Large deformation micromechanics of particle filled acrylics at elevated temperatures.

摘要

The main aim of this study is to investigate stress whitening and associated micro-deformation mechanism in thermoformed particle filled acrylic sheets. For stress whitening quantification, a new index was developed based on image histograms in logarithmic scale of gray level. Stress whitening levels in thermoformed acrylic composites was observed to increase with increasing deformation limit, decreasing forming rate and increasing forming temperatures below glass transition. Decrease in stress whitening levels above glass transition with increasing forming temperature was attributed to change in micro-deformation behavior. Surface deformation feature investigated with scanning electron microscopy showed that source of stress whitening in thermoformed samples was a combination of particle failure and particle disintegration depending on forming rate and temperature. Stress whitening level was strongly correlated to intensity of micro-deformation features. On the other hand, thermoformed neat acrylics displayed no surface discoloration which was attributed to absence of micro-void formation on the surface of neat acrylics. Experimental damage measures (degradation in initial, secant, unloading modulus and strain energy density) have been inadequate in describing damage evolution in successive thermoforming applications on the same sample at different levels of deformation.;An improved version of dual-mechanism viscoplastic material model was proposed to predict thermomechanical behavior of neat acrylics under non-isothermal conditions. Simulation results and experimental results were in good agreement and failure of neat acrylics under non-isothermal conditions ar low forming temperatures were succesfully predicted based on entropic damage model.;Particle and interphase failure observed in acrylic composites was studied in a multi-particle unit cell model with different volume fractions. Damage evolution due to particle failure and interphase failure was simulated by implementing imperfect interphase within particle agglomerates and imperfect interphase between filler and matrix through a user defined interphase model. In parametric studies, influence of interphase strength, interphase stiffness and interparticle distance was studied to determine conditions that will favor particle and/or interphase failure between matrix and filler. Composite elastic modulus results from finite element analysis results of unit cell models were in good agreement with experimental results and analytical model predictions at different temperatures for various volume fractions of fillers. A temperature dependent strength criterion for initiation of particle failure in acrylic composites was determined based on comparison of finite element analysis results of unit cell model with expereimental results for acrylic composites.