An application of non-destructive structure status inspection and online monitoring based on ultrasonic

Application

Fast Ultrasonic-based non-destructive structural material inspection and online damage monitoring.

Challenge

Most ultrasonic inspection technologies now in use are ultrasonic body wave oriented. Limited by its transmission mechanism, ultrasonic inspection must be made spot-by-spot. This leads to many disadvantages including low efficiency and high cost, as well as limited application in structure status monitoring for its spot-by-spot scanning.

The ultrasonic is a stress wave formed by reflected and stacked body waves and propagates along the structure wall. Featuring low attenuation and long propagation distance, it can be used to do non-destructive inspection against objects of large structure and regular shape. It possesses the potential of wireless application for online structure status monitoring.

Relative to body wave, the ultrasonic is more complex in two aspects: The first is that multi modals of ultrasonic may coexist in single frequency and the other is its dispersion where the same modal of waves propagates with different speeds in different frequencies. This complexity brings demanding requirements on inspection platform and method.

Solution

The ultrasonic inspection is an active method composed of signal triggering and receiving. Employing a single modal wave as the inspection signal requires the inspection platform to suppress other modals with respect to signal triggering and receiving through sensor sizes, signal triggering frequency, and optimized pairing to get triggers of a single wave modal.

An ultrasonic applicable phase control array and signal processing algorithm based on the principles is developed to facilitate fast imaging inspection against material damages.

• Application background

With increasing attention on the structural safety of large scale equipment, non-destructive inspection technology has become one of the indispensable tools to ensure the manufacturing and use of contemporary structural equipment in a range of industries including aerospace, power generation, and petrochemical transport and processing. The structural equipment employed in these fields are commonly subject to tough working conditions and result in internal damages and reduced structural safety caused by wear, corrosion, fatigue, and creepage. The non-destructive inspection technology is critical in online monitoring and diagnosis for this equipment.

Commonly adopted non-destructive industrial inspection methods are: Penetrant, magnetic particle, eddy current, ultrasonic, and X-ray. The ultrasonic inspection is the one most adopted for its wide application range (applicable with both metal and non-metal material) and harmless to the human body. Most ultrasonic inspection technologies now do their inspections spot-by-spot as the body wave they employ limits the effective inspection area to that under or very close to the probe. This leads to low efficiency and high costs that can be avoided by the ultrasonic inspection method.

The ultrasonic is a stacked up combination of regular ultrasonic body waves. In a medium of limitless uniformity and full-direction isotropic elasticity only longitudinal and transverse body waves can exist. The two propagate at a specific speed without wave form coupling. When propagated in limited wave media (e.g. plates and tubes) these two would transform into ultrasonic along the boundary caused by boundary restriction and modal conversion. The ultrasonic is a stress wave formed by longitudinal and transverse body waves reflected and overlaid between upper and lower boundaries that propagate along the wave media.

As a stress wave propagating in a solid with upper and lower boundaries, it attenuates as being absorbed by the media. This leads to a propagation distance proportional attenuation. On the contrary, the ultrasonic body wave pervades in three dimensions from its trigger point and attenuates in proportion to the square of the distance traveled. Attenuation of ultrasonic is much less than that of the body wave and so can propagate along the wave media for a very long distance.

This enables the ultrasonic to enjoy several meters of non-destructive inspection and results in huge improvements over conventional spot inspection approaches. For the shielded and underground structure often found in power generation and petrochemical industries, ultrasonic inspection technology's meters-length inspection from a single entry point without fully exposing the structure saves huge costs and improves efficiency significantly.

With a long inspection distance and extensive coverage, as well as online application potential, the ultrasonic inspection is ideal for structure health monitoring (SHM).

• Challenges

As an ultrasonic results from the ultrasonic body wave’s reflection and overlaying in the wave media, it is more complicated that the latter in its multi-modal and dispersion characteristics.

For plate structure with the same property in every direction of free boundary condition surface, its dispersion relation can be expressed as:

Here, h = half thickness of the plate wall, ω = angular frequency, k = number of waves, VL and VS = velocities of longitudinal and transverse waves in the material. When α=0 this is a symmetric modal and non-symmetric when α=π/2.

The wave dispersion curve can be derived from this and is shown in Figure 1. Multiple wave modals can be found with the same frequency. Take frequencies below 800 kHz: Modals A0, S0, and SH0 coexist. The number of coexisting modals increases along with rising frequencies; there are 8 propagating waves existing at the frequency of 2MHz. This complicates the flaw reflecting signals at the receiving end and hampers its inspection application.

