CN108181190B - Method for rapidly predicting fatigue limit of spot-welded joint made of dissimilar materials - Google Patents
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Abstract
本发明公开了一种异种材料点焊接头疲劳极限快速预测方法，涉及焊接结构疲劳可靠性评估技术领域。该方法包括：通过确定第三阶段中每个载荷水平所对应的疲劳温升斜率；对多个疲劳温升斜率按指定规则进行划分，得到两组疲劳温升斜率数据；对两组疲劳温升斜率数据分别进行线性拟合得到两直线，将两直线的交点所对应的载荷水平值作为异种材料点焊接头的疲劳极限预测值，也即，将温升斜率出现转折所对应的载荷大小作为疲劳损伤机制发生改变的临界点，并以此预测异种材料点焊接头的疲劳极限，通过本发明的方法能够实现异种材料点焊接头疲劳极限的快速预测，其结果具有真实性、可靠性。
The invention discloses a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials, and relates to the technical field of fatigue reliability evaluation of welded structures. The method includes: determining the fatigue temperature rise slope corresponding to each load level in the third stage; dividing multiple fatigue temperature rise slopes according to specified rules to obtain two sets of fatigue temperature rise slope data; The slope data are linearly fitted to obtain two straight lines, and the load level value corresponding to the intersection of the two straight lines is used as the fatigue limit prediction value of the spot welded joint of dissimilar materials, that is, the load corresponding to the turning point of the temperature rise slope is used as the fatigue limit value. The critical point at which the damage mechanism changes, and the fatigue limit of the dissimilar material spot welded joint can be predicted based on this. The method of the invention can realize the rapid prediction of the fatigue limit of the dissimilar material spot welded joint, and the result has authenticity and reliability.
Description
技术领域technical field
本发明涉及焊接结构疲劳可靠性评估技术领域，更具体的涉及一种异种材料点焊接头疲劳极限快速预测方法。The invention relates to the technical field of fatigue reliability evaluation of welded structures, and more particularly to a method for quickly predicting the fatigue limit of spot welded joints of dissimilar materials.
背景技术Background technique
高速、重载、节能、安全、舒适是现代铁道交通运输的主要特点，而结构轻量化是实现上述目标的有效途径。SUS301L奥氏体不锈钢列车凭借其自重轻、防腐蚀性好、抗拉强度高、美观安全等优点获得广泛应用；Q235低碳钢可焊性好，且具有淬硬性，绝大多低碳钢的焊接接头和热影响区硬度都高于母材。部分车体部件采用不锈钢与碳钢异种材料点焊的连接方法，既能综合利用二者优势，又能克服传统薄板电弧焊因焊接变形大而导致接头强度降低的问题。但是两种材料的过载力学性能和疲劳性能，以及断裂机制完全不同，故二者点焊接头疲劳极限的可靠预测是其结构设计过程中极其重要的一个环节。High speed, heavy load, energy saving, safety and comfort are the main characteristics of modern railway transportation, and lightweight structure is an effective way to achieve the above goals. SUS301L austenitic stainless steel is widely used due to its advantages of light weight, good corrosion resistance, high tensile strength, beautiful and safe; The hardness of the joint and heat affected zone is higher than that of the base metal. Part of the body parts adopts the spot welding method of stainless steel and carbon steel dissimilar materials, which can not only comprehensively utilize the advantages of both, but also overcome the problem that the joint strength of traditional thin-plate arc welding is reduced due to large welding deformation. However, the overload mechanical properties, fatigue properties and fracture mechanisms of the two materials are completely different, so the reliable prediction of the fatigue limit of the two spot welded joints is an extremely important link in the structural design process.
近年来，红外热像法凭借其全场、实时、非接触和非破坏等优点逐步受到国内外学者的青睐，并将其应用到疲劳极限的研究当中。而现有基于红外热像技术的疲劳预测方法主要基于Risitano单线法和Luong双线法，Risitano等经过十五年的潜心研究，发现在高于疲劳极限的载荷作用下，材料疲劳破坏过程中温度稳定阶段的温度值和载荷大小有近似的线性关系；而在低于疲劳极限的载荷作用下，材料的温度变化很小，材料的疲劳极限可以通过绘制不同载荷水平下温度稳定阶段的温升值与载荷之间的直线关系来确定。Luong等研究发现，尽管当外加载荷低于疲劳极限时不会发生疲劳破坏，但是非塑性效应(如粘性效应)同样会引起温度变化，通过将疲劳极限之上和之下的两组温度数据进行线性拟合，得到两条直线的交点就是材料的疲劳极限。In recent years, infrared thermal imaging has gradually been favored by scholars at home and abroad due to its advantages of full-field, real-time, non-contact and non-destructive, and has been applied to the study of fatigue limit. The existing fatigue prediction methods based on infrared thermal imaging technology are mainly based on the Risitano single-line method and the Luong double-line method. After fifteen years of painstaking research, Risitano et al. There is an approximate linear relationship between the temperature value in the stable stage and the size of the load; and under the load below the fatigue limit, the temperature of the material changes very little. The linear relationship between the loads is determined. Luong et al. found that although fatigue failure does not occur when the applied load is below the fatigue limit, non-plastic effects (such as viscous effects) can also cause temperature changes, by comparing two sets of temperature data above and below the fatigue limit. Linear fitting, the intersection of the two straight lines is the fatigue limit of the material.
