Non-destructive inspection (NDI) of additive manufactured (AM) metal alloys

Additive manufactured (AM) metal alloys has enabled the fabrication of lightweight and high complexity components for the aerospace and automotive industry. Similar to traditional processing methods (e.g. casting, welding), porosity is a common concern in AM metal alloys.

X-Ray Computed Tomography (CT) scanning is the “ultimate” tool for porosity measurement in metal alloys. However, the equipment is costly, bulky and the data requires large storage size and long processing time. Therefore, there is a need to develop alternative non-destructive testing (NDT) techniques that are reliable yet less costly and impose lesser demand on post-measurement processing.

In this work, we reported the use of infrared thermography (IRT) and ultrasonic testing (UT) for porosity measurement of AM metal alloys. The samples-under-test are AM metal specimens with different degree of porosity. Consequently, we obtained the correlation between the IRT and UT results with respect to the degree of porosity. We believe that our method will be beneficial to researchers or engineers working on the NDI of AM metal alloys.

Modeling of 3D Electro-Mechanical Wave Propagation in 1-3 Piezocomposite based Ultrasonic Sensors: Simulation and Experimental Comparison

Quantitative modeling tools are important in design and fabrication of ultrasonic sensors with optimum characteristics and reduce the number of experimental efforts. This paper describes a computationally efficient 3D time-domain finite element modeling of electro-mechanical wave propagation in 1-3 piezocompoiste based ultrasonic sensors using the PZFlex software. First, we demonstrate the full 1-3 piezocomposite transducer design, which includes the standard dice-and-fill technique, electrical impedance matching layers, backing materials, electrical circuit of a coaxial cable and the coupling medium. The accuracy of electrical, mechanical and dielectric properties of a piezoceramic material are verified and optimized by comparing the simulated electrical impedance results with experiments, quantitatively. The acoustical properties of two-phase backing materials are analyzed to obtain the optimum volume fraction of heavy metal inclusions in the epoxy matrix. These acoustical properties are further incorporated in the transducer model to improve the accuracy of simulation results. The simulated ultrasonic pulse-echo results are compared with laboratory experiments and a good quantitative agreement is achieved. Finally, we discuss the applications of developed ultrasonic sensors in the Oil & Gas industry.

The interaction of the fundamental symmetric Lamb wave mode with material discontinuity: a finite element analysis

This study investigates the propagation behavior of the fundamental symmetric Lamb wave mode (S0 mode) upon interaction with weld lines of dissimilar materials. The effect of the angle of incidence and propagation direction on the reflection and transmission behavior of the S0 mode was explored. An intact AA6061-T6/AZ31B dissimilar joint was employed for the investigation, and the interaction of the transmitted wave with the weld interface was scrutinized using “COMSOL Multiphysics” finite element software. The study was conducted as the wave propagates from AA6061-T6 to AZ31B and vice versa, to examine the effect of different propagation directions.

Lamb waves were excited, through the welded plate, at different incident angles (0, 15, 30, and 45 degrees). The S0 mode reflection and transmission were assessed through a direct comparison of the wave amplitudes along the incident and reflection directions. The wave attenuation curves of the S0 mode were constructed in order to have a better understanding of the effect of the interface. A method for filtering the reflection caused by the material interface was proposed, based on the frequency-wavenumber filtering technique.

It was observed that the incidence angle has minimal effect on the S0 reflection from the material interface. Further, no mode conversion was detected upon crossing the material discontinuity.

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.

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.

Requirements and possibilities to guarantee the working order of automated ultrasonic imaging systems

In order to guarantee reliable results of the imaging ultrasound test, it is necessary to check the test system itself regularly, partly before and after each test. Possible errors in the test system arise, e.g. by earing the probes, wear of the mechanics, misalignment or defects of the UT system. A common method is the examination of a defined reference specimen and the comparison of the result with previous scans by the examiner. By subtracting the master C-scan and displaying the result, differences can be assessed more easily, thus saving time and improving reliability. Partially, the auditor can also be dispensed with by means of automatic analysis. Corresponding tools are available for our testing set-ups and will be presented in the paper.

It is important for the meaningfulness of this test method that the test parameters, the amplitude dynamic range and the structures of the reference measurement to be mapped correspond to those of the subsequent component test. This requirement can be easily achieved in the examination of serial parts, but not in the case of various tests, e.g. in the laboratory.

Since the imaging properties are checked during the reference test but not all possible system functions are tested, a periodic calibration of the UT system is necessary. The characteristics of the test system are systematically recorded, documented and checked for compliance with limit values. For our test systems, we also offer this as an on-site service.

