# A New Method for Detecting Weld Stability Based on Color Digital Holography
## 1 Introduction  
With the booming development of the manufacturing industry, welding, as a common permanent connection method, is widely used in fields such as construction [1], bridges [2; 3], aerospace [4; 5; 6], etc. During the welding process, changes in many factors can have an impact on the formation of welds, thereby affecting welding quality and product performance. The weld stability is directly related to the safety and reliability of engineering, so the study of weld stability detection is of great research significance.  
At present, the testing methods for welding quality can be divided into two categories: destructive testing [7; 8; 9] and non-destructive testing. Destructive tests include tensile tests, bending tests, and impact tests. In practical testing, destructive testing methods are widely used due to their low cost, but these methods can cause irreversible damage to the tested object and cannot measure the weld stability of the workpiece in service. Therefore, it cannot provide extensive, long-term, and stable testing for low-yield, high-cost, and structurally complex objects. Non-destructive testing methods include visual inspection [10], penetration testing [11], magnetic particle testing [12], radiographic testing [13; 14; 15], and ultrasonic testing [16]. Among them, visual inspection is the simplest and most commonly usednon-destructive testing method, but its testing results are subjective and unreliable. The penetration testing method marks the surface cracks of the tested object with a penetrant, which can observe surface defects more clearly, but cannot reflect the internal condition of the tested object. Magnetic particle inspection is only applicable to ferromagnetic materials. The ultrasonic testing method is not suitable for objects with large particle sizes. The detection accuracy of radiographic testing methods is high, but they are harmful to human health and have high costs.  
Color digital holographic imaging is an imaging technique based on coherent light interference [17; 18; 19], which is widely used in engineering inspection [20], biomedical inspection [21; 22; 23], optical imaging [24; 25; 26], and other fields. According to the inherent characteristics of interferometric imaging technology, speckle noise often accompanies the imaging process of digital color holography. It causes the boundaries of the wrapped phase to become blurred and discontinuous, making phase unwrapping very challenging. If speckle noise is not removed, the calculated deformation field will be severely distorted. When the surface of the test object is smooth and the stress distribution area is large, traditional spatial frequency domain methods can reduce speckle noise [27]. On the other hand, the surface of the weld seam is relatively rough and the stress distribution area is concentrated, which requires high noise reduction technology. Traditional spatial and frequency domain algorithms cannot meet the noise reduction requirements. Therefore, due to the limitations of traditional denoise technology, there are currently no literature reports on the use of color digital holography technology for detecting weld stability.  
The aim of the present work was to propose a new method to detect the weld stability of aluminum alloy plates in service based on color digital holography. A color digital holography optical path was designed to capture the holograms of aluminum alloy plates under varying loads. Deep learning techniques were employed to eliminate speckle noise in wrapped phases. Finally, the coefficient of variation of lap-jointed, butt-jointed, and defective butt-jointed aluminum alloy plates was calculated and used to evaluate the weld stability.