The dispersion curves also indicate that the velocity of waves of the same modal change at different frequencies. This disperses component signals of different frequencies along with propagation distances and leads to extended excitation signal time domain and reduced amplitudes. In the excitation wave packet change process of a wave with a center frequency of 200 kHz and modal A0 propagated in a 2mm thick steel plate, see Figure 2, it is clear that wave packets suffer extended time domain and sharply reduced amplitudes along with their propagation distance. This would lead to signal overlay/mixing and attenuation which, in turn, results in unrecognized defect characteristics.

Figure 1: Frequency dispersion curve in a 2mm thick steel plate
(Elasticity modulus 216.9GPa, Poisson's ratio 0.28, and density 7.9×103kg/m3)

(a: excitation signal, b: wave form after 1000mm propagation, c: wave form after 1500mm propagation, d: wave form after 2000mm propagation)

Multi-modals and dispersion of a wave complicate its signal excitation, particle vibration, propagation, propagation, receiving, and signal retrieval comparing to that of normal ultrasonic inspection. This, in turn, requires ultrasonic inspection specific methods and techniques before it can be put into actual use.

• Solution

3.1. Single modal ultrasonic excitation

The number of modals of ultrasonic increases along with rising frequency. This complicates the signal and hampers the detection of defect characteristics signal. Single modal wave excitation is required for inspection application.

The wave dispersion characteristics curve indicates that only three 0-tier wave exist below the cutoff frequency of high level wave modal (810 kHz for 2mm thick steel plate) including the symmetric modal S0, non-symmetric modal A0, and horizontal shear modal SH0. That is, we can cut the number of waves down to three if the frequency of excitation signal can be controlled below the cutoff frequency of high level wave.

Patterns of S0, A0, and SH0 modal waves differ from each other. The A0 modal is off-plane shift oriented as shown in Figure 3(a) while the S0 and SH0 modals shift primarily within a plane with S0 directed parallel with the propagation direction, Figure 3(b) and SH0 perpendicular to the propagation direction, Figure 3(c).

The excitation of ultrasonic is actually to couple the stress wave associated with the modal in the object subject to inspection. To get a single wave modal the surface stress distribution required by the desired modal must be optimized with transducers and while those not required be suppressed at the same time.

Transducers available for coupling wave stress fields in a structure under inspections now are: piezoelectric transducer, electromagnetic acoustic transducer (EMAT), magnetostrictive transducer, and laser ultrasonic transducer. The piezoelectric transducer employs the piezoelectric effect and the inverse piezoelectric effect of crystal materials for wave excitation and detection sensor. The popular piezoelectric materials now in use are PZT and flexible PVDF. The former one features better piezoelectric conversion efficiency and lower cost at the expense of lacking of flexibility. The OVDF material is flexible yet comes with lower piezoelectric effect. The EMAT excites wave stress field with Lorenz force by changing electromagnetic field in metal structure. The magnetostrictive transducer (MT), first created by H. Kwun et al., excites wave stress field with the magnetostrictive effect. The laser ultrasonic transducer generates thermal stress vibration on the surface of the structure to be inspected with laser pulse beam to excite ultrasonic. Suffered by bulky footprint and higher costs, it is now used in lab environments rather than field inspection in most cases.

Out of these wave transducers the PZT piezoelectric chips are applied in structure health status monitoring for its compactness, light weigh, and low costs. Most research teams around the world adopt the PZT piezoelectric chip for wave excitation and receiving transducer.

3.2 Optimization of wave's excitation wave form

Different velocities of component wave packets of different frequencies lead to dispersed ultrasonic. Severe dispersion results in mixed inspection signals and defect characteristics retrieval failure. This requires wave excitation frequency and wave form optimization.

The ultrasonic is commonly excited with the Hanning window modulated 5 cycle sine waves. The Hanning window functions to reduce the frequency sidelobe caused by abrupt beginning and ending of wave forms to concentrate energy at the excitation frequency. The framed modulation to excitation signal reduces its bandwidth and dispersion effect. See Figure 4 for waveform and associated spectrum of 200 kHz sine waves before and after frame modulation.

(a) Original signal,
(b) spectrum of original signal,
(c) Hanning window modulation signal,
(d) modulated signal spectrum

3.3 Ultrasonic inspection platform

Differing from regular ultrasonic's spot-by-spot inspection; the ultrasonic can inspect a large area at a fast speed. It requires precise accurate inspection data to position defects more accurately. An ultrasonic inspection platform addressing its characteristics (e.g. specific excitation signal and ceramic piezoelectric transducer) is created as shown in Figure 5:

Output signals from the arbitrary function generator can be applied directly to the two poles of a piezoelectric chip transducer to drive the piezoelectric ceramic for piezoelectric effects. The piezoelectric signal is then converted to vibration signal of the same frequency and propagates in the structure to be inspected. The voltage signal generated by the arbitrary wave form signal generator at amplitudes between 10mVP-P-10VP-P is too low to drive the piezoelectric ceramic for ultrasonic excitation in the structure. A custom-made high voltage amplifier is added in the platform to amplify the excitation voltage from the ceramic excitation voltage transducer. It linearly amplifies the input signal from the signal generator to 180Vp-p with up to 2MHz linear amplified frequency at 180Vp-p output.

