Debonding of particles

Major results Figure 14.16 shows the effect of mean particle size of spherical silica (flame-fused synthetic silica) on acoustic emission. The emission increases with the particle size of filler increasing. The source of this increase in acoustic emission is thought to be related to the fracture of particles, debonding of particles from... 

The saturation pressure seems to be proportional to the elastic modulus of the propellant (Figure 10). Although there is no solid experimental evidence, it is suspected that the positive hydrostatic pressure acts as a retardation parameter in cavitation or in debonding of particles from the polymeric matrix, as described by Gent and co-workers. Qualitatively, the effect of applied pressure during a tensile test is believed to delay the occurrence of vacuoles and to decrease their number. This assumption may be sustained by simultaneous volume-expansion measurements taken during tensile tests under different pressures. For an increase in pressure, the relative measured volume decreases (see Figure 11). 

Fig. 13.38 (a) The cavitated layer just underneath the flank of the fracture plane in an Izod impact sample of a rubber-particle-modified HDPE blend (Bartczak et al. (1999a) courtesy of Elsevier), (b) The cavitated layer, again just underneath the fracture plane, in an Izod sample of a CaCOs-particle-modified HDPE blend, showing the extensive debonding of particles prior to fracture (from Bartczak et al. (1999b) courtesy of Elsevier). 

Matrix-rubber particle adhesion is an important parameter for rubber toughening. For effective rubber toughening, rubber particles must be well bonded to the thermoset matrix. The poor intrinsic adhesion across the particle-matrix interface causes premature debonding of particles, leading to catastrophic failure of the materials. Nearly all the studies [9, 193, 2-10] have been concerned with reactive rubbers as toughening agents, and showed that dispersed particles have interfacial chemical bonds as a consequence of chemical reactivity. 

Crazes are similar to cracks, the opposite sides of which are connected with fibres of oriented polymer material. In filled polymers, crazes are caused by debonding of particles and by stretching of the polymer in... 

Figure 6.18 A stereoscan of a fracture surface of a dental silicate cement. The debonded glass particle is to be identified by its pitted surface, the result of selective acid attack. Note the particulate nature of the matrix (Wilson et at., 1972).

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

Under uni-axial tension, the effect of particles is mainly to be stress concentration sites. During crack propagation, the stress field is tri-axial debonding with void growth accommodates the volume expansion of the material at the crack front. The stress re-distribution in the matrix... 

This equation shows that debonding stress increases with adhesion and filler fraction and decreases with particle size. Figure 7.27 shows the effect of particle size on prediction of yield stress based on the debonding simulated by an equation derived from Eq 7.27. Decreasing particle size increases the stress required for debonding. 

In mixtures of particles, the stress of debonding is not uniform. Higher stress is needed to debond from smaller particles." Adhesion is inversely proportional to the cube root of the diameter of the particles. " Experiments confirmed that large particle sized filler decreased the tensile strength of composites. The filler concentration effect is not linear. Up to a certain concentration, filler did increase the tensile properties but beyond certain level there is a reverse effect. This may relate to the interactions described in previous sections where the quality of bonding (weak or strong) depended on filler concentration. 

The effect of simply having a void, or a hole, in the epoxy cell was also ascertained in order to determine the effect of a cavitated, or interfacially debonded, rubber particle. 

Finally, it should be noted that stress-whitening arises because these voids, created by cavitation and debonded particles, scatter light. Hence, it is observed that the most intense stress-whitening is present in the recipe that undergoes the greatest extent of particle cavitation and debonding. 

Also included in Table I are the true fracture stress (of) calculated from the cross section of the fractured specimen, and the fracture strain (ef). The fracture stress dropped significantly, from 171 MPa for LLDPE to 100 MPa and 35 MPa for 10% and 25% PS, respectively. The fracture stress of the 37.5% PS blend was even lower, 8.5 MPa, but was still higher than the oy value of 5.9 MPa, so the blend deformed in a ductile manner. The blend with 50% PS fractured in a quasi-brittle manner at a stress of 7.0 MPa, which was slightly lower than cry for this composition. The large decrease in Of with increasing PS concentration was consistent with debonded PS particles that were not load-bearing during plastic deformation. 

Figure 3. Schematic representation of particle debonding and void growth during deformation of Type I blends.

Figure 4. Schematic of the cubic array of partially debonded, spherical particles. The effective cross-sectional area (A ) is defined.

Figure 6.25 shows SEM pictures (fractured surface) of a typical specimen (a) after initial fracture and (b) after the first healing. Solid CP particles are clearly visible in Figure 6.25(a). In Figure 6.25(b), solid particles are not seen but traces of melted, deformed, and debonded CP particles are identified. Also, the surface in Figure 6.25(b) is smoother compared to that in... 

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