Investigation of aluminium foam shear behavior by DIC analysis

Aluminum foams represent a new class of materials characterized by a porous structure that confers them remarkable potentiality in advanced engineering application thanks to their interesting value of strength/weight ratio. Cellular structure consists of a solid and a gaseous phase and cells morphology identifies two type of foam: open and closed cells foams. This peculiarity allows employing these materials in a large range of industrial fields: aeronautic, automotive, biomechanical and nautical. Nevertheless, applications are yet limited since the behavior characterization of these new materials is inadequate and incomplete. In particular, the literature is very poor regarding to the study of foam shear behavior [1-6]. In this work, a large number of tests have been planned on several kinds of foam having different density and thickness. New systems for shear tests are conceived, compared and evaluated. Moreover, all the tests are recorded by a CCD camera in order to obtain a series of images representing the deformed configuration of the specimen during the test. These images are used to determine the full-field displacement data and then to calculate the local strain in each point of the foam. This experimental technique is based on Digital Image Correlation (DIC). DIC measurements are full-field and non-perturbative technique and it is particularly useful in all the analysis in which heterogeneity of behavior is characterizing the material. DIC is an appealing technique for studying the heterogeneities that evolve into a material during a quasi-static or dynamic application of load, because under opportune conditions it shows a precision and versatility that is difficult to get employing other techniques [7, 8]. It is an optical method based on image correlation and path recognition of markers of a virtual grid superimposed to the images of tested specimen. The CCD camera records the images with 256 grey levels. Due to their particular morphology and structure, aluminum foams can present strongly localized phenomena of deformation that can evolve in time and space. Because of the statistical laws that govern this behavior it is impossible to make reference to a reliable analytical or numerical model for making sharp prevision. Load systems adopted are represented in Fig. 1. They are designed in manner to be mounted on a MTS conventional testing machine generally used for tensile and compressive tests. The eccentric grip system produces a shear stress in the longitudinal section, indicated by a red line in figure, when a tensile load is applied at the grips. The biaxial load system applies simultaneously a tensile stress by the vertical arms gripped in the MTS machine and a compressive stress by the horizontal arms. Tensile and compressive stress can be assumed equal if it supposes that the stiffness of the specimen is the same in tension and compression [1]. In this condition, shear stress state is obtained in a 45° plane respect to the vertical direction. Curves obtained by these two kinds of test show similar shape: an initial elastic trend followed by yielding and a peak load. Results prove that also in the shear behavior density is the principal factor that affects the foam properties as shear modulus and strength. The use of DIC technique to determine strain field allows evaluating if a shear stress state is really obtained. Besides, it is possible to evaluate the behavior of inner core of specimen and to understand if a phenomenon is superficial or it interests all the thickness of stressed specimen. Finally, a comparison of shear modulus values obtained using an analytical relationship [4] reveals a good agreement especially with the data obtained by biaxial device.