45冷轧钢拉应力作用的力-磁效应研究

在不同的最大拉应力作用下，反复加载．卸载，测量了45冷轧钢试件在加载和卸载过程中试件表面某确定点漏磁场随拉应力的对应关系。试验结果表明，当最大拉应力＜610MPa时，磁感应强度随应力的变化关系为线性关系。当最大拉应力处于610MPa与屈服点653MPa之间时，磁感应强度随应力的关系由直线关系变为折线关系，并且折线极值点的位置由较小应力处迅速变为较大应力处位置。在极值点位置向高应力方向移动过程中，磁感应强度的变化量迅速增大。当最大拉应力大于屈服点时，磁感应强度的变化量保持较大的状态，折线极值点的位置基本保持不变。得到了试件在拉应力作用下的磁化关系，同时证实了可以通过磁信号的大小对试件的安全性做出评估。     

铁磁试件缺口种类对磁记忆信号的影响研究

为研究铁磁试件不同缺口对磁记忆信号的影响,利用ANSYS有限元仿真软件,结合力磁耦合模型,在拉应力和地磁场的共同作用下对3种不同缺口的20#钢板进行仿真,分别提取磁记忆信号切向分量和法向分量。仿真结果表明：在地磁场作用下,随着拉应力的增大,3种试件在各自缺口处均出现应力集中现象,在拉应力大于120 MPa后应力集中现象愈发明显。受到同样大小的拉应力时,V形缺口试件倾向于裂纹的扩展,而U形缺口试件倾向于裂纹的形成,U形缺口的应力集中范围更大,因此双关联缺口试件在U形缺口处更易断裂。双关联缺口试件磁记忆信号法向分量约为检测路径上两个缺口处磁记忆信号值的叠加之和,故试件被近似磁化成一个磁铁。双关联缺口试件检测路径中间点的磁记忆信号切向分量并没有极值,因此用其判断双关联缺口试件的应力集中区不再适用。缺口试件在弹性阶段,磁记忆信号随拉应力变化幅度较小;缺口试件在塑性阶段,磁记忆信号值随拉应力变化幅度较大。因此可以利用试件弹、塑性阶段磁记忆信号变化,对工程试件的缺陷损伤进行早期识别。     

高强钢焊接构件工字形横截面残余应力试验及统一分布模型研究

为研究不同强度等级高强钢焊接工字形截面残余应力的变化规律,采用分割法对960MPa钢材试件进行测量,得到了残余应力分布。将试验结果与大量其他强度等级(包括235MPa、345MPa、460MPa以及690MPa)钢材焊接工字形截面的残余应力进行对比分析,研究了残余拉应力和残余压应力随钢材强度的变化规律。研究结果表明,焊缝和翼缘焰切边附近残余拉应力与截面几何尺寸无明显相关性,且呈现随钢材强度增大而增大的变化规律,并始终低于钢材强度;翼缘和腹板的残余压应力与钢材强度无明显关系,但随板件宽厚比及板厚的增大而明显减小。基于这一规律及大量试验数据,提出了不同强度等级钢材焊接工字形截面残余拉应力和压应力的计算方法,确定了分布范围,制定了统一分布模型,为不同强度等级高强钢结构的研究提供了基础条件。     

拉应力对45钢磁滞与磁致伸缩曲线的影响规律

针对弱磁致伸缩特性(峰值应变约十几微应变)的碳钢材料,集成微型拉伸装置、高精度电阻应变仪、交流磁化器和磁测量模块,构建单轴拉伸状态下材料磁滞和磁致伸缩曲线的同步测量系统,实验测试拉应力对45号钢材料磁滞与磁致伸缩曲线的影响规律。结果表明:随拉应力增加,特征参量矫顽力(H_c)、磁致伸缩最大值(λ_(MAX))呈指数下降趋势,饱和磁感应强度(B_s)近乎线性降低;磁参量剩余磁感应强度(B_r)、最大磁导率(μ_(max))和磁致伸缩曲线下极值位置(H_(cλ))均呈现先快速增长后缓慢下降的两阶段变化规律,与应力的关系均服从三次多项式方程。标定得到的上述特征参量与应力的关系方程,可用于应力无损表征,从表征能力和测试系统的工程应用性角度对比讨论基于磁滞与磁致伸缩曲线的应力检测方法的特点。     

