A cut-ply specimen for the mixed-mode fracture toughness and fatigue characterisation of FRPs

Abstract

The characterization of mixed-mode fracture toughness and fatigue delamination growth in fibrereinforced composites is crucial for assessing the integrity of structural elements in service. An asymmetric cut-ply coupon (ACP) loaded in four-point bending is here proposed to carry out the aforementioned characterisations. Analytical expressions of the energy release rate and mode-mixity for the ACP are derived and validated by means of finite element analysis. A fracture toughness and fatigue characterisation of the carbon/epoxy material IM7/8552 is carried out via ACP specimens. It is proved that the material data obtained from ACP specimens match those generated using ASTM standard mixed-mode bending (MMB) coupons. The main reason for the introduction of the ACP test resides in its applicability to characterisation scenarios where measuring the delamination length with optical means, as required for MMB coupons, is difficult. Such scenarios include the investigation of static and fatigue delamination growth at low and high temperatures, which requires the usage of environmental chambers. This poses significant constraints in terms of volume available for the test rigs, and, most importantly, limitations on visual access to observe delamination propagation.However, the manufacturing of ACP coupons is more complex than for MMB specimens and the testing requires several additional precautions that are here discussed in detail.

Temperature effects on mixed mode I/II delamination under quasi-static and fatigue loading of a carbon/epoxy composite

“…Both quasi-static and fatigue tests were repeated at À50°C, 20°C, 50°C and 80°C. The results from 20°C (RT) are introduced in an earlier paper [21] and are also presented here for comparison purposes.…”

“…The fabrication process of the ACP specimens is described in detail in Ref. [21] and the nominal dimensions of the ACP coupons used for this research study are shown in Fig. 1.…”

“…The ACP specimens [21] consist of two ply-stacks of UD plies, one of continuous plies and one of ''cut-plies''. In the latter, the 0°plies are cut mid-length, perpendicular to fibre direction, as illustrated in Fig.…”

“…All tests were performed using the ACP specimens [21] loaded in four point bending and were repeated at À50°C, 20°C (room temperature), 50°C and 80°C under both quasi-static and fatigue loading. This temperature range was selected since it is representative of operating temperatures experienced by composite materials in aeronautical applications.…”

“…A novel methodology for the generation of delamination onset and propagation data under quasi-static and fatigue loading has been previously introduced [21]. This is based on using Asymmetric Cut Ply (ACP) specimens, whereby ERR values can be determined solely from the load-displacement data [21], so that no visual monitoring of the delamination front is required.…”