PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 206–213 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

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determine the service life in the specific surrounding environment. Thus, the first aim is to analyse a wide set of results of the ATT and the final aim of the analysis is to compute the time to failure of the structural member depending on the externally applied loading level, i.e., to obtain curves of time to failure versus applied stress. The structural element considered in the present analysis is a cylindrical bar or wire subjected to tensile loading in axial direction. This is considered to be representative of many axially-loaded engineering members in the form of wires, bars, strands, tendons, etc. Possible damage in the component may happen by transverse cracking, i.e., when a surface mode I crack – probably of quasi-elliptic shape – appears in the plane perpendicular to the main axis of the bar, caused by the combined effect of the mechanical and environmental actions. This crack may produce the tension failure of the structural element. The problem under analysis has two parts: an environmental process (of physico-chemical nature) and a localized fracture phenomenon (of mechanical character). The environmental process consists of hydrogen diffusion from the surrounding environment into the metal, and it is assumed to have cylindrical symmetry. The fracture mechanics phenomenon, on the other hand, has a localized nature and the damage is assumed to be concentrated in the form of a part-through crack perpendicular to the bar axis, with a loss of cylindrical symmetry. 3. Experimental bases The ATT (FIP-78 1981) is a simple test, easy to perform, and realistic from the engineering point of view, since the test sample is the real prestressing steel wire with no previous detectable damage (just as it should be used in construction) and the cracking process is environmentally induced (as happens in the real situation due to corrosion processes in prestressed concrete). The ATT is used in this work to analyze the influence of internal residual stress distributions on the HE susceptibility of prestressing steels. To this end, four different commercial prestressing steel wires – with different residual stress laws induced by manufacturing (cold-drawing) in each case – were tested at different stress levels and test temperatures. The test results are shown in Fig. 1 for the four steels and the two test temperatures (35 and 50ºC). For each level of stress (externally applied) the average time to failure and the interval corresponding to the standard deviation are plotted. The experimental scatter – measured in time to failure – is very high and it clearly increases as the applied stress descends. The explanation of this experimental behaviour should be sought in any variable able to modify the externally applied stress state during the test, and the internal residual stress distribution is the candidate.

Fig. 1. Experimental results of the ATT at 35ºC (left) and 50ºC (right).

4. Theoretical bases 4.1. Diffusion equations

A computer model was developed to predict the wire life in ammonium thiocyanate by obtaining curves of applied stress vs. time to failure. The model is formulated on the basis of axisymetric hydrogen diffusion in a cylinder (Fig. 2a) and damage localization in the form a surface part-through crack (Fig. 2b) which is created when a