Volume 55, Issue 4

A series of research studies have been conducted over the course of five years venturing into the fields of in-service inspections (ISI) in nuclear power plants (NPPs) and inspection of manufactured components to be used for permanent nuclear waste disposal. This paper will provide an overview of four research studies, present selected experimental results and suggest ways for optimization of the NDT process, procedures, and training. The experimental results have shown that time pressure and mental workload negatively influence the quality of the manual inspection performance. Noticeable were influences of the organization of the working schedule, communication, procedures, supervision, and demonstration task. Customized Failure Mode and Effects Analysis (FMEA) was used to identify potential human risks, arising during acquisition and evaluation of NDT data. Several preventive measures were suggested and furthermore discussed, with respect to problems that could arise from their application. Experimental results show that implementing human redundancy in critical tasks, such as defect identification, as well as using an automated aid (software) to help operators in decision making about the existence and size of defects, could lead to other kinds of problems, namely social loafing and automation bias that might affect the reliability of NDT in an undesired manner. Shifting focus from the operator, as the main source of errors, to the organization, as the underlying source, is a recommended approach to ensure safety.

Die Fehlerauffindwahrscheinlichkeit oder “Probability of Detection” ist zu einer wichtigen Qualitätskennzahl von zerstörungsfreien Prüfsystemen geworden. Die Einschätzung von nur künstlichen Fehlern ist jedoch als Bewertungsgrundlage meist nicht ausreichend. Die später verwendeten Prüfobjekte mit den real vorkommenden Fehlern sind die zu betrachtende Basis, für die der Endanwender eine gültige Qualitätseinschätzung der angewandten zerstörungsfreien Prüfung fordert. Jedoch ist bei dem Fall, dass die Datenmenge gering ist, auch die Aussage der POD beschränkt. Es muss daher eine Zwischenlösung gefunden werden, in der die Informationen von beiden Prüfungen kombiniert werden. Die Mathematik ermöglicht mit der Bayes'schen Statistik, diese Informationen miteinander zu kombinieren und somit beide Informationen in die Bewertung einfließen zu lassen. Dabei spielt jedoch der Zusammenhang zwischen Signal und Fehlerausprägung eine wichtige Rolle. Praktisch angewendet und erprobt wird diese Vorgehensweise für die Bewertung der zerstörungsfreien Prüfungen der Firma POSIVA, die sich mit dem Bau eines Endlagers für hochradioaktiven Abfall in Finnland beschäftigt.

The aim of this publication is to provide an overview of new methodologies for evaluating the reliability of NDE systems in accordance with the specific requirements of industrial applications. After a review of the substantive issues of the previous decades, the go forward guidance is concluded. For high safety demands a quantitative POD (Probability of Detection) created from hit/miss experiments or signal response analysis and ROC (Receiver Operating Characteristics) are typically created. The modular reliability model distinguishes between the pure physical-technical influence, industrial application factors, and human factors. It helps to learn which factors can be determined by modeling and by open or blind trials. A new paradigm is offered to consider the POD or reliability of the system as a function of the configuration of input variables and is used for optimization rather than for a final judgement. New approaches are considered dealing with real defects in a realistic environment, affordable but precisely like the Bayesian approach or model assisted methods. Among the influencing parameters, human factors are of high importance. A systematic psychological approach helps to find out where the bottlenecks are and shows possibilities for improvement.

In line with the damage tolerance design principles, an adequate non-destructive inspection system is needed to ensure structural integrity. Yet, when pushed to the limits of their detection capability, non-destructive evaluation systems do not produce consistent hit/miss indications. Their capability of detecting flaws is therefore expressed as a probability of detection. In the conventional signal response reliability analysis, the probability of detection is expressed as a function of the flaw size. The adequacy of the inspection system is verified by comparing the size of the flaw that is reliably detected with the maximum allowable flaw size, which will not undermine the structural integrity. Analyses of modern structures show that other parameters also determine the severity of the flaw for the structure. Within the multi-parameter reliability analysis, the probability of detection can be expressed as a function of diverse influencing parameters. When determining the adequacy of the non-destructive evaluation system, the capability of detecting a flaw has to be expressed and checked against the critical value of exactly that parameter that determines flaw severity for the structure. Failing to do so can lead to a rejection of the healthy, or acceptance of the bad part. The principle is demonstrated on the transmit-receive longitudinal ultrasonic inspection data of the iron cast component with surface, semi-elliptical, crack-like flaws.

Ein detailliertes Wissen über die Werkstoffeigenschaften stellt für Konstrukteure und Fertigungsingenieure die Grundvoraussetzung zur Gewährleistung eines sicheren und wirtschaftlichen Betriebs von Bauteilen dar. Ziel dieser Untersuchung ist es, den nur wenig untersuchten Konstruktionswerkstoff Vulkanfiber hinsichtlich des Einflusses der Verformungsgeschwindigkeit und des Feuchtegehaltes auf die quasistatischen Eigenschaften mikrostrukturbasiert zu charakterisieren und zu bewerten. Neben materialografischen und mikroskopischen Einrichtungen wurden ein Ultramikrohärteprüfsystem sowie zur mechanischen Charakterisierung eine elektromechanische Universalprüfmaschine, ein taktiles und ein Videoextensometer, Thermoelemente und ein Thermografiesystem sowie eine Hochgeschwindigkeitskamera zur Detektierung des Rissbildungs- und -ausbreitungsverhaltens eingesetzt.