然而点焊接头尤其是异种材料的点焊接头，一方面容易产生熔核偏移；另一方面由于循环载荷的作用点位于熔核中心内部，熔核表面在疲劳过程中的温度演化现象难以监测。上述因素使得异种材料点焊接头疲劳温升演化规律与典型的金属材料及对接接头规律存在诸多差异。首先，整个疲劳过程的疲劳温升值不大，这是由于点焊接头本身的结构特点及试样的几何尺寸所决定的，点焊接头承受循环剪切拉伸的作用点即热源点位于熔核中心，再经过板厚方向传热到试样表面，散热较多，只能监测到几度甚至零点几度温升值，但这不影响疲劳温度演化整体趋势及由此而展开的深入研究；再次，经过第一阶段温度快速升高所形成的峰值远低于最终断裂对应的峰值，这是由于点焊接头与纯金属或对接接头试样相比，承载能力低，施加载荷小，外界输入的机械能少；另外，未出现温度稳定的平台阶段，对于占据整个疲劳过程绝大部分周次的第三阶段，温度是一个稳定升高的过程，且整个第三阶段的疲劳温升斜率是一个稳定值，并未出现典型金属材料“三个阶段”的第二阶段平台，因此Risitano单线法和Luong双线法不能适用异种材料点焊接头疲劳极限快速预测。However, spot-welded joints, especially those of dissimilar materials, are prone to nugget offset on the one hand; on the other hand, since the action point of cyclic load is located inside the center of the nugget, it is difficult to monitor the temperature evolution of the nugget surface during the fatigue process. . The above factors make the evolution law of fatigue temperature rise of dissimilar material spot welded joints different from those of typical metal materials and butt joints. First of all, the fatigue temperature rise of the entire fatigue process is not large, which is determined by the structural characteristics of the spot welded joint itself and the geometric size of the sample. In the center, heat is transferred to the surface of the sample through the plate thickness direction, and there is a lot of heat dissipation. Only a few degrees or even a few tenths of a degree temperature rise can be monitored, but this does not affect the overall trend of fatigue temperature evolution and the in-depth research carried out therefrom; again; , the peak value formed by the rapid temperature increase in the first stage is much lower than the peak value corresponding to the final fracture, which is due to the fact that the spot welded joint has lower bearing capacity and lower applied load compared with pure metal or butt joint samples, and the external input There is little mechanical energy; in addition, there is no plateau stage of temperature stability. For the third stage, which occupies most of the cycles in the entire fatigue process, the temperature is a process of stable increase, and the fatigue temperature rise slope of the entire third stage is a stable , the second-stage platform of the "three stages" of typical metal materials does not appear, so the Risitano single-line method and the Luong double-line method cannot be used to quickly predict the fatigue limit of spot welded joints of dissimilar materials.
发明内容SUMMARY OF THE INVENTION
本发明实施例提供一种异种材料点焊接头疲劳极限快速预测方法，用以解决现有技术Risitano单线法和Luong双线法不能适用异种材料点焊接头疲劳极限快速预测的问题。The embodiment of the present invention provides a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials, which is used to solve the problem that the prior art Risitano single-line method and Luong double-line method cannot be applied to the rapid prediction of the fatigue limit of a spot welded joint of dissimilar materials.
本发明实施例提供一种异种材料点焊接头疲劳极限快速预测方法，包括：The embodiment of the present invention provides a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials, including:
S1、在疲劳试样表面喷一层均匀的黑色哑光漆，所述黑色哑光漆的辐射率为0.9；其中，所述疲劳试样为异种材料点焊接头制备而成；S1. Spray a layer of uniform black matte paint on the surface of the fatigue sample, and the emissivity of the black matte paint is 0.9; wherein, the fatigue sample is prepared by spot welding joints of dissimilar materials;
S2、利用红外热像仪实时监测异种材料点焊接头的不锈钢侧熔核及塑性环表面局部热点的温度；S2. Use an infrared thermal imager to monitor the temperature of the stainless steel side nugget and the local hot spot on the surface of the plastic ring of the dissimilar material spot welding joint in real time;
S3、确定不同载荷水平下疲劳温升与循环周次的原始关系；其中，所述疲劳温升为异种材料点焊接头的不锈钢侧熔核及塑性环表面局部热点的最高温度与环境最高温度的差值；S3. Determine the original relationship between the fatigue temperature rise and the cycle times under different load levels; wherein, the fatigue temperature rise is the difference between the maximum temperature of the stainless steel side nugget and the local hot spot on the plastic ring surface of the dissimilar material spot welded joint and the maximum temperature of the environment difference;
