基于应力波因子的金属材料表面塑性损伤检测

提出了一种基于应力波因子的金属材料表面塑性损伤检测方法。将AZ31镁铝合金试件加载到不同的拉伸载荷后,利用Ritec SNAP非线性超声测试系统激发和接收Rayleigh表面波,通过实验测试在不同载荷作用后基于应力波因子的声学非线性系数与应力的关系。研究结果表明,当加载应力接近材料的屈服极限时,基于应力波因子的声学非线性系数随着应力的增加明显增大,因此可以利用基于应力波因子的声学非线性系数对金属材料表面的塑性损伤进行非线性超声无损评价。     

0Cr17Ni4Cu4Nb不锈钢早期损伤非线性驻波法检测

针对非线性超声纵波检测中信号幅值较低、二次谐波不易被有效激发且易受干扰的问题,提出非线性驻波检测方法。分别对0Cr17Ni4Cu4Nb马氏体不锈钢拉伸试件进行非线性超声纵波法和非线性驻波法检测,检测结果表明,相比于非线性超声纵波法,非线性驻波法能够提升信号幅值,有效激发材料内部由于非线性相互作用产生的二次谐波,提高超声信号的信噪比和频谱的频率分辨率,且试验结果与有限元仿真结果基本一致。对0Cr17Ni4Cu4Nb马氏体不锈钢三点弯曲疲劳试件进行非线性驻波检测,在疲劳寿命50%以前,不同疲劳应力下归一化超声非线性系数均随疲劳寿命增加而增大。因此,非线性驻波法能够更好的检测0Cr17Ni4Cu4Nb不锈钢早期损伤。     

20~#钢不同晶粒度试件非线性超声特性实验研究

传统的线性超声检测技术对微观缺陷的检测不敏感,而非线性超声检测是利用信号的频域特征来进行缺陷判别,具有独特的优势。采用RAM-5000 SNAP非线性超声检测系统,对不同晶粒度20#钢试件进行非线性超声检测实验。通过测量不同试件在相同激励条件下的基波与二次谐波幅值,得到其相关非线性系数,分析材料不同晶粒度对非线性超声特性的影响。结果表明:随着材料晶粒度的增大,超声非线性效应增强,非线性系数值不断增加。因此,利用非线性系数可以表征材料微观晶粒度的变化情况,验证非线性超声检测技术在判别材料微观组织变化中的有效性。     

金属磁记忆检测技术定量评估构件疲劳损伤研究

对18CrNi4A钢缺口试件在三级应力水平下进行疲劳试验和磁记忆信号检测,研究金属磁记忆信号在疲劳过程中的变化规律和磁记忆检测技术对构件疲劳损伤的定量估评.结果表明:在稳定循环阶段,磁信号随疲劳循环周次增加无显著改变,疲劳裂纹萌生后,磁信号逐渐增加,并在断裂后发生激变;磁信号特征参量Kmax,Hp(y)max,Hp(y)min和Hp(y)sub值与应力水平和疲劳损伤程度存在强烈的相关性,特征参量绝对值随应力水平或疲劳损伤程度的增加而逐渐增加;磁信号特征参量Kmax平均值法可较准确地定量评估构件疲劳损伤,该方法判据为:当m(m=Kmax/KAVmax)＞1,试件存在严重的疲劳损伤.     

一种韧性断裂模型的理论研究和实验验证

为研究高强钢板成形过程中的损伤破裂机理,更准确地预测高强钢的断裂失效行为,基于细观损伤力学的空穴理论,并在屈服函数就是塑性势函数的通用性假设基础上推导了各向同性的韧性断裂模型;同时引入Lode参数以反映不同应变状态下空穴形核、长大以及聚合的差异,提出了一种包含应力三轴度和Lode参数的新模型.在Hill正交各向异性屈服假设下,描述了平面应力状态下应力比值、r值与应力三轴度、等效塑性应变的关系.最后,针对DP590进行了参数确定和实验验证.结果表明:应力三轴度在高强钢韧性断裂中仍然起主导因素,在低应力三轴下,材料主要是剪切型破坏,空穴的长大及聚合方式主要受剪应力影响,高应力三轴下,空穴损伤主要受拉应力影响,断裂是韧窝形的;Lode参数决定了应力组成形式,也间接地反映了应变状态,它与应力三轴度共同决定了空穴损伤的发展.新的模型能较准确地预测DP590的成形极限.     

