Deformation and fracture toughness of a bulk amorphous Zr–Ti–Ni–Cu–Be alloy

The flow behavior and fracture toughness of two different plate thicknesses (i.e. 4 and 7 mm) of a bulk amorphous Zr–Ti–Ni–Cu–Be alloy was investigated. It is shown that the flow/fracture stress was independent of superimposed hydrostatic pressure over the range 50–575 MPa, suggesting that the flow behavior follows the von Mises criterion. However, the macroscopic orientation of the fracture plane relative to the stress axis was strongly affected by changes in stress state, suggesting some normal stress dependence to the flow/fracture behavior. The fracture behavior was also studied on both notched and precracked bend bars for both plate thicknesses. The average fracture toughness obtained from seven fatigue precracked specimens taken for both plate thicknesses was 17.9±1.8 MPa√m, while the notched toughness obtained on specimens with notch root radii ranging from 65 to 250 μm taken from both plate thicknesses were in the range of 9l–131 MPa√m.     

Determination of dynamic mode I fracture toughness of rock at ambient high temperatures using notched semi-circular bend method

To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock, dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system. Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves. A high-speed camera was used to check the fracture characteristics of the specimens. The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave. The dynamic force balance is achieved even when the specimens are at a temperature of 400 °C. The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25?400 °C, and high temperature has a strengthening effect on the dynamic fracture toughness.     

基于单边缺口拉伸试样的环缝接头阻力曲线确定方法对比

近些年单边缺口拉伸(SENT)试样被用于管道基于应变的设计与评估,并且相关机构发展了一些断裂韧性的测试标准,包括DNVGL-RP-F108,CANMET,BMT,USP以及英国国家标准BS 8571等. 但相关测试标准之间在测试原理、测试方法以及断裂韧性计算方法上还存在较大差异,有必要对它们之间的区别进行进一步的研究. 文中以API X80管线钢环缝接头为研究对象,对夹持式单边缺口拉伸(SENT)试样的裂纹尖端张开位移(δ)和J积分阻力曲线不同确定方法进行了对比. 结果表明,通过拟合计算J-R曲线、δ-R曲线以及初始断裂韧性值J_0.2和δ_0.2,给出了SENT试样断裂韧性阻力曲线计算的推荐公式.     

MoSi_2复合材料断裂韧性的测量及评价

对压痕法和单边切口梁法所测MoSi2 基复合材料的断裂韧性作了研究和分析 ,提出了压痕法测量MoSi2 基复合材料断裂韧性的计算修正式。结果表明 :单边切口梁法的测量结果随试样切口宽度的增加而增大 ,压痕法测量的结果随计算式和试验载荷的不同而异。使用上述修正式将缩小压痕法与SENB法的测量偏差     

Loss of crack-tip constraints for shallow cracks in bending

Single-edge notched bars (SENB) in bending with a/W ratios ranging from 0.05 to 0.5 are examined for fracture toughness in terms of the J-integral approach. The corresponding results indicated that, for a a/W ratios less than 0.3, there is a significant loss of J-dominance, which is attributed to the impact of plastic deformation on the cracked face. For larger a/W ratios, J-dominance is maintained into a large-scale yielding regime. According to the recently developed two-parameter criterion (J,Q), compressive Q-stress was interpreted as an indication of low crack-tip stress triaxiality for shallow cracks, whereas positive g-stress was associated with high crack-tip stress triaxiality for deep cracks. For the material properties and specimen geometries considered herein, a fracture toughness locus is constructed in terms of the (J-Q) parameters for each of the a/W ratios. The overall fracture data, which are in agreement with those predicted by other approaches, appear to provide a rigorous framework for interpreting the effect of loss of crack-tip constraint in elastic-plastic fracture analysis.     

Fracture Properties and Fracture Surface Morphologies in Rubber-PMMA Composites

Correlations between fracture properties and microscopic features were investigated using notched (both blunt notched and sharp notched) specimens of rubber-PMMA composites in very low-to-medium range of cross-head speeds. It is found that: (1) the decreasing trend of crack initiation region size with increasing speed correlates with the fracture elongation; (2) the decreasing trend of SWZ size with increasing speed and decreasing rubber content can be correlated with that of calculated plastic zone and the trend correlates also with that in modulus of toughness; (3) the order of degree of material plastic flow at notch tip in the polymers correlates with the corresponding order of their K values.     