S4、对多个所述疲劳温升与循环周次的原始关系进行滤波处理，得到的不同载荷水平下疲劳温升与循环周次的演化趋势；其中，所述疲劳温升与循环周次的演化趋势包括四个阶段，第一阶段为温度快速升高阶段，第二阶段为温度下降阶段，第三阶段为温度稳定升高阶段，第四阶段为冷却阶段；S4. Perform filter processing on the original relationship between the fatigue temperature rise and the cycle times, and obtain the evolution trend of the fatigue temperature rise and the cycle times under different load levels; wherein, the fatigue temperature rise and the cycle times The evolution trend includes four stages, the first stage is the stage of rapid temperature increase, the second stage is the stage of temperature drop, the third stage is the stage of stable temperature increase, and the fourth stage is the cooling stage;
S5、确定第三阶段中每个载荷水平所对应的疲劳温升斜率；其中，所述疲劳温升斜率为循环周次循环百万次的疲劳温升改变量；S5. Determine the fatigue temperature rise slope corresponding to each load level in the third stage; wherein, the fatigue temperature rise slope is the change in fatigue temperature rise for one million cycles of cycles;
S6、对多个疲劳温升斜率按指定规则进行划分，得到两组疲劳温升斜率数据；S6. Divide multiple fatigue temperature rise slopes according to the specified rules, and obtain two groups of fatigue temperature rise slope data;
所述对多个疲劳温升斜率按指定规则进行划分，得到两组温升斜率数据包括：The multiple fatigue temperature rise slopes are divided according to specified rules, and two sets of temperature rise slope data are obtained, including:
对温升斜率小于等于指定阈值的划分为一组；Divide the temperature rise slopes less than or equal to the specified threshold into a group;
对温升斜率大于指定阈值的划分为一组；Divide the temperature rise slopes greater than the specified threshold into a group;
S7、对两组疲劳温升斜率数据分别进行线性拟合得到两直线方程，并将两直线的交点所对应的载荷水平值为异种材料点焊接头的疲劳极限预测值；S7. Perform linear fitting on the two sets of fatigue temperature rise slope data to obtain two straight line equations, and use the load level corresponding to the intersection of the two straight lines as the predicted fatigue limit value of the dissimilar material spot welded joint;
所述对两组疲劳温升斜率数据分别进行线性拟合得到两直线方程分别为：The two linear equations obtained by performing linear fitting on the two groups of fatigue temperature rise slope data are:
θ1＝0.000278567F+0.12346θ 1 =0.000278567F+0.12346
θ2＝1.12259F-6.12676θ 2 =1.12259F-6.12676
其中，F为载荷水平，θ1为第一温升斜率，θ2为第二温升斜率。Among them, F is the load level, θ 1 is the first temperature rise slope, and θ 2 is the second temperature rise slope.
较佳地，所述步骤S7之后还包括：Preferably, after the step S7, it also includes:
S8、利用阶梯法对异种材料点焊接头进行预测，得到疲劳极限试验值；S8. Use the ladder method to predict the spot welded joint of dissimilar materials, and obtain the fatigue limit test value;
S9、将所述疲劳极限预测值与所述疲劳极限试验值进行比较，得到误差值。S9. Compare the predicted fatigue limit value with the fatigue limit test value to obtain an error value.
较佳地，所述异种材料由厚度均为4mm的SUS301L不锈钢和Q235B低碳钢组成。Preferably, the dissimilar materials are composed of SUS301L stainless steel and Q235B low carbon steel with a thickness of 4mm.
较佳地，所述红外热像仪的灵敏度小于0.03℃，温度范围为-20℃～1200℃，图像捕获频率为9Hz。Preferably, the sensitivity of the infrared thermal imager is less than 0.03°C, the temperature range is -20°C to 1200°C, and the image capturing frequency is 9 Hz.
本发明实施例中，通过确定第三阶段中每个载荷水平所对应的疲劳温升斜率；对多个疲劳温升斜率按指定规则进行划分，得到两组疲劳温升斜率数据；对两组疲劳温升斜率数据分别进行线性拟合得到两直线，将两直线的交点所对应的载荷水平值为异种材料点焊接头的疲劳极限预测值，也即，将温升斜率出现转折所对应的载荷大小作为疲劳损伤机制发生改变的临界点，并以此预测异种材料点焊接头的疲劳极限，由于该预测值与试验值进行对比得出的误差为5.21％，具有较高的一致性，因此，本发明能够实现异种材料点焊接头疲劳极限的快速预测，其结果具有真实性、可靠性。In the embodiment of the present invention, by determining the fatigue temperature rise slope corresponding to each load level in the third stage; dividing a plurality of fatigue temperature rise slopes according to specified rules to obtain two sets of fatigue temperature rise slope data; The temperature rise slope data are linearly fitted to obtain two straight lines, and the load level corresponding to the intersection of the two straight lines is the predicted fatigue limit value of the dissimilar material spot welded joint, that is, the load corresponding to the turning point of the temperature rise slope. As the critical point where the fatigue damage mechanism changes, the fatigue limit of spot welded joints of dissimilar materials is predicted based on this. Since the error between the predicted value and the experimental value is 5.21%, it has a high consistency. The invention can realize the rapid prediction of the fatigue limit of the spot welded joint of dissimilar materials, and the result has authenticity and reliability.
