Analysis of damage in 5083 aluminum alloy deformed at different strainrates

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Abstract

This paper presents a study on damage evolution in 5083 marine-grade Aluminum alloy while deformed under different strain rates. The concept of continuum damage mechanics (CDM) was utilized to evaluate the evolution of damage throughout the deformation process of 5083 Aluminum alloy. Tensile tests with several unloading and reloading stages were conducted at room temperature to generate true stress–true strain curves at three different strain rates of 0.001 s−1, 0.01 s−1, and 0.1 s−1. A scanning electron microscope (SEM) was utilized to characterize the damage at several strain levels for each strain rate. SEM results showed that void area fraction increases with strain rate and with accumulation of plastic strain. Reduction in stiffness with accumulation of plastic strain was also evaluated. Isotropic damage values were predicted using the hypotheses of strain equivalence and strain energy equivalence, as well as a recently developed energy-based model. The predicted damage values were verified via comparisons with the present SEM results. The energy-based model showed good comparisons, while the other two hypotheses overestimated the damage, especially at higher strainrates.

Introduction

The emergence of some global economic forces has influenced the Aluminum industry, with effects on consumption, production and supply. Aluminum materials are exposed to different types of loadings, leading to deterioration and change in their structural/mechanical properties. This progressive physical process of degradation in the mechanical properties with continuous loss of stress carrying capacity is commonly referred to as damage. The material damage may initiate at some points in the deformation process in the form of micro-cracks and/or micro-voids evolving at its latest stage, to the accumulation of macro-cracks and consequently leading to material failure. Therefore, systematic understanding of the ductile failure mechanism due to accumulation of plastic deformation and damage is needed to enable proper structural design and hence provide better serviceability.

A great deal of effort has been investigated in understanding the plasticity and damage behavior and its effect on the flow stress of different Aluminum alloys. This understanding is essential for the modeling and analysis of numerous processes including high-speed machining, impact, penetration and shear localization. The widely used approach for damage modeling is the use of the thermodynamic framework of the continuum damage mechanics (CDM) describing the isotropic and anisotropic damaged-plastic (or viscoplastic) behavior of materials. Lemaitre–Chaboche's approach [1] is extensively used in many research laboratories and industrial developments. Moreover, ductile damage, due to microvoids evolution, occurs especially in well plastically deformed zones where the stress triaxiality is high. The latter has a significant effect on the void growth rate. Void initiation and growth have been extensively studied by means of micromechanics analysis. In 1977, Gurson [2] proposed a pioneered model of damage by cavitation based on an approximation analysis of spherical voids with only one yield function for porous ductile perfectly plastic matrix. The initial Gurson model shows some limitations. In fact, it over-predicts the evolution of microvoids under monotonic loading conditions. Any type of ratcheting under cyclic loading paths cannot be predicted since the yield function depends only on a single yield function. Therefore, several extensions have been made. The most important ones are either based on the improvement the predictions at low volume fraction of voids [3] or modified its yield function in order to describe the rate sensitivity, necking instabilities and better description of the final voids coalescence[4]. Classically, damage can be viewed as microcracks, microvoids, and other local defects. Scalar isotropic and tensorial anisotropic damage concepts are generally adopted in modeling. Anisotropic induced by damage is shown by a number of microcrack distributions and their growth within metals. Several damage models considering isotropic and/or anisotropic expressions for the damage variable have been proposed by many authors [5], [6], [7], [8], [9], [10]. Although it is anisotropic in nature, assuming isotropic damage in many cases is not too far from reality, at least in the deformation range up to the maximum engineering stress [11], [12], [13], [14].

There are many methods to experimentally measure the damage process, which is reflected in a progressive material deterioration that can be evaluated through the decrease of strength, stiffness, toughness, etc. Lemaitre and Dufailly [15] used the concept of effective stress and recommended different direct and indirect experimental methods to evaluate damage. The indirect ones involve destructive methods such as measurement of change in the elastic modulus and ultrasonic wave propagation, and non-destructive methods like for example the hardness variation measurement and the electric potential. The direct measurements, on the other hand, include the observation of micrographic pictures using digital microscopes to measure the areas of cracks and the density variation measurement. The latter approach has been widely used to evaluate damage in metals by finding the fraction area of voids and cracks at different strain levels. The crack and void measurements are conducted using enhanced modern image analysis tools such as the scanning electron microscope (SEM), which is relatively easy to work with. The analysis of SEM images which can be converted to any suitable format does not lead to volume loss of the sample.

The main objective of this research is to examine the evolution of damage throughout the deformation process of 5083 Aluminum alloy under different rates. A series of tensile tests is carried out on Aluminum sheet specimens at three different strain rates of 0.001 s−1, 0.01 s−1 and 0.1 s−1 in order to obtain the stress–strain curves which in turn allow obtaining their elastic, strain hardening and damaging characteristic parameters. SEM image analysis is also conducted at different levels of plastic strains for each strain rate. The present SEM results as well as the stress–strain results are utilized to characterize and model the damage process in the 5083 Aluminumalloy.

Section snippets

Experimental procedure

In this work, 5083 marine grade Aluminum alloy in the form of a 4.0 mm thick sheet was used. This alloy was chosen due to its high strength to weight ratio and excellent corrosion resistance. A flat type coupon specimen was selected. The experimental specimens were fabricated according to ASTM E8 standards [16] with the only modification being a shorter gauge length to enable higher strain rates. A sample and a sketch of dimensions (in millimeters) of the test specimens are shown in Fig. 1.

A

Theoretical description of damage

The isotropic damage definition which is widely employed in literature due to its simplicity and efficiency is adopted in this study to characterize and model the degradation process of 5083 Aluminum alloy. A damage parameter φ which represents the deterioration of a material state before the initiation of macro-cracks is introduced using the effective stress concept given as follows:φ=AA¯Awhere A¯ is the effective net area and A is the damaged area. Eq. (1) shows that, when φ=0, the material

Results and discussions

True stress–true strain curves and direct damage measurements obtained using SEM are presented in this section. Experimental damage results are also compared with the damage values predicted using the energy-based damage model, and the strain equivalence and elastic energy equivalence hypotheses.

Conclusion

In this study, uniaxial tensile tests for 5083 Aluminum alloy samples were performed at different strain rates. Specimens were unloaded at different plastic strain levels, and a representative element was cut from each specimen to quantify the void fractions using SEM. An isotropic damage variable is used within the context of CDM to indicate the deterioration of the material capability to carry loads. Degradation in elastic moduli with the accumulation of plastic strain, and plastic strain

Acknowledgment

This work was supported by American University of Sharjah (Faculty Research Grant no. FRG 10–40).

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