2.6 Mechanical properties of HTS bulk Ë 43Eis defined by Hooke’s law,휎=E휀, where휎and휀are tensile stress and strain,
respectively.
The shear modulusGin a certain deformation range is defined by the relation
휏=G훾, where휏is the shear stress and훾is the shear strain.
Fracture toughnessKICis 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 strength휎f. 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 invariant quantity, but
is rather 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 thea-bplane is relatively low,
ranging from 10 to 30 MPa [135]. 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 material휎yscan be obtained using the following expression
pm=1.1휎ys, where the mean contact pressurepmbetween the indenter and the material
is the applied load divided by the contact area.
The mechanical properties for YBCO bulks are listed in Tab. 2.5. The mechanical
properties of YBCO bulks have been investigated and measured using various me-
thods. It is difficult to get standard data. This table lists only the present representative
values for the reader’s reference. The data for the Young’s modulus and shear modulus
show a weak increase slightly with decreasing temperature.
Fujimoto [143] reported the 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
toughnessKICwas 1.5 for the standard bulk and 1.7 for the densified bulk.
Fujimoto and Murakami [144] made a thorough study on the mechanical proper-
ties of a low-void-density Gd123 bulk and a conventional Gd123 bulk with voids evalua-
ted at 77 K. The fracture toughnessKICaveof the low-void-density bulk was higher than
those of the porous bulk. TheKICaveof the porous bulk was 1.36 MPa⋅m1/2for as-grown
and 2.27MPa⋅m1/2for annealed.KICaveof 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. The increase in the Young’s modulus
and flexural strength at 77 K, compared to those at room temperature, was ascribed to
the decrease in inter-atomic distance by cooling.