Refining Automated Ultrasonic Inspections with Simulation Models

Computer models of ultrasonic beams can be used to accurately predict fields radiated from transducers [1,2]. Given these fields and reciprocity relations [3] the responses from reflectors of known shape can be calculated. Often scan sensitivity for an inspection is quantified relative to the response from a flat bottomed hole (FBH). Because the FBH is a simple known shape, a computer simulation with an ultrasonic measurement model [4] can be used to model and refine the inspection

A Comparison of Laser Ultrasonics and EMAT Texture Measurements in Aluminum Alloys

Ultrasonic techniques, which measure elastic anisotropy, have been used to study texture and plastic anisotropy of sheet materials. Ultrasonic velocity measurements can determine the orientation distribution coefficients (ODCs) W400, W420, and W440 which are used to describe crystallographic orientation distributions [1]. For steel sheets, strong correlations have been observed between ultrasonic velocity and the formability parameters r̄ and Δr [3]. The results for aluminum show a relationship between the ODC W440 and the degree of earing [2–3].</p

Mechanical Reciprocity Principles and Ultrasonic Measurement Models

An electromechanical reciprocity relation derived by Auld [1] has become a powerful tool for modeling many ultrasonic NDE experiments. Auld’s relation has also served as the foundation for developing more explicit models of ultrasonic systems, such as the quasi-plane wave measurement model of Thompson and Gray [2], which has been used for a variety of quantitative calibration, classification, and flaw sizing applications. Here, we will develop a relationship similar to that of Auld’s but using simpler mechanical reciprocity relations. One side benefit of this mechanical reciprocity approach will be an explicit statement of the manner in which ultrasonic transducers are mechanically reciprocal to one another.</p

Disbond Detection and Characterization Using Horizontally Polarized Shear Waves and EMAT Probes

Horizontally polarized shear waves offer high sensitivity for inspection of adhesively bonded structures. They produce a strong shear deformation on the adherent-adhesive interface allowing a relatively direct estimation of the adhesive failure (disbonds). Compared to localized conventional point-by-point ultrasonic waves, horizontal shear guided waves can be launched over long distances and larger areas of structural parts can be covered. Guided horizontal shear waves have a multimodal character and do not exhibit mode conversion at interfaces. The first fundamental symmetric shear mode (HS0) propagates non-dispersively in isotropic materials, while higher orders modes are dispersive and behave as guided Lamb waves

Application of Ultrasonic Pod Models

The ability to quantify the reliability of nondestructive evaluation (NDE) inspection techniques is required to integrate inspectability into the component design process. Inspectability is typically evaluated on the basis of the design engineer’s experience and knowledge of NDE. While this approach can yield adequate designs with regard to inspection reliability, the potential for uninspectability remains. There is also the possibility that the designer’s knowledge of the reliability of NDE techniques may be limited to “standard” approaches which may be be inadequate for new component geometries or materials. This could lead the design engineer to imagine that a given component is inadequately inspectable and to redesign the part when the correct solution is either to modify the inspection protocol or to select a different technique. Alternatively, models which predict inspection reliability could be used to weigh the trade-offs and risks associated with selection among candidate NDE techniques to be applied to inspection of a given component design and to identify NDE system configurations for optimal reliability. This approach is, in fact, a key feature of the Unified Life Cycle Engineering concept currently being developed by the Air Force[l]

Tensile Overload and Stress Intensity Shielding Investigations by Ultrasound

Growth of a fatigue crack is modified according to the development of contacts between the crack faces [1,2] creating shielding, thus canceling a portion of the crack driving force. These contacts develop through a number of mechanisms, including plastic deformation, sliding of the faces with respect to each other and the collection of debris such as oxide particles [3]. Compressive stresses are created on either side of the partially contacting crack faces resulting in opening loads that must be overcome in order to apply a driving force at the crack tip. In this way, the crack tip is shielded from a portion of the applied load, thus creating the need for modification [1] of the applied stress intensity range from ΔK = KImax − KImin to ΔKeff = KImax − KIsh. Determination of the contact size and density in the region of closure from ultrasonic transmission and diffraction experiments [4] has allowed estimation of the magnitude of Kish on a crack grown under constant ΔK conditions. The calculation has since [5] been extended to fatigue cracks grown with a tensile overload block. The calculation was also successful in predicting the growth rate of the crack after reinitiation had occurred. This paper reports the further extension to the effects of a variable ΔK on fatigue crack growth. In addition, this paper presents preliminary results on detection of the tightly closed crack extension present during the growth retardation period after application of a tensile overload as well as an observation of the crack surface during reinitiation of growth that presents some interesting questions

