Abstract: In this paper, authors established the approximate relative expression of tearing instability stress and plastic collapse stress under simple loading for circumferential throughwall cracks in pipes. Based on the relative expression, the equation (σ_ins,s/σ_l,s) ≌ (σ_{ins þ}/σ_{l þ}) was obtained which possesses sufficient Precision in engineering assessment and can be extended to circumferential part-throughwall flaws subjected to Pure bending or combined tension and bending.