Infrared Thermography assisted evaluation of static and fatigue Mode II fracture toughness in FRP composites

Abstract

The work proposes the combined use of a Modified Transverse Crack Tension (MTCT) test coupon and Infrared Thermography, to evaluate the static and fatigue behaviour of Fibre Reinforced Polymer composites under Mode II delamination. Artificial delaminations starters are added to the TCT coupon, whose effects on the Strain Energy Release Rate are discussed. Infrared Thermography and Thermoelastic Stress Analysis are implemented to investigate stresses and delaminations growths on two FRP materials: a pre-preg IM7/8552 carbon fibre-epoxy and a glass-fibre reinforced epoxy laminates. The thermographic, thermoelastic and second harmonic signals have been determined and used to monitor the onset of instable crack growth under monotonic loading, and the delamination growth under fatigue loading. The use of such full-field non-contact thermographic parameters allows a further and effective insight on the behaviour of TCT specimens, providing useful information for the characterisation of the Mode II delamination behaviour under both static and fatigue conditions.

A methodology for the rapid characterization of Mode II delamination fatigue threshold in FRP composites

“…transverse cut plies with respect the crack increment, ∆ , is always null, while the derivative of the R-curve, at the same point, is positive in the rising part of the R-curve, attaining a null value only in the plateau. Hence the critical load will occur when the crack driving force curve is tangent to the plateau of the R-curve, and therefore the experimental determination of critical load allows to determine the steady-state value of the critical SERR (see [21,24] for a more in-depth discussion).…”

“… The specimen type and loading rig allow for an easy implementation of optical, non-contact, Experimental Stress Analysis, and NDT techniques (see e.g. [21] for the case of Thermoelastic Stress Analysis).…”

“…An advantage over the TCT is that this last specimen may easily lead to unsymmetrical fatigue crack growth among the four expected delamination fronts, due to non-homogeneities in the transverse notch. For this reason the preparation of the transverse notch requires a special care in the TCT, that is instead not necessary in the MTCT [21].…”

“…Regarding fatigue tests, the sampled thermograms are post-processed in the frequency domain with a lock-in filter [21,30], thus obtaining the thermoelastic signal. This, in particular, is correlated to the stress field, and allows to identify the evolution of the delamination by considering the special thermoelastic signatures activated by the cracked zones.…”

“…In the MTCT version, artificial delaminations are also added across the transverse notch tips during the manufacture. When the MTCT is subject to tensile cyclic loading, pure mode II fatigue interlaminar delamination is favored and activated from the tips of the artificial delaminations [17,21].…”