title: The Energy-absorbing Characteristics of Novel Contoured Core Sandwich Structures
Creators: name: Haldar, AK
Publisher: name: University of Liverpool Repository
Year: 2016

description: Automobile and aerospace industries are facing problems more and more on reducing the weight and manufacturing cost of a structure, but guaranteeing an equal level of comfort with satisfactory structural performance of components. To overcome these contradictory requirements traditional designs and materials must be revised. Therefore, this research study aims to design, manufacture and characterise the properties of novel contoured-core sandwich structures to obtain strong, stiff and lightweight structures including air ventilation to reduce the danger of deterioration and humidity retraction. Two different contoured profiles, named flat-roof and spherical-roof contoured-cores, were designed to investigate structural response under quasi-static and dynamic loading conditions. Flat-roof and spherical-roof structures were made from a glass fibre reinforced plastic (GFRP) and a carbon fibre reinforced plastic (CFRP). The composite contoured cores were fabricated using a hot press moulding technique and then bonded to skins based on the same material, to produce a range of lightweight sandwich structures. Testing was initially focused on establishing the influence of the number of unit cells, thickness of the cell wall, width of the cell and the core filled with foam on their mechanical behaviour under quasi-static loading. Fibre fracture and matrix cracking in the composite systems, as well as debonding between the skins and the core, were observed during the compression. The compression strength and modulus were shown to be dependent on the number of unit cells and the cell wall thickness. It has also been shown that the specific energy absorption capacity of the panel increases nonlinearly with increasing the cell wall thickness, with the spherical-roof cores outperforming their flat-roof counterparts. Moreover, the foam filling on the composite contoured-core systems improved the strength as well as specific energy-absorbing characteristics of the structures. Low velocity impact loading was subsequently performed on the sandwich structures and showed that the values of energy absorption were slightly higher than the tests conducted at quasi-static loading, as a result of the rate-sensitive effects on the damage resistance of the composite material. In addition, blast tests were undertaken to subject the core materials to a much higher strain-rate. Extensive crushing of the contoured cores was observed, suggesting that these structures are capable of absorbing a significant amount of energy under the extreme loading condition. Finally, the results of these tests were compared with previously-published data on a range of similar core structures. The energy absorbing characteristics of the current spherical-roof systems are shown to be superior to other core structures, such as aluminium and composite egg-box structures. The finite element models using ABAQUS/explicit were further developed to simulate the quasi-static and low velocity impact response of woven carbon and glass fibre contoured-core designs. Initially, a two dimensional model with Hashin’s failure criteria was developed to compare with the experiment. Following this, a user defined material subroutine (VUMAT) was implemented to model the through thickness damage of the contoured-core structures using Hashin’s 3D failure criteria. The FE models were validated against the experimental results in terms of the stress-strain responses, the specific energy absorption and the failure mode, with reasonably good correlation. The models developed could be further used for parametric studies to assist in designing and optimising the structural behaviour of contoured-core sandwich structures.