表面质量和缺口对Ti80合金弯曲性能的影响

利用数字图像相关方法研究了Ti80合金表面质量和缺口对其三点弯曲性能的影响，采用扫描电镜及电子背散射衍射方法对断口进行观察。结果表明：表面抛光试样的抗弯强度最大，表面打磨及缺口试样的抗弯强度略有降低，没有明显的缺口强化效应，断裂应变随着表面质量的下降及缺口尺寸的增加而迅速减小；应力集中系数越大，试样的变形区域越小，应变强化能力越弱；Ti80合金的断口呈韧窝形貌，存在大量的小角度晶界。     

温度对聚氯乙烯缺口冲击强度及断面粗糙度的影响

研究了温度对聚氯乙烯ＰＶＣ缺口冲击强度，断口形貌特征及断面粗糙度的影响，结果表明，ＰＶＣ缺口冲击强度和断面粗糙度参数ＲＳ聚温度变化，在脆化温度Ｔｂ处存在极小值，前者取决于真实断裂表面积大小，后乾受控于局部高弹变形及裂纹分叉扩展综合作用，低温冲击断面上易观察到裂纹分叉扩展形成的弧形条纹及分层。     

温度对聚氯乙烯缺口冲击强度及断面粗糙度的影响

研究了温度对聚氯乙烯 PVC 缺口冲击强度、断口形貌特征及断面粗糙度的影响。结果表明,PVC 缺口冲击强度和断面粗糙度参数 R_s 随温度变化,在脆化温度 T_h 处存在极小值,前者取决于真实断裂表面积大小;后者受控于局部高强变形及裂纹分叉扩展综合作用。低温冲击断面上易观察到裂纹分叉扩展形成的弧形条纹及分层。     

航空钛合金板胶接表面粗糙度的超声测量

采用一种新的超声频谱法快速测量航空钛合金板经喷砂预处理后的胶接表面粗糙度,并引入表面面积均方根粗糙度系数ST来表征材料表面三维微观形貌.以航空Ti-6Al-4V钛合金胶接板为测量试样,超声换能器接收测试试样表面的反射回波,并计算得到AR参数谱.以反射脉冲AR参数谱和声波镜面反射理论为基础,建立了有关脉冲波声反射系数和表面均方根粗糙度系数ST的数学模型.利用表面粗糙度系数理论模型.数值计算反射回波的理论频谱曲线,并与实测反射回波AR谱进行拟合.利用最小值搜索算法,处于最佳拟合时的表面粗糙度系数ST即为试样胶接表面的测量结果.实验表明,超声反射频谱法测量结果与轮廓仪测量结果符合得很好,该测量方法在材料或零部件表面粗糙度在线测量中具有广泛的应用前景.     

航空钛合金板胶接表面粗糙度的超声测量

采用一种新的超声频谱法快速测量航空钛合金板经喷砂预处理后的胶接表面粗糙度,并引入表面面积均方根粗糙度系数Sr来表征材料表面三维微观形貌．以航空Ti-6Al—4V钛合金胶接板为测量试样,超声换能器接收测试试样表面的反射回波,并计算得到AR参数谱．以反射脉冲AR参数谱和声波镜面反射理论为基础,建立了有关脉冲波声反射系数和表面均方根粗糙度系数Sr的数学模型．利用表面粗糙度系数理论模型．数值计算反射回波的理论频谱曲线,并与实测反射回波AR谱进行拟合．利用最小值搜索算法,处于最佳拟合时的表面粗糙度系数Sr即为试样胶接表面的测量结果．实验表明,超声反射频谱法测量结果与轮廓仪测量结果符合得很好,该测量方法在材料或零部件表面粗糙度在线测量中具有广泛的应用前景．     

