X80管线钢的氢脆性能

制备了单边缺口拉伸(SENT)试样,在0.5 mol/L H2SO4+3 g/L NH4SCN溶液中,以10 mA/cm2的电流密度充氢4 h,采用拉伸试验和三点弯曲试验考察了氢脆对X80管线钢断裂韧性的影响.结果显示:充氢SENT试样的断裂韧性和承载力显著低于母材SENT试样的,使用三点弯曲试验测定的断裂韧性结果较为...     

单边缺口拉伸试样的断裂韧性计算方法对比

在工程临界评估（engineering critical assessment）中,裂纹尖端张开位移（CTOD值）的精确性将极大影响设计安全裕度与服役寿命.单边缺口拉伸（SENT）试样裂纹尖端的应力应变场与管道在实际服役状况下相似,被认为比较适用于测量管道CTOD值.目前有几种针对SENT断裂韧性的计算方法,但是并没有统一的标准.文中采用API X70管线钢,进行SENT试样断裂韧性试验并对比各计算方法与双刀口法之间的区别.采用Crackwise 4.0评估了不同计算方法得到韧性值对裂纹极限尺寸的影响.结果表明,几种CTOD计算方法相较于双引伸计法都有较大误差,断裂韧性的精确度对于ECA评估极限裂纹尺寸有极大影响.     

残余应力对管线钢韧性断裂的影响

以弹塑性断裂力学理论为基础,应用ABAQUS有限元软件,采用固有应变法引入残余应力,并基于细观塑性损伤模型研究了残余应力对管线钢韧性裂纹扩展阻力曲线的影响.模型中选择了具有不同深浅裂纹的单边缺口弯曲（SENB）试样和DNV推荐准则中用于管线钢断裂评估的单边缺口拉伸（SENT）试样,并对这两种断裂力学试样在大范围屈服条件下的裂纹扩展阻力曲线（CTOD-R曲线）行为进行了对比分析.结果表明,对于深裂纹试样,残余应力降低了韧性裂纹扩展阻力曲线,而浅裂纹试样受残余应力的影响可忽略不计.     

基于拘束的X80钢裂纹扩展阻力曲线构建

对于管道中的裂纹,由于其裂纹尖端拘束水平低,使用单边缺口弯曲试样、紧凑拉伸试样所测得的裂纹扩展阻力曲线对管道进行断裂评估,得到的结果趋于保守. 文中采用裂纹深度不同的单边缺口拉伸试样,探讨了裂纹深度对裂纹扩展阻力曲线的影响. 采用有限元分析,计算了裂纹深度不同的单边缺口试样的裂纹尖端拘束水平. 最后,基于裂纹尖端的拘束水平,所构建的裂纹扩展阻力曲线能更加准确地表征管道中裂纹的扩展阻力曲线.     

面外拘束效应对单边缺口拉伸试样断裂韧性的影响

采用API X90管线钢,对不同厚度的单边缺口拉伸(SENT)试样进行断裂韧性实验,并结合全场应变测量技术和断口分析,研究SENT试样面外拘束(试样厚度)对裂纹尖端张开位移(CTOD)的影响。结果表明,在相同的载荷水平下,随着试样厚度的增加,最大纵向应变峰值急速下降,高纵向应变区域由裂纹尖端迁移至距裂纹面一定距离的未开裂侧,裂纹尖端的塑性变形能力降低。CTOD值对厚度变化很敏感,随着试样厚度的增加而降低。当试样厚度宽度比≥4时,临界CTOD值达到下平台并保持基本不变。因此,在管线工程设计中,可将试样厚度宽度比等于4作为SENT试样断裂韧性与试样厚度无关的一个参考试样尺寸。     

EH36钢埋弧焊焊缝金属组织临界断裂韧性

通过对典型海洋结构用钢EH36埋弧焊焊缝金属进行单边缺口拉伸试验(single edge notched tensile,SENT)研究焊缝金属中针状铁素体(acicular ferrite,AF)和先共析铁素体(proeutectoid ferrite,PF)的临界断裂韧性. 结果表明,AF的断裂韧性要高于PF,主要原因在于AF内部具有较高密度的位错及位错能,在外力作用下,裂纹尖端极易发生塑性变形而钝化;相反,PF组织较纯且均匀,位错密度较小,裂纹尖端并未发生钝化. 这种组织的不均匀性导致了焊缝金属断裂韧性较大的分散性,主要归结于试样预制裂纹尖端位置的微观组织性能.     

