L. Guerra Rosa

Fracture toughness of different types of granite

The optimisation of processes such as drilling, cutting, grinding or polishing, demands understanding of the in uence of material related properties in the overall process [1]. For example, the fracture mechanisms involved in material removal (and their dependence on microand macro-defects inevitably present) plays a decisive role in practical rock engineering and fragmentation processes (as those referred above). As a result, a signi®cant number of research studies is currently being undertaken in order to further clarify the in uence of certain fracture parameters (such as the fracture toughness of the stone material) to the fracture mechanisms, as well as to develop accurate methods for their determination [2±4]. Fracture toughness can be de®ned as a measure of the ability of a material to resist the growth of a preexisting crack or aw under stress, and has been extensively used to design fracture safe structures. Using linear elastic fracture mechanics (LEFM) it is possible to establish a relationship between the strength of a cracked structure and the materials fracture toughness, crack size, geometry and location. Fracture toughness determination methods are therefore necessary for safe design and accurate failure analysis. Most of these methods permit evaluation of the lower bound fracture toughness, i.e. the planestrain fracture toughness, which is the crack extension resistance under conditions of crack-tip plane strain. In brittle materials, fracture toughness in pure mode I loading (KIc) is related with the critical value of strain energy release rate (also termed crack extension force), Gc, according to the expression: KIc x88 x81x81x81x81x81x81x81x81x81x81x81x81x81x81x81x81x81 EGc x851y u2x86 s

Prediction of threshold and ultra-low fatigue crack growth rates

Abstract A model was derived to predict the true threshold value for fatigue crack growth in the absence of crack closure. The model, based only on the tensile and cyclic properties of the material, was successfully verified against a set of experimental data on medium and high strength steels and one aluminium alloy. Good agreement with experimental results was also obtained for Region I of the d a/ d N vs ΔK curve using a fatigue crack growth rate equation based on the same model. Fatigue crack growth data obtained from the medium strength steel CK45 in the normalized state and two heat-treated conditions were analysed. Good data correlation was shown using a previously developed normalizing parameter, φ = (ΔK 2 −ΔK 2 th )/(K 2 c −K 2 max ) , in the entire range of fatigue crack growth rates and for stress ratios ranging from 0.1 to 0.8.

Integrated assessment procedure for determining the fracture strength of glass components in CSP systems

The structural integrity and reliability of glass components are key issues for concentrated solar power (CSP) systems. For example, the glass windows in a solar furnace may suffer catastrophic fracture due to thermal and structural loadings, including reaction chamber pressure cycling. Predicting design strength provides the basis for which the optical components and mounting assembly can be designed so that failure does not occur over the operational lifetime of a given CSP system. The fracture strength of brittle materials is dependent on the size and distribution of cracks or surface flaws. Due to the inherent brittleness of glass resulting in catastrophic failure, conservative design approaches are currently used for the development of optical components made of glass, which generally neglect the specific glass composition as well as subcritical crack growth, surface area under stress, and nature of the load – either static or cyclic – phenomena. In this paper, several methods to characterize the strength of glass are discussed to aid engineers in predicting a design strength for a given surface finish, glass type, and environment. Based on the Weibull statistical approach and experimental data available on testing silica glass rod specimens, a theoretical model is developed for estimating their fracture strength under typical loading conditions. Then, an integrated assessment procedure for structural glass elements is further developed based on fracture mechanics and the theory of probability, which is based on the probabilistic modelling of the complex behaviour of glass fracture but avoids the complexity for calculation in applications. As an example, the design strength of a glass window suitable for a solar furnace reaction chamber is highlighted.

Fatigue Threshold Behaviour

For many engineering structures an extensive fatigue live is one of the basic requirements. It is well known that longevity depends on many factors, primarily on low stress intensity ranges. In the present analysis of various tests specifically designed for the study of ultra-low growth, some relevant aspects of the fatigue process and associated effects will be described. For these processes the Linear Elastic Fracture Mechanics (LEFM) method is generally applicable, any plastic deformation is limited to a very small zone close to the tip of the crack. Attention is therefore focused on the fatigue mechanisms involved, leaving aside other factors, such as environment, temperature and frequency. The test methods used comply with the available standards (mostly ASTM). It was considered advantageous to investigate steels in preference to other materials, because they are used in large quantities in the majority of the engineering constructions. However, it should be noted that other metals, such as aluminium and titanium have also been tested and satisfactory correlation with prediction models achieved.