Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGUre 2.21
fracture fixation plates (Problems 2.12 and 2.13).
A. The elastic modulus of plate
A
is higher than that of
B
.
B. The bending stress of plate
A
is higher than that of
B
.
C. Plate
A
will fail at a lower level than plate
B
.
D. Plate
A
will bend less than plate
B
.
E. Plate
A
will fail in shear while
B
will fail in tension.
ANSWER:
A. False, since modulus is intrinsic. It is proper to say that
A
is stiffer
than
B
, although the moduli are the same.
B. False. If “bending stress” means bending moment, these are iden-
tical, as they are determined by the test apparatus and the applied
force. If “bending stress” means σ
of
, then σ
of(
A
)
= 1/4σ
of(
B
)
.
C. False. Whether fail means plastically deform or fracture (if the
bending moment applied is high enough),
B
will fail before
A.
D. True. Deformation is linearly related to
M
/
E
*
I
; thus, it varies
inversely as
I
and
I
A
= 8
I
B
.
E. False. Both plates will deform if σ
of
exceeds the tensile yield stress
and fracture if it exceeds the ultimate tensile strength. Changing
I
does not change the failure mode.
PROBLEM 2.13
If plate
B
(Figure 2.21) were to be doubled in thickness, which of the
following statements would be true?
A. Axial stiffness in tension or compression would increase four times.
B. Axial stiffness in tension or compression would increase eight
times.
C. Elastic modulus would increase two times.
D. Bending stiffness would increase four times.
E. Bending stiffness would increase eight times.
ANSWER:
A and B. Both false as axial stiffness depends on cross-sectional area
and would increase two times.
C. False. See Figure 2.12, answer
A
above.
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