Alternative method for detecting defects in composite materials

In 2017, researchers developed a two-step inverse problem that allowed for three-dimensional thermographic imaging. Known as the virtual wave concept, the method allows for the full multidimensional evaluation of temperature data. Thummerer et al. extended the method to develop a protocol for testing composite materials, which could provide quicker results on defects in anisotropic materials.

“We wanted to introduce a fast and reliable multidimensional thermographic defect detection method for anisotropic material, such as composites,” said author Gregor Thummerer. “During preliminary theoretical considerations, we found that using the virtual wave concept, we can establish a totally new thermophysical parameter estimation method.”

The virtual wave concept transforms the thermographic data into “virtual” ultrasonic data and uses ultrasonic reconstruction methods to detect defects. Based on this idea, Thummerer et al. established a relationship between the virtual speed of the sound tensor and the thermal diffusivity tensor to estimate the anisotropic thermal diffusivity tensor. Using the estimated tensor allows, in a second inversion step, the reconstruction of the initial temperature of a material after it had been heated by laser.

In anisotropic materials such as an airplane wing made of carbon fibers, heat may diffuse at different speeds along different directions. This distorts the appearance of defects seen in thermographic data of the material. Using the presented parameter and image reconstruction method, the anisotropic thermophysical parameters could be estimated and used to correct this distortion.

The authors tested their approach on simulated and experimental data conducted on a unidirectional carbon fiber reinforced polymer. They hope the method can be used to improve the testing of manufactured parts in aerospace and automotive industries that use composite components.

Source: “Photothermal testing of composite materials: Virtual wave concept with prior information for parameter estimation and image reconstruction,” by G. Thummerer, G. Mayr, and P. Burgholzer, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0016364 .