Development of Motorized Azimuthal Scanners for Ultrasonic NDE Of Composites

Composites are a material class for which nondestructive material property characterization is as important as flaw detection. Fiber reinforced composite laminates often possess strong in-plane elastic anisotropy attributable to the specific fiber orientation and layup sequence. Many of these elastic anisotropies may be investigated using ultrasound [1–6], among which angular measurements are often used. Hsu et al [3,4] used angular scan of acousto-ultrasonic signals to investigate fiber reinforced composite laminates. By placing and rotating two contact transducers on the same side of crossedplied composite laminates, the angular dependence of the acousto-ultrasonic signal was measured and found to have good correlation with the fiber orientation of the sample. Angular measurement of normal-incident shear wave has also been used to detect errors in layup sequence and ply orientation in both green (before cured) and cured composites [4–6]. The transmitted signals of normal incident shear wave in a “crossed polarizer” configuration were found to be particularly sensitive to ply misorientation and layup sequence in a laminate. For green composites, sandwiched between aluminum delay lines, EMATs (electro-meganetic transducer) were used so that the problem of changing coupling condition during the angular scan was avoided. This technique was believed to hold good potential as a practical NDE tool for detecting layup errors during the manufacturing process [5]

Multiviewing Ultrasonic Transducer System for Flaw Reconstruction

The characterization of failure-initiating flaws in materials and components is an important task in the assurance of structural integrity. Based on ultrasonic signals scattered from the flaw in a number of directions, inverse scattering algorithms may be used to reconstruct the size, shape and orientation of the flaw. One of the more successful methods for elastic wave inverse scattering and flaw sizing has been the inverse Born algorithm in the long and intermediate wavelength regime

Analysis of Ultrasonic Backscatter for Porosity Characterization in Graphite-Epoxy Composites

This paper summarizes recent work on the use of ultrasonic back-scatter for the estimation of porosity levels in continuous-fiber-reinforced, layered graphite-epoxy composites. This work is a continuation of previous work described in Refs. 1–3, which addresses the effect of porosity on (1) azimuthal angle backscatter scans, and (2) the spectral characteristics of backscatter

Ultrasonic Characterization of Porosity in Composites

The determination of levels of porosity is important in the engineering uses of graphite fiber/polymer matrix composites, since the interlaminar shear strength can be greatly reduced by excessive porosity [1]. Research in making nondestructive evaluations using ultrasonics as the probing energy has taken many directions. Hsu [2] has successfully modeled the frequency dependent attenuation to predict porosity levels in composites. Kline [3] has extended the work of Hashsin and Rosen [4] to determine the porosity and fiber volume fraction of composites by solving for the elastic coefficients of the composite structure. The propagation of leaky Lamb waves [5] has also been used to model porosity levels

Application of Shear Waves for Composite Laminate Characterization

The layup sequence in a composite laminate greatly effects its properties. If one ply is misaligned in the layup sequence, it can result in the part being rejected and discarded. At the present time, most manufacturers cut a small coupon from the waste edge and use a microscope to optically verify the ply orientations on critical parts. This can add a substantial cost to the product since the test is both labor intensive and performed after the part is cured. A nondestructive technique which could be used to test the part after curing and require less time than the optical test would be very beneficial, and one that could be performed prior to curing would be extremely desirable. Preliminary tests demonstrate a high probability that the model and tests developed in this paper can be used for characterizing uncured layups as well.</p

Apropriate Ultrasonic System Components for NDE of Thick Polymer-Composites

In certain marine applications, thick polymer-composite materials may have to endure different operating environments than those experienced in traditional aerospace applications. In particular, structures made of such materials may experience very large compressive and bending forces. To prevent in-service failure, appropriate NDE methods and instrumentation are needed to characterize the state of the material. Specifically, in addition to detecting high-contrast anomalies (cracks and delaminations) it may be of interest to determine the pore content, measure the fiber volume, assess the severity of fiber waviness, and the like [1]

