Recent works made by the authors' research group on magnetic field-induced martensitic transformations are reviewed, which are concerned with some ferrous alloys and steels. Main results of the works are as follows: The effect of magnetic fields on martensitic transformation start temperature is quantitatively explained by cumulative three energies, that is, magnetostatic energy, high field susceptibility energy and forced volume magnetostriction energy. Antiferromagnetic order in the parent phase suppresses the martensitic transformation. The appearance of magnetoelastic martensitic transformation is newly found in an ausaged Fe-Ni-Co-Ti alloy. Futhermore, directional growth of magnetic field-induced martensite plates is observed parallel to the magnetic field.
Design and development of superconducting structures require basic research on electromagnetic fracture mechanics. In the theory of brittle fracture in a strong magnetic field, usually we consider linear electromagnetic elastic solutions of crack problems. Examples including analysis of electromagnetic elastic crack problems are reviewed to show the effect of magnetic field on the mechanical behavior of cracked materials. Many electrically conducting materials are used in magnetic and electric current fields. First, the electromagnetic elastic problems of conducting cracked materials are analized to show the effect of electromagnetic force. The current flow is disturbed by the presence of the crack and the electromagnetic body force is caused by the interaction between the magnetic field and the disturbed current. If the electrically conducting material is used in the strong magnetic field, we must consider the effect of induced current. Secondly, scattering of time harmonic waves by a crack in an electrically conducting material under a magnetic field is analized to show this effect. If a ferromagnetic material is used in a magnetic field, we must consider the effect of induced magnetization. Lastly, the fracture and deformation of a soft ferromagnetic material in a magnetic field is investigated to show this effect.
A change of an energy system, an advance and a development of a cryogenic engineering are going to yield some new type cryogenic structures or equipments. Many materials have been used and are planed in application to these structures, and at the same time, a material development to aim to satisfy a certain property has been conducted. In this review report, the cryogenic structural materials are summarized, and their joining method and joint properties are related briefly.
A high precision fused silica differential dilatometer has been developed with a basis on a TMA (Thermo-Mechanical Analyser) in order to measure the linear thermal expansion coefficient (LTEC) of solids in the temperature range of 20 to 400K with total uncertainties of 10nm and 50mK in length and temperature, respectively. To improve the performance of the measurement system, the sensitivity of LVDT (Linear variable differential transformer) is absolutely determined by using a laser interferometer, the temperature of LVDT system is highly stabilized to minimize the zero point drift of the system, and the temperature of the specimen is accurately measured by a combination of a calibrated PRT (Platinum resistance thermometer) and a differential AuFe-Chromel thermocouple. LTECs were measured under a constant heat/cool mode to improve the measurement efficiency. An error analysis shows that the length and temperature measurements under such a nonequilibrium condition maintain the same accuracy as the measurements under the equilibrium condition. The performance of the apparatus was evaluated by measuring copper (SRM 736) and tungsten (SRM 737) certified by NIST (National Institute of Standards and Technology). The present data of LTEC agree with the NIST reference values within 0.1×10-6/K on both specimens.
A new type of cryogenic temperature fatigue testing machine was installed in 1983 at the National Research Institute for Metals, Tukuba Laboratories. This machine is featured by the continuous operation at 4K without replenishing liquid helium. A closed-loop helium recondensation system enables a long-term fatigue test at 4K longer than 1, 000h. The total operation time reached 5, 500h in these seven years.
Significant temperature rise occurs in specimen during mechanical tests at liquid helium temperature. We carried out measurements and an analysis of specimen temperature rise in order to get appropriate testing conditions during constant amplitude stress and strain cycling tests in liquid helium on typical austenitic stainless steels, titanium alloy and OFHC Cu. The magnitude of temperature rise increased with the increase of the test frequency and the strain range. The obtained testing conditions from specimen temperature measurements are as follows: i) In load-controlled fatigue tests, for tension-tension mode; less than 5Hz at a stress level of lower than yield strength and less than 1Hz at a stress level of higher than yield strength. For tension-compression mode; less than 1Hz at a stress level of lower than yield strength and less than 0.1Hz at a stress level of higher than yield strength, ii) In strain-controlled fatigue tests, less than 4×10-3s-1 at a higher strain level.
We developed Nb3Sn superconductors for AC use and fabricated a coil. The Nb3Sn superconductors were manufactured using the internal tin diffusion process. To reduce AC losses, the spacing between Nb filaments was designed to be 0.5μm, sequentially, the space factor of Nb filaments was 6%. The diameter of a strand is 0.2mm, and the diameter of a Nb filament is 0.4μm. AC losses in the strand were 180kW/m3 at 0.5T, and were 18kW/m3 at 0.1T (60Hz, peak value). A coil was made by the Wind and React method using conductors composed of 7×7 strands. Specifications of the coil are as follows. The inner diameter is 156mm, outer diameter is 188mm, height is 34mm, and the number of turns is 17 turns×4 layers. To reduce wire motions, the coil is impregnated with epoxy resin. The first quench current for DC operation was 1, 130A. The third quench current was 1, 280A, and the maximum magnetic field of conductors was 1.6T. The magnet was tested under AC 60Hz operation. The quench current was 340A (rms). The cause of quenching is supposed to be the temperature rise of conductors due to coupling losses among the strands.
Persistent Current Switch (PCS) is one of the important part of persistent current mode magnet system. The PCS has some degradation: it quenches at far below the critical current. Magnet system is restricted by the PCS quench. This paper studies the characteristics of PCS wound by a thinner CuNi matrix conductor such as 0.3mm in diameter for various magnetic field. It is obtained that its quench currents decrease with current sweep rate. In order to explain this dependence of current sweep rate, following instability model is developed: self-field AC losses increase PCS winding temperature thus PCS is quenched when current reaches its critical current determined by PCS's temperature. Numerical calculation on the model was performed and its result agreed with the experiment data.