Predicting the fatigue life of polycarbonate

In this study a constitutive modelling approach is used to predict the fatigue life of polycarbonate. After a thorough investigation on the heating effects, accurate lifetime predictions are made under isothermal as well as non-isothermal conditions. Moreover, it is shown that cyclic fatigue has an accelerated effect on the ageing behaviour

Modelling large-strain deformation of thermo-rheologically complex materials : characterisation and validation of PMMA and iPP

In this study an attempt is made to describe the thermorheologically complex deformation behaviour of the glassy polymer PMMA and semi-crystalline polymer iPP, by using a constitutive modelling approach [1]. For both polymers, it is shown that this approach successfully captures the thermorheologically complex behaviour of PMMA and iPP. Moreover, the model is capable of predicting yield stress and creep lifetime of PMMA using only one parameter set

An analytical method to predict fatigue life of thermoplastics in uniaxial loading: sensitivity to wave type, frequency and stress amplitude

A method is presented that allows fatigue life predictions on the basis of creep life data. The approach is based on the assumption that the time-dependent failure of polymers is determined by the intrinsic strain softening that is initiated when a critical threshold value of the plastic strain is surpassed. To facilitate fatigue predictions, an acceleration factor is defined that indicates how much faster plastic strain is accumulated by a cyclic signal compared to its static mean stress. Analytical solutions of the acceleration factor are presented for triangular and square waves, which predict that only the stress amplitude of the cyclic signal and the material’s stress dependency affect fatigue life, whereas frequency plays no role. Verification using several glassy and semicrystalline polymers demonstrates that this method yields accurate quantitative lifetime predictions not only for polymers that exhibit ductile failure but also for those that display brittle fracture, provided that fracture is preceded by (localized) plastic flow

An analytical method to predict fatigue life of thermoplastics in uniaxial loading: sensitivity to wave type, frequency and stress amplitude

A method is presented that allows fatigue life predictions on the basis of creep life data. The approach is based on the assumption that the time-dependent failure of polymers is determined by the intrinsic strain softening that is initiated when a critical threshold value of the plastic strain is surpassed. To facilitate fatigue predictions, an acceleration factor is defined that indicates how much faster plastic strain is accumulated by a cyclic signal compared to its static mean stress. Analytical solutions of the acceleration factor are presented for triangular and square waves, which predict that only the stress amplitude of the cyclic signal and the material’s stress dependency affect fatigue life, whereas frequency plays no role. Verification using several glassy and semicrystalline polymers demonstrates that this method yields accurate quantitative lifetime predictions not only for polymers that exhibit ductile failure but also for those that display brittle fracture, provided that fracture is preceded by (localized) plastic flow

Fatigue life predictions for glassy polymers : a constitutive approach

Long-term failure under constant or cyclic load is governed by the same process as short-term failure at constant rate of deformation. Failure proves to originate from the polymer’s intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study we developed, and validated, a 3D constitutive model that is capable to predict the occurrence of these plastic instabilities, yielding quantitative predictions of the lifetime of polycarbonate under constant load.1 Here we demonstrate that the same approach is also applicable to predict the life span of polycarbonate under cyclic loading conditions, over a large range of molecular weights and thermal histories, with a single parameter set only. The model incorporates the influence of physical aging, accelerated by the applied cyclic stress. For low cycle fatigue, at large stress amplitudes, where failure is thermally dominated, it is shown that the current constitutive model has to be extended to a multirelaxation time expression to properly describe the (evolution of the) energy dissipation

Predicting the long-term failure of polycarbonate: A constitutive approach

In this study, an in-house numerical model is employed to predict the time-to-failure of polycarbonate under long-term constant or cyclic loading. This elasto-viscoplastic model captures the intrinsic deformation behavior of the glassy polymer. The model parameters are determined from uni-axial compression testing. In the case of long-term loading, however, special attention should be given to the intrinsic deformation behavior, as this might change within the time-scale of the experiment. Therefore, an elaborate investigation is performed to implement so-called aging kinetics in this model. It is shown that the application of approach leads to accurate predictions of time-to-failure under both constant loading and cyclic loading conditions. Moreover, the current model is based on a single parameter set and is molecular weight independent