.5mm厚的CHSP试样(直线缺口小冲杆试样)是4种试样类型中最适合用于此方法的试样类型。同时证明了用基于Beremin模型的小冲杆试验技术估算材料的Weibull参数的方法是可行的,并通过该方法实现了用小试样获取材料在韧脆转变区的累积失效概率。     

评铝—锂合金断裂韧性测试方法

本文对铝-锂合金断裂韧性的多种测试方法进行了比较和分析.建议采用Kahn型撕裂试验来研究铝-锂合金的断裂韧性.因为所需试样小,操作方便,而且测得的单位面积裂纹扩展能对材料断裂性能敏感,可适用于各种不同强度、韧性水平的材料性能测试,并与平面应变断裂韧性参数K_1c存在相当精确的关系.     

工作介质中材料的应力腐蚀性能

为了研究某材料在工作介质中的应力腐蚀问题,开展了光滑试样和中心穿透预裂纹试样在温度为T_0、不同试验介质中、施加一定预应力水平的恒载荷试验,测试试样的力学性能,分析其金相组织,并测量了预裂纹试样在恒载试验过程中裂纹的扩展量.结果表明:在不同试验介质中,施加(90～95)% σ0.2载荷的光滑试样和施加(30～90)% K_c载荷的预制裂纹试样,经过约13000 h的试验后,没有试样发生滞后断裂,试验前后材料的抗拉强度、断裂韧性没有发生显著性变化;预制裂纹试样在恒载过程中裂纹向前扩展,扩展速率在10~(-5) mm/h的数量级;材料在工作介质中不存在应力腐蚀.     

基于小冲杆试验技术评估充氢锆合金材料的拉伸性能

锆合金由于具有良好的机械性能、耐腐蚀性能等而被用作反应堆内燃料元件的包壳材料。然而,Zr-4已不能满足核电技术在更高燃耗条件下的使用要求,因此,通过调控合金成分开发新型锆合金具有重要意义。本研究对Zr-4以及其他两种新型锆合金材料在室温、315℃条件下进行常规拉伸试验,结合计算结果分析了成分差异造成的沉淀相改变,提出了析出强化机制对于锆合金性能提升的重要意义。首次利用小冲杆试验对锆合金原样的力学性能进行测试,该工作确定了常规拉伸试验与小冲杆试验结果之间的经验公式中与锆合金材料本身相关的系数数值,验证了小冲杆试验用于锆合金拉伸性能评估的可行性。利用气相渗氢法在400℃对Zr-4以及其他两种新型锆合金进行充氢处理,并利用小冲杆试验对锆合金充氢试样的力学性能进行测试。结果显示,充氢锆合金在载荷-位移曲线的塑性失稳阶段出现“平台区”特殊现象。本论文对该现象进行探究,用金相分析表征了氢化物的形貌特征并对其含量进行了定量估算,推测氢化物与基体断裂韧性上的差异、氢化物相特殊的长链构型以及强取向性与这一现象有着重要关联。     

全层片组织TiAl基合金的室温断裂及断裂韧性

采用标准试样，测试了全层片组织Ｔｉ３３Ａｌ３Ｃｒ０．５Ｍｏ（质量分数，％，下同）合金的室温断裂韧性（以下简称断裂韧性）。在ＳＥＭ、ＴＥＭ下原位观察了板状和薄膜状合金试样中裂纹产生及扩展的动态过程。发现试样的断裂韧性因层片板条取向的不同而呈现各向异性，α２相层片板条对裂纹的扩展具有阻碍作用并使裂尖发生钝化。基于以上原因，使得层片板条取向呈现随机性的合金试样具有高的室温断裂韧性。     

在周期载荷下阻蚀涂层对д16T合金板材裂纹临界长度和断裂韧性的影响

航空技术装备的使用效率在很大程度上取决于航空结构中所采用的材料性能,及其抵制疲劳裂纹发展的能力。材料抵制裂纹能力的指标之一是在给定载荷下的临界裂纹长度和平面应变断裂韧性K_(1c)。本文研究的是在阻蚀液中进行冷磷化处理对Д16T合金5毫米厚板材试样临界裂纹长度和断裂韧性的影响。用30个阳极氧化的(供货状态)和30个阴蚀处理的试样(由同一板材制成)做了对比试验,试样宽45毫     

紧凑拉伸Be试样应力和断裂行为研究

利用X射线应力分析仪和在线加载装置测试了紧凑拉伸(compacttension,CT)Be试样缺口前端的应力分布,利用万能拉伸试验机、引伸计和扫描电镜研究了紧凑拉伸Be试样的断裂行为.研究表明:样品加工缺陷如缺口在载荷作用下形成应力集中,样品首先在此开裂,获得了紧凑拉伸铍试样的载荷-裂纹张开曲线和平面应变断裂韧性为15.4MPa√m.扫描电镜观察表明Be断口呈现解理特征,紧凑拉伸试样断口呈现出3个特征区.裂纹尖端扩展观察表明沿基滑移面形成解理微裂纹,这些微裂纹长大后受主应力控制.利用断裂韧性评估了Be材存在微裂纹时的断裂强度.     

