Naotake Ohtsuka

Application of fracture surface topographic analysis technique to thermal fatigue crack extension in stainless steel vessel

Abstract The fracture surface topographic analysis (FRASTA) technique is applied to investigate the propagating process of fatigue crack in a model vessel made of type 304 stainless steel due to repetitive thermal shocks for 1000 cycles. FRASTA is a procedure for reconstructing the process of crack extension by comparing topographic features of coupled areas of opposing fracture surfaces via computer. Based on the result, fracture mechanics analysis is made to estimate the relationship between crack depth and applied number of thermal cycles. During the thermal transient test, acoustic emission (AE) is monitored using four channel AE sensors. The estimated number of initiating cycles and propagating behavior of macroscopic crack by fracture mechanics analysis is compared with the result of AE monitoring examination, and the usefulness of the FRASTA technique is discussed.

What Kind of Capsule Endoscope Is Suitable for a Controllable Self-Propelling Capsule Endoscope? Experimental Study Using a Porcine Stomach Model for Clinical Application (with Videos)

Background We have been developing the Self-Propelling Capsule Endoscope (SPCE) that allows for controllability from outside of the body and real-time observation. What kind of capsule endoscope (CE) is suitable for a controllable SPCE is unclear and a very critical point for clinical application. We compared observing ability of three kinds of SPCEs with different viewing angles and frame rates. Methods Eleven buttons were sewed in an excised porcine stomach. Four examiners controlled the SPCE using PillCamSB2, -ESO2, and -COLON2 (Given Imaging Ltd., Israel), for 10 minutes each with the aim of detecting as many buttons and examining them as closely as possible. The ability to find lesions was assessed based on the number of detected buttons. The SPCE-performance score (SPS) was used to evaluate the ability to examine the lesions in detail. Results The SPCE-ESO2, -COLON2, and -SB2 detected 11 [interquartile range (IQR): 0], 10.5 (IQR, 0.5), and 8 (IQR, 1.0) buttons, respectively. The SPCE-ESO2 and -COLON2 had a significantly better ability to detect lesions than the -SB2 (p < 0.05). The SPCE-ESO2, -COLON2, and -SB2 had significantly different SPS values of 22 (IQR, 0), 16.5 (IQR, 1.5), and 14 (IQR, 1.0), respectively (p < 0.05 for all comparisons; SPCE-SB2 vs. -ESO2, -SB2 vs. -COLON2, and -ESO2 vs. -COLON2). Conclusions PillCamESO2 is most suitable in different three CEs for SPCE for examining lesions in detail of the stomach.

Nonincremental Evaluation of Modified J-R Curve

A method to calculate the deformation theory J, J D , which takes into account the influence of crack growth on J, and a modified version of the J-integral, J M , to allow a large relaxation of restrictions on the amount ofcrack extension and specimen configuration were proposed by Ernst et al. In this work, simple and nonincremental formulas for J D , J M , and crack extension, based on the single-specimen key-curve method using blunt U-notched specimens are proposed. The methods are applied to the determination of J-R curve for three-point bend type and compact tension type specimens. The accuracy of the methods in the determination of the J-R curves is discussed, emphasizing the differences between J-R curves for J D and J M .

Combined Effect of Temper and Hydrogen Embrittlement on Threshold for Hydrogen–Induced Fracture in Cr-Mo Steels

This paper presents the results of the research activities of the subcommittee on Hydrogen Embrittlement of the Japan Pressure Vessel Research Council (JPVRC). The combined effect of temper embrittlement and hydrogen embrittlement in Cr-Mo steels is discussed. It has been recognized that Cr-Mo steels used widely in the refining and petrochemical industry are quite susceptible to temper embrittlement. Although the synergetic relation between temper embrittlement and hydrogen embrittlement is a matter of major concern, studies regarding this subject are rarely encountered. Task group VIII (TG8) of the JPVRC conducted fracture toughness tests for three kinds of 2.25Cr-1Mo steels and 2.25Cr-1Mo-0.3V steel. These steels were prepared by subjecting them to normalizing, tempering and post-weld heat treatments which simulated actual conditions. Some specimens were embrittled by step cooling (Socal −1 treatment). It was found that the threshold for hydrogen-induced fracture was lowered when the specimen was exposed to pressurized hydrogen gas (15MPa) for 48 hours at 450 °C and there was no marked indication of the synergetic action concerning this embrittlement.© 2012 ASME

