NDE of lumber and natural fiber based products with air coupled ultrasound

Due to the porous nature of wood and natural fiber based products, conventional fluid or gel coupled ultrasonic inspection is unsuitable. Air‐coupled ultrasonic transmission scanning, being non‐contact, is ideally suited for inspecting lumber, wood and natural fiber based products. We report here several successful applications of air‐coupled ultrasound for the inspection of wood. Air‐coupled ultrasonic scan at 120 kHz can easily detect “sinker‐stock” lumber in which bacterial damage of ray tissue cells had occurred during anaerobic pond storage. Channels in ash lumber board caused by insect bore were imaged in transmission scan. Delamination and material inhomogeneities were mapped out in manufactured wood and natural fiber products including medium density fiberboards, compression molded shredded waste wood with formaldehyde resin, and acoustic panels molded with kenaf fibers. The study has demonstrated some of the capabilities of air‐coupled ultrasound in the NDE of forest products

Damage Tolerance and NDE of Polymeric Composites

As composites are being used as primary load bearing members, the increased understanding of damage tolerance of the material is becoming important. When load is applied to a structure containing a crack or damage, a small area ahead of the crack undergoes very high stress. These stresses cause further damage in this small zone. Initially this damage is at the microlevel: in isotropic materials the microdamage is in the form of slippage and microcracks at grain boundary; in composites the microdamage constitutes of matrix microcracks, matrix-fiber interface cracks and fiber pull out. The size of this damage zone varies depending on the material type and the damage tolerance i.e. energy absorbed by the damage zone is directly proportional to the damage zon

Ultrasonic NDE of Tubing Pinch Welds

Resistance tube closure welds, referred to as pinch welds, are used to reliably seal small stainless steel tubing. The pinch welding process, discussed in detail by Thomas et al [1], involves applying a load to force electrodes together to collapse the tube, sometimes in the presence of confining dies which prevent lateral expansion. Electric current is injected through the electrodes to soften the tube through Joule heating. The resulting hot plastic deformation causes contact and welding of the inner surfaces of the tube

Development of a portable mechanical hysteresis measurement and imaging system for impact characterization in honeycomb sandwich structures

Honeycomb sandwich materials are commonly used for aero‐structures, but because the outer skins are typically thin, 2–10 plys, the structures are susceptible to impact damage. NDI methods such as tap tests, bond testers and TTU ultrasound are successfully deployed to find impact damage, but identifying the type∕degree of damage is troublesome. As the type∕degree of impact damage guides decisions by the maintenance, repair and overhaul (MRO) community regarding repair, the ability to characterize impacts is of interest. Previous work demonstrated that additional impact characterization may be gleaned from hysteresis loop area, as determined from an out‐of‐plane load‐vs‐displacement plot, where this parameter shows a correlation with impact energy. This presentation reports on current work involving the development of a portable hysteresis measurement and imaging system based on an instrumented tapper. Data processing and analysis methods that allow production of the load∕displacement data from a single accelerometer are discussed, with additional reporting of tests of software to automatically vary pixel size during scanning to decrease C‐scans inspection time

Field and Beta-Site Testing of the Dripless Bubbler Ultrasonic Scanner

The Dripless Bubbler technique [1–4] has demonstrated in both laboratory and field trials the ability to identify adhesive disbonds and quantify the metal loss due to corrosion in aircraft fuselage structures. In the latest round of field trials, this technique was successfully applied to aid in characterizing exfoliation corrosion around fasteners in thick wing skins (0.190 – 0.500 inches). In two Beta-site tests, the technique was used to identify delaminations, verify ply drop-offs and evaluate repairs in aircraft composite structures such as rudders, spoilers and flaps

A Novel Fabrication Technique for Prescribed Interior Cracks in a Metal

One of the major concerns in nondestructive evaluation (NDE) is the detection and characterization of cracks in structural materials. In the development of NDE methodologies, laboratory samples containing known flaws are very useful for the experimental verification of models and algorithms. Simulated flaws made to specific design serve to check the correctness of various aspects of an NDE technique under development. In addition, model flaws are also used in the calibration of nondestructive testing equipments. A familiar example is the ubiquitous flat bottom hole test blocks

Characterizing Shear Wave Contact Transducers by Immersion Scanning

Shear wave ultrasonic transducers for contact mode testing are commercially available. These packaged transducers usually contain a circular piezoelectric element and the mounting is such that the polarization direction of the shear vibration is defined by a line passing through the center of the transducer and the electrical connector. For measure ments in isotropic materials, the polarization direction is not important. For quantitative shear wave measurements in anisotropic materials such as composites1–3, it is important to know the polarization direction of the transducer with a good precision. Unfortunately, commercial shear wave transducers are usually not very accurate in their mounting; it is not uncommon to have +/- 10° of error in their polarization direction and much greater errors have also been encountered