Bao Mi

An ultrasonic method for dynamic monitoring of fatigue crack initiation and growth

Attached ultrasonic sensors can detect changes caused by crack initiation and growth if the wave path is directed through the area of critical crack formation. Dynamics of cracks opening and closing under load cause nonlinear modulation of received ultrasonic signals, enabling small cracks to be detected by stationary sensors. A methodology is presented based upon the behavior of ultrasonic signals versus applied load to detect and monitor formation and growth of cracks originating from fastener holes. Shear wave angle beam transducers operating in through transmission mode are mounted on either side of the hole such that the transmitted wave travels through the area of expected cracking. Time shift is linear with respect to load, and is well explained by path changes due to strain combined with wave speed changes due to acoustoelasticity. During subsequent in situ monitoring with unknown loads, the measured time of flight is used to estimate the load, and behavior of the received energy as a function of load is the basis for crack detection. Results are presented from low cycle fatigue tests of several aluminum specimens and illustrate the efficacy of the method in both determining the applied load and monitoring crack initiation and growth.

Simulation and Measurement of Ultrasonic Waves in Elastic Plates Using Laser Vibrometry

The propagation of Lamb waves in elastic plates is analyzed both numerically and experimentally. A Scanning Laser Doppler Vibrometer (SLDV) is here used to detect and visualize transient waveforms propagating in an elastic plate at low ultrasonic frequencies. The waves are excited by a piezoelectric crystal glued to the plate surface and actuated by sinusoidal pulses of varying frequency. The pulse sequence is triggered by the SLDV internal controller so that phase and delay information are preserved. Such information allows visualization of the waveform pattern as it propagates over the plate surface. The experiment produces animated displacement maps where the interaction with discontinuities in the plate such as defects becomes apparent. This capability suggests application of the SLDV technique as part of an overall damage detection methodology which combines the recognized sensitivity of ultrasonic waves with the localization of damage via wavefield visualization. The interpretation of the experiment...

Development of automated real-time data acquisition system for robotic weld quality monitoring

Abstract The lack of reliable non-contact, non-destructive, online sensors with the ability to detect defects as they form and with the capacity to operate at high temperatures and in harsh environments is a big obstacle to fully automated robotic welding. This paper presents a non-contact automated data acquisition system for monitoring a robotic gas–metal arc welding process based on laser ultrasonic technology. While a robot welds between two 1040 steel strips, a Nd:YAG Q-switched pulse laser generates ultrasound on one side of the weld by ablation, and a non-contact electro-magnetic acoustic transducer (EMAT) placed on the opposite side of the weld detects ultrasound transmitted through the weld bead. Ablation is employed because high temperature specimens require strong signals to compensate for attenuation within the bulk of the material. The data is then analyzed to determine the time required for ultrasound to travel from the laser source to the EMAT, termed as the time of flight (ToF). When experimental ToF is compared to theoretical ToF, it is determined that surface waves are detected by this system. Therefore, this system can measure weld bead reinforcement distance. In most cases, weld bead geometry is an indication of the weld quality, and can be used as feedback to control a welding process.

DAMAGE DETECTION IN PLATE STRUCTURES USING SPARSE ULTRASONIC TRANSDUCER ARRAYS AND ACOUSTIC WAVEFIELD IMAGING

A methodology is presented for health monitoring and subsequent inspection of critical structures. Algorithms have been developed to detect and approximately locate damaged regions by analyzing signals recorded from a permanently mounted, sparse array of transducers. Followup inspections of suspected flaw locations are performed using a dual transducer ultrasonic approach where a permanently mounted transducer is the source and an externally scanned transducer is the receiver. Scan results are presented as snapshots of the propagating ultrasonic wavefield radiating out from the attached transducers. This method, referred to here as Acoustic Wavefield Imaging (AWI), provides an excellent visual representation of the interaction of propagating ultrasonic waves with the structure. Pre‐flaw and post‐flaw ultrasonic waveforms are analyzed from an aluminum plate specimen with artificially induced damage, and the AWI results show the location and spatial extent of all of the defects.

Parametric Studies of Laser Generated Ultrasonic Signals in Ablative Regime: Time and Frequency Domains

A laser pulse incident on a material may generate ultrasound by means of two different phenomena: thermoelastic effect at low power density and ablation effect at high power density. Ablative generation of ultrasound is necessary for some critical applications such as on-line weld quality monitoring in which strong signals are required to compensate the elevated temperature and the long path length. While the waveform in time domain has been discussed extensively in the literature, there is little knowledge about the frequency components of laser ultrasound, although this information is necessary for practical applications. In this paper, analytical results from both thermoelastic and ablative regimes are reviewed. Laser ultrasonic signals (longitudinal waves and surfaces waves) generated by laser ablation are measured in a number of metal samples (2024 A1, 6061 A1, 7075 A1, mild steel, and copper) with a broadband laser interferometer. The frequency spectra are analyzed and compared for different thicknesses (50.8 mm, 25.4 mm, 12.7 mm, and 6.4 mm) and for different power densities. Hanning windowing is applied to the longitudinal pulses in time domain before frequency analysis is performed. The experimental data match the theoretical predictions very well. The results show that the frequency spectrum extends from 0 to 15 MHz, while the center frequency occurs near 2 MHz. The detailed distribution of the spectrum is dependent on the material, thickness, and laser power density.

