Experimental and numerical study of composite sandwich panels for lightweight structural design

Abstract

Composite sandwich panels are commonly considered as lightweight materials for structural design in many applications including automotive, aerospace and defence systems. These panels must be developed and validated for its mechanical strength characteristics under different loading conditions before deploying them for the practical application. Hence, the present paper deals with the experimental and numerical studies on composite sandwich panels subjected to quasi-static edgewise and flatwise compression loads. The composite face sheet was developed using 2 × 2 bi-directional 200GSM twill carbon fibre with hot cure benzoxazine resin. Sandwich panels were made using the developed composite face sheets with aluminium honeycomb core. The developed composite face sheets were tested for tensile and flexural conditions and the sandwich panels for edgewise and flatwise compression loads. Three samples for each of these tests were conducted in Shimadzu UTM with 50 kN capacity machine. A consistent load-displacement profile and the failure pattern were observed for all the samples tested in each test category. It is noted from these tests that the composite face sheets exhibited an ultimate tensile strength of 444 MPa and a flexural strength of 842 MPa which is pretty to close to the epoxy based composites used in the aerospace applications; sandwich panels showed an edgewise compressive strength of 610 MPa and flatwise compressive strength of 3.5 MPa. Tensile test coupons initially had undergone certain delamination before failure and failed close to the specimen holding end. Flexural test specimens were shown delamination of only one layer at the loading face. However, the specimen was well intact together even after failure. Bending mode of failure was noted during edgewise compression test of the sandwich; severe delamination was seen in the tension side of the sandwich whereas a line-folding crack with failure was exhibited at the mid-plane of the specimen. In case of flatwise compression, no evident of damage was observed in the sandwich structure. The second part of the paper describes the numerical simulations of these test cases including tensile, flexural, edgewise compression and flatwise compression using an explicit finite element solver viz. LS-DYNA. Material type 22 and 26 was selected to define the material behaviour of composites and honeycomb respectively. The delamination was modelled using *CONTACT_AUTOMATIC_SURFACE_SURFACE_TIEBREAK card with option 9 that works under B-K linear fracture mechanics law to define the normal and shear failure of cracks. The numerical results were found to match well with experimental observations for the individual coupons as well as the sandwich samples. Therefore, it is concluded from this study that the developed LS-DYNA simulation methodology can be used as a tool for composite sandwich panels based lightweight structural design.