The fabrication, evaluation and several applications of metallic buffer rods consisting of a core and a cladding are presented. The clad rods can have a taper shape and the cladding is fabricated by thermal spray techniques. Clad rods as long as 1 m have been fabricated. Experimental results show that the ultrasonic signals detected at 5 MHz with these buffer rods have very high signal to noise ratio (>35 dB) for both longitudinal and shear waves in a reflection mode. Applications to the measurement of the thickness of hot plates and molten metals up to 960°C and to the on-line monitoring of polymer extrusion are reported.

Experimental evaluation of clad metallic buffer rods for high temperature ultrasonic measurements

Bulk thickness measurements were performed at elevated temperatures on magnetite coated low carbon steel pipe and aluminium samples, using a permanent magnet electromagnetic acoustic transducer (EMAT). The design presented here exploits the non-contact nature of EMATs to allow continuous operation at elevated temperatures without physical coupling, sample preparation (in the form of oxide scale removal), or active cooling of the EMAT. A non-linear change in signal amplitude was recorded as the magnetite coated sample was heated in a furnace, whereas a steady decrease in amplitude was observed in aluminium. For a magnetite coated pipe sample, after a dwell time of 3 h, a SNR of 33.4 dB was measured at 450 °C, whilst a SNR of 33.0 dB was found at 25 °C. No significant EMAT performance loss was observed after one month of continuous exposure to 450 °C. EMAT-sample lift-off performance was investigated at elevated temperature on magnetite coated steel; a single-shot SNR of 31 dB for 3.0 mm lift-off was recorded at 450 °C, highlighting the suitability of this design for scanning or continuous fixed point inspection at high temperature.

High temperature EMAT design for scanning or fixed point operation on magnetite coated steel

In-line ultrasonic monitoring of polymer co-extrusion and twin-screw extrusion are presented. Co-extrusion of high density polyethylene (HDPE) and a thermoplastic elastomer based on polypropylene-EPDM (ethylene-propylene-diene monomer) has been investigated by ultrasonic sensors consisting of piezoelectric transducers and clad buffer rods. One extremity of the rod (probing end) was installed flush with the die surface so as not to disturb the material flow. The other end was air cooled in order to protect the transducer from excessive heating. This approach has been demonstrated to be quite convenient for monitoring and controlling industrial material processes: first, it can work at temperatures up to 1000°C; second, the clad buffer rod probing end can be machined to the same shape as those of commercial temperature and pressure sensors commonly used in the extrusion process. Therefore, no modifications are required for the installation in the original equipment. The information obtained includes the position of the interface between polymers and the stability of the process. The same ultrasonic probe has also been installed on a barrel of a twin-screw extruder. This study was performed using polyethylene and polystyrene. It has been verified that the ultrasonic sensor can be successfully operated along the extruder screw and that the ultrasound can give access to the material properties while the polymer is being processed. This means that the technique can be exploited to monitor and control in situ the characteristics of the polymer being transformed in operations typically performed on twin-screw extruders, such as compounding, visbreaking or reactive extrusion.

Ultrasonic in-line monitoring of polymer extrusion

Acoustic emission (AE) testing is a widely used nondestructive testing (NDT) method to investigate material failure. When environmental conditions are harmful for the operation of the sensors, waveguides are typically mounted in between the inspected structure and the sensor. Such waveguides can be built from different materials or have different designs in accordance with the experimental needs. All these variations can cause changes in the acoustic emission signals in terms of modal conversion, additional attenuation or shift in frequency content. A finite element method (FEM) was used to model acoustic emission signal propagation in an aluminum plate with an attached waveguide and was validated against experimental data. The geometry of the waveguide is systematically changed by varying the radius and height to investigate the influence on the detected signals. Different waveguide materials were implemented and change of material properties as function of temperature were taken into account. Development of the option of modeling different waveguide options replaces the time consuming and expensive trial and error alternative of experiments. Thus, the aim of this research has important implications for those who use waveguides for AE testing.

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C. A. Loong

Papers

On-line ultrasonic monitoring of a die-casting process using buffer rods

In-line ultrasonic monitoring of semi-solid magnesium die casting process

Production and Die Casting of Semi-Solid Magnesium Alloy AZ91D