There is a need for innovative methods to provide accurate sizing of discontinuities in structures, to guide maintenance actions and better inform engineering of the structure state. This paper highlights the development of the capability to size the length and depth of cracks in multilayer fastener sites using bolt-hole eddy current (BHEC) techniques. Technical efforts include improvements to model calibration, liftoff compensation, and the inversion process. In addition, an expanded set of surrogate models was developed that address crack sizing for titanium, aluminum, and steel structures across multiple frequencies and for varying hole diameters. A comprehensive crack sizing evaluation study was performed under a wide range of test conditions, demonstrating improved sizing capability over using peak amplitude. Crack length estimates were found to have less error than crack depth estimates, although crack depth is the more critical parameter for informing maintenance actions.

This paper presents progress on the characterization of discontinuities in multilayer fastener sites using bolt hole eddy current techniques with model-based inversion. Recent improvements have been made to the model calibration, liftoff compensation and inversion steps, now addressing crack sizing in titanium, aluminum, and steel for a range of hole diameters and at multiple frequencies. Results are presented from a comprehensive crack sizing evaluation study for a wide range of varying test conditions.

well as the lack of effective data for training supervised machine learning algorithms. This paper presents an unsupervised machine learning approach for calling possible flaw indications based on a combination of expert knowledge, segmentation, and anomaly detection. The algorithm requires no prior labeled training data and performs robust analysis even in the presence of complex, noisy data.

This paper presents progress on the characterization of discontinuities in multilayer fastener sites using bolt hole eddy current (BHEC) techniques with model-based inversion. Improvement to the inversion process have been made, however, some model discrepancy remains resulting in greater inversion error for smaller corner discontinuities. Sensitivity to probe liftoff and the calibration process was also observed during experimental testing, indicating the need to estimate and compensate for varying probe state and certain adjacent material conditions.

A comprehensive approach is presented to perform model-based inversion of crack characteristics using bolt hole eddy current (BHEC) techniques. Data was acquired for a wide range of crack sizes and shapes, including mid-bore, corner and through-thickness crack types, and from both standard eddy current hardware and a prototype BHEC system with z-axis position encoding. Signal processing algorithms were developed to process and extract features from the 2D data sets, and inversion algorithms using VIC-3D generated surrogate models were used for inverting crack size. New model results are presented, which now address the effect of having a corner crack at an edge and a through crack adjacent to two edges. A two-step inversion process was implemented that first evaluates the material layer thickness, crack type and location, in order to select the most appropriate VIC-3D surrogate model for subsequent crack sizing inversion step. Inversion results for select mid-bore, through and corner crack specimens are presented where sizing performance was found to be satisfactory in general, but also depend on the size and location of the flaw.

This paper describes a novel method for determining the specific action to melt the metals, and reports the values for action to melt that measure for several elements and three alloys: aluminum 2024, aluminum 6061, and C27400 brass. We electrically heat small diameter wires (127 $\mu{\rm m}$ ) to the point of vaporization using a slow regime exploding wire experiment. Using high-resolution voltage and current data, we compute the derivative of electrical resistivity with respect to specific electrical action. Features in the plot of this derivative clearly show the onset of melting for many of the materials we tested. We compare our results for copper, silver, aluminum, molybdenum, and titanium to those published by Tucker and Toth in the 1970s. Our data agree with their published values for silver and molybdenum, but not with those for copper, aluminum, and titanium. This paper presents our results and discusses possible reasons for the discrepancies between some of our measurements and those of Tucker and Toth.

The objective of this paper is to present a comprehensive approach to image processing for Giant Magnetoresistive (GMR) array sensors with sheet current sources for the inspection of fatigue cracks in complex metallic structures. The approach incorporates optimal phase angle adjustment, array sensitivity compensation, fastener site identification, an adaptive edge evaluation and removal algorithm and a classification metric. Processing examples are presented that highlight the benefits of these algorithms for improving crack detection for challenging edge and fastener spacing conditions.

Giant Magnetoresistive (GMR) sensing arrays have been developed to detect fatigue cracks in thick, multi-layered metallic structures. As part of a program conducted by the U. S. Air Force Research Laboratory, fatigue crack specimens were fabricated to provide inspection targets for a GMR array. These specimens were mounted to simulate a wing structure and inspected using a Boeing Mobile Automated Scanner (MAUS). Probability of Detection (POD) from inspections and the results of capability studies are presented.

This paper describes temperature measurements made on the high-energy medium-caliber launcher at the Institute for Advanced Technology. Simulations performed in Maxwell 3-D and E-Physics showed that Joule heating from current diffusing into the rails accounts for most of the temperature rise in the conductors. Temporal skin effects increase thermal dissipation significantly over what would be expected by the ohmic losses under fully diffused conditions. Based on this analysis, Joule heating is the overwhelmingly dominant source of heating in low-speed tests. As the velocity of the armature increases, Joule heating remains the dominant source of heat; however, additional mechanisms-which may include frictional heating, arcing energy, aluminum deposition, and temperature-dependent properties-are required to more satisfactorily explain the temperature profile obtained.

