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.

Hit/Miss POD With Model Assisted and Emulated Flaws

The determination of the reliability of an inspection is of high significance. In particular, it is important to determine what is the largest crack, which could conceivably be missed during the in-service inspection. This information is utilized in order to choose
the most effective method for different situations. Probability of detection (POD) curves are used to quantify the inspection effectiveness. However, major obstruction for POD curves is the requirement for a lot of data points in order to give reliable estimates of the lower limit performance, rendering reliable POD curves highly expensive to produce.
In this study, POD curve is estimated using only few thermal fatigue cracks, which is insufficient to produce a POD curve. In the present study the idea is to emulate the amplitude response from the measured crack in a way that represents an amplitude response from a certain crack size and in addition CIVA simulation is used to produce amplitude response data from similar simulated cracks. Both amplitude responses are then in turn converted to a B-scan image for inspectors to evaluate whether there is a crack or not. Then a POD curve is generated from the achieved hit/miss data. The idea is to decrease the amount of needed real flaws and also to determine if an inspector can tell the difference between real, emulated and a simulated flaw.

Electromagnetic metamaterials sensor for structural integrity of zirconia doped on rare earth coating on stainless steels

Thermal barrier coating (TBC) is used as thermal protective layer dedicated to metallic components in high temperature region, on engines and gas turbines, leading to high component reliability and operating temperature, resulting in higher efficiency and better environmental benefits. Zirconia doped with rare earths oxides is considered a good TBC material due of its low thermal conductivity, refractory, chemical inertness, and compatible thermal expansion coefficient with metallic support. The base material (substrate) is an AISI 316L stainless steel. The ceramic zirconia top-coating is nonconductive and nonmagnetic and therefore behaves like an air gap between the electromagnetic (EM) sensor and the conductive substrate and only creates a probe lift off effect for this method. The substrate AISI 316L stainless steel is nonmagnetic, when is formed only by an austenite type phase, with magnetic permeability equal to 1, allowing electromagnetic test.
The paper purpose is to emphasize the improvement of Zr coating-based ceramics properties as a function of addition of Y2O3 in the structure of the original ceramics. The structural investigations proposed in this paper cover destructive and nondestructive methods (EM, EA, SEM, X-ray, neutron diffraction ) offering the first indication of the variation of the phase composition and the structural parameters, micro-hardness measurements as well in order to analyze the structural properties of these materials with upmost importance in fields such as thermal protective layer dedicated to metallic components in high temperature region, on engines and gas turbines.

Measurement of Stress in Mechanical Components with a Novel Laser-Ultrasonic Measurement System

In this paper, the propagation of Rayleigh waves on substrates with applied or residual stress is investigated. For this purpose the propagation of Rayleigh waves was measured with a Laser-Ultrasonic measurement system. It consists of a pulsed laser which excites broadband Rayleigh waves in a thermoelastic regime and a two-wave-mixing interferometer with a photorefractive GaAs crystal for robust detection of the surface waves. Both angular and frequency dispersion are considered theoretically and experimentally. It was found that there are clear indications of non-homogeneous stress states in the measurement results which qualitatively correspond with the theoretical models considered. However, some deviations between measurements and simulations still prevail, which may be attributed to effects not considered in the models. For grinded specimens, a clear dependence of a stress parameter on the grinding level was found which is also sensitive to annealing. This study is a work towards broader industrial application of Laser-Ultrasonic measurement systems.

Micro and macro microstructure modelling for the influence of grain size on magnetic properties in structural steel and modelled commercial sensor outputs

The mechanical properties of steels are controlled by their microstructural parameters, such as grain size, phase balance and precipitates, which are developed during thermal mechanical processing. It is desirable to be able to monitor microstructural changes during processing, allowing in-situ feedback control, or to characterize microstructure in steel products in a non-contact and non-destructive manner. Electro-magnetic (EM) sensors are now widely employed in the steel industry, for example to monitor phase transformation during strip cooling (EMspec) and strength in cold rolled strip (IMPOC, HACOM, 3MA), where a combination of empirical correlations and fitted models are used. Models have recently been developed to relate the sensor signals to magnetic properties and, separately, to relate magnetic properties to microstructure.
In this work an ultra-low carbon steel sample was used to generate ferrite grain sizes between 2.5 – 80 µm. EBSD data was used to generate 3D voronoi based microstructure models representing the grain structures, which have been used in multi scale magnetic modelling approaches to predict the magnetic properties. Both micro (EMicroM) and macro (COMSOL Multiphysics based) scale models have been developed and applied to determine magnetic hysteresis curves and low field permeability values. These have been compared to experimentally determined hysteresis curves and magnetic parameters. The modelled and, where available, measured magnetic data have then been used in EM sensor (IMPOC, 3MA and EMspec) models to determine sensor sensitivity to grain size measurements.

