APPLICATIONS OF INFRARED THERMOGRAPHY IN NONDESTRUCTIVE EVALUATION

IRT for NDE is aimed at the discovery of subsurface features (such as subsurface thermal properties, presence of subsurface anomalies/defects), thanks to relevant temperature differences observed on the surface with an infrared (IR) camera. Figure 1 illustrates the general concept.IRT is deployed along two schemes, passive and active. The passive scheme tests materials and structures which are naturally at different (often higher) temperature than ambient while in the case of the active scheme, an external stimulus is necessary to induce relevant thermal contrasts(which are not available otherwise, e.g. specimen at uniform temperature prior to testing).The first law of thermodynamics concerns the principle of energy conservation and states that an important quantity of heat is released by any (industrial) process consuming energy because of the law of entropy. Temperature is thus an essential parameter to measure in order to assess proper operation. Common applications of the passive scheme in NDE are for buildings, components and processes, maintenance, medicine and properties evaluation. Table 1 lists recent reported applications.In these applications, abnormal temperature profiles indicate a potential problem relevant to detect. In passive thermography, the key words if the temperature difference with respect to the surrounding, often referred to as the ’delta-T’ or the “hot spot.” A delta-T of 1 to 2 °C is generally found suspicious while a 4°C value is a strong evidence of abnormal behavior. In most of the applications, passive thermography is rather qualitative since the goal is simply to pinpoint anomalies of the type go/no-go. These applications are generally based on empirical rules applied by experienced personal (know-how of the trade).Some investigations are however more sophisticated and provide quantitative measurements. In these cases, a direct thermal modeling is necessary.For instance in a reported application,such approach is proposed in the case of needles used to sew fabrics in the automobile industry (seat cushions and backs, airbags, etc.). In this study a model is developed to simulate the needle heating during high speed sewing. This help to better understand needle heating which is further confirmed by experimental measurements on the plant floor, Figure 2. Heating (up to100-300°C) comes from friction between the needle and the fabric and increases at high speed causing serious problems such as worn or broken thread, fabric scorching, tempering and weakening needle (sewing is performed in between 1000 to 3000 rpm). Understanding mechanisms of needle heating allows to take actions to optimize sewing operations through for instance needle redesign and needle cooling with significant economic and quality benefits due to the million of sewed products daily.

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