Formalisation and Validation Of a Novel Method Suitable For Designing Notched Metallic Components Against Proportional/Non-Proportional Constant/Variable Amplitude Multiaxial Fatigue.

ABSTRACT The construction industry is constantly being challenged to build more complex modern structures using lighter components (thus saving material) but involving not only more complex geometries but also multiaxial loadings. This creates a need to devise an efficient methodology for the design and manufacture of elements that can sustain a wide range of service loads. The investigation of the elasto-plastic deformation of engineering components against multiaxial fatigue load, however, has largely been limited to simple situations where the cyclic load is of a relatively constant amplitude. When both the geometry and load history are more complex, however, inaccuracies might accumulate so that the predicted outcomes end up differing significantly from the experimental result. Multiaxial fatigue is defined as a localised structural problem involving a nominal system of complex cyclic loads that create a corresponding multiaxial local stress/strain history. In addition, any component with a geometrical feature (for instance, notches) under uniaxial fatigue load results in triaxial stress/strain states at any point inside the material. This thesis aims to develop,applyandvalidateanumericaldesigntechniquebasedonacriticalplanetheory, suitable to predict the longevity of notched geometries against multiaxial cyclic load involving a random loadandthepresenceofzero/non-zeromeanstress. Thehypothesisisbasedonthecombinationof both the Modified Manson-Coffin Curve Method MMCCM and the Theory of Critical Distances that applied in terms of Point Method (PM) to evaluate fatigue damage in the low/medium-cycle fatigue regime on metallic materials. Notch geometries with three different notch root radii were modelled and analysed under uniaxial/multiaxial complex fatigue loadings using elasto-plastic finite element ANSYS® software. Both in-phase and out-of-phase fatigue loading were considered. A Kinematic Hardening criterion was used to describe the mechanical behaviour of the material in the plastic regime. The additional non-proportional hardening that accompanies out-of-phase loading was also included in the fatigue model. Then, by taking advantage of the TCD, a local effective stress/strain history was determined in the vicinity of notch root. A key feature of the TCD is that the critical elasto-plastic stress/strain states liable to provoke fatigue damage can be estimated at a specific distance from the notch root for different notch geometry. Such a distance is known as the Critical Distance CD. According to the TCD, the critical distance CD is a material property that changes according to neither the geometrical feature of a component nor the investigated loading paths. To apply the MMCCM correctly, the direction of the critical plane was indicated among that orientation experiencing the Maximum Variance(MV) of the calculated shear strain by using a multi-variable optimisation method called the Gradient Ascent Method. The MMCCM is based on the concept that fatigue cracks initiate on a material plane with the largest shear strain amplitude. The developed approach was then extended to account for the effect of the mean stress/strain and the proportionality of the load sequences. Finally, the number of cycles to failure (Nf.e) was determined through the Modified Manson-Coffin Curve. With regard to a Variable Amplitude(VA) cyclic loading, a classic Rainflow method was used as a cyclecountingschemetoreducethecomplexirregularshearstrainhistoryintoaseriesofconstant amplitudes. Then, the equivalent shear strain amplitude was used to predict fatigue damage Nf.e. The efficiency and reliability of the developed fatigue approach were systematically validated by conducting a series of low/medium-cycle fatigue experimental work using low carbon steel 080M40. Overall, 24 plain samples and 108 notched samples were machined and tested. Three different root radii for the notches (1.5, 3.0, 6.0 mm) were considered and then classified according to the notch root radius type (Sharp, Intermediate & Blunt). The above-mentioned tests were performed under uniaxial/multiaxial, constant/variable amplitude in-phase and out-of-phase loading conditions, with similar/different frequencies. In addition, the experimental fatigue loading signals were described through both zero-mean and non-zero mean stress values. In conclusion, the validation process exhibited very good agreement between the experimental and estimated results. Interestingly, the physical results achieved were quite satisfying and prove that the developed technique is a powerful engineering tool and a successful design criterion to evaluate multiaxial fatigue damage in cases of notched geometry. The content of this research work has been reported and published in various publications and conferences by the author as listed in the Publication section of this thesis.