考虑材料微观塑性损伤的焊接接头延性裂纹扩展行为分析

通过考虑材料微观塑性损伤，定量研究了焊接接头中强度匹配、试件几何形状对接头抗延性裂纹扩展行为的影响。对管线用钢焊接接头，测试了标准三点弯曲试样、同种接头及不同匹配状态的双边裂纹拉伸试样的阻力曲线。通过考虑材料塑性损伤的数值模拟，由标准三点弯曲试样的阻力曲线测试结果，得出反映材料微观损伤的特殊单元模型控制参量，再根据该参量对同种接头及不同接头强度匹配下双边缺口拉伸试件的阻力曲线进行了定量预测，其结果与试验结果相当吻合。结果表明，考虑塑性损伤的特殊单元模型能很好地描述接头的抗延性裂纹扩展阻力特性。     

Al—Li合金在慢拉伸条件下的形变与断裂行为

研究了Ａｌ－Ｌｉ合金在单向受力状态下的形变特征与断裂行为，测试了不同时效状态合金的慢拉伸裂纹扩展速率与应力强度因子的关系。结果表明，不同时效合金的范性形变机制、微观断裂模式和慢拉伸裂纹扩展阻力不同，它们视微观组织和晶界性态而异，应变局部化是低塑性的根源。     

室温剧烈冷变形和细化AZ31镁合金的加工硬化行为(英文)

在室温下,挤压态镁合金丝材最大累计面积减少61%,并对所得材料进行退火处理以细化晶粒。在室温下以恒定的应变速率对拉拔态和退火态试样进行拉伸试验,分析每个试样的拉拔面积减少量和平均晶粒尺寸与整个应力—应变曲线的关系。结果表明:冷变形试样具有恒定的弹性模量,但应力明显依赖变形程度。相应的θ—σ曲线(θ代表加工硬化速率,dσ/dε)表现为加工硬化扩展阶段Ⅱ和抑制阶段Ⅳ。再结晶试样随着晶粒的细化屈服应力增高且表现出典型的多晶材料加工硬化阶段:Ⅱ,Ⅲ,Ⅳ和Ⅴ。此外,随着晶粒的细化,由于晶界滑移的作用,第Ⅳ阶段出现下降。在冷拉和再结晶材料中,不同的硬化行为显示出不同的硬化机理。     

脆性材料的亚临界裂纹扩展和双向应力影响的数值模拟

脆性材料的破坏通常是瞬态快速断裂,亚临界慢裂纹扩展特别是宏观慢扩展,很难在实验中观察.它一定程度上制约了玻璃陶瓷等材料的失效过程分析和阻力特性研究.本研究利用MFPA2D(Material Failure Process Analysis)软件成功地模拟了玻璃类脆性材料在平面应力状态下承受单向、双向应力时的亚临界裂纹扩展全过程,着重研究了脆性材料的慢裂纹扩展和扩展速度所受双向应力的影响,并讨论这种影响的机理和作用.通过声发射特性的数值模拟得到脆性材料在单向及双向应力状态下的亚临界裂纹扩展长度-荷载步曲线.结果表明,平行于裂纹面的拉应力对裂纹扩展有一定程度的阻碍作用,而平行于裂纹的压应力对裂纹有驱动效果.该数值实验结果与相关的实验结果取得了较好的一致,表明数值实验的可靠性,从而为脆性材料的可靠性和寿命评价提供理论基础和手段.     

粉末冶金BeAl材料疲劳性能宏微观分析

用原位SEM观测研究了粉末冶金BeAl材料应力控制的疲劳机制.断裂时,低应力疲劳比高应力的平均累积塑性应变小得多.前者的累积塑性应变变化速率随着循环次数增大明显减小,后者则接近线性变化规律,表明高应力水平下的疲劳损伤累积更接近线性假设.低应力疲劳可从表面观测到一条疲劳主裂纹,由微观尺度下的累积塑性应变控制;高应力水平疲劳加载初期很快有宏观尺度的累积塑性应变,使Be与Al结合相界面同时产生较多裂纹.裂纹主要沿Al和Be结合强度较弱的表面路径扩展,随后沿纵深方向扩展,在材料内部裂纹扩展方向会变为与加载方向平行的Be基滑移面方向.当裂纹达到临界尺寸,局部塑性应变控制变为主应力方向控制,促使裂纹向前方扩展,宏观断口呈现出有层次的撕裂型.     

