Analysis of Failure Emanating from a Notch
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ANALYSIS OF FAILURE EMANATING FROM A NOTCH G. Pluvinage Notch-like defect assessment cannot be done by using classical fracture mechanics (mechanics of cracks). In order to prevent overconservatism, the concepts of notch fracture mechanics, such as notch stress intensity factor or notch energy integral J ρ are generally used for this purpose. A local stress criterion, named volumetric method, defines effective stress. It is used for more advanced assessment when introducing geometry and constraint effects. These effects are taken into account for notch fracture toughness and constraint a modified notch failure-assessment diagram. Keywords: notch, fracture mechanics, failure-assessment diagram.
Failure Emanating from Notches Failure is governed by the critical conjunction of three elements: defect, loading, and fracture resistance. A large variety of defects can promote failures. Crack is the most severe among these defects as ideally considered with an infinite sharpness. Notches are considered as less severe with finite sharpnesses. Notch severity strongly depends on the notch sharpness as the inverse of the notch radius. Notches are responsible for numerous failures of the components. As an example, a case of failure emanating from a column with square thread of a friction screw press is shown in Fig. 1. The thread was not rounded off in the roots and the flanks exhibited striations. Both these factors enhanced the notch effect. Striations are particularly numerous at the point A (Fig. 1), which also denotes the point of fatigue initiation. In the present paper, special attention is paid to notches appearing at oil (or gas) pipe surfaces. These notches or scratches are generally created by excavators when the works are carried out on the nearby pipes. These incidents are called external interferences. They are considered to be responsible for more than 50% of pipe failures in the Western Europe [1]. Prediction of failures emanating from a notch cannot be done by using classical fracture mechanics, which considers failure-promoting defects as crack-like defects. Therefore, this approach is too much conservative. Past methods were based on the stress concentration factor, and it was considered that failures are initiated at the notch root, which is true only in the case of brittle fracture and for a particular approach to the mechanism of failure. It is assumed that the fracture driving force is concentrated at one point and, therefore, its density is infinitely high. Thus, the notch fracture mechanics (NFM) has been developed to overcome this difficulty, [2]. The notch sharpness is taken into account through the stress distribution at the notch tip and various failure criteria (local or global). Since Neuber [3] or Novozilov [4], it has been admitted that fracture needs a process volume in which the necessarily available energy is stored. Université de Lorraine, École Nationale d’Ingénieurs de Metz, Metz, France; Fiabilité Mécanique, Conseils, Lorraine, France; e-mail: [email protected]. Published in Fizyko-Khimichn
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