Self‐Calibrating Ultrasonic Methods for In‐Situ Monitoring of Fatigue Crack Progression

Ultrasonic sensors permanently affixed to aluminum coupons are used to monitor progression of damage during fatigue testing with the long term goal of structural health monitoring for diagnostics and prognostics. Necessary for success are proper design of the ultrasonic testing methods, robust transducer mounting techniques, and real‐time signal processing for determining the state of the structure. It is also highly desirable for the overall system to be self‐calibrating with built‐in diagnostics in order to detect and compensate for sensor degradation or failure. Self‐calibrating ultrasonic techniques are applied for monitoring of cracks initiating and propagating from the inaccessible inner diameters of rivet holes where the transducers are mounted on the accessible specimen surface. Angle beam ultrasonic methods are utilized that are suitable for detecting small defects in critical local regions of high stress. Results are presented for aluminum coupons subjected to low cycle fatigue and demonstrate u...

Three-dimensional ray tracing of laser ultrasound for weld penetration sensing

The time-of-flight (TOF) method is an ultrasonic nondestructive testing (NDT) technique. The TOF of an ultrasonic wave can be correlated to weld penetration depth, and hence weld quality. Changes in material properties due to temperature gradients will cause ultrasonic speed to vary during welding, which causes a curved propagation path. A ray tracing algorithm is required in order to study how ultrasound propagates within a weld sample. In this paper, a three-dimensional (3-D) ray tracing algorithm based on Fermat’s principle is presented. First, ray equations are derived using the calculus of variation. Then, a numerical algorithm is developed to solve the derived ray equations and obtain the curved propagation path. This algorithm includes finite element analysis (FEA) to obtain the transient temperature distribution during the welding and shooting method to solve the boundary value problem. After the curved ray path is obtained, the TOF can be found by integrating the time variable along the ray path....

In-situ ultrasonic monitoring of crack growth under static and dynamic loading conditions

Successful in-situ monitoring of crack initiation and growth is a necessary prerequisite for applying ultrasonic methods to structural health monitoring. For conventional ultrasonic testing methods, a focused beam may be used to directly image the crack tip; however, this method is difficult to apply during fatigue testing because of access limitations and couplant contamination issues. However, ultrasonic sensors can be permanently attached to a specimen to detect signal changes due to crack initiation and growth if the wave path is properly directed through the area of critical defect formation. The dynamics of cracks opening and closing during the fatigue process modulate the amplitude of ultrasonic waves propagating across these crack interfaces. Thus, even very small cracks can be reliably detected using permanently mounted sensors if the ultrasonic response can be measured as a function of load. A methodology is presented here that uses this behavior to detect and monitor crack formation and growth. This methodology may also be applied to structures subjected to unknown dynamic loads by using the ultrasonic signal to both estimate the instantaneous dynamic load and interrogate the integrity of the structure. Essential to the success of this method is an initial calibration on the undamaged structure where ultrasonic response is measured as a function of known static load. Results are presented from several aluminum specimens undergoing low cycle fatigue tests, and the dynamic loading results are shown to be comparable to the static ones in terms of the response of the ultrasonic signal to crack progression.

Ultrasonic Monitoring of Fastener Holes Using Load Modulated Energy Algorithms for Early Detection of Fatigue Cracks

Ultrasonic techniques have the potential to provide early detection of fatigue induced cracks originating from fastener holes with the long term goal of structural health monitoring. This paper considers energy‐based algorithms for analyzing ultrasonic waveforms obtained using fixed angle beam transducers in a through‐transmission configuration. The transducers were located on either side of the fastener hole with the sound beam oriented along the loading direction, and waveforms were obtained as a function of applied load. Previous work has shown that the ratio of the received ultrasonic energy at under tensile loading to the energy with the sample under no load is a robust method for determining the presence of cracks. The reason for the effectiveness of this metric is that the energy reduction from the crack opening is accentuated by the applied load. For structural health monitoring applications, the earliest possible detection of cracking is desired, and the total energy ratio method does not utilize...

Frequency spectrum analysis of laser generated ultrasonic waves in ablative regime

In this paper, laser ultrasonic signals generated in ablative regime are measured in a number of metal samples (2024 Al, 6061 Al, 7075 Al, mild steel, and copper) with a broadband laser interferometer. The frequency spectra are analyzed and compared for different thicknesses (50.8 mm, 25.4 mm, 12.7 mm, and 6.4 mm), and for different power densities. Hanning windowing is applied before frequency analysis is performed. The experimental data match the theoretical predictions very well. The results show that the frequency spectrum extends from 0 to 15 MHz, while the center frequency occurs near 2 MHz. The detailed distribution of the spectrum is dependent on the material, thickness, and laser power density.