烧结NdFeB永磁合金的断裂韧性

用Zwick万能实验机测量了烧结NdFeB永磁合金样品的三点弯曲强度、断裂韧性KIC及弹性模量E。在三点弯曲试验中测得塑变功为0，而断裂功与最大消耗功相等，表明烧结NdFeB合金具有极大的缺口敏感性。缺口敏感的脆性材料用单边切口粱法（SENB）测量KIC时要求较高的抛光精度，以消除表面划痕的影响，比较而言，压痕法测量KIC简便迅速。但由于所用NdFeB合金为烧结复相材料，其微观结构的不均匀性使实验数据较为分散。本研究借助Palmqvist裂纹系统半椭圆裂纹模型HV-KIC的计算公式，采用开微缺口的单边缺口粱试样的KIC和压痕硬度及系列压痕裂纹参数，经回归分析确定了该公式中的经验常数，使之能够较为准确地表征烧结NdFeB永磁合金的断裂韧性。     

Charpy裂纹试样的启裂点确定与动态断裂韧性

本文研究了用应变片测定Charpy裂纹试样动态断裂启裂点的方法,在不同的试验温度下,用双通道高速采样磁盘存储示波器同时测定了冲击试样的P—t曲线和裂纹尖端区域的名义ε—t曲线,结合动态有限元对Charpy裂纹试样冲击过程的模拟结果,分析了动态断裂韧性的测定方法,并指出了以往动态断裂韧性测定中存在的问题。     

CRACK INITIATION POINT DETERMINATION AND DYNAMIC FRACTURE TOUGHNESS FOR CHARPY PRE-CRACKED SPECIMEN

The method to detect the crack initiation point of Charpy pre-cracked specimen under dy-namie loading was studied using strain gauge.The load-time curve and nominal strain-timecurve at the crack tip for impact testing specinens may be shnultaneously measured bytwin-channel oscilloscope with high speed sampling and diskette storing.Based on the dynam-ic finite element simulation of impact response of Charpy specimen,the measuring method ofdynamic fracture toughness was analysed and some problems in previous dynamic fracturetoughness measurement were discussed.     

Simulation of the toughness of partially sintered ceramics with realistic microstructures

Increasing the pore volume fraction of porous ceramics enhances their functionality for a wide range of applications. However, the increased functionality comes at the expense of their toughness. Discrete element method (DEM) three-dimensional simulations, which operate at the length scale of individual particles, are used to investigate the toughness of microstructures typical of partially sintered ceramics. The method is first validated by comparing DEM simulations to the linear elastic fracture mechanics elastic solution at the crack tip of a pre-cracked numerical sample. The toughness of realistic random microstructures is then obtained using DEM simulations and compared to experimental data. Green density is shown to have a significant effect on toughness. Discrete simulations also suggest that the linear relationship between toughness and Young’s modulus is primarily linked to the size of the solid necks formed during sintering of particles.     

Fracture and fatigue crack-growth behavior of single crystal NiAl

The plane strain fracture toughness for pre-cracked, single crystal NiAl + 0.25%Fe was determined to be -6.5 MPa√m for the {110}t Ol crack orientation. The fatigue crack-growth behavior for this orientation was also studied. Fractography indicates the possibility of fatigue crack propagation on planes other than the preferred cleavage plane. Fatigue on these alternate planes also resulted in increased plastic deformation on the fracture surface. Finally, the transient crack-growth response following compressive overloads have been shown to be effective in reinitiating fatigue crack-growth. This effect may be due to a reduction in shielding from roughness induced crack closure.     

