Edge crack damage analysis of AZ31 magnesium alloy hot-rolled plate improved by vertical roll pre-rolling

In the present study,through vertical roll pre-rolling of AZ31 magnesium alloy hot-rolled plate at room temperature,the effect of different vertical roll pre-rolling reduction on edge crack of the plate during flat rolling was systematically studied.The evolution of microstructure and texture in the edge and middle of the plate after vertical roll pre-rolling,heating and rolling was analyzed by using EBSD technology.The results show that during the vertical roll pre-rolling,{10–12}primary twins and{10–12}-{10–12}secondary twins dominated the edge deformation,while{10–12}primary twins dominated the middle deformation.With the increase of vertical roll pre-rolling reduction,the twin volume fraction of the edge and the middle increased,and the difference between them decreased gradually.After heating,the twin orientation caused by vertical roll pre-rolling was still maintained,and there was a significant difference in grain size between the edge and the middle.When the reduction rate of flat rolling was 30%,cracks have already appeared in the initial plate,while the vertical roll pre-rolled samples showed extremely high rolling performance.When the reduction rate of flat rolling was 50%,the 8PR sample still had no crack initiation.In addition,after flat rolling of 50%,the initial plate showed a strong basal texture,while the maximum pole density of the 8PR plate was small,and the texture distribution was very scattered,showing the characteristics of weak orientation.

Limit Load Solution for Electron Beam Welded Joints with Single Edge Weld Center Crack in Tension

Limit loads are widely studied and several limit load solutions are proposed to some typical geometry of weldments.However,there are no limit load solutions exist for the single edge crack weldments in tension（SEC（T））,which is also a typical geometry in fracture analysis.The mis-matching limit load for thick plate with SEC（T） are investigated and the special limit load solutions are proposed based on the available mis-matching limit load solutions and systematic finite element analyses.The real weld configurations are simplified as a strip,and different weld strength mis-matching ratio M,crack depth/width ratio a/W and weld width 2H are in consideration.As a result,it is found that there exists excellent agreement between the limit load solutions and the FE results for almost all the mis-matching ration M,a/W and ligament-to-weld width ratio（W-a）/H.Moreover,useful recommendations are given for evaluating the limit loads of the EBW structure with SEC（T）.For the EBW joints with SEC（T）,the mis-matching limit loads can be obtained assuming that the components are wholly made of base metal,when M changing from 1.6 to 0.6.When M decreasing to 0.4,the mis-matching limit loads can be obtained assuming that the components are wholly made of base metal only for large value of（W-a）/H.The recommendations may be useful for evaluating the limit loads of the EBW structures with SEC（T）.The engineering simplifications are given for assessing the limit loads of electron beam welded structure with SEC（T）.

Elastic behaviour of an edge dislocation near a sharp crack emanating from a semi-elliptical blunt crack

The interaction between an edge dislocation and a crack emanating from a senti-elliptic hole is dealt with. Utilizing the complex variable method, closed form solutions are derived for complex potentials and stress fields. The stress intensity factor at the tip of the crack and the image force acting on the edge dislocation are also calculated. The influence of the morphology of the blunt crack and the position of the edge dislocation on the shielding effect to the crack and the image force is examined in detail. The results indicate that the shielding or anti-shielding effect to the stress intensity factor increases acutely when the dislocation approaches the tip of the crack. The effect of the morphology of the blunt crack on the stress intensity factor of the crack and the image force is very significant.

Effect of variable thickness cross rolling on edge crack and microstructure gradient of AZ31 magnesium alloy

The hot rolling experiment of AZ31 magnesium alloy was carried out by laying anoverlay mold at the initial temperature of 400℃.According to the Mizushima automatic plan view pattern control system(MAS)rolling theory and the cross rolling process,different reductions in the middle and edges of the magnesium alloy were realized,and the influence of the regional controlled reduction rolling on the edge cracks and microstructure gradient of the magnesium alloy were analyzed.It is shown that this rolling approach has reduced the maximum edge crack depth of the rolled piece by 56.85%,and there is a weakening tendency in the base surface texture of the strip edge,the base surface texture density drops from 23.97 to 17.48 after ordinary flat rolling.It exhibits basal texture gradients from the edge to the middle of the sheet along the RD direction,which reflected the uneven deformation of the sheets.It is suitable for the processing of metal molds that require large edge reductions such as mobile phone shells,and provided a theoretical basis for the variable thickness rolling of the magnesium alloy strip.