Modeling of Ultrasonic Signals from Weak Inclusions

Recent research efforts aimed at improving the detection of hard-alpha inclusions have emphasized the need for accurately modeling the responses from such weakly-reflecting inclusions. The need arises because of the rare natural occurrence of hard-alpha inclusions, and consequently, the lacks of suitable experimental samples. These difficulties lend impetus to the application of signal modeling to augment and extend the experimental data in assessing detectability. Currently, a new approach is being developed for the purpose of predicting time-domain echoes from inclusions of specified morphology. This work is the continuation of our previous study of flat-bottomed holes [1–2] in constructing a methodology for estimating the probability of detection of flaws in titanium alloys based on a combination of physical and statistical models

Analytic Diffraction Corrections to Ultrasonic Scattering Measurements

Ultrasonic theories generally predict a scattering amplitude which relates a spherically spreading, far-field scattered wave to an incident plane wave. In ultrasonic immersion measurements, the frequency and angular dependences of the scattering amplitude are convolved with those of the transmitting and receiving transducers and the propagation through the liquid-solid and solid-liquid interfaces. This paper presents a set of approximate corrections for these effects for the cases of angle beam inspection through planar, spherically curved or cylindrically curved surfaces. The primary parameters in the correction are the function D, which corrects for the diffraction effects occurring during a transducer calibration experiment, and the function C, which describes the on-axis pressure variation of the beam. Values of C and D are available in the literature for the case of a piston transducer radiating into an infinite fluid medium. The major portion of this paper is concerned with the extension of those results to the aforementioned two media problems in which mode conversion, refraction, diffraction, and focussing all play interrelated roles. Results of preliminary experiments to test the corrections are also included

Characterization of Microstructural Effects on Fatigue Crack Closure

The growth of a fatigue crack is modified by the development of contacts between the crack faces1,2creating shielding and thus canceling a portion of the applied load. These contacts develop through a number of mechanisms, including plastic deformation, sliding of the faces with respect to each other and the creation and collection of debris such as oxide particles3. Compressive stresses are created on either side of the partially contacting crack faces resulting in opening loads that must be overcome in order to apply a driving force to the crack tip for growth. In this way, the crack tip is shielded from a portion of the applied load, thus creating the need for modification1 of the applied stress intensity range from ΔK = KImax — KImin to ΔK = KImax — KIsh. Determination of the contact size and density in the region of closure from ultrasonic transmission and diffraction experiments4has allowed estimation of the magnitude of KIsh on a crack grown under constant ΔK conditions. The calculation has since5 been extended to fatigue cracks grown with a tensile overload block. The calculation was also successful in predicting the growth rate of the crack after reinitiation had occurred. This paper reports the results of attempts to define the amount of retardation remaining before reinitiation of crack growth in terms of the parameters used by the distributed spring model

A Generalized Model of the Effects of Microstructure on Ultrasonic Backscattering and Flaw Detection

The influence of microstructure on ultrasonic inspection is well known. Familiar examples include the attenuation of ultrasound due to scattering from grain boundaries and the anisotropies in velocity that are associated with preferred grain orientation. Less commonly discussed are the creation of backscattered noise, which can mask flaw signals, and the modification of transducer radiation patterns, e.g. the modulation of the phase fronts in a beam, which can cause fluctuations in signals reflected from surfaces [1]. The latter influence the measurement of attenuation as well as the strength of signals reflected from flaws. The goal of this work is to develop a unified basis for understanding these phenomena, as can be used in the analysis of the performance of ultrasonic flaw detection systems. Of interest are correlations of noise in time as well as the variance of noise signals (about their mean of zero) and reflected signals (about a non-zero mean).</p