IJE TRANSACTIONS B: Applications Vol. 27, No. 5, (May 2014) 803-810 International Journal of Engineering Journal Homepage: www.ije.ir The Investigation of Modeling Material Behavior in Autofrettaged Tubes Made from Aluminium Alloys K. Aliakbari*, Kh. Farhangdoost Department of Mechanical Engineering, Ferdowsi University of Mashhad, Mashhad, Iran PAPER INFO Paper history: Received 25 September 2012 Accepted in revised form 07 November 2013 Keywords: Thick-Walled Tube Autofrettage Nonlinear Mathematical Model Bauschinger Effect Aluminum Alloys ABSTRACT In this research, a nonlinear strain hardening mathematical model is proposed for 7075 aluminium alloy (A7075). Uniaxial tension-compression experimental data are used to figure out a suitable model to study the Bauschinger effect factor, BEF. Hence, uniaxial tension-compression tests on specimens having 12.5 mm and 6 mm diameters were carried out by an Instron servohydraulic machine and the results were compared. In this paper several factors including Young’s modulus, the amount of offsets to determine yield point and BEF were studied. Besides, BEF in the aluminium tubes made from A5083 and A7075 alloys were compared. This model will be employed to predict residual stress and fatigue life in autofrettaged tubes. doi: 10.5829/idosi.ije.2014.27.05b.17 1. INTRODUCTION 1 There are many industrial uses of thick-walled cylindrical pressure vessels such as power plant boilers, nuclear and chemical reactors, gun barrels and reactor pressure vessels. These are now used in renewable energy industries and also food preparation industries. Thick-walled cylinders are not only resisting against high internal pressures, but also are bearing pressure fluctuations, heat shock, and a corrosive environment, which all cause crack initiation and fatigue crack growth [1]. Many comprehensive researches have been carried out on crack propagation in thick-walled pressure tubes reflecting its importance [2]. The pressure in tubes is mostly of cyclic type and can cause cracks due to the fatigue which in turn causes the leakage of the under-pressure fluid, or eventually leads to the bursting of tubes [3]. One of the effective methods of increasing the pressure capacity in thick- walled tubes is applying the process of autofrettage. In this method, making the plastic zone of the walls up to a *Corresponding Author Email: karimaliakbari@yahoo.com (K. Aliakbari) certain radius is done by an internal pressure (hydraulic autofrettage or swage autofrettage). After releasing the internal pressure, what remains is residual stress distribution in the tube that consequently increases the loading capacity and resistance against the growing rate of internal cracks. As the autofrettage process is affected by a loading-unloading cycle, the material behavior should be dealt with carefully [4]. The first analytical model of the autofrettage process was proposed by Hill [5] in 1950. This model assumes plane strain, elastic-perfectly plastic material, von Misses yield criterion and the condition incompressibility without considering the Bauschinger effect factor. The ratio of the decreased yield stress in reversed loading to the initial yield stress from a certain amount of plastic deformation is called Bauschinger effect factor (BEF). It was discovered by Bauschinger [6] in 1881. The autofrettage process is affected by plastic deformation of loading-unloading cycle. Hence, to analyze the autofrettage process it is essential to calculate the BEF accurately. Perry et al. [7] have generalized the Bauschinger effect (BE) to other parts of unloading behavior and attribute them to the effects including variable Young’s modulus related to unloading phase