The theory of damage mechanics is applied to pressure vessel components operating at high temperatures. It is suggested that some existing design procedures can be readily adapted to deal with high temperature problems.
When load bearing components operate at temperatures in excess of 0.25 Tm where Tm is the absolute melting temperature, materials suffer time-dependent deterioration which may be in the form of transgranular cracks or voids which grow in the grain boundaries. The material deterioration can grow to such an extent that rupture takes place after a certain time of exposure to stress. The damage can also give rise to a reduction of ductility, so that materials with large ductilities under tests of short duration may have greatly reduced ductility for tests involving long exposure times. The failure of a structure may be conveniently divided into two stages. The first stage is the initiation of damage at the point of highest stress. This is followed by a propagation stage in which zones of failed material spread across the component until sufficient material has failed for plastic collapse to occur. The problem of determining the failure time is clearly complex if the advance of the damage front is to be calculated. This approach will require the use of nonlinear finite element procedures in conjunction with suitable constitutive equations which describe the growth of damage and the effect of damage on deformation rates. These procedures have been developed but the calculations place heavy demands on the computer and are expensive. Consequently, they have diminished attraction for the designer for whom simple yet realistic calculations are essential tools at the early stages of design. Attempts will be made in this report to cover both the approximate and the more precise procedures which are currently in use.
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| Number of Pages : | 36 |
| Published : | 1985 |