Quantitative evaluation of fracture,healing and re-healing of a reversibly cross-linked polymer

The repeated fracture–healing characteristics of 2MEP4F polymer, a cross-linked polymer based on Diels–Alder cycloaddition, are systematically and quantitatively evaluated using a sample geometry that allows for controlled incremental crack growth so that the cracked sample remains in one piece after the test, improving our ability to realign the fracture surfaces prior to healing. The specimens have been pre-cracked to a repeatable length to enable accurate comparison between virgin and healed fracture loads, and hence fracture toughness. Moreover, multiple data points are extracted from a single sample. In this paper, we report the results of our repeated fracturing and healing cycles. We have shown that healing at temperatures between 85 and 95 °C, after repeated fracture–healing cycles, results in full fracture toughness recovery and no dimensional changes due to creep. We have then calculated the fracture toughness after each fracturing and healing cycle, using a previously developed and tested model, arriving at consistent results for repeatedly healed 2MEP4F polymer. These results show a fracture toughness of about 0.71 MPa m^1/2 for this material at room temperature. We have also examined healing for shorter periods, as little as 30 min, using the same temperature–pressure cycle, arriving at essentially the same final results, i.e., full toughness recovery. That the healing process is largely independent of time is in contrast to the diffusion-controlled healing observed in the welding of thermoplastic polymers. Rather, it shows that the healing in the 2MEP4F polymer results from the repair of the broken Diels–Alder (cross-linking) bonds that seems to restore a molecular structure similar to that of the original virgin polymer with the same (or even slightly improved) overall macroscopic fracture resistance.     

Fracture mechanics tests for measuring the adhesion of magnetron-sputtered TiN coatings

Experiments for determining the adhesion strength between TiN coatings and high speed steel substrates have been performed by using the three-point bending test and a modified shear test. Sample preparation is easier for the modified shear test; however, the interpretation of results is more complicated. The three-point bending test yields fracture mechanics data such as the interface fracture energy G_c and the fracture toughness K_c. The most critical problem is to prepare a sample with a notch which initiates crack propagation at the film-substrate interface. Various sublayers with weak adhesion, such as carbon, oxide and metals, have been tested as a notch, as well as thin mechanical slits. The results obtained so far demonstrate that the adhesion strength depends on the substrate cleaning treatment and on the impurity gas content in the receiver. Typical values of the fracture toughness K_c for substrates that have not been sputter cleaned are in the range 1–3 MN m^-3/2. Advantages and disadvantages of the various methods used are analysed and critically discussed with respect to sample preparation, reproducibility and data evaluation.     

X70管线钢不同温度下断裂韧性实验研究

以三维弹塑性断裂理论为基础，对有限厚度板裂纹端部应力场、三维应力约束进行了分析，通过对不同厚度、不同初始裂纹长度在不同温度下三点弯曲试件的断裂韧性测试断口观测和理论分析获得如下结果：离面约束对裂尖应力场及断裂韧性有强烈的影响；断口均产生分层裂纹，其位置、大小和数量与试样厚度、温度和裂纹初始长度有关；温度较低时，分层裂纹距主裂纹根部一定距离，分层裂纹宽度较小，对厚度效应影响较小；温度较高时，分层裂纹首先出现在主裂纹根部，分层裂纹宽度较大且充分张开，降低了试样的有效厚度，对X70管线钢进行性能评价时必须考虑管道壁厚、层裂和环境温度的耦合作用。     

Interface fracture toughness and fracture mechanisms of thermal barrier coatings investigated by indentation test and acoustic emission technique

Interface fracture toughness and fracture mechanisms of plasma-/sprayed thermal barrier coatings (TBCs) were investigated by interfacial indentation test (IIT) in combination with acoustic emission (AE) measurement. Critical load and AE energy were employed to calculate interface fracture toughness. The critical point at which crack appears at the interface was determined by the IIT. AE signals produced during total indentation test not only are used to investigate the interface cracking behavior by Fast Fourier Transform (FFT) and wavelet transforms but also supply the mechanical information. The result shows that the AE signals associated with coating plastic deformation during indentation are of a more continuous type with a lower characteristic frequency content (30-60 kHz), whereas the instantaneous relaxation associated with interface crack initiation produces burst type AE signals with a characteristic frequency in the range 70-200 kHz. The AE signals energy is concentrated on different scales for the coating plastic deformation, interface crack initiation and interface crack propagation. Interface fracture toughness calculated by AE energy was 1.19 MPam_1/2 close to 1.58 MPam_1/2 calculated by critical load. It indicates that the acoustic emission energy is suitable to reflect the interface fracture toughness.     

MECHANICAL PROPERTIES OF A TYPE OF Al-ALLOY WELD

1.IntroductionIntheasrospaceindustry,Al-alloysarewidelyusedinstructuressuchasliquefiedgascontainersandcryogenicstabilizers.Asthesestructuresmustwithstandhighl0adsundercry0genicconditions,goodqualityALalloyweldisessentiaI.Theweldpropertiesincludingstrength,stiffnessandt0ughnessmustmeetthepracticalrequirements.Manypeople[1)2]havemadepromisingpr0gressesinthestudyofAl-alloyweldingtechniquesandAl-alloyweldproperties.However,therearestillgapsintheknowledgedatabaseandtheneedtocorrectthisunsatisfac…