Mechanisms of inelastic behavior of fiber reinforced polymer composites

Sammanfattning: In the present thesis, the sources of linear/non-linear viscoelastic and viscoplastic behaviour in polymer composite materials are under study. The significance of this work is related to the nature of all composite materials. All polymer composites tend to indicate a time-dependent behaviour. This behaviour can be either linear or nonlinear. No matter what it is, is very important to be taken into account in the analysis, since it is related to strain rate effects, microdamage induced to the structure of the composite and/or irreversible plastic strains.This microdamage is usually caused due to the application of high stresses or high strain. For that reason additional stiffness degradation experiments were performed. In these tests, samples were subjected to high stress levels. Such high stress levels are also responsible for irreversible phenomena that were mentioned before. Then, a material model was used to study the viscoelastic and viscoplastic behaviour. This model assumes that the viscoelastic and viscoplastic responses may be decoupled; the micro-damage influenced viscoelastic strain response can be separated from viscoplastic response which is also affected by damage. In this thesis, three materials were studied, each one corresponding to a submitted/published scientific article. The first paper entitled "Time dependent nonlinear behaviour of recycled PolyPropylene (rPP) in high tensile stress loading" studied the behaviour of recycled polypropylene and recycled polypropylene with the addition of Maleic Anhydride grafted PolyPropylene (MAPP). The time dependent response was decomposed into nonlinear viscoelastic and viscoplastic parts and each of them was quantified. It was found that the elastic properties did not degrade due to high loading. The addition of MAPP did not change the mechanical properties of the rPP. Then the material model was applied and the involved parameters were identified.In the second article, entitled "Mechanical properties of a recycled carbon fibre reinforced MAPP modified polypropylene composite", the previously studied rPP/MAPP matrix was used to form a composite by using recycled carbon fibres. It was found that in creep tests, the time and stress dependence of viscoplastic strains follows a power law, which makes the determination of the parameters in the viscoplasticity model relatively simple. What is more, the viscoelastic response of the composite was found to be linear in the investigated stress domain. The material model was validated in constant stress rate tensile tests. Finally, in the third article, entitled "The sources of inelastic behaviour of GF/VE NCF [45/-45]s laminates" a glass fibre non-crimp fabric laminate was studied. The viscoelastic and viscoplastic material model parameters were calculated and it was found that the material indicates no linear region. This fact was also attributed to the fibre orientation. Loading the fibres in an off-axis direction caused shear stresses, which are responsible for microdamage (related to the fibre-matrix interface and intralaminal cracks) which is considered to be an important source of non-linearity.