Small punch testing for determining the cryogenic fracture properties of 304 and 316 austenitic stainless steels in a high magnetic field

This paper examines the effect of magnetic field on the fracture properties of austenitic stainless steels at liquid helium temperature (4 K). Small punch tests were performed on cold-rolled 304 and 316 austenitic stainless steels. Previously proposed correlation for small punch and elastic-plastic fracture toughness test methods was applied to predict a small punch test-based fracture toughness from equivalent fracture strain.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Interlaminar shear properties of composite insulation systems for fusion magnets at cryogenic temperatures

This paper reports the cryogenic interlaminar shear properties of composite insulation systems for the superconducting magnet coils in the International Thermonuclear Experimental Reactor (ITER). Short beam shear tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) on three insulation systems consisting of woven glass fiber reinforced plastic (GFRP) composites with different polymer resins and polyimide films, and the dependence of their apparent interlaminar shear strength on the temperature and the polymer resins was discussed. A detailed observation of failed specimens was made to verify the failure mechanisms. Insulation systems subjected to gamma irradiation at room temperature were also considered, and the effect of irradiation on the cryogenic interlaminar shear properties was examined.     

Cryogenic fatigue behavior of plain weave glass/epoxy composite laminates under tension-tension cycling

This paper focuses on understanding the tension-tension fatigue behavior of woven glass fiber reinforced polymer laminates at cryogenic temperatures. Tension-tension fatigue tests at frequencies of 4 and 10 Hz with a stress ratio of 0.1 were conducted at room temperature, 77 and 4 K. The fatigue stress versus cycles to failure (S-N) relationships and fatigue limits for 10⁶ cycles were obtained. Fractured specimens tested under fatigue tests were also examined with optical microscope.     

Tension-tension fatigue behavior of GFRP woven laminates at low temperatures

This paper describes an experimental study on the fatigue damage behavior of GFRP woven laminates in terms of stiffness degradation and residual strength under cyclic loading at low temperatures. Uniaxial, load-controlled, tension-tension fatigue tests were conducted at room and low temperatures. The applied stress versus cycles to failure (S-N) relationships and fatigue limits were obtained for the GFRP woven laminates and the microcrack evolution due to fatigue loading was characterized using optical microscopy. Temperatures were also measured using a thermocouple embedded in the center of the specimens.     

Effect of cryogenic treatment on microstructure, mechanical and wear behaviors of AISI H13 hot work tool steel

This paper focuses on the effects of low temperature (subzero) treatments on microstructure and mechanical properties of H13 hot work tool steel. Cryogenic treatment at −72 °C and deep cryogenic treatment at −196 °C were applied and it was found that by applying the subzero treatments, the retained austenite was transformed to martensite. As the temperature was decreased more retained austenite was transformed to martensite and it also led to smaller and more uniform martensite laths distributed in the microstructure. The deep cryogenic treatment also resulted in precipitation of more uniform and very fine carbide particles. The microstructural modification resulted in a significant improvement on the mechanical properties of the H13 tool steel.     

Analysis of mixed-mode interlaminar fracture and damage behavior of GFRP woven laminates at cryogenic temperatures

The objective of this work is to investigate the interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates loaded in a mixed-mode bending (MMB) apparatus at cryogenic temperatures. The finite element analysis (FEA) is used to determine the mixed-mode interlaminar fracture toughness of MMB specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A FEA coupled with damage is also employed to study the damage distributions within the MMB specimen and to examine the effect of damage on the mixed-mode energy release rate. The technique presented can be efficiently used for characterization of mixed-mode interlaminar fracture and damage behavior of woven laminate specimens at cryogenic temperatures.     

Mode III fatigue delamination growth of glass fiber reinforced polymer woven laminates at cryogenic temperatures

This paper investigates the cryogenic fatigue delamination behavior of glass fiber reinforced polymer woven laminates under Mode III loading. Fatigue delamination tests were conducted using split cantilever beam specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A finite element analysis was also employed to calculate the energy release rate. The temperature dependence of the fatigue delamination growth rate vs. energy release rate range is discussed. Fracture surfaces were examined by scanning electron microscopy to identify the delamination mechanisms under fatigue loading. The important conclusion we reach is that the Mode III fatigue delamination growth rates of woven laminates at cryogenic temperatures are lower than that at room temperature.     