节镍型高氮奥氏体不锈钢的动态再结晶行为

对节镍型高氮奥氏体不锈钢在不同应变速率、不同变形温度下进行热变形模拟试验,并根据试验数据绘制应力-应变曲线。利用加工硬化率θ与应力-应变σ的曲线拐点和-dθ/dσ-σ曲线最小值点判定动态再结晶开始状态。确定动态再结晶临界应力σ_c和临界应变ε_c。同时计算出临界应变ε_c与峰值ε_p间的关系:ε_c≈0.378ε_p。构建出节镍型奥氏体不锈钢动态再结晶临界应变预测模型:lnε_c=0.026 85lnZ-4.7358。     

微观结构对板条马氏体启裂和裂纹稳态扩展的影响

研究了具有典型板条马氏体组织的低碳合金结构钢启裂和裂纹早期扩展阻力与微观组织结构、断裂过程中消耗功之间关系。结果表明：板条马氏体微观组织结构决定裂纹尖端塑性约束程度；微观组织结构特征不同，裂纹尖端塑性约束程度不同，启裂过程和裂纹早期扩展过程中消耗的弹性能和塑性功不同，导致材料抵抗裂纹稳态扩展的阻有较大差异。     

Ag/Ni20纤维复合材料触点制备过程中的塑性变形

以大变形Ag/Ni20纤维复合材料触点为实验样品,以扫描电镜等为分析手段,根据触点内部纤维组织的变形和分布情况,通过建立5区域应力应变模型,研究大变形金属纤维复合材料触点制备中的塑性问题。结果表明,触点在制备过程中存在压缩和拉伸两种变形,位于钉头与钉角处的第3区和钉头表面的第1区应力应变最集中,为拉伸变形,平均变形量在80%,是裂纹容易产生的地方。材料塑性应满足这两个区域的塑性拉伸变形要求,才能制备出高质量的触点     

7003铝合金应力腐蚀裂纹扩展的电化学阻抗谱分析

采用慢应变拉伸试验、扫描电镜(SEM)观察以及电化学阻抗谱分析,研究峰时效(PA)、双峰时效(DPA)、回归再时效(RRA)状态下的7003铝合金在3.5%Na Cl(质量分数)溶液中应力腐蚀裂纹扩展的特点。结果表明:应力腐蚀裂纹的过程分为两个阶段,即一次裂纹阶段与二次裂纹阶段,其中二次裂纹扩展阶段决定材料的应力腐蚀敏感性。裂纹扩展截面图及阻抗谱拟合结果显示:二次裂纹扩展速率vPAvDPAvRRA。断口形貌表明:峰时效状态二次裂纹沿晶倾向最为严重,双峰时效次之,回归再时效最轻微。结合应力-应变曲线分析:3种时效状态下的应力腐蚀敏感性为I_(SCC)(PA)I_(SCC)(DPA)I_(SCC)(RRA)。     

σ相在核电一回路主管道不锈钢中的脆化机理

研究了σ相对核电一回路主管道Z3CN20.09M不锈钢冲击韧性的影响,利用原位拉伸、显微硬度、断口形貌等手段分析了σ相的脆化机理.结果表明,σ相显著降低一回路主管道不锈钢的冲击韧性,时效处理Z3CN20.09M不锈钢中以σ相为主的由铁素体共析分解生成的(σ+γ2)结构的硬度远高于奥氏体基体,两者变形协调性差,(σ +γ2)结构阻碍位错滑移,提高材料强度,同时降低塑性;(σ+γ2)结构内部存在大量高能量σ/γ2和a/σ/γ2非共格界面,变形时应力在此处集中,成为潜在裂纹源,易萌生裂纹.高应变速率下,裂纹迅速在其内部产生、扩展是材料韧性降低、变脆的本质原因.     

DUCTILE CRACK INITIATION AND STEADY-STATE PROPAGATION OF HIGH STRENGTH STRUCTURAL STEEL

The resistance to crack propagation at earlier stage for a high strength structural steel withcertain ductility and its correlation to microstructures,stress states,deformation history andstrain characteristics have been investigated.The resistance to crack propagation is mainly de-termined by the plastic constrain ahead of the crack tip,the elastic energy and plastic workabsorbed in the stress-strain field.These are connected with the state function of triaxialstress.The deformation history and strain characteristic during deformation of material aredescribed by the flow line in which the deformation history and strain characteristic restrainthe crack initiation at stage Ⅱ and the crack propagation at stage Ⅲ.The strain hardeningrate may sensitively reflect the stress distribution and micro-fracture mechanism in the interi-or of material.     

