Magnetic NDT Technology for Characterizing Materials – A State of the Art Survey

Per definition, magnetic NDT can be applied at ferromagnetic material only. However, most of the materials we use today still are iron-based steels with bcc lattice and therefore magnetic. The magnetic properties of these materials can be utilized in NDT for defect detection and sizing as well as for materials characterization in terms of mechanical properties determination, also in on-line process-controlled systems. MT is old and one of the most applied NDT techniques in the world for detecting surface-breaking cracks by using magnetic particles. Nowadays the technique can be mechanized and the interpretation of powder indications as findings is performed by intelligent pattern recognition software, i.e. the drawback to be working with a high human factor influence can be eliminated. However, in complex shaped geometries, for instance pusher beams of steering gears in car industry, the existence of pseudoindications prevent the application of MPI. Based on new magnetic sensors, i.e. GMR, an automatic detection with high sensitivity became possible

Early Fatigue Damage from the NDE Point of View

The motivation for an early fatigue damage characterization by using NDE is easy to understand. Technical components are designed observing in most of the cases conservative design rules for a certain lifetime under fatigue loads. However, during service a mechanical and/or thermal overloading can occur, unexpected at the design phase, and the component locally sometimes is damaged. During further operation the component does not follow the predicted fatigue behaviour, lifetime is shortened and risk for early failure has to be taken into account. Therefore, as a part of the inservice inspection, it is relevant to detect locations and regions of early damage and to quantify the state of damage is the NDE challenge, i.e. to predict the actual lifetime. Repairing the component or by adjustment of the loads the component is made to follow a safe behaviour again up to the designed end of life

Recent Developments in Eddy Current Testing

From the point of material fatigue and corrosion the objective and quantitative inspection of surfaces and near-surface zones in metallic components by NDT methods are of interest. Eddy-current methods in particular are suitable to solve this inspection problem. In the last years the electric current perturbation was investigated with the special case of yoke magnetization and signal pick-up by magnetic tapes. Finite element codes were applied to bring better interpretation in eddy-current testing, i.e. for the modeling of special interpretation problems in steam generator tube inspection

Modeling and Measurement of Field and Flux of MPI

Magnetic particle inspection (MPI) is one of the oldest NDT techniques and it is the most common method used to detect surface defects in ferromagnetic materials. But since the beginning MPI has been based on experiments, experience and on subjective judgements. This contribution shows for the first time how MPI of complex shaped parts can be designed objectively by applying Maxwell equations and finite element calculations. The comparison between modelling and measurement shows substantial agreement. Results are given and discussed especially for automotive components like crankshaft and hub under DC- as well as AC-magnetization using coils and yokes

Early Fatigue Damage from the NDE Point of View

The motivation for an early fatigue damage characterization by using NDE is easy to understand. Technical components are designed observing in most of the cases conservative design rules for a certain lifetime under fatigue loads. However, during service a mechanical and/or thermal overloading can occur, unexpected at the design phase, and the component locally sometimes is damaged. During further operation the component does not follow the predicted fatigue behaviour, lifetime is shortened and risk for early failure has to be taken into account. Therefore, as a part of the inservice inspection, it is relevant to detect locations and regions of early damage and to quantify the state of damage is the NDE challenge, i.e. to predict the actual lifetime. Repairing the component or by adjustment of the loads the component is made to follow a safe behaviour again up to the designed end of life.</p

Prawdopodobieństwo detekcji - podejście do łączenia probabilistycznej mechaniki pękania z NDT - gdzie jesteśmy?

The contribution presents the detail discussion on various problematic aspects of utilization of both fracture mechanics based methodology and NDT technology to conduct the assessment of failure of components. Consequently, it shows a software tool concerning Probability of Detection – Probability of Sizing Concept, which allow to bring both methodologies (Fracture Mechanics and NDT ) together and to meet in a joint approach. The contribution introduces in the application of one software approach, which allows modeling and simulation of real scenarios in detail, based on a variety of properties of different relevant materials from practice.Praca ta przedstawia szczegółową dyskusję na temat różnych aspektów problematyki jednoczesnego wykorzystania metodyki opartej na mechanice pękania i technologii badań nieniszczących (NDT ) do przeprowadzania oceny i predykcji uszkodzenia komponentów. W konsekwencji, przedstawiono oprogramowanie narzędziowe odnoszące się do koncepcji prawdopodobieństwa wykrycia (Probability of Detection– Probability of Sizing), które pozwalają na połączenie obu metodologii (mechaniki pękania i NDT ) i działanie w ramach wspólnego podejścia. Praca przedstawia implementację w ramach oprogramowania podejścia, które umożliwia modelowanie i szczegółową symulację rzeczywistych sytuacji, przy uwzględnianiu wielu właściwości różnych materiałów używanych w praktyce

Dynamic magnetostriction for material characterisation of micro structure states of degraded structural steel

The magnetostriction effect has been known for over 100 years. The first observation was made by Joule in 1842; while applying a magnetic field to a nickel rod specimen the length changed. Magnetostriction is the change of the dimensions and the Young modulus of a magnetic material caused by a change in its magnetic state. Magnetostriction is classified as linear or longitudinal; transverse and volume magnetostriction. Linear magnetostriction is also referred to as Joule magnetostriction, where deformation occurs in the direction of the applied field while the volume of the distorted magnetic domains remain constant. During transverse magnetostriction the dimension change is perpendicular to the applied field. Volume magnetostriction is mainly caused by the increase of distance between the atoms by the applied magnetic field. If the volume of an iron-based material expands with an increase in magnetisation the volume magnetostriction is defined as positive. The magnetisation state of a material can also be changed by temperature variations or by applied mechanical stress. Spins in excited state can transfer their excess energy to the lattice so a disturbance of the coupled magnetic and crystal lattice affects the magnetostriction. Moreover, in Joule magnetostriction the dimensional change is associated with the distribution of distorted magnetic domains so it could be expected that domain movement hindrance would affect magnetostriction. Magnetisation state changes can cause a strain just as strain can cause a change in the magnetisation state. Thus magnetostriction effects are reciprocal causing a magnetostrictive material to behave as piezoelectric materials due to magneto elastic interaction. A sound wave is generated by the change in the magnetic state while the sound wave strains change the magnetisation state of the material. This principle of the reciprocity of magnetic state - strain was used as the theoretical background of the dynamic magnetostriction test. By applying the dynamic magnetostriction test the effect of the precipitates on the crystalline of a ferromagnetic polycrystalline can be recognised by its interaction on the magnetisation state. Dynamic magnetostriction tests have been further developed at the Fraunhofer Institute for NDT, Saarbrücken, in order to define material properties. A 15NiCuMoNb5 copper content ferritic steel material was submitted to isothermal thermal ageing and the material properties changing because of copper precipitation were studied. Mechanical and physical properties, e.g. hardness, electrical conductivity were tested. The objective was to provide a basis for developing a non-destructive testing technique for this type of steel, which is widely used in the energy industry as pipeline and vessel material. This contribution discusses the physical background, the NDE-approach and the results