Validation of impact simulations of a car bonnet by full-field optical measurements

Innovative designs of transport vehicles need to be validated in order to demonstrate reliability and provide confidence. The most common approaches to such designs involve simulations based on Finite Element (FE) analysis, used to study the mechanical response of the structural elements during critical events. These simulations need reliable validation techniques, especially if anisotropic materials, such as fibre reinforced polymers, or complex designs, such as automotive components are considered. It is normal practice to assess the accuracy of numerical results by comparing the predicted values to corresponding experimental data. In this frame, the use of whole field optical techniques has been proven successful in the validation of deformation, strain, or vibration modes [1]. The strength of full-field optical techniques is that the whole displacement field can be visualized and analyzed. By using High Speed cameras, the Digital Image Correlation (DIC) method can be applied to highly non-linear dynamic events and deliver quantitative information about the three-dimensional displacement field [2]. The objective of the present paper is to integrate full-field optical measurement methodologies with state-of-the-art computational simulation techniques for non-linear transient dynamic events, in order to improve both methods. Whilst the impact of homogeneous panels is a relatively straightforward task, the simulation of impact and subsequent development of damage in a composite panel is probably at the leading edge of current knowledge. In this frame, flat rectangular composite plates of dimensions 0.2 x 0.1 m, as well as, a car bonnet frame structure of dimensions about 1.8 x 0.8 m, shown in Fig.1a and Fig.2a, respectively, are considered. Both structures have been manufactured of PolyPropylene (PP) and PolyAmide (PA) glass fiber reinforced thermoplastic materials. Aiming to assess the panels and bonnet energy absorbing capability, they have been tested in hard-body low velocity, low energy, mass-drop impact loading, in a drop-tower, with impact energies ranging from 20.J to 200J. In parallel, simulation models of the plates and the car bonnet frame have been developed, as shown in Fig. 1b and 2b. Both structures are modelled using layered shell elements. A node-to-surface contact definition is introduced for modelling the physical contact of the impactor to the bonnet. A High Speed Image Correlation system [3] was used to record full field optical measurement data, which were used to correlate experimentally recorded and numerically calculated displacement / strain histories at various points of the structures and different time intervals o the event. In such a way, experimental validation of simulations of the dynamic event of the flat plates and the car bonnet frame, by using full-field optical methods of deformation measurement were performed. Comparisons are based on both ‘point-to-point’ comparisons of displacement and strain data, as well as comparison of displacement or strain snapshot plots at certain time intervals of the dynamic event, based on an image ‘shape descriptor’ approach.