PSI - Issue 7

Šulko Miroslav et al. / Procedia Structural Integrity 7 (2017) 262–267

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Šulko Miroslav et Al./ Structural Integrity Procedia 00 (2017) 000–000

1. Introduction The most of the operating loading of the structures are neither static nor deterministic. The real loading of the structures is time-varying and often different from the conditions on which the structure was designed. In such case where time-varying loadings cause time-varying stresses in the material, it is necessary to evaluate and monitor the fatigue damage of the material. Some of operational modes might be marked as “non-standard”, in such cases there is unknown the influence of such loading modes on the reliability and the service life of the structure. However, it is not always possible to determine these processes by calculation, especially if there is no detailed record of the previous load and operating conditions. However, in some cases, where it is necessary to obtain an notion about the state of material degradation due to fatigue process, especially in the case of costly structures (e.g. in the gas industry, the chemical industry, nuclear power industry, ...) as well as in the cases of structural parts or structural elements that are critical from the perspective of structure design as well as to life or environment threatening structures in the case of an accident. It would be appropriate to develop a method that would allow the accumulated fatigue damage in the material to be diagnosed and that way to avoid different types of accidents. Or, on contrary, to extend projected service time.

Nomenclature HV0,1

Vickers micro hardness, load 100g Vickers micro hardness, load 30g Vickers micro hardness, load 20g Vickers micro hardness, load 10g

HV0,03 HV0,02 HV0,01

HB1/30/10

Brinell hardness, diameter of indenter 1 mm, loading force 300 N , duration time 10 s

2. Microstructure of cyclically loaded material The driving force of progressive fatigue damage in the material is a cyclic plastic deformation at the critical point † of construction, whose work of damage is expressed by hysteresis energy. In cases, where the load parameters are not known at this point, we can only try to track their exposure and consequences in cyclic loaded material. In the microstructure of the material, cyclic plastic deformation causes the movements of the grid defects (e.g., dislocations) - in mainly their density and configuration (Fig. 1).

Fig. 1. The surface structure of the steel specimens C55 for different effects of cyclic deformations (1 – the highest amplitude of loading, 3 the lowest amplitude od loading).

† As critical place indicates the locality, point or cross-section whose lifetime is given by lifetime of the overall structure.