# Spot Weld Inspections Using Active Thermography
## 1 Introduction  
Performing quality inspections during manufacturing has increased enormously in the last decades, and more and more manufacturers are discovering the use of non-destructive inspection methods. Using non-destructive testing, it is no longer needed to manufacture additional parts in order to submit them to destructive inspections. Those inspections are performed in a variety of fields, such as aircraft construction, metal structure manufacturing, composite manufacturing, etc. Several different techniques are being used in order to examine the structural integrity of the manufactured parts. [PERSON] et al. [1] performed ultrasonic testing on riveted joints in aviation construction, [PERSON] et al. [2] compared different non-destructive inspection techniques to assess the quality of welded joints and [PERSON] et al. [3] performed ultrasonic testing on complex titanium and carbon fiber composite joints. It is proven that non-destructive testing can offer a reliable inspection method in order to investigate the structural integrity of materials and joints. In the automotive industry, many spot welds are used to combine parts together. On average, the body work of a car alone consists already of 5000 spot welds [4]. Nowadays, the quality of the spot weld is investigated using destructive research, resulting in many disadvantages. If the quality of the spot weld is not sufficient, a larger batch of spot welds will be disappproved since not every single spot weld can be tested. Therefore, car manufacturers search for alternative solutions in non-destructive inspections. An easy inspection method can be found in visual inspections; however, these examinations can only be performed by someone skilled in the art. An alternative can be found in penetrant testing. This technique is suitable to detectcracks, but is not capable of showing defects inside the material. Besides penetrant testing, another surface examination technique can be found in eddy current testing [5; 6; 7]. Performing subsurface inspections is possible using ultrasonic testing, but extreme accuracy is needed to place the transducer in the middle of the spot weld. Performing visual surface inspection of spot welds is not sufficient since a spot weld can look decent although no internal structure change has occurred. As explained in [8], a proper spot weld contains multiple sections. The weld nugget has a different internal structure in comparison to the region around it. Therefore, it is important to inspect the surface of a spot weld as well as the internal structure. An alternative approach can be found in active thermography for non-destructive inspections [8; 9; 10]. Several efforts have been made regarding the use of active thermography for spot weld inspections [11; 12; 13; 14]. Scientific research often focuses on the ability to inspect spot welds using a specific measurement setup. [PERSON] and [PERSON] [5] and [PERSON] et al. [14] performed thermography using light sources as external heating, and [PERSON] et al. [12] and [PERSON] et al. [13] combined thermography with laser excitation. However, in a manufacturing process, not every measurement technique can be used. It is, for instance, possible to inspect spot welds using laser thermography, but due to safety issues, it is not suitable to be used in the production hall without extensive safety precautions. This manuscript focuses on finding the most optimal measurement setup to perform active thermography inspections during the manufacturing process. Multiple excitation methods are described in this manuscript, divided in several paragraphs based on the used heating method. For each measurement setup, a variety of parameters is tested and compared. Besides a variation in excitation parameters, a comparison between two post-processing techniques is performed for each excitation type. The used measurement setups can be found in Section 2, Materials and Methods. Section 3 describes the influence of the different parameter sets and post-processing techniques.