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Deformation studies of aluminum alloy 7075-T651 in equi-biaxial tension

Thursday (09.11.2017)
11:35 - 11:55
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Light alloys, especially aluminum alloys, finds extensive applications in aerospace industries because of its high strength to weight ratio in combination with excellent corrosion resistance. More often, such structural applications (pressure vessels and propellant tanks) are designed based on its uniaxial properties alone, although it encounters biaxial stress conditions during its service. Hence, biaxial tensile testing of metallic materials is of direct relevance to improve the structural integrity and reliability under such stress state. Many experimental techniques were reported for testing materials under biaxial stress state such as hydraulic bulge test, combined axial loading and internal pressure, Marciniak punch test and cruciform testing (cross-shaped). However, cruciform testing gained much interest because of its ability to test under in-plane condition and also offers the possibility of testing under various biaxial loading ratios without changing the specimen configuration.

Biaxial tensile testing machine of 250 kN capacity was in-house custom designed to carry out biaxial tensile testing of metallic materials till failure using cruciform specimen geometry. The test rig consist of hydraulic power pack, double acting hydraulic cylinders and loading frames which can apply desired tensile force along X and Y directions respectively. Strain-smart system 5000 data acquisition system (DAQ) was used for recording load cell as well as strain gage readings. Non-contact digital image correlation (DIC) technique was employed for obtaining strain distribution in the gage section of the cruciform specimen. Scanning electron microscopy (SEM) was employed to understand the influence of stress state on the mode of fracture.

It is observed that for instance, severe reduction in ductility with marginal improvement in strength under biaxial tension in contrast to its uniaxial counterpart. Fractographic studies revealed increased occurrence of grain boundary cracking under biaxial stress state. Though, the material displayed near-isotropic yield behavior, reduction in ductility is attributed to inherent crystallographic texture coupled with difficulty in accommodating through thickness deformation during biaxial loading.

Prof. Dr. Ravi Kumar Nelamane Vijayakumar
Indian Institute of Technology Madras
Additional Authors:
  • Srinivasan Nedunchezhian
    Indian Institute of Technology Madras
  • Prof. Dr. Ramachandran Velmurugan
    Indian Institute of Technology Madras
  • Satish kumar Singh
    Vikram Sarabhai Space Centre,Trivandrum
  • Dr. Bhanu Pant
    Vikram Sarabhai Space Centre, Trivandrum