Multiaxial fatigue damage parameters:Literature overview: Traditional stress-based models versus stress-based critical plane approaches

The global reach within transportation industry seems to be towards ever lighter load carrying components e.g. by using higher strength weldable steels. Structural weight reduction and/or the increase in payload of on-road and off-road vehicles may increase the magnitude of operational stresses for a given set of operating and environmental conditions, resulting often in reduced margins of safety factors. Designers worlwide are currently employing fatigue design code approaches (fatigue damage parameters) that are mainly derived from uniaxially loaded small scale laboratory samples. In contrast to the uniaxial loading assumptions of the fatigue design codes, extensive field measurements to capture structural responses indicate that most vehicular structural components are subjected to nonproportional multiaxial loads. At the same time, recent developments in laboratory fatigue tests using more realistic (multiaxial) loads often indicate that the design code-based fatigue damage parameters are unable to predict the increased fatigue damage caused by nonproportional multiaxial loads. Therefore, prior to reducing the factors of safety by increasing the magnitude of structural responses, the nature of operational loads (uniaxial vs. multiaxial) should be accounted for and suitable fatigue damage parameters should be developed and experimentally verified. The conventional stress-based fatigue damage parameters - most of which are extensions of static yield criteria - are summarized in this paper. Most of these parameters have been derived from laboratory fatigue tests where small-scale specimens have been subjected to either axial push-pull, cantilever bending or three/four-point bending loading (proportional and in-phase loading). More recent "critical plane" concepts are also introduced, since these parameters that employ the interaction of normal and shearing stresses have been shown to possess better life predictive capability than the conventional methods for multiaxially loaded components. This document forms a background study for the subsequent definition of multiaxial hot spot-approach that will employ the interaction of normal and shearing hot spot stress quantities in the vicinity of weld toes. The multiaxial hot spot approach for the multiaxial fatigue damage assessment of structural steel weldments subjected to realistic (non- proportional and out-of-phase) multiaxial loads will be published in a separate document