Experimental determined temperature and strain field correlation in high-speed LFT characterization

The coupling of deformation and conversion of deformation work into heat is topic of interest for nearly two centuries. The measurement technology in the last decades was mainly based on the use of local thermocouples, arrays of them or arrays of single infrared detectors. The application of high-speed infrared (IR) cameras overcomes the restrictions of measuring only local thermal information. The reliability and application of thermocouples at high loading rates like crash and impact at long fiber reinforced thermoplastic (LFT) samples or in zones of high strains or localization is very restricted. Based on high-speed IR measurement, a new method of correlating field information of deformation from digital image correlation (DIC) and heat from impact loaded material was introduced. Strain rate dependent mechanisms of LFT were investigated regarding the influence of heating during the deformation. Correlated experimental information of heating and deformation from high strain rate loading experiments enable to analyze LFT more in detail than before. Relations of thermal und mechanical work have been investigated, based on the same control volume. A strain and strain rate dependent change of the differential heat transition value (𝛽𝑑𝑖𝑓𝑓) was found for the investigated LFT. With increasing strain rates 𝛽𝑑𝑖𝑓𝑓 decreases for LFT. Furthermore, local fibers induce hot spots can be localized and qualitatively and quantitatively determined. It was found that the zones on the specimens, where local hot s pots occur, increase with the strain rate. Also the distribution of strain and heating at certain states of loading can easily be extracted from the correlated field data. It was found that critical states of local strain are reached at higher global strain. An increased local temperature and an expansion of the deformation zone, seems to be responsible. The higher energy absorption capacity of LFT at higher strain rate can be explained with the change in the thermo-mechanical behavior on a global scale. Also effects on the micro scale were found and quantified by analyzing the experimental thermo-mechanical data from correlated optical strain and infrared measurements.