Prediction of Strain Hardening and Fatigue Life of Cyclically Unstable Materials

The data on cyclic deformation of materials with different cyclic properties are analyzed. The relationship between the level of additional strain hardening and the mechanical characteristics of a material is established. The proposed models take into account the influence of the form of stressed state on strain hardening and durability under conditions of nonproportional cyclic deformation. The models are constructed on the basis of the analysis of the experimental data on cyclic deformation of 304 stainless steel. The prediction of strain hardening and durability is performed with separate analysis of the influence of shape of the cycle and the form of stressed state. To take into account the influence of shape of the cycle, we use the coefficient of disproportionality of the cycle and the parameter of sensitivity to disproportionality. To take into account the form of stressed state, we use the coefficient of the form of stressed state and the corresponding parameter of sensitivity to the form of stressed state. The results of prediction of both strain hardening and durability of materials on the basis of the proposed phenomenological models reveal their fairly high efficiency. __________ Translated from Problemy Prochnosti, No. 5, pp. 118 – 130, September – October, 2005.     

Determination of Residual Stress in Beams Under Bauschinger Effect Using Surface Strain Measurements

Abstract:  The paper explores the use of surface strain measurements to determine residual stresses induced in plastically bent beams under Bauschinger effect using minimal stress–strain data. The hardening behaviour of some materials in tensile and compressive are dissimilar specially under Bauschinger effect. It is therefore ideally necessary to obtain both tensile and compressive stress–strain data in order to determine residual stresses in such materials. The paper shows that the compression stress–strain data can be implicitly determined from tensile and bending stress–strain results by using equilibrium considerations. This removes the need for separate compression tests to be carried out. The agreement of the derived stress–strain data in compression and of residual stress with experimental results is encouraging.     

带状组织对微合金钢应变局部化及应变强化行为的影响

目的 研究不同加载方向下带状结构对微合金钢塑性变形及应变强化行为的影响。方法 将X80管线钢加热到1200 ℃后分别进行炉冷和空冷热处理,得到多边形铁素体/退化珠光体和粗/细铁素体晶粒带状组织以用于实验研究。将单轴微拉伸实验和数字图像相关方法相结合,通过零变形实验分析系统精度,获取RD、45°和TD取向带状组织试样的拉伸变形规律。同时,在DIC全场变形测量结果与实验数据统计分析结果的基础上,提出了可描述应变强化程度的参量K_f,用于对比和分析不同加载方向下带状结构对应变强化行为的影响。结果 零变形实验中位移平均值和标准差的最大值分别为0.004 pixels和0.006 pixels,系统精度较高。加载方向会影响带状组织的塑性变形演化情况,45°试样中产生了明显与带状取向一致的应变分布特征。在相同应力下45°试样首先达到了较大的应变强化程度,其抗变形能力最低。结论 不均匀带状组织对局部变形的影响程度与2个组成带的力学性能差异有直接联系,参量K_f可在一定程度上描述应变局部化引起的各向异性特征。     

Strain Hardening of Initially Isotropic Metals under Loading along Slightly Curved Trajectories

By the example of martensitic steel we study regularities of strain hardening under loading along two-link broken paths corresponding to slightly curved strain paths. It is shown that the loading surface separating the domains of elastic and elastoplastic strains (yield surface) is displaced in the direction of a vector connecting the surface center with the loading path image point, while the shape of its frontal part remains unchanged. The yield surface center displacement versus the intensity of accumulated plastic strains is described by a curve invariant to the loading path.     

Low-Temperature and Strain Hardening of Austenitic Steels at Temperatures of about 4.2 K

We analyze the results of experimental investigations of the complex effect of various modes of prestraining and deep cooling and the strain rate on the mechanical characteristics of alloys used in cryogenic equipment. It is demonstrated that the strength characteristics of some materials can be significantly improved by strain and low-temperature hardening while preserving rather high levels of plasticity. The changes in the mechanism of deformation of metals at a temperature of 4.2 K caused by the phenomenon of intermittent yield are responsible for the anomaly of the Bauschinger effect.     

