Showing papers in "Journal of Nondestructive Evaluation in 2007"

Single Mode Tuning Effects on Lamb Wave Time Reversal with Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Abstract: Lamb wave time reversal method is a new and tempting baseline-free damage detection technique for structural health monitoring. With this method, certain types of damage can be detected without baseline data. However, the application of this method using piezoelectric wafer active sensors (PWAS) is complicated by the existence of at least two Lamb wave modes at any given frequency, and by the dispersion nature of the Lamb wave modes existing in thin-wall structures. The theory of PWAS-related Lamb wave time reversal has not yet been fully studied.

Monitoring the Microstructural Changes During Tempering of Quenched SAE 5140 steel by Magnetic Barkhausen Noise

Abstract: The aim of this work is to characterize the microstructures of quenched and tempered steels non-destructively by a diverse set of parameters of the Magnetic Barkhausen Noise method (MBN fingerprint, frequency spectra, pulse height distribution, root-mean-square, and total number of pulses). Identical specimens from a SAE 5140 steel bar were prepared. All specimens were austenitized at 860°C for 30 minutes and water-quenched identically. The quenched specimens were then tempered at various temperatures between 200°C and 600°C. The microstructures were characterized by metallographic examinations and hardness measurements. Pulse height distributions, noise signal envelopes and frequency spectra were used to evaluate Barkhausen activity. The results show that as the tempering temperature increases, the Barkhausen activity increases due to the enhancement of domain wall displacement with softening of the martensite. An excellent correlation was found between Barkhausen parameters and hardness values.

Characterization of Dual-Phase Steels Using Magnetic Barkhausen Noise Technique

Abstract: The aim of this work is to nondestructively characterize the dual phase steels using the Magnetic Barkhausen Noise (MBN) method. By quenching of AISI 8620 steel specimens having two different starting microstructures, from various intercritical annealing temperatures (ICAT) in the ferrite-austenite region, the microstructures consisting of different volume fractions of martensite with morphological variations have been obtained. The microstructures were first conventionally characterized by metallographical investigations and hardness tests. Then, the MBN measurements were performed using a μSCAN commercial system. Good correlations between the martensite volume fraction, hardness and MBN emission have been obtained. MBN signal height clearly decreased as the ICAT, therefore the volume fraction of martensite increased. The effect of the initial microstructure prior to intercritical annealing has also been differentiated by the MBN measurements. It has been concluded that MBN method can be used as a useful tool for nondestructive characterization of dual phase steels.

Computational Characterization of Adhesive Layer Properties Using Guided Waves in Bonded Plates

Abstract: This research develops a guided waves technique to nondestructively characterize the stiffness properties of bonded engineering components. This study first quantifies the influence of the relevant adhesive layer properties—Young’s modulus, Poisson’s ratio and bond thickness—on the dispersion curves of a two-layer bonded system, an aluminum plate with an adhesive tape layer bonded to its lower surface. Both a commercial finite element (FE) code (ABAQUS/Explicit) and the global matrix method (GMM) are used to determine the dispersion relationships of this bonded plate system in the form of frequency-wavenumber and slowness-frequency relations. These dispersion curves are then used to determine a set of adhesive tape parameter sensitive points, whose frequency coordinates represent the solution criteria for a proposed inversion procedure. This inversion is based on the GMM and assumes the three adhesive tape properties are unknown. The performance of this inversion procedure depends on the number of input time-domain signals; it is possible to solve the inverse problem for all three of the unknown adhesive tape properties if multiple input signals are known.

Feature Extraction for Crack Detection in Rain Conditions

Abstract: The solution of the features selection problem is critical for robust detection of crack signals in noisy environment, varying from short-time impulses such as raindrops to the wide-band white Gaussian noise. In this paper, two novel feature selection methods were used to reduce an initial set of 90 features, 67 estimated in the time domain and 23 in the frequency domain, decreasing significantly the memory requirements and the computational complexity of a Radial-Basis-Function (RBF) cracks detector. The evaluation process is carried out in a database including of more than 6000 cracks, raindrops and simultaneous crack and raindrops signals. Additive white Gaussian noise is used to distort the real signals at −20 to 20 dB Signal to Noise Ratio (SNR). The experimental results show that the number of features can be reduced to approximately 25, without affecting the classification rate of cracks and raindrops in the noisy signals, if the SNR is better than 0 dB. In noise-free environment a classification rate of 91% for a single crack/raindrop event is achieved using only five features. A different set of five features reaches a rate of 85% at 10 dB SNR.

