Location

San Diego, CA

Start Date

1-1-1985 12:00 AM

Description

Electromagnetic fields associated with surface cracks in metals have been widely studied in recent years in connection with eddy current and a.c. field techniques for detecting and sizing fatigue and corrosion cracks. In our own work, studies have been made of thin-skin fields for cracks of various plan forms using unfolded field techniques 1,2,3, and of the two-dimensional crack (i.e. the large aspect ratio crack) in a semi-infinite material when the ratio of crack depth to skin depth is arbitrary4,5. These studies were prompted by the need to obtain accurate interpretation of surface probe readings from an a.c. potential difference instrument which was developed at University College London and is marketed under the name Crack Microgauge. The instrument was originally designed to operate at a frequency of 5.6 kHz, but further development has produced a dual-frequency version with a second and lower frequency of 560 Hz. This enables an operator to make a choice of skin depth § to suit his particular test material and crack geometry. A reduction in the frequency has the effect of increasing § which can be practically useful in several ways. For example in examining for subsurface flaws it is necessary to increase the penetration of the field into the material so that it reaches to the depth of the flaw. A further application which provides the motivation for the present paper is to the situation where a crack occurs on one face of a plate of known thickness. There are many instances, for example in investigating structures fabricated from sheet material, or in the examination of pipe-work, where it would be useful to be able to detect and measure cracks on the inside surface by testing on the outside. Testing from the blind side in this manner evidently requires that the frequency should be low enough for the skin depth to be comparable with the plate thickness. In this paper our main aim is to report the results of a theoretical and experimental study of the field in a plate which has a two-dimensional normal crack on one face. The theory allows arbitrary values of the ratio of skin depth § to plate thickness b. In particular we examine the signal on the face opposite to the crack which would enable the presence of the crack to be detected.

Book Title

Review of Progress in Quantitative Nondestructive Evaluation

Volume

4A

Chapter

Chapter 2: Eddy Currents

Section

Modeling

Pages

349-357

DOI

10.1007/978-1-4615-9421-5_39

Language

en

File Format

Application/pdf

Share

COinS
 
Jan 1st, 12:00 AM

Developments in Surface Crack Detection by the A.C. Field Technique

San Diego, CA

Electromagnetic fields associated with surface cracks in metals have been widely studied in recent years in connection with eddy current and a.c. field techniques for detecting and sizing fatigue and corrosion cracks. In our own work, studies have been made of thin-skin fields for cracks of various plan forms using unfolded field techniques 1,2,3, and of the two-dimensional crack (i.e. the large aspect ratio crack) in a semi-infinite material when the ratio of crack depth to skin depth is arbitrary4,5. These studies were prompted by the need to obtain accurate interpretation of surface probe readings from an a.c. potential difference instrument which was developed at University College London and is marketed under the name Crack Microgauge. The instrument was originally designed to operate at a frequency of 5.6 kHz, but further development has produced a dual-frequency version with a second and lower frequency of 560 Hz. This enables an operator to make a choice of skin depth § to suit his particular test material and crack geometry. A reduction in the frequency has the effect of increasing § which can be practically useful in several ways. For example in examining for subsurface flaws it is necessary to increase the penetration of the field into the material so that it reaches to the depth of the flaw. A further application which provides the motivation for the present paper is to the situation where a crack occurs on one face of a plate of known thickness. There are many instances, for example in investigating structures fabricated from sheet material, or in the examination of pipe-work, where it would be useful to be able to detect and measure cracks on the inside surface by testing on the outside. Testing from the blind side in this manner evidently requires that the frequency should be low enough for the skin depth to be comparable with the plate thickness. In this paper our main aim is to report the results of a theoretical and experimental study of the field in a plate which has a two-dimensional normal crack on one face. The theory allows arbitrary values of the ratio of skin depth § to plate thickness b. In particular we examine the signal on the face opposite to the crack which would enable the presence of the crack to be detected.