Developing an Air-Coupled Ultrasonic C-Scan Imaging System to Detect Composite Defects Using LabVIEW

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"Easy-to-program LabVIEW combined with NI-SCOPE, the LabVIEW Advanced Signal Processing Toolkit, and third-party drivers reduced development time by 60 percent and development cost by 50 percent. The success of this project has given us confidence to meet new challenges for future needs."

- Thatisetty Murugesh, Advanced System Laboratory (ASL)

The Challenge:
Developing a noncontact air-coupled ultrasonic C-scan imaging system for fast and reliable detection of defects in thick and multilayered composites that continuously acquires data from an ultrasonic transducer for C-scan imaging of the sample.

The Solution:
Using NI motion control, an ultrasonic pulser/receiver, and an NI DAQ board programmed with NI LabVIEW software to create an automated scanning system with a mechanical X-Y scanner using motion control.

Author(s):
Thatisetty Murugesh - Advanced System Laboratory (ASL)
K Srinivas - Advanced System Laboratory (ASL)

ASL, under the Defence Research and Development Organisation located in Hyderabad, India, fabricates composite structures for aerospace applications. Carbon and glass fiber reinforced polymer matrix composites are widely used for various structural applications, including load-bearing structural elements. Defects such as air gaps, delaminations, and porosity may manifest during composite processing. These defects affect the performance of the composite structures.

We can perform nondestructive testing (NDT) on composites to identify defects before using them for final applications. Among many NDT methods, ultrasonic inspection is a well-known technique for the reliable detection of defects in composites. Composite structures are highly attenuative and anisotropic compared to metals, so they require special techniques for shop-floor applications. Contact ultrasonic composite inspection is not preferable due to restrictions on using a couplant (water or gel) on the surface. Noncontact (air as couplant) ultrasonic composite inspection through transmission inspection is a suitable tool to inspect composites because no couplant is required, so inspection becomes faster and probes can quickly move over the test surface without hindrance due to contour, surface, and roughness.

System Description

Conventional ultrasonic systems use spike and pulse for transducer excitation with a short percentage of duty cycle. However, this energy is not sufficient for exciting air-coupled ultrasonic transducers, which require a tone burst with at least a few cycles for transmission through the air and composite part. To overcome these limitations, we selected a high-voltage sinusoidal tone burst pulser and receiver with a high-gain preamplifier. For improving transmission through composite structures and achieving high signal-to-noise ratio (SNR), we used tailor-made pulses based on mathematical equations. Figure 1 shows the block diagram of the air-coupled C-scan imaging system.

The system transmits custom radio frequency (RF) signals to the arbitrary waveform generator using LabVIEW software. The generated signals are input to the high-power ultrasonic pulser for generating the desired tone burst excitation for transmission through transmission mode. Another transducer connected to the receiver through a preamplifier receives the ultrasonic signal. The receiver RF output is connected to an NI PCI-5152 digitizer board for data acquisition. LabVIEW combined with motion control and a PCI-5152 fully controls the air-coupled C-scan imaging system. The mechanical scanner used for the air-coupled C-scan imaging is shown in Figure 2.

The ultrasonic imaging system has two distinct parts: the scanning system and the imaging system. Figure 3 shows a flowchart representing the order of events in the scanning process. The scanning system consists of a mechanical X-Y scanner precisely controlled with LabVIEW software. The scanning system uses ultrasonic techniques to collect and store information on defects and information coming from a given area of a specimen, so the imaging system can construct a visual representation of the specimen and any defects present.

The A-scan data is recorded at equally spaced positions across the surface of the material that needs to be examined. The LabVEW GUI controls the scanning equipment, ultrasonic hardware, data collection, and storage. The system achieves the transmission and reception of the ultrasonic sound waves using a commercial ultrasonic transducer and pulser/receiver. During scanning, the transducer is above and below the specimen at an exact height determined by the focal length of the transducer. The transducer moves parallel to the surface of the material using two stepper motors that control movements in the X and Y directions.

Before scanning, the coordinate system is set to the home position, which is defined as the coordinate origin (X=0, Y=0). The area of the specimen to scan is then selected by moving the transducer to three locations on the extremes of the desired scan area. From these positions, we can specify a rectangular scan area in terms of Xstart, Xstop, Ystart, and Ystop. The motion controller sends out a series of pulses to the stepper motors to move the transducer in the X and Y directions.

The system sends a trigger pulse to the pulser/receiver to generate a tone burst that excites the ultrasonic transducer. The ultrasonic wave propagates through the specimen from the front surface and collects data from the back surface of the specimen. The ultrasonic wave propagates through the specimen and is attenuated. If any defect is present in the specimen, the system further attenuates the wave. Each time ultrasonic hardware is triggered this way, the system captures the resultant A-scan waveforms using the PCI-5152.

During scanning, the transducer moves back and forth between the minimum and maximum X coordinates. At each measurement point, the system can record A-scans and extract peak amplitudes. The system can derive the coordinates of any point from its position relative to the first point for that scan line, the coordinates of which the system recorded.

From the data stored on the computer, the system can extract the X and Y coordinates of each measurement point and the resolution of the scanning, which is useful for the quantification of defect size. Figure 4 shows the front panel of the LabVIEW GUI for control and data acquisition. The amount of data stored depends on the size of the specimen and resolution used during scanning.

We developed a LabVIEW GUI to display all three basic forms of data, called A-scan, B-scan, and C-scan. In Figure 5, the front panel of the LabVIEW GUI shows the A-scan, B-scan, and C-scan representation of composite laminate. All-time versus amplitude signals are called A-scans at a point of the specimen, B-scan represents a cross-sectional view of the specimen, and C-scan represents a planar view of the specimen.

Easy-to-Use Tools for Rapid Development

The automated C-scan imaging system we created eliminates manual errors and improves the inspection speed of composite structures. LabVIEW, NI-SCOPE, and the LabVIEW Advanced Signal Processing Toolkit helped us quickly and easily develop the LabVIEW GUI.

Easy-to-program LabVIEW combined with NI-SCOPE, the LabVIEW Advanced Signal Processing Toolkit, and third-party drivers reduced development time by 60 percent and development cost by 50 percent. The success of this project has given us confidence to meet new challenges for future needs.

Author Information:
Thatisetty Murugesh
Advanced System Laboratory (ASL)
Advanced System Laboratory (ASL),DRDO Complex,Kanchanbagh
Hyderabad
India
Tel: 9866463341
tmg_md@yahoo.co.in

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