Metal pipes and plates are prone to cracks, corrosion and other material defects. Typically, cracks develop as relatively shallow defects caused by, for example, material fatigue or crystal defects. Over time, cracks become longer and deeper, and, given enough time, cracks may compromise the structural integrity of the pipe/plate. Therefore, metal parts are from time to time inspected to detect the presence and severity of crack. Additionally, layered composite materials may have flaws such as delamination (lack of adhesion between the layers). Some inspection technologies use ultrasonic waves to inspect for thesuch flaws.
FIG. 1 is a schematic view of defect detection in accordance with prior art. Some conventional technologies use piezoelectric transducers or electromagnetic acoustic transducers (EMAT) to generate ultrasonic waves in a solid material 6 (e.g., a metal plate). The conventional piezoelectric transducer includes a crystal 2 (e.g., a piezoelectric element) and a couplant 4 (e.g., gel or fluid) that transfers vibrations onto the solid material 6 (e.g., a steel plate). With another conventional technology, the EMAT 15 produces vibrations in a conductive and/or paramagnetic solid material 6. The EMAT 15 includes a permanent magnet 10 coupled with a coil 12. When the alternating current (AC) flows into the coil 12, magnetic field of the permanent magnet 10 interacts with magnetic field created by the AC current in the coil 12 to generate eddy currents in the solid material 6. The energy of these eddy currents are transferred to the crystal lattice of the solid material, producing ultrasonic waves.
When the ultrasonic waves reach a crack or flaw 5, reflected ultrasonic waves are generated. These reflected waves can be detected by a receiver that is also a piezoelectric element or an EMAT receiver. For example, at the receiving EMAT (not shown), the interaction of the reflected ultrasonic waves with the magnetic field of the receiving EMAT induces electrical currents in the receiving EMAT coil circuit. These induced currents can be measured, and further analyzed to characterize the crack 5.
The ultrasonic waves can be broadly classified into two categories: bulk waves and guided waves. Bulk waves, as the name suggests, can be generated into the bulk of the material at very high frequencies. Guided waves propagate at lower frequencies when compared to bulk waves for a given wall-thickness. Guided waves are characterized by multimodality, which is further characterized by the propagation of multiple packets of waves at distinct velocities for a given band of frequencies, each of which may be identified as a guided wave mode. Guided waves are typically employed in long distance inspection of structures. When applied to ultrasonic non-destructive inspection or testing of structures, the multimodality of guided waves can cause the corresponding signals to be unreadable or difficult to interpret.
FIG. 2 is a partially schematic, isometric view of defect detection in pipes using EMATs in accordance with prior art. Illustrated defect detection system 50 includes several EMAT transmitters (TX-es) 15-T interspersed with several EMAT receivers (RX-es) 15-R. These EMAT transmitters/receivers are distributed over the inner surface of a pipe 1. The individual EMAT transmitters 15-T generate ultrasound waves 40-F and 40-B in the material of the pipe 1 (i.e., the ultrasound waves guided in the forward and backward direction about the circumference of the pipe), as explained with reference to FIG. 1. When the ultrasound waves encounter the crack (defect) 5, the reflected ultrasound waves are generated and detected by one or more EMAT receivers 15-R. A distance from the EMAT receiver 15-R to the crack can be calculated based on the known time difference between the time when the ultrasound waves were transmitted by an EMAT transmitter 15-T and the time when the reflected ultrasound waves were received by an EMAT receiver 15-R. Therefore, the illustrated system emits guided waves about the circumference of the pipe 1. In other conventional technologies, the piezoelectric transmitters/receivers (collectively, transceivers or TRX-es) are used instead of the EMATs.
FIG. 3 is a schematic view of an EMAT 15 with multiple coils in accordance with prior art. The illustrated EMAT 15 uses multiple coils 12-1 to 12-4 that are overlaid and displaced as an array. Collectively, the multiple coils represent a coil transducer 22 of the EMAT 15. When the coils 12-1 to 12-4 are individually driven with phase-delayed ultrasonic pulsers, the combined ultrasonic waves result in lower modal noise when compared to an EMAT having a single coil 12. This method results in a wave having a fixed range of wavenumbers.
FIG. 4 is a schematic view of piezoelectric fibers 21 used in defect detection systems in accordance with prior art. Collectively, the multiple piezoelectric fibers 21 represent a piezoelectric transducer 22. In operation, the individual piezoelectric fibers 21 are excited by electrodes 50, making the piezoelectric fibers 21 expand or contract, depending on the polarity of the electrodes. In the example illustrated in FIG. 4, the piezoelectric fibers 21 expand in the direction 22e when excited (energized) by the electrodes 50. For the transducer 22 with four piezoelectric elements 21, four sources of excitation (pulsers) can be used, one per each piezoelectric element 21, to generate an arbitrary sequence of excitation. With some conventional technologies, the amplitude may be varied across the transducer array elements, to achieve an effect that is equivalent to traditional phased array transduction. This conventional method is sometimes referred to as “amplitude control of guided waves.”
However, for the transmitter having a large number of elements (either coils for the EMAT systems, or piezoelectric elements for piezo-based systems), a relatively large number of pulsers and their supporting electronics are required. Alternatively, a single source of excitation can be used for all transmitter elements in parallel, however, resulting in poor control of the wave direction.
Exciting the transmitter elements simultaneously by their corresponding pulsers is called a “real-time mode” of excitation. With some conventional technologies, individual transmitters (e.g., the coils 12 or piezoelectric crystals 2) of an arbitrary array are excited sequentially in time, and, after reflecting from the defect in the specimen, the reflected ultrasonic waves are also acquired sequentially off the individual receivers. The received data are filtered to select the preferred modes. The sequential excitation of the individual transmitters/receivers is called a “synthetic mode” excitation. Compared to the real-time mode excitation, the synthetic mode excitation requires less pulsers, but it also lowers the energy of the ultrasonic waves in the specimen. In many applications, especially when inspecting “lossy” substrates such as polymers, polymer coated metals or carbon fiber reinforced polymer (CFRP) structures, reducing the energy of the ultrasonic waves is generally undesirable.
Accordingly, there remains a need for defect inspection systems that can produce strong guided waves with reduced number of transmitter/receiver (transducer) elements, pulsers and/supporting electronics.