
576  Chapter 13:  Mechanical Properties 
Uemura  and  Takayanagi  (1966)  developed  expressions  for  the  shear 
modulus  and  Poisson's  ratio  in  an  elastic  two-phase  system;  Reddy 
et  al. 
(1995), taking the inclusions to be voids, applied those expressions and found 
G  (7 -  5Vo) (1  -  p) 
=  (24) 
G O  15(1 - 
Vo) p  + 
(7 -  5Vo)(1 -  p) 
B  4G0(1  -p) 
9  (25) 
Bo  4Go +  3Bop 
Ledbetter 
et  al. 
(Ledbetter  and  Datta,  1986;  Ledbetter 
et  al., 
1994)  applied 
scattering theory to  a homogeneous composite material  consisting  of a random 
distribution  of spheroidal  particles  in  a  continuum matrix.  By  considering  the 
inclusions to be massless voids with zero elastic resistance to shear and dilatation, 
the  shear and bulk moduli were expressed as 
1 
Go -  2--~1 [-A2 
+  (A 2 -  4A1A3) '/2] 
(26) 
4G~ 
(27) 
B~ -- 4(1  - 
p)G o -  3pB 
A 1 -8(1  -p), 
A 2 -  (3 - 
2p)B  -  (8 +  4p)G,A 3 _ 
-3(1 
+p)BG. 
(28) 
d.  Property  Data 
Data for elastic properties are given in Tables  13.2 to  13.5.  The elastic modulus 
did not exceed 200 GPa for any material in Table  13.2, and the typical value was 
on the order of only 100 GPa. The Bi : 2212 and Bi(Pb) : 2223 materials tended to 
have  an  elastic  modulus of less  than  50 GPa,  with the  exception  of one  study 
(Chang 
et  al., 
1993)  that  found  highly  anisotropic  results  for  highly  textured 
Bi:2212  (nearly  120GPa  in the  ab-plane  and  about 45GPa  along the  c-axis). 
Values of Poisson's ratio,  Tablel3.5, were in the range from 0.1  to 0.3. 
C 
Strength 
High-T c superconductors are predominantly brittle materials; that is,  in a plot of 
tensile stress vs strain, as illustrated schematically in Fig.  13.1, there is a critical 
strain at which the material fractures. This situation is distinct from the behavior 
of metals  that  deform  plastically  prior  to  the  occurrence  of material  failure. 
Conceptually, brittle  fracture  is the  catastrophic propagation of a  crack through 
continguous regions of a material body resulting in the rupture of that body into