Experimental Validation of a Phased Array Probe Model in Ultrasonic Inspection

New manufacturing technologies such as additive manufacturing facilitate flexible and complex designs and production of components. However, these new techniques should not compromise the safety aspect, which imposes higher demands on the integrity insurance and inspection methods. Phased array ultrasonic testing (PAUT) provides advanced inspection and evaluation processes, whereas qualification is still needed when applied together with new manufacturing techniques. Numerical modeling, as one of the potential qualification methods, has been developed for decades and should be validated before practical applications. This paper presents an experimental validation work of the phased array probe model implemented in a software, simSUNDT, by comparing the maximum echo amplitudes between the physical experiments and simulations. Two test specimens with side-drilled holes (SDHs) and different materials are considered for validation and practical purposes. An experimental platform with a mechanized gantry system, which enables stabilized inspection procedure, is built and applied during the validation work. Good correlations can be seen from the comparisons and this model is concluded as an acceptable alternative to the corresponding experimental work. The relation between depth and beam angle is also noticed and investigated, which is essential to guarantee an accurate inspection

Comparison between three mathematical models of three well defined ultrasonic NDT cases

Ultrasonic nondestructive testing (NDT) is commonly used for in-service inspection in different areas. But reliability of NDT method is highly dependent on the equipment and crack features. Although, it is possible to use thoroughly validated mathematical models to avoid complicated and costly experimental work, when it is necessary to qualify new procedures. Finite Element Model (FEM) is a powerful tool, which is commonly used for such cases. In this paper three mathematical models of three well defined cases will be compared with each other

Experimental Validation and Application of a Phased Array Ultrasonic Testing Model on Sound Field Optimization

In safety dominant industries, nondestructive evaluation (NDE) is crucial in quality assurance and assessment. Phased array ultrasonic testing (PAUT) as one of the NDE methods is more promising compared with conventional ultrasonic testing (UT) method in terms of inspection speed and flexibility. To incorporate PAUT, the techniques should be qualified, which traditionally is performed by extensive physical experiments. However, with the development of numerical models simulating UT method, it is expected to complement or partly replace the experiments with the intention to reduce costs and operational uncertainties. The models should be validated to ensure its consistency to reality. This validation work can be done by comparing the model with other validated models or corresponding experiments. The purpose of current work focuses on the experimental validation of a numerical model, simSUNDT, developed by the Chalmers University of Technology. Validation is conducted by comparing different data presentations (A-, B- and C-scan) from experimental and simulated results with some well-defined artificial defects. Satisfactory correlations can be observed from the comparisons. After the validation, sound field optimization work aiming at retrieving maximized echo amplitude on a certain defect can be started using the model. This also reveals the flexibility of parametric studies using simulation models

Simulation-Based Investigation of a Probability of Detection (POD) Model Using Phased Array Ultrasonic Testing (PAUT) Technique

Probability of detection (POD) as a metric for quantifying the capability of inspection procedures in nondestructive evaluation (NDE), has been applied and evolved in industries since 1970s. Progress had been noted when certain statistical functions were brought up to model POD behavior, including log-normal model (also referred as Probit model). This model had been concluded to be the best fit and therefore has been widely used in many studies, while the involved assumptions and conditions were not carefully addressed and explored. To make flexible POD datasets available for specific inspection procedures and reduce the number of expensive experiments needed, model-assisted POD (MAPOD) is an alternative. This paper addresses a pure simulation-based POD procedure of an inspection scenario involving phased array ultrasonic testing (PAUT) on lack-of-fusion defects in additive manufactured (AM) components. The mathematical simulations are performed by an ultrasonic testing (UT) simulation software, simSUNDT, developed at Chalmers University of Technology in Sweden. Resulted inspection datasets with the proposed data processing steps are evaluated in terms of the assumptions and conditions of log-normal POD model, with the purpose of discussing the POD model validity under different circumstances. Simulation-based POD curves are finally compared with several discrete POD values at some defect sizes, calculated through massive computations from physics-model based metamodel. Comparisons and observations confirm satisfactory application of log-normal POD model despite some violations in model hypotheses

In-situ detection of redeposited spatter and its influence on the formation of internal flaws in laser powder bed fusion

