= 5 % material loss at 0.4 s after the heat flash. Along with the advantages in data acquisition, the low frequency technique is relatively insensitive to minor surface curvature and to ultrasonic interference effects caused by adhesive bondline thickness variations in the lap splice. With the low frequency technique, interface echoes of the lap splice are not resolved and gating of the signal is unnecessary this in itself makes the technique simple to implement and saves considerable more ยป time in data acquisition. Low frequency, in this context, refers to a wavelength that is greater than the lap splice`s layer thicknesses. C-scans produced by this technique are color representations of the received signal`s peak-to-peak amplitude (voltage) taken over an (x, y) grid. The inspection technique uses a computer-controlled data acquisition system to produce a C-scan image of a radio frequency (RF) waveform created by a low frequency, broadband, focused beam transducer, driven with a spike voltage pulser. The ultrasonic prototype consists of a normal incidence, low frequency inspection technique, and a scanning adapter that allows focused immersion transducers to be operated in a direct contact manner in any inspection orientation, including upside-down. Other effects, such as temperature and potential, as well as the impact of the environment on fatigue crack growth have also been studied.This thesis is a collection of research efforts in ultrasonics, conducted at the Center for Aviation Systems Reliability located at Iowa State University, as part of the Federal Aviation Administration`s ``Aging Aircraft Program.`` The research was directed toward the development of an ultrasonic prototype to inspect the aluminum/adhesive fuselage lap splices found on 1970`s vintage Boeing passenger aircraft. The extent and morphology of the attack in artificial lap joints has been compared to studies of corroded samples from actual aircraft. Laboratory studies have also included exposure tests involving artificial lap joints exposed to various simulated bulk and crevice environments. The statistically significant ions have been used to create a second generation solution. Additional tests have determined the relative importance of each of the detected ions in model solutions used for future predictive tests. Electrochemical analyses of the behavior of AA2024-T3 in these solutions gave corrosion rates of up to 250 microns per year (10 mpy). After determining the species present and their relative concentrations, the resultant solution was reproduced in bulk and electrochemical tests were performed to determine the corrosion rate. Measurements of pH of wetted corroded surfaces indicated an alkaline occluded solution. Over twenty different ions have been detected. CE analysis has been performed on over sixty corrosion product samples removed from both civilian and military aircraft. Capillary electrophoresis (CE) is used to identify the ionic species contained in corrosion product samples removed from fuselage lap splice joints. A protocol for collecting and identifying the chemistry of airframe crevice corrosion has been developed. The objective of this work is to develop a laboratory corrosion test protocol to identify the chemistry to which lap joints are exposed and to develop a model of the corrosion within the joints. Fuselage lap-splice joints are a particularly important structural detail in this regard because of the difficulty associated with detection and measurement of corrosion in these occluded regions. Determination of the Corrosive Conditions Present within Aircraft Lap-Splice Joints The complexity of airframe structure lends itself to damage resulting from crevice corrosion.
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