ENERGY DAMAGE SUMMATION METHODS FOR CRACK INITIATION AND GROWTH DURING BLOCK LOADING IN HIGH-TEMPERATURE LOW-CYCLE FATIGUE

Strain-controlled total endurance tests have been carried out on smoot h specimens of 21/4Cr-1Mo steel at 550 degrees C in continuous cycling , and also using interspersed blocks of ''major'' and ''minor'' cyclin g. Similar low-cycle fatigue crack growth tests were also carried out on cylindrical specimens of 1/2CrMoV steel containing a 0.2 mm deep st arter defect using interspersed blocks of ''major'', ''intermediate'' and ''minor'' cycling, their order being changed in separate tests. Da mage summation in the smooth specimen tests at failure was found to be less than unity (irrespective of the failure criterion) when Miner's rule was employed, and the results are compared with limited previous data. Damage was also summed on an energy-expended basis and this was found to be (i) independent of strain range, and (ii) independent of c ycle type (i.e. continuous loading or block loading). The nearer the c hosen failure criterion to crack initiation, the closer was the agreem ent in accumulated energies. Similarly, in the crack growth tests, the number of complete loops to ''failure'' (10% load drop) was independe nt of block order over a wide range of applied strain ranges, but was affected by the number of cycles in each block. However, the energy cr iterion was again able to rationalise all the data.