The pervasive adoption of laser powder bed fusion (LPBF) as an industrial manufacturing technique relies on the improvement of its repeatability, currently limited by the stochastic formation of flaws. Considering that large flaws can form randomly and despite the optimization of process parameters, an in-situ monitoring technique suitable for detecting deviations that originate these critical flaws is paramount. The redeposition of spatters on the build area has previously been identified as one of the factors responsible for the rise of internal flaws, but so far limited are the efforts towards their detection. This study aims to detect spatter redeposits via in-situ monitoring and to couple the detections to lack of fusion. For that, long-exposure near-infrared in-situ monitoring associated with image analysis is employed to determine the exact locations and quantify the incidence of spatter redeposits across three full builds performed at varying layer thicknesses. The existence and distribution of internal flaws is verified ex-situ by means of ultrasonic inspection and metallography. The formation of internal flaws is attributed to spatter redeposits after detailed characterization of size, particle and surface morphology of spatter and identification of particles with identical characteristics on the fracture surface in the adjacencies of lack of fusion. It is found that spatters preferentially redeposit on the adjacencies of the gas outlet, but that the affected portion of the build area and the prevalence of detections is heavily dependent on the powder layer thickness employed in the manufacturing process. The monitoring system setup preferentially acquires signal from spatters redeposited on print regions, making it particularly suitable for flaw detection

Ultrasonic NDT in Defect Detection - Mathematical modelling

This thesis is concerned with the mathematical modelling of the ultrasonic nondestructive testing situation. The first part of the thesis treats a simplified model of an ultrasonic transducer and its implementation into a T-matrix method-based solution to a crack scattering problem. The source is an acoustic piston-like model of an unangled compressional transducer acting in pulse echo situation with its behaviour as a receiver treated with reciprocity. The considered crack is a freely oriented penny-shaped crack-like flaw, partly closed due to an external background pressure. In order to evaluate and verify the numerical results, comparisons with numerical calculations, provided by a GTD method-based program package, are performed. The elastodynamic equivalence to the above is then considered. A more general probe model is proposed and modelled as boundary conditions on an elastic half-space. The action of the probe as a receiver is treated with a reciprocity argument. The electric signal is then obtained essentially as a product of the spherical expansion coefficients of the transmitting probe, the transition matrix of the defect and the spherical expansion coefficients of the receiving probe. The ultrasonic contact probe and its options as a transmitter are thoroughly investigated and several numerical examples are provided in the middle of the thesis. Numerically computed contour plots of the displacement field beneath the probe are given as snapshots, with varying contact conditions, frequency, bandwidth, angles and types. In order to enable experimental verification, a model for the ultrasonic probe evaluation situation, including the transmitting probe, an elastic plate and the receiving probe, has been developed. The reflection matrix representing reflection from the backwall is included and a model of the backwall measurement situation is deduced. Numerical examples are provided with varied couplants, shapes of effective area, traction distributions and different input signals. The final part of the thesis treats the ultrasonic inverse problem of determining the crack given the input and output electric signals. The numerical solution is achieved by applying optimisation techniques to the mathematical model of the ultrasonic NDT situation. The inverse problem is reduced to minimisation of a non-linear least squares problem and is performed with a quasi-Newton algorithm consisting of a locally convergent SVD-Newton method combined with a backtracking line search algorithm. Numerical examples for three specified realistic NDT situations are presented

The implementation and validation of a phased array probe model into the simSUNDT software

Ultrasonic phased array technique has gone from being a unique technique with few and very specific purposes into being an established tool and replacing conventional contact technique in a broad field of applications. Even though the far-field behaviour is more or less identical to single crystal techniques the knowledge in e.g. how it differentiates in interaction with defects and geometry closer to the probe is rather limited. A thorough validated mathematical model based on the physics has the ability to overcome this lack of understanding and is the only realistic alternative in the development of new procedures based on this technique. The simSUNDT software consists of a Windows\uae-based preprocessor and postprocessor together with a mathematical kernel (UTDefect), dealing with the actual mathematical modelling. The model employs various integral transforms and integral equation and enables simulations of the entire ultrasonic testing situation. In the latest released version (2.0) a model of phased array probe has been incorporated. Each of its elements is in the model represented by the boundary conditions that generate a plane wave, at a certain angle, in the far field. These boundary conditions (i.e. the pressure on the surface under the element) are then translated into the main coordinate system and after superposition they built up a phased array wave front (constructive phase interference) with prescribed nominal angle. Modelling has been identified as an effective tool and in a previous work the developed methodology based on simulations of a well defined procedure was validated. In this paper a new model of phased array technique is implemented in the simSUNDT software and thoroughly validated