Issue 35

A. Tzamtzis et alii, Frattura ed Integrità Strutturale, 35 (2016) 396-404; DOI: 10.3221/IGF-ESIS.35.45 396 Focussed on Crack Paths Fatigue crack growth prediction in 2xxx AA with friction stir weld HAZ properties A. Tzamtzis, A. T. Kermanidis Laboratory of Mechanics and Strength of Materials, Department of Mechanical Engineering, University of Thessaly, Leoforos Athinon, Pedion Areos 38334, Volos, Greece atzam@uth.gr , akermanidis@mie.uth.gr A BSTRACT . An analytical model is developed to predict fatigue crack propagation rate under mode I loading in 2024 aluminum alloy with FSW HAZ material characteristics. Simulation of the HAZ local properties in parent 2024 AA was performed with overaging using specific heat treatment conditions. The model considers local cyclic hardening behavior in the HAZ to analyze crack growth. For the evaluation of the model, the analytical results have been compared with experimental fatigue crack growth on overaged 2024 alloy simulating material behavior at different positions within the HAZ. The analytical results showed that cyclic hardening at the crack tip can be used successfully with the model to predict FCG in a material at overaged condition associated with a location in the FSW HAZ. K EYWORDS . Fatigue Crack Growth Analysis; Aluminum Alloy; Friction Stir Weld; Heat Affected Zone; Cyclic Hardening. I NTRODUCTION atigue crack growth rate (FCG) is influenced by several complex interactive processes that involve microstructural and mechanical load variables. Models used for analysis of crack growth rate under constant amplitude stress include a hierarchy of major simplifications, with most important being the application of the linear elastic fracture mechanics (LEFM) concept and are mainly empirical or semi-empirical because they rely on fitting experimental material crack growth data [e.g 1]. Early models based on damage accumulation at the crack tip have a more physical background but use parameters that are difficult to determine experimentally [2-4]. In more recent modeling attempts [5-8], authors have used the cyclic material behavior at the crack tip to derive analytical expressions for the fatigue crack growth rate. Again, most of the models include geometrical (crack tip blunting radius) or material parameters (stress intensity factor threshold), which cannot be derived from simple tests. In [9], a fatigue crack growth model is developed, which considers a strip plastic zone with material hardening at the tip of a crack, under low cycle fatigue conditions, which uses few experimental parameters for fatigue crack growth analysis. In the above approaches a homogeneous material is considered and properties used for prediction are average material properties of the material volume examined. If a crack grows within a material zone with varying microstructure the analysis is more complicated and local material behavior needs to be implemented. In the early work by Reifsnider et al. [10] it was found that the existence of a strength gradient at the tip of the crack, depending on its slope, may accelerate or retard crack growth. This is relevant in the case of the heat affected zone (HAZ) in a weld region, where the aged material consists of a modified microstructure with local F