Temperature profiles obtained in thermoelastic stress test for different frequencies

Abstract

Purpose Maintenance is one of the most critical and expensive operations during the life cycle of metallic structures, in particular in the aeronautic industry. However, early detection of fatigue cracks is one of the most demanding operations in global maintenance procedures. In this context, non-destructive testing using image techniques may represent one of the best solutions in such situations, especially thermal stress analyses (TSA) using infrared thermography. The purpose of this paper is to access and characterize the main stress profile calculated through temperature variation, for different load frequencies. Design/methodology/approach In this paper, a cyclic load is applied to an aluminum sample component while infrared thermal image is being acquired. According to the literature and experiments, a cyclic load applied to a material results in cyclic temperature variation. Findings Frequency has been shown to be an important parameter in TSA evaluations, increasing the measured stress profile amplitude. The loading stimulation frequency and the maximum stress recorded show a good correlation (R2 higher than 0.995). It was verified that further tests and modeling should be performed to fully comprehend the influence of load frequency and to create a standard to conduct thermal stress tests. Originality/value This work revealed that the current infrared technology is capable of reaching far more detailed thermal and spatial resolution than the one used in the development of TSA models. Thus, for the first time the influence of mechanical load frequency in the thermal profiles of TSA is visible and consequentially the measured mechanical stress.

Temperature patterns obtained in thermoelastic stress test at different frequencies, a FEM approach

“…However, to correlate the stress and the temperature changes, another coefficient is needed, K TSA , as shown in equation (25). This value depends on the parts’ material, and knowing both components are made of aluminum, as already said, this value is known (Silva et al. , 2017).…”

“…TSA information is normally presented as a vector, where the modulus is a magnitude that is proportional to the temperature variation experienced by the specimen and the phase denotes the angular shift between the thermoelastic signal and the reference. Calibration is important because a small change in values such as emissivity or the thermoelastic coefficient can lead to big changes in the obtained stresses (Sharpe, 2008;Silva et al, 2017;D ıaz et al, 2004a).…”

“…The temperature attenuation occurs, but its values go down below the ambient temperature, meaning that at some points in time the specimen is compressing at the crack tip, which it shouldn't. The tests performed in (Silva et al, 2017) prevent that by applying a load that guarantees that the specimen is submitted to traction at all times. Some simulations were ran in order to change the range of the sine used in the thermal simulation (change it so it doesn't vary from À1 to 1, bur from 0 to 2 or even never be zero) but this problem was not corrected.…”

“…Using image (or field) nature techniques enables the identification of strain concentrations and damage more accurately. This could be done for identifying the presence of defects or to quantify them (Sharpe, 2008;An et al, 2014;Silva et al, 2017;Farahani et al, 2020;Ancona et al, 2015). One of the most interesting thermographic techniques is thermoelastic stress analysis (TSA).…”

Lock-In Thermal Test Simulation, Influence, and Optimum Cycle Period for Infrared Thermal Testing in Non-Destructive Testing

“…This provides a high and uniform emissivity (0.98), thus enabling the usage of optical stimulations. This procedure was also used in previous works [ 33 ].…”