40CrNiMoA钢不同微观组织超声疲劳寿命研究

应用超声疲劳试验技术,对三种不同微观组织状态的40CrNiMoA钢在超声振动载荷(f＝20kHz,R＝-1)下的疲劳性能进行测试分析.结果表明:在超声振动条件下,具有强韧配合的微观组织超声疲劳寿命得到显著提高;不同微观组织的40CrNiMoA具有两种类型的S-N曲线:持续下降型和阶梯下降型,试样在107以上仍会发生疲劳断裂.扫描电镜对疲劳断口的分析表明:在超声疲劳载荷循环下,疲劳裂纹起源于试样表面.初步探讨了超高周疲劳断裂机理.     

超声冲击对纯钛材焊接接头疲劳性能的影响

目前,有关超声冲击法对工业纯钛焊接接头疲劳性能改善效果的影响研究报道不多。采用超声冲击处理方法对工业纯钛TA2焊接接头进行全覆盖强化处理,通过疲劳对比试验分析了不同超声冲击处理时间对TA2焊接接头疲劳性能的影响;通过接头微观组织、断口形貌观察分析了超声冲击处理提高TA2焊接接头抗疲劳断裂性能的微观机理。结果表明,在200 MPa应力条件下,原始焊态试样平均历经11 645次循环就已断裂,经超声冲击处理3 min的焊接试样平均历经23 424次循环后才断裂,疲劳寿命提高约1倍;在超声冲击过程中,试样表层受到大的应变量、高应变速率和重复载荷的作用,使得形变孪晶不断增加;重复载荷的作用,可以再次在内部产生更加细小的孪晶,孪晶与孪晶间重复交割导致晶粒进一步细化。     

超声冲击处理MB8镁合金十字接头的表层组织及疲劳性能

采用超声冲击方法对MB8镁合金十字接头焊趾处进行超声冲击处理,对比测试焊态及冲击处理态接头的表层组织和疲劳性能。结果表明:超声冲击处理可以在MB8镁合金焊接接头表面获得纳米晶组织。在循环寿命为2×106条件下,焊态试样的条件疲劳强度为32.07MPa,冲击处理态试样的条件疲劳强度为41.88MPa,提高了30.59%。未冲击接头疲劳断裂大多发生在焊缝缺陷处,冲击处理后的接头则发生在热影响区。超声冲击处理不仅可以大幅提高MB8镁合金十字接头的疲劳寿命,还可以改变接头疲劳断裂位置。此外,热影响区也是MB8镁合金十字接头疲劳断裂的薄弱区域,这与热影响区晶粒粗大有很大的关系。     

铁素体钢断裂韧度尺寸效应的小冲孔试验

对六种不同厚度的试样进行了小冲孔试验,并进行了有限元模拟,此外还通过扫描电镜观察并分析了断裂面的微观结构。试验结果显示,试样的SP断裂变形能和断裂韧度随试样厚度的增大而增大。提出了裂纹起始遵循断裂应变准则及裂纹扩展遵循断裂能密度准则的观点。有限元模拟结果与试验结果基本吻合。断裂面呈典型的韧性断裂特征,试样变形后的半球状外表面布满微小的褶皱状突起和微裂纹。     

300M钢拉伸试样异常断裂原因分析

300M钢在进行拉伸试验时,在试样夹持头过渡区发生异常断裂。通过化学成分分析、宏观检验、金相检验、硬度测试与扫描电镜分析的方法,对试样异常断裂的原因进行了分析。结果表明:该试样的异常断裂与试样热处理过程中产生增碳现象和试样夹持头台阶设计不合理有关。     

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     

Q690D高强钢基于连续损伤模型的断裂破坏预测分析

高强钢材在实际钢结构中已经开始逐步得到应用,该文针对Q690D高强度结构钢材,进行了钢材圆棒试样在单调加载和超低周循环加载下的断裂破坏试验,研究了试样裂纹的起始位置,分析了加载制度对试样承载能力和变形能力的影响,并采用扫描电镜观察了试样断口的微观形貌,断口呈现韧窝形式的延性断裂特征。基于Q690D钢材缺口圆棒试样的单调拉伸试验结果,结合有限元分析,标定了钢材的连续损伤模型参数。最后,应用钢材的连续损伤模型,对圆棒试样和带初始间隙试样在不同加载制度下的断裂破坏进行预测分析,得到试样的裂纹起始位置、荷载-位移曲线、断裂位移和疲劳寿命均与试验结果吻合良好。     

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。     

铸态稀土镁合金热拉伸变形断裂机理

采用扫描电镜对稀土镁合金Mg-13Gd-4Y-2Zn-0.4Zr在不同温度下的拉伸试样进行断口分析。结果表明:稀土镁合金在热拉伸温度越高时,塑性越好。同一温度下,随着试样角度的增大,最高载荷逐渐减小,断裂由韧性断裂逐渐变成解理断裂。0°试样在热拉伸时发生的断裂属于韧性断裂,主要是由正应力引起的。45°和90°试样的断裂属于剪切断裂,主要是由剪应力引起的。     