自密实轻集料混凝土双轴受力力学性能

为探究自密实轻集料混凝土双轴力学性能,采用三轴试验机对其进行双轴压-压和双轴拉-压试验,得到不同工况下的应力-应变曲线及其破坏形态,通过提取应力-应变曲线峰值应力和峰值应变,并与相关文献普通混凝土与轻集料混凝土研究成果对比,分析自密实轻集料混凝土双轴力学性能。研究结果表明:双轴压-压工况下,当侧向压应力较低时,试件主要呈现剪切破坏形态;当侧向应力较高时,试件呈劈裂破坏形态。双轴拉-压工况下,试件主要呈劈拉破坏形态,与侧向应力无关。随着侧向压应力的提高,自密实轻集料混凝土主压应力相对比无侧向应力工况明显提高,峰值应力最大提高均值幅度为68.08%,主拉应力随侧向压应力的提高逐步降低,最大降低幅度为62.35%。应用Kupfer双轴受力破坏准则验证自密实轻集料混凝土受侧向应力影响变化规律较为保守,同时基于Kupfer提出自密实轻集料混凝土双轴力学性能破坏准则,所得到的破坏准则方程具有良好的适用性。      

玄武岩纤维增强泡沫混凝土的单轴拉伸及准静态压缩性能

为研究玄武岩纤维增强泡沫混凝土的力学性能,共设计了52组试件,讨论了玄武岩纤维体积掺量和纤维长度对各密度试件的拉伸和压缩性能的影响。结果表明：玄武岩纤维可显著提高试件的抗拉峰值应力(最大提升达到737%)和峰值应变(最大提升达到833%),可有效改善中高密度试件的受拉失效模式,使其出现伪应变硬化现象,提升了试件的抗拉承载能力和变形能力。试件抗拉峰值应力和峰值应变随纤维体积掺量增大而增大,随纤维长度增长先增大后降低;另一方面,玄武岩纤维能改变试件的受压破坏模式,使其从纵向劈裂破坏转变为斜向剪切破坏和横向压溃破坏,显著提高了中低密度试件的抗压承载力和吸能能力(最大提升达到328%)。试件的吸能能力随纤维体积掺量增大而增强,随纤维长度增长先提升后降低。     

焊接残余应力磁记忆信号特征的研究

针对焊接残余应力与金属磁记忆信号存在一定联系的现象,设计了Q345R焊接试件.以X射线法检测应力值为参考,研究焊接试件横向和纵向残余应力与试件焊接残余磁信号的对应关系.结果表明,焊接试件残余压应力和试件纵向残余拉应力与试件的磁信号梯度值,在数值和变化趋势上均无明显相关性;磁信号梯度值能用于较大横向拉应力的定位,且梯度最大值点与横向残余应力的最大值点存在一定的对应关系.     

带有橡胶垫层的混凝土接触特性试验及其内聚力模型

针对带有橡胶垫层的混凝土试件,通过直剪试验研究了带有橡胶垫层的混凝土接触摩擦特性。采用PPR内聚力模型表征接触面的接触摩擦特性,对试验结果进行了模拟分析。试验结果表明:在带有橡胶垫层的混凝土接触面剪切过程中,剪切应力与剪切位移的变化过程可分为弹性、弹塑性硬化和应变软化变形阶段。当轴向应力在1.5 MPa~13 MPa范围内时,残余强度与剪切强度比在55%~65%,当轴向应力为17 MPa和21 MPa时,残余强度与剪切强度比大约分别为70%、80%。橡胶垫层在混凝土之间起到良好的缓冲作用。在轴向应力较大时,接触面的应力变形会伴有明显的软化变形阶段。利用Archard非线性幂次准则描绘了剪切峰值应力与轴向应力的关系,准则中常数k和m分别为0.97和0.33。PPR内聚力模型计算表明剪切应力随剪切位移变化关系曲线与试验结果基本吻合,为研究盾构管片块体间的接触摩擦作用研究提供借鉴。     

