ON THE USE OF NANO INDENTATION TO PREDICT CONSTITUTIVE BEHAVIOR OF EPOXY RESINS

LUCAS SANCHEZ FELLAY; CARLOS MOLINA; P.M. FRONTINI

Simposio; XXIV International Materials Research Congress; 2015

Many investigations in the past were concerned with the problem of extracting time independent elastic and plastic response of materials using depth sensing indentation tests. However, it is widely known that polymers also exhibit a strong time dependent mechanical behavior. This time dependency can be observed in indentation tests. For example, the displacement of the punch increases with time while the force remains constant (creep). In order to represent accurately mechanical behavior of polymers model should account for hydrostatic pressure, strain rate, and strain hardening. In this work we explore the possibility of extracting constitutive behavior of glassy polymers using nanoindentation experiments. To this aim a RTM6 aerospace grade epoxy resin was chosen. Its constitutive behavior was first established in uniaxial compression at different strain rates. It is shown that the nonlinear viscoplastic behavior of the resin can be well represented by a tailored parallel network model according to Bergström proposal (polymerfem.com/content.php?79-parallel-network-model) consisting of one with eight-chain, and in parallel other with a linear elastic spring in series with a power law dashpot with exponential evolution of yield. Then, a strategy which combines physical and numerical experiments is proposed in an attempt to extract material parameters from indentation tests. The experiments were carried out with a Hysitron nanoindenter system (TriboIndenter 950).Material is subjected to a three-step load history. The first step is loading, performed at different velocities; after that, a constant-load step is used, varying the holding time; and finally the unload step is carried out at .In order to tune up methodology virtual experiments were performed using the parameters fitted in compression. Using an inverse analysis technique, the mechanical response is deconvoluted by matching the experimental curve with the ones computed by finite element modeling.