Modeling Ultrasonic Beam Propagation in Graphite Composites

A continuing project at the Center for NDE in Ames involves the development of models which predict the probability of detecting flaws using a given inspection system.[1] Our general approach as it applies to through-transmission immersion inspections is as follows. With the two transducers to be used in the inspection, a reference experiment is performed to determine relevant information concerning equipment characteristics and transducer efficiencies. This may be done by placing a calibration specimen into the ultrasonic beam and measuring the time-domain electrical signal in the output cable of the receiver. Using models, we then predict how this received electrical signal would be changed if the calibration specimen were removed and unflawed and flawed components were placed into the beam in turn. The two components are assumed to have identical geometries except for a hypothetical flaw of given type, size, and orientation. If the difference between the predicted output signals for the two components is sufficiently large, compared to system noise, then the hypothetical flaw is said to be detectable

Sensitivity Analysis of the Ultrasonic Response from a Non-Normal Surface-Breaking Crack

The use of computer simulations is becoming an increasing popular strategy for designing ultrasonic inspections. There are many benefits of accurate simulations, the most important one being their cost effectiveness. In many cases, before inspection procedures are finalized, it is possible to simulate the competing inspection plans, and to use the outputs of simulation trials to choose the best plan. This strategy is particularly useful when there is limited accessibility to the components that need to be inspected, as in the extreme case when the inspection procedure requires that operating equipment be removed from service. In such cases, it is best to be fully prepared before taking the inspection equipment to the test site and computer simulations can play an important role in such preparation, often at a significantly reduced cost with respect to traditional methods

Model-Based Software for Simulating Ultrasonic Pulse/Echo Inspections of Metal Components

The use of models to simulate inspections has played a key role in UT NDE R&D efforts. Over the years, a series of wave propagation models, flaw response models, and microstructural backscatter models have been developed at CNDE to address inspection problems of interest. One use of the combined models is the estimation of signal-to-noise ratios (S/N) in circumstances where backscattered echoes from the microstructure (grain noise) act to mask sonic echoes from internal defects. Such S/N models have been used to address questions of inspection reliability, such as how to optimize the choices of transducer properties and inspection design to insure that critical defects are reliably detected. Under the sponsorship of the National Science Foundation\u27s Industry/University Cooperative Research Center at ISU, an effort was initiated in 2015 to repackage existing research-grade software into user friendly tools for the rapid estimation of S/N for ultrasonic inspections of metals. This presentation provides an overview of the ongoing modeling effort, with emphasis on recent developments. The software can now treat both normal and oblique-incidence immersion inspections of curved metal components having equiaxed microstructures in which the grain size varies with depth. Both longitudinal and shear-wave inspections are treated. The model transducer can either be planar, spherically-focused, or bi-cylindrically-focused. A calibration (or reference) signal is required, and is used to deduce the measurement system efficiency function. This can be “invented” by the software using center frequency and bandwidth information specified by the user, or, alternatively, a measured calibration signal can be used. Defect types include flat-bottomed-hole (FBH) reference reflectors, and spherical pores and inclusions. Simulation outputs include estimated defect signal amplitudes, RMS grain noise amplitudes, and S/N ratios as functions of the depth of the defect within the metal component. At any particular depth, the user can view a simulated A-scan displaying the superimposed defect and grain-noise waveforms. The realistic grain noise signals used in the A-scans are generated from a set of measured “universal” noise signals whose strengths and spectral characteristics are altered to match predicted noise characteristics for the simulation at hand. Examples are presented comparing measured and predicted A-scan signals for FBHs in Nickel-alloy components. We also discuss efforts currently underway to generate a simulated C-scans (including grain noise speckle) corresponding to inspections in which the model transducer is scanned above the defect. As will be demonstrated as part of this poster presentation, the software typically requires only a few seconds to complete a simulation when running on a typical laptop computer

Survey of Ultrasonic Grain Noise Characteristics in Jet Engine Titanium

In ultrasonic inspections of titanium billets and forgings, grain noise echoes are routinely observed. These arise from the scattering of the incident sound beam by the metal microstructure, and can limit the detection of small or subtle defects. We report on a survey of grain noise characteristics in fourteen billet and forging specimens supplied by aircraft engine manufacturers. All specimens were examined in a similar manner using a 5-MHz focussed transducer, with pulse/echo noise measurements made through three orthogonal sides of each specimen. Emphasis is placed on describing two related probability density functions (PDF’s) which characterize aspects of the backscattered noise seen in a scanning experiment. The first PDF describes the RF noise voltages seen at a fixed observation time t; the second describes the gated peak-to-peak noise voltages seen for time gates of various durations. The PDF for the RF noise voltages is expected to be Gaussian if a large number (\u3e10) of grains contribute appreciably to the noise at time t [1], but non-Gaussian behavior is seen in some specimens. The use of K-distributions to describe the non-Gaussian cases is examined. This work is in support of efforts described in a companion article [2] to develop methods for predicting gated peak noise (GPN) distributions

