Safety assessment of allowable through-wall crack angles for pipes subjected to tensile loading

Abstract

Fully plastic collapse stresses for high toughness ductile pipes with circumferential cracks subjected to tensile loading can be predicted by means of Limit Load Criteria based on the net-section stress approach. The Limit Load Criteria are provided by the ASME Code Section XI. Allowable membrane stresses for part-through cracks were determined by plastic collapse stresses in combination with safety factors. The allowable stresses decrease with increasing angles of the part-through cracks. When crack angles are large, the allowable stresses of the part-through cracks are larger than the collapse stresses of through-wall cracks. For such large cracks, allowable stresses greater than the collapse stresses cause instability, and are thus detrimental to pipe integrity, especially in thin-wall pipes, even though the stresses are allowed. In order to avoid the anxiety associated with this, it is necessary to establish maximum allowable crack angles. This paper proposes maximum allowable crack angles for allowable stresses based on the collapse stresses of through-wall cracks. The maximum allowable crack angles can be expressed by equations for various service level conditions.