Investigation On Plastic Failure Mechanism And Ultimate Load Bearing Capacity Of Local Wall-thinning Penstocks

The defect of local wall-thinning can be found on surface of penstocks after those penstocks have been operated for a long time and been eroded by water and silt.The ultimate load bearing capacity will be reduced by the defect,which may lead to plastic failure under internal prssure.Based on the early research findings of our research group,research works are performed on plastic failure mechanism and ultimate load bearing capacity of plain penstock and penstock with stiffener ring in this paper.The main contents are as follows:(1)Calculation model,incremental iterative technique,converging rule,failure criteria are investigated,and the appropriate elasto-plastic incremental analysis(EPIA)for evaluation ultimate load bearing capacity analysis of local wall-thinning penstock is providedgiven and validated in terms of some experimental data.Some suggestion are given:material strain hardening and true stress-strain curve should be adopted;0.9σ’u and σu’ can be used as the failure reference stress for low strength penstock and mid-high strength penstock respectively.(2)The plastic failure mechanism of plain penstock with local wall-thinning and main influence factors are discovered,then the simplified formula of ultimate load bearing capacity for low strength and mid-high strength penstocks are proposed.Some conclusions are drawn that there are three failure modes of plain penstock with local wall-thinning decided by material parameters and defect size;the main influence factors on normalized ultimate load bearing capacity are c,a and b,their affecting degree minished in order;the regression formula of ultimate load bearing capacity about P0,a,b and c is proposed.(3)The plastic failure mechanism of local wall-thinning penstock with stiffening rings and main influence factors are discovered,then the simplified formula of ultimate load bearing capacity is proposed.Some conclusions are drawn that plastic failure mechanism of penstock with stiffener rings is same with plain penstock;stiffener ring near local wall-thinning has a significant impact on ultimate load bearing capacity;pL of penstock with stiffener rings is considerably larger than plain penstock,but main influence factors are the same,the regression formula of ultimate load bearing capacity is proposed on the basis of plain penstock.