回火处理对一种新型钴基合金表面结构和压缩力学性能的影响

本文对钴基合金试样进行了不同时间的回火处理,对压缩应力-应变曲线进行了分析,通过对回火前后钴基合金的微观组织和断口形貌的扫描电镜观察,解释了该钴基合金回火处理前后的压缩断裂过程变化和压缩性能差异。研究发现,回火处理前后的压缩试样断裂过程有较大差异:未经回火处理的试样,其压缩断裂过程表现为典型的脆性断裂,断裂面与压缩方向呈45°夹角;而经过回火处理的压缩试样,首先在试样表面产生平行轴线方向的裂纹,同时在试样顶端形成一个45°锥形断裂面,最后试样断裂。回火过程中生成的氧化层是回火试样压缩断裂过程变化的主要原因,但是对整个压缩曲线的变化起主导作用的仍然是材料的基体组织。回火后的钴基合金组织中fcc-Co相比例显著增加,Cr_(23)C_6碳化物颗粒尖角消失,形状变得圆滑,显著改善了该钴基合金的韧性。     

CFRP中孔隙几何形貌与超声衰减系数关系的研究

通过金相和超声衰减系数表征CFRP层压板中的孔隙,研究孔隙形貌对超声衰减的影响。结果表明:超声衰减系数随孔隙几何形貌参数(平均长、宽、面积)的增大而增大;平均孔隙长度随孔隙率的增加程度大于平均孔隙宽度随孔隙率的增加程度;超声衰减系数随主要尺寸区域内的孔隙所占比例的增加,总体趋势减小;同一孔隙率下,大尺寸孔隙所占的比例越多,其超声衰减系数越大。总之,本试验中孔隙形貌对超声衰减有一定的影响且不能忽略。     

Characterization of Grain Size and Yield Strength in AISI 301 Stainless Steel Using Ultrasonic Attenuation Measurements

AISI 301 stainless steel samples were annealed over the temperature range of 800?C1200°C for 60?minutes to produce different grain sizes. These samples were characterized by ultrasonic immersion technique, tensile test, and optical microscopy. The attenuation of ultrasonic waves measured at a frequency of 20?MHz showed a good correlation with average grain size, hardness, and yield strength of AISI 301 stainless steel samples. A?new equation was derived for calculation of yield strength on the basis of ultrasonic wave attenuation and Hall-Petch relation.     

Determination of grain size in uranium-chromium alloys by ultrasonic attenuation

Ultrasonic attenuation was measured in cylindrical samples of uranium-chromium alloys used for nuclear fuel elements. The grain structure of the samples was previously refined by proper heat-treatment and the grain size in the range of 50 to 200 μm was determined by conventional metallographic methods. The attenuation coefficient,γ, was correlated with the mean grain diameter,ˉD, for three ultrasonic frequencies: 4, 6 and 12 MHz. The experimental results were compared with existing theories and good agreement was found with the curves based on Merkulov's analysis for cubic metals. A more practical result of this work was the development of a non-destructive testing procedure for grain-size determination in nuclear fuel elements cast from these alloys. The highest frequency (12 MHz) is normally used as it is more sensitive in the lower grain-size limit (50 to 100 μm) and only in border-line cases would one switch to the other frequencies (6 and 4 MHz) which are more applicable to the higher grain-size limit (100 to 200 μm).     

球形孔通孔和闭孔泡沫铝合金的超声衰减性能

研究了球形孔通孔和闭孔泡沫铝合金在1 MHz～10 MHz的超声衰减性能.结果表明:泡沫铝合金的超声衰减性能决定于其孔结构;通孔泡沫铝合金的超声衰减系数α随着孔径d的减小、孔隙率Ps减小和比表面积Sv的增加而增大;闭孔泡沫铝合金的超声衰减系数α随孔径d的减小、孔隙率Ps的增加和比表面积Sv的增加而增大;当孔径d、孔隙率Ps相近时,闭孔泡沫铝合金的超声衰减性能优于通孔泡沫铝合金;在1 MHz～10 MHz二者是具有良好阻尼性能的轻质材料.其衰减机制为在弹性范围内超声应力波在具有大量孔隙界面的泡沫铝合金中的衰减.     

Quantitative mapping of pore fraction variations in silicon nitride using an ultrasonic contact scan technique

An ultrasonic scan procedure using the pulse-echo contact configuration was employed to obtain maps of pore fraction variations in sintered silicon nitride samples in terms of ultrasonic wave propagation parameters. Ultrasonic velocity, attenuation coefficient, and reflection coefficient images were obtained simultaneously over a broad band of frequencies (e.g., 30 to 110 MHz) by using spectroscopic analysis. Liquid and membrane (dry) coupling techniques and longitudinal and shear-wave energies were used. The major results include the following: ultrasonic velocity (longitudinal and shear wave) images revealed and correlated with the extent of average through-thickness pore fraction variations in the silicon nitride disks. Attenuation coefficient images revealed pore fraction nonuniformity due to the scattering that occurred at boundaries between regions of high and low pore fraction. Velocity and attenuation coefficient images were each nearly identical for machined and polished disks, making the method readily applicable to machined materials. Velocity images were similar for wet and membrane coupling. Maps of apparent Poisson's ratio constructed from longitudinal and shear-wave velocities quantified Poisson's ratio variations across a silicon nitride disk. Thermography images of a disk indicated transient thermal behavior variations that correlated with observed variations in pore fraction and velocity and attenuation coefficients.     

