1. 1. FURI Project for Spring 2015 Investigating the Role of Stress Triaxiality in High Strain Rate Behavior of an AZ31 Magnesium Alloy Introduction Magnesium alloys are of increasing interest due to their low-density, moderate specific strength and stiffness, recyclability and high damping among other properties [1]. In particular, these properties are of direct relevance to automobile, rail and aerospace applications, and thus have attracted the interest of the civilian and defense transportation sectors for fuel use reduction. Of utmost importance is the ability to increase strength and creep resistance [2] of the Mg-alloys to meet the structural needs of some components; however, meeting the rigorous demands for fuel efficiency will require a variety of lightweight components with energy efficient formability to economically strengthen and manufacture sheet products and complex shaped parts. This exposes a clear weakness on the part of HCP Mg-alloys; the room temperature ductility is intrinsically poor and the strain rate behavior (especially under a car crash scenario, 35 MPH frontal impact which translates to a strain rate of ~103 s-1 ) is relatively unknown. Hence, in this FURI proposal, high strain rate testing under a tensile condition will be performed using Bridgeman (cylindrical sample with different notch radii) notch specimens to vary the stress triaxiality and to investigate stress-state effects on the high strain rate behavior of an AZ31 magnesium alloy. AZ31 magnesium alloy is the most common Mg alloy consisting of mainly magnesium with 3 wt. % aluminum and 1 wt. % zinc [2]. High strain rate testing has been performed extensively in the past using a split Hopkinson pressure bar (SHPB) a diagram of which can be found in Figure 1a [3]. The SHPB uses a series of strain gauges on two bars that are designed to not experience any permanent deformation on impact. The material specimen is placed between the two bars. Strain data from the gauges can be used to calculate the force and displacement in the specimen during loading. The SHPB in the multiphysics lab at Arizona State University (Figure 1b) is capable of performing tensile tests at strain rates from 103 s-1 to 104 s-1 and will, therefore, be used for this investigation. Objective The primary objective of this research is to investigate the role of stress-triaxiality on the high strain rate behavior of lightweight Mg-alloys and subsequently provide a materials data base for the finite element high strain rate material model development. Figure 1. a) Schematic of a tensile SHPB b) SHPB apparatus at Dr. Solankis lab at ASU.
2. 2. Approach Rolled AZ31 Mg alloy has been purchased from Source1Metals and machined in house with varying notch sizes (0.8 mm, 1.45 mm, and 2.75 mm). The varied notch size will change the stress triaxiality. The specimens will be prepared from the rolling, transverse, and normal directions. The effects of impact will be analyzed through quasistatic (10-3 s-1 to 10-1 s-1 strain rates) and high strain rate mechanical testing. Low strain rate tests will use an electromechanical tester capable of measuring up to a 10 kN load. High strain rate tests will be performed on a 12.7 mm diameter, maraging steel SHPB. The SHPB has been setup for tensile testing using a gas gun to fire a steel striker tube toward a flange causing a tensile wave to travel through the bar upon impact (Figure 1a). The sample will be glued between the two bars of the SHPB using an ultra-high strength epoxy which has been proven effective for high strain rate testing of metals. Strain waves generated from striker impact will travel through the specimen and be measured by strain gauges located on the two bars. The combination of low and high strain rate mechanical testing will allow determination of the effect of varied strain rates on stress triaxiality. Force and displacement data will be obtained from the tests and used to generate a true stress-strain curve which, when low rate stress and strain are compared to high rate stress and strain, will show the effects of strain rate on stress triaxiality. In addition to studying the stress and strain response, fractured surface images will be obtained using scanning electron microscopy (SEM). As a result of stress triaxiality, shear strains will be generated in the material leading to reduced ductility in the specimen. Results from SEM will show how the impact of shear changes for each condition tested (i.e. notch size, directionality, and strain rate). Expected outcomes The purpose of investigating the stress triaxiality of Mg at elevated strain rates is to develop better models for crash simulations allowing more efficient use of materials to enhance vehicle performance (e.g. fuel efficiency) without sacrificing safety or longevity of the vehicle. To this end, our investigation will determine how the material behaves under various stress states. These findings will provide the necessary data to develop improved simulations of material impact. This work directly interfaces with industrial development of important lightweight alloys for improved fuel economy, strength, and performance. Impact Testing the high strain rate triaxial stress response of AZ31 Mg alloy will result in a greater understanding of the failure properties of components under extreme environment (crash). This gained knowledge will pave the way for further research, improvements, and optimizations of structural components. As a result, this research will aid in the betterment of the environment as well as the general public as fewer emissions will be generated from lighter weight, more fuel efficient vehicles. By improving simulations, optimization of more efficient magnesium structures will improve the performance of electric vehicles and will require less fuel from standard gas using vehicles. All the while, safety standards and vehicle longevity will not be sacrificed. This research can improve our understanding of an established material, opening different possibilities and applications for it. This research falls under the two Ira A. Fulton School of Engineering research themes of sustainability and energy. Improving the simulations of a material that comprises many different structures in the transportation industry contributes to a lighter structure thus using less energy to move and emits fewer emissions from the engine.
3. 3. References [1] Edgar RL, Magnesium Alloys and their Applications, K.U. Kainer pub., France, (2000) [2] ASM Specialty Handbook: Magnesium and Magnesium Alloys, Eds Avedesian MM, Baker H), ASM International, USA 1999 [3] B. A. Gama, S. L. Lopatnikov, and J., John W Gillespie, Hopkinson bar experimental technique: A critical review, Appl. Mech. Rev., vol. 57, no. 4, pp. 223250, Oct. 2004 PERSONAL STATEMENT: Motivation My motivation to do this research project is to gain experience and knowledge that is unobtainable in a lecture room setting. With this research I hope to use and improve my current skill set and engineering knowledge. In addition to understand the material, structural, and mechanical reasoning behind the results of our tests. Career Planning This research opportunity will give me a taste of what a true engineer is. I hope to use this as a tool to increase my knowledge of the industry. As I study to become an engineer I will use the knowledge gain to aide in my future career path and ambitions. Coursework Although I am only I sophomore and have yet to be enrolled in specialized engineering based courses, I have the tenacious mindset to learn the principles and methods that will lead to this researchs success. This research opportunity will give me a huge learning curve above my peers. Work Experience I will be relying heavily on my experience in building longboards for it will aid in my abilities to modify the current Hopkinson bar system, and quickly pick up the skills necessary to work in the machine shop. With my previous experience in working with composites and wood, working with metals wont be as foreign. Summary Overall I am ecstatic about this research project. Personally this is something that fascinates me to the core. I am always eager to solve issues when faced with a new challenge and I feed off of the rush of excitement when an assembly of parts are laid before me. I hope my passion will drive me to this projects success, for this experience will help aid me in my journey to become a true engineer. Timeline: Tasks Timeline (week) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Preliminary literature review SHPB setup High rate tensile testing Quasistatic tensile testing Data analysis Report and presentation