Palmer Miner, Compressive peak loads and high R-values
The ultrasonic peening technique consists of introducing a groove at the weld toe, “cleaning” the weld toe from possible crack prone sites as well as introducing compressive stresses during one and the same working operation. The effect of “cleaning” weld toe region as well as introducing a weld toe groove have undisputed benefits towards the fatigue life which are also easy to quantify. However, for a reliable assessment of the long term benefit of the ultrasonic peening treatment it is necessary to quantify the influence on the induced compressive stresses by the following (variable amplitude load) features:
It is a well known fact that the type of spectrum or the distribution of loading history in compression and in tension has an effect on Miner’s rule and this effect may also apply for improved welds. The loading history interacts with the compressive stresses induced by the UP treatment.
Compressive stresses induced by UP treatment could be relaxed by external loads, either by compressive peak loads or negative R-values and moderate stress ranges, suffered by the structure during its remaining service life. The question will then be if we could still count on the degree of improvement assessed on the basis of constant amplitude fatigue testing with certain R-value from laboratory results.