附图说明Description of drawings
图1为本发明实施例提供的异种材料点焊接头试样尺寸；Fig. 1 is the sample size of the spot welding joint of dissimilar materials provided by the embodiment of the present invention;
图2为本发明实施例提供的一种异种材料点焊接头疲劳极限快速预测方法的疲劳试验测温系统；FIG. 2 is a fatigue test temperature measurement system of a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials provided by an embodiment of the present invention;
图3为本发明实施例提供的一种异种材料点焊接头疲劳极限快速预测方法的流程示意图；FIG. 3 is a schematic flowchart of a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials according to an embodiment of the present invention;
图4本发明实施例提供的疲劳试样在载荷水平为7.0KN时的疲劳温升与循环周次关系曲线图；4 is a graph showing the relationship between the fatigue temperature rise and the cycle times of the fatigue sample provided by the embodiment of the present invention when the load level is 7.0KN;
图5为本发明实施例提供的劳试样在不同载荷水平下的疲劳温升与循环周次关系曲线图；5 is a graph showing the relationship between the fatigue temperature rise and the cycle times of the fatigue sample under different load levels provided by the embodiment of the present invention;
图6为本发明实施例提供的一种异种材料点焊接头疲劳极限预测图；Fig. 6 is a fatigue limit prediction diagram of a spot welded joint of dissimilar materials provided by an embodiment of the present invention;
图7为本发明实施例提供的利用阶梯法对异种材料点焊接头进行预测的试验结果图。FIG. 7 is a graph of the test results of predicting spot welded joints of dissimilar materials by using a step method according to an embodiment of the present invention.
具体实施方式Detailed ways
下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整地描述，显然，所描述的实施例仅仅是本发明一部分实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都属于本发明保护的范围。The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.
本发明实施例的试验材料与设备Test materials and equipment for embodiments of the present invention
试验选用厚度均为4mm的SUS301L不锈钢和Q235B低碳钢，进行异种材料的搭接双面单点焊，其名义化学成分及主要力学性能如表1、表2所示。为了保证焊接接头质量，施焊前需对焊件表面进行清理，以除掉表面脏物与氧化膜，获得小而均匀一致的接触电阻，这是避免电极粘结、喷溅，保证点焊质量和试验稳定性的主要前提。点焊试验在悬挂式点焊机上进行，电极材料为CrZrCu，电极直径22mm，电极端头球面半径为100mm，电极行程为30mm。点焊后按照JIS Z3140-2000、JIS Z3139-2000标准进行外观检验、平滑度检验及断面检验。In the test, SUS301L stainless steel and Q235B low carbon steel with a thickness of 4 mm were selected for lap joint double-sided single-spot welding of dissimilar materials. The nominal chemical composition and main mechanical properties are shown in Table 1 and Table 2. In order to ensure the quality of the welded joints, the surface of the weldment should be cleaned before welding to remove the surface dirt and oxide film to obtain a small and uniform contact resistance, which is to avoid electrode sticking and splashing and ensure the quality of spot welding. and the main premise of test stability. The spot welding test was carried out on a hanging spot welding machine. The electrode material was CrZrCu, the electrode diameter was 22 mm, the spherical radius of the electrode tip was 100 mm, and the electrode stroke was 30 mm. After spot welding, the appearance inspection, smoothness inspection and cross-sectional inspection are carried out according to JIS Z3140-2000 and JIS Z3139-2000 standards.
表1试验材料名义化学成分Table 1 Nominal chemical composition of test materials
表2试验材料主要力学性能Table 2 Main mechanical properties of test materials
本发明实施例中，选用SUS301L-Q235B异种材料点焊接头作为疲劳试样，且疲劳试样的疲劳温度演化试验在PLG–200D高频疲劳试验机上进行，依据ISO14234-2003标准，疲劳试样尺寸如图1所示(单位：mm)，试样纵向为板材轧制方向，板状试验片边缘需适当修整，试件要求对称和具有足够的精度。In the embodiment of the present invention, SUS301L-Q235B dissimilar material spot welded joint is selected as the fatigue sample, and the fatigue temperature evolution test of the fatigue sample is carried out on the PLG-200D high-frequency fatigue testing machine. According to the ISO14234-2003 standard, the size of the fatigue sample is As shown in Figure 1 (unit: mm), the longitudinal direction of the sample is the rolling direction of the plate, the edge of the plate-shaped test piece needs to be properly trimmed, and the test piece requires symmetry and sufficient accuracy.
具体地，为了提高金属表面的辐射率，在疲劳试样表面喷上一层均匀的黑色哑光漆，其辐射率为0.9。疲劳试验时应力比R＝0.1，以正弦波方式加载，指定寿命为2x106次。Specifically, in order to improve the emissivity of the metal surface, a uniform layer of black matte paint was sprayed on the surface of the fatigue specimen, and its emissivity was 0.9. In the fatigue test, the stress ratio R=0.1, the load is in the form of sine wave, and the specified life is 2×10 6 times.
