No abstract

The effects of the filler volume fraction and strength of adhesion on the mode of tensile failure of a particulate reinforced polypropylene (PP) are investigated using finite element simulation (FES). When there is perfect adhesion between constituents, an upper bound for tensile yield strength is found to be 1.33 times the matrix yield strength above a critical volume of particulate concentration. Utilizing Sjoerdsma's model for interacting stress concentration fields, one can determine the concentration dependence of the yield strength below the critical filler volume fraction. When there is no adhesion between constituents, a modified version of an equation by Nicolais and Narkis adequately describes a lower bound for the tensile yield strength. The particulate concentration and the matrix ductility are the prime factors in controlling the brittle failure of the composite. Upper and lower bounds for brittle failure strength are characterized using a strength‐of‐materials approach and stress concentration factors for both “perfect” and “zero” adhesion. The properties of calcium carbonate filled PP homopolymer were measured over a wide range of filler volume fractions. CaCO3 was either treated with stearic acid to prevent adhesion between constituents or used as received. Maleic anhydride grafted PP (MPP) was used to promote adhesion. For filler volume fractions below 0.2, yielding of the composite by combined microcavitation and shear deformation was the principal failure mechanism. At vf above 0.35, a brittle failure mechanism dominated. In the range between 0.2 and 0.35, both failure modes were observed in the populations tested. Good agreement was found between the experimental results and the proposed model.

The effect of adhesion on the strain energy release rate (Gc) and Charpy notched impact strength (NIS) of calcium carbonate (CaCO3)‐filled polypropylene (PP) at room temperature is investigated over a wide interval of particulate filler volume fractions. The concentration dependence of Gc and NIS are discussed in terms of competition between the effects of increasing stiffness, decreasing effective matrix cross section, and the transition from a plane strain to a plane stress mode of failure. In all cases the plane stress and plane strain limits of the critical strain energy release rate for initiation of cracks were not affected by the presence of the filler and are the same as those for neat matrix. In the case of no adhesion between components, the size of the crack tip plastic zone increases with increasing filler volume fraction (vf) because of the reduction of the material yield strength. In the region 0 < vf < 0.12, there is a mixed mode of failure, and the measured value of Gc for crack initiation increases steadily as the sample cross section approaches a fully plane stress state. The reduction in yield strength also results in the increase in Gc for crack propagation as reflected by an increase in NIS. Above vf= 0.12, the specimen cross section is in a fully plane stress state, and further increase in filler volume fraction (decrease in matrix effective cross section) has the net effect of reducing both Gc and NIS. In the case of “perfect” adhesion, the yield strength increases only slightly with vf. In the region 0 < yr < 0.05 there is also a mixed mode of failure, but the increase in Gc is much less than that for the no‐adhesion case since the size of the plastic zone in front of the crack is much smaller. Above vf= 0.05, the combined effects of increasing stiffness, reduction of the size of the plastic zone, and decreasing matrix cross section dominate the behavior, causing a steady reduction in both Gc and NIS. Good agreement was found between experimental data and calculations based on fracture mechanics principles.

The mechanical properties of an orthodontic wire pultruded from S2-glass-reinforced polyethyleneterephthalate glycol (PETG) were measured using two experimental devices simulating clinical conditions. A comparison of moduli measured in the clinically relevant devices with those measured in a standard flexural test reveals that data obtained using small cross-section, short span length clinical specimens require corrections associated with clamping and shearing effects. The clamping effect dominates and is caused by the softening of the material near the clamps. The shear effect becomes important at high fibre volume fractions and small span/thickness ratios. With adoption of these corrections, good agreement between moduli calculated using rule of mixtures and those measured in clinical tests is achieved. The analytical base developed for prediction of the stiffness of the orthodontic wire for different span/thickness ratios improves .the procedure for design of dental appliances.