Deformation - Orthopaedic Biomaterials in Research and Practice - page 46

σ:

σ T :

FIGUre 2.19 stress-concentrating design features.

C. Two large holes, with “saddle” broken out: 2.1

D. Single large hole: 2

Thus, the best answer is D ; the best way to take a bone biopsy is with
a trephine or a Craig needle. Extending the defect proximal-distal as
in C causes a small secondary structural weakening effect not directly
related to stress concentration. This is a small effect, since the bone is
a thick-walled tube. The values for A and B are less than the theoretical
calculations would suggest (≈10 and 4, respectively) for homogeneous
defect-free material. Cortical bone has such a high density of inherent
defects (lacunae, Haversian canals, cement lines, etc.) that the stress con-
centration factor cannot practically exceed 4, even for holes with sharp
corners. Finally, in living bone, the stress concentration effect of a hole
may be removed by local remodeling, even if the hole persists, as in a
sequestrum. (For discussion, see Clark et al. 1977.)
In addition to obvious cracks and defects in materials or hard tissue,
engineering design may contribute to stress concentration by providing
internal holes, external retaining notches, or simply by abrupt changes
in cross-sectional area (Figure 2.19). One design response to this is the
“constant strength” fracture fixation plate, which has a greater thickness
at screw hole locations to compensate for stress concentration effects.
Acquired defects, such as scratches, nicks, and so on, are also points of
stress concentration. For this reason, it is advisable not to use fracture
fixation components with obvious surface flaws.

additional problems

PROBLEM 2.10
For the two materials in Figure 2.20, select the best answer:

A. b is equal in strength to a .
B. b is stronger than a since it can undergo more strain before failing.
C. b , while having a lower yield stress than a , is more ductile and
therefore absorbs less energy before failure.