Evaluation of Fatigue Specimens Using EMATs for Nonlinear Ultrasonic Wave Detection

Abstract: It is suggested that the waveform of the received ultrasonic wave is slightly different from that of the incident ultrasonic wave when the displacement of an ultrasonic wave and the opening width of a crack are almost same. Thus, as regarding the incident ultrasonic wave, its harmonic frequency components change as it penetrates the crack. Since, the nonlinearity of a solid material is very small compared to that of a liquid medium with a high nonlinear efficiency when used as a coupling medium, we applied an electromagnetic acoustic transducer (EMAT), which does not require a coupling medium. In addition, we tried to develop the EMAT that could alternately drive S0-Lamb wave and fundamental Shear Horizontal (SH0)-plate wave to detect any nonlinearity in an ultrasonic wave. We actually tested the performance using the fatigue specimens we fabricated. As a result, we observed that the harmonic components changed when we used the specimen with a specific loading condition and a specific ultrasonic mode. This is indicated that the harmonic components detection using the trial EMAT could also provide useful information on the degree of damage to any structures or any materials.

Infrared Detection of Defects in Powder-Metallic Compacts

Abstract: Electric Joule heating of low-conductivity media through direct current excitation can be used to generate a temperature profile throughout a powdermetallic (P/M) compact. When recording the surface temperature distribution with an infrared (IR) camera important information regarding the integrity of the sample can be gained. Unlike most existing IR techniques, this research concentrates on obtaining the temperature distribution and heat flow behavior in P/M parts when they are subjected to active electric current injection. The practical measurements are supported and complemented by a simple theoretical model that serves as a calibration tool to aid in the evaluation of the infrared signatures that are recorded over the sample surface and correlated with the detection of surface and subsurface flaws. In this paper we will report on the design of the active infrared detection system and a basic theoretical testbed that is suitable for calibration. Specifically, we state the governing equations and boundary conditions, followed by devising numerical solutions that enable a comparison to the measured thermal response. In addition, the numerical modeling approach can also serve as a method to model various flaw sizes and orientations in an effort to practically determine flaw resolution limits as a function of minimally detectable temperature distribution. Preliminary measurements with controlled and industrial samples indicate that this IR testing methodology can successfully be employed to inspect both green-state and sintered P/M compacts.

Evaluation and Correlation of Residual Stress Measurement in Steel

Abstract: The stress patterns relevant for the assessment of the post production service behaviour of plain carbon hot strip mill plates were evaluated on the surface and at various depths. The hole drilling strain gage (HDSG) is a standard, well proven, semi-destructive technique and is employed for the purpose. However, this technique requires skill and is slow in operation. The magnetic Barkhausen technique (MBE) is non destructive and is still in an experimental stage, but has tremendous potential and has been tried here in steel plant environment. The objective is to establish whether MBE technique could be used as an alternative to HDSG technique. The calibration for MBE results was done under controlled compressive and tensile loading. The stress profiles were determined at six locations along rolling and transverse directions using these two techniques. The rolling stresses using HDSG method were found to be tensile near the surface, increase with depth and saturate at 0.5 to 1 mm depths. The transverse stresses were low tensile or compressive near the surface and saturate to low tensile values at similar depths. The stresses (averaged over a certain depth) by MBE are tensile in rolling direction and compressive in transverse direction. Correlation plots between residual stresses by HDSG method up to different depths and average stress by MBE show that that a good correlation exists at 0.35 mm depth. Beyond this depth, correlation is not good. It is true that for the material of the sheet and the frequency employed, the MBE results are not relevant beyond this depth. Also, the equivalent uniform stress as measured by HDSG method would deviate more and more from the actual stress as we go deeper.

Remote Optical Non-Destructive Inspection Technique for Metal and Composite Structures

Abstract: This paper presents a laser based non-destructive inspection technique which is purely non-contact. An acoustic device is used to remotely excite the material under inspection while a scanning laser vibrometer is employed to measure the resulting vibrations. The test materials, mostly aluminum or composite aircraft structures, have been successfully inspected from a standoff distance of at least a meter. The NDI system clearly shows the defect locations overlaid on the images of the test objects for ease of the end user. Metal and composite delamination and subsurface corrosion in aluminum structures are among the defects that were detected.