Spline Fictitious Boundary Element Alternating Method for Edge Crack Problems with Mixed Boundary Conditions

The alternating method based on the fundamental solutions of the infinite domain containing a crack,namely Muskhelishvili’s solutions,divides the complex structure with a crack into a simple model without crack which can be solved by traditional numerical methods and an infinite domain with a crack which can be solved by Muskhelishvili’s solutions.However,this alternating method cannot be directly applied to the edge crack problems since partial crack surface of Muskhelishvili’s solutions is located outside the computational domain.In this paper,an improved alternating method,the spline fictitious boundary element alternating method(SFBEAM),based on infinite domain with the combination of spline fictitious boundary element method(SFBEM)and Muskhelishvili’s solutions is proposed to solve the edge crack problems.Since the SFBEM and Muskhelishvili’s solutions are obtained in the framework of infinite domain,no special treatment is needed for solving the problem of edge cracks.Different mixed boundary conditions edge crack problems with varies of computational parameters are given to certify the high precision,efficiency and applicability of the proposed method compared with other alternating methods and extend finite element method.

Analysis of strip edge cracks during cold rolling of thin strip

In this paper,the mechanics of strip edge cracks and its propagation has been studied,and the effects of strip edge drop and stress intensity factor(SIF) on edge crack defections during cold rolling of thin strip have been discussed.An experimental investigation was presented into the effect of strip edge drop on edge cracks during cold rolling of thin strip.The edge crack increases significantly due to more inhomogeneous deformation and work hardening at the strip edge.The effective stress intensity factor range is important as it represents the major physical cause of the crack propagation.The efficiency and reliability of the SIF analytical model has been demonstrated in the study.The proposed method for predicting strip edge crack is helpful in producing defect-free products and providing an understanding of the mechanics of edge crack propagation in cold rolling of thin strip.

AN EDGE CRACK PROBLEM IN A SEMI-INFINITE PLANE SUBJECTED TO CONCENTRATED FORCES

An oblique edge crack problem in a semi-infinite plane is discussed. Re concentrated forces are applied on the edge crack face, or on the line boundary of the cracked semi-infinite plane. The rational mapping function approach is suggested to solve the boundary value problem and a solution in a closed form is obtained. Finally, several numerical examples with the calculated results are given.

The edge crack problem for an orthotropic functionally graded strip under concentrated loads

The plane crack problem of an orthotropic functionally graded strip under concentrated loads is studied. The edge crack is perpendicular to the boundary and the elastic property of the material is assumed to vary depending on thickness. By using an integral transform method, the present problem can be reduced to a single integral equation which is solved numerically. The influences of parameters such as the nonhomogeneity constant and the geometry parameters on the stress intensity factors (SIFs) are studied. It is found that the nonhomogeneity constant has important influences on the SIFs.

Prediction of crack location and propagation in stretch flanging process of aluminum alloy AA-5052 sheet using FEM simulation

The stretch flanging process is significantly affected by various geometrical,material and process parameters.The punch-die clearance and initial flange length are main parameters which have major effects on the edge crack location and strain distribution along die profile radius in the flange.Non-axisymmetric stretch flanging process of AA-5052 sheet metal blanks was carried out by numerical simulation to predict the deformation behavior of flange,location and propagation of crack in flange and to investigate the effect of punch die clearance,flange length,die and punch profile radius and friction in the stretch flanging process.The experimental investigations were made to validate the simulations results.The results reveal that the crack length increases with the increase in the flange length.It is found that the flange length has a significant effect in circumferential direction as compared with the radial direction.The punch die clearance has the most significant effect in crack propagation in comparison with flange length.The circumferential strain is found to be larger in the case of punch having the profile radius less than the die profile radius,which leads to faster edge crack propagation.A close agreement is found between simulation and experimental results in terms of location of edge crack and forming load.