L245无缝钢管在4MPa氢气环境下的氢脆敏感性研究

为了考察低钢级管线钢在中压氢气环境下的氢脆敏感性，采用光滑和缺口试样慢拉伸试验方法，结合宏观和微观断口表征与分析，研究了L245钢在空气和4 MPa氢气中的氢脆敏感性。结果表明：L245钢光滑试样在4 MPa氢气中慢拉伸的抗拉强度、断面收缩率和延伸率的损失率分别为5.2%、4.8%和-0.1%,而缺口试样在4 MPa氢气中慢拉伸的抗拉强度、断面收缩率和拉伸位移的损失率分别达到19.1%、45.6%、15.4%,即发生了氢脆。L245钢光滑试样在空气介质中的慢拉伸断裂方式为韧性断裂；开缺口后起裂源开始于缺口位置，然后以准解理断裂的方式向内部扩展，加速试样断裂过程，但整体仍然表现为韧性断裂。相比之下，当L245钢光滑试样在4 MPa氢气环境中时，会在颈缩位置产生裂纹源；开缺口后，从缺口位置向内部产生的准解理断裂区域显著增加，使得断裂过程急剧加快，而心部则发生了氢致韧断。开缺口后L245钢在4 MPa氢气中的断裂方式为准解理断裂与氢致韧断相结合。     

缺口形状对铁基烧结材料冲击韧度的影响

冲击韧度是评价材料性能的一个重要指标,同种材料在不同缺口形状下的冲击韧度是不同的。对不同缺口形状的铁基烧结材料进行了室温冲击试验,研究了缺口形状对冲击韧度、冲击断口形貌以及断口附近裂纹微观形貌的影响,试样的缺口形状有半圆型、U型、Ⅰ型、V型和无缺口5种,其中V型缺口的缺口角度又有45°,90°,120°3种。结果表明:无缺口试样的冲击韧度最大,其平均值为9.86J·cm~(-2),远远超出有缺口试样的;半圆型缺口试样的冲击韧度为2.70J·cm~(-2),U型缺口试样的冲击韧度为2.46J·cm~(-2),较半圆型缺口试样的略小;Ⅰ型和3种V型缺口试样的冲击韧度相差甚小,且比U型缺口试样的要低;无缺口试样的冲击断口有明显塑性变形,断口附近产生的裂纹多,分布范围较广;有缺口试样的冲击断口塑性变形不明显,断口附近裂纹少,且分布范围局限在断口处。     

JFE-HITEN610U2L钢组织形貌及其对韧性的影响

对JFE-HITEN610U2L钢的显微组织和断口形貌进行了分析。该钢的显微组织以针状铁素体为主，具有良好的强韧性。当针状铁素体比较粗大时，冲击试样的断裂方式从韧窝断裂转变为解理断裂。从材料使用的安全性考虑，建议对该材料增加显微组织、晶粒度和冲击试样断口形貌等内容的检验。     

SiC_p与SiC_w增强铝基复合材料的冲击试验

采用仪器化冲击试验对碳化硅颗粒(SiC_p)与碳化硅晶须(SiC_w)增强铝基复合材料的冲击能量与缺口形状、摆锤刃口半径的关系进行了研究,并利用扫描电镜和能谱仪对冲击断口的形貌及成分进行了分析。结果表明:与不开缺口试样相比,采用U型缺口试样进行试验时,得到的冲击总能量较稳定,不同摆锤刃口半径之间的冲击总能量相差较小,推荐采用U型缺口试样进行SiC_p与SiC_w增强铝基复合材料的夏比冲击试验;SiC_p增强铝基复合材料的断裂以SiC颗粒解理脆性断裂为主,SiC_w增强铝基复合材料的断裂是脆性断裂+韧性断裂的混合型断裂。     

Stellite12钴基合金热循环冲击前后拉伸断裂机理研究

对不同缺口的Stellite12钴基合金试样(700℃/20℃进行不同次数的热循环冲击和未冲击)进行原位拉伸,并结合试验数据的分析以及断口形貌的扫描电镜观察,分析了Stellite12钴基合金热循环冲击前后的拉伸断裂过程和断裂机理。结果发现:热循环冲击后不同半径试样的断裂过程略有不同,热循环冲击后的小圆弧缺口试样在缺口根部产生表面微裂纹,试样边缘及微裂纹两侧产生氧化微孔;原位拉伸时,该试样热冲击过程产生的裂纹先向试样厚度方向扩展,待厚度方向贯通,然后裂纹尖端的基体发生变形、黑相(白相)穿晶开裂、少量沿氧化微孔裂开,试样瞬间发生断裂;而经历热循环冲击后的大圆弧试样表面并未产生明显的裂纹,拉伸加载过程经历大圆弧根部基体变形、黑白相内开裂、边缘氧化微孔张开,试样突然断裂;对于未冲击试样,在加载过程中,试样的断裂过程经历基体变形、黑白相内部开裂,能量聚集到一定程度试样突然断裂。对于未热冲击的三种不同试样其断裂过程基本类似,仅仅是由于小圆弧半径的试样应力集中程度更大,从而使得其断裂应力低于平板以及大圆弧试样。     