Ultrasonic's excitation signal drives the piezoelectric transducer to generate an ultrasonic for propagation in the channel after being amplified by the power amplifier. When arriving at the receiving end of the wave, it employs the inverse piezoelectric effect of piezoelectric ceramic to convert vibration to voltage output. As the inverse piezoelectric effect of piezoelectric ceramic is very weak, at 100Vp-p driving voltage the output voltage signal at the receiving end is at the millivolt order of magnitude, the received signal need be amplified with the preamplifier before entering the signal collector. This platform employs a custom made preamplifier with adjustable gain between -4.5dB-525dB. The input impedance of the preamplifier is set to 6K Ohm as the piezoelectric chip is of very high impedance and very low output signal power.

The signal collector employs the PCI-9846 high speed digitizer from ADLINK. The latter's high sampling rate and resolution is ideal for wave signal collection. Its four channels concurrent log capacity also reduces wave array signal collection time sharply.

The multi-circuit switch unit functions switching the excitation and the receiving transducers. It switches excitation transducers as the piezoelectric transducer has only one excitation end and there exists multiple transducers. It employs relays controlled by single-chip computer for signal channel on/off by connecting the single-chip computer to your PC's serial port.

3.4 Phased transducer array

Transducer array can be found in sonar and radar devices. Its scores of sensors enable it to point-by-point scan different positions in space with phased array algorithm. We can use ultrasonic's long distance propagation capability to image the object to be inspected with point-by-point scanning based on the phased array concept adopted by the radar system. That is, a wave radar.

One of the key elements of ultrasonic radar is its phased array and the associated algorithm. This application example adopts a phased array in closely spaced circle as shown in Figure 6. The array is composed of 16 piezoelectric chip units, in dimension of Φ7×0.2mm, equally spaced on a circle of 50mm diameter. It can scan imaging in the space of 0-360°around the array.

The phased array contains 16 independent wave transducers. To use it for defect imaging inspection, you need to excite one transducer and record the wave signals collected by all 16 of them. Repeat this for each of the 16 transducers to get a 16×16 array of time domain signals with each one of them corresponding to a pair of excitation-receiving transducers.

As the ultrasonic is featured with dispersion the processing of signals collected by the phased array must be designed specifically. The first is to convert each circuit of time domain signal to frequency domain through FFT and then to wave number domain amplitude based on the pattern dispersion characteristic relationships to get another 16×16 signal array. To scan in different directions the array control algorithm is used to sum up signals from each circuit multiplied by specific phase control coefficient according to the direction to be scanned. The last step is to Fourier transform every row of the array from wave number to distance domain and to image the defect for imaging inspection.

• Inspection example

In this example an imaging inspection is exerted to detect defects in plate-like components with phased array of wave as described above, 16 Φ7×0.2mm piezoelectric chips evenly spaced at a circle of 50mm diameter. The object to be expected is a 2mm thick steel plate with a defect of 2mm through hole 500mm away from the array center. The wave is excited by a Hanning window modulated 5 cycle sine wave with center frequency at 200 kHz.

The inspection process starts using one transducer for excitation and collects 4 receiving signals with the 4 channels of PCI-9846, select another 4 transducers for signal collection with the multi-circuits switch unit until receiving signals from the 16 transducers are all collected. Select the next transducer for excitation and repeat the process until all 16 transducers have been excited once.

Process the 256 circuits of signal with the phased array signal processing method described earlier to get the distribution image of defects as shown in Figure 7.

As illustrated with this example, the ultrasonic can be used for non-destructive material inspection and the damage distribution imaging with ultrasonic phased array is relatively more accurate.

(The array is located at the origin point, the simulated damage is a through hold of 2mm diameter and is 500mm away from the array center)

• Conclusion

This application example indicates that an ultrasonic phased array can image inspecting damage of the material in a plate shape. This inspection method can inspect a larger area with array at a tiny zone. It is suitable not only for non-destructive inspection but also for online monitoring.

This method requires an ultrasonic array with multiple transducers and each and every transducer is required to be subject to signal excitation and collection. This may lead to relatively long collection time. In this example, a single channel collector mandates 256 times of signal collection. Thanks to 4 collection channels of ADLINK PCI-9846 digitizer the same inspection can be finished in quarter of the time required otherwise. This is very significant in online damage monitoring that mandates timeliness.

Related ADLINK Links:

• More about ADLINK Modular instrument (digital instrument)
• More about PCI-9816/9826/9846