On the Issue of the Admissibility of Defects in the Lower Chord of Crane Beams

Abstract

Crane beams are among the most damaged structures in the buildings with heavy, and very heavy-duty overhead cranes. Their destruction can lead to the death of people working in the workshop, and to significant material losses. The durability of crane structures is determined by the intensity of work of the overhead cranes. The main defects and damages of metal crane beams are regulated by the standard STO-22-05—04, according to which all the cracks in the crane beams are divided into groups according to the risk of destruction. It is believed that the presence of cracks in the lower (stretched) chord of crane beams is unacceptable. Modern methods of non-destructive testing allow to detect defects with a minimum size from 2 mm (ultrasound) to 1 µm (capillary method). These cracks may be due to the inhomogeneity of the chemical composition of the alloy, or be of a structural nature, manifesting itself, in contrast to fatigue cracks that grow in the most loaded areas of the structure, in any part of the lower chord of the beam. If any crack is found in the lower chord of the crane beams, it is required to stop the operation of the crane and carry out a major overhaul of the crane runways. However, following these formal rules is not always economically and logically justified. The paper considers an example of rationing by the criterion of non-straining through cracks in the lower stretched chord (cracks of the first group). Based on the criteria of linear fracture mechanics, coefficients are proposed for all steels from which the crane beams are made, allowing for their technical inspection to give an express assessment of the admissibility of the detected through cracks. It is also shown that it is necessary to normalize the permissibility of the parameters of semi-elliptical cracks in the stretched zone (in the lower shelf of the I-beam crane beam), since these cracks can eventually develop into through defects.