Sapieta Milan, Dekýš Vladimír, Štalmach Ondrej, Sapietová Alžbeta, Svoboda Martin
Faculty of Mechanical Engineering, University of Zilina, Unierzitna 1, 010 26 Zilina, Slovakia.
Faculty of Mechanical Engineering, Jan Evangelista Purkyně University, 400 96 Ústí nad Labem, Czech Republic.
Materials (Basel). 2021 Sep 17;14(18):5359. doi: 10.3390/ma14185359.
The aim of this work is to verify the presence of deformation in the metal specimen from the material AISI 316L by means of lock-in thermography. The specimen was cyclically loaded by the three-point bending in the fatigue testing machine. A response of the specimen to such excitation can be detected in the infrared spectrum and to determine temperature changes during a loading cycle. By means of the lock-in method, an increased signal to noise ratio (radiation energy detected by an infrared camera) was achieved. Besides, the temperature changes were determined on the basis of amplitudes of radiant energy changes detected by the camera. The temperature change (all radiant energy) corresponds with the first invariant of the tensor of deformation and, after a calculation and regarding the material parameters, also the invariant of the stress tensor. The proportionality between the signal from the camera and the first deformation invariant is achieved if the specimen load is an adiabatic event. This process is achieved by choosing a sufficiently high load frequency. In case of a presence origin of plastic deformations, there takes place only part of radiant energy. When we accept the hypothesis of a presence of just elastic deformations and plastic deformation is also present in the monitored process, then the evaluated thermograms based on the assumption of the presence of elastic deformation present anomalies in a distribution of the determined tensor invariant of deformations. These anomalies are caused by a presence of plastic deformations. Based on the anomalies, plastic deformation can be detected and subsequently analyzed. For the tested specimen and the applied load, the calculation of stress tensor was performed. It confirmed a congruence of results obtained by the analysis of the physical process in the infrared spectrum of the mid-wave infrared camera.
这项工作的目的是通过锁相热成像技术验证AISI 316L金属试样中是否存在变形。该试样在疲劳试验机中通过三点弯曲进行循环加载。可以在红外光谱中检测试样对这种激励的响应，并确定加载循环期间的温度变化。通过锁相方法，实现了更高的信噪比（红外相机检测到的辐射能量）。此外，根据相机检测到的辐射能量变化幅度确定温度变化。温度变化（所有辐射能量）与变形张量的第一不变量相对应，经过计算并考虑材料参数后，也与应力张量的不变量相对应。如果试样加载是一个绝热事件，则相机信号与第一变形不变量之间实现比例关系。这个过程通过选择足够高的加载频率来实现。在存在塑性变形源的情况下，只发生部分辐射能量。当我们接受仅存在弹性变形的假设，而在监测过程中也存在塑性变形时，基于弹性变形存在的假设评估的热成像图在确定的变形张量不变量分布中会出现异常。这些异常是由塑性变形的存在引起的。基于这些异常，可以检测并随后分析塑性变形。对于测试的试样和施加的载荷，进行了应力张量的计算。它证实了通过对中波红外相机红外光谱中的物理过程进行分析所获得的结果的一致性。