Effect of copper on edge cracking behavior and microstructure of rolled austenitic stainless steel plate

Cu is known to affect the edge cracking characteristics of austenitic stainless steel as it causes embrittlement.The hot rolling test of four kinds of austenitic stainless steel with different copper content(0,2.42,3.60 and 4.35 wt.%)was carried out to examine the effect of hot rolling cracks on steel containing different copper contents.The evolution of crack and microstructure was analyzed using the scanning electron microscope,energy-dispersive spectrometer,electron back scattered diffraction and transmission electron microscope.Experimental results showed an upward trend in edge cracking degree when Cu content was 4.35%,and the crack extended from the edge of the steel plate to the middle by about 14 mm.Besides,severe oxidation was observed inside the crack by fractography.With the increase in copper content at 1250℃,the content of{110}<112>brass and{112}<111>copper textures decreased.When the content of copper was 4.35%,the decrease was most significant,and{112}<111>copper texture content decreased to only 0.5%.Generally,the textures of 2.42%Cu and 3.60%Cu 304L steel changed little,while a large change in the texture of 4.35%Cu 304L steel was observed.To conclude,the increase in rolling temperature can prevent edge crack and its propagation effectively.

Mechanical properties of AISI 1045 ceramic coated materials by nano indentation and crack opening displacement method

Abstract： An effective approach was conducted for estimating fracture toughness using the crack opening displacement （COD） method for plasma enhanced chemical vapor deposition （PECVD） coating materials. For this evaluation, an elastoplastic analysis was used to estimate critical COD values for single edge notched bending （SENB） specimens. The relationship between fracture toughness （Kic） and critical COD for SENB specimens was obtained. Microstructure of the interface between AleO3-TiO2 composite ceramic coatings and AISI 1045 steel substrates was studied by using scanning electron microscope （SEM）. Chemical compositions were clarified by energy-dispersive X-ray spectroscopy （EDS）. The results show that the interface between of Al203-TiO2 and substrate has mechanical combining. The nanohardness of the coatings can reach 1 200 GPa examined by nanoindentation. The Klc was calculated according to this relationship from critical COD. The bending process produces a significant relationship of COD independent of the axial force applied. Fractographic analysis was conducted to determine the crack length. From the physical analysis of nanoindentation curves, the elastic modulus of 1045/AI2O3-TiO2 is 180 GPa for the 50 μm film. The highest value of fracture toughness for 1045/A1203-TiO2-250 μm is 348 MPa·mv2.

EFFECT OF Mg AND Zr ADDITION ON CREEP CRACK GROWTH BEHAVIOUR OF A Ni-BASE SUPERALLOY

The creep crack growth(CCG)and the time to carck initiation and rupture of specimen (t_(rc)and t_r)were measured by means of electrical potential method on single edge notched specimens at 700℃.The field near the crack tip under steady-state creep was represented by energy rate integral(C~*),and the CCG rate as a function of C~* has been obtained.The agreement between the predicted and observed t_(rc)values is quite good.It was found that the addition of small amount of Mg and Zr in the alloys causes t_(rc)and t_r of the specimens to in- crease significantly.