图2为本发明实施例提供的一种异种材料点焊接头疲劳极限快速预测方法的疲劳试验测温系统，该系统采用美国Fluke公司生产的Ti450红外热像仪记录点焊接头不锈钢侧熔核及塑性环局部热点的温度变化，红外热像仪灵敏度不大于0.03℃，温度范围-20～1200℃，图像捕获频率为9Hz，红外热像仪放置在距离试样30cm处，垂直试样表面拍摄。Fig. 2 is a fatigue test temperature measurement system of a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials provided by an embodiment of the present invention. For the temperature change of the local hot spot of the plastic ring, the sensitivity of the infrared thermal imager is not greater than 0.03 °C, the temperature range is -20 to 1200 °C, and the image capture frequency is 9 Hz.
力学性能测试，为了得到US301L-Q235B异种材料点焊接头的拉伸性能参数以确定疲劳试验所加初始载荷的水平，分别取三组试样进行静载拉伸试验，测得其抗拉强度如表3所示。可以看出，在试验所选取的工艺参数下，由于点焊搭接接头本身特点所导致的偏心剪切拉伸作用，点焊接头的抗拉强度低于母材。Mechanical property test, in order to obtain the tensile property parameters of US301L-Q235B dissimilar material spot welded joints to determine the level of initial load applied in the fatigue test, three groups of samples were taken for static load tensile test, and the tensile strength was measured as follows: shown in Table 3. It can be seen that under the process parameters selected in the test, the tensile strength of the spot welded joint is lower than that of the base metal due to the eccentric shear tensile effect caused by the characteristics of the spot welded lap joint itself.
表3 US301L-Q235B异种材料点焊接头静载拉伸性能Table 3 Static tensile properties of US301L-Q235B dissimilar material spot welded joints
图3为本发明实施例提供的一种异种材料点焊接头疲劳极限快速预测方法的流程示意图。如图3所示，该方法包括:FIG. 3 is a schematic flowchart of a method for quickly predicting the fatigue limit of a spot welded joint of dissimilar materials according to an embodiment of the present invention. As shown in Figure 3, the method includes:
S1、在疲劳试样表面喷一层均匀的黑色哑光漆，该黑色哑光漆的辐射率为0.9；其中，该疲劳试样为异种材料点焊接头制备而成。S1. Spray a layer of uniform black matte paint on the surface of the fatigue sample, and the emissivity of the black matte paint is 0.9; wherein, the fatigue sample is prepared by spot welding joints of dissimilar materials.
S2、利用红外热像仪实时监测异种材料点焊接头的不锈钢侧熔核及塑性环表面局部热点的温度。S2. Use an infrared thermal imager to monitor the temperature of the stainless steel side nugget and the local hot spot on the surface of the plastic ring of the dissimilar material spot welding joint in real time.
在实施例中，对SUS301L-Q235B异种材料点焊接头疲劳试样在不同载荷水平下进行高频拉-拉载荷疲劳试验，分别选取5.0KN，5.5KN，6.0KN，6.5KN，7.0KN，7.5KN，8.0KN共七个载荷水平。试验同时，借助高性能红外热像仪对点焊接头SUS301L不锈钢侧熔核及塑性环表面的局部热点进行监测，记录下点焊接头在整个疲劳试验过程中的温度演化数据。In the examples, high-frequency tensile-tensile load fatigue tests were performed on SUS301L-Q235B dissimilar material spot welded joint fatigue samples under different load levels, and 5.0KN, 5.5KN, 6.0KN, 6.5KN, 7.0KN, 7.5KN were selected respectively. KN, 8.0KN a total of seven load levels. At the same time, the high-performance infrared thermal imaging camera was used to monitor the local hot spots on the SUS301L stainless steel side nugget and plastic ring surface of the spot welded joint, and record the temperature evolution data of the spot welded joint during the entire fatigue test process.
S3、确定不同载荷水平下疲劳温升与循环周次的原始关系；其中，该疲劳温升为异种材料点焊接头的不锈钢侧熔核及塑性环表面局部热点的最高温度与环境最高温度的差值。S3. Determine the original relationship between the fatigue temperature rise and the cycle times under different load levels; where the fatigue temperature rise is the difference between the maximum temperature of the stainless steel side nugget and the local hot spot on the plastic ring surface of the dissimilar material spot welded joint and the maximum temperature of the environment value.
S4、对多个该疲劳温升与循环周次的原始关系进行滤波处理，得到的不同载荷水平下疲劳温升与循环周次的演化趋势；其中，该疲劳温升与循环周次的演化趋势包括四个阶段，第一阶段为温度快速升高阶段，第二阶段为温度下降阶段，第三阶段为温度稳定升高阶段，第四阶段为冷却阶段。S4. Perform filter processing on the original relationship between the fatigue temperature rise and the cycle times, and obtain the evolution trend of the fatigue temperature rise and the cycle times under different load levels; among them, the evolution trend of the fatigue temperature rise and the cycle times It includes four stages, the first stage is the stage of rapid temperature increase, the second stage is the stage of temperature drop, the third stage is the stage of stable temperature increase, and the fourth stage is the stage of cooling.
