The base temperature influences the thermal shock behavior in two ways. First, a higher base tempera­ture influences the damage, cracking, and melting threshold. All of them are essential because they limit the operational conditions and when exceeded cause enhanced material degradation. Therefore, life­time estimates based on RT data will yield unrealistic conclusions.

Second, crack formation strongly depends on the plastic deformation at high temperatures and even more on the stress developed during cool down. To understand the influence of a higher base temperature, one has to be aware of the typical shape of the yield and tensile strength curve for W or a W alloy.105,157 While the decrease in strength is rather high at low temperatures, the curve flattens at high temperatures despite a drop in strength when exceed­ing the recrystallization temperature. As a result, the high temperature plastic deformation induced by the combination of a heated surface and ‘cool’ base material can be significantly reduced by a small increase in base temperature. Combining this effect with the increased ductility of W at the given base temperature, brittle crack formation can be avoided when heating

the material above a certain threshold.82’157’164’165 This temperature threshold is related to the DBTT but is not necessarily identical to it.

4.17.4.1.2 Repetition rate

In addition to the parameters mentioned above, the damage, cracking, and melting thresholds are deter­mined by the number of load repetitions, because of continuing material degradation such as hardening and recrystallization. This is of particular interest for short transient events with a high repetition rate in magnetic (ELMs) and inertial fusion devices. Up to now the simulation of submillisecond events (ELMs, IFE) has been performed only up to a rela­tively low number of cycles; large numbers of pulses (e. g., >106 ELM pulses during the life-time of the ITER divertor) are not feasible in the majority of the above-mentioned test facilities.