拉伸应变硬化指数的测试及方法比较

针对IS010275：2007及GB／T5028-2008中对于材料在拉伸时呈现锯齿屈服效应曲线如何计算拉伸应变硬化指数n值的问题,就未呈现锯齿屈服效应的曲线和呈现锯齿屈服效应的曲线,采用国际线性回归法计算行nline ,并用Zwick试验机采用回归法测试行ntest 以及用差分平均法测试nsingle,同时对三种测试方法进行比较。结果表明：对于不呈现锯齿屈服效应的曲线,可以同时采用上述三种方法;而对于呈现锯齿屈服效应的曲线,不能采用差分平均法测试nsingle,只能采用国际线性回归法计算nline 以及采用Zwick试验机用回归法测试nrest。且对于呈现锯齿屈服效应的曲线测试nline和ntest,回归区间不宜太小。     

超高韧性水泥基复合材料多缝开裂特性及其自生愈合

应变硬化水泥基复合材料(Strain hardening cement-based composites,SHCC)是超高性能水泥基材料的一种。通过三点弯曲加载试验,分别对普通砂浆和SHCC试件诱导开裂,对两种材料的开裂特性及其裂缝分布规律进行了研究。结果表明,SHCC材料试件与普通水泥砂浆试件相比具有更高的承载力和应变能力,初始开裂荷载约为普通砂浆试件的2.5倍。SHCC材料中的PVA纤维的桥接作用有效延迟了裂缝的产生与扩展,将普通砂浆试件中数量少而大的裂缝转化为多而细的微小裂缝,呈现多缝开裂特性,产生的裂缝宽度符合正态分布规律,且90%的微小裂缝小于30μm。这些微小裂缝在潮湿环境中产生一定程度的自生愈合,在水中的愈合程度和速度均高于在潮湿空气中。     

Strain Softening and Hardening Behavior in AZ61 Magnesium Alloy

The deformation behavior of AZ61 Mg alloy during hot deformation has been investigated in wide temperature and strain rate range by a Gleeble simulator. Specimens are deformed in compression in the temperature range of 523～673 K and at strain rates of 0.001～1 s-1. It is found that the flow curves exhibit a peak and then decrease towards steady-state of classical DRX, which decrease with rising temperature and decreasing strain rate. The deformation behavior of the specimens can be attributed to the occurrence of strain hardening and softening. As stress decreases, the strain hardening rate declines at a fast rate when temperature rises or strain rate decreases. The shapes of θ-σ curves indicate some important features such as subgrain formation, the critical stress, the peak stress and steady stress. The onset of DRX can be determined by the point of inflection on θ-σ or Inθ-σ curves.     

10CrNiMo结构钢低周疲劳过程中的应变滞后特性

在6级轴向逐级增加的系列恒应变幅(0.3%~1.0%)控制下,室温空气中,采用光滑圆形横截面试样,研究了10CrNiMo结构钢在低周疲劳作用下的应力-总应变及应力-塑性应变的滞后回线特性。结果表明:在小应变幅(0.3%~0.4%)控制下,虽然弹性应变占绝对优势,但塑性应变并不为零,且逐级增长;而在大应变幅(0.6%~1.0%)控制下,塑性应变增长较快,并逐渐占主导地位。无论应变幅大小,只须几个循环,材料便进入稳定状态,而初始循环与失稳循环却表现出不稳定的特征。     

多边形铁素体/针状铁素体双相管线钢的应变硬化行为

对X70管线钢进行临界区热处理,制备出四种铁素体/针状铁素体(PF/AF)体积分数不同的双相管线钢。用电子背散射衍射(EBSD)分析了PF含量对这种双相管线钢的晶粒尺寸、大角度与小角度晶界的比例以及几何必要位错密度(GND)的影响;通过Hollomon和修正C-J方程分析了这种钢的应力比与应变硬化指数(n值)的关系,以及不同PF体积分数双相管线钢的塑性变形和应变硬化的机理。结果表明,PF/AF双相管线钢的应变硬化能力几乎与应力比无关,而应变硬化指数与均匀延伸率表现出特定的线性关系。随着PF体积分数的提高,这种钢的颈缩点后移且应变硬化行为由两阶段向三阶段转变。PF体积分数的改变,对其第I和第II阶段的应变硬化能力有显著的影响。     