Hardness and toughness investigations of deep cryogenic treated cold work die steel

In consideration of good results about the application of deep cryogenic treatment (DCT) on materials, the effect on the microstructure and properties (hardness, toughness and the content of retained austenite) of a new developed cold work die steel (Cr8Mo2SiV) was examined. The execution of the deep cryogenic treatment in different processes showed a varying effect on materials. It was shown that the hardness of the DCT specimens was higher (+0.5HRC to +2HRC) whereas the toughness was lower when compared with the conventionally treated specimens (quenching and tempering). Following the DCT process retained austenite transformed into martensite, however, not completely.     

Low temperature radiative properties of materials used in cryogenics

Radiative heat transfer between two parallel surfaces, a sample surface and a black surface, was measured. One of the surfaces was cooled with liquid helium to about 5 K and the other one was step by step heated to temperatures ranging between 30 and 140 K. As a result, the total hemispherical absorptivity and emissivity of the sample surface were determined in dependence on the temperature of the heat radiation. Aluminium samples were made of Al sheet, Al foil and aluminized mylar. Further measurements were performed on sheets of aluminium alloy, Cu, zinc brass and stainless steel. The influence of different types of sample treatment such as chemical and mechanical surface finishing and material annealing on the radiative properties is presented.     

Analytical and finite element investigations of shear/compression test fixtures

Insulation systems are critical components of the international thermonuclear experimental reactor (ITER). They must meet the super conducting magnets design requirements, including mechanical strength under combined shear and compressive stresses at cryogenic temperatures. Past cryogenic magnet systems often relied on woven glass/epoxy materials for insulation. An important point is to find a reliable shear/compression test method for these materials. The present work investigates a commonly used shear/compression setup and aims at measuring the reliability of the obtained test results. Therefore, the stress and failure analysis is performed analytically and numerically using the finite element method. The model is based on woven glass fiber reinforced materials which are subjected to combined shear and compressive stresses as well as to thermal loading, that results from cooling from 293 K to the test temperature of 77 K. A short analytical section shows the problems of common failure criteria which are used to describe the interaction of the shear and compression stresses. The numerical—finite element—section is based on three-dimensional linear elastic finite element models under thermo-mechanical loading. The locations of high stress gradients are investigated using an average stress criterion. Three different model geometries (15°, 45°, and 70°) are analyzed and finally compared with respect to their reliability.     

Development of superconducting and cryogenic technology in the Institute for Technical Physics (ITP) of the Research Center Karlsruhe

Since the early 1970s the Institute for Technical Physics of the Research Center Karlsruhe has been involved in the development of superconductivity for research and industrial applications. A broad program with a focus on the superconducting magnet technology was established to include large magnets for nuclear fusion, high-field magnets for nuclear magnetic resonance spectrometers, ore separation and energy storage magnets. Research and development work was performed in collaborative projects with other national as well as international institutions and industry. The success of these projects has been supported by a broad foundation of engineering science in superconductor development, electrical and cryogenic engineering. Several well known test facilities like TOSKA, STAR, HOMER, MTA along with well equipped laboratories for conductor development, materials at cryogenic temperatures, cryogenic high-voltage engineering have made substantial contributions to in-house, national and international projects. A strong cryogenic infrastructure with two refrigerators and sophisticated cooling circuits from about 4.5 K down to 1.8 K assure the reliable operation of these large facilities. Last but not least, cryogenic research, including vacuum pumps for International Thermonuclear Experimental Reactor, improvements in thermal insulation, cryogenic instrumentation and small on board refrigerators has supported progress in this field. High-temperature superconductivity projects for low AC loss conductors, a 70 kA current lead and a fault current limiter are currently in progress.     

Optimization of the duration of cryogenic processing to maximize wear resistance of AISI D2 steel

The wear properties, hardness values and the microstructural characteristics of AISI D2 steel cryotreated at 77 K for different soaking durations (0-132 h) have been examined to find out the optimized soaking time in cryogenic processing for maximization of its wear resistance. Examination of the structure-property relations of differently treated specimens indicates that the best wear resistance is obtained for specimens cryogenically processed for 36 h. This result has been substantiated using the nature of the associated variations of different microstructural features, hardness values, topography of worn surfaces and characteristics of wear debris.     

