Measurement of the thermoelastic response of short-fibre-reinforced polyamide 6 with application to a component under operating load

Abstract

Thermoelastic stress analysis (TSA) is a powerful technique to measure the temperature changes induced in a cyclically loaded material. For a quantitative analysis of the strain field from the measured temperature change a calibration of the measured TSA-signal is necessary. In the calibration of the fibre-reinforced materials, the anisotropic and inhomogeneous nature of the material has to be taken into account. The measured TSA-signal is usually provided in camera units (CUs). To calibrate the thermoelastic signal against the material's physical properties, a temperature calibration of the TSA-signal has to be performed. This is achieved using DeltaVision software by assigning the measured CUs to a temperature value, taking the infrared sensor's non-linear response into account.A calibration is conducted against a measurement on uniaxially loaded specimens with respect to the observed material's microstructure and its resulting mechanical and thermoelastic properties. The calibration constant evaluated is compared with a calibration against the material's physical properties and applied to a component under a working load.From the measurement results, it appears that the influence of the middle stress predicted by the higher-order theory of the thermoelastic effect is significant for the investigated material.

Identification of the critical load for cyclic loading of shortfibre reinforced PA6 GF30 by a quantitative analysis of the thermoelastic response

“…the mechanical properties of the specimens are not influenced as it was the case using for example strain gauges [7]. On the other hand is the stiffness of viscoelastic materials under quasistatic conditions lower than under dynamic loading, see the values measured in [5]. In the following only the Poissons' ratios are needed, which are nearly identical for quasi-static and dynamic loading.…”

“…The different layers have different mechanical and thermoelastic properties, due to the different fibre orientations. The global mechanical properties of the composite material are dominated by the layers with high fibre orientation 3 and 5, resulting in orthotropic mechanical properties of the composite material [5].…”

“…In the strain oriented approach, the measured thermoelastic signal is not calibrated against the sum of the principal stresses in the surface layer of the material, but against the sum of the principal strains, according to equation 2 [5,8]:…”

“…In [5] the calibration constant of PA6 GF 30 was evaluated with 0.41 %/K, dependent on the fibre orientation. This was assumed to be caused by a mean stress applied to the material.…”

“…In the ongoing research it came out, that the difference of the calibration constants between the two fibre orientations can be ascribed to the used reference measuring system. In [5] strain gauges were used to measure the sum of the principal strains. The composite material PA6 GF30 has a comparatively low stiffness.…”