In solid-armature railguns, hypervelocity gouging damage can significantly limit the useful life of rails. Most solutions to this problem have involved the use of hard rail materials, which tend to be poor electrical conductors. However, it has also been observed that precoating the conductors of a railgun can significantly delay the onset of gouging. This is usually accomplished by launching several aluminum-alloy armatures on a set of rails below the gouging threshold. When an aluminum-alloy armature is launched at high speeds on copper-alloy rails, a thin (tens of microns) layer of aluminum is deposited on the surface of the rail. Subsequent tests at higher speeds often result in the delay or absence of gouging at speeds where it is normally expected to occur. However, this effect is not particularly robust, since gouging can still occur with sufficient lateral loads. In the experiments reported here, tests were conducted on UNS C15725 (Glidcop Al-25) copper rails to examine the effect of a controlled precoating applied with an electroplating process. Aluminum thicknesses of 2, 5, 25, and 50 μm were tested, providing a well-characterized aluminum layer as opposed to the rough variable-thickness layer typically deposited with an aluminum armature. Results of the experiments and analysis are reported in this paper.

The Institute for Advanced Technology has been conducting experiments on plasma-armature railguns for the past several years. To control the damage mechanisms associated with this class of railgun, an electrothermal (ET) launcher preinjects the projectile into the plasma railgun bore milliseconds before the arc is energized. During the initial testing of the ET launcher, it was found that injection velocities were significantly lower than expected for a given input energy. Subsequent experiments using photonic Doppler velocimetry to resolve velocity as a function of time confirmed this finding. Using an ablative capillary discharge code written by Powell and Zielinski in 1991, the velocity discrepancy is analyzed, and the properties of the propellant gas-such as ionization fractions, the mass of the propellant gas, and the magnitude and decay of the pressure pulse seen by the projectile-are determined.

Material ejected from aluminum armatures at the rail-armature interface has been identified as a mechanism that degrades both rails and insulators in a railgun, significantly reducing the bore lifetime. With the goal of controlling the onset of armature ejecta, a series of single-shot tests was conducted in a small railgun with a bore cross section of 22 × 44 mm. The tests utilized channels of various sizes and geometries machined into the rail contact surface of the armatures to see if ejecta could be controlled. These tests identified several channel patterns as having the potential to delay the onset of armature ejecta. A series of multiple-shot tests was subsequently conducted in a larger railgun having a bore cross section of 38 × 76 mm. The goal of these experiments was to see if the channel patterns that delayed armature ejecta had a significant impact on rail erosion at start-up. Three test series were conducted. In the first series, three armatures with a nested circular channel pattern were tested. In the second series, three armatures with a large centrally located channel were tested. Both armature designs used equivalent contact areas. These results were then compared to a standard armature contact face with no modifications in a third test series.

This paper describes a series of experiments to study the efficiency of an electrothermal (ET) launcher. The launcher serves as a pre-accelerator for a plasma-driven hypervelocity railgun experiment at the Institute for Advanced Technology. The ET launcher is designed to accelerate small polycarbonate projectiles to about 1 km/s. The objective of this study was to understand how pulse duration affects the efficiency with which the pressure of the ET discharge is coupled to the projectile. The study consisted of multiple tests for which the total energy of the discharge was kept constant, but the pulse length was varied. The dependent variable in the tests was the muzzle velocity of the projectile. The experimental results are presented and compared with gas-dynamic considerations.

A large-caliber railgun was developed to demonstrate the supersonic launch of 120 mm projectiles. A trade study that evaluated over 70 different railgun configurations and geometries resulted in the selection of a high-inductance-gradient, multi-turn configuration as the best overall choice. Subscale tests were performed on both the railgun and launch package configurations. A full-scale laboratory system was installed that included a dedicated high-current, large-caliber breech and gunline. A full-scale launcher was successfully tested to beyond the design conditions.

Summary form only given. This paper reports on the design and fabrication efforts of a supersonic, high-mass electromagnetic launcher. The program goal was to efficiently accelerate an existing M933/934 mortar round to over 400 m/s. The basic electromagnetic architecture chosen was a railgun, with a detailed system study undertaken to determine the optimal railgun configuration. Operational limits required that the overall launch package have a mass of less than 18 kg and a length of less than one meter. The peak acceleration was required to be less than 10 kG, and no launch package components could be discarded in flight. In addition, the program required a laboratory demonstration, which when coupled with the schedule and budget constraints, required that the existing IAT electromagnetic launch facility (ELF) be capable of providing the prime power.

Summary form only given. Research in the area of plasma armature railguns is currently underway at the Institute for Advanced Technology (IAT) as part of an Air Force MURI. The program is aimed at investigating the possible use of an electromagnetic launcher for the rapid and affordable launch of microsatellites (~1 to 10 kg) into low-earth orbit. In the experiment, the IAT is developing a plasma-driven railgun to launch low-mass projectiles of roughly 7 g to a velocity in excess of 7 km/s. To accomplish this goal requires overcoming the problem of bore ablation, which has been linked to an observed velocity ceiling of about 6 km/s in plasma armature launchers. Bore ablation is a direct consequence of the intense heat radiated by plasma armatures. Controlling bore ablation requires a coordinated approach that includes: 1. using magnetic augmentation to reduce power dissipation in the plasma, 2. using high-purity alumina insulators to raise the ablation resistance of the bore, 3. using pre-acceleration to prevent ablation of the bore materials at low velocity, and 4. using a synchronously driven, distributed power supply to electrically isolate stages. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and results obtained from the hardware that has been designed and tested so far.