Compressive Failure of Impacted NCF Composite Sandwich Panels - Characterisation of the Failure Process

Abstract

In the present study, non-crimp fabric (NCF) composite face sheet sandwich panels have been tested in compression after impact (CAI). Damage in the face sheets was characterised by fractography. Compression after impact loaded panels were found to fail by plastic fibre microbuckling (kinking) in the damaged face sheet. Studies of panels for which loading was interrupted prior to failure revealed extensive stable kink band formation at several positions and in numerous plies. Kink bands initiated and propagated within a wide region close to the point of impact. In addition, kink bands initiated in zones with high shear stresses, away from the impact centre line. Consequently, the fractographic results from this investigation do not support the assumption of modelling the impact damage as an equivalent hole. To achieve accurate predictions of kink band initiation, the stress field must be known. The results from this study imply that bending effects caused by remaining dent or material eccentricities in the damaged region must be considered.

Damage tolerance assessment of composite sandwich panels with localised damage

“…For some panels, the compression loading was interrupted just before complete failure and the panel was sectioned, and the surfaces polished and inspected in a microscope. Microbuckling was seen in this fractographic investigation with lengths up to 20 mm [6]. These lengths corresponded very well with the lengths of localised strain growing from the damage as observed by DSP measurements.…”

“…A cross-section of the impact zone is shown in Fig.2(a). However, ultrasonic C-scan investigations revealed significant overlapping delaminations with a diameter 2R = 15-20 mm for the thin panels and 2R = 20-30 mm in the thick panels [6]. The sharp type impact created a slit in the face sheet, as shown in Fig.2(b).…”

Damage Tolerance of Naval Sandwich Panels

“…A cross-section of the impact zone is shown in Fig.16a. However, ultrasonic C-scan investigations revealed significant overlapping delaminations with a diameter 2R = 15-30 [35]. The sharp type impact created a slit in the face sheet, as shown in Fig.…”

“…As seen herein, the dent growth is very rapid just before failure and the panel fail by compressive failure of the face sheet laminate. The same type of panels was used to study two types of impact damages, one resulting from a blunt impactor and one form a sharp [2,35,36]. These are shown in Fig.16.…”

“…Some tests were interrupted just prior to failure [35], and small pieces of the laminate close to the impact point was cut out, ground and polished stepwise in the thickness direction in order to identify location of failure. Even though the panel had not yet failed, this fractographic investigation revealed numerous local compressive failures manifested by kink bands (micro-buckling failures).…”