Abstract
This paper focuses on the identification of failure causes of a bolted joint used for the bedding of a primary column of an aerial platform. The tests conducted to determine chemical composition and mechanical properties have confirmed the 12.9 grade of the bolts. Also, mechanical properties tests and metallographic examination did not indicate any hidden defects caused by heat treatment (e.g. decarburisation etc.). On the other hand, visual and metallographic examination of cross-section as well as electron microscope scanning confirmed fatigue fracture of the bolts. Therefore, further analyses were done to check the dimensioning and the installation of the bolts. The effects on stress states in the bolts are presented from the standpoint of rated operating regimes, stiffness of joined elements, lubrication conditions of thread surfaces and temperature incompatibility between the bolts and elements being joined. Pre-tightening torque value incompatible with lubrication conditions was found to be the main factor that led to high stress values that initiated and later intensified the fatigue process which ended in fatigue failure of the bolts.
Highlights
Izvorni znanstveni članak U radu se razmatra problematika identifikacije uzroka otkaza vijčanog spoja uležištenja osnovnog stupa platforme za rad na visinama
Various approaches have been proposed to identify the failure causes of bolted joint used for bedding of the primary column of an aerial platform
Based on values from Tab. 5, it can be concluded that consolidated stress response, including all partial effects, equals 1186,4 MPa, which practically corresponds to bolt material yield strength (Re = 1200 MPa)
Summary
Aerial platform operating conditions and working principle define operating regimes characterised by distinct stochastic nature regarding the level and the nature of loads on whole construction [1]. Considering extremely low number of hours the platform was used as well as the frequency of bolt replacements, activities of other part of the research were simultaneously performed, with the aim to evaluate correct dimensioning of the subject bolted joint [11] These researches and analyses were directed towards recognising the potential partial effects on load regimes and stress states of the bolts in the critical joint from the perspective of lubrication conditions of thread surfaces while forming the bolted joint, bolt pre-tightening, temperature incompatibility between the bolts and the elements being joined, platform rated operating regimes, as well as the stiffness of the elements in the critical joint regarding load distribution within the bolts
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