E. Vitale’s research while affiliated with University of Pisa and other places

Abstract— An overview is presented of studies conducted at the University of Pisa on the Weight Function technique as applied to Fatigue Crack Grown analysis. The fundamental theoretical aspects of the technique are summarised, discussing some recent methods for the determination of the Weight Function. The application of the technique to non-linear (contact) problems and to the evaluation of the crack tip stress field is also discussed. It is shown that the Weight Function method allows one to efficiently consider many crack propagation problems, some examples of which are provided.

A qualified nodalization for WWER-1000 is available at DCMN (Dipartimento di Costnizioni Meccaniche e Nucleari) of University of Pisa that is suitable for running with the thermohydraulic system code elap5/mod3.2. The nodalization consists of about 1400 hydraulic nodes and more than 5000 mesh points for conduction heat transfer. The four loops of the NPP are separately modelled. Detailed information about the plant hardware has been got from contacts with Eastern Organizations in Bulgaria, Russia and Ukraine. The qualification of the nodalization has been achieved at a steady state level utilizing a procedure available at DCMN and at a transient level on the basis of operational (planned) transients performed in the Bulgarian Kozloduy-5 NPP and of the unplanned transient occurred at the Ukrainian Zaporosche NPP (April 1995). Data measured in steam generators have also been utilized. The nodalization has been widely applied to the analysis of accident scenarios in WWER-1000, including Large Break LOCA, Small Break LOCA, ATWS, Loss of Feedwater and Station Blackout. The present activity aims at evaluating the potential for PTS (Pressurized Thermal Shock) following a steam line break accident. The thermalhydraulic results were employed as input for a parametric Fracture Mechanics analysis based on conservative hypothesis of the shape and localization of a pre-existing defect. Stress analysis evidenced the effect of partial cooling of the vessel and gave some general indications of the risk for unstable crack propagation under the simulated PTS conditions.

Si-alloyed pyrolitic carbon (PyC) is currently employed in many biomedical devices, due to its fairly good biological compatibility and non biodegradeability. For prosthetic heart valve applications, required to operate safely for many years, the resistance to abrasive wear is one of the limiting factors which must be accurately evaluated. The present study reports on abrasive wear testing of Ti/PyC and PyC/PyC sliding couples. For both couples it was found that the wear behaviour can be shifted from a low wear regime, characterised by very small wear rates and reduced scatter, to a high wear regime, characterised by high wear rates and high scatter, due to the presence of particle contamination coming from the environment and/or from the specimen polishing process. Actual biomedical devices, particularly heart valves, should not experience the high wear regime, due to the absence of any hard particle contamination source. The wear observed in these items is in fact minimal and may depend on mechanisms other than abrasive wear. In these conditions the experimental evaluation of the wear behaviour should definetely be performed by tests on actual devices.

The results of a study on the creep behaviour of the vessel lower head under severe accident conditions are reported. An experimental program aimed at the evaluation of the creep properties of A533grB steel at high temperature (800-1100°C) and under biaxial loading is summarized and the main results reported. A Finite Element simulation of the lower head under severe accident conditions allows to show the effect of the main parameters affecting the time to rupture.

Abstract— The paper discusses the results of fatigue crack growth rate tests conducted in the presence of residual stresses. Three different residual stress distributions, obtained by laser welds, were employed in order to characterize the crack propagation behaviour under different conditions, producing either an increase or a reduction of the stress intensity factor due to external loads. Test results are analysed by means of a non-linear numerical model (based on the weight function method) and a knowledge of the fatigue crack growth properties of the base material, free from residual stresses. The results of the analysis are discussed with reference to experimental trends, in order to clarify the predictive capabilities of the method and aspects needing further investigation.

A method is illustrated for the evaluation of the stress distribution ahead of a crack under Mode I loading. The method is based on the knowledge of the weight function, whose general properties are employed to derive an integral equation. The solution of this integral equation, having the stress distribution as the unknown function, is reached through a numerical technique. Comparison with theoretical solutions shows that the proposed method allows an accurate reproduction of the stress distribution for every distance from the crack tip. The possibilities offered by the use of the method in combination with local fracture criteria are discussed.

The paper illustrates the damage evolution observed in carboresin specimens with a central hole subjected to cyclic tensile loading. Damage monitoring was conducted via a purpose-designed automatic Ultrasonic (US) scanning apparatus, whose responses were qualified by means of micrographical observations. Observed delamination onset and growth are discussed with reference to the results of three-dimensional finite element analyses. In particular, a model simulating the presence of delaminations and their partial closure through non-linear contact calculations is employed in order to evaluate the distribution of the energy release rates along the delamination boundaries. The results indicate that the observed evolution of delamination may be explained by numerical models, provided that closure effects are taken into account.

The paper gives a complete report on a five-year research program concerning the fracture behaviour of the pressure vessel steel A533-B under thermal shock conditions. Five tests on 140-mm thick plates carrying a surface through-thickness crack were performed with different material conditions and thermal histories. The results confirmed the applicability of the Linear Elastic Fracture Mechanics approach and of the arrest toughness criterion. Moreover, the confidence limits achievable by a material characterization procedure based on Charpy-V tests were assessed. The prevention of crack initiation under a decreasing stress intensity factor and crack-tip temperature (‘simple’ warm pre-stressing effect) was largely confirmed, while reinitiation prevention after thermal-mechanical histories including complete unloading could not be demonstrated.

The paper deals with the development of a numerical method for determining Weight Functions in two-dimensional problems. After a short review of some recent numerical techniques an original approach is presented. The method is based on Finite Element calculations with coarse meshes and on the knowledge of some values of the Stress Intensity Factor for one reference loading condition. The validity of the method is demonstrated for a theoretical case and its accuracy and suitability are discussed with reference to practical applications.

This paper illustrates the general organization and the preliminary results of a comprehensive research program aimed at the characterization of fatigue delamination growth in laminated carboresin composites. The study included two types of test, conducted to determine both the basic material properties (under single fracture mode conditions) and the behavior of a simple structural component, i.e., a laminated plate specimen with a central hole. Results obtained in the two types of test are presented and discussed, with reference to the problem of predicting the behavior of actual laminated components on the basis of the results of commonly employed laboratory tests.