Laser ultrasonics applied to surface and subsurfacic flaws detection in sphere, example of c-crack detection in nitride ceramic bearing balls

The generation and detection of ultrasound using laser is not widespread in industry as it remains a costly technology while constraining due to security aspects. However, it constitutes one of the only efficient means to generate ultrasound contactless, and is thus well suited for very specific applications such as hot piece inspection or complex geometry inspection. Contactless generation and detection is also a major benefit for surface waves applications, as the detection measurement will not alter the wave propagation phenomena.

Based on theses assets, an inspection method dedicated to surface and subsurface flaw detection in spherical elements such has bearing balls is proposed. The method uses the propagation of Rayleigh waves confined to the sphere equator and extracts flaw signature thanks to a specific signal processing algorithm. The method is a potential solution for the inspection of ceramic bearing balls, which is of crucial importance for aeronautic industry as they ensure the proper functioning of jet engine.

The developed method is evaluated for the detection of c-crack, critical bearing balls notches of a few hundred micrometers long, less than 1µm wide and forming an arc. Results on nitride ceramic bearing balls are presented.

Robotized Laser-Ultrasonic NDT system for large aircraft structures

In this presentation, we will show the actual development stage of a new technical approach for a laser based NDT system for fast and contactless testing of large carbon fiber reinforced polymer (CFRP) aircraft structures. The approach is based on a non-contact laser ultrasound technique with delivery of both the laser ultrasound excitation and detection pulses through flexible optical fibers. The backscattered light is also collected into a fiber. The measurement head, which contains the twobeam outputs and the light collection optics is scanned over the surface by a 6-axis lightweight robot arm. The excitation and detection lasers are based on diode pumped Nd:YAG lasers which enable a low profile casing with low weight and very long lifetime with little maintenance and high scanning speed of around 500Hz. Both lasers are based on commercial laser designs by Innolas and are specially redesigned for the required specifications. For the demodulation of the ultrasonic waves, both a balanced two wave mixing interferometer (B-TWM) and a dual confocal Fabry-Perot Interferometer (D-CFPI) are tested. In the presentation, the results of their suitability to different defect types and CFRP layer structures will be shown.
The overall goal of the shown developed system is to obtain the optimum technology for the non-destructive inspection of both present and future generation hybrid aircraft and thick composite structures, containing acoustic damping materials, which highly attenuate ultrasonic waves. In the follow-up of this Clean Sky 2 project “ACCURATe”, the prototype system will be validated via deployment to inspect a long barrel demonstrator, which is to be developed in the Clean Sky 2 program using hybrid materials technology.

Acknowledgment: The Project (“ACCURATe”) leading to these results has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation program under Grant Agreement n°755616.

Ultrasonic characterisation of microstructures damaged by high temperature hydrogen attack

High temperature hydrogen attack (HTHA) of carbon and low alloy steels is a critical problem in the refinery industries. A number of techniques have been used in the past to detect and characterise the existence of this in-service damage mechanism using conventional ultrasonic instrumentation. Their capability to detect the initial emergence of damage is currently under review in industry and the availability of advanced array probe imaging techniques potentially offer enhanced capabilities. This paper will consider the issues surrounding early-stage detection of HTHA in susceptible steels, assess the capabilities of established conventional techniques and explore potential advantages of using array ultrasonic imaging techniques. Evidence from theoretical simulations of the inspection scenario as well as data collected on a specimen containing HTHA damage will be presented.

Damage detection by using features of nonlinear ultrasonic modulation in vibrating structures

Due to the high need for structural health monitoring in aerospace applications, numerous, quite mature linear ultrasonic NDT techniques are available for the detection of defects. Linear ultrasonic NDT e.g. by using guided waves is based on mode conversion and reflection of probe waves by a delamination or a defect, provided the defect is open, resulting in an acoustic impedance mismatch. However, in practical applications defects are often ‘closed’ when not under substantial stress.

Moreover, in most nonlinear ultrasonic NDT techniques the lack in differentiation between sources of nonlinearity makes defects undistinguishable from e.g. nonlinearity induced by mechanical contacts.

Here, we aim to detect damage, addressing the practical difficulties of monitoring vibrating structures. Defects were created and detected in aluminum plate-like samples, using PZT transducers to generate and detect probe waves. The presented diagnostic algorithms compare features of the nonlinear relation between the amplitude of the transmission probe wave and the load on the sample with a threshold value, in order to assess the state of the sample. The applications are robust to environmental changes, are based on durable components, while being sensitive to vibrating defects.

Part of the research leading to these results has received funding from the European Community’s Seventh Framework Programme [FP7/2007-2013] under grant agreement n°212912 and the “NDTonAIR” project (Training Network in Non-Destructive Testing and Structural Health Monitoring of Aircraft structures) under the action: H2020-MSCA-ITN-2016- GRANT 722134.