Integration of NDE Reliability and Fracture Mechanics

Abstract

The Pacific Northwest Laboratory is conducting a four-phase program for measuring and evaluating the effectiveness and reliability of in-service inspection (lSI} performed on the primary system piping welds of commercial light water reactors (l..WRs). Phase I of the program is complete. A survey was made of the state of practice for ultrasonic rsr of LWR primary system piping ·Nelds. Fracture mechanics calculations 'Nere made to establ-ish required nondestru:tive testing sensitivities. In general, it was found that fatigue flaw·s less t11an 25% of wall :hic!<ness would not grow to failure within an inspection interval of 10 years. However, in s011e cases failure could occur consi·dera~ly faster. Statistical methods for predicting and measuring the effectiveness and reliability of lSI were developed and will be applied in the 01 Round Robinof Phase II. Methods were also developed for the production of flaws typical of those found in service. Samples fabricated by these 1nethods'Nil I be us~d in Phase II to test inspection effect h·eness and re 1 i ability.Measurements were made of the influence of flaw characteristics {i.e., roughness, tightness, and orientation) on inspection reliability. These measurernents, as well as the predictions of a statistical model fJr inspection reliability, indicate that :urrent reporting and recording sensitivities are inadequate.i i i

Crack Characteristics and Their Importance to NDE

“…The difficulty of the inspection is caused by typical characteristics of cracks, which affect, e.g., propagation, reflection, diffraction, transmission, attenuation and diffusion of ultrasonic energy [2,10]. Such flaw characteristics have been stated to be, amongst others, location, orientation and size of a crack (e.g., [19]), the opening of a crack and crack tip (e.g., [1,20,24]), the remaining residual stresses in the material (e.g., [8,11]), fracture surface roughness (e.g., [13,20]), plastic zone (e.g., [16]), and filling of the crack with some substance (e.g., [2]). …”

“…This effect was seen for certain inspection geometries in the PISC-II exercise, where small rough flaws were found to have higher detectability than the smooth flaws of the same size. Increased frequency of the used probe increases the scattering effect due to fracture surface roughness [2]. Furthermore, if the surface has a regularly shaped fracture surface roughness profile, reflection of the incident sound wave may favor certain directions thereby decreasing the testing repeatability and reliability [9].…”

“…In their results, the echo height from crack opening significantly changed 4 Relation between fatigue crack surface opening width and echo height of three different sizes of cracks (3.7 mm, 6.1 mm and 8.2 mm) [24] with the changed surface opening width. Furthermore, echo height with the same opening width increased with increasing crack depth ( In the studies of detection sensitivity of mechanical fatigue cracks the effect of material condition to the interdependence of the opening of the crack and obtained ultrasonic echo amplitude height have been studied, e.g., by Becker et al [2]. Ultrasonic amplitude height obtained from mechanically loaded fatigue cracks showed different behavior with cold-worked and annealed materials.…”

“…Furthermore, the total signal drop was with the cold-worked material 32 dB and with the annealed material 22 dB, and both were reached at the yield stress of the material. Becker et al [2] attribute the difference in the total amplitude drop and the magnitude of the drop to the force the fracture surfaces bear. In the annealed material much less force is available, because the yield strength of the annealed material is lower, being half of that of the cold-worked material.…”

“…Studies on the effect of residual stresses have been performed experimentally with external mechanical loading, e.g., by Iida et al [11], Yoneyama et al [24], Becker et al [2], Ibrahim et al [10] and Denby et al [5]. Theoretical studies on this topic have been published, e.g., by Temple (1985) and Wirdelius (1992).…”