Gerd Dobmann

Nondestructive characterization of materials A growing demand for describing damage and service-life-relevant aging processes in plant components

Abstract The nondestructive detection and characterization of damage to materials poses a challenge to instrumentation and inspection technology. A reliable method has yet to be developed anywhere in the world which is sufficiently rugged to be used on components in an industrial environment. The application-oriented studies conducted to date have demonstrated the complex overlapping of influences exerted on nondestructive measurement variables by the damage to be detected and by other material states and properties. The nondestructive detection and characterization of material damage will not offer any realistic chance of success until the material and component state is adequately characterized prior to the occurrence of the damage and variables are used which provide various types of information on the states and properties resulting from various interactions with the material. The process approaches and measurement techniques developed by the Institute for Nondestructive Testing are presented and evaluated below. The characterization of the initial state of a component with respect to its homogeneity and isotropy of its properties is possible by way of ultrasonic and magnetic techniques. Both techniques are also successfully employed in characterizing surface and bulk stress states in components. Adaptation of the sensor systems and material-specific preliminary testing are possibly required prior to actual application under practical conditions in individual cases. The same applies to the analysis of the anisotropy of material properties. Detecting areas of plastic deformation is possible via ultrasonic and magnetic techniques: quantification requires calibration, e.g. via a tensile test. By contrast, positron annihilation—a technique which is still in the laboratory stage—offers the advantage of being independent of residual stress for the most part. The detection and characterization of creep damage with the required degree of detection sensitivity is not possible to date. The potential of ultrasonic and magnetic techniques for the early detection of damage, i.e. for reliably detecting advanced porosity, is limited. It seems necessary not only to improve methods for detecting pore formation, but also to utilize other creep-induced structural changes via instrumentation and monitoring technology. Experimental studies for detecting hydrogen-induced embrittlement and stress-corrosion cracking demonstrate the possibilities offered by electric and magnetic techniques. In summary, the present level of knowledge and state of the art have to be evaluated so that the possibilities and limitations of the individual methods are recognized and the detection potential improved by the combined utilization of several techniques.

Robust solutions of inverse problems in electromagnetic non-destructive evaluation

Non-destructive testing (NDT) is the application of physical measurement technology based on energy interaction with the material and its nonconformities. The materials response is sensed by transducers and sensors which?in most cases?scan the component and document the results in inspection images. However, NDT measures a physically defined quantity or even an intrinsic property. The difference between non-destructive evaluation (NDE) and NDT is in the interpretation of the inspection data. NDE has to discuss the inspection results in terms of quality elements and characteristics which are relevant to describe the fitness of the material for use. In the case of macroscopic defects these are the kind of defect (cracklike, globular) and its size and orientation to the main stress directions; in the case of material property determination the parameters are mainly mechanical properties. Therefore, in NDE one has to solve inverse problems. The solution of inverse problems based on mathematical procedures such as integral equations is a strong developing discipline and most of the articles prepared for this special issue of the journal have the objective of discussing the latest state of the art in that field. However, practical NDE needs robust and quick solutions which are to be applied mainly online. Therefore, we present here inversion procedures based on multiple linear regression algorithms applied to inspection data. We describe the calibration procedure to fit the free parameters of the model functions and give examples of practical applications in industry.

Nondestructive characterization of materials (ultrasonic and micromagnetic techniques) for strength and toughness prediction and the detection of early creep damage

Abstract In recent years, nondestructive testing techniques for materials characterization have been developed in Germany under the sponsorship of the Ministry of Research and Development, as part of the Reactor Safety Research Programme, in order to provide techniques for PSI and ISI that are sensitive and reliable, in particular with respect to the prediction of strength and toughness. As ferritic steels (pressure vessels and pipelines in the primary circuit) are of special interest, R&D was concentrated on micromagnetic techniques which are sensitive to the microstructure and its changes under service and/or repair conditions. In order to characterize microstructural states superimposed by residual stresses in an unambiguous way, numerical modelling was applied using advanded tools of mathematical approximation theory, i.e. multiregression algorithms and neural networks. For the detection of early creep damage in fossil power plant applications, i.e. micropores and their subsequent development to linked pores and microcracks, besides the micromagnetic techniques an ultrasonic techniques was also applied and optimized for in situ applications on components such as pipe bends. Whereas the ultrasonic technique is sensitive to pore concentrations as small as about 0.2%, the parameters of the micromagnetic techniques are mainly influenced by temperature- and load-induced microstructural changes occurring in service, dependent on the steel quality. The techniques are applied at two pipe bends (steel grades 14 MoV 6 3 and X 20 CrMoV 12 1) loaded under near practical conditions during seven inspection intervals between 2048 h and 21 000 h to evaluate the progress of damage.

Aging material evaluation and studies by non-destructive techniques (AMES-NDT) — a European network project

This paper presents results obtained in a round-robin action organized in a concerted action of ten partners in the EURATOM program of the European Community. The objective of the research was to document the state of the art of available non-destructive testing (NDT) techniques in order to characterize material aging phenomena based on a reduction of Charpy-V energy and a shift in the fracture appearance transition temperature. Therefore, samples from the Japanese nuclear reactor pressure vessel JRQ-steel (ASMT Standard A533-B Class 1) have been thermally treated at 700°C for 18 h with a subsequent water quenching. Besides micromagnetic and electromagnetic NDT, the positron annihilation technique, ultrasonic reverberation by using Laser ultrasonics and the thermo-electrical power have been applied to characterize the aged material states.