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。     

ADI的冲击韧度和断裂韧度

论述了ADI的冲击韧度和断裂韧度。室温有缺口和无缺口ADI的冲击韧度比铸钢,锻钢要略差,但约是普通珠光体球铁的3倍,ADI的冲击韧度虽随温度降低而降低,但在-40℃条件下仍保持大约室温冲击韧度的70%。断裂力学性能是更重要的安全设计和失效分析依据,无论哪一种断裂韧度,ADI的试验数据都好于普通球铁,相当于或好于强度相当的铸钢和锻钢。     

Fracture of severely plastically deformed Ta and Nb

A comparative study focusing on the fracture characteristics of severely plastically deformed (SPD) Nb and Ta is presented. Since many SPD-processes lead to typically elongated microstructures, orientation dependent fracture toughness measurements with three different crack orientations were performed. Among the various testing orientations, two exhibit a considerably high fracture toughness in both materials. The high fracture resistance in these orientations will be discussed on the basis of the significance of crack deflection and delamination processes. In both materials the lowest fracture toughness was found in the third testing orientation parallel to the grain elongation. However, between Nb and Ta the fracture toughness differs in this orientation by a factor of three, rendering Nb even parallel to the grain alignment to a quite fracture resistant material.     

氧化铝基陶瓷材料断裂韧性的测量与评价

用压痕法和单边切口梁法测量了氧化铝基陶瓷材料的断裂韧性,对这两种方法所测得的结果进行了分析对比,确定了最适用的压痕法计算公式。研究发现,原料粉末的起始粒度对用压痕法计算材料KIC的最优公式选择并无影响,但对材料的断裂韧性及抗弯强度有不同程度的影响,亚微米第二相增强微米基体的材料综合力学性能最佳;TiN的加入使Al2O3-TiC基体材料的断裂韧性有所增加;在较小的压痕载荷下计算出的材料的断裂韧性值更接近实际;在用不同的公式计算同一材料的断裂韧性值时,断裂韧性值与压痕载荷之间有相似的关系曲线。     

温度对焊接热模拟X80管线钢断裂韧性的影响

通过试验和3D有限元模拟相结合的方法分析了焊接热模拟X80管线钢在不同温度（-90,-60,-30和0℃）下的断裂韧度.文中选取标准三点弯曲试样在不同温度下进行断裂韧度测试,同时对测试件进行三维有限元实体建模与分析计算.结果表明,X80管线钢的断裂韧性随温度减低显著减小,并使其倾向于脆性断裂.材料在不同温度下的真实应力-应变曲线行为从光滑拉伸到断裂力学试样具有良好的可传递性,温度对材料的硬化行为没有明显影响,有限元计算的结果表明,应用该方法可以准确地计算X80钢焊接热影响区不同温度下的CTOD值.     

Accurate Evaluation of the Mechanical Properties of Grey Cast Iron

Abstract

Grey cast iron has been regarded, in some circles, as being brittle, low in reliability and one where the usual design equations cannot be applied. The reason is usually attributed to the presence of randomly distributed graphite flakes. This presentation discusses, both experimentally and theoretically, the notch strength, bending strength, and the scattering of tensile strength of grey cast iron, and suggests a new approach based on plasticity theory for the accurate evaluation of these characteristics. Experiments showed that grey iron has a very low notch sensitivity in circumferentially notched bars, but that strength decreases in edge-notched plates. At the temperature of liquid nitrogen, notch sensitivity is very high. In ductile iron, strength increases in notched specimens. These results have been interpreted by the stress distribution, taking into account non-elastic stress/strain behaviour, and using a fracture criteria with an over-stressed depth, δ· The δ is a necessary region for fracture and its minimum value is related to the graphite eutectic cell size. Nominal bending strength, which is about twice that of the tensile strength and which varies with the beam height and sectioned geometry, was also explained by these considerations. The scatter in strength, expressed by the coefficient of variation, was 2–9% in grey iron, a figure not as large as expected. The value was higher in the case of small test pieces, whilst the average strength was unchanged. These results were easily explained by the strength- hardness relationship and by a relationship derived from the strength theory of bundled threads, in which the fracture process is similar to that of cast iron.

In the analyses, the fracture behaviour of cast iron was assumed to be non-elastic, and ductile rather than elastic and brittle. It is shown that reasonably accurate strength evaluation of cast-iron structures is possible by considering these characteristics.     

Effects of tungsten on the hydrogen embrittlement behaviour of microalloyed steels

The effects of tungsten (W) additions (0, 0.1, 0.5 and 1 wt.%) on the hydrogen embrittlement behaviour of microalloyed steels were systematically investigated by means of slow strain rate tests on circumferentially notched cylindrical specimens, and the mechanism of hydrogen-induced embrittlement was discussed. W addition is found to increase the activation energy of hydrogen desorption. Microstructural features affect the hydrogen embrittlement behaviour and fracture modes of microalloyed steels. It is suggested that the hydrogen-induced embrittlement in the studied microalloyed steels with different W additions is caused by the combined effects of decohesion and internal pressure in the presence of hydrogen.     