The electrochemical behaviour of a carbon steel in deaerated acid media with concentrations of 20 ppm, 50 ppm, 100 ppm and 200 ppm imidazoline at different temperatures and with different stirring rates has been evaluated by using anodic and cathodic polarization curves (Tafel) and electrochemical impedance spectroscopy (EIS) to compare the corrosion inhibition efficiency. The experimental results suggest that imidazoline is a good corrosion inhibitor at 50 ppm concentration.

In this study, several heat treatments were applied to DIN 100Cr6 steel to obtain different matrices. In the first stage of the study, solution annealing treatment was applied to the steel and cooling was carried out in various media (furnace, oil, and salt bath). In order to eliminate the stresses after transformation from austenization, a low temperature tempering treatment was applied to the quenched samples. All heat treated samples were examined using light microscopy after metallographic preparations. In the second step, ‘ball-on-disc’ type tribometer was used to determine the friction coefficient of the steels depending on the matrix phase. Weight loss was recorded and the friction coefficient versus distance was plotted for each steel. Worn surfaces of the steels were examined using scanning electron microscopy to characterize the wear mechanisms. It turned out that (i) pearlitic, bainitic and martensitic matrices could be obtained depending on the cooling medium, (ii) martensitic matrix had higher wear resistance based on its weight loss, (iii) abrasive and adhesive wear tracks were present on the worn surfaces of the steels.

The effect of different surface preparation methods such as mechanical, chemical, and electrochemical surface preparation as well as ultrasonic cleaning on the formation, stability, and deterioration of surface films formed on austenitic 316 stainless steel was investigated in Tyrode's solution. The methods of cyclic polarization, AC impedance measurements and surface techniques were used. A hysteresis loop in a cyclic polarization curve was obtained that indicates a delay in re-passivation of an existing pit when the potential is scanned cathodically. Nyquist diagrams consist of two strongly overlapped capacitive semicircles which are assigned to charge transfer and passive film resistance. Electrolytic polishing improves corrosion resistance by increasing the value of the corrosion potential and breakdown potential of the passive layer as well as the pit initiation potential. After chemical passivation and acid cleaning, no perfect passivation region was observed. Change in surface fractal is in good agreement with the result obtained from height roughness factor of AFM.

In this research, the mordenite based catalysts for direct decomposition of N 2 O have been prepared and characterized for their direct decomposition performance at the operating temperature of 300–550 °C commonly encountered in the nitric acid plants tail gases. The zeolite catalyst was prepared by 2 steps: 1. Precursor wet ion exchange, 2. Precursor impregnation. The bi-metallic catalyst thus prepared shows an excellent N 2 O decomposition performance. The total conversion efficiency of Pt-Fe-MOR catalyst has been achieved at 390 °C. XPS analysis confirmed platinum on mordenite supports in two forms: PtO and PtO 2 species interact with oxygen ions at the metal/support interface. Deposition of Pt as a promoter on the surface of Fe-mordenite results in a considerable enhancement of catalytic activity.

Mordenite based catalysts for direct decomposition of N 2 O have been successfully prepared and their direct decomposition performance characterized at the operating temperature of 300–400 °C commonly encountered in the nitric acid plants tail gases. The catalyst was prepared in 2 steps: 1. Precursor wet ion exchange, 2. Precursor impregnation. In addition to the previous contribution, it is shown here that the bimetallic Rh-, Pd-, Ir-Fe-MOR catalysts thus prepared exhibited an excellent N 2 O decomposition performance: total conversion efficiency of Rh-Fe-MOR catalyst has been achieved at 400 °C and between 7500 h −1 and 10 000 h −1 space velocity.

This paper presents a process to determine the relationship between acoustic emission (AE) signals and the strain signal pattern for estimating material fatigue life. In addition, it establishes the usefulness of AE in predicting the fatigue life of metallic components. This study was carried out to investigate the relationship between the strain and AE signals to ensure that AE can also be used as a tool to predict the fatigue life of metallic specimens. Two types of sensors, i. e., the AE piezoelectric transducer and the strain gauge, were attached to SAE 1045 steel specimens during the fatigue cyclic test when the specimens were under constant loading. Two types of signals were obtained at three different applied loads (6.0 kN, 6.4 kN and 6.8 kN). Both signal types were then analyzed using a statistical model, and the results were used to correlate the fatigue life and AE signals. The fatigue life values were calculated using strain-life models. The correlation between the experimental and predicted values of fatigue life was then established. The results showed that the correlation between the r.m.s. AE values and the calculated fatigue life ranged from 74.0 % to 98.5 %, while the correlation between the kurtosis AE values and the predicted values was found to be between 95.0 % and 99.9 %. These correlation values showed that the AE technique can be used to predict the fatigue life of metallic specimens.