Jinyi Lee

THE DETECTION PROBABILITY IMPROVEMENT OF THE FAR-SIDE CRACK ON THE HIGH LIFT-OFF USING THE MAGNETIC CAMERA

To evaluate cracks using magnetic flux leakage (MFL), the distribution of magnetic field (DMF) near the crack should be quantified. A narrow spatial resolution of DMF is demanded around the crack. Also the lift-off should increase over several mm to protect the sensors from environments such as high temperature, dust, and vibration. Furthermore, the MFL must be amplified to detect a far-side crack because the MFL will concentrate in the ferromagnetic specimen and will be weak. The magnetic camera and magnetic lens could be useful for high lift-off conditions such as in nondestructive testing of a far-side environment. The far-side crack detection probabilities, Max(∂B/∂x) and RMS(∂B/∂x), were improved by about 20~60% by using the magnetic lens. The crack can be easily detected by using the 1/4RMS algorithm. Finally, an equation that expresses the relationship between the volume of crack and Max(∂B/∂x) is proposed. This equation is based on the principle that the magnetic field is inversely proportional to the square of the lift-off and is useful for determining the crack volume when the lift-off is known.

Theoretical Consideration of Nondestructive Testing by use of Vertical Magnetization and Magneto-Optical Sensor

This paper describes a new magnetization method for non-destructive testing with magneto-optical sensor (denoted as MO sensor) which have the following characteristic; high observation sensitivity, independence of the crack orientation, and precise imaging of a complex crack geometry such as multiple cracks. When a magnetic field is applied normally to the surface of a specimen which is significantly larger than its defects, approximately the same magnetic charge per unit area occurs on the surface of the specimen. If there is a crack in the specimen, magnetic charge per unit area has the same value at the bottom of the crack. The distribution of the vertical component of the magnetic flux density, Bz, is almost uniform over the no-crack area (denoted as BZ, BASE), while the magnetic flux density is smaller in the surroundings of the crack (denoted as BZ, CRACK) : If BZ, BASE is a bit larger than the saturated magnetic flux density of the MO sensor (BS), then small magnetic domains occur over the crack area and a large domain over the non-crack area because BZ, CRACK is smaller than Bs.

Integration of Hall and Giant Magnetoresistive Sensor Arrays for Real-Time 2-D Visualization of Magnetic Field Vectors

In this study, we combined an array of giant magnetoresistive (GMR) sensors with an array of Hall sensors to measure all three components of magnetic vectors with a high spatial resolution and visualize the magnetic field distribution on a 2-D plane. The sensors were arrayed in a matrix arrangement such that the limitation to the number of sensors could be overcome. To test the device, we generated a magnetic field using a pancake-type magnetizer and a cracked magnetized ferromagnetic specimen and imaged the vectors. We obtained a spatial resolution of 0.78 mm in the z-direction with 1024 Hall sensors and 2.34 mm in the x-direction with 100 GMR sensors. Moreover, the x and z components could be imaged at 6 fps. The proposed device is promising for application in magnetic flux leakage detection.

Scan Type Magnetic Camera Images with a High Spatial Resolution for NDT Obtained By Using a Linearly Integrated Hall Sensors Array

Magnetic flux leakage testing (MFLT), which measures the distribution of a magnetic field on the magnetized specimen by using a magnetic sensor such as the Hall sensor, is an effective nondestructive testing (NDT) method for detecting surface crack on magnetized ferromagnetic materials. A scan type magnetic camera, based on the principle of MFLT, uses inclined Hall sensors array on the printed circuit board (PCB) to detect small cracks in high speed. However, the waveforms appear in the direction perpendicular to the scan, because the sensors are bonded at different gradients and heights on a PCB in spite of careful soldering. In this paper, the linearly integrated Hall sensors (LIHS) on a wafer are proposed to minimize these waves and to improve the probability of crack detection. The specimen took from a billet is used to determine the effectiveness of the LIHS in the multiple cracks detection.

Bobbin-Type Solid-State Hall Sensor Array With High Spatial Resolution for Cracks Inspection in Small-Bore Piping Systems

Bobbin coil and bobbin-type Hall sensor arrays are proposed as an alternative for crack inspection inside a small-bore piping system. The cracks can be imaged at high speed without using a scanner since the electromagnetic (EM) field is distorted by the cracks. An array of 32 × 32 Hall sensors with 0.78 mm spatial resolution was set in a cylinder with diameter of 15 mm and length of 24.96 mm. A bobbin coil operating at 5 kHz of alternating current was positioned inside of a piping system and the sensor array outside the cylinder. Distorted EM fields around outside diameter stress corrosion cracking was imaged at 1 frame/s.