Evaluation of grain refiners influence on the mechanical properties in a CuAlBe shape memory alloy by ultrasonic and mechanical tensile testing

This work carried out a non-destructive evaluation of grain size influence on the mechanical properties of a CuAlBe shape memory alloy with and without grain refiners. Ultrasonic signal processing, considering only the longitudinal velocity, was used for the non-destructive evaluation. Therefore, the average modulus of elasticity values found for the CuAlBe shape memory alloy was 45.7 GPa and 57.3 GPa with and without grain refiners, respectively. The corresponding values obtained by conventional mechanical tensile testing were equal to 43.2 GPa and 52.6 GPa, respectively. Additionally the mechanical tensile testing verified that the addition of grain refiners increases the stress of the alloy but has a slight effect on the alloy’s ductility. Thus, the modulus of elasticity and consequently the ultrasonic velocity, as well as the stress and strain values of CuAlBe alloy are fully dependent on its grain size. The ultrasonic analysis shows that this alloy is an excellent sound, vibration and mechanical wave absorber, presenting a high attenuation coefficient related to the wave scattering through the grains. In addition, the ultrasonic signal processing method used here confirms its main advantages of fastness and reliability.     

超声衰减谱法测量含蜡原油中蜡晶粒度

利用超声衰减法测量了不同频率不同温度下南海原油、丘陵原油和白油中的声衰减系数。同时,结合超声耦合相模型(Harker-Temple模型)和声散射模型(Bouguer-Lambert-Beer-Law模型)进行了数值研究,模拟出油样在两个不同频率下的声衰减系数以及它们的比值随蜡晶粒度的变化关系,较为准确地计算预测含蜡原油中声衰减系数。经过比较理论预测获得的声衰减模型与实验获取的声衰减系数,计算出了原油中所含蜡晶在不同温度下的平均粒度大小。结果表明,随着温度的降低,蜡晶的平均粒度大小呈增大趋势。测量结果与显微镜法测量所得结果进行对比,表明此方法应用到含蜡原油中蜡晶粒度大小的测量是可行的。     

基于频谱能量的材料晶粒尺寸表征方法

研究了利用超声频谱能量对材料晶粒尺寸进行表征的方法。通过不同的热处理方式获得了不同晶粒尺寸的奥氏体不锈钢材料,再分别利用衰减系数法、声速法和频谱能量法对材料的晶粒尺寸进行表征。结果表明:频谱能量法得到的衰减系数与平均晶粒尺寸呈非线性关系,晶粒尺寸的预测误差在4%~15%以内,优于传统的分析方法,证明了新方法的有效性。     

铜箔抗拉强度及延伸率的尺寸效应研究

为了研究尺寸参数对金属箔材的抗拉强度和延伸率的影响规律,采用不同厚度和晶粒尺寸的铜箔进行室温单向拉伸试验.试验结果表明:铜箔的抗拉强度和延伸率同时受厚度和晶粒尺寸的影响,这种尺寸效应的描述必须引入无量纲的厚度晶粒尺寸比(T/D)作为比较参数.抗拉强度在不同厚度晶粒尺寸比区间内的变化规律不同;而延伸率在厚度晶粒尺寸比相同时都随厚度的减小而降低.拉伸断口的扫描电镜分析显示箔材的延伸率随着厚度的减小出现的突降和断裂机制的变化有关.     

Electromechanical properties of fine-grain, 0.7 Pb(Mg(1/3)Nb(2/3))O3-0.3PbTiO3 ceramics

A fine grain, relaxor-based piezoelectric ceramic 0.7 Pb(Mg(1/3)Nb(2/3))O3-0.3PbTiO3 (PMN-30% PT) has been investigated, which was fabricated using the columbite precursor method. The complete set of electromechanical properties of the piezoceramic at room temperature is determined using a combination of ultrasonic and resonance techniques. This fine-grain ceramic (grain size < or = 2.5 microm) exhibits ultra-high dielectric permittivity (epsilon33(T)/epsilon0 approximately 7000) and a high coupling coefficient k(33) (= 0.78). Ultrasonic spectroscopy was used to measure the dispersion of the phase velocity and attenuation for the longitudinal wave propagating in the poling direction. Lower attenuation and smaller velocity dispersion were observed compared to modified Pb(Zr(x)Ti(1-x)O3 (PZT-5H) ceramics. The measurement results show that this fine-grain PMN-30% PT ceramic is a very good material for making ultrasonic array transducers.