High Temperature Shear Strength of T650-35/HFPE-II-52 Polyimide Matrix Unidirectional Composite
The shear strength of a 4-ply unidirectional composite laminate consisting of carbon fibers (T650-35) and a polyimide matrix (HFPE-II-52) was measured over a temperature range of 25 to 315°C. The tests were performed using a Iosipescu test sample, modified to provide a more uniform shear stress distribution across the gauge section and loaded with an Arcan type test fixture. The test specimen design is based on the results of an extensive finite element study. Shear strength tests were performed on dry, 50% RH and 100% RH moisture saturated samples. Results of the experiment show that shear strength decreases from approximately 120 MPa at 25°C to 60 MPa at 315°C and that moisture saturated samples have a 5–10 MPa lower shear strength than dry samples.
Coupled temperature and strain rate effects on non-linear mechanical behaviour of amorphous polymers : Experimental characterisation and modelling of strain rate-temperature superposition
The present PhD thesis proposes a simplified and accurate strategy for characterising and modelling the mechanical behaviour of amorphous polymers from the quasi-fluid state up to the solid state.The study was carried out on PMMAs with different molar masses and crosslinking degree.First, we addressed the mechanical behaviour in the linear viscoelastic domain using DMTA and rheological tests. Results showed that increasing the molar mas and crosslinking degree increased the elastic and loss moduli as the α-transition. In parallel, using the time-temperature superposition principle allowed determining “equivalent strain rates at reference temperature”.Then, we performed uniaxial tensile and shear uploading-unloading tests at high temperature and coupled with DIC, to characterise the mechanical behaviour at large strain. Additionally, strain rate and temperature effects were coupled by means of the “equivalent strain rate at reference temperature” extracted from observations in the linear domain. Results showed that targeting the same equivalent strain rate lead to the same stress-strain curves, i.e. same mechanical response. This allows reducing the number of experimental tests needed to characterise the mechanical behaviour of amorphous polymers.Finally, a constitutive modelling based in a thermodynamics framework, was used to reproduce the mechanical response of the PMMAs at large deformation. The model presented a good agreement with the experimental data, being able to reproduce viscoelastoplastic, viscoelastic, hyperelastic and viscohyperelastic behaviours for cyclic loading tensile.
Derivation of Boundary Conditions for Micromechanics Analyses of Plain and Satin Weave Composites
Efficient 3D analysis of periodic structures depends on identifying the smallest region to be modeled and the appropriate boundary conditions. This paper describes systematic procedures for deriving the boundary conditions for general periodic structures. These procedures are then used to derive the boundary conditions for plain and satin weave composites.