Influence of thermomechanical interaction effects on the failure behaviour of polymer foam cored sandwich panels

Abstract

The paper presents an experimentally based and numerically supported investigation of the collapse behaviour of polymer foam cored sandwich beams subjected to combined mechanical and thermal loading. Recent analytical and numerical modelling results available in the literature have ascertained that collapse may be due to loss of stability induced by nonlinear interactions between mechanical loads and thermally induced deformations, when accounting for the reduction of the polymer foam core mechanical properties with increasing temperature. In the paper, experiments are devised whereby a thermal gradient is introduced into a sandwich beam specimen loaded in three point bending. The experiments cover a range of temperatures where one face sheet is heated from room temperature (25°C) to just below the glass transition temperature of the polymer foam core (70°C) and the other face sheet remains at room temperature. Digital image correlation (DIC) is used to obtain the local displacement field of the sandwich beam and its temperature is monitored using an infrared detector and thermocouples. The experimental results are compared with the predictions of both a generally nonlinear finite element model and an analytical so-called high-order sandwich panel theory (HSAPT) model. It is important to note that the HSAPT model has clear limitations as it takes into account only the geometric nonlinearity and thermal degradation of the foam core elastic properties, and it further assumes that the sandwich constituents are linear elastic with infinite straining capability. The HSAPT model predicts the occurrence of a strongly nonlinear load response leading to loss of stability (limit point Palleti, H.N.K.T., Zhang, S., Dulieu-Barton, J.M., Fruehmann, R.K. and Thomsen, O.T., "Influence of thermomechanical interaction effects on the failure behaviour of polymer foam cored sandwich panels", Journal of Sandwich Structures and Materials, 17, 2015, 308-331. DOI: 10.1177/1099636214567602. 2 behaviour). However, the experiments show that for the investigated sandwich beam configuration it is necessary to include the nonlinear material properties in the modelling, as the nonlinear beam response leading to failure and collapse is significantly influenced by plastic deformations in the constituent materials. Thus, core indentation and extensive plasticity precedes the transition to loss of stability driven by geometric nonlinearity and thermomechanical interaction effects. Finite element analyses that include both geometric and material nonlinearities provide results that correlate closely with the experimental observations. The work presented lays the foundations of a methodology for validating complex thermomechanical behaviour in sandwich structures using non-contact full-field measurement systems, and it demonstrates that analytical or numerical models based on the assumption of linear elastic material behaviour (such as the HSAPT model referenced in the paper) generally cannot adequately describe the thermomechanical be...

The effect of elevated temperatures on the bending behaviour of foam cored sandwich structures

“…The temperature profile from the specimen side surface was captured immediately after the removal of the insulation. A linear temperature distribution through the core thickness was observed (a detailed verification of the temperature distribution in this set-up has been reported in Palleti et al 19 ). However, over a short period of time, the temperature profile became nonlinear due to heat losses at the side surface.…”

Indentation failure of foam cored sandwich structures subjected to elevated temperatures

“…Based on accurate measurements of the temperature dependent foam core properties, Zhang et al [6,7] looked at the implication of temperature reduced/ degraded foam core properties on structural components and investigated the failure mode of face sheet wrinkling and the bending behaviour of foam cored sandwich structures leading to core shear failure under combined mechanical and thermal loading. In parallel, Palleti et al [8] investigated the load-response of a foam cored sandwich beam subjected to elevated temperatures with a special emphasis on the use of a nonlinear finite element (FE) material model based on the generalized Hill anisotropic plasticity model to capture the foam core's highly nonlinear response at elevated temperatures. In [6][7][8], foam cored sandwich beams were loaded in three-or four-point bending, while a through-thickness temperature gradient was induced by an infrared lamp pointing at the top face sheet.…”

“…The tensile value of the corresponding material axis is then set to the same value as the compressive value to ensure the fulfilment of the incompressibility requirement. Tensile and compressive parameters in the remaining material axes (x and y) are set according to the tensile properties derived in [2][3][4][5][6][7][8]. An overview of the nonlinear material model input is given in Table 3.…”

“…However, visco-elastic effects were not included in the modelling conducted. Hill's generalized anisotropic plasticity model was adopted as proposed in [8]. The technical details of the material model can be found in the ANSYS documentation [21].…”

“…al [8] investigated the loadresponse of a foam cored sandwich beam subjected to elevated temperatures with a special emphasis on the use of a nonlinear FE material model based on the generalized Hill anisotropic plasticity model to capture the foam core's highly nonlinear response at elevated temperatures. In [6][7][8] foam cored sandwich beams were loaded in three-or four-point bending while a through-thickness temperature gradient was induced by an infrared lamp pointing at the top face sheet. This is a very realistic scenario that can be found wherever a heat source, such as sun radiation or machinery, is positioned on one side of a sandwich panel.…”