Gary L. Burkhardt

Detection and Analysis of Electric-Current Perturbation Caused by Defects

The electric-current perturbation technique consists of establishing an electric-current flow in the material to be examined and then detecting localized perturbations of this current flow at inhomogeneities such as inclusions or cracks. The current perturbation is sensed by using a small noncontacting probe to detect the associated magnetic-flux perturbation at the surface of the specimen. In the work reported here, analysis of the electric-current perturbation signals was based on an analytic solution for the change in current density caused by a slot of infinite length and finite depth. Comparisons were made with experimental data for electric-discharge machining (EDM) slots of various depths in Incoloy 901 where the electric current was introduced by ohmic contact. Excellent agreement between theory and experiment was obtained for deep slots, although the model predicts a more rapid decrease in signal amplitude with decreasing slot depth than is experimentally observed. Additional measurements made with fatigue cracks show that cracks as small as 0.45 mm in length are easily detected. It was concluded that the technique is not only a sensitive method for flaw detection, but also offers the possibility of determining flaw characteristics through analysis of signal shapes and amplitudes.

Nondestructive measurement of stress in ferromagnetic steels using harmonic analysis of induced voltage

Abstract A method for nondestructively measuring mechanical stresses in ferromagnetic steels was explored by applying a sinusoidal magnetic field to the materials and analysing the harmonic content of the resulting voltage induced in a sensing coil. The uniaxial stress dependence of the third harmonic amplitude was investigated along with the feasibility of separating stress effects from the effects of variations in material properties such as hardness, grain size and tempering temperature. The results obtained from SAE 4340 steel and AISI 410 stainless steel with various grain sizes, hardnesses and yield strengths showed that it is feasible to reduce the effects of material property variations by using the anisotropy in the harmonic amplitude as a stress indicator and thus to measure stresses in components made of ferromagnetic steels.

Conformable Eddy Current Array for Mapping External Pipeline Corrosion

In a new project sponsored by the U.S. Department of Energy’s National Energy Technology Laboratory, feasibility is being determined for a convenient, affordable method to map corrosion on the outside surface of pipelines. The goal of the project is a system that can produce a contour map of a corroded area and be easily deployed in the field by pipeline maintenance personnel. The collected data will support assessment algorithms such as B31G and RSTRENG (Remaining Strength), which rely on three-dimensional corrosion sizing. The array will be made up of multiple eddy current sensors, scanned electronically to collect data that represent local wall-loss measurements. The work is being performed at Southwest Research Institute and is building upon the results of an earlier project concerned with graphitic corrosion in cast iron pipe. Technical assistance is being provided by Clock Spring Company, who is cofunding the project and planning to commercialize the system if it proves feasible. This paper presents the basic measuring technology, results of laboratory testing of breadboard coils and a description of the proposed field operating procedure.Copyright

Relationship between reflected signal amplitude and defect size in rope inspection using a transverse-impulse vibrational wave

Abstract The relationship between defect size (reduction in cross-section and length) and reflected signal amplitude of a transverse-impulse vibrational wave in a rope was investigated experimentally. Results obtained from a 4.8 mm diameter wire rope with simulated defects of various sizes agreed well with the theoretical reflection coefficient. The lower boundary of the detectable defect size and a possible means of further extending the boundary are also discussed.

Eddy Current Probe Design for Second-Later Cracks under Installed Fasteners

The United States Air Force has an operational need to reliably detect second-layer cracks around fastener holes in two-layer airframe structures with the fasteners in place. Because access to the second layer is usually not available, the inspection must be performed by placing a probe on the outer surface of the structure and detecting cracks through the first layer. Eddy current methods have been applied to this inspection problem [1–6], and have met with some success; however, much improvement is still needed to achieve the desired sensitivity to cracks and rejection of signals caused by the geometry of the structure under inspection.

Feasibility of nondestructive evaluation of synthetic or wire ropes using a transverse-impulse vibrational wave

Abstract The feasibility of the nondestructive evaluation (NDE) of wire and synthetic ropes using a transverse-impulse vibrational wave was investigated experimentally. The experiment was conducted using a 2.38 mm diameter stainless-steel wire rope and a 6.35 mm diameter double-braided nylon rope. Each sample was approximately 14 m long. Broken strands produced readily detectable partial reflection of the wave. The velocity of the wave propagation followed the well-known equation ( F/M) 1 2 , where F is the applied load and M is the mass per unit length of the rope. The results indicate that the transverse-impulse vibrational wave method can provide a fast and simple means of detecting flaws and determining both the load level in and the average mass per unit length of a rope.

REALTIME MONITORING OF PIPELINES FOR THIRD-PARTY CONTACT

Third-party contact with pipelines (typically caused by contact with a digging or drilling device) can result in mechanical damage to the pipe, in addition to coating damage that can initiate corrosion. Because this type of damage often goes unreported and can lead to eventual catastrophic failure of the pipe, a reliable, cost-effective method is needed for monitoring and reporting third-party contact events. The impressed alternating cycle current (IACC) pipeline monitoring method consists of impressing electrical signals on the pipe by generating a time-varying voltage between the pipe and the soil at periodic locations where pipeline access is available. The signal voltage between the pipe and ground is monitored continuously at receiving stations located some distance away. Third-party contact to the pipe that breaks through the coating changes the signal received at the receiving stations. In this project, the IACC monitoring method is being developed, tested, and demonstrated. Work performed to date includes (1) a technology assessment, (2) development of an IACC model to predict performance and assist with selection of signal operating parameters, (3) investigation of potential interactions with cathodic protection systems, and (4) experimental measurements on operating pipelines. Based on information recently found in published studies, it is believed that the operation of IACC on a pipeline will cause no interference with CP systems. Initial results on operating pipelines showed that IACC signals could be successfully propagated over a distance of 3.5 miles, and that simulated contact can be detected up to a distance of 1.4 miles, depending on the pipeline and soil conditions.

NDE OF AGING AIRCRAFT STRUCTURE USING ORTHOGONAL-AXIS EDDY CURRENT PROBES

Detection of subsurface cracks around fasteners and hidden corrosion in multi-layer aluminum structure is a critical requirement for aging aircraft inspection. Of particular concern is the Navy P-3 aircraft which is currently undergoing refurbishment In this aircraft, multi-layer structure with thicknesses up to 0.3 inch must be inspected nondestructively to identify areas in need of repair. In order to meet productivity requirements, eddy current testing (ECT) using the McDonnell Douglas MAUS III system will be performed.

Nondestructive Testing of Ropes Using the Transverse Impulse Vibration Method

Rope consists of a group of strands of fibers or wires, twisted or braided together in various constructions and sizes [1,2]. Fibers used for fabrication of synthetic ropes are made of various polymers including nylon, aramid, polyester, and polyethylene. Wires used for fabrication of wire ropes are typically made of carbon steel. Other metals such as stainless steel and monel are also used for wire materials.

Probability of Detection of Flaws in a Gas Turbine Engine Component Using Electric Current Perturbation

In an exploratory development program, the Electric Current Perturbation (ECP) method was optimized for inspection of typical F-100 gas turbine engine components (disks and seals).1,2 A primary objective was to achieve high reliability for the detection of flaws (fatigue cracks) at the retirement-for-cause (RFC) target flaw size of 0.010 in. long x 0.005 in. deep. Probability of detection (POD) data for surface flaws in blade slots of an F-100 first stage fan disk were estimated from experimentally determined probability density functions (PDF’s) for background and flaw signals. The POD as a function of flaw size was estimated from these data.