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。     

Q235钢疲劳损伤的非线性Rayleigh波检测技术研究

该文采用非线性Rayleigh波检测方法,研究Q235钢拉伸疲劳损伤和腐蚀疲劳损伤的测试与评价过程;搭建非线性Rayleigh波检测系统,分别在不同拉伸频率的循环拉伸载荷和腐蚀疲劳载荷下,采集非线性声波时域信号并进行频谱分析,测量非线性系数随加载载荷周期数的变化趋势,并分析拉伸频率和腐蚀环境对非线性系数的影响。实验结果表明:Q235钢试件的超声非线性系数与疲劳周数具有一定的单调递增关系,超声非线性系数可以用来表征材料的表面疲劳损伤程度;拉伸频率对试件的疲劳损伤影响小,不同频率拉伸疲劳下,钢材的非线性系数相差不大;腐蚀环境会加剧试件的疲劳损伤程度,在腐蚀疲劳载荷下,非线性系数会增大。实验结果可为研究Q235钢拉伸疲劳损伤和腐蚀疲劳损伤的测试与评价过程提供一定的指导。     

锈损冷弯薄壁钢板的断裂机制与断裂模型

为研究锈蚀对冷弯薄壁钢板断裂机制的影响,本工作选取在工业环境中服役多年的C形钢檩条,从其平板部位和弯角部位截取标准试件进行单调拉伸试验,并通过有限元数值分析研究了锈坑深度、深径比和截面损失率对其应力三轴度和等效塑性应变的影响.拉伸试验结果表明:随锈蚀程度增大,试件的塑性硬化阶段和颈缩阶段逐渐缩短,屈服阶段逐渐消失;试件的屈服强度、极限强度和断裂应变逐渐减小.有限元分析结果表明:应力三轴度随锈坑深度和深径比增加逐渐增大,随截面损失率的增加变化不显著;等效塑性应变随锈坑深度和截面损失率增加明显增大,随锈坑深径比增加呈现先增大后减小的趋势.本工作还建立了应力三轴度和等效塑性应变相对值与锈坑深度、深径比和截面损失率的拟合公式,根据拟合结果,对空穴增长模型(VGM模型)和应力修正临界应变模型(SMCS模型)进行修正,从而推导出点蚀损伤和全面腐蚀损伤下冷弯薄壁钢板的断裂模型.     

基于塑性应变能的多轴低周疲劳寿命预测模型

在分析多轴疲劳损伤机理的基础上,提出一个新的多轴低周疲劳寿命预测模型。此模型以临界平面上的塑性应变能作为疲劳损伤参量,分析了临界平面的特点并给出了损伤参量的计算过程。利用该模型预测了不同加载路径下的304不锈钢试件的疲劳寿命,并与试验值进行比较。结果表明,该损伤参量具有明确的物理意义,能够适用于多轴比例与非比例等各种复杂的加载情况。     

水泥砂浆塑性收缩开裂本构方程初探

采用自行设计的塑性抗拉强度及塑性毛细管收缩应力测试装置分别测试了不同灰砂比和不同聚乙烯醇纤维掺量时砂浆的塑性抗拉强度及塑性毛细管收缩应力,对实验结果进行了线性回归分析.结果表明,当抗裂指数K大于1.62时,试件不开裂,当K值小于1.38时,试件开裂,当K值介于1.38和1.62之间时,试件有一定概率开裂.灰砂比和纤维掺量对塑性抗拉强度以及塑性毛细管收缩应力的线性影响是显著的,抗裂指数关于灰砂比及纤维掺量的本构方程分别为Kc/s=-0.00207Rc/s+0.0383/0.00879Rc/s+0.0227和Kf=0.00379Vf+0.0359/-0.00373Vf+0.0345.经实验检验,这2个方程的计算结果与实验结果相吻合.     