预应变对工业纯钛TA2疲劳裂纹尖端应变场的影响

钛制承压设备在制造和服役过程中会产生塑性变形,影响其抗疲劳性能。为了研究预应变对工业纯钛TA2疲劳裂纹扩展行为的影响,本文对原始材料、预应变量为10%、20%和30%的材料进行疲劳裂纹扩展试验,结合数字图像相关(Digital image correlation, DIC)技术获取裂纹尖端应变场,研究预应变对裂纹尖端应变场的影响。结果表明：随着预应变量的增加,稳态疲劳裂纹扩展阶段的扩展速率减小,裂纹张开位移减小,裂纹张开载荷增大,裂纹尖端塑性区、塑性变形量以及循环塑性应变累积量均减小。因此预应变抑制了裂纹尖端的塑性变形及循环塑性累积,致使裂纹闭合效应愈加明显,从而抑制了裂纹的扩展。研究结果对钛制承压设备的安全评定具有参考意义。     

Low Cycle Fatigue Behavior of HT250 Gray Cast Iron for Engine Cylinder Blocks

The strain-controlled low cycle fatigue properties were evaluated on specimens of HT250 gray cast iron (GCI) at room temperature. The material exhibited cyclic stabilization at a low strain amplitude of 0.1% and cyclic softening characteristic at higher strain amplitudes (0.15-0.30%). At a representative total strain amplitude (0.30%), the hysteresis loops of HT250 GCI were asymmetric with a large amount of plastic deformation in the compressive phases. Furthermore, the hysteresis loop became larger in both width and height with increasing total strain amplitude (from 0.10 to 0.30%), and tended to exhibit a clockwise rotation. The fatigue crack propagation mechanisms were different at various total strain amplitudes, where high stress concentration due to dislocation pile-up favored fatigue crack initiation in the examined HT250. Finally, the roughness-induced crack closure was a key to determine the crack growth rate as well as fatigue life.     

Deformation near parallel cracks close to interface in bonded dissimilar plate

Tensile tests are carried out by using rectangular plate specimens extracted from the explosion clad plate. Artificial parallel cracks are made perpendicularly to the explosive interface in each specimen. When a tensile load is applied perpendicularly to the crack plane, strain, plastic deformation near cracks and crack opening displacement are examined by experiments as well as elasto-plastic finite element analysis. The effects of the material inhomogeneity, the change of material characteristics, the residual stress and the interaction between parallel cracks on the deformation behavior are revealed. The plastic zone around parallel cracks, at relatively low applied stress level, grows mainly ahead of and between two cracks. The interaction between parallel cracks on the crack opening displacement can be explained from the square of stress intensity factor at the low applied stress level and from the plastic deformation at the high applied stress level.     

金属材料在循环载荷下塑性变形传播对疲劳性能的影响

对疲劳裂纹萌生寿命和扩展速率的研究表明，４０Ｃｒ钢调质和正火态存在加载频率效应，而１Ｃｒ１８Ｎｉ９Ｔｉ钢固溶处理态未发现加载频率效应。引起加载频率效应的实质是，随加载频率不断提高，金属材料塑性变形传播速度不断减小，从而改变塑性区特性。     

Zum Einfluß der mechanischen Belastung auf die interkristalline Spannungsrißkorrosion von unlegiertem Stahl in Carbonat/Bicarbonat-Lösung

Effect of mechanical stress on intergranular stress corrosion cracking of unalloyed steel in carbonate/bicarbonate solution Intergranular stress corrosion cracking of a carbon steel in 0.5 M NaHCO₃ + 0.5 M Na₂CO₃ solution at 70°C was examined by normal, interrupted and cyclic CERT testing. Within the critical potential range, the lengths of intergranular cracks increase with increasing strain rate, and decrease when the material is cold worked. The fracture time indicates that the crack velocity increases with increasing strain rate. This is also observed with tensile specimens subjected to interrupted and cyclic strain rates. Detailed information on crack propagation and threshold stress levels can be obtained by interrupted and cyclic CERT tests. Intergranular cracks arise if surface region of the material undergoes plastic deformation in excess of a definite amount. In the case of a 60°-notch specimen the plastic zone must be greater than 0.3 mm. For cyclically strained specimens the crack velocity can markedly decrease with time or can become even zero as a consequence of strain hardening. From this one can conclude that stress corrosion cracking can only occur if stress levels are very high.     