Comparison of edge cracking and tensile cracking in in-situ deformation at 150℃of Mg-2Zn-1.5Mn alloy sheet

The Mg-2Zn-1.5Mn alloy(ZM21)sheets were hot-rolled in a single pass at 150℃with different reductions through on-line heating rolling.Microstructures and edge cracking behavior of the rolled sheets were investigated.The in-situ tensile tests at 150℃were also carried out,the crack initiation and propagation were compared with the edge cracking behavior of ZM21 sheets prepared by on-line heating rolling.The results reveal that the edge cracks are most likely to originate in the rolling direction and normal direction(RD-ND)plane due to the secondary tensile stress along RD.Edge cracking becomes more severe with an increasing reduction.The edge cracks mainly initiate and propagate in the fine recrystallized grain areas and the junction of recrystallized grains and sub-grains with hard orientation.The in-situ tensile test indicates that micro-cracks mainly initiate at the triple junction of grain boundaries where grains have hard orientation with low basal Schmid factor(SF).Meanwhile,those cracks are more likely to propagate along the grain boundaries with maximum difference in basal Schmid factor.Besides,the crack initiation and propagation during the in-situ tensile deformation at 150℃are found not to be associated with the recrystallization.

白垩系弱胶结砂岩TBM掘进刃宽参数研究

TBM刀盘滚刀的机械参数是影响扰动围岩裂纹扩展的主要因素之一。为了揭露白垩系砂岩的内部结构,探明破岩机理,针对中国西部某隧道工程,开展白垩系砂岩微观结构XRD试验、SEM试验、偏光显微镜试验。选择不同刃宽滚刀,采用PFC2D数值模拟贯入试验,用Fish语言提取裂纹特征、贯入力幅值变化、裂纹数量及胶结破坏比能耗进行比较分析,探究裂纹形成机理与破岩效率。结果表明:弱胶结砂岩石英和长石矿物磨圆度低,分选性差,胶结物少,胶结物松散,微孔隙多,而这些特征所形成的岩石微结构恰好构成了裂纹扩展的通道;同等贯入度工况下,刃宽12 mm呈现出诸多优势,此时水平裂纹最接近自由面,使得破岩过程更加高效;大岩块最为突出,表明破岩效果显著,中间裂纹最少,减少了能量的不必要损耗,破岩能量利用最为有效;白垩系弱胶结砂岩掘进选择刃宽12 mm较为合理,这是由于刃宽超过12 mm后破岩机理发生改变,由拉剪破坏转向压剪破坏,破岩力大幅增加,比能耗上升,破岩效率下降。本研究结果符合相关理论模型,并在中国西部某隧道工程得以成功验证。

基于Sobel算子桥接的双编码器路面裂缝检测网络

为提高道路路面裂缝的检测精度,针对路面裂缝的多态性和噪声复杂等问题,提出了一种基于Sobel算子桥接的双编码器路面裂缝检测网络,双编码器由原图编码和梯度编码两部分组成,以解决单编码器容易丢失梯度信息的问题。首先,原图编码结果通过桥接Sobel算子计算8个方向产生梯度编码的编码信息;然后,将原图编码结果与梯度编码结果通过一个多尺度的边缘信息弥补模块,以增强裂缝的边缘信息;最后,引入动态通道图卷积获得通道之间存在的拓扑关系,以突出重要通道的语义特征。研究结果表明:所提出的方法在DeepCrack、CamCrack789和CFD这3个基准数据集上取得较好的结果;综合指标ODS在DeepCrack、CamCrack789和CFD数据集分别为87.75%、85.05%、78.83%。

中厚板边部裂纹的原因及措施

板坯连铸机设备改造后钢板边部裂纹发生率异常增高,对钢板裂纹微观形貌进行分析得出:钢板边部裂纹来源于连铸坯。对连铸坯表面宏观形貌、保护渣、钢水成分、结晶器冷却、连铸设备维护以及结晶器振动参数等因素进行了对比与分析,结果表明:连铸坯四个角振痕不均匀、不规则、有时错乱且分布不规律,结晶器振动参数不合理、连铸机工况不佳是钢板边部裂纹产生的主要原因。通过调整结晶器振动起始位置到中位、延长结晶器振幅变化的斜坡时间、提高连铸机工况条件等,使钢板边部裂纹发生率由5%~50%降低至1%以内。

