2.6　Mechanical properties of HTS bulk

In addition to the thermal properties above, mechanical properties of the HTS bulks are also extremely important for industrial applications. HTS bulks are brit tle materials with poor mechanical properties.This is because a number of voids are produced by oxygen formation or gas trapped in the MTG YBCO process.In addition to this, material fractures are caused by the existence and propagation of micro-cracks in superconductor.In particular, thermal stress and fatigue are produced with repeated and rapid changes between room and low operation tem peratures, and hence cause failure of YBCO superconductors.When the highest trapped fields are generated by the external magnetic fields, large stress/strains will be induced and cause more serious rupture, which can be very dangerous.A number of researches have been carried out to improve the mechanical properties of melt-textured YBCO.The vickers hardness test is one of the convenient methods to estimate the mechanical properties of materials.The mechanical properties of HTS bulk associated with the application are briefly discussed in this section.Detailed data of the mechanical properties for YBCO bulks can be found elsewhere63，134.

The mechanical properties for YBCO samples can be determined using the nanoindentation technique. Stress-strain curves（σ-ε）and mechanical properties such as hardness（H），elastic modulus（E），fracture toughness（KIC ），fracture strength（σf ），and yield stress（σys ）can be obtained from the applied load（P）vs.indentation depth（h）data and with the corresponding tip indenter.

The density of the HTS materials depends strongly on the preparation and processing technique. The density of the HTS bulk materials prepared by melt texture growth（MTG）is larger than that by the sintering process.The density of HTS films and single crystals is the largest.For either MTG or the sinteringprocess, some porosity problems in the interior of bulk materials are inevitable.The density of the HTS bulk ceramic materialsρdepends strongly on their porosity.The magnitude of the elastic modulus of ceramic samples depends strongly on their densityρ，which is less than the theoretical maximum densityρmax =6.383 g/cm3.The ratioρ/ρmax is denoted the relative density.

The hardness（H）correlates with yield strength and Young's modulus of the material. Vickers hardness is the preferred method since the indentation traces are small, typically 10-100μm.

The elastic modulus E of an object is defined as the slope of its stress-strain curve in the elastic deformation region. The elastic modulus（Young's or elastic modulus）E is defined by Hooke's law，σ=Eε，whereσandεare tensile stress and strain, respectively.

The shear modulus G in a certain deformation range is defined by the relationτ=Gγ，whereτis the shear stress andγis the shear strain.

Fracture toughness KIC is described as the ability of a material containing a crack to resist fracture. Fracture toughness reveals the resistance against crack propagation.

The normal stress at the beginning of fracture at the elastic limit is defined as the fracture strengthσf.It is calculated from the load at the beginning of fracture during a tension test and the original cross-sectional area of the test sample, i.e.ratio of the maximum stress and fracture cross sectional area at material breakage.HTS RE123 bulks are very brittle ceramics.Their fracture strength is not an invari ant quantity, but rather is controlled by the microcracks within the bulk material.These microcracks set a limit for the fracture strength of the material because they start to propagate when the external tensile stress reaches the fracture toughness of the material.The fracture strength of YBCO-123 was reported to range from 40 MPa to 200 MPa, depending on the sample quality.

The tensile strength（Rm）of bulk YBCO along the a-b plane is relatively low, ranging from 10 MPa to 30 MPa135. The high trapped fields of HTS bulk samples are limited primarily by the tensile strength, because there is a relatively large Lorentz force in the HTS bulk.

The yield stress of the materialσys can be obtained using the following expression pm =1.1σys ，where the mean contact pressure pm between the indenter and the material is the applied load divided by the contact area.

The mechanical properties for YBCO bulks are listed in Table 2. 6.1.The mechanical properties of YBCO bulks have been investigated and measured using various methods.It is difficult to get standard data.This Table lists only resent representative values for the reader’s reference.The data for Young’s modulus and shear modulus show a weak increase slightly with decreasing temperature.

Table 2. 6.1 The mechanical properties for REBCO bulk materials

Fujimoto143reported mechanical properties of densified bulk single domain GdBCO. The average flexural stress of the specimens in this experiment was 72 MPa for densified bulks, and 58 MPa for standard bulks.The Young's modulus was 145 GPa for the densified bulk and 120 GPa for the standard bulk.The measured fracture toughness KIC was 1.5 for the standard bulk and 1.7 for the densified bulk.

Fujimoto and Murakami144made a thorough study on the mechanical properties of a low void density Gd123 bulk, and a conventional Gd123 bulk with voids evaluated at 77 K. The fracture toughness KICave of the low void density bulkwas higher than those of the porous bulk.The KICave of the porous bulk was 1.36 MPa·m1/2for as-grown, and 2.27 MPa·m1/2for annealed.KICave of the densified bulk was 1.43 MPa·m1/2for as-grown, and 2.44 MPa·m1/2for annealed.Obviously, the fracture toughness of annealed bulks was higher than that of as-grown bulks.Increase of the Young's modulus and the flexural strength at 77 K, compared to those at room temperature, was ascribed to the decrease of the inter-atomic distance by cooling.

Murakami et al.145，146reported the mechanical properties of the low porosity DyBCO bulks with different Dy211 contents, and found that the bending strength and the Young’s modulus of the low porosity bulks were higher than those of porous Dy123 bulks.

The evaluation on the mechanical properties of an REBCO large single grain bulk of diameter 150 mm at liquid nitrogen temperature（77 K）was carried out by Murakami et al.147in 2013. Fracture strength data at 77 K were obtained through bending tests for planar specimens.The average fracture strength（σfave ）was about 70 MPa.Theσfave of the large single-grain bulk at 77 K was slightly higher than that at room temperature.

Another important problem in the engineering applications of HTS bulks is the rapid and irreversible degradation due to exposure to ambient atmospheric con ditions. In addition, other environments, such as fabrication, handling, transport and storage, would also cause similar irreversible degradation.In particular, the degradation is very serious when HTS REBCO bulks are exposed to humid atmo sphere environments.In order to prevent the performance degradation for REBCO bulk materials exposed to harmful environments, it is very necessary to investigate protection methods for REBCO bulk materials，Rod et al.148studied the degradation of textured YBCO under operation in high humidity conditions, and experimentally proved that the YBCO samples after degradation contain a high amount of Y-211 phases in the degraded region due to the reaction of YBCO with water. The results show that the initial formation of barium hydroxide and the non-superconducting green phase, Y2 BaCuO5 ，induces the surface degradation and the degradation of mechanical properties.

In the research and development of the first man-loading HTS Maglev testvehicle149in the world in last century, thin copper box sealing technology was used. YBCO bulks were placed in a copper box, then sealed by welding, vacuum pumped, injected the dry nitrogen with a pressure slightly larger than atmosphere, and then permanently sealed.Dry nitrogen improves heat conduction.The eight HTS Maglev equipments are composed of 344 blocks of YBCO.The magnetic levitation forces of each were measured every six months.The results showed that there was almost no change of the magnetic levitation forces after more than ten years.After opening the copper boxes enclosing the HTSC, the YBCO bulks were almost unchanged.For permanent applications, this may be a very effective method（see section 6.24 and 6.29）.There is another method that uses the epoxy and cryogenic adhesive to seal the YBCO bulks, however, the long-term protection is not as effective as the above mentioned methods.