分形维数与D6AC钢的韧性

本文研究了超高强度钢Ｄ６Ａｃ的断裂韧性、冲击韧性和拉伸面缩率与断口分形维数的关系。分别用二次电子线扫描和数字图象法测定断裂韧性试样的断口分形维数，得出试样的韧性与分形维数Ｄｓｅ、Ｄ_Ｈ和Ｄ_Ｌ成正比关系，即韧性随分形维数增大而增加。试样断口的粗糙度由夹杂物引起的差异小于金相组织不同引起的差异时，使用数字图象法测得的分形维数与韧性的线性关系优于二次电子线扫描的结果。     

用示波落锤试验法评价14Mn VTiRe钢的韧脆断裂行为

采用示波落锤试验法对 5mm热连轧 1 4MnVTiRe钢的韧脆断裂行为进行了研究并对落锤撕裂试样的动态断裂特征进行了评价。结果表明 ,当裂纹扩展功远大于裂纹萌生功时 ,试样处于韧性断裂状态 ;当裂纹扩展功小于裂纹萌生功时 ,试样处于脆性断裂状态 ;当裂纹扩展功与裂纹萌生功相当时 ,试样处于混合断裂状态。该结果与按标准测定的剪切面积百分数有良好的对应关系 ,说明示波落锤试验法可作为一种评定材料韧脆断裂特征的实用方法     

Influence of stress intensity and crack speed on fracture surface topography: mirror to mist to macroscopic bifurcation

The development of roughness on the fracture surfaces of a brittle, glassy, epoxy resin from the mirror-to-mist transition to macroscopic bifurcation has been investigated using optical microscopy, scanning electron microscopy (SEM) and contact and non-contact laser profilometry. Most of the observations were made on specimens fractured in edge-notched tension. In a series of tests the initial crack length was varied to obtain fracture surfaces formed by accelerating and decelerating cracks without macroscopic bifurcation (specimen A) and by cracks which accelerated continuously to macroscopic bifurcation (specimen B). Some observations were made on specimens tested in compact tension to study changes in fracture surface topography associated with crack arrest in stick-slip fracture. There was a close correlation between the topographical detail revealed by the different techniques. In specimen A the roughness increased progressively from the mirror-to-mist transition and reached a maximum before decreasing as the crack decelerated. The topographical features revealed by optical microscopy and SEM were the same for accelerating and decelerating cracks at the same roughness value. In specimen B the roughness increased continuously to macroscopic bifurcation. There was a close similarity between the topographical features at all levels of roughness. A simple model for the basic step involved in roughness formation is presented which involves an element of the crack tip tilting out of the plane of the main crack before stopping (micro-bifurcation). The scale of micro-bifurcation ranged from 3 m in the early stages of mist, when the crack velocity was close to 10% of the shear wave velocity, to the full width of the specimen (6 mm) at macroscopic bifurcation. The micro-bifurcation process develops from crack surface undulations and does not involve micro-crack nucleating ahead of the main crack. It is concluded that the relationships between crack velocity and dynamic stress intensity, and the value of the limiting crack velocity, must be interpreted in terms of micro-mechanical processes at the crack tip which are strongly dependent on specific material characteristics.Emeritus Goldsmiths' Professor Metallurgy, University of Cambridge, U.K.     

凝聚包覆法粉末NBR对SAN的增韧作用

用凝聚包覆法制得的粉末NBR(PNBR)与SAN共混,测定了共混体的Charpy冲击强度,研究了PNBR的性质对共混体冲击韧性的影响。结果表明,含5份包覆剂的线型PNBR对SAN有最佳的增韧效果,当其用量为30份,其共混体的无缺口冲击强度达96kJ/m2。NBR大分子的交联作用及包覆剂含量大于5份都会降低PNBR对SAN的增韧效果。用扫描电子显微镜对共混体冲击断面形貌的分析表明,共混体在无缺口冲击试验条件下出现脆韧转变,其增韧机理以剪切屈服为主兼有银纹化;在缺口冲击试验条件下则为脆性断裂,其增韧机理以裂纹终止为主兼有银纹化。     

Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.     

