Hidden Corrosion Research Articles

Pulsed Eddy-Current Measurements of Corrosion and Cracking in Aging Aircraft

ABSTRACTPulsed, or transient eddy-current methods are an effective tool for quantitative characterization of hidden corrosion and cracking in multi-layer aircraft structures. Eddy currents are the method of choice for this task, since they penetrate multiple layers of metal, whether or not the layers are mechanically bonded. The pulsed eddy-current technique is an important advance over conventional eddy-current methods because it rapidly acquires data over a wide range of frequencies, thereby providing more information than a conventional, single-frequency eddy-current instrument. We have combined a pulsed eddy-current instrument with a portable two-axis scanner to produce an instrument capable of rapidly scanning aircraft lap splices in situ, producing pseudo-color images that reveal hidden corrosion or cracking. A unique feature of time-domain eddy-current data is the ability to selectively filter clutter from the image by time-gating the pulsed signal. Time-gating permits the user to select the inspection depth, thereby eliminating interference from upper layers, air gaps, lift-off variation and fasteners. By using a theoretical model of the pulsed eddy-current system, it is possible to interpret the data quantitatively, yielding quantitative maps of corrosion damage. Some of the same advantages of the pulsed eddycurrent technique apply to the characterization of hidden fatigue cracks as well, although the tieory for crack signals is less advanced.

Techniques for depth-selective, low-frequency eddy current analysis for SQUID-based nondestructive testing

Conventional eddy current techniques are widely used for detection of surface-breaking cracks in metal structures. These techniques have limited success in the detection of deep, nonsurface-breaking flaws that require low frequency eddy currents, for which inductive pick-up probes have drastically reduced sensitivity. High resolution, Superconducting QUantum Interference Device (SQUID) magnetometers, which are very sensitive to do or low frequency magnetic fields, have been developed for detection of subsurface flaws. We have now extended SQUID NDE by utilizing a sheet inducer to produce an extended eddy current parallel to the surface in a conducting plate. The magnitude of the induced current density inside the plate reduces with the depth; however, the current component at a certain phase angle may increase with the depth. At a particular phase angle, the current density on the surface becomes zero, while the current inside the plate is large, so that the magnetic signal at that phase angle due to the surface structures can be minimized. With this method, we have detected simulated cracks in the sides of plugged holes in a thick plate, a hidden corrosion area in a specimen which consisted of two painted aluminum plates joined with sealant, as well as crack defects adjacent to fasteners in the second layer of lap joined aluminum plates. We present a theoretical model for simulation of the phase-related magnetic signal due to a flaw, which shows the relation between the phase angle and the depth of the flaw. The theoretical phase analysis is compared with the experimental results.

LASER-GENERATED LAMB WAVES TO DETECT HIDDEN INTERFACIAL CORROSION IN THIN-SKIN AIRCRAFT COMPONENTS

Abstract One of the major challenges in extending the life of ageing aircraft is to develop improved nondestructive inspection techniques for the rapid large-area detection of various types of hidden corrosion in aircraft components. In this paper, a new one-sided ultrasonic technique is proposed to solve the generic problem of detecting hidden corrosion in thin metallic aircraft skins, where the corrosion has originated at the interface between the skin and a second layer in an aircraft component. The technique involves the use of laser-generated ultrasonic Lamb modes whose attenuation in a thin skin is increased by the presence of corrosion. This paper presents a successful demonstration of the technique on a thin-skin elevator component. The potential of the Lamb wave technique for rapid large-area scanning for hidden corrosion is also described.

Detection of hidden corrosion damage in aviation structures by the eddy current method

Detection of hidden corrosion damage in aviation structures by the eddy current method

A method of eliminating fretting corrosion in metal fatigue testing

1. We have developed a method of eliminating the hidden corrosion which appears in the fatigue testing of metals. 2. This method differs from earlier methods in its simplicity and reliability at temperatures between 20 and 800 °C; it has been used for fatigue tests with symmetrical and asymmetrical loading cycles on samples of various metals and has yielded reliable results, without undue waste of time, on bases of up to 108−109 cycles at temperatures of up to 800 °C.

CORROSION PREVENTION ASPECTS OF PACKAGING EXPORT MACHINERY

Whether a company sends its machines to export packers or whether it packs them in its own works, early attention must be given to corrosion prevention. This attention greatly assists the packer in doing his job; hidden corrosion cannot easily be overcome as it cannot be seen at the time of packing.