Laser Speckle Photometry for Stress Measuring at Industrial Components

Excessive deformations, cracks or failures occur in industrial practice when components are overloaded. It is therefore desirable to continuously monitor the mechanical stress state of industrial components. Due to their high thermal conductivity, direct copper bonded (DCB) substrates are used in power electronics to meet the high reliability requirements. These DCBs usually carry residual stresses, especially at the edges of the copper layers. The stress concentration can increase under operating conditions or even during the manufacturing process, resulting in cracks and fractures. In order to avoid the critical cracking situation, it is necessary to know the residual stresses. The article presents a possible approach to the measurement of stress states. A new optical method, the Laser Speckle Photometry (LSP), was used in the laboratory on ceramics exposed to bending stress. Laser Speckle photometry is a fast and contactless method for the measurement of the spatial-temporal dynamics of speckle fields with high temporal resolution after mechanical or thermal excitation. During the experiment laser light illuminates a defined surface region. Due to the optical rough surface of the sample, a so-called speckle pattern is reflected and recorded by a CMOS camera system. The speckle pattern depends on the sample surface condition and the mechanical strain condition. The stress-induced changes in the materials structure lead to changes in the speckle field, which is formed by a probing laser. The shift of speckle-field is analysed by statistical methods, using co-occurrence matrix and correlation functions. Correlations between stress condition and measurement signal were observed and evaluated. This resulting measurement signal was then calibrated using mechanical stresses determined using finite element (FE) simulations. The authors present the transferability of previous local stress measurements on Al2O3 ceramics to LTCC ceramics. It is shown, that laser speckle photometry is a suitable instrument for non-destructive characterization and monitoring of stress states in ceramics.