Role of stress triaxiality in elastoplastic characterization and ductile failure prediction

The triaxiality of the stress state is known to greatly influence the amount of plastic strain which a material may undergo before ductile failure occurs.During tensile load histories, the necking induces significant stress triaxiality modifications which in turn affect the experimental stress-strain measurements needed for the characterization of ductile metals.In this paper, the recently proposed “MLR” model of necking effect is used to obtain the flow curves of various metals by correcting the experimental data of tensile tests. Finite elements simulations of the experimental tests are performed to calculate the stress triaxiality evolution on various notched and unnotched specimens. A ductile failure criterion, due to Bao and Wierzbicki, is then applied to evaluate the material damage and predict failure. This procedure is applied to a set of 20 specimens series made of six metals with 10 different notch shapes.The damage calculations also indicate the material points where failure initiates. These predictions are confirmed by micrographic observation of voids on polished fragments of the broken specimens.     

利用非线性表面波评价材料疲劳损伤

采用非线性Rayleigh表面波检测方法,实现了不同疲劳阶段下钢试样拉伸和腐蚀疲劳损伤的测试与评价;基于楔块\换能器激发与接收声波方式,搭建非线性Rayleigh波检测系统,测量了不同激励水平下基波幅值平方与二次谐波幅值间的线性关系以及Rayleigh表面波二次谐波的累积效应;分别在拉伸载荷和腐蚀疲劳载荷下,采集非线性时域信号并进行频谱分析,测量声学非线性系数在不同疲劳阶段下变化趋势,并分析不同疲劳载荷对钢试样声学非线性系数的影响。实验结果表明:超声非线性系数与疲劳周期数呈单调递增关系,可以用声学非线性系数来表征材料的表面疲劳损伤程度;相比较周期性拉伸疲劳损伤,腐蚀疲劳试样的声学非线性系数会增大,是由于腐蚀环境会加重实验中钢试样的疲劳损伤程度。研究成果可为疲劳损伤无损检测与评价提供一定的指导意义。     

Comparison of different stress-state dependent cohesive zone models applied to thin-walled structures

Two different approaches that explicitly incorporate the stress triaxiality into cohesive zone models applicable to thin-walled structures are compared to identify the relative merits and limitation of these models. The number of model parameters involved, the ease of parameter determination and the predictive capabilities of the models over a wide range of thin-walled geometries are investigated. The first model, proposed recently by the authors, uses basic elastic–plastic constitutive equations combined with a model parameter depending on the average triaxiality in plane stress conditions. The second model incorporates stress-state through exponential dependence of cohesive strength on triaxiality, similar to plane strain studies earlier. The respective parameters for both models are identified and subsequently applied to several notched and precracked specimens. It is shown that in contrast to stress-state independent models, both constraint dependent models are able to predict well failure of a wide range of structures. While the model incorporating triaxiality dependent cohesive parameters has more parameters to be determined, it is not restricted to any specific stress condition and therefore can be extended to arbitrary three-dimensional stress-states.     

On the numerical implementation of a shear modified GTN damage model and its application to small punch test

Gurson-type models have been widely used to predict failure during sheet metal forming process. However, a significant limitation of the original GTN model is that it is unable to capture fracture under relatively low stress triaxiality. This paper focused on the fracture prediction under this circumstance, which means shear-dominated stress state. Recently, a phenomenological modification to the Gurson model that incorporates damage accumulation under shearing has been proposed by Nahshon and Hutchinson. We further calibrated new parameters based on this model in 22MnB5 tensile process and developed the corresponding numerical implementation method. Lower stress triaxiality were realized by new-designed specimens. Subsequently, the related shear parameters were calibrated by means of reverse finite element method and the influences of new introduced parameters were also discussed. Finally, this shear modified model was utilized to model the small punch test (SPT) on 22MnB5 high strength steel. It is shown that the shear modification of GTN model is able to predict failure of sheet metal forming under wide range of stress state.     

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality.The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.     