磁场对铁磁试件力磁耦合关系的影响研究

为研究磁场对铁磁试件力磁信号分布规律的影响,对不同附加环境磁场下的45#钢试件进行静载拉伸试验,检测试件表面的磁记忆信号B的大小,并对试件在不同附加环境磁场下的力磁耦合关系进行分析讨论。试验结果表明:在磁场环境作用下B随应力增大而增大,在屈服点附近达到最大值,此后各点B值基本保持稳定;附加环境磁场并未改变力磁信号的变化规律,但可增加磁记忆信号的大小数值,且磁化状态变化量D随着附加环境磁场的增大而增大,当H为240 A/m时达到最大,随后开始减小;在塑性变形阶段,力磁信号表现为先减小后增大的磁化反转现象。故附加环境磁场不影响金属磁记忆技术的定性评价,但影响定量评价,因此可用磁记忆检测技术来判断铁磁试件的力磁耦合特征,对金属磁记忆定量检测的进一步研究具有重要意义。     

铁磁材料零件疲劳损伤磁记忆检测方法的实验

通过对优质碳素钢做疲劳拉伸实验,研究在交变应力作用下,铁磁材料的表面磁场强度垂直分量的变化.实验研究表明:磁场强度垂直分量过零点不能完全表征应力集中位置.磁场强度梯度的变化与疲劳循环次数有一定关系.随着加载及应力循环次数的增加,沿试件轴线方向各点的磁场强度出现一定的规律性变化,前期各点变化趋势基本一致,当试件出现微小裂纹时,以裂纹点为分界出现相向变化,在远离裂纹的测点也存在同样的趋势.扩展成宏观裂纹时,裂纹处及端面达到磁饱和状态,产生较强的漏磁场,磁场强度在裂纹两侧出现正负两个区域,出现过零点.     

The influence of stress-controlled tensile fatigue loading on the stress–strain characteristics of AISI 1045 steel

This study analyses the influence of fatigue loading on the residual tensile properties of AISI 1045 steel. The fatigue tests were carried out under stress-controlled tensile loadings at a stress ratio equal to 0. The maximum applied stresses were within the range from 550 MPa to 790 MPa. An analysis of ratcheting strain and plastic strain amplitude evolution due to fatigue loading was performed on the experimental data. In the next stage of this study, the initial fatigue loadings were introduced. Two maximum stresses, 550 MPa and 750 MPa, and three cycle lengths, 25%, 50% and 75% of the total number of cycles required to fracture the material at a given stress, were used. The pre-fatigued specimens were subjected to tensile testing at strain rates from 10⁻⁴ to 10⁰ s⁻¹. A large number of fatigue cycles, equal to 75% of the fatigue life, induces material softening as well as a drop in elongation and a reduction of area. Pre-fatigue at maximum stress equal to 550 MPa results in the increase of the elastic limit and offset yield point as well. Both parameters reach almost constant value after number of cycles equal to 25 % of the fatigue life. The further increase in the number of cycles does not affect elastic limit and offset yield point in a clearly visible way. The increase of maximum stress of the initial fatigue loadings up to 750 MPa induces similar but stronger effect i.e. increase and stabilization of elastic limit and offset yield point values, however decrease of both parameters value is observed at large number of pre-fatigue cycles corresponding to 75% of the fatigue life.     

