Materials and methodology 124

8.2.1

The piezoelectric transducers that generate and detect the Lamb waves were made of lead zirconate titanate (PZT). The PZTs were positioned in specific areas on the sample under investigation in order to e a le opti ised p o a ilit of dete tio (POD). These surface acoustic Lamb waves that are excited on the surface of a solid material are also called Rayleigh waves named by the John William Strutt, 3rd Baron Rayleigh, who was the first who discussed about their existence [203]. The whole concept of generation and detection of ultrasonics in technical applications is mostly based on piezoelectricity. Piezoelectricity is a phenomenon where mechanical stress is transformed into an electric field and vice versa. It can be found on the surfaces of specific materials. When they are subjected to a vibration or stress, the internal structure of the molecules, that for the ate ial s crystal, is deformed. It is observed that this deformation of the molecules creates positive and negative charges and therefore an electrical polarity is generated [204]. Figure 8.5 below, shows a round PZT attached near a pseudo-defect on the aluminium plate as well as the variety of pseudo-defects that were used for the tests. The height of the round pseudo defects was 15 mm and the height of the rectangular pseudo defects was 10 mm while their length and width varied between 5 mm and 40 mm.

Figure 8.5:PZT and pseudo-defect attached on the plate (left) and different shapes and sizes of pseudo-defects (right).

PZTs are acting here as transducers that excite and receive acoustic vibrations. In order to attach them but also to remove and re-attach them for the test purposes, grease or paraffin was used as a coupling agent between the PZTs and the surface of the aluminium plate. This technique has been previously proved successful in order to achieve low noise and clear wave signals from the PZTs [205]. The PZTs were provided from Meggit PLC, Helsingør, Denmark. Meggit PLC is one of the industrial partners of the AISHA II European project.

8.2.2

In order to hold the 1x1m plate and to press the pseudo-defects on both sides a metallic frame with adjustable clamps was used. Figure 8.6 below, shows the design of the frame and the two clamps that are used to press the pseudo-defects against the plate. The plate was placed inside the frame using four non-conductive holders at each edge.

Figure 8.6:Holding equipment for 1x1m plates and pseudo-defects.

The pseudo defects were positioned between the clamps at various positions on the surface of the plate. An illustration of the set-up using a rectangular pseudo defect and 2 PZTs for the investigation of the acoustic signal is presented in the figure 8.7 below.

Figure 8.7:Layout and position of sensors and pseudo defect. The circles are the PZTs and the rectangular is the pseudo defect.

In the previous figure, when the source and receiver are located next to each other and opposite the pseudo defect, then the reflection of the Lamb wave is investigated. In contrary, when the pseudo defect is located between the source and the receiver, the transmission of the Lamb wave is investigated.

8.2.3

A point that needed to be investigated was the amount of pressure that had to be applied on the pseudo defects in order to get the best measurements possible in point of acoustic reflection or transmission. The more force was applied on the pseudo defect, the more it was pressed on the aluminium plate and therefore, it was affecting the reflection or transmission of the acoustic signals from the PZTs. A screwdriver with adjustable torque settings was used for the above reason. The investigation of this behaviour within the AISHA II research was first studied by Professor Vitalij Pavelko from the Riga Technical University (RTU), Latvia. The results in Riga but also in Leuven showed that above a specific value of torque and further, the amplitude of the acoustic signals was not changing. This value of torque was adequate to rely on and it was used to press the pseudo defects on the plate. Table 8.1 below, shows the amount of force applied on the pseudo defects and figure 8.8 shows how the acoustic signal stays stable above 400 N.

Torque Force Peak-to-Peak RMS

Nm N mV mV 0 0 3.46 1.16 2 235.3 2.66 1.15 2.5 294.2 2.64 1.12 3 352.9 2.58 1.07 3.5 411.8 2.54 0.864 4 470.6 2.54 0.86 4.5 529.4 2.54 0.861

Figure 8.8:Acoustic signal remains relatively stable after an amount of force applied on the pseudo defect. The arriving acoustic signal is transmitted through the pseudo defect from the actuator to the receiver

The investigation on the reflection and transmission of the Lamb waves by the pseudo defects was performed through a series of tests. Different shapes of pseudo defects were used and they were positioned in different angles in order to examine their influence on the acoustic response. Usually, the pseudo defect was positioned in the centre of the plate and the PZTs were allocated around it. One PZT was the source exciting guided waves and another PZT was the receiver, receiving the acoustic signal. The acoustic signal from the wave generator was at a frequency of 350 kHz with 5 counts of burst at an output voltage of 10 Vpp. It was

initiated from an Iwatsu Pulse/Function Generator and it was finally analyzed in a Tektronik DPO 4034 Digital Phosphor Oscilloscope, which was generating the relevant waveforms showing the amplitude on the Y axis versus time on the X axis.