P92钢高温断裂韧性的试验

文中以P92耐热钢为研究对象，研究了不同尺寸的开侧槽和不开侧槽的紧凑拉伸试样在630 °C下的断裂韧性，得到了相应的的阻力曲线及断裂韧度J_Q. P92钢在高温下为典型的韧性断裂机制. 基于三维有限元计算对侧槽的拘束效应进行表征，结果表明，侧槽可明显提高试样的拘束水平，试样尺寸越小，J阻力曲线差异越明显. 随载荷增大，非侧槽试样的拘束变化更明显，开侧槽将导致试样阻力曲线不同. 试样尺寸及结构的改变，对韧性材料的阻力曲线影响较大，而对断裂韧度值影响较小. 试样开侧槽之后裂纹扩展更平齐，可优化断裂韧性试验过程.     

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.     

PZT压电陶瓷的应力腐蚀

使用单边缺口拉伸试样对PZT-5压电铁电陶瓷进行恒载荷下的应力腐蚀研究,介质分别为水、甲醇和甲酰胺,结果表明, PzT-5压电陶瓷在这三种介质中均会发生应力腐蚀开裂.归一化应力腐蚀门槛应力强度因子分别为KISCC/KIC=0.66(水),0.73(甲醇)和0.75(甲酰胺).其中断裂韧性:KIC=(1.34±0.25)MPa·m1/2.     

准静态断裂韧度标准GB/T 21143—2014中几处改动的讨论

最新版本的准静态断裂韧度标准GB/T 21143—2014与上一版本GB/T 21143—2007相比进行了较大改动。通过试验对两个版本的标准进行了比较和讨论,对比了阻力曲线中J与δ的计算公式。发现新标准将系数g2(a0/W)改为g2(a/W),造成阻力曲线与J0.2BL的结果较旧标准偏大,但是特征值J0.2BL与美标相比依然较为保守。同时发现,新标准在δ0与δ的计算公式中增加了旋转修正,这一改动会使二者的结果更加准确。本次标准修订还增加了被测试样的形状和新的测量方法,这为小尺寸试样的测试带来了极大的便利。新标准对测量裂纹长度方法的示意图进行了修改,但在测量高韧度材料试样时仍然存在无法观测到裂纹的问题,建议参照BS 7448的裂纹测量方法进行修改。     

高速列车用6061和7N01铝合金焊接接头断裂韧性分析

按照国家标准GB/T 21143-2007《金属材料准静态断裂韧度的统一试验方法》求得高速列车用6061和7N01铝合金焊接接头中焊缝、热影响区和母材3个区域的CTOD值δ_c和J积分值J_c,借助于数理统计的方法对焊接接头的断裂韧性进行了分析,并结合金相组织和断口形貌分析了它们之间关系.结果表明,对数正态分布对小样本断裂韧性数据拟合程度最好;6061和7N01铝合金焊接接头中各区域δ_c值和J_c值热影响区最大,焊缝次之,母材最小;6061和7N01铝合金焊接接头比较,7N01铝合金焊接接头中母材和焊缝的δ_c值和J_c值都优于6061铝合金焊接接头的相同区域;热影响区的δ_c值6061铝合金优于7N01铝合金;而热影响区的J_c值7N01铝合金优于6061铝合金.     

Out-of-Plane Constraint Effect on the Fracture Toughness of Single Edge Notch Tension SpecimensEI北大核心CSCD

The crack-tip stress and strain fields of single edge notch tension (SENT) specimen are similar to those of the full-scale pipe containing surface cracks under longitudinal tension and/or internal pressure. It is well known that material's fracture toughness is not constant, and the specimen size has a significant influence on fracture toughness. It is thus essential to consider the transferability from fracture specimens in laboratory testing to practical structures, i.e., size effects or constraint effects. However, the specimen dimensions for SENT specimens recommended by current design procedures have not validated the out-of-plane constraint effect on the fracture toughness. In this work, the effect of specimen thickness on the crack tip opening displacement (CTOD) of SENT specimen was investigated using an API X90 grade steel. Full-field deformation measurement by digital image correlation (DIC) technique and stretching zone width (SZW) examination were performed to analyze the size effects on fracture toughness. The results show that the critical crack initiation toughness is highly sensitive to specimen thickness, and decreases significantly as specimen thickness increases until the thickness-to-width ratio (B/W) equals to 4, beyond which the effect of specimen thickness becomes relatively weak. As the specimen thickness increases, the maximum longitudinal strain and stretching zone width decrease sharply, and the location of high-strain zones changes significantly; when B/W >= 3, strain is initiated from the area oppo-site the cracked side rather than from the crack tip, indicating a strong loss of plasticity for thicker specimens. A dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint into account for SENT specimen.     

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.     