Finite element limit analyses of under-matched tensile specimens

This paper quantifies the effect of under-matching on plastic limit loads and fully plastic stress triaxialities for mismatched flat plate (in plane strain and plane stress) and round bar tensile specimens, via parametric finite element (FE) limit analyses based on elastic-perfectly-plastic materials. It is found that the effect of the strength mismatch ratio (the ratio of the yield strength of the weld zone to that of the base material) on plastic limit loads and fully plastic stress triaxialities can be significant for plane strain plate and round bar specimens, but much less significant for plane stress plate specimens. Its effect is dependent significantly on the slenderness of the weld zone (defined by the ratio of the weld zone width to the specimen width). The effect of the slenderness of the weld zone is apparent only when the weld width is less than the specimen width or diameter. In particular, the stress triaxiality in the softer weld zone can increase significantly with decreasing the ratio of the weld zone width to the specimen width (radius). Based on the present limit load results, a simple method to extract intrinsic tensile properties of the under-matched weld zone from test results of under-matched tensile specimens is proposed.     

板状构件粘结强度的非线性超声导波检测

相较于铆接、螺接、焊接等连接方式,板状粘结构件具有质量轻、应力分布均匀等特点,广泛运用于航空航天、车辆制造等工业领域。板状粘结构件在服役过程中出现的粘结强度退化、弱粘结等会影响其服役可靠性及安全性,因此对粘结强度进行检测十分必要。非线性超声导波对材料微观结构特征变化比较敏感,可用于粘结构件的粘结强度检测。采用非线性超声导波对铝合金-环氧树脂-铝合金板状构件进行检测,通过不同的固化工艺制备粘结结构件模拟不同粘结强度,检测结构件中传播的非线性超声导波,计算超声非线性参量,获得超声非线性参量在不同固化工艺下的变化趋势。通过拉伸实验测得粘结强度,进而构建超声非线性参量与粘结强度的关系。实验结果显示,粘结强度越大,超声导波的非线性参量越小。该研究表明,非线性超声导波可有效检测板状构件的粘结强度,为工业检测板状结构粘结强度提供了有效方法。     

45号钢受热损伤的超声非线性检测实验研究

45 steel is a common material for parts manufacturing, and it is important to study its thermal damage detection for parts failure assessment.Here, in order to study ultrasonic nonlinear effects of 45 steel with different degrees of thermal damage, specimens with different degrees of thermal damage were made.Ultrasonic nonlinear detection tests were conducted for specimens using the RAM-5000 system made by RITEC company to obtain relation curves between ultrasonic nonlinear coefficients and heating temperature.These curves were deeply analyzed combining with the material’s metallographic microstructure evolution laws.Results showed that the ultrasonic nonlinear coefficient of 45 steel firstly increases, then decreases and increases again with increase in heating temperature;when heating temperature reaches 600 ℃, the ultrasonic nonlinear coefficient begins to increase; when temperature reaches 800 ℃, itreaches the peak value;thenit drops sharply, this is consistent with the coherent strain state of 45 steel between 700-800 ℃; finally, it at 1 300 ℃ rebounds slightly;the correlation between variation of ultrasonic nonlinear coefficient of 45 steel and its metallographic microstructure varying provides atest basis for the ultrasonic nonlinear detection of 45 steel thermal damage and the evaluation of its metallographic microstructure change.     

Ductile shear failure or plug failure of spot welds modelled by modified Gurson model

For resistance spot welded shear-lab specimens, interfacial failure under ductile shearing or ductile plug failure are analyzed numerically, using a shear modified Gurson model. The interfacial shear failure occurs under very low stress triaxiality, where the original Gurson model would predict void nucleation and very limited void growth. Void coalescence would therefore be largely postponed. However, using the shear modification of the Gurson model, recently introduced by Nahshon and Hutchinson (2008) [1], failure prediction is possible at zero or even negative mean stress. Since, this shear modification has too large effect in some cases where the stress triaxiality is rather high, an extension is proposed in the present study to better represent the damage development at moderate to high stress triaxiality, which is known to be well described by the Gurson model. Failure prediction and tensile response curves for an interfacial shear failure or a ductile plug failure, are here compared when using either the original Gurson model, the shear modified model, or the extension to the shear modified model. The suggested extension makes it possible to use the shear modified model as a simple way of accounting for damage development under low triaxiality shearing, without further increasing the damage rate in regions of moderate to high stress triaxiality.