Detection of local stress concentration zones in engineering components – the lacking link in the NDT system

It is known that the main sources of damage during operation of engineering components are local stress concentration zones (SCZs) that are formed under the action of workloads primarily on defects of metallurgical and process nature. Sizes of these stress concentration zones vary from several tens of microns to several millimeters. Furthermore, it is unknown where these local areas are situated and how they can be detected. At the same time the reject norms of the applied NDT methods (ultrasonic testing, X-ray, MPI and other methods) at manufacturing plants significantly exceed the sizes of metallurgical defects.
At present a fundamentally new NDT method based on the use of the magnetic memory of metal (MMM) is more and more commonly applied in practice. It uses the natural magnetization formed during the components’ fabrication. The results are based on resent experiences.
The article considers the capabilities of the MMM method during the diagnostics of metallurgical and process manufacturing defects in new components.

Magnetic Estimation of Stresses in Welded Joint Zones with Different Initial Plastic Strains

During assembly or transportation to the site of assembly, as well as in operation, structural components may suffer additional uncontrollable plastic strain. This may change the initial level of informative test parameters.
In order to determine the capabilities of magnetic methods for the estimation of the stress-strain parameters of the material of individual zones of welded metal structures, in view of their his-tory, the effect of preliminary plastic strain on the magnetic behaviour of metal in different zones of a welded pipe under subsequent elastic tension/compression is studied.
Three groups of flat tensile test specimens cut out from the base metal, the weld and the heat-affected zone (HAZ) of a pipe made of the X70 steel are studied. In the first stage, the speci-mens were subjected to uniaxial tension to different values of plastic strain. In the second stage, the specimens plastically tensioned to different values were subjected to elastic tension/compression.
A correlation has been found between the magnetic characteristics of the metal in the differ-ent zones of the welded X70 steel pipe and the amount of plastic strain in these zones.
The magnetic behaviour of the base metal, the materials of the weld and the HAZ of a pipe has been determined as dependent on applied uniaxial tensile/compressive stresses, in view of the material history in the form of plastic strain to different degrees.
The magnetic characteristics of the materials are shown to vary uniquely in the range of ap-plied elastic uniaxial stresses between ‒200 and 120 MPa, and this makes them usable for the evalu-ation of the stress-strain state of the individual zones of welded X70 steel pipes.
The obtained results testify to the necessity of taking into account the initial SSS of metal structures when developing magnetic methods for the determination of their SSS parameters.

Reference blocks generated by laser treatment for grinding burn inspection

Grinding burn is defined as a result of unintentional heat impact during processing of hardened steel surfaces such as claimed surfaces of gear parts. Depending on heat intensity local tempered zones or re-hardened zones are generated. The occurrence of grinding burn within the production process is a risk for the safety of the components. Different test methods are applied to verify grinding burn. Surface temper etching (STE) is the most commonly used method for detecting grinding burn up to now. It is the only standardized testing method (ISO 14 104:2014). However, industrial automation of this method is limited and the evaluation of etched parts is performed visually by an operator. For this reason, non-destructive methods are of high interest as they allow reproducible detection of grinding burn without influence of human factors. The 3MA-methode, the Barkhausen noise analysis and in recent times, eddy current testing are already applied.
Components with defined defects of different characteristics are required for the evaluating of non-destructive test methods as well as for STE according to ISO 14104. Generation of reproducible grinding burn on components cause problems regarding to size and depth of the influenced area. An alternative is the generation of artificial defects. Artificial defects have to be fabricated reproducibly in size, location and intensity and they have to show similar physical behavior like real defects.
Experiences and results of manufacturing and assessment of reference blocks with artificial defects generated by laser treatment for grinding burn detection are presented. The artificial defects meet the requirements mentioned above and may be used for non-destructive testing methods as well as for STE. The reference blocks are used for calibration of the test equipment, especially the required test sensitivity. In addition, reference blocks are applied in certain intervals within the process in order to guarantee testing reliability.

