Journal of Sandwich Structures & Materials

Marine grade hybrid sandwich panels are designed in accordance with BS EN ISO 12215-5:2008 using a wet lay-up and cured under vacuum pressure. The high fibre content composite laminate skins use marine grade orthophthalic polyester, POLYLITE® 440-M850, resin and chopped strand mat (CSM)/woven E-glass; the core is DIVINYCELL® H100 closed cell linear polyvinyl chloride (PVC) foam. Impact damage testing followed American Society for Testing and Materials (ASTM) D7766-11 procedure C and ASTM D7136/D7136M-05. Impact damage was sustained by the default hemispherical indentor and further ‘standard’ geometrical indentor rocks – conical, pyramid and cylindrical. The investigation reviews the current state of affairs in impact sandwich testing procedures. A 50–80% through thickness penetration criteria is proposed following noted shortcomings in the standard originally intended for laminates. It is shown that the panel overall flexural rigidity to thickness ratio better describes the transition between ‘thin’ to ‘thick’ impact response; force measurements indicate that strain rate effects need to be considered; dent indentation can only be assessed through data acquistion; destructive damage observation sectioned specimen observations describe the impact damage sustained under the various indentor conditions, as well as the roles played by both the face composite skins and the core material.

The load response and failure behaviour of ‘grid-scored’ sandwich panels used in wind turbine blades have been investigated. This paper presents the results of a combined experimental and numerical investigation of the load response and failure behaviour of a specific grid-scored foam cored composite sandwich panel configuration subjected to multiaxial quasi-static loading conditions that are representative for realistic loading conditions present in wind turbine blades. From the experimental evidence a criterion based on fracture mechanics has been proposed for predicting the onset of fracture in the resin grid. The criterion can be applied directly in conjunction with finite element modelling based on 3D solid elements where the resin grid in situ the core is fully modelled. However, since most full-scale blade models are based on first-order shear deformation theory where the core properties normally are homogenised a strain-based failure criterion is also proposed. The input for this failure criterion is the allowable resin grid strain, which can be obtained from a simple uniaxial tension test of a grid-scored sandwich beam specimen. The predictions of the criteria have been compared with the experimental observations, and a good correlation has been found.

This paper focuses on the free vibration characteristics of functionally graded sandwich cylindrical shells resting on Pasternak elastic foundation. The free vibration analysis is based on three-dimensional elasticity theory and the natural frequencies are obtained by means of generalized differential quadrature method. The two-constituent functionally graded shell consists of ceramic and metal. These constituents are graded through the thickness according to a generalized power-law distribution with four parameters. The benefit of using the considered power-law distribution is to illustrate and present useful results arising from symmetric, asymmetric and classic profiles. A detailed parametric study is carried out in order to reveal the effects of different profiles of ceramic volume fraction, two-parameter elastic foundation modulus, different geometrical parameters such as the mid radius-to-thickness ratio, length-to-mean radius ratio and the thickness of face sheets on the vibrational characteristics of the functionally graded sandwich cylindrical shell.

The structural stiffness is explored for box-section beams in three-point bending. Four hollow tubes of various topologies are designed, manufactured and tested. The observed results show that a significant local compliance exists at the centre of the upper tube wall under the loading patch, in addition to the macroscopic bending of the hollow tube. The predicted and measured stiffnesses are in good agreement for a hollow tube with monolithic walls, and a hollow tube with foam-cored sandwich walls. However, the predicted stiffnesses are much greater than the measured values for two hollow tubes with sandwich walls comprising lattice corrugated cores. This can be ascribed to the imperfections introduced by the manufacturing route either by welding or adhesive bonding, and to the local yielding and large plastic deformations of the upper tube walls under the loading patch.

This paper presents the impact and compression after impact behavior of PVC foam cored /E-glass reinforced/vinylester sandwiches used in a four-seat amphibian aircraft. The impact damage was sorted in three categories: barely visible impact damage; visible impact damage; and clearly visible impact damage. It was observed that when increasing impact energy, the extension of the damage from barely visible impact damage to visible impact damage corresponds to a significantly high rate of energy absorption as depicted by the absorbed energy/impact energy ratio. Sandwich coupons were modeled with the finite element analysis ANSYS software to predict the critical failure load in presence of damage zones equivalent to those observed experimentally. The finite element model predicts consistently the compression after impact strength of undamaged coupons and this result confirms the model used to represent the woven fabric by an equivalent cross-ply laminate model. However, the finite element overestimates the compression after impact strength of impacted ones. It is suggested that the induced out-of-plane displacements generate stress concentrations in the tip of cracks located at the borders of the damaged zone.