Emat Pipe Inspection with Guided Waves

Electro Magnetic Acoustic Transducers (EMATs) are a well-known type of ultrasonic probes used for nondestructive evaluation (NDE) of electrically conductive materials. Conventional Ultrasonic Testing (UT) is performed by longitudinal waves and mode converted shear waves generated by piezoelectric probes. EMATs are broadening the range of usable wave modes by the direct conversion of polarized shear waves with normal and angle beams and the selective generation and detection of nearly all types of guided waves. Despite their limitations (low efficiency, lift-off sensitivity, limited frequency range etc.) they have the big advantage to perform UT without couplants. The dry coupling allows UT at elevated temperatures, in media which do not tolerate liquids (e.g. natural-gas pipelines) or on sensitive and coated surfaces of blanks used for car bodies in the automotive industry etc. Ultrasonic Testing (UT) is still concentrated on the ‘classical’ wave modes generated by piezoelectric probes. Other wave modes as the Shear Horizontal (SH) waves and the wide class of guided waves offer new solutions for UT not yet widely used due to the lack of availability of appropriate probes and equipment. EMATs are the most far developed ultrasonic probes for UT using SH waves and guided waves. This contribution will present new solutions for UT of pipes using guided waves such as Shear Horizontal waves: for weld inspection, for long range inspection of pipes, for screening UT for hidden corrosion, for crack inspection in gas pipelines. The latest developments of the probe design are briefly reported. The equipment and their integration in production lines for in-service application are shown together with examples of inspection results during their practical applications.

Cyclic deformation behaviour of AISI 321 austenitic steel and its characterization by means of HTC-SQUID ☆

Abstract AISI 321 austenitic steel forms martensite due to quasi-static and cyclic loading. This presupposes the exceeding of the threshold value of cumulated plastic strain. The main aim is to determine the fatigue damage of austenitic steel by characterizing the martensitic structure with the help of the SQUID measuring technique. Several specimen batches were evaluated and thereby the load amplitudes and the test temperatures were varied (room temperature and 300°C). The experiments result in characteristic curves of the SQUID signals according to the fatigue damage which could be confirmed with comparative measurements with different methods, such as, e.g. ultrasonic absorption measurements. The extremely sensitive SQUID measuring technique allows also detection of information in specimens fatigued at a temperature of 300°C in which the phase fractions of strain-induced martensite are extraordinarily low.

Monitoring of the Friction Stir Welding Process to Describe Parameter Effects on Joint Quality

With the industrial use of FSW growing steadily, non-destructive testing methods that can detect the impending formation of flaws during welding must be developed. The present work accomplished two goals. First, the distribution of oxide fragments within aluminium welds could be correlated with certain welding process variables. Consequently, an approach was suggested to prevent the conglomeration of oxide fragments in the weld by reducing the surface roughness of the abutted edges. Second, welding forces can be used to predict the formation of elongated cavities inside the weld. This project showed that by monitoring the welding forces, the ability to change the welding variables in real time could prevent the formation of flaws in friction stir welds.

Micro-Magnetic Evaluation of Micro-Residual Stresses of the IInd and IIIrd Orders

Micro-residual stresses (MRS) of the IInd and IIIrd orders play an important role in the fracture mechanical analysis of thermally-cycled materials and thus in lifetime analysis of such affected components. In multi-phase materials there can exist two kinds of MRS: thermally-induced MRS of the IInd order and coherent MRS of the IIIrd order. The first appear when individual material phases exhibit different thermal expansion coefficients and the second occur when the lattice parameter of the second phase particles which are embedded coherently in the matrix and the lattice parameter of the matrix are different. The main emphasis of the presented research work is the development of a micro-magnetic non-destructive technique for quantitative characterization of MRS of the IInd and IIIrd orders in iron-based materials. Forthat goal Fe-Cu-(Ni-Mn) samples were manufactured and characterized by means of a non-destructive procedure based on the tensile load dependence of the maximum Barkhausen noise amplitude.

THE ROLE OF PROBABILISTIC APPROACHES IN NDT DEFECT-DETECTION, -CLASSIFICATION, AND -SIZING

Starting with the damage tolerance design of structures in aerospace applications it became obvious: NDT procedures, utilized according to standards in the regulated area and according to technical reports, guidelines and handbooks in non-regulated applications have to take into account the uncertainties of the techniques which are system features influenced by the type of product to be inspected, as well as its individual design and manufacturing techniques, the type of non-conformity or irregularity to be detected, the type of ND technique selected, the hardware (transducer and system electronics), the software reliability and — the human factor of the inspector or user. All of the influences are summarized in a feature which is called POD — probability of detection. If there is a certain probability for detection, then there is also a PND, a probability for non-detection. As each technical system is not complete and not ideal the indication of a “finding” or an “alarm” by the system can be a real hit but also a false indication, or also called, false alarm. In the same way, in the case of a “calm” system, i.e. no alarm is observed and no finding is indicated, the system answer can be true or false. For each of the four cases the knowledge of the probability has to be known. POD distributions as function of the size of an individual material irregularity are relevant influencing parameters concerning the probability of the failure of a component evaluated according to a failure assessment diagram so far as a probabilistic approach is applied. The contribution introduces into the state of the art of a software development which especially in the future will be applied to evaluate pipeline integrity after performing in-line non-destructive inspections.

Early detection of creep damage by ultrasonic and electromagnetic techniques

Abstract Residual lifetime analysis of components of power plants requires information on the degree of damage in the material. In the case of creep damage in components such as pipe bends, it is necessary to detect damage at the stage of micropore formation in order to ensure safe operation. Based on the influence of porosity on physical material properties (density, elastic moduli, electrical resistivity, coercivity), the potential of several NDT techniques for the detection of creep cavities is discussed. Changes in density and elastic moduli can be traced by ultrasonic velocity measurements. Experimental results obtained so far under laboratory conditions show rather good agreement with theoretical estimations. The practical applicability of the techniques used has still to be demonstrated, which is the objective of further work.