Effect of local damages on the buckling behaviour of pyramidal truss core sandwich panels

Abstract

Truss core sandwich panels have been widely investigated due to their superior mechanical performances. However, local defects or damages during preparation and service may reduce the strength significantly. The objective of this paper is to examine the imperfection sensitive of this kind of structures under in-plane compression. The elastic and plastic buckling behaviour of pyramidal truss core sandwich panels with local damages under in-plane compression are studied experimentally and numerically. Local damages including unbound nodes between lattice truss and the facesheet, missing lattice cells and holes in the facesheet are considered. In-plane compression tests of truss core sandwich panels with prefabricated local damages are conducted, and then a finite element model in conjunction with random number is developed to simulate the buckling behaviour of the panel with randomly distributed damages in a specific region. Experimental and numerical results show that, besides the damage extent, the location of unbound nodes and missing lattice cells have significant effect on the buckling strength of the pyramidal truss core sandwich panel. In addition, the local damage sensitiveness of sandwich panel with round holes in the facesheet is lower than that with square holes.

Introduction

Truss core sandwich panels, which possess combinations of load capacity and multi-functionality, are advanced structures that can be applied in industrial sectors such as ships, aircrafts, civil engineering and aerospace engineering. With the development of preparation technique, various types of lattice truss materials have been fabricated and have been extensively investigated for their basic mechanical properties and applications in energy absorption [1], [2], [3], [4], [5], [6], [7], [8], [9]. Nevertheless, the stiffness and strength of the truss core sandwich panel may decrease due to a variety of imperfections and local damages caused by their own structural complexity, immaturity of manufacturing process and severe service environment and load. Therefore it is necessary to examine the sensitivity of truss core sandwich panel to local damages, especially on their mechanical properties such as the buckling strength.

There have been some experimental and theoretical works focused on the behaviour of sandwich structures under bending and in-plane compression [10], [11]. Hu et al. [11] reviewed and assessed various theories for modelling sandwich composites. By ignoring the bending stiffness of the core, the critical buckling load of the sandwich panel can be analytical solved, and it is a simple yet effective approach for predicting overall buckling load for sandwich panel [12]. However, for sandwich panels under in-plane compression, local buckling or other complex failure mode may happen except for the global buckling, and the finite element method with appropriate kinematic model for instability analysis is imperative [13], [14], [15]. When sandwich structures have initial imperfections, local damages have been a subject of major concern in engineering applications because of the associated problems of reduction in load-bearing capacity. The presence of the this kind of damages, which causes reductions in the bending stiffness and shear stiffness, will leads to the undesirable loss in the buckling strength. There have been many relevant studies on the imperfection sensitivity of the buckling behaviours of sandwich structures to local damages. Somers et al. [16] developed a theoretical model to predict the buckling load and the post-buckling behaviour of delaminated sandwich beams. It can be found from their studies that the sandwich construction is very sensitive to the presence of delaminations situated at the core-facesheet interface. Kwon et al. [17] analysed the compression behaviour of sandwich beams which have holes and delaminations between the skin and the core. Rasmus et al. [18] analysed the behaviour of the compression loaded sandwich beams that contains a debond by using a geometrically non-linear finite element model. The finite element model reveals that the sandwich column is very sensitive to the initial debond length and the local facesheet imperfection. The sensitivity results from two mechanisms: (a) interaction of local debond buckling and global buckling and (b) the development of a damaged zone at the debond crack tip. In addition, similar conclusions can be found from the related works [19], [20], [21], [22].

Recently some works was also reported on the imperfection sensitive of lattice truss materials with missing lattice cells [23], [24], [25], [26]. It can be found from these studies that the lattice truss material is more tolerant to local damages for the compression behaviours than open-cell foams [23], [26]. Moreover, local damages take the form of unbound nodes also have less effect on the compression stiffness and peak strength of truss core sandwich panels. But shear properties of truss core sandwich panels are significantly degraded due to this kind of local damages [27]. As a result, this kind of local damages may reduce the buckling strength of truss core sandwich panels, which have poor shear stiffness. Yuan et al. [28] analysed the thermal buckling behaviour of pyramidal truss core sandwich panels experimentally. It can be found from the experimental result that the local damage during fabrication has a great effect on the critical thermal buckling temperature and the buckling mode of the sandwich panel. For other types of local damages, Sebaey et al. [29] studied the behaviour of pyramidal truss core sandwich panels with notched facesheet under biaxial compression through numerical simulation.

However, it is noticed that the sensitivity of the buckling behaviour of truss core sandwich panels to the extent and the type of damages has been rarely studied, especially when missing lattice truss cells and unbound nodes are located in a specific region. In the present paper, the response of pyramidal truss core sandwich panels with unbound nodes, missing lattice truss cells and holes in the facesheet subjected to in-plane compression are studied experimentally and numerically. The outline of this paper is as follows. In Section 2, the fabrication process of the specimen with prefabricated local damages and the experimental procedure are described. In Section 3, the finite element model in conjunction with random number is developed, and a series of numerical analysis are carried out to investigate effects of damage extent, damage type and damage location on the critical buckling load. Finally, some findings are collected and summarised in Section 4.