A “Toolbox” for Forensic Engineers

292 Forensic Materials Engineering: Case Studies
Figure 9.2 Optical micrograph of fracture surface on femoral stem of total hip
device. Crack growth from medial corner (A), and clamshell markings emanating
from lateral corner (B).
and exhibited clam-shell markings typically found on fatigue fracture surfaces
(Figure 9.2). The boundary between the two crack fronts was identified
by a step near the central portion of the trapezoidal cross section.
Based on microscopic observation and stress analysis, Rimnac proposed
the following sequence of events to explain the fracture of the femoral stem.
Residual tensile stresses induced by large compressive overload on the medial
side of the stem resulted in the growth of a fatigue crack prior to a second
crack nucleation on the lateral side. With crack growth, there is a progressive
reduction in residual tensile stresses, along with an increase in the extent of
crack closure, ahead of a fatigue crack subjected to cyclic compressive loads.
As a result, the medial cracks slowed down or stopped after a certain amount
of crack advance. The nucleation of medial cracking increased the stress level
to which the lateral side of the stem was exposed. With time and cyclic loading
in service, the two crack fronts eventually met up. This coalescence of crack
fronts produced a step on the fracture surface. Rimnac et al. concluded that
the failure of the femoral stem was a consequence of the combined effects of
the trapezoidal cross-sectional design and of the materials properties of the
stainless steel used to fabricate the component.
9.4.2 Failure in Femoral Stem of Total Hip Prosthesis
Total hip prostheses do have a finite lifetime within the body environment
and will therefore simply wear out with time. This can be aptly demonstrated
by the following case where a person falling had apparently caused breakage
of a total hip prosthesis.