核电压力容器600合金小裂纹应力腐蚀开裂扩展速率定量预测

小裂纹应力腐蚀开裂(SCC)在核电关键构件(NPPs)的全寿命衰减过程有重要影响。通过将薄膜滑移-溶解/氧化模型与弹塑性有限元(EPFEM)相结合,定量预测核电反应堆压力容器(RPV)中小裂纹SCC扩展速率。根据裂纹尖端力学场的分析,确定以裂纹尖端应变率来表征小裂纹的萌生和扩展,并通过距离扩展小裂纹尖端特定的r₀处的塑性应变(de_p/da)来近似表征裂纹尖端应变率。提出了基于弹塑性断裂力学的动态裂纹扩展和准静态裂纹扩展两种方法计算塑性应变(de_p/da),并进行两种计算方法比对塑性应变随裂纹长度变化的敏感性,得到两种计算方法之间差异的同时,也确定小裂纹扩展的塑性应变变化比长裂纹更为敏感。小裂纹的SCC扩展速率大于长裂纹的SCC扩展速率,距裂尖的特征距离r₀是重要的影响因子,鉴于特定距离r₀难以确定,建议通过将相同环境和相同材料下的SCC实验数据结合单边拉伸试样的有限元数值计算结果来确定。研究结果能够实现核电关键结构材料的SCC扩展速率定量预测及服役压力容器的安全评价。     

Corporate Effects of Temperature and Strain Range on the Low Cycle Fatigue Life of a Single-Crystal Superalloy DD10

Low cycle fatigue behavior of a nickel-based single-crystal superalloy DD10 was investigated at 760 and980 °C under different strain ranges. Results show that the fatigue life(Nf) of DD10 alloy exhibits different temperature dependence under various strain ranges. Under low strain range, the alloy exhibits a longer Nfat 760 °C than that at980 °C. However, under high strain range, a reverse result is obtained. This difference can be attributed to the change of dominant damage modes under various test conditions, which is manifested in different modes of crack initiation(crack nucleation and its early propagation). At 760 °C, the crack initiates at pores in subsurface due to local stress concentration.This process is mainly controlled by plastic amplitude and plastic property, but not affected by oxygen-induced damage before the crack propagates to the surface. At 980 °C, the crack initiates at surface instead of pores due to the more homogeneous plastic deformation and the disharmony between the external oxidation layer and the bulk material when the strain amplitude is high. At that temperature, the process is mainly controlled by oxidation damage and strain amplitude simultaneously. Therefore, under high strain range, the crack initiation is much easier at 760 °C due to plastic deformation and the poor plasticity, while under low strain range obvious oxidation damage at 980 °C may accelerate the crack initiation.     

安全端焊接接头镍基合金600SCC裂纹扩展行为研究

镍基合金作为压水堆一回路安全端焊接接头焊缝的常用材料,由于严苛的服役环境以及焊缝处材料力学性能的不均匀使得镍基合金极易发生应力腐蚀开裂现象,对核电安全运行造成很大影响。为了解材料宏观结构参量变化(包括材料塑性性能以及应力强度因子K)对SCC裂纹扩展速率的变化,本文通过建立镍基合金600不同宏观结构参量下的SCC裂纹扩展有限元模型,分析了镍基合金600不同塑性以及载荷参数变化对裂尖塑性区和拉伸塑性应变的影响,结果表明塑性区尺寸及裂尖拉伸应变受到裂尖应力强度因子、屈服强度及硬化指数的影响,其中裂尖应力强度因子的影响较大,同时与屈服强度成反比,应力强度因子和硬化指数成正比；通过比较不同应力强度因子下计算所得SCC扩展速率结果和高温水环境下SCC扩展速率实验,获得了符合镍基合金600的特征距离r₀的取值范围；研究结果能为核电镍基合金600的高温水环境下SCC速率预测提供一定的科学依据。     

慢加载条件下氢对裂纹萌生与扩展的作用

在扫描电镜原位拉伸条件下，研究了氢原子对正火态的Ｃｒ－１Ｍｏ钢裂纹萌生与扩展的作用结果发现：氢原子使缺口根部韧性裂纹萌生时的临界塑变降低，并使裂纹的扩展方式从沿最大切应力方向的连续扩展改变成沿最大主应力面的跳跃扩展．试验未发现明显的氢致塑变特征．文中对氢原子的影响机理进行了分析．