Study of the fracture of ferritic ductile cast iron under different loading conditions

This work is a continuation of the studies presented in a recent paper by the authors, where the fracture surfaces of pearlitic ductile cast iron under different loading conditions were exhaustively analysed. In this study, fracture surfaces of ferritic ductile cast iron (or ferritic spheroidal graphite cast iron) generated under impact, bending and fatigue loading conditions were characterised and compared. The fracture surfaces were characterised qualitatively and quantitatively from the observation under a scanning electron microscope. The fracture mechanisms in each case were identified. For impact tests, as test temperature increases, the dominant fracture mechanism changes from brittle to ductile. For bending tests, a fully ductile fracture micromechanism dominates the surface. In fatigue tests, the surface shows a mix of flat facets that appear to be cleavage facets and ductile striations, but the typical fatigue striations are not easily found on the fracture surface. Methodologies for the determination of the macroscopic direction of main crack propagation in both ductile and brittle failure modes are proposed. These allow identifying main crack propagation direction with good approximation. The results are potentially useful to identify the nature of loading conditions in a fractured specimen of ferritic spheroidal graphite cast iron. The authors believe that it is necessary to extend the methodologies proposed in samples with different geometry and size, before they can be used to provide additional information to the classical fractographic analysis.     

A grain-scale study of spall in brittle materials

The evolution of spall for a brittle material is investigated under variance of anisotropy, grain boundary fracture energy, and loading. Because spall occurs in the interior of the specimen, fundamental studies of crack nucleation and growth are needed to better understand surface velocity measurements. Within a cohesive approach to fracture, we illustrate that for anisotropic materials, increases in the fracture energy cause a transition in crack nucleation from triple-points to entire grain boundary facets. Analysis of idealized flaws reveals that while crack initiation and acceleration are strong functions of the fracture energy, flaws soon reach speeds on the order of the Rayleigh wave speed. Finally, simulated surface velocities of spalled configurations are correlated with microstructural evolution. These fundamental studies of nucleation, growth, and spall attempt to link atomic separation to the macroscopic spall strength and provide a computational framework to examine the evolution of spall and the impact on the simulated surface velocity field.     

沥青基炭／炭复合材料的弯曲断裂特征

以1KPAN基高强度炭纤维为增强体、以调制中温煤沥青为基体前躯体,采用压力浸渍－炭化工艺制备出了不同密度二维沥青基炭／炭复合材料。经过对复合材料试样进行的弯曲试验表明,其弯曲断裂特征与材料密度具有密切的联系。根据弯曲强度－位移曲线,高密度复合材料表现为脆性断裂,而低密度复合材料表现为韧性断裂。从弯曲断面的SEM图片来看,脆性断裂时的断面比较平整,韧性断裂时断面上有大量炭纤维拔出。炭／炭复合材料的断裂破坏过程实质上就是基体裂纹在材料内的扩展过程,其扩展的途径与界面结合状况有关。裂纹沿界面的扩展将引起基体与纤维的脱粘,脱粘又导致纤维与基体之间的相对滑动,这种相对滑动将吸收相当一部分能量,从而可以延缓材料的断裂过程,起到韧化作用。     

Effects of heat treatment processes on microstructure and properties of 2·25Cr–1Mo–0·25V HSLA steels

Abstract

In the present paper, the effects of the heat treatment processes with two conditioning treatments and four quenching–tempering processes on the mechanical properties of 2·25Cr–1Mo–0·25V high strength low alloyed (HSLA) steel are investigated. The results show that the conditioning treatments have obvious effects on the low temperature impact energy but little effect on the tensile strength. The elevation of the final austenitising temperature increases the strength, whereas it results in the decrease in the low temperature impact energy due to the coarse microstructure. The results of the fracture surfaces analysis further make sure that the fracture surfaces of tensile specimens all exhibit ductile characters with a lot of dimples. However, the fracture surfaces of impact specimens exhibit two typical fracture characters, i.e. the ductile and brittle fracture surface corresponding to the fine and coarse microstructures respectively. In addition, the elongation and reduction in area seem to be insensitive to the heat treatments. Meanwhile, the impact fracture mode is more sensitive to the grain size and not to the low temperature impact energy.