Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 73

additional problems
PROBLEM 3.4
A brittle material
A. Is stronger in tension
B. Is stronger in bending
C. Is fatigue resistant
D. Is stronger in compression
E. Is easily shaped
ANSWER:
The correct answer is D since compression will close preexisting cracks
and inhibit crack propagation. Brittle materials are weaker under other
types of loading. They are hard and not easily shaped.
PROBLEM 3.5
Creep phenomena in materials (true or false)
1. Are of no concern in orthopaedics
2. Occur in many materials including metals
3. Occur only if a material is viscoelastic
4. Result in fracture
5. Occur under cyclic load
ANSWER:
F, T, F, F, T. Creep is of concern in orthopaedics as it has been shown to
contribute to failure in polyethylene components. Polymers, tissue, and met-
als (at elevated temperatures) display creep. Viscoelasticity (deformation
depending on both load and load rate) is not a precondition for creep. Creep
results in fracture (or rupture) only in the tertiary stage. Static loads can pro-
duce creep; creep may also occur under cyclic loading conditions and may
be greater than expected for the same static average load.
PROBLEM 3.6
The material for a fracture fixation plate should be as hard as possible
(true or false).
ANSWER:
False. Surface hardness is desirable to prevent marring or deformation of
the plate by the underside of the screw heads. However, since hardness is
related directly to σ y , a fully hard plate would be difficult to deform and
be quite brittle. Thus, a better combination of properties would be a soft
interior combined with a hard surface; this combination can be obtained
by processing.