Properties of ZrO2–Al2O3 composite as a function of isothermal holding time

Zirconia and alumina based ceramics present interesting properties for their application as implants, such as biocompatibility, good fracture resistance, as well as high fracture toughness and hardness. In this work the influence of sintering time on the properties of a ZrO₂–Al₂O₃ composite material, containing 20 wt% of Al₂O₃, has been investigated. The ceramic composites were obtained by sintering, in air, at 1600 °C for sintering times between 0 and 1440 min. Sintered samples were characterized by microstructure and crystalline phases, as well as by mechanical properties. The grain growth exponents, n, for the ZrO₂ and Al₂O₃ were 2.8 and 4.1, respectively, indicating that different mechanisms are responsible for grain growth of each phase. After sintering at 1600 °C, the material exhibited a dependency of hardness as function of sintering time, with hardness values between 1500 HV (120 min) and 1310 HV (1440 min) and a fracture toughness of 8 MPa m^1/2, which makes it suitable for bioapplications, such as dental implants.     

退火工艺对激光选区熔化成形Ti6Al4V合金组织及室温力学性能的影响

以Ti6Al4V球形粉末为原料,利用激光选区熔化成形方法制备了Ti6Al4V合金试样,采用光学显微镜、扫描电镜及力学性能测试等手段,研究了退火工艺对Ti6Al4V合金室温力学性能及组织的影响规律。结果表明: SLM成形沉积态Ti6Al4V合金室温抗拉强度超过1200 MPa,而平均断后伸长率仅为4.0%;在650 ℃下进行真空退火处理,合金的抗拉强度仍保持在1200 MPa左右,规定塑性延伸强度R_p0.2高于1150 MPa,但试样的断后伸长率<10%;而在750及800 ℃下进行真空退火处理,合金试样的抗拉强度降至1100 MPa左右,规定塑性延伸强度高于1050 MPa,伸长率达到甚至超过10%,材料的综合强韧性得到明显提升。随着真空退火加热温度和保温时间的增加,SLM成形Ti6Al4V合金原始β晶界逐渐变模糊,晶粒趋向于等轴化。与此同时,快速冷却转变的α′针状马氏体未出现明显地粗化。     

亚温淬火对NiCrMo-3堆焊层耐磨性能的影响

研究了亚温淬火对NiCrMo-3堆焊层显微组织、力学性能及耐磨性能的影响。结果表明,与未淬火试样相比,亚温淬火后NiCrMo-3堆焊层的晶粒较小,呈絮状分布。堆焊层表面的主要物相为金属间化合物CrO₂和NiFe₂O₄,基体间存在Ni、Cr元素形成的间隙化合物。亚温淬火后堆焊层的硬度较高并具有良好的耐磨性能,且强度提高,但韧性下降。亚温水淬时的摩擦因数在摩擦过程中减小,拉伸断口中的脆硬相之间有韧性带状组织。     

淬回火对10Cr5MoVRE钢组织性能的影响及其第二相析出行为

以10Cr5MoVRE钢为研究对象,对轧态及淬回火(QT)态试样组织性能进行对比研究,利用TEM和能谱仪对QT态试样析出物进行观察分析。力学性能检测结果表明,QT态试样获得了较好的强韧性配比,相较于轧态试样,QT态试样伸长率提高了93.8%,0 ℃条件下的冲击吸收能量提高了6倍以上。借助SEM观察的轧态及QT态10Cr5MoVRE试验钢低温冲击断口形貌,轧态试样的低温冲击断口形貌为脆性解理断裂,QT态的为韧性韧窝断裂。借助OM观察分析,轧态试样组织为粒状贝氏体而QT态为回火索氏体。QT态10Cr5MoVRE钢第二相析出物的TEM观察分析结果表明,碳化物析出相主要是碳化钒、碳化钼及碳化铬形成的复合相,显微夹杂物析出相则是Al₂O₃、MnS、Ca₃(PO₄)₂及稀土化合物等组成的复合夹杂物。     

Ti对Nb-20Si-5Al超高温合金性能的影响

采用放电等离子烧结法(SPS)制备了Nb-20Si-5Al-xTi(x=0,18,20,22,摩尔分数)超高温合金,研究了Ti加入量对Nb-20Si-5Al合金的室温断裂韧度和高温抗氧化性的影响。结果表明,随着Ti加入量的增加,超高温合金的相组成由Nbss、Nb₅Si₃和Al3Nb相转变成为Nbss、(Nb,Ti)5Si3和Ti相。Ti能明显改善Nb-20Si-5Al超高温合金的断裂韧度和高温抗氧化性能,随着Ti加入量的增加均先提高然后降低,在Ti加入量为20%时,合金断裂韧度最大,为7.41 MPa·m^1/2,相比未加Ti时提高了约56.9%,其高温氧化速率最低,为0.72×10^-4g²/(cm⁴·h)。添加Ti元素后,其氧化产物中出现Ti₂Nb₁₀O₂₉、TiNb₂O₇、TiO₂等,可以提高其氧化膜的致密性,从而提高高温抗氧化性能。