Analysis of the spectra of a multi-frequency electromagnetic sensor to determine the magnetic permeability of steel strip

Multi-frequency electromagnetic sensors, such as EMspec, are now being used to non-destructively inspect the properties of strip steel on-line during industrial processing. These sensors measure the relative permeability of the strip during process operations such as controlled cooling and the permeability values are analysed in real time to determine important microstructural parameters such as the transformed fraction of the required steel phases. These parameters are critical to achieving the desired mechanical properties in the strip product.
The inductance spectra produced by the sensor are not only dependent on the magnetic permeability of the strip, but is also an unwanted function of the electrical conductivity and thickness of the strip and the distance between the strip steel and the sensor (lift-off). The confounding cross-sensitivities to these other parameters must be rejected by the processing algorithms applied to inductance spectra.
The paper considers cross-sensitivity of the complex EMspec spectra to the four variables, permeability, conductivity, thickness and lift-off. The paper then goes further to consider the solution of the inverse problem of determining unique values for the four variables from the spectra. Both the finite element method and the Dodd and Deeds formulation are used to solve the forward problem during the inversion process. Results from an industrial scale EMspec sensor are presented including the effects of noise. The results are backed up with measurements of selected cases.
The results are significant because they show for the first time that the inductance spectra can be inverted in practice to determine the key values for the thermal processing of steel strip.

Connecting the mechanics of strength with ferromagnetism: theoretical basis of complex stressed state inspection

Magnetic techniques of stressed state inspection of steel pieces and elements of structures use semi-empirical dependencies of structure-sensitive magnetic parameters on mechanical strain of metal value. A challenge to obtain their symbolic description in general cases of complex stressed state of metal (two- or three-axial) still have not any strict solution.

In order to simplify structural strength calculations the complex stressed state of metal in theory of strength is usually reducing to equal one-axial strain of metal. Applying this approach to existing symbolic expressions for structure sensitive magnetic parameters of linearly strained metal, we expanded ones in common case of complex stressed state.

Proceeding from the law of energy conservation the equation between increments of magnetized metal energy after and before deformation, and specific potential energy of deformation of metal, depending on the principal stresses obtained.

Solving the composed equation in some approximation allowed within ferromagnetism the symbolic expressions obtained, concluding to dependence of residual magnetization and residual magnetic field of metal on the components of complex stressed state.

The dependence verified experimentally throughout the range of deformation from quiescent state to destruction of metal in scientific researches carried by different authors. Its behavior influenced by large number of external factors are considering in dependence obtained: magnetic properties, relation between mechanical and magnetic states of metal, rate of principal stresses interconnection, their signs and bulk symmetry.

Obtained symbolic expressions, based on common hypothesis of mechanics of strength which suggests that any complex stressed state may be represented by equal one-axial stress, expands on structure-sensitive magnetic parameters: the mechanical strain value determined on empiric dependence of residual magnetic field on one-axial strain – this is total effective mechanical strain of metal in complex stressed state.

COMPARISON OF TECHNOLOGICAL CAPABILITIES OF DIFFERENT MAGNETIZING DEVICES FOR MAGNETIC PARTICLE TESTING

The paper provides a comparative analysis of constant magnet magnetizing devices (MD), applied for magnetic particle testing (MPT) of complex geometry parts, Flawtinder and TVA-5 with similar pole sizes and equal number of magnets in each device. The difference lied only in structure of the yoke joining constant magnets. Flawtinder magnetizing device represents itself two columns with magnets joined by flexible cable. Section of ferromagnetic mass of the magnetic column and joining cable differs approximately two times. The magnetizing device with yoke in from of cable is almost broken magnetic circuit. Another magnetizing device TVA-5 is a flexible magnetic circuit on pivots, which has section similar to constant magnet section. TVA-5 devices in contrast to Flawtinder can be easily taken off, since the poles are spring-loaded, and moved over tested object surface.
The tests were carried out on standard specimen following all technological MPT operations. Thus, two types of MD, which can provide magnetic particle testing of parts of any geometry, were compared under maximum similar conditions of work. It was determined as a result of experiment that MD of Flawtinder type with joining cable has not very high indices of defect detection due to very developed normal component Hn of magnetic field. It is known that the lower relationship of normal and tangential components Ht, the higher technological properties of MD are. As a result of tests it was determined that TVA-5 with flexible yoke, section of which equals poles section, has better indices than that in the device with joining cable.
This difference in Hn/Ht value significantly depends on distance between the magnet poles.