其中，为了统一分析比较，将每一时刻点焊接头试样SUS301L不锈钢侧熔核及塑性环表面局部热点的最高温度与环境最高温度的差值作为疲劳温升ΔT，以循环周次N为横坐标，以疲劳温升ΔT为纵坐标，建立异种点焊接头疲劳过程的原始温度演化曲线。以载荷水平为7.0KN的试样为例。由于弹性效应的存在，异种材料点焊接头疲劳过程的原始温度是一个不断震荡的过程，需借助Matlab软件对其进行滤波处理，以消除弹性效应获得点焊接头疲劳过程温度演化的整体趋势。经Matlab滤波后得到的整体温度演化趋势如图4所示，其中(a)为异种材料点焊接头试样在可见光下拍摄的照片，(b)到(f)为关键点所对应的红外热像图。可以看出，SUS301L-Q235B异种材料点焊接头在循环载荷作用下熔核及塑性环表面局部热点的温度变化分四个阶段，即由(b)到(c)的温度快速上升阶段、由(c)到(d)的温度下降阶段、由(d)到的(f)温度稳定上升阶段以及随后的自然冷却阶段，这与报道中有关纯金属材料“三个阶段”及对接接头“五个阶段”的典型特征存在较明显的差异。首先，整个疲劳过程的疲劳温升值不大，这是由于点焊接头本身的结构特点及试样的几何尺寸所决定的，点焊接头承受循环剪切拉伸的作用点即热源点位于熔核中心，经过4mm板厚传热到试样表面，散热较多，只能监测到几度甚至零点几度温升值，但这不影响疲劳温度演化整体趋势及由此而展开的深入研究；再次，经过第一阶段温度快速升高所形成的(c)处峰值远低于最终断裂对应的(f)处峰值，这是由于点焊接头与纯金属或对接接头试样相比，承载能力低，施加载荷小，外界输入的机械能少；另外，未出现温度稳定的平台阶段，对于占据整个疲劳过程绝大部分周次的第三阶段，温度是一个稳定升高的过程，且整个第三阶段的疲劳温升斜率是一个稳定值。Among them, for the purpose of unified analysis and comparison, the difference between the maximum temperature of the SUS301L stainless steel side nugget and local hot spot on the plastic ring surface of the welded joint sample at each time point and the maximum temperature of the environment is taken as the fatigue temperature rise ΔT, and the cycle number N is the horizontal Coordinate, take the fatigue temperature rise ΔT as the ordinate, establish the original temperature evolution curve of the fatigue process of dissimilar spot welded joints. Take a sample with a load level of 7.0KN as an example. Due to the existence of the elastic effect, the original temperature of the fatigue process of the dissimilar material spot welding joint is a process of constant oscillation, which needs to be filtered with the help of Matlab software to eliminate the elastic effect and obtain the overall trend of the temperature evolution of the fatigue process of the spot welded joint. The overall temperature evolution trend obtained after filtering by Matlab is shown in Figure 4, where (a) is the photo of the dissimilar material spot welded joint sample taken under visible light, (b) to (f) are the infrared heat corresponding to the key points. image. It can be seen that the temperature changes of SUS301L-Q235B dissimilar material spot welded joints under the action of cyclic load of nugget and local hot spots on the surface of plastic ring are divided into four stages, namely the stage of rapid temperature rise from (b) to (c), and the stage from (b) to (c). c) the temperature drop stage from (d) to (d), the stable temperature rise stage from (d) to (f) and the subsequent natural cooling stage, which are related to the “three stages” of pure metal materials and the “five stages” of butt joints reported in the report. There are obvious differences in the typical characteristics of stage". First of all, the fatigue temperature rise of the entire fatigue process is not large, which is determined by the structural characteristics of the spot welded joint itself and the geometric size of the sample. In the center, the heat is transferred to the surface of the sample through the thickness of 4mm, and the heat dissipation is large, and only a few degrees or even a few tenths of a degree temperature rise can be monitored, but this does not affect the overall trend of fatigue temperature evolution and the in-depth research carried out therefrom; again, The peak at (c) formed by the rapid temperature increase in the first stage is much lower than the peak at (f) corresponding to the final fracture, which is due to the lower bearing capacity of spot welded joints compared with pure metal or butt joint specimens. The applied load is small, and the mechanical energy input from the outside is small; in addition, there is no plateau stage of temperature stability. For the third stage, which occupies most of the cycles in the entire fatigue process, the temperature is a process of stable increase, and the entire third stage is The fatigue temperature rise slope is a stable value.
为了深入研究疲劳温升与疲劳极限的内在关系，将七个点焊接头试样的疲劳温升与循环周次的关系曲线建立在同一坐标系下，如图5所示。可以看出，随着载荷水平的升高，疲劳温升的峰值逐渐升高，且温度变化率即单位周次升高的温度越大。其中，载荷水平为5.0KN和6.5KN的试样在循环周次达到200万的预设次数时未发生断裂，其疲劳温升是一个震荡平台，温度改变量较小；而发生疲劳断裂的试样均出现了显著温度变化。因此，尝试将温升斜率出现转折所对应的载荷大小作为疲劳损伤机制发生改变的临界点，并以此预测异种材料点焊接头的疲劳极限。In order to deeply study the internal relationship between fatigue temperature rise and fatigue limit, the relationship between fatigue temperature rise and cycle times of seven spot welded joint specimens was established in the same coordinate system, as shown in Figure 5. It can be seen that with the increase of the load level, the peak value of the fatigue temperature rise gradually increases, and the temperature change rate, that is, the temperature increase per unit cycle, is larger. Among them, the samples with load levels of 5.0KN and 6.5KN did not fracture when the cycle times reached 2 million preset times, their fatigue temperature rise was an oscillation platform, and the temperature change was small; while the fatigue fracture occurred in the samples. There were significant temperature changes in all samples. Therefore, an attempt was made to take the load corresponding to the turning point of the temperature rise slope as the critical point for the change of the fatigue damage mechanism, and use this to predict the fatigue limit of spot welded joints of dissimilar materials.
