Length-scale effects on damage development in tensile loading of glass-sphere filled epoxy

Abstract

Particle-reinforced polymers are widely used in load-carrying applications. The effect of particle size on damage development in the polymer is still relatively unexplored. In this study, the effect of glass-sphere size on the damage development in tensile loaded epoxy has been investigated. The diameter of the glass spheres ranged from approximately 0.5-50 lm. The first type of damage observed was debonding at the sphere poles, which subsequently grew along the interface between the glass spheres and epoxy matrix. These cracks were observed to kink out into the matrix in the radial direction perpendicular to the applied load. The debonding stresses increased with decreasing sphere diameter, whereas the length to diameter ratio of the resulting matrix cracks increased with increasing sphere diameter. These effects could not be explained by elastic stress analysis and linear-elastic fracture mechanics. Possible explanations are that a thin interphase shell may form in the epoxy close to the glass spheres, and that there is a length-scale effect in the yield process which depends on the strain gradients. Cohesive fracture processes can contribute to the influence of sphere size on matrix-crack length. Better knowledge on these underlying size-dependent mechanisms that control damage development in polymers and polymer composites is useful in development of stronger materials. From a methodology point of view, the glass-sphere composite test can be used as an alternative technique (although still in a qualitative way) to hardness vs. indentation depth to quantify length-scale effects in inelastic deformation of polymers.

Size effect in particle debonding: Comparisons between finite fracture mechanics and cohesive zone model

“…According to equation (1), it should be notice that peak stress value does not depend on the size of the particle. It is observed experimentally that the damage initiation stress depends on particle size, 8 whereas the stress concentration does not. Lauke 9 asks two crucial questions about the debonding parameters for a spherical particle“First: Is the debonding stress, σ c , dependent on the particle diameter?…”

Length scale dependence in elastomers – comparison of indentation experiments with numerical simulations

“…In [11,13,17] the length scale dependence in polymers has been experimentally observed in elastic deformation, which is in contrast to metals, where length scale effects were observed in plastic deformation and usually attributed to geometrically necessary dislocations [2]. Although there is mounting experimental evidence for size effects in polymers [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], compared to metals, length scale dependent deformation in polymers is arguably not well understood and there are only few length scale dependent theories suggested in the literature [5,6,21,22] for polymers.…”

Indentation size effect of multiple orders of magnitude in polydimethylsiloxane

“…There is mounting evidence that polymers exhibit size‐dependent deformation. Size‐dependent deformation has been observed in microbeam bending,1, 2 indentation tests,3–10 composite materials,11 and foams12 at micrometer to submicrometer length scales. In contrast to metals, however, where size‐dependent deformation at these length scales is commonly associated with geometrically necessary dislocations (see, e.g., Han et al13 and the references listed therein), the origin of size‐dependent deformation in polymers is still not clear.…”