PSI - Issue 19

Giovanni M. Teixeira et al. / Procedia Structural Integrity 19 (2019) 175–193 Author name / Structural Integrity Procedia 00 (2019) 000–000

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The equivalent structural stress parameter, as described in Equation 7, takes into account the plate thickness ( t ), the loading mode effect ( r ) and the stress parameter (Δ σ s ) that captures adequately the stress concentration effect, which are the most important aspects to be accounted for in fatigue of welded joints. The (ΔS s -N) fatigue master curve shown in Fig. 5 is able to collapse the SN data available for the various joint types, thicknesses and loading modes into a single narrow band.

Fig. 5. Master SN Curve and its parameters.

3. Time Domain Fatigue The Verity® method as described in the previous lines is basically a set of steps designed to provide a reliable and consistent damage parameter out of nodal forces and moments obtained from FEA. Equation 7 shows how structural stress ranges Δ σ s are turned into equivalent structural stress parameters ΔS s so that number of cycles to failure can be calculated. Verity® does not specify how cycles should be counted, although a path dependent cycle counted method (PDMR) has been proposed (Dong et al. (2006)) to address variable amplitude multiaxial loading scenarios. It is clear from the proposed PDMR (and other publications from the same authors) that the input loads are expected to be represented in the time domain, and therefore the finite element results (in terms of forces and moments) expressed as a function of time. Random vibration fatigue can be addressed in either time or frequency domains. The advantages of the latter will be clear in the next paragraphs. 4. Fatigue of Welds in the Frequency Domain When a family of signals are represented by a PSD (power spectral density) histogram (e.g. Fig. 6) the underlying assumption is that those signals (e.g. Fig. 7) share some properties (spectral moments) which can be used as statistical descriptors or, in another words, define the shape of a probability density function (PDF). In a stationary stochastic process the n th -order spectral moment of a one-sided PSD G(f) is defined as