On the effect of the residual stresses on the crack opening displacement in a cracked sheet

The residual stress and displacement fields caused by localized plastic flow near a mode I crack tip in a sheet under plane stress conditions are investigated. The present study founds on the classical Dugdale scheme of the plastic flow localization. The residual stress field is considered to be induced by reversed plastic flow near the crack tip caused by an unloading. As it is found the residual stresses around the crack compress the crack tip, while the residual tensile stresses in a distant from the crack tip zone occur. It is shown the maximum residual tensile stresses can reach the significant value of the one third of the yield limit. The length of the compressed plastic zone and the residual displacement distributions are obtained. The exact formula for the residual crack opening displacement to estimate the crack closure is found. Then the next loading of the cracked plate is considered. It is shown that the second loading causes the origin of two plastic zones localized near the crack tip and at the point, where the maximum residual tensile stresses are concentrated. Again, according to the Dugdale scheme of the plastic localization, both the plastic flow zones are modelled as narrow stripes on the line extending the crack. To determine three non-dimensional parameters, characterizing the position of the segment-like plastic flow zones, a non-linear system of equations is obtained and analyzed. The exact formula for the crack opening displacement after a loading–unloading cycle is obtained. An asymptotic analysis (as the linear dimension of the distant plastic flow zone compared with the actual crack length is small) is given. It shows that the effect of the distant plastic flow zone appears as some complementary crack closure.     

Aspects of residual thermal stress/strain in particle reinforced metal matrix composites

The influence of inclusion geometry and thermal residual stresses and strains on the mechanical behaviour of a 20 vol% Al₂O₃ particulate reinforced 6061-T0 Al alloy metal matrix composite is investigated through finite element analysis. The introduction of residual thermal stresses/strains prior to external loading leads to a decrease of the proportional limit, 0.2% offset yield stress and the apparent stiffness. The residual stresses/strains are shown to have a greater effect on the composite behaviour under compressive loading than tensile loading. The residual stresses/strains have little effect on the cyclic behaviour of the composite. In only the second cycle, the difference between the cyclic curves, with and without a thermal history, was 2 MPa. Use of a cube shaped particle, with sharp corners and edges, in the unit cell model led to much greater initial hardening behaviour than spherical inclusions, and therefore a greater 0.2% offset yield stress due to stress/strain localisation at the particle corners and edges. This results in regions of constrained plasticity and high stress triaxiality in the matrix around the particle, producing improved load transfer in the composite. It is shown that inclusion aspect ratio, in the range of 0.5–2.0, has an impact on the yield stress. A minimum yield stress occurred at an aspect ratio of approximately 0.9 with significant increases on either side of this point. The influence of residual stress/strain had a similar effect throughout the aspect ratio range except tensile loading, following thermal treatment, on unit cells with inclusion aspect ratios greater than 1.5 resulted in the highest yield stresses.     

Synchrotron diffraction measurement of residual stresses in friction stir welded 5383-H321 aluminium butt joints and their modification by fatigue cycling

This paper presents two‐dimensional information on the residual stresses in 8 mm 5383‐H321 aluminium plates joined by double pass (DP) friction stir welding (FSW). It considers the inherent variability in residual stress magnitudes along 0.5 m lengths of weld pass, and their modification under a sequence of applied fatigue loads. This represents one of a planned series of experiments aimed at illuminating the effects of fatigue cycling on residual stress fields. In this particular case, the magnitudes of the bending fatigue loads (R= 0.1) were chosen to correlate with the measured proof strengths of the weld metal (approximately 160 MPa) and the parent plate (approximately 260–270 MPa). In four‐point bend S–N tests at R= 0.1 on 40 mm wide FS welded specimens of this alloy and plate thickness, these peak stress levels correspond to lives of around 10⁵ cycles and 10⁷ cycles, respectively. Results from the work indicate that significant variability exists among welded plates in peak compressive stress magnitudes (a range of perhaps ?50 MPa to ?140 MPa), although peak tensile stresses were relatively low and more consistent (from around 0 to 30 MPa). Fatigue loading accentuates the peak‐to‐valley stress change and causes an overall translation of the stresses to become more positive. Peak tensile stresses increase several‐fold during fatigue cycling.     

