Virtual Testing & Applications (Multiscale Modeling & Simulation)

Virtual Testing & Applications (Multiscale Modeling & Simulation)

Abstract

Fracture toughness and fatigue crack growth rate data are two key parameters which are necessary for conducting the safe life analysis of fracture critical parts used in space and aircraft structures. Currently, these allowables are obtained through the ASTM testing standards which
are costly and time consuming. In many occasions, due to budget limitations and deadlines set forth by the customer, it is not possible to conduct fracture related tests in time. A proposed numerical approach has been developed by B. Farahmand that is based on the extended Griffith theory and can predict fracture allowables for a variety of alloys. The simplicity of the concept is based on the use of basic, and in most cases available, uniaxial full stress-strain data to derive material fracture toughness values. The fracture toughness value is thickness dependent and its value is used to predict region III of the fatigue crack growth rate curve. Regions I & II of the da/dN versus ∆K curve can be estimated separately and will be connected to region III to establish the total fatigue crack growth rate data. As the result of this work two computer codes, fracture toughness determination (FTD) and fatigue crack growth (FCG), were generated under the NASA contract. Results of fracture toughness and fatigue crack
growth rate data calculated by this approach were compared with numerous test data in the NASGRO database. Excellent agreements between analyses and test data were found, which will validate the FTD and FCG methodology. This novel approach is referred to as the virtual testing
technique. It enables engineers to generate fracture allowables analytically by eliminating unnecessary tests. Yet, there is another innovative approach in the virtual testing arena that relies on the multiscale modeling and simulation technique. This technique is becoming popular in the field of computational materials, where the failure mechanism is described from the bottom up approach. The methodology is based on the abinitio concept where it is assumed that the failure of material will initiate from the atomistic level and grow to a visible crack. Under this
condition nanocracks will grow under the applied load (as the result of weak interface) and advance toward the micro and macro size, which thereafter will cause total structural failure.