Statistical Analysis and Computer Generation of Spatially Correlated Acoustic Noise

Abstract: In ultrasonic NDE, simulation studies can play an important role in complimenting experimental validation of techniques under development. The utility of such simulations depends, in part, on the degree to which the simulated defect and noise signals are representative of the measured signals. In this paper, we describe an approach for generating simulated acoustic noise with a spatial correlation coefficient distribution and maximum extreme value (MEV) distribution which matches those distributions for measured acoustic noise. The procedure for generating noise signals is outlined for a line scan and for a raster scan. The basic approach forces the correlation of neighboring signals to the desired correlation by creating each signal as the sum of appropriately scaled neighboring signals plus a new random signal. For the line scan where each interior position has only two neighbors, this process is done sequentially without iteration. For the raster scan where each interior point has four nearest neighbors, iteration is required to simultaneously achieve the desired correlations with row and column neighbors. The MEV distribution is controlled in an outer iterative loop with the shape and position of the distribution dictated by spectral content of the noise signals and by controlling the signal energy, respectively. Results are shown which demonstrate the effectiveness of the approach. With this approach, a limited number of measured signals can be used to establish the correlation coefficient and MEV distributions which drive the computer generation of a large number of simulated acoustic noise signals.

A Backscatter Radiography Simulation Study

Abstract: Backscatter radiography has become a well known inspection technique for the cases where the inspection is to be carried out only for one sided configurations. Compared to the conventional projection radiography, the backscatter radiography has more parameters to be adjusted for a well implemented inspection procedure. The interrogating beam and detector collimation, beam and detector orientation and the distance to the target surface are among those parameters that need to be adjusted. The adjustment of these parameters is not straightforward. A simulation based approach can provide good insight into adjusting the parameters. In this article, we show how simulations can be used for understanding the impact of various parameters. In doing that, we also document how the deterministic methods can prove to be very beneficial since the computations can be done only once and the scattered flux can be computed for various detector configurations without going back to transport computations. Some computational results are provided.

Transient Elastic Impact Response of Slender Graphite Rods

Abstract: Graphite rods are manufactured by extruding the mixture of calcined petroleum coke and coal tar pitch into the desired shape and baking the cooled specimens at about 800°C. Cracking can occur in rods during the manufacturing process. It is useful to be able to detect the presence of such cracks in the rods prior to their being machined and put into use as electrodes or cathodes or thermal insulator. In an effort to develop a nondestructive testing approach to evaluation of the rods, transient elastic impact was determined for slender rods. Theory for solid, slender rods provided an important starting point for this work. Subsequently, numerical models were developed and simulation was used to determine the response of rods containing cracks. Experiments on graphite rods with and without cracks were conducted and the internal condition determined from the recorded signals. The rods were then cut lengthwise to reveal the internal condition and verify the predicted results. In all cases the knowledge gained from simulation allowed for the presence of cracks to be detected.

Effects of Broadband Laser Generated Ultrasound on Array Gain

Abstract: Array gain is a common parameter used in laser phased array research. This paper will present a new parameter called the frequency modulation of laser phased array (FMLPA). The array gain model for laser phased arrays was derived using an assumption that ultrasound from each array member interferes with each other. This would be always true if laser generated ultrasound is narrow band. However, laser generation of ultrasound is broad band. Broad band ultrasound signals have short duration in the time domain. If the time delay between generated wave fronts from each array member is longer than the duration of the broad band ultrasound signal from each array member, the ultrasound signals from each array member will not interfere with each other. The time delay between generated wave fronts from each array member is 0 s at a laser phased array’s beam steering angle and increases away from the beam steering angle. Therefore, ultrasound from each array member always interfere at angles close to the beam steering angle. However, ultrasound from each array member may not interfere at angles away from the beam steering angle depending on the time delay between generated wave fronts and duration of the broad band ultrasound signal. A theoretical model of the FMLPA was developed and experimentally verified for use when ultrasound from each array member does not interfere with each other. It was experimentally verified that current array gain equations still apply when ultrasound from array members interfere with each other. The FMLPA can be used to create new techniques for measuring weld penetration depth, crack location, and dimensions of objects.