S5、确定第三阶段中每个载荷水平所对应的疲劳温升斜率；其中，该疲劳温升斜率为循环周次循环百万次的疲劳温升改变量。S5. Determine the fatigue temperature rise slope corresponding to each load level in the third stage; wherein, the fatigue temperature rise slope is the change in fatigue temperature rise for one million cycles per cycle.
S6、对多个疲劳温升斜率按指定规则进行划分，得到两组疲劳温升斜率数据。S6. Divide a plurality of fatigue temperature rise slopes according to a specified rule, and obtain two groups of fatigue temperature rise slope data.
其中，所述对多个疲劳温升斜率按指定规则进行划分，得到两组温升斜率数据包括：Wherein, the plurality of fatigue temperature rise slopes are divided according to specified rules, and two sets of temperature rise slope data are obtained, including:
对温升斜率小于等于指定阈值的划分为一组。Divide the temperature rise slopes less than or equal to the specified threshold into a group.
对温升斜率大于指定阈值的划分为一组。Divide into a group the temperature rise slopes greater than the specified threshold.
所述指定阈值为经验值，在本发明实施例中，可将该阈值设定为0.5。The specified threshold is an empirical value, and in this embodiment of the present invention, the threshold may be set to 0.5.
S7、对两组疲劳温升斜率数据分别进行线性拟合得到两直线方程，将两直线的交点所对应的载荷水平值为异种材料点焊接头的疲劳极限预测值。S7. Perform linear fitting on the two sets of fatigue temperature rise slope data to obtain two straight line equations, and use the load level corresponding to the intersection of the two straight lines as the fatigue limit prediction value of the dissimilar material spot welded joint.
其中，建立第三阶段温升斜率与载荷水平间的关系图，如图6所示。从图可不难看出，载荷水平为5.0KN、5.5KN和6.5KN所对应的温升斜率很微小，小于指定阈值0.5，其余四个载荷水平的温升斜率发生了显著变化。为了确定这种温升斜率出现转折所对应的载荷大小，并以此作为疲劳极限预测值，分别对两组疲劳温升斜率数据进行线性拟合，可得两直线方程分别为：Among them, the relationship between the slope of the temperature rise in the third stage and the load level is established, as shown in Figure 6. It is not difficult to see from the figure that the temperature rise slopes corresponding to the load levels of 5.0KN, 5.5KN and 6.5KN are very small, less than the specified threshold of 0.5, and the temperature rise slopes of the other four load levels have changed significantly. In order to determine the load corresponding to the turning point of the temperature rise slope, and use it as the predicted value of the fatigue limit, the two sets of fatigue temperature rise slope data were linearly fitted, and the two straight line equations were obtained:
θ1＝0.000278567F+0.12346 (1)θ 1 = 0.000278567F+0.12346 (1)
θ2＝1.12259F-6.12676 (2)θ 2 =1.12259F-6.12676 (2)
其中，将(1)式跟(2)式联立，两直线交点所对应的横坐标为5.569KN。因此，得到异种材料点焊接头的疲劳极限预测值为5.569KN。Among them, the formula (1) is combined with the formula (2), and the abscissa corresponding to the intersection of the two straight lines is 5.569KN. Therefore, the predicted fatigue limit value of the spot welded joint of dissimilar materials is 5.569KN.
另外，为了验证本发明疲劳极限预测方法的真实性与可靠性，随后进行了阶梯法疲劳试验。In addition, in order to verify the authenticity and reliability of the fatigue limit prediction method of the present invention, the ladder method fatigue test was subsequently carried out.
S8、利用阶梯法对异种材料点焊接头进行预测得到疲劳极限试验值。S8, using the ladder method to predict the fatigue limit test value of the dissimilar material spot welded joint.
S9、将所述疲劳极限预测值与所述疲劳极限试验值进行比较，得到误差值。S9. Compare the predicted fatigue limit value with the fatigue limit test value to obtain an error value.
