Influence of notch geometry and interface on stress concentration and distribution in micro-tensile bond strength specimens

Abstract 

Objectives

Describe stress distribution and compare stress concentration factor (K_t) for homogeneous micro-specimens with different notch geometries and stick-shaped homogeneous and bimaterial specimens by means of finite element (FE) analysis.

Methods

Axisymmetric models were created for homogeneous specimens with different notches and for stick-shaped homogeneous and bimaterial specimens. FE mesh was refined at areas of expected stress concentration and boundary conditions included an applied tensile stress in the axial direction. Linear elastic analysis was used.

Results

For hourglass homogeneous specimens, K_t equaled 1.32 and 1.12 for a notch radius of 0.6mm and 3.3mm, respectively. A non-uniform axial (σ_zz) stress distribution was found in the notch cross-section, with values at the outer edge being 78% and 25% larger than at the center. In addition, a triaxial stress state was generated. Stick-shaped and dumbbell homogeneous specimens presented K_t=1 and a uniform, uniaxial stress distribution along the entire cross-section. Shear stresses were zero for all homogeneous specimens. When an adhesive interface was added to the stick-shaped specimen, an area of localized axial stress concentration (K_t=1.55) was detected at the bimaterial joint near the outer edge. Normal stresses σ_rr and σ_θθ and shear stress τ_zr were also non-zero at the free-edge.

Conclusions

Dumbbell or stick-shaped specimens are favored for μTBS testing, as they do not present stress concentrations due to geometry. However, dissimilar mechanical properties of joint components will lead to stress concentrations and non-uniform multi-axial stresses, although to a lesser extent.

Keywords: Stress concentration factor, Notches, Micro-tensile bond strength, Finite element analysis