Estudio teórico-experimental del comportamiento a impacto de baja velocidad de piezas estructurales fabricadas con termoplásticos

Resum

The substitution of metals by thermoplastic materials is increasing in structural applications in which lightness is essential. In many of these applications impact behaviour is a key factor, because impact produces the catastrophic rupture of components, risking the physical integrity of users. The complex impact behaviour of these materials makes difficult the use of engineering design methods like finite element method, that alow to reduce manufacturing costs and deadlines. This problem has two main facts: on one hand, the lack of routine tests methods for measuring material properties in impact conditions, required as input data by finite element codes. on the other hand, most of the constitutive models available in finite element codes have been developed for other materials that don’t show the features of thermoplastics in impact conditions. In the present work a new characterisation method of impact behaviour of thermoplastics has been proposed, based on instrumented tensile-impact experimental tests. This method has allowed the determination of the stress/strain curves of polypropylene at constant strain rates. This information has been introduced in a finite element code through simple constitutive models available in it. The characterisation tensile-impact tests have been simulated, as well as indentation-impact tests. The correlation of numerical and experimental results show that the method is valid for predicting the evolution of force signal in time, as well as the minimum energy value for producing the rupture of samples in tensile impact configuration. Moreover, it has been demonstrated to be a great difference between the impact behaviour of the material in tensile-impact and indentation-impact configurations. The correlation of numerical and experimental displacement-time curves in both configurations has permitted to detect some main facts to be considered: the unloading behaviour of the material, as well as the dependency of Young’s modulus and elastic limit with strain rate, must be taken into account both in the charaterisation method and constitutive modelling.