August EWJ 24 - Flipbook - Page 15
entanglements and bridging, which means one striation doesn't necessarily correspond to one loading
cycle.
Fracture Surface Features and Common Failure
Mechanisms
In addition to understanding the theoretical aspect of
material deformation and failure, identifying key fracture surface features helps close our iterative learning
loop about how a material behaves under stress, as
many of the features help us reconstruct the cause and
sequence of failure events.
Creep, which involves the slow disentanglement of
polymers, occurs when a material continues to stretch
under a constant load. Therefore, creep usually manifests in failures associated with elongation and crosssection reduction.
As a side note, when it comes to failure analysis, one
does not have to be a material expert to identify key
fracture surface features as fracture features generally
possess unique characteristics and are frequently comparable among different materials.
Environmental failure, or environmental stress
cracking (ESC) for plastics, is the result of the material
coming into contact with incompatible chemicals in
the presence of constant loading or, for metals, environmental agents that could cause corrosion/oxidation. ESC tends to show signs of degradation at the
initiation points and is followed by crack propagation.
Fracture Surface Features
The first critical fracture surface feature is the
initiation point. Failure initiation normally happens
instantaneously, like an explosion event. Therefore,
the fracture surface tends to be flat and smooth. Additionally, like explosions, the subsequent fracture
propagation marks will naturally radiate outwards
from the origin. Fracture propagation marks, such as
radial, chevron, and river marks, will all point back towards the initiation point. So, when examining a fracture surface, the first thing to do is to trace the fracture
propagation marks back to the origin of initiation,
visually like tracing the rays back to the sun.
Factors to Consider in Diagnosing Material
Failures
Material Selection
Material selection is without a doubt the number one
task when it comes to developing any commercial
products. On top of structural integrity requirements,
application environment (likelihood of exposure to
detrimental chemicals or not), expected life span, and
overall cost target are the first level of considerations
that will help define the probable material candidates.
However, it is not unusual to see cases where the cost
target overrides the structural integrity and application environment requirements. So, a not-so-unusual
landmine to watch out for when dealing with field
failure cases.
Once the initiation point is identified, one can begin to
infer what might have happened. For example, if the
initiation point is located on the outer surface of a
component, then the fracture initiation is more likely
due to surface damage, sharp corners, over-crimping,
or exposure to UV or chemicals externally. If the initiation point is located on the inner surface of a component, the failure initiation is likely due to contact
with incompatible chemicals and environmental factors, such as heat and pressure. If the initiation appears from within the component, then
manufacturing defects, such as impurities, trapped air
bubbles, or cracking due to shrinkage, could be the
culprit.
Material Design
Once the proper material is selected, then comes the
part design. Achieving desired structural integrity in a
component requires not only inherent material
strength but also structural stiffness, which is geometry-dependent like wall thickness. On top of wall thickness, sharp corners can cause stress concentration and
lead to failure at lower than target nominal applied
stress. Consequently, proper radiuses to avoid sharp
corners and proper bottom thread design for effective thread engagement are some of the seemingly
trivial yet critical factors to look out for in a failed part.
Mechanisms of Failures
After the initiation point is identified, understanding
the subsequent eventual failure mechanism is vital to
comprehend what leads to the material failure. Overloading, fatigue, creep, and environmental are some
common material failure mechanisms.
Material Fabrication
After proper selection of material and appropriate
design considerations, parts then get fabricated. Not
surprisingly, proper fabrication process parameters
will also impact the quality and end performance of
the components. For example, some of the critical
processing parameters for plastic injection molding
are adequate raw material drying, molding temperatures, injection speed, mold design for efficient material flow path, mold cavity air venting, and cooling
speed to minimize residual stress. For extrusion, temperatures for different process zones, proper screw elements configuration for materials to be processed,
and extrusion speed are some of the important parameters to pay attention to. Localized stress concentration due to air entrapment associated with
inadequate raw material drying or poor injection
mold cavity air venting is regular issues encountered
in injection molding. Therefore, signs of air bubbles,
Overloading happens when the stress on an application exceeds the material's strength, often resulting in
quick fracture surfaces. Overloading can be caused by
over-tightening plastic connectors or using mechanical tools on parts designed for hand tightening. Inadequate radius and improper dimensions are common
design flaws that can lead to local stress exceeding the
critical level.
Fatigue, on the other hand, results over time from
cyclic load, leading to local failure accumulation and
sustained propagation. Due to its cyclic nature, fatigue
failures are characterized by beach marks or striations
observed on the fracture surface. It is, however, worth
noting that polymers differ from metals due to their
EXPERT WITNESS JOURNAL
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AUGUST 2024
