Damage characterization of quasi-statically indented composite sandwich structures

Abstract

The nature of quasi-static indentation damage is studied in aluminum honeycomb core sandwich panels with eight ply, quasi-isotropic, graphite/epoxy face sheets. Parameters that are varied include the core thickness, core density, face sheet layup, and indentor diameter. The majority of induced damage is in the vicinity of the barely visible threshold. The permanent dent in the panel is found to be always larger than the contact area of the indentor, and specimens with denser cores exhibit smaller dent diameters for a given dent depth. Regardless of specimen layup, delaminations occur essentially only at the 3rd, 5th, 6th, and 7th interfaces. Stiffer cores, either in terms of a higher density or, for those cores considered, a smaller thickness, result in lower dent depths, smaller dent diameters, and more face sheet delamination for a given indentation event. The manner in which core orthotropy influences the size and pattern of the delaminations is shown to depend on face sheet layup. Regardless of the core, larger delaminations occur in face sheets that contain 90° angle changes between the adjacent plies in comparison to those that contain only 45° angle changes. Results are compared to those previously reported in the literature, and mechanisms that are related to plate boundary conditions are described that reconcile what would appear to be conflicting results obtained by different studies. Findings are then discussed in the context of choosing the most damage-resistant structural configuration and how this translates to damage tolerance.

Damage resistance of aluminum core honeycomb sandwich panels with carbon/epoxy face sheets

“…The objective is to determine the effects of core thickness, core density, face sheet stacking sequence, face sheet thickness, and indentor diameter on the nature and extent of damage, to establish the various physical mechanisms at work, and to deduce certain guidelines that may be useful in the design of damage resistant structures. Many of the results already published in Singh et al 21 for the eight ply case are presented again in combination with the 16 ply results in this paper. This approach is taken in order to help make comparisons easier between the eight and 16 ply results.…”

“…Perhaps due to the large overall sandwich panel thickness, the through-transmission approach produced relatively poor results. For the pulse-echo technique, transducer frequencies from 5 to 50 MHz were evaluated and the results compared to each other and to destructive examinations, similar to those described in Singh et al 21 Interestingly, the 50 MHz transducer was found to produce the best overall assessments for both face sheet thicknesses. It gave the most information near the indented surface, the most clarity for the delaminations observed, and it penetrated essentially the same depth into the specimen (approximately 11 plies) as the lower frequency transducers.…”

“…The c-scan's software was configured to capture the time of flight between the transducer's pulse and the first wave reflection from the specimen's surface, and this result was used along with the wave speed in water to determine the variation in depth over the indented region. Complete details of the measurement technique and its validation are described in Singh et al 21 for the eight ply case. Identical procedures were adopted for the 16 ply case.…”

“…This is likely due to the somewhat wavy nature of the ply that is adjacent to the honeycomb core and the associated difficulty with obtaining a clear reflected signal from the back surface of the face sheet. 21 Thus, in what follows, all interlaminar delamination assessments are performed using the SU-CML c-scan unit with a 50 MHz pulse-echo transducer.…”

Analytical solution of some delamination scenarios in thick structural sandwich plates

“…[1]. Delamination and debonding -in general -is induced by impact [2][3][4][5][6][7][8][9][10], blast loading [11], drilling and perforation [12,13] or indentations [14][15][16] or other special loads [17,18]. Moreover, delaminations can change significantly the buckling behaviour of laminated structures [19][20][21][22][23][24].…”