板坯连铸机系统改造后边裂问题分析与改进

板坯连铸机改造后,铸坯连续出现南北两侧振痕不均匀和边裂问题,严重地影响了正常的生产流程,对产品质量控制也造成了很大影响。针对铸坯边裂,依据生产数据和生产跟踪情况,从工艺、设备工况和操作等方面分析,排除了生产工艺和操作因素所造成的影响,最终确定是由于结晶器起振中心位置不合适导致的。结晶器起振中心位置调整之后,边裂问题明显减少,铸坯质量得到改善,边裂发生率由10.5%降低到0.5%以下。

补片形式对蒙皮复合材料孔边裂纹修复效果的影响

针对蒙皮复合材料孔边裂纹,提出了共固化补片修复和预固化补片修复两种修复方式,并设计了与之适配的修复方案,研究了修复方式对试样抗拉强度恢复效果的影响,阐明了完好试样和各类修复试样的断裂失效机制。结果表明:相同补片尺寸下,预固化补片修复试样的强度恢复率均高于共固化补片修复试样,预固化补片修复试样强度恢复率最高为81.9%,且试样断口平齐,补片与复合材料层压板之间的断裂机制为纤维纵向拉断;共固化补片修复试样强度恢复率最高为78.6%,主要断裂机制为复合材料层压板分层。证明了相比于共固化补片修复,预固化补片修复试样能够恢复更高的抗拉强度,为蒙皮复合材料修复提供了具体的工艺方案和理论基础。

基于DBSCAN算法与GMM理论的铝合金板孔边疲劳裂纹萌生监测

针对航空结构中多孔铝合金板在疲劳载荷作用下的孔边裂纹萌生监测问题,以光纤传感系统为基础结合小波分解、含噪声的密度空间聚类以及高斯混合模型,提出了一种孔边裂纹萌生监测方法。首先以光纤光栅传感系统采集循环加载条件下多孔铝合金板孔边裂纹萌生至结构断裂全程中孔边微应变并构建孔边微应变曲线。对孔边微应变曲线进行小波分解,得到微应变曲线的低频分量与高频分量,并以低频分量最小值及高频分量突变作为孔边裂纹萌生特征。在分析裂纹萌生时引入DBSCAN算法与GMM理论用于计算孔边裂纹萌生时的疲劳加载循环数并进行比较与分析得到多孔铝合金板孔边主裂纹的萌生位置以及孔边主裂纹裂纹萌生时的疲劳加载循环数。试验结果表明:此监测方法能够准确定位出孔边主裂纹的萌生位置,计算孔边主裂纹萌生时的疲劳加载循环数,且疲劳加载循环数计算误差在5%以内。在未来可应用于全机地面疲劳试验、结构健康监测等多种场景。

压载荷下Ⅰ-Ⅱ复合型中心裂纹板弹塑性屈曲载荷研究

为了研究含裂纹结构承载能力,采用有限元方法建立压载荷下Ⅰ-Ⅱ复合型中心裂纹板模型,计算裂纹板弹塑性屈曲载荷,讨论相对裂纹长度(a/w)、裂纹倾角(θ)、板宽(w)、板厚(t)、边界条件对弹塑性屈曲载荷及其变化规律的影响。对于上下夹持中心裂纹板,随着裂纹长度和裂纹倾角的增大,板宽越大,中心裂纹板屈曲载荷对裂纹越敏感,中心裂纹板承载能力受到裂纹长度和裂纹倾角的影响越明显。弹塑性屈曲载荷随着板厚的增大而增大,并且随着裂纹倾角的增大,板厚越大,屈曲载荷的下降程度越显著。上下简支中心裂纹板与上下夹持中心裂纹板的屈曲载荷变化规律基本一致,且上下夹持中心裂纹板的屈曲载荷显著大于上下简支中心裂纹板的屈曲载荷。给出不同边界条件下弹塑性屈曲载荷与相对裂纹长度、裂纹倾角的回归关系式。通过中心裂纹板弹塑性屈曲试验发现,采用压缩力学性能计算弹塑性屈曲载荷更接近试验结果,试验屈曲载荷的变化规律与有限元模拟计算结果一致。研究结果对含裂纹结构的承载能力评价具有参考价值。