Failure Mechanisms

· Cracks act as "stress concentrators" in the material.
· For the idealized elliptical crack shown below, the maximum stress at the crack tip σ_m is related to the applied stress σ₀ by: σ_m = {1 + 2(a/ ρ)^0.5}σ₀ . For: a = 10^-6m and ρ = 10^-10m, σ_m~ 200σ₀ .

· The photoelastic crack model in a polycarbonate sample shows "fringes" of equal stress contours. High fringe density shows high stress concentration.