Destructive and Ultrasonic Investigations of Damage Development in Metallic Materials
Damage development due to creep under uniaxial tension at elevated temperatures is assessed using destructive and non-destructive methods in steels, commonly used in power plants or chemical industry, and in aluminium alloy used in aircrafts for responsible elements. The results obtained using two different destructive methods for assessments of damage development are critically discussed. In the first method the specimens of steel after different amounts of creep prestraining were stretched up to failure and variations of the selected tension parameters were taken into account for damage identification. In the second one, a damage degree was evaluated by studying variations of an initial yield locus position in the stress space and by determination of the yield loci dimensions. The ultrasonic investigations were selected as the non-destructive method for damage development evaluation.
A fatigue damage indicator parameter for P91 chromium-molybdenum alloy steel and fatigue pre-damaged P54T carbon steel
Two grades of structural steel were subjected to fully reversible, constant stress amplitude cyclic loading. The local strain response of the material was measured and recorded during the test, with the applied testing technique enabling the monitoring of hysteresis loop variation for the narrowest cross-section of the hourglass specimen. Changes in hysteresis loop width, representing the local inelastic response of the material, were recorded in order to monitor the density of structural imperfections. Material ratcheting behaviour was observed as changes in the mean strain for selected load cycles. Ratcheting was attributed to local deformation of the material in the vicinity of imperfections such as voids or inclusions, as well as deformation induced by the propagation of microcracks. Definitions of a damage indicator parameter and damage parameter were proposed. The fatigue behaviour of the two investigated grades of steel was finally illustrated in the form of damage curves for different stress amplitudes and for undamaged and fatigue pre-damaged material.
Creep fracture characteristics of weld-repaired cast Ti-6Al-4V
Constant load creep tests were conducted with cast Ti-6Al-4 V bars in air and vacuum at 315† and 650†C. Some of the bars were notched in their gauge section and heliarc-welded to simulate a weld repair in a casting. Results indicate that the welding has little influence on the time to rupture of the bars. Fractographic analysis of the bars indicates minor differences in the welded and unwelded fracture surfaces. However, these differences do not affect the microstructural influence on the creep and fracture processes. The prior beta grain, size and the alpha platelet size are shown to control the creep fracture of this alloy. These data show that weld repair of titanium castings for creep applications should be acceptable.