International Journal of Aerospace and Lightweight Structures

doi: 10.3850/S2010428615100114

Probability of Detection – A Concept To Combine Nondestructive Testing and Evaluation with Probabilistic Fracture Mechanic Approaches

Gerd Dobmann^a, Jochen Kurz and Dragos Cioclov Fraunhofer-Institute for Non-destructive Testing, Saar-University, 66123 Saarbrücken, Campus E 3 1, Germany. ^aGerd.Dobmann@t-online.de

ABSTRACT

The here presented contribution is aiming at the increase of understanding between NDT and fracture mechanic experts. The results documented are on the basis of the software application PVrisk which was developed to analyze cracks and other types of irregularities, named material defects, to assess pipelines and cylindrical pressure-vessels. The software and its application as tool essentially is in the concept of the quantitative non-destructive evaluation (QNDE). The meaning and advantage of QNDE is in the combination and synergy of reliable inspection results based on NDT on one hand and procedures for lifetime evaluation of components on the other. However, since the QNDE-concept requires the knowledge about the geometrical dimensions of the irregularity to be assessed, only such NDT techniques are in consideration at the discussion which possess the ability to deliver these values through the inspection procedure. Since the quality of reliability is a demand additionally, this reliability must be known for the proposed procedures. By taking into account the variety of the product-forms and the materials used, the different material-loads applied (mechanically static and dynamic, temperature, corrosion, etc.) the demand can be fulfilled alone by utilization of statistical methods and characteristics. PVrisk can assess both, deterministic and probabilistic approaches. For the probabilistic treatment, the statistical fuzziness of the geometrical inspection result can be taken into account equally like the observation of the scatter in the relevant material properties. Knowledge about the real statistic in the inspection data presupposes this both in the NDT data and in the fracture mechanics data. The discussion of real NDT data shows that it is not possible to detect each of the possible irregularities with the same likelihood and to assess them in size. This fact lets itself describe quantitatively through the concept of likelihood (POD-probability of detection), which strongly is influenced by the type of the NDT technique.

Keywords: Probability of detection, Fracture mechanic, Deterministic, Probabilistic, Failure assessment diagram, Sensitivity study, Material properties influence, Crack geometry influence.

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