Design and development of a measuring method to determine the strain state during hot crack initiation

在过去的几十年里，激光焊接技术得到了显著的发展，并被确立为一种高效、经济的工业工具。凝固开裂以及材料的可焊性多年来一直是一个备受争议的问题，特别是关于热裂纹形成的原因。本提案的目的是发展一种二维光学技术，以确定在激光焊接过程中凝固裂纹形成所需的局部临界应变和局部临界应变率。为了实现这一目标，首先，焊接池及其附近的视频质量，特别是对于焊接过程中在热裂纹敏感区域凝固的材料，必须通过选择合适的照明和摄像传感器来优化。之后，需要在Open Source Computer Vision Library （OpenCV）中实现最相关的测量算法，以确定拉伸载荷金属试样表面的二维瞬态应变。结果将相互比较，并与ARAMIS系统（数字图像相关系统）的参考测量结果进行比较。测量原理和评价算法应具有足够的精度、较低的计算时间和不需要任何特殊表面处理的评价能力。将选择最适合这些要求的算法进行进一步的开发。下一步是选定算法的发展，以获得糊状区，即在焊接熔池后面的临界温度范围内的位移和应变。挑战在于：如何在运动图像序列的定义区域内进行应变评估。为了克服这一挑战，将测试两个概念。然后，必须开发能够实时分析应变的测量算法。为了降低算法的计算成本，需要对图像分辨率、所需的最小帧速率、所使用的滤波器功能和对硬件平台的要求等几个因素进行优化。主要是由于没有测量技术可以直接确定焊缝附近的位移或应变，因此将使用FEM模型验证所开发算法获得的结果。最后，在定义了测量系统的硬件要求以及测量结果的验证之后，必须修改算法，以便能够自动评估临界应变条件，这可能会导致材料的热裂。

Over the last few decades the laser beam welding technology developed significantly and was established as an efficient and economic tool in industry. Solidification cracking as well as the weldability of materials is still for many years a highly contentious issue, particularly regarding the causes of the hot crack formation. The aim of this proposal is development of a 2-dimentional optical technique to determine the local critical strain and local critical strain rate required for solidification crack formation during laser beam welding. To realize this aim, firstly, the video quality of the weld pool and its vicinity, particularly for a solidified in a hot crack susceptible zone during the welding process material must be optimised with choice of the right lighting and camera sensor. After that, the most relevant measurement algorithms should be implemented in the Open Source Computer Vision Library (OpenCV) to determine the 2D transient strain at the surface of tensile loaded metallic specimens. The results will be compared to each other as well as to the reference measurements from the ARAMIS system (Digital Image Correlation system). The measurement principle and evaluation algorithm should perform with sufficient accuracy, low calculation time and the capability of evaluation without any special surface preparation. The most suitable algorithm corresponding to these requirements will be selected for further development. The next step is the development of the selected algorithm to obtain the displacements and the strains in the mushy-zone i.e. in the critical temperature range behind the weld pool. The challenge is: How can the strain evaluation perform in defined region in moving images sequences. Two concepts will be tested to overcome that challenge. Afterwards, the measurement algorithm must be developed to be able to analysis the strain in realtime. Several factors as the image resolution, the minimum required frame rate, the used filters functions and the requirements for the hardware platform should be optimised to reduce the computational cost of the algorithm. Mainly due to the fact that there is no measuring technique that allows direct determination of displacements or strains next to the weld, the result obtained by the developed algorithm will be validated using the FEM model. Finally, after the definition of hardware requirements for the measuring system as well as validation of the measurements, the algorithm must be modified in order to enable automatic evaluation of the critical strain conditions, which could potentially lead to hot cracking in the material.