CHARACTERIZATION OF COMPOSITE MATERIALS USING ACOUSTIC EMISSION TEST

The paper is the result of a 2 years research made in collaboration with Aerospace company Leonardo.
The object was to evaluate the behavior of different type of composite with carbon fiber and glass fiber.
The samples were tested on a three point bending and monitoring continuously during the application of load and full test up to the failure.
Characteristic of the test was to maintain constant the load and verify after two and three days if really no fractures happen.
Only in these conditions the load will slowly increased and again time over.
The research has taken to the following conclusions: there is a load (Delay Cracking Load) at which a slowly cracking phenomena start also maintaining constant the load.
The importance of the research was that only through acoustic emission is possible to determine for each type of composite the DCL (Delay Cracking Load).

Safety Performance of Se-75 Radiography Sources

Se-75 is a versatile gamma radiography nuclide. Its energy is optimum for the range of materials and thicknesses of common interest to gamma radiographers and it is an excellent choice for Small Controlled Area Radiography (SCAR). Its use is continuing to grow with the introduction of new Se-75 exposure devices, enabling the SCAR technique to be implemented simply, safely and effectively. SENTINEL has developed an optimized, third-generation Se-75 source using metal-alloy selenium for use with both SCAR and conventional exposure devices. This source has improved near-spherical focal geometry and is available in higher activities up to 120Ci. Its focal dimension of 3.5mm is the smallest on the market at this activity level.
• Highest specific activity and smallest focal dimension: better images
• Inherent safety with source tested to 1250C – intrinsically safer
A radiological incident in Europe in 2016 resulted in the rupture and leakage of 75Se from a commercial source that had been made using elemental selenium. Due to the ensuing high financial and commercial costs associated with such an incident, it prompted SENTINEL to carry out a new series of high temperature integrity tests to verify the superior performance of SENTINEL’s proprietary metal-alloy source technology and to compare the results with sources made using elemental selenium. This paper describes the thermal test program and discusses the implications for the safety and performance of gamma radiography using Se-75.

New technique for ultrasonic inspection of multi-pass welds with EMAT guided waves

Multi-pass welding is typically employed to join thick-walled plates or pipes. This welding technique is very common in many industries such as ship building, pressure vessels, nuclear or oil & gas. Normally, these welds can only be inspected at room temperature using conventional non-destructive testing techniques, which are slow and time-consuming processes. Once a defect is located in the weld, the weld needs to be grinded down or cut out and re-worked which is not only expensive but also adds additional wastage of time. This paper presents a novel technique using Rayleigh and Shear Vertical waves generated with electromagnetic acoustic transducers (EMAT) which aims to control the quality of partially completed girth welds and may be considered in the future for the inspection of each weld pass during welding process in a relatively hot state, providing real-time inspection and quality control.

Ultrasonic array design optimisation for defect characterisation

Ultrasonic arrays are now increasingly used in many industrial inspections. Alongside this development, recent research has provided improved array image quality through Full Matrix Capture (FMC) and the use of a wide range of post-processing algorithms. Arrays have the capability to not only detect and locate defects, but also to provide information about the characteristics of the defect. For a crack, this information is size and orientation angle. But it is also important to be able to distinguish between cracks and other defects, such as volumetric voids and inclusions, which are often much less significant from a structural integrity perspective. An array insonifies a defect from a range of angles and thereby measures part of the matrix of defect scattering matrix. In this way, the scattering matrix encodes the defect characterisation information. Typically, this detail is lost when the FMC data is transformed into an image. However, here we extract this scattering information and use it to characterise small defects. Firstly, we show that under certain conditions, it is possible to determine the defect type, distinguishing between cracks and volumetric defects. Secondly, once the defect type is determined, we show that it is possible to accurately extract parameters such as size, orientation and aspect ratio. We show that this characterisation information is inherently probabilistic and introduce defect probability maps which reveal the most probable defect and the probability landscape. Finally, we show how such knowledge can lead to the design of new arrays, optimised specifically for characterisation. In this step, it is assumed that the defect has been detected and the requirement is now purely to determine its characteristics to the highest possible accuracy. We show that improved characterisation accuracy can be achieved with this optimised approach and suggest that this concept will have benefits for some safety critical applications.