August EWJ 24 - Flipbook - Page 14
Navigating the Complexities
of Material Failures
by Jack Huang, Ph.D, Envista Forensics
As modern lifestyle utilizes various materials, such as plastics, metals, ceramics, and their
composites and designers continue to stretch the envelope of how to engage these materials in new
applications, there are bound to be occasions where the materials don’t live up to their projected
expectations.
Material failures occur regularly in our daily lives, and
it is natural to ask, “Why can’t designers specify the
materials and manufacturers fabricate the parts properly for the intended applications?” Well, in real life, it
is through the iteration of prescribing the materials of
interest, analyzing how the materials fail, and adjusting design specs that material developers and designers learn how to best use the new materials. So,
understandably, failure analysis, in this cycle of iterative learning, is a crucial part of forensic science as
there is much detailed information that can be extracted from the analysis. Today, we will touch on the
complexity and intricacies of material failures and
their analysis.
The Basic Principles of Material Failures
Fundamentally, material failures, at the macroscopic
level, are determined by the magnitude of the imposed stress. Failures occur when the stress state of a
material system or component exceeds critical stress.
In solid mechanics, the stress-strain curve, which illustrates a material's deformation trajectory, prescribes
the material’s deformation behavior when under
stress. Most materials exhibit a linear range during
initial deformation or stretching, where stress
and strain correlate. Beyond a specific point, often
referred to as yield or fracture, the material risks
breaking, leading to permanent deformation.
Nominal stress is the overall applied stress on the
material. Although material failures are primarily determined by the nominal stress, local stress, and residual stress are equally important. Local stress could be
higher due to factors such as design, geometry, or dimension, known as stress concentrators. Residual
stress is a result of the fabrication process and can be
seen as a baseline offset. So, while nominal stress may
be lower than the failure critical stress, there might have
been local failure initiated due to elevated stress resulting from stress concentrators and/or residual stress.
Material Deformation Characteristics
With the stress-strain curve prescribing how a material
behaves when under stress, there are commonly two
extreme deformation-failure scenarios: brittle failure,
where the material breaks within or shortly post the
linear range, and ductile failure, where the material
undergoes significant stretching before breaking. Coincidentally, brittle and ductile failure patterns frequently find typical examples in metals and polymers.
Metals, with crystalline structures, tend to exhibit high
elasticity and can absorb energy and revert to their
original shape once the load is removed. So, they tend
to demonstrate a more proportional relationship, linearity, between stress and strain. Contrarily, due to
polymers' microstructure, like a bowl of spaghetti noodles with each noodle representing a polymer chain,
they display viscoelastic behavior. The viscoelastic nature of polymers, including both elastic and viscous
properties during deformation as when the polymer
chains were stretched, they may disentangle and slide
against each other, often resulting in a ductile failure
manner. In real life, materials tend to fail in a manner
between extremely brittle and ductile patterns.
