Analysis of peculiar failure of 7050 aluminum bolts used in water-sprinkled environment

Abstract

Structures assembled using profiles and bolts made of aluminum (Al) alloy can be recycled without dismantling, as loosening and electrical corrosion of the bolts are prevented by fastening them between homogeneous materials. However, reports on practical applications are scarce. This study analyzes the breakage of Al bolts in a structure used for an extended period in a farm growing cloud-ear mushrooms under a water-sprinkled environment. The site investigation revealed that the fracture surface of bolts was inclined and that the farm water had a weakly basic pH value. Mechanical, chemical, and crystallographic properties were measured through tensile testing, fractography, X-ray diffraction, Fourier transform infrared spectroscopy, and electron back-scatter diffraction analysis on both broken and unbroken Al bolts recovered from the site. The results showed brittle mechanical properties due to stress-corrosion cracking from aluminum hydroxide formation caused by prolonged water exposure. Failure analysis was conducted using maximum principal stress and Mises stress-failure criteria based on actual, tightening-torque values. The bolts were judged to fail based on the maximum principal-stress criterion. These findings suggest that optimizing the environment and tightening torque is crucial for the effective use of structures fastened with Al bolts.

Keywords

• 7050-aluminum bolts
• aluminum hydroxide
• stress corrosion cracking
• brittle fracture
• maximum principal stress criterion
• electron backscatter diffraction (ebsd)