Giuseppe Pandarese

Experimental investigation by laser ultrasonics for high speed train axle diagnostics

The present paper demonstrates the applicability of a laser-ultrasonic procedure to improve the performances of train axle ultrasonic inspection. The method exploits an air-coupled ultrasonic probe that detects the ultrasonic waves generated by a high-power pulsed laser. As a result, the measurement chain is completely non-contact, from generation to detection, this making it possible to considerably speed up inspection time and make the set-up more flexible. The main advantage of the technique developed is that it works in thermo-elastic regime and it therefore can be considered as a non-destructive method. The laser-ultrasonic procedure investigated has been applied for the inspection of a real high speed train axle provided by the Italian railway company (Trenitalia), on which typical fatigue defects have been expressly created according to standard specifications. A dedicated test bench has been developed so as to rotate the axle with the angle control and to speed up the inspection of the axle surface. The laser-ultrasonic procedure proposed can be automated and is potentially suitable for regular inspection of train axles. The main achievements of the activity described in this paper are: – the study of the effective applicability of laser-ultrasonics for the diagnostic of train hollow axles with variable sections by means of a numerical FE model, – the carrying out of an automated experiment on a real train axle, – the analysis of the sensitivity to experimental parameters, like laser source – receiving probe distance and receiving probe angular position, – the demonstration that the technique is suitable for the detection of surface defects purposely created on the train axle.

Advanced ultrasonic non-destructive testing for damage detection on thick and curved composite elements for constructions

In this work, the problem of non-destructive testing on composite components with complex shapes for civil constructions and transport infrastructures is analyzed. In such applications at the state of the art main challenges are related with the inspection of thick sandwiches with low density cores (below 80 kg/m3) and curved panels. After a review of suitable non-destructive testing techniques, an original set-up for low frequency (100 kHz) ultrasonic inspection is proposed, which combines different solutions in through-transmission mode. The set-up is based on a hybrid configuration coupling a contact emitting probe with a non-contact air-coupled receiver. The use of a contact probe in emission is necessary to have enough energy to analyze thick components with low density core. The contact between probe and surface is made small (spot of 1 mm) and smooth using a spherical cap to increase lateral resolution at low frequency and to allow scan on irregular surfaces sometimes present in curved parts. To improve understanding this cap has been tested here also with a single probe in pulse echo mode. The non-contact probe in reception allows a better inspection flexibility on curved and thick components, where pulse echo is not feasible at all. The system is mainly developed for inspection after production in an industrialized production process, where through-transmission testing is possible. The analysis of results on two different samples (one thick sandwich with low density 40 kg/m3, 50 mm thick PUR core and one curved laminate panel) shows that the proposed methods can efficiently inspect construction composites of complex shape with satisfactory signal-to-noise ratio (usually SNR > 15 dB) and lateral resolution (2–3 mm).

Similar and Dissimilar FSWed Joints in Lightweight Alloys: Heating Distribution Assessment and IR Thermography Monitoring for On-Line Quality Control

The heating distribution assessment on similar and dissimilar friction stir welded joints in AA6082 and AA5754 aluminium alloy sheets was investigated. The FSW experiments were carried out using constant rotational and welding speeds of 1500 rpm and 60 mm/min, respectively. Temperature was locally measured by means of K-type thermocouples inserted into thin grooves located on the bottom side of the sheets, in fixed positions, very close to the welding line. It was observed that the mechanical properties of joints are related to the heat distribution. In order to obtain a completely non intrusive temperature monitoring, that was able to follow the process dynamic, a non-contact measurement system based on infrared thermography was also developed. Such system, used for the experimental evaluation of temperature on the upper surface of the joints, is also able to detect the presence of flow defects with a non-destructive method, demonstrating its effectiveness as a diagnostic instrument for the on-line quality control of welded joints.

Optimization of the excitation and measurement procedures in nondestructive testing using shearography

This paper deals with the development of optimal procedures for nondestructive testing (NDT) inspections using shearography. In the new proposed method a parameter is adopted, the contrast-to-noise ratio (CNR), which allows the quantification of the contrast of the defect to the background in the image. During the calibration of the technique, on samples with known defects, the CNR also takes into account the size and location of the identified defects, compared to those expected. The optimal measurement and loading conditions (e.g., excitation temperature level, time between image acquisitions) are determined by experimental parametric analyses aimed at maximizing the CNR on specimens with known defects. In the present work the developed methodology is described and applied to the definition of best practices for the NDT analysis of aeronautical sandwich composites structures (used in the production of helicopters) by shearography inspection with thermal excitation. In this case the attention is focused on optimizing the thermal loading procedures, but it can be clearly extended to other types of excitation methods.

The development of a shock-tube based characterization technique for air-coupled ultrasonic probes.

The present paper proposes a new characterization technique for air-coupled ultrasound probes. The technique is based on a shock tube to generate a controlled pressure wave to calibrate transducers within their operating frequency range. The aim is to generate a high frequency pressure wave (at least up to 200 kHz) with the low energy levels typical of commonly used air-coupled ultrasound probes. A dedicated shock-tube has been designed and tested to assess calibration performances. The sensor transfer function has been measured by using a pressure transducer as reference.

Laser ultrasonics for bulk-density distribution measurement on green ceramic tiles

In this paper a Laser Ultrasonics (LUT) system is developed and applied to measure bulk density distribution of green ceramic tiles, which are porous materials with low heat conductivity. Bulk density of green ceramic bodies is a fundamental parameter to be kept under control in the industrial production of ceramic tiles. The LUT system proposed is based on a Nd:YAG pulsed laser for excitation and an air-coupled electro-capacitive transducer for detection. The paper reports experimental apparent bulk-density measurements on white ceramic bodies after a calibration procedures. The performances observed are better than those previously achieved by authors using air-coupled ultrasonic probes for both emission and detection, allowing to reduce average uncertainty down to about ±6 kg/m3 (±0.3%), thanks to the increase in excitation efficiency and lateral resolution, while maintaining potential flexibility for on-line application. The laser ultrasonic procedure proposed is available for both on-line and off-line application. In this last case it is possible to obtain bulk density maps with high spatial resolution by a 2D scan without interrupting the production process.

Dynamic measurement of speed of sound in n-Heptane by ultrasonics during fuel injections

&NA; The paper presents a technique to measure the speed of sound in fuels based on pulse‐echo ultrasound. The method is applied inside the test chamber of a Zeuch‐type instrument used for indirect measurement of the injection rate (Mexus). The paper outlines the pulse‐echo method, considering probe installation, ultrasound beam propagation inside the test chamber, typical signals obtained, as well as different processing algorithms. The method is validated in static conditions by comparing the experimental results to the NIST database both for water and n‐Heptane. The ultrasonic system is synchronized to the injector so that time resolved samples of speed of sound can be successfully acquired during a series of injections. Results at different operating conditions in n‐Heptane are shown. An uncertainty analysis supports the analysis of results and allows to validate the method. Experimental results show that the speed of sound variation during an injection event is less than 1%, so the Mexus model assumption to consider it constant during the injection is valid. HighlightsMethod for measurement of speed of sound in n‐Heptane has been developed.Time resolved estimate of speed of sound during injection has been provided.Ultrasound measurement system provides reliable signals in dynamic conditions.Linear dependence of speed of sound vs pressure during injection is observed.Characterization of speed of sound with temperature has been accomplished.