Assessing the risk of brittle fracture of structures with holes made of old mild steel

Abstract

In the safety assessment of existing structures made of old mild steel and for the decision on necessary repair and reinforcement measures, the verification of sufficient material toughness (safety against brittle failure) is essential. The normatively regulated assessment methods to avoid brittle failure in DIN EN 1993-1-10 were developed based on extensive fracture mechanics investigations. They apply to welded structures and modern steels with generally high toughness. The quantification of the material toughness in material standards is realized by Charpy impact tests. The relationship between the transition temperature T_27J of the notch impact energy and the reference temperature T₀ in fracture toughness is realized by the modified Sanz-correlation which has been derived for these steels.

The method is not suitable for mild steel structures with holes for riveted or bolted joints. On one hand, the stress concentrations and residual stresses of welded and riveted structures and thus toughness requirements are significantly different. On the other hand, the toughness properties of old mild steels have significantly larger variations.

In the present dissertation, experimental, analytical and numerical studies of the brittle fracture behavior of structures with holes made of old mild steel are presented. Extensive material analyses to determine the fracture toughness in the brittle-ductile transition region according to the Master Curve concept (ASTM E1921) are an essential part of this thesis. The evaluations confirm that, depending on the manufacturing process, different grades of steel can be defined. To analyze the influence of punching the holes on the brittle fracture behavior of old mild steels, structural examinations and microhardness measurements are performed.

Based on an extensive analysis of typical construction details of existing structural steelwork, fracture mechanical finite element calculations of connections of angle profiles are performed to determine their toughness requirements. The obtained results of the stress intensity factor are revised for further use by modification of known solutions from the specialized literature.

Based on the studied construction details, a practical method for assessing the hazard of brittle fracture of riveted and bolted components is derived in the framework of fracture mechanics safety analysis. Using statistical methods variations of strength and toughness properties of mild steels are considered and transferred to a semi-probabilistic assessment concept after the verification by component tests.