TY - BOOK

T1 - Advanced Characterization of the Tensile and Compressive Behavior of PP and PP Composites

AU - Jerabek, Michael Josef

N1 - no embargo

PY - 2009

Y1 - 2009

N2 - A basic understanding of the micro-mechanisms of deformation and failure related to the mechanical properties of polypropylene (PP) and particulate filled PP composites is of crucial importance for appropriate material and structural component modeling. Especially PP composite materials offer a great potential for tailor-made properties by specific selection and definition of filler type, content and surface modification. As the polymeric matrix exhibits viscoelastic behavior, mechanical composite properties also strongly depend on time, rate of testing, temperature and stress state. Thus, the main objective of the dissertation was to investigate the effect of several of these parameters on the behavior of neat and particulate filled PP. As to the filler types, two glass beads with different size distributions and talc were used in two volume fractions. Debonding stress decreased with increasing particle size leading to a decreased composite yield stress. Because of their different temperature dependence, the debonding stress, which is substantially below the yield stress at low temperatures, approached the yield stress with increasing temperature. In the low temperature regime, debonding, crazing and micro-cracking were determined as the dominating deformation micro-mechanisms, leading to brittle fracture, which changed to shear yielding and crazing of the matrix as the main mechanisms leading to ductile failure as temperature increased. Changing of the dominating mechanism was observed also in composites filled with talc, which led to different composition dependences in various temperature regimes. As to the local stress state around the filler particles, the pressure dependent yield behavior of the neat PP was characterized applying various test methods from uniaxial tension and compression to multiaxial tension and confined compression. An excellent linear relationship of equivalent stress vs. mean stress was found for all temperatures studied. To determine the pressure sensitivity as an intrinsic material parameter, normalized yield stresses were calculated with respect to the uniaxial tensile test. As expected, increasing pressure sensitivity values were found with increasing temperature, which was explained in terms of the free volume theory. The 3D Drucker-Prager yield function was fitted to the yield stresses and an average error between the predictions and the measurement of 8 % was obtained.

AB - A basic understanding of the micro-mechanisms of deformation and failure related to the mechanical properties of polypropylene (PP) and particulate filled PP composites is of crucial importance for appropriate material and structural component modeling. Especially PP composite materials offer a great potential for tailor-made properties by specific selection and definition of filler type, content and surface modification. As the polymeric matrix exhibits viscoelastic behavior, mechanical composite properties also strongly depend on time, rate of testing, temperature and stress state. Thus, the main objective of the dissertation was to investigate the effect of several of these parameters on the behavior of neat and particulate filled PP. As to the filler types, two glass beads with different size distributions and talc were used in two volume fractions. Debonding stress decreased with increasing particle size leading to a decreased composite yield stress. Because of their different temperature dependence, the debonding stress, which is substantially below the yield stress at low temperatures, approached the yield stress with increasing temperature. In the low temperature regime, debonding, crazing and micro-cracking were determined as the dominating deformation micro-mechanisms, leading to brittle fracture, which changed to shear yielding and crazing of the matrix as the main mechanisms leading to ductile failure as temperature increased. Changing of the dominating mechanism was observed also in composites filled with talc, which led to different composition dependences in various temperature regimes. As to the local stress state around the filler particles, the pressure dependent yield behavior of the neat PP was characterized applying various test methods from uniaxial tension and compression to multiaxial tension and confined compression. An excellent linear relationship of equivalent stress vs. mean stress was found for all temperatures studied. To determine the pressure sensitivity as an intrinsic material parameter, normalized yield stresses were calculated with respect to the uniaxial tensile test. As expected, increasing pressure sensitivity values were found with increasing temperature, which was explained in terms of the free volume theory. The 3D Drucker-Prager yield function was fitted to the yield stresses and an average error between the predictions and the measurement of 8 % was obtained.

KW - Polypropylene Particulate filled composites Viscoelasticity Drucker-Prager yield function Micro-mechanisms Debonding

KW - Polypropylen Partikel gefüllte Composites Viskoelastizität Mikromechanik Debonding Drucker-Prager Fließkriterium

M3 - Doctoral Thesis

ER -