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Monitoring Failure Mechanisms By Ray Garvey, R&D Engineer
Why do components fail? What can we do about it? This article explains eight common
failure mechanisms, types of equipment to which each applies, and recommends non-
intrusive monitoring techniques to discover why components are in various stages of
progressive failure.
This article builds on earlier publications on the failure mechanismsi, wear rates ii, stress wave
analysis iii, the role shear plays in failure mechanismsiv, and wear out failure mechanismsv.
Sonic and ultrasonic stress wave analysis using microphone and radio wave sensors are
featured in this article. It will be shown how these novel techniques supplement,
complement, and advance the state of the art regarding condition monitoring for several
failure mechanisms.
Eight Failure Mechanisms Abrasion, corrosion, fatigue, and adhesion, cavitation, erosion, electrical discharge and
deposition are failure mechanism from the referenced literature. Characteristics of each one
are stated below and in Table 1.
1. Abrasion
Silica dust particles are transported by lubricant to a narrow clearance between moving
surfaces. The hard particles too large to pass through embed into one surface and cut the
other. Shear force between the lubricated hard particles and the moving surface cut a v-
notch into the moving metal surface. This cutting process emits a spectrum of mechanical
vibration from the point of abrasion and generates abrasive wear debris which is carried
away by the lubricant. This mechanism is generally not self-propagating and easily offset
by particulate contamination control. It affects nearly all mechanical systems. This
mechanism can be triggered by a surge in circulating system or by a defective breather.
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Illustrations : abrasive wear particles
2. Corrosion.
A corrosive substance attacks metal and convert surfaces from strong, thermally and
electrically conductive metal into soft, electrically and thermally resistive oxide. The
resulting oxide is easily rubbed off by shear which exposes fresh metal for sustaining
oxidation. This mild rubbing emits stress waves from vicinity and wipes soft metal oxides
into the lubricant, exposing metal to the oxidation process. This mechanism is offset by
moisture contamination control. It can be triggered by process contamination, coolant
leak or defective desiccating breather. Corrosion affects nearly all electrical and
mechanical systems and is synergistic with all of the other mechanisms.
Illustration : corrosive wear particles
3. Fatigue
Roller bearings and gears often fail due the process of rolling contact which eventually
results in material fatigue cracks and spall. Compression between rollers and races and
between gear teeth produces sub-surface Hertzian contact shear that eventually work
hardens the metal until microcracks originate, grow, interconnect, and then release metal
debris typically in forms of chunks, platelets, and needles. This emits stress waves from
impacts and releases the metal debris into the lubricant. This mechanism is offset by
minimizing dynamic loading from imbalance, misalignment and resonance, by static load
reduction, and by other good maintenance practices. It can be triggered by improper fit
or thermal growth. It affects mechanical systems with loaded bearings and gears. As
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described below, the mechanism of cavitation also causes cyclic sub-surface shear
resulting to material fatigue cracks and spall.
Illustrations : fatigue wear particles
4. Adhesion (or boundary wear)
A metal-to-metal contact occurs when the lubricant film designed to eliminate friction
and separate roller from race or journal from shaft fails due to inadequate lubrication.
The increase in friction and shear causes mixed mode and boundary mode lubrication
regimes. The contact emits stress waves. Compression with mixed mode and boundary
lubrication results in shear and friction that causes intense heating, melting, and
discoloration. It releases metal debris and metal oxides into the lubricant and emits a
spectrum of vibration. This mechanism is offset by maintaining correct lubricant at the
correct level and operating at design speed and load. This mechanism may be triggered
by too slow speed, too high load, too low viscosity, and inadequate lubricant delivery. This
mechanism affects nearly all mechanical systems with loaded components. Adhesive wear
and other boundary wear damage is progressive, self-propagating, and accelerates
corrosion.
Illustrations : severe sliding wear particles
5. Cavitation
Liquid cavitation leading to solid surface damage is stimulated by cyclic fluid flow dynamic
pressure variation in vicinity of the surface. In a slow part of the pressure cycle suction
enables evacuated micelle nucleation originating from solid surface irregularities. Highly
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saturated dissolved gas from surrounding liquid may diffuse into expanding bubbles.
Later in the pressure cycle, suction is released, bubbles implode toward the nucleation
surface cycle. The implosion causes a shape-charge supersonic surface impulse, analogous
to the pop from the end of a bull whip, transfers compression and shear stress waves
following the collapse. Subsurface shear from the fluid-structure stress wave dislocates
sub-surface material morphology. Eventually the dislocations lead to fatigue cracks and
then spall. Note that when the bubbles contain partial pressure gases diffused from near-
saturation surrounding liquid, then there is also localized intense heating from the
compressed gases. The cavitation impulses from cavitation events emit stress waves
dislodge particulate debris. This mechanism typically occurs on impellers, pumps, valves,
other flow devices supporting the described cavitation damage process. Cavitation
damage is offset by fluid flow design, control, speed and surface treatment. It triggered
by pressure, flow, and speed variation. Cavitation damage is normally progressive, self-
propagating, and often leads to fatigue cracking and stress corrosion cracking.
6. Erosion
High velocity particulate liquid or solid matