ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Objectives
Long bone failure characteristics under
impact bending: an in-depth study
Mike WJ Arun, A Chawla, S Mukherjee, Debasis Sahoo, Sourabh Barouah
The objective of the current work is to exper-
imentally study the tolerance of long bones
bending under impact and to understand the
mechanism which causes their failure.
Materials & Methods
Twelve isolated human humerus were tested
in three point bending mode in the anterior
to posterior direction. Bone specimens were
“potted” with fixtures using a quick setting
bone cement (Poly Methyl Methacrylate, PM-
MA).
Figure 1 Drop tower setup
An instrumented drop tower was used to im-
pact the bone. The drop tower consists of a car-
riage of mass 31 kg that can fall freely through
guided slots. The height of the drop can be ad-
justed to vary impact velocity.
Results
Figure 2 shows the force vs time plot for the ex-
periments. All plots are plotted till the fracture
initiation as observed in high-speed videos.
Indian Institute of Technology, Delhi, India
1.5 m drop 1.0 m drop
0.5 m drop Average
Figure 2 Force-Time plots
The average load taken by the bones for both
1.5 m and 1.0 m drop was around 2.7 kN, while
for 0.5 m drop it was 1.9 kN . Thus there was
a 30% increase in maximum load at higher
drop heights (i.e. higher strain rates), which is
a well-documented behavior of bones. In 1.0
m drop tests, the force vs time plots had two
peaks before failure. This phenomenon was
due to the inertial effect. For 1.0 m drop, the
average time to fracture was 0.57 ms and for
1.5 m drop, it was 0.3 ms. The slope of the force
history during the first peak was observed to
be the highest (20.1 kN/ms) in the 1.5 m drop
case while the 0.5 m drop tests recorded the
lowest (10.4 kN/ms). This shows an increase
in bone stiffness with increase in impact ve-
locity and signifies the strain rate dependen-
cy of bones. It was observed that the average
time for fracture propagation across the ten-
sile side of bending was 0.1 ms, while across
the compressive side was 0.8 ms. This is due
to the asymmetric strength of bone in ten-
sion and compression. Eleven fractures were
oblique and one fracture was transverse in na-
ture. Eight of the oblique fractures were of the
pattern as shown in Figure 3.a, where there
was a crack diversion in the compression side
of impact. These fractures are observed only in
1.5 m and 1.0 m drop experiments. The other
three were of the pattern as shown in Figure
3.b, where there was no crack diversion in the
compression side of impact. This pattern is ob-
served only in 0.5 m drop tests. The transverse
fracture is shown in Figure 3.c.
b. Oblique fracture with no
crack diversion
Figure 3 Observed fracture types
a. Oblique fracture with
crack diversion
c. Transverse fracture
The fracture event captured by high speed
camera is shown in Figure 4.
a. at time=0.26ms b. at time=0.37ms c. at time=1.1ms
Figure 4 Fracture captured by high speed camera
Conclusion
A detailed experimental work had been carried
out to study the tolerance and the mechanism
of fracture in long bones. Interesting observa-
tions have been made, which were not previ-
ously reported. This information can be used
effectively to derive material models that can
be used in finite element simulations.