应变速率对一种无铼单晶高温合金低周疲劳性能的影响

研究了一种无铼镍基单晶高温合金在1223 K、不同应变速率(5×10^-4s^-1、1×10^-3s^-1、5×10^-3s^-1、1×10^-2s^-1)条件下的低周疲劳行为。结果表明:在四种应变速率条件下,合金均表现出循环稳定。随着应变速率的增加,合金的疲劳寿命逐渐增加,且其半寿命稳定滞后回线环内面积逐渐减少,表明低应变速率合金更容易积累蠕变塑性变形。疲劳裂纹源均萌生于试样表面,随着应变速率的增加,疲劳过程中产生的塑性变形越来越少,疲劳裂纹扩展区的面积逐渐增大。低应变速率时,较大的塑性变形导致合金取向发生明显的偏转,诱发多滑移系开动进而形成位错网;反之,高应变速率时,合金没有产生明显的塑性变形,只有单一方向的位错塞积形成位错束。     

厚壁方矩形管冷弯效应对比分析

为了对厚壁型钢管的冷弯工艺改进、性能分析、结构强度设计和数值模拟结果的验证提供重要依据,进行了冷弯厚壁钢管上截取的平板件、弯角件、母材和相应方、矩形截面短柱的力学性能试验研究,获得了短柱全截面屈服强度相对于母材强度的提高值。分别运用北美等国厚壁冷弯型钢规范、国内薄壁冷弯型钢规范以及相关文献中的修正规范,对我国厚壁冷弯方矩形型钢管全截面强度进行设计和对比分析。结果表明:因冷弯工艺不同,国外公式计算的结果高于国内短柱实测值,国内厚壁冷弯型钢的强度设计不能照搬现有的国外厚壁冷弯型钢设计规范;国内薄壁公式能否适用要视型钢的冷作硬化效应程度决定;原料的强屈比和冷弯应变程度越大,则冷作硬化效应越大;焊接热使板件受到低温"退火"的作用不可忽略,其常常导致竖直配辊冷弯厚壁矩形型钢时,两竖直侧平板件强度低于母材。     

利用反射式衍射光栅进行瞬态应变测量

介绍了利用反射式衍射光栅进行瞬态应变测量的方法,在一维细长杆中,通过测量杆未端的绝对速度,就可以算出动态应变和应变灵敏度系数K,文章对该方法进行了理论计算和实验结果分析,给出了结论和进一步研究方向。     

应变比对定向凝固高温合金DZ125低循环疲劳行为影响的研究

对定向凝固镍基高温合金DZ125在850℃和980℃应变控制的低循环疲劳(LCF)行为进行研究。对比分析应变比R=-1和R=0情况下的应力-应变迟滞环、循环平均应力以及断裂循环次数(寿命)的差异。结果表明:在所研究的两个温度水平下,相同应变范围条件下应变比R=-1和R=0的低循环疲劳寿命差别不大。利用扫描电子显微镜对不同应变比作用的试样断口进行观测,初步分析裂纹萌生机理。     

沉淀硬化不锈钢弹簧低周疲劳性能研究

研究了Ｃｒ１２Ｍｎ５Ｎｉ４Ｍｏ３Ａｌ沉淀硬化不锈钢弹簧低周疲劳性能和疲劳断口，指出在５２０℃时效时，由δ铁素体析出的大片状Ｊ相和α＇相诱发疲劳裂纹是降低疲劳寿命的主要因素；害３００℃时效处理可大幅度提高疲劳寿命。