Fitness for Purpose of Mechanical and Structural Components
Research focus
The Peer review has evaluated this group as Good
Fitness for purpose is an advanced engineering approach for the assessment of mechanical components residual strength and life, considering the presence and the effect of different non linearity and non-homogeneity of the materials. It is concerned with several aspects related to: - material characterization (toughness, crack propagation parameters in presence or not of aggressive environmental, low-cycle fatigue); - local stress and strain state; - material damage mechanisms; - material damage and behaviour modelling. Experimental investigations and numerical and theoretical studies allow to individuate a model able to predict the residual life and the effective criticality of the mechanical component. The research group focused his activity in all these directions, with a particular attention to the following topics: Defect Tolerant Design: One of main activities of the group is ‘Defect Tolerant Design’ is based on estimating the fatigue strength of threshold stress for the non-propagation of small cracks emanating from the defects (inclusions, inhomogeneities, casting defects). In particular the activity of the group has been focused onto the analysis of threshold conditions under multiaxial state of stress and the application of these techniques to fatigue design of helicopter components and automotive components. One of the important points for a successful application of ‘Defect tolerant design’ to components is the ability to estimate the prospective distribution of maximum defect in a component. About this subject there is an important cooperation with Prof. Y.Murakami (Kyushu University) and Prof. C.W. Anderson (Univ. of Sheffield) within the activity of ESIS TC20 (www.esisweb.org) which has led to a novel method for analyzing the presence of multiple inclusions. Crack propagation modelling: In order to determine the residual life in presence of defects it is important to analyze and model fatigue crack growth. In particular the activity has been focused in improving the performances of ‘Strip Yield’ algorithm. This improvements are part of the participation of some members of the research group to ‘NASGRO Consortium’ and this activity is done in cooperation with Prof. M.Skorupa (AGH, Krakow). Failure analysis: A failure analysis was carried out on a 12”-non-integral pipeline connector, joining a 12” pipeline with an offshore platform riser. During service the failure of this connector occurred. In order to understand how and why the failure occurred a numerical 3-D model was realized and fracture surface was investigated by means of X-ray diffraction. Multiaxial fatigue: Research activities were conducted in both the high-cycle and the low cycle fatigue regimes with focus on the definition of assessment methods for structural integrity of mechanical components. In the context of high cycle fatigue, extended and in-depth experiments concerned the influence of mean shear stresses upon torsional fatigue strength of steels and the influence of different frequency ratios upon fatigue strength in combined axial load and torsion, with the aim of choosing and developing multiaxial fatigue criteria suitable for the application to metallic materials. Special focus has been put on the development of criteria based on the critical plane approach. A calibration method for one of these criteria by using experimental results from uniaxial tests was developed for the application to rolling contact fatigue. In order to apply the multi-axial criteria more efficiently in a design environment, as post-processor of FEA stress analyses, numerical tools were developed allowing for calculating the relevant quantities in shorter times, especially in the case of non proportional loading. In the perspective of low-cycle fatigue, a new low–cycle multiaxial fatigue life prediction methodology based on the concept of an effective shear strain has been developed. The proposed model, which is formulated as a generalised equivalent strain, takes into account the effect of non–symmetrical loading cycles. In the view of the application of the aforementioned methodologies, multi-axial fatigue experiments were carried out at temperatures corresponding to those experienced by the gas turbine materials during service, with the purpose of performing life prediction calculation for propulsion system components under cyclic multi-axial, thermo-mechanical loading (high temperature, variable amplitude loading). Additionally, two different methods for the fatigue design of pressure vessel nozzles, designed with the same safety coefficient, according to ASME and VSR 1995 standards were considered. Material behaviour: Experimental toughness tests on specimens constructed by means of an alloy Mg-Al-Mn were performed. This investigations evidenced the toughness characteristics of this alloy.
Dipartimento di afferenza
Docenti afferenti
Full Professors
Stefano Beretta
Paolo Clerici
Piermaria Davoli
Laura Vergani
Associate Professors
Marco Giglio
Assistant Professors
Andrea Bernasconi
Michele Carboni
Mauro Filippini
Antonietta Lo Conte