The Effect of Mechanical Stress on Barkhausen Noise from Heat-Treated Permalloy Plates

The effect of mechanical stress on Barkhausen noise from 45-permalloy plate is reported for as-rolled plate and plates which are annealed at various temperatures up to 1000 K. The data are compared with data of magnetic permeability, coercive force and Vickers hardness. The Barkhausen noise energy detected has been found to increase linearly with nominal stress. The larger the annealing temperature, the less the rate of increase was observed. On the other hand, the magnetic permeability has been found to decrease linearly with nominal stress in a small stress region. Overall features are explained qualitatively by a model in which tensile stress makes the number of effective pinning sites which have appropriate activation energies for discontinuous motion of domain wall more, while annealing process makes just the number of pinning sites such as defects and grain boundaries less.

Nondestructive evaluation of bonding strength of silver soldered bits for civil engineering works

In this paper the bonding area of silver soldered tungsten cemented carbide bits was evaluated nondestructively by using electric potential and ultrasonic methods and was compared with the bonding strength measured by shearing off test. In the electric potential method, voltage was measured by changing a distance of electric terminals which were connected across the bonding plane. In the ultrasonic method, a transducer was attached to a lubricated tip surface and the amplitude of back surface echo signals was measured. It was concluded that the electric potential method is considered to be applicable as a primary nondestructive inspection of the bits by improving the configuration of tip of the terminals and applying constant contact pressure. The ultrasonic method can be used to certify the caught bits in a net of the primary inspection.

Topographic analysis of fracture surface after fatigue test at elevated temperature.

The fracture surface topography analysis (FRASTA) technique was applied to investigate the fatigue cracking of SUS304 stainless steel specimen at elevated temperatures. The specimen with single edge notch was subjected to 2603 cycles of fatigue at a constant strain range at 600°C. After the test, the topographic features of the conjugate fracture surfaces were compared using the FRASTA technique and fracture mechanics analysis was conducted. The analysis showed that a macroscopic fatigue crack was initiated at around 1000 cycles and then propagated at nearly a constant rate. These features and the expected crack growth rate coincided well with the experimental observation. This indicates that the FRASTA technique is applicable to the estimation of fatigue cracking at elevated temperatures.

Evaluations of KISCC, KIH and subcritical crack growth rate by holding-load and fractography test.

The threshold stress intensity factor, KISCC for stress-corrosion cracking or KIH for hydrogen assisted cracking, is generally determined by one of the cantilever constant-load test, constant-displacement test and rising-load test. In the present paper, problems encountered with these tests, especially on the accuracy, test duration, and/or measurement of crack growth rate, were discussed first. Then the holding-load and fractography test was proposed as a way to improve the existing procedures. An outline of the new testing procedure is as follows: Several precracked specimens exposed in test environments or precharged with hydrogen are loaded constantly at different KI values for a relatively short predetermined test duration. The specimens are then unloaded and separated apart by cleavage fracture or fatigue loading. The amount of subcritical crack growth during the test, ΔaH, is measured by an electron microscopic observation of the fracture surface. The value of KISCC or KIH at the onset of the crack growth and the growth rate are determined from the relation of ΔaH and its rate per test duration to KI value.The test procedure was used to predict the values of KIH and growth rate for hydrogen assisted cracking of 1/2 Mo steel and 2-1/4Cr-1Mo steel. The results indicate that the usual test procedure estimates higher KIH than the proposed one. Thus, it is considered that the reexamination of the existing KISCC and KIH data is required.