Magnetoresistance up to 50 T of highly strengthened Cu-Ag conductors for pulsed high field magnets

The resistance of cold worked Cu-x wt.%Ag alloys (x = 7 and x = 24) is measured in dependence of magnetic field and temperature. The magnetoresistance (MR) in the field range 0 T ? B ? 50 T is positive and increases with magnetic field. If the magnetic field B is applied perpendicular to the transport current I, the magnetoresistance increases in the temperature range from 77 K to 300 K. The highest value of 14% for the MR is measured at 77 K and 50 T. For I‖B it was found that the MR is independent of the temperature in the investigated range from 77 K to 199 K. The MR is attributed to the microstructure of the alloys and appears to be independent of the Ag content of the alloys under investigation.     

Effect of different treatments of copper surface on its total hemispherical absorptivity bellow 77 K

Total hemispherical absorptivity of copper surfaces treated with standard industrial methods was measured in dependence on the temperature of thermal radiation, varying from 25 K to 300 K. The sample temperature was typically from 5 K to 40 K and did not exceed 70 K. Usability of chemical and mechanical Cu surface finishing as well as Cu plating with Ni and Au for cryogenic design is discussed. As an example of practical application of our results, the cryogenic design of a LN₂ trap is presented.     

A new apparatus for mechanical Q-factor measurements between 5 and 300 K

Today’s laser interferometric gravitational wave detectors are sensitivity limited by thermal noise of the optical components within the detection band of about 0.1-1 kHz. Cooling down these parts to cryogenic temperatures is a promising technique to increase the sensitivity of the gravitational wave detectors by at least one to two orders of magnitudes. Cooling substantially increases the material Q-factor contributing to reduced thermal noise. This article describes a new cryogenic apparatus which allows the measurement of the mechanical Q-factor - as a measure of internal losses - in a temperature range from 5 K up to 300 K. The requirements for cryogenic Q-factor measurements and their realization are shown. The measuring technique as well as the key parameters are discussed. Exemplary, measurements on crystalline quartz and silicon (1 0 0) are given to characterize the setup.     

The effects of the air gap between pancake windings on the central magnetic field in a high temperature superconducting magnet

In an HTS magnet consisting of pancake windings, an air gap between the pancake windings can be used to increase the central magnetic field because the air gap increases the critical current of pancake windings. The effects of an air gap on the central magnetic field of an HTS magnet are discussed in this paper according to the various number of turns and also to the number of pancake windings. Results of calculation show the air gap could increase the central magnetic field and the field uniformity simultaneously. The optimum air gap which maximized the central magnetic field was about 4 mm at eight pancake windings and 50 turns per pancake winding. The central magnetic field increased 6.2% from 0.225 T (no air gap) to 0.239 T (4 mm air gap) at that case.     

Mechanical tests of ATLAS Barrel Toroid tie rods

An installation has been designed and manufactured to test the tie rods of superconducting magnets of Atlas Barrel Toroid at tension load 2.5 MN (about 520 MPa maximum tensile stress) and with temperature gradient along the tie rod from 10 K to 270 K. All the 64 tie-rods have withstood the loading and remained in an elastic mode without any traces of plastic behavior.     

A shieldless method for cryogenic cold–vapor supply usage: Theory and practice

We have proven by numerical analysis and experiment that with the use of the SRDB developed shieldless method for cryogenic vapor usage maximum vapor–cold usage is achieved. It is shown that evaporation is decreased in cryovessels and cryostats by using this method equal to 45 times for helium, 5 times for hydrogen and 1.7 times for nitrogen.     

Fiber-optic Bragg gratings as magnetic field-insensitive strain sensors for the surveillance of cryogenic devices

While conventional electrical resistance strain gages show increasing cross-sensitivities to temperature and magnetic field with decreasing temperature down to liquid helium, it has been found that fiber-optic Bragg grating strain sensors show negligible thermo-optic and magneto-optic effects in cryogenic environments; therefore, they allow reliable strain measurements. These specific application advantages of optical fiber Bragg grating sensors at low temperatures, together with the electrical isolation and low electro-magnetic interference, low thermal conductivity and their multiplexing capability, make them attractive for structural health monitoring in cryogenic devices such as superconductive magnets. In this paper we present low temperature characteristics of fiber Bragg grating-based sensors and address application-based side effects such as induced birefringence.