Ultrasonic Crack Characterization: A Constrained Inversion Algorithm

Obtaining flaw geometry and orientation information from ultrasonic measurements often involves time consuming scanning and data processing functions. One possible way of reducing the amount of information needed for flaw characterization purposes is to require that the fine details of the flaw not be resolved but instead to obtain the best “equivalent” flaw geometry and orientation that fits a predefined simple flaw shape of unknown size and orientation. Hsu et al. [1] have implemented such a procedure for isolated voids and inclusions by using the Born approximation and obtaining a non-linear least squares estimation of the best ellipsoid that fits the data. Here, we will demonstrate that a similar constrained inversion procedure can be developed using the Kirchhoff approximation [2-4] for a flat elliptical crack. Using the results of our previous paper [5], an explicit coordinate-invariant expression that relates the time difference, At, between the arrival of the waves diffracted from the flashpoints of the crack and the crack shape and orientation is obtained. This expression, together with At measurements in different scattering directions is placed into a regression analysis to obtain a set of equivalent flat elliptical crack parameters. A series of tests of this method using noisy synthetic data are considered and the sensitivity of the results to number of measurements and the viewing aperture of the transducer set-up is discussed

Development of Ultrasonic Inspection for a Bonded Superalloy Blade

Directionally solidified multigrain and single crystal airfoils have been used in aircraft gas turbines for over ten years and are currently found in aircraft engine-derivative gas turbines used for land-based power generation. However, the adoption of this technology for large land-based gas turbines is just underway and is not a simple scale-up. One approach to produce the large blades required for this application involves casting the blade in two separate halves and then bonding these halves together using the transient liquid phase bonding (TLPB) process [1]. This process results in a number of internal bond surfaces at the ribs. The condition of these bond surfaces must be determined prior to the blade entering service.</p

Validating pore size estimates in a complex microfiber environment on a human MRI system

PURPOSE: Recent advances in diffusion-weighted MRI provide "restricted diffusion signal fraction" and restricting pore size estimates. Materials based on co-electrospun oriented hollow cylinders have been introduced to provide validation for such methods. This study extends this work, exploring accuracy and repeatability using an extended acquisition on a 300 mT/m gradient human MRI scanner, in substrates closely mimicking tissue, that is, non-circular cross-sections, intra-voxel fiber crossing, intra-voxel distributions of pore-sizes, and smaller pore-sizes overall. METHODS: In a single-blind experiment, diffusion-weighted data were collected from a biomimetic phantom on a 3T Connectom system using multiple gradient directions/diffusion times. Repeated scans established short-term and long-term repeatability. The total scan time (54 min) matched similar protocols used in human studies. The number of distinct fiber populations was estimated using spherical deconvolution, and median pore size estimated through the combination of CHARMED and AxCaliber3D framework. Diffusion-based estimates were compared with measurements derived from scanning electron microscopy. RESULTS: The phantom contained substrates with different orientations, fiber configurations, and pore size distributions. Irrespective of one or two populations within the voxel, the pore-size estimates (~5 μm) and orientation-estimates showed excellent agreement with the median values of pore-size derived from scanning electron microscope and phantom configuration. Measurement repeatability depended on substrate complexity, with lower values seen in samples containing crossing-fibers. Sample-level repeatability was found to be good. CONCLUSION: While no phantom mimics tissue completely, this study takes a step closer to validating diffusion microstructure measurements for use in vivo by demonstrating the ability to quantify microgeometry in relatively complex configurations

NDE Requirements for Thick Marine Composites

There is an increasing interest in the use of thick (more than 25 mm) polymer-matrix composite materials in marine structures. At the present time, the U.S. Navy is employing state-of-the-art polymer-matrix composite materials in the construction of submarine bow domes and is interested in the use of more-advanced materials in submarine pressure hulls and specialized sonar transducers. Concurrently, there appears to be considerable interest in the commercial sector, particularly in off-shore construction, where polymer-matrix composites are used in buoyancy materials, ropes and risers