基于8.8级螺栓疲劳性能的预紧力优化

测量了8.8级螺栓的拉伸性能,根据螺栓材料的强度极限和屈强比研究了预紧力分别为强度的10%、30%和50%的极限条件下材料的疲劳性能。结果表明,当8.8级螺栓的预紧应力从10%强度极限提高到50%强度极限时,其疲劳极限由370 MPa降低到263 MPa。根据有效应力(σˉσˉ)参数法处理预紧应力对8.8级螺栓疲劳曲线的影响,得到了疲劳极限处的有效应力(σˉ10^7=562.75MPaσˉ10^7=562.75MPa)。当有效应力σˉ<σˉ10^7σˉ<σˉ107时预紧的8.8级螺栓不会发生疲劳失效,由此得到了8.8级M6和M27两种螺栓在不同应力比下所对应的最大预紧力和预紧扭矩曲线。     

An investigation of residual stresses in brazed cubic boron nitride abrasive grains by finite element modelling and raman spectroscopy

Joining cubic boron nitride (CBN) abrasive grains and tool body made of steel using brazing always creates residual stress due to thermal mismatch of the components when cooling down from the brazing temperature. A large tensile stress perhaps causes grain fracture during the grinding process with single-layer brazed CBN abrasive tools. To evaluate the residual stresses occurring in brazed CBN grains, values and distribution of residual stresses are calculated using the finite element method. Effects of bonding materials, embedding depth, gap thickness and grain size on brazing-induced residual stresses are discussed. Results show that the Cu–Sn–Ti bonding alloy always results in a larger tensile stress in the CBN grains, when compared to Ag–Cu–Ti alloy during the cooling phase of the brazing process. The maximum tensile stress is obtained at the grain–bond junction region irrespective of the choice of bonding material and embedding depth. When the grain side length is 100 μm, gap thickness is 10 μm and grain embedding depth is 30%, the maximum magnitude of the tensile stresses is obtained. The maximum stress is 401 MPa with Ag–Cu–Ti alloy and 421 MPa with Cu–Sn–Ti alloy. The brazing-induced residual stresses have been finally measured experimentally by means of the Raman spectroscopy. The current simulated results are accordingly verified valid.     

Measurement of fatigue-induced surface plasticity

A method is presented for measuring at a surface the localized plastic strains induced by fatigue within individual grains. The technique uses mica flakes distributed on a sample surface as reference gauges, relative to which strains in the surface can accurately be determined. An application of the method to the study of fatigue induced microplasticity in an Al 2219-T851 alloy is discussed. On an unfatigued specimen, subjected to applied stresses less than the yield strength, deformation is elastic over gauge lengths comparable with the grain size. After fully-reversed cyclic loading at a peak tensile stress of 75% of the yield strength for 20×10³ cycles, the larger grains in the alloy exhibit a residual tensile strain after a tensile loading cycle. Neighbouring smaller grains are driven into elastic compression to accommodate this tensile plastic deformation. Peak localized tensile plastic strains may exceed 0.5% at the surface. This technique will be useful in evaluating models of fatigue crack initiation and surface damage accumulation.     

Influence of shot peening on fatigue durability of normalized steel subjected to variable amplitude loading

The influence of shot peening on the fatigue durability of normalized carbon steels subjected to variable amplitude loading has been investigated. The relaxation of residual stresses was recorded during the fatigue life time. Strain amplitude spectra were extracted from real spectra recorded from components in service. The results were compared with data achieved from constant amplitude testing. In both types of tests parallel studies were made on both peened and unpeened specimens. Shot peening leads to pronounced increase in life time, especially for smaller amplitudes. For both variable and constant amplitude loading shot peened specimens exhibit longer life provided the residual stresses during fatigue loading do not relax more than to about 60% of their initial value. To get an improvement in life time of at least a factor two for peened specimens, the stress amplitude in constant amplitude loading or the maximum stress amplitude in variable amplitude history must not be more than 20% larger than the magnitude of the initial residual stresses. This limit corresponds to 1.2 times the yield strength of the unaffected material.