Micro-Scale Fatigue Damage Assessment of CFRP Laminates Using Lock-in Thermography

This study proposes a new micro-scale damage assessment method of laminated carbon fiber-reinforced plastics based on the thermal diffusivity measurement. This measurement was conducted by the laser-spot-periodic-heating method using lock-in thermography. Measured samples were subjected to the tension fatigue test at a relatively low load and high cycle. As a result, the thermal diffusivity showed a decreasing trend with an increase in the load cycles. It was shown that this method can detect the effect of the minute fatigue damage at a level that cannot be seen with a microscope.

Algorithms for thermal diffusivity measurement of heterogeneous 1D materials based on Infrared Microscopy Enhanced Angstrom Method

An infrared microscopy enhanced Angstrom method has been develpoed to measure the thermal diffusivity. Infrared microscopy technique can acquire temperatures of multiple points at one shot. Two algorithms for calculating thermal diffusivity were proposed and compared in practice. One is based on global temperature data and the other is based on local temperature data. The according calculated thermal diffusivities are denoted as α n G and α n L . Three 1D materials of different heterogeneity (Cu wire, Ni-Cu wire and PVA-CNT fiber) were measured on the experimental platform. The calculated α n G and α n L values show that for homogeneous material such as Cu, these two algorithms give similar results, while for heterogeneous ones (Ni-Cu and PVA-CNT), they come to be discrepant. The data fluctuation analysis of f n L zooms in the discrepancy and verifies that α n L is more sensitive to local property change and more competent in revealing heterogeneous properties.

Uncertainty Assessment for Very High Temperature Thermal Diffusivity Measurements on Molybdenum, Tungsten and Isotropic Graphite

The French National Metrology Institute LNE has improved its homemade laser flash apparatus in order to perform accurate and reliable measurements of thermal diffusivity of homogeneous solid materials at very high temperature. The inductive furnace and the associated infrared (IR) detection systems have been modified and a specific procedure for the in situ calibration of the used radiation thermometers has been developed. This new configuration of the LNE’s diffusivimeter has been then applied for measuring the thermal diffusivity of molybdenum up to 2200 °C, tungsten up to 2400 °C and isotropic graphite up to 3000 °C. Uncertainties associated with these high temperature thermal diffusivity measurements have been assessed for the first time according to the principles of the “Guide to the Expression of Uncertainty in Measurement” (GUM). Detailed uncertainty budgets are here presented in the case of the isotropic graphite for measurements performed at 1000 °C, 2000 °C and 3000 °C. The relative expanded uncertainty (coverage factor k = 2) of the thermal diffusivity measurement is estimated to be between 3 % and 5 % in the whole temperature range for the three investigated refractory materials.

Quantifying Uncertainty in Pulsed Thermographic Inspection by Analysing the Thermal Diffusivity Measurements of Metals and Composites

Pulsed thermography has been used significantly over the years to detect near and sub-surface damage in both metals and composites. Where most of the research has been in either improving the detectability and/or its applicability to specific parts and scenarios, efforts to analyse and establish the level of uncertainty in the measurements have been very limited. This paper presents the analysis of multiple uncertainties associated with thermographic measurements under multiple scenarios such as the choice of post-processing algorithms; multiple flash power settings; and repeat tests on four materials, i.e., aluminium, steel, carbon-fibre reinforced plastics (CFRP) and glass-fibre reinforced plastics (GFRP). Thermal diffusivity measurement has been used as the parameter to determine the uncertainty associated with all the above categories. The results have been computed and represented in the form of a relative standard deviation (RSD) ratio in all cases, where the RSD is the ratio of standard deviation to the mean. The results clearly indicate that the thermal diffusivity measurements show a large RSD due to the post-processing algorithms in the case of steel and a large variability when it comes to assessing the GFRP laminates.