Quantitative passive thermography for evaluation of fatigue damage in an intrinsic hybrid composite

In consequence of recent developments in lightweight design, structures composed of different materials have become of high interest for structural applications. Combining a variety of advantageous material properties, metal-carbon fiber reinforced polymer (CFRP) hybrid-composite are one particular class of those. However, the challenging production process and mechanical loading often result in imperfections and defects. Due to recent research and technological improvements, non-destructive testing methods are capable to detect such damages and imperfections with reasonable effort and there fore are often applied in industry. However, it is questionable how to evaluate the severity of detected defects in terms of the structural integrity or mechanical performance. Within this work a new approach is proposed to evaluate fatigue damage using passive infrared thermography, which is applied during mechanical testing. This technique is well known to capture the dissipated heat due to damage. Best results for damage detection capabilities and values of dissipated heat can be obtained using the lock-in transformation, as it is less affected by camera calibration coefficients and experimental setup leading to the characterization of damage mechanisms and the measurement of defect size. The cross correlation of the measured defect size with mechanical values (e.g. dynamic stiffness, mechanical hysteresis) allows defining the residual bearable load and describing the dependency between the degradation of mechanical properties and the defect size.