Development and assessment of energy methods for structural durability

Fatigue life assessment is the key task of the design of mechanical components subjected to service loads for avoiding failure occurring in the form of incipient cracks which may cause damages to the entire mechanical system or even worse to people. The nowadays industrial applications increasingly require mechanical components having complex geometry subjected to complex loading conditions. Considering the guidelines of the fatigue design of welded joints as an example, the standards report several stress-based fatigue design curves each one related to the most common welded structural details under a given loading direction. For this reason, by adopting the nominal approach, if the welded detail is different from those reported in the standards, the choice of the proper design fatigue curve might not be done for certain. This dissertation deals with fatigue assessment of metallic material and components by adopting local energy-based parameter which can quantify the local damage due to stress gradient caused by notches as well as defects. More precisely, the extension of the applicability of three energy-based approaches to several factors that influence the fatigue strength of material and components in addition to those already covered is the aim of the present dissertation. The first energy-based method, the so-called Peak Stress Method deals with the fatigue assessment of welded joints by means of a numerical FE-oriented application of the Notch- Stress Intensity Factors (N-SIFs). The equivalent peak stress (the fatigue damage parameter used for assessing the fatigue strength of welded joints) can be obtained by invoking the averaged Strain Energy Density (SED) criterion. The second one deals with the fatigue characterization of metallic component by assuming the specific heat loss per cycles as a fatigue damage parameter which can be evaluated in a standard constant amplitude fatigue test by adopting an easy experimental technique based on temperature measurements of material surface. The third one deals with the fatigue characterization of metallic materials produced by additive manufacturing, one of the most attractive and studied technology nowadays. Since these materials are affected by the presence of irregular defects, energy-related fracture mechanics approaches seem to be the most suitable for fatigue life assessment. In the first chapter, the state of the art of energy-based methods for fatigue and fracture mechanics characterization are described along with their theoretical frameworks. In the second chapter, the theoretical framework for extending the applicability of Peak Stress Method to the fatigue strength assessment of welded joints subjected to multiaxial loading conditions has been presented. Then, several multiaxial fatigue data taken from the literature relevant to both aluminum and steel welded joints were analysed by using the PSM for validating the theoretical prediction. The equivalent peak stress has shown to correlate with good approximation about all the experimental data. The third chapter deals with the analysis of the thermal energy dissipated during fatigue tests on severely notched AISI 304L stainless steel specimens. For the first time, the specific heat loss per cycle (Q parameter) was evaluated experimentally on 4-mm-thick, hot-rolled AISI 304L stainless steel specimens, characterized by 3, 1 and 0.5 mm notch tip radii by means of a FLIR SC7600 infrared camera during fully reversed axial fatigue tests. The new fatigue test results were successfully included in the existing heat energy-based scatter band previously calibrated on plain and bluntly notched specimens. Finally, an analysis of the heat energy dissipated around the notch tip has been presented and discussed with the aim of proposing a semi-automatic procedure to evaluate the thermal energy dissipated distribution. The fourth and fifth chapters deal with the analysis of the thermal energy dissipation on both AISI 304L and C45 steels specimens subjected to multiaxial loads. The specific heat loss per cycle was measured during constant amplitude multiaxial fatigue tests adopting two different phase shift angles of the applied loads and two biaxiality ratios. All the fatigue test results on both materials are in good agreement with the relevant scatter band previously calibrated except for the out of phase multiaxial fatigue results relevant to the AISI 304L steel. These results seem to be justified by the strain-induced martensitic transformation in metastable austenitic stainless steel, significantly present in out of phase cyclic loading condition. In the sixth chapter, the influence of the defect on fatigue behaviour of maraging steel specimens has been investigated. Axial fatigue tests were carried out on three batches of AMed maraging steel specimens produced by two different AM systems. Furthermore, axial fatigue tests were carried out on wrought maraging steel specimens both in annealed and in aged condition. After failure, the √area of the killer defects was examined by SEM observations of the fracture surfaces. A stress intensity factor-based design curve for all the test series was obtained taking into account the short crack effect by means of the El-Haddad-Smith-Topper model. Due to the lack of expensive experimental data to determine the relevant material length parameter a0, a novel rapid method to approximately evaluate a0 has been proposed. In particular, it consists in matching El-Haddad-Smith-Topper model with Murakami’s expression of the threshold range of mechanically short cracks. The advantage of the adopted engineering approach is that only Vickers hardness of the material is necessary. Theoretically, this rapid method can be also adopted to estimate the size of the control volume of the averaged SED approach due to the analogy of the latter to the material length parameter a0. In the end, the stress intensity factor-based design curve was adopted to estimate the fatigue strength of sharp V-shaped notches characterized by a reduced notch opening angle.