The combined use of magnetoacoustic and magnetic parameters for evaluation of structure and strength of steels subjected to plastic deformation and heat treatment

The influence of excitation and detection conditions on the magnetoacoustic emission (MAE) parameters of ferromagnets of different chemical composition was investigated. It is established that the dependence of the MAE amplitude on the remagnetizing field frequency has nonmonotonic character and that the maximum of the MAE amplitude of all investigated ferromagnet materials corresponds to a field frequency of 3–5 Hz. The decrease of MAE amplitude with following increase of the alternating field frequency caused by the action of eddy currents. It is shown that the value of the field corresponding to the MAE maximum, which for a given time dependence of the remagnetizing field can be determined by the time shift in an oscillogram, can be a new parameter of materials characterisation of ferromagnets.

The magnetic properties of steels of different grades subjected to various thermal treatments have been investigated. It is shown that the residual magnetic induction and induction of coercive return of steels is a sufficiently universal parameters of temperature testing of tempering and annealing of steels. These testing parameters can be used both separately (single-parameter testing) and jointly (two-parameter testing). Presence of correlation between residual magnetic induction and amplitude of magnetoacoustic emission of annealed steels allows to recommend MAE amplitude as a testing parameter in scanning systems of ferromagnetic steels materials characterisation.

Eddy current array : an alternative to conventional surface NDT methods

A large number of Non-Destructive Techniques (NDT) have been developed and successfully used over the past decades for the detection of surface breaking flaws in various materials, such as: cracks, pitting, corrosion, etc. Those are more or less sophisticated, but all provide valuable information on the integrity of the component being inspected, and come with specific advantages and limitations.
Established methods such as penetrant testing (PT) and magnetic particle inspection (MPI) are effective but can lack practicality in some applications. Others, such as conventional eddy current inspection technique (ECT) mainly deploy single element probes resulting in protracted inspection times. Results are also typically greatly affected by operator’s skills, material properties and geometry.
Advances in electronics enabled the development of more modern inspection techniques like Eddy Current Array (ECA), increasing the reliability of surface inspection over traditional methods. Indeed, being able to tailor coil designs and multiplexing patterns allows users to optimize the acquisition chain to their specific application. Moreover, by multiplexing and leveraging advanced data processing capabilities, ECA solutions allow inspections to be carried out quickly, often with less surface preparation. They also provide additional benefits such as state-of-the-art imaging (e.g. 2D and 3D C-Scan displays), improved surface coverage, ease of deployment and data archiving. Finally, on top of defect detection, ECA technology can also provide quantitative sizing.
This paper describes the eddy current array method along with variations on the theme, inclusive of their benefits and limitations. The deployment of ECA on real components, subject to representative field conditions is also discussed. Typical applications are presented, providing valuable insights on the use of ECA in lieu of more conventional techniques.

NDT of austenitic steels – Evaluation of spot weld nugget diameters by imaging analyses of the residual magnetic flux density

Non-destructive testing (NDT) of resistance spot welding compounds continues to pose a great challenge. The nugget diameter is the most important quality criterion. Established ultrasonic-based methods reach their limits when evaluating the nugget diameter of combinations with significantly different sheet thicknesses and with deep electrode indentations. In these cases, the imaging analysis of residual flux density shows high potential. Previous results of this NDT method concentrate on spot weld combinations of ferromagnetic steels. Hence, latest research at Technische Universität Dresden focuses on the application of the imaging analysis of residual magnetic flux density on spot welds using austenitic steels. Due to the high cooling rates after welding delta ferrite with ferromagnetic properties is generated inside the nugget, and therefore the residual magnetic flux density can be measured.

The talk will show the measuring concept of the NDT method for spot welds and presents the results of its application on two and three sheet metal combinations of austenitic steels.