Application of a NDI Method Using Magneto-Optical Film for Micro-Cracks

Leakage magnetic flux is occurred in the cracked area of magnetized specimens, and also it changes the magnetic domain area of the magneto-optical film positioned on the specimen. It causes the change of the optical permeability of the magnetic domain on the crack area. So crack images can be obtained easily using this principle. On the other hand, utilizing a laser in this method makes possible to perform a remote sensing by detecting the light intensity contrast between cracked area and normal area. This paper shows the application of non-destructive inspection system taking advantage of magneto-optical method for micro-cracks and presents examples applied to the several types of specimens having fatigue cracks and fabricated cracks using this method. Also the authors prove the possibility of this method as a remote sensing system under the oscillation load considering application to real fields.

Magnetic Leakage Testing Using Linearly Integrated Hall and GMR Sensor Arrays to Inspect Inclusions in Cold-Rolled Strips

Inclusions in steel are entrapped foreign materials, which may be metallic or non-metallic; they may be distributed throughout the volume of steel strips during the rolling process and may propagate as narrow cracks in steel strips. In this paper, for inspecting inclusions in steel strips, we propose the use of magnetic flux leakage testing involving a magnetizer and magnetic field sensor arrays, such as linearly integrated Hall sensors (LIHaS) in differential mode and linearly integrated giant magnetoresistance sensors (LIGiS). In the proposed method, images of the distribution of Hall voltage (VH) and giant magnetoresistance voltage (VG) due to the magnetic leakage field around an inclusion are obtained by means of LIHaS and LIGiS, respectively. The magnetic leakage field images are obtained at different magnetization directions relative to the rolling direction by rotating the magnetizer. Different data processing techniques, such as differentiation, noise reduction, and fusion of image data of the inclusion obtained by LIHaS and LIGiS, are applied to enhance the detectability of the inclusion. The effectiveness of the proposed technique was experimentally verified by detecting actual inclusions situated in the rolling direction in a cold-rolled steel strip.

A hand held magnetic camera system for real time crack inspection

Paramagnetic metals like aluminum alloys are used in aircraft due to their light weight. Magneto-optical eddy current imager (MOI) is a nondestructive magneto-optical technique that is used to inspect fatigue cracks and corrosion in the aluminum alloy of aged aircraft. Unfortunately, the quantitative distribution of distorted magnetic flux leakage (MFL) from crack tips is difficult to measure, and therefore, the shapes and sizes of cracks are difficult to evaluate quantitatively. Additionally, the inspected results vary significantly according to the skill level of the worker. Hand-held magnetic camera using Hall sensor array for crack inspection in paramagnetic materials is developed by research center for real time NDT. Distorted alternate currents (DAC) are produced around the crack tips when the sheet type induced current (STIC) is induced on the specimen in the hand-held magnetic camera system. The alternate MFL resulting from the DAC is detected using a Hall sensor array. The root-mean-squared MFL after the use of high pass filter is converted to a digital signal by an analog-to-digital converter (ADC). And it is stored in a computer, and processed by interface and operation software. An algorithm for the quantitative nondestructive testing and evaluation of cracks of different shapes and sizes on the aluminum alloy, Al7075, is proposed in this paper.

Nondestructive Testing of Train Wheels Using Vertical Magnetization and Differential-Type Hall-Sensor Array

This study proposes a differential-type Hall-sensor array and vertical magnetizers used to improve the detection ability of crack in express-train wheels. In addition, exclusive parallel signal-conditioning circuits, analog-to-digital converting circuits, and interface were developed to reduce measurement time. The permanent magnet applies a vertical magnetic field on the wheel, while the differential-type linearly integrated Hall-sensor array measures the distribution of the magnetic field around a crack. The proposed method was verified by experiments.

Eddy current imager based on bobbin-type hall sensor arrays for nondestructive evaluation in small-bore piping system

A bobbin coil with bobbin-type solid-state Hall sensor arrays was proposed for eddy current testing. A 32×32 matrix of InSb Hall sensors was set on a cylinder 15 mm in diameter and 25 mm long. The distorted alternating magnetic fields around inner diameter (ID) and outer-diameter stress corrosion cracks (ODSCCs)were imaged at 1 fps. The effectiveness of the proposed techniquewas verified with a standard copper alloy specimen with hole-type circumferential ID-and ODSCCs.