Yasumitsu Tomita

Nondestructive estimation of fatigue damage for steel by Barkhausen noise analysis

Abstract Magnetic Barkhausen noise (BHN) signals are detected when magnetized domain walls move discontinuously in a ferromagnetic material. This non-uniform motion is sensitive to the microstructures of material which vary continuously with the increase of applied stress and loading cycles. In this paper, the possibility of evaluation of residual fatigue life under cyclic loading conditions is discussed based on BHN analysis. The material used in the test is mild steel. Both static loading tests and load-controlled axial pulsating fatigue tests are performed on plain plate specimens. The BHN signals are measured for each increment of tensile load during static tests and for each interval of cyclic loading during fatigue tests. The main results obtained are as follows: (1) When the applied stress range is below the fatigue limit, the BHN signal keeps nearly constant. (2) When the applied stress range is above the fatigue limit, the ratio of the BHN peak voltage of the stress level S to 0 MPa at the Nth cycle, V ns V n 0 , varied clearly with loading cycles. (3) The procedure of residual life prediction is proposed.

Fatigue crack growth behavior under random loading model simulating real encountered wave condition

Abstract The purpose of this study is to clarify a random loading model for fatigue strength analysis of ship structural members, and to examine fatigue crack growth behavior under the above random loading condition. We analyzed data collected from 38 ships that sailed in the North Pacific and 11 ships in the Japan-Indian Ocean over a period of 14 years; and 6 naval ships in the North Pacific near Japan. The main points are as follows 1. 1) the distribution function of long-term wave-induced load and stress can be described by the Weibull distribution function, with an expected shape parameter value close to unit; 2. 2) the time history of wave conditions during service sailing can be divided into two types—calm sea condition and various storm conditions. These two kinds of conditions appear in random order; 3. 3) based on the above results, a random loading model—‘storm model’ for the fatigue design of a ship is proposed. Then, welded joints simulating side longitudinals of ship structural member were tested under constant and storm loading model. Fatigue crack growth behavior such as crack growth rate, retardation phenomenon etc. were examined.

Study on Heat Transfer Between Gas Flame and Plate During Line-Heating Process

This paper discusses how the transient 3-dimensional distribution plate surface of the spot heating gas flame is measured in detail by a high performance L.I.F. measurement system. It has been found that the thermal-flow field within the combustion flame remains almost unchanged regardless of the temperature increase in the steel plate. A new hypothesis on the heat transfer during line heating process is built up based on the results of the L.I.F. experiment. This hypothesis is that the distribution of gas temperature near plate surface and local heat transfer coefficient depend only on the distance from the torch. The appropriateness of this hypothesis is proven by the performance of an inverse heat conduction analysis of a spot heating experiment.

Study on Fatigue Crack Initiation Process by Using Crystalline FEM analysis (2 nd Report)

A crystalline FEM theory which incorporates the Bausingers effect is applied to investigate the relationship between the crystal direction and the progress of plastic deformation localization and irreversible slip generation, which are part of the fatigue crack initiation process, by calculating the cyclic deformation behavior of a f.c.c. crystal with high Schmids factor which is buried in a large crystal with low Schmids factor.As results, the followings are found : 1) In the same manner as the results for f.c.c. single crystal analyzed in the previous reports, plastic deformation localization and irreversible slip generation in the course of loading history can be simulated by using the crystalline FEM code developed in the previous reports when the buried crystal is set in the specific direction.2) Plastic deformation localization and irreversible slip generation appears only when all of the following conditions are satisfied : a) direction of crystal is set so that the coplaner or non-coplaner double slip occurs ; b) the slip directions of two activated systems meet the free surface at the same angle ; c) the slip planes of two systems meet the free surface at the same angle.3) The properties of cyclic deformation behavior in the coplaner double slip cases are affected more seriously by the deviation of loading direction than the ones in the non-coplaner double slip cases.

Three-dimensional Numerical Investigation on Thickness-through Fatigue Crack Closure

In order to establish the calculation method of the fatigue crack opening stress level for the surface cracks in ship structural members, the validity of various crack-node release schemes and opening / closure stress definitions are investigated by 2- and 3-dimensional F.E. analysis of the crack closure behavior of a thickness-through crack. As results, followings are found;1) The measured and calculated opening stresses defined by the compliance method agree well when the crack tip node is released at the maximum load and the element edge length at the crack tip is chosen so that it is about 1/10 the size of the plastic zone.2) The variation of the crack propagation rate with the depth which gives a good account of the changing nature of the crack edge shape of a real thickness-through crack can be derived when the opening stress is defined as the remote stress at the time the compressive local stress at the crack tip node changes to a tensile stress.3) It is advisable to define the opening stress by the sign change of the local stress at the crack tip whenKopis evaluated locally by 3-dimensional F.E. analysis.

The Study of Fatigue Design Method based on Fatigue Crack Growth Analysis (2nd Report)

In the previous paper, authors have proposed a new fatigue design method based on storm model in the North Pacific using crack growth analysis. Following results were obtained: fatigue strength of ship structural members is strongly influenced by the number, the order, and the size of sea state. In order to develop the storm model for ocean route, many data are needed such as wave scatter diagram, a number of storm, a duration of storm and relative angle in storms. When we can obtain these data, we can make storm model according to our proposed procedure. However, in general, there are few data in ocean route.In this paper, authors propose a procedure to develop storm model in any ocean route where there is only wave scatter diagram. Fatigue crack growth life is analyzed under following three different cases of relative angle and duration of storm: 1) relative angle is kept constant during a sea state, 2) relative angle is kept constant until reaching maximum wave height and then is reversed by 180 degree, 3) relative angle is changed every wave, and for duration of storms, 2 days, 3.5 days, and 4.7 days. Referring to the results of crack growth simulation, authors propose a procedure to develop storm model in any ocean route.

Fatigue Life under Random Loading Condition (4th Report)

At present, the most reliable fatigue design for structures subjected to service loading is considered to be one based on fatigue strength diagrams. These diagrams would have to be obtained from actual service loading fatigue tests or random loading fatigue tests simulating service loading. However, it would take a very long time to complete the necessary experiments, so long as to be impossible.In the previous paper, the authors proposed a new method of developing fatigue strength diagrams under random loading conditions. This method combines random loading test plus an equation, and promises to reduce by 95-99 percent the time required for ordinary random loading fatigue tests with the Weibull distribution.In this paper, using the proposed method, fatigue strength diagrams for butt and fillet welds of mild steel were obtained for load-controlled axial pulsating tension random loading with the Weibull distribution. Shape coefficient in the Weibull distribution was chosen to be 0.75, 1.0 and 1.25. Strength diagrams with these welds were also derived by means of UKDOE Guidance and AWS Code.The main results obtained in this study are as follows.1) Fatigue strength diagrams, represented by shape coefficients as a parameter, are approximated by paralell straight lines in semi-log coordinate.2) Fatigue strength decreases by about ten precent or fatigue life about one-hundredth as shape coefficient increases by 0.25, from 0.75 to 1.25.3) Strength diagrams for both welds, obtained by those two organizations, lead to divergent final designs.