Factors affecting the detectability of voids by infrared thermography

During the manufacturing of laminar plastics, debondings between layers can occur due to bad curing. Two inspection techniques by infrared thermography have to be considered: Impulse or transient thermography and lock-in thermograpy. For both techniques, the detectability of a defect and the factors affecting it are major issues. The first investigated material is isotropic polyvinyl chloride, the second is anisotropic unidirectional carbon fiber­ reinforced plastic. This article will discuss the above mentioned factors and will derive some practical rules about the detectability of a defect. For the two base materials analysed, the complex contrast is studied as a function of the defect diameter to depth ratio and the modulation frequency of the heating source. Practical rules will be derived and a comparison between the two techniques will be made. Laminar plastics such as woven fiber-reinforced plastics are becoming more and more widely used due to their good mechanical properties and their ability to be tailored to the intended application. During their manufacturing, debondings between layers can occur due to bad curing. Fiber-reinforced plastics in form of sheets are also used to reinforce civil engineering structures like bridges [1]. The presence of air inclusions or delaminations in the bonding layer between the sheet and the structure can drastically reduce the mechanical strength of the reinforcement. Therefore, a fast testing method is needed in order to detect such flaws. Until now, only the time consuming ultrasonic inspection method came into consideration. Infrared thermography is a particularly well suited inspection method, because it is fast and gives an image of the location of the defects. Two inspection techniques by infrared thermography have to be considered: the impulse or transient thermography [2] and the lock-in thermography [3].