Effect of Particle and Carbide Grain Sizes on a HVOAF WC-Co-Cr Coating for the Future Application on Internal Surfaces: Microstructure and Wear

The use of nanoscale WC grain or finer feedstock particles is a possible method of improving the performance of WC-Co-Cr coatings. Finer powders are being pursued for the development of coating internal surfaces, as less thermal energy is required to melt the finer powder compared to coarse powders, permitting spraying at smaller standoff distances. Three WC-10Co-4Cr coatings, with two different powder particle sizes and two different carbide grain sizes, were sprayed using a high velocity oxy-air fuel (HVOAF) thermal spray system developed by Castolin Eutectic-Monitor Coatings Ltd., UK. Powder and coating microstructures were characterized using XRD and SEM. Fracture toughness and dry sliding wear performance at three loads were investigated using a ball-on-disk tribometer with a WC-Co counterbody. It was found that the finer powder produced the coating with the highest microhardness, but its fracture toughness was reduced due to increased decarburization compared to the other powders. The sprayed nanostructured powder had the lowest microhardness and fracture toughness of all materials tested. Unlubricated sliding wear testing at the lowest load showed the nanostructured coating performed best; however, at the highest load this coating showed the highest specific wear rates with the other two powders performing to a similar, better standard.     

On the role of martensitic transformation on damage and cracking resistance in TRIP-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J_R curves and CTOD measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite.     

TMCP钢焊接热影响区局部脆化区断裂韧度测试

针对强度(屈服点应力)级别为420-460MPa的三种TMCP钢,探讨通常的焊接热影响区局部脆化区断裂韧度测试方法的可行性及存在问题。由于多层焊热影响区组织复杂,需进一步研究确认试验中得到的断裂韧度能否代表局部脆化区的韧度。试验结果发现,对多层焊热影响区试样,试验后必须进行刨面(Sectioning)分析,首先检查疲劳裂纹前沿是否位于所测试的目标区域;其次当有延性裂纹扩展时,检查延性裂纹扩展是否偏离目标区域。当满足这些条件时,所测试的断裂韧度才能代表目标区域的韧度值。同时也试验研究了焊缝强度匹配对热影响区断裂韧度的影响。     

焊接热循环对X80管线钢粗晶区韧性和组织的影响

利用焊接热模拟技术、光学金相、透射电子显微镜和示波冲击韧度试验、断裂韧度试验研究了焊接热循环对X80管线钢粗晶区韧性和组织的影响。试验结果表明,在六种热循环参数下,X80管线钢模拟粗晶区具有不同的显微组织,当焊接热循环参数较小时,以下贝氏体和板条马氏体为主,随着热循环参数的增大,以粒状贝氏体为主,且其中的M—A岛的形态由细短条状转变成大长条状或大块状,分布由晶界转向晶内,同时数量增多,韧性恶化。     

两种Al2O3结构陶瓷断裂强度的统计分析

研究了结构陶瓷材料的统计分布规律。首先通过实验测试了两种Ａｌ２Ｏ３陶瓷光滑试件和不同缺口半径试件的断裂强度;对实验数据用正态分布和Ｗｅｉｂｕｌｌ分布分别进行了分析,发现对于缺口件,用正态分布描述其断裂强度分散性要比Ｗｅｉｂｕｌｌ分布好一些。这也说明在对陶瓷材料结构件进行设计时,应考虑用正态分布表述其断裂强度的分散性和存活概率。     

Toughness and microstructural parameters in thermomechanically processed 2091 Al-Li alloy

The effect of microstructural features on the toughness of 2091 A1-Li alloys processed with various thermomechanical schemes was analyzed. Grain size and the distribution of precipitates were varied by thermomechanical processing. Dynamic fracture toughness tests were done to evaluate the toughness of these materials using a Computer Aided Instrumented testing system. The δ (A1₃Li) and S’ (Al₂CuMg) precipitates, which are primary strengthening precipitates, were confirmed by TEM analysis in the under-aged 2091 Al-Li alloy. From analysis of the relation between toughness and the microstructural factors induced by thermomechanical processing, the toughening of the alloy was found to be associated with the refinement of the subgrains. The strength and toughness parametzrs were well related to subgrain size.