Coupling Microstructure Outputs of Process Models to Ultrasonic Inspectability Predictions

The efforts of the materials community can be characterized as the study of the relationship of processing, structure, properties and performance, as schematically illustrated in Figure 1. Added, in parentheses, are quantities of importance when these ideas are applied to ultrasonic NDE. It would be highly desirable if one could start from models of processes such as rolling, casting and extrusion; predict the microstructural features produced, such as grain size or shape, texture (preferred grain orientation), or the two-point correlation of elastic constants (to be discussed later); predict the resulting ultrasonic properties such as velocity v, attenuation a and backscattering coefficient η; and ultimately determine the inspectability of the part. Such a capability would allow NDE to be considered explicitly during the selection of material processing procedures

Baseline UT measurements for armor inspection

Some prototype armor panels are fabricated from several layers of dissimilar material bonded together. These may include ceramics, graphite composites, fiberglass composites and rubber. The ultrasonic properties of these layers influence inspections for armor defects. In this paper we describe measurements of ultrasonic velocity, attenuation, sound beam distortion and signal fluctuations for the individual layers comprising one armor prototype. We then discuss how knowledge of these properties can be used when choosing an optimum frequency for an ultrasonic pitch∕catch immersion inspection. In our case an effective inspection frequency near 1.5 MHz affords: (1) adequate strength of through‐transmitted signals in unflawed armor; (2) adequate lateral resolution for detecting small disbonds at interfaces; and (3) low levels of UT signal fluctuations due to the natural inhomogeneity of certain armor layers. The utility of this approach is demonstrated using armor panels containing artificial disbonds at selected interfaces

Feasibility Study for Detection and Quantification of Corrosion in Bridge Barrier Rails

Technical challenges exist with infrastructure that can be addressed by nondestructive evaluation (NDE) methods, such as detecting corrosion damage to reinforcing steel that anchor concrete bridge railings to bridge road decks. Moisture and chloride ions reach the anchors along the cold joint between the rails and deck, causing corrosion that weakens the anchors and ultimately the barriers. The Center for Nondestructive Evaluation at Iowa State University has experience in development of measurement techniques and new sensors using a variety of interrogating energies. This research evaluated feasibility of three technologies—x-ray radiation, ground-penetrating radar (GPR), and magnetic flux leakage (MFL)—for detection and quantification of corrosion of embedded reinforcing steel. Controlled samples containing pristine reinforcing steel with and without epoxy and reinforcing steel with 25 percent and 50 percent section reduction were embedded in concrete at 2.5 in. deep for laboratory evaluation. Two of the techniques, GPR and MFL, were used in a limited field test on the Iowa Highway 210 Bridge over Interstate 35 in Story County. The methods provide useful and complementary information. GPR provides a rapid approach to identify reinforcing steel that has anomalous responses. MFL provides similar detection responses but could be optimized to provide more quantitative correlation to actual condition. Full implementation could use either GPR or MFL methods to identify areas of concern, followed by radiography to give a visual image of the actual condition, providing the final guidance for maintenance actions

Predicting ultrasonic grain noise in polycrystals: A Monte Carlo model

A Monte Carlo technique is described for predicting the ultrasonic noise backscattered from the microstructure of polycrystalline materials in a pulse/echo immersion inspection. Explicit results are presented for equiaxed, randomly oriented aggregates of either cubic or hexagonal crystallites. The model is then tested using measured noise signals. Average and peak noise levels and the distribution of the noise voltages are studied as the density of grains changes

Modeling the Effects of Beam Size and Flaw Morphology on Ultrasonic Pulse/Echo Sizing of Delaminations in Carbon Composites

The size and shape of a delamination in a multi-layered structure can be estimated in various ways from an ultrasonic pulse/echo image. For example the -6dB contours of measured response provide one simple estimate of the boundary. More sophisticated approaches can be imagined where one adjusts the proposed boundary to bring measured and predicted UT images into optimal agreement. Such approaches require suitable models of the inspection process. In this paper we explore issues pertaining to model-based size estimation for delaminations in carbon fiber reinforced laminates. In particular we consider the influence on sizing when the delamination is non-planar or partially transmitting in certain regions. Two models for predicting broadband sonic time-domain responses are considered: (1) a fast "simple" model using paraxial beam expansions and Kirchhoff and phase-screen approximations; and (2) the more exact (but computationally intensive) 3D elastodynamic finite integration technique (EFIT). Model-to-model and model-to experiment comparisons are made for delaminations in uniaxial composite plates, and the simple model is then used to critique the -6dB rule for delamination sizing