阶梯法疲劳验证试验Ladder method fatigue verification test
根据GB/T15111-94《点焊接头剪切拉伸疲劳试验方法》标准，采用2X106次作为判别标准开展疲劳试验，当试样超过预设循环周次2X106而未产生明显的宏观裂纹，视为“通过”，否则视为“不通过”。根据经验及点焊接头静载拉伸试验所得抗拉强度的大小确定疲劳试验初始载荷大小，疲劳试验中施加的疲劳载荷幅度从7KN开始，若在前一次试验中试件没有通过，则在下一次试验中将载荷降低0.5KN；反之，则增加0.5KN。按照此规则在相同的加载频率与应力比下重复进行疲劳试验，根据GB/T24178-2009《金属材料疲劳试验数据统计方案与分析方法》，最少选取8根试样用于解释试验，在4个等间距的载荷水平下，每个载荷水平测试两个试样，试验结果如图7所示。According to GB/T15111-94 "Spot Welded Joint Shear Tensile Fatigue Test Method", 2X106 times are used as the criterion to carry out the fatigue test. When the sample exceeds the preset cycle times 2X106 without obvious macro cracks, the It is "passed", otherwise it is regarded as "failed". The initial load of the fatigue test is determined according to the experience and the tensile strength obtained from the static load tensile test of the spot welded joint. The fatigue load amplitude applied in the fatigue test starts from 7KN. In the test, reduce the load by 0.5KN; otherwise, increase it by 0.5KN. According to this rule, the fatigue test is repeated under the same loading frequency and stress ratio. According to GB/T24178-2009 "Statistical Scheme and Analysis Method of Fatigue Test Data for Metal Materials", at least 8 samples are selected for interpretation test. Under equally spaced load levels, two specimens were tested for each load level, and the test results are shown in Figure 7.
对试验数据进行统计分析，可以得到该异种材料点焊接头在2X106循环周次下所对应的疲劳极限为：Statistical analysis of the test data shows that the fatigue limit of the dissimilar material spot welded joint under 2× 10 6 cycles is:
其中，利用本发明方法疲劳极限预测值与利用阶梯法得到的疲劳极限试验值相比较，并计算其误差：Wherein, the fatigue limit predicted value using the method of the present invention is compared with the fatigue limit test value obtained by using the ladder method, and the error is calculated:
因此，δ为疲劳极限预测值与疲劳极限试验值的误差，且δ为5.21％，具有较高的一致性，能够实现异种材料点焊接头疲劳极限的快速预测，同时，本发明所提出方法有望为不等厚异种材料等非均质焊接结构服役行为的研究提供理论指导和技术支持。Therefore, δ is the error between the predicted value of fatigue limit and the test value of fatigue limit, and δ is 5.21%, which has high consistency and can realize rapid prediction of fatigue limit of spot welded joints of dissimilar materials. At the same time, the method proposed in the present invention is expected to Provide theoretical guidance and technical support for the research on service behavior of heterogeneous welded structures such as unequal thickness dissimilar materials.
本发明实施例将SUS301L-Q235B异种材料点焊接头在循环载荷作用下熔核及塑性环表面局部热点的温度变化分为四个阶段，随着载荷水平的升高，疲劳温升的峰值逐渐升高，且温度变化率即单位周次升高的温度越大。且整个第三阶段的疲劳温升斜率是一个稳定值。因此利用通过确定第三阶段中每个载荷水平所对应的疲劳温升斜率；对多个疲劳温升斜率按指定规则进行划分，得到两组疲劳温升斜率数据；对两组疲劳温升斜率数据分别进行线性拟合得到两直线，将两直线的交点所对应的载荷水平值为异种材料点焊接头的疲劳极限预测值，也即，将温升斜率出现转折所对应的载荷大小作为疲劳损伤机制发生改变的临界点，并以此预测异种材料点焊接头的疲劳极限，得到点焊接头疲劳极限的预测值为5.569KN，并且进行了阶梯法疲劳验证试验，得到该异种材料点焊接头在2X106循环周次下所对应的疲劳极限为5.875KN，预测值与试验值之间的误差为5.21％，具有较高的一致性。In the embodiment of the present invention, the temperature change of the SUS301L-Q235B dissimilar material spot welded joint under the action of cyclic load is divided into four stages. The peak value of the fatigue temperature rise gradually increases with the increase of the load level The higher the temperature, the greater the temperature change rate, that is, the temperature that increases per unit cycle. And the fatigue temperature rise slope of the whole third stage is a stable value. Therefore, by determining the fatigue temperature rise slope corresponding to each load level in the third stage; dividing multiple fatigue temperature rise slopes according to the specified rules to obtain two groups of fatigue temperature rise slope data; for the two groups of fatigue temperature rise slope data Two straight lines are obtained by linear fitting respectively, and the load level corresponding to the intersection of the two straight lines is the predicted value of the fatigue limit of the dissimilar material spot welded joint, that is, the load corresponding to the turning point of the temperature rise slope is taken as the fatigue damage mechanism. The critical point of the change was used to predict the fatigue limit of the dissimilar material spot welded joint, and the predicted value of the fatigue limit of the spot welded joint was 5.569KN, and the ladder method fatigue verification test was carried out to obtain the dissimilar material spot welded joint at 2X106. The fatigue limit corresponding to the cycle times is 5.875KN, and the error between the predicted value and the experimental value is 5.21%, which has a high consistency.
以上公开的仅为本发明的几个具体实施例，本领域的技术人员可以对本发明进行各种改动和变型而不脱离本发明的精神和范围，倘若本发明的这些修改和变型属于本发明权利要求及其等同技术的范围之内，则本发明也意图包含这些改动和变型在内。The above disclosures are only a few specific embodiments of the present invention, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention, provided that these modifications and modifications of the present invention belong to the rights of the present invention The present invention is also intended to include these changes and modifications within the scope of the claims and their equivalents.
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