Nowadays most industrial realities undergo a strong push to improve cost-effectiveness, productivity and quality of manufactured products. In particular we focussed our attention in the area of design of plastic structural components, including both optimization of existing structures and design of new ones. In this case, but the following considerations have a more general value, these needs could be translated into demanding requirements of cost-effectiveness, weight reduction, reduced time-to-market with guarantee reliability. From a material perspective this means demanding mechanical performances, attention to safety margins and need of a better control of key design parameters. To obtain these results, we need to develop a new approach and effective tools in the design of plastic materials and components aimed at tailoring part behaviour to endurance and performance requirements. The target of the project is to find effective tools for predicting life endurance and damage evolution of plastic materials and components under mechanical/thermal service loading, in order to support the development of new material formulations and the design and optimization of structural components. In a particular way, we focussed our work in the characterization and modellization of materials durability and damage mechanisms. One of the main problems related to materials durability is due to fatigue failure. Fatigue process is a progressive weakening of a component with increasing time under load such that loads to be supported satisfactorily for short duration produce failure after long durations [1, 2, 3]. Fatigue failure should not be thought only as the breaking of the specimen into two separated pieces, but as a progressive material damage accumulation [2]. Material damage during fatigue loading manifests as progressive reduction of stiffness and as creep [5]. As standard fatigue testing are expensive in terms of money and time, it is essential to develop new approaches less time consuming and simpler to be implemented. One of the most important goals of the present work is the setting of an investigation method (Accelerated Fatigue Test) very simple to be implemented that is able to differentiate damage accumulation and durability performances of various material formulations in reduced time.

In this investigation the relationship between J-integral and CTOD is studied considering a Compact Tensile (CT) and Singe edge notched bend (SENB) specimens using finite element analysis. The magnitude of CTOD is estimated by 90o-intercept method and also by plastic hinge model. The results indicate that there exists a discrepancy in estimation of CTOD by 90o-intercept method and by plastic hinge model. The CTOD values obtained by both the methods are found to be linearly proportional to J-integral. The linear proportionality constant dn between CTOD and J is found to strongly depend on the method of estimation of CTOD, specimen geometry and a/W ratio of the specimens.

The problem of fatigue characterization of materials by small data samples is here considered and a Bayesian procedure for data analysis is presented. For both hot rolled and quenched and tempered carbon steels, on the basis of some literature data and by well-established correlations between fatigue and tensile properties, a prior probability density function, able to model most of the available information on material fatigue strength, is defined. Prior information, when fused by means of Bayes theorem with information from experimental data, allows to identify with greater accuracy the fatigue properties of the tested steel. A Montecarlo study is carried out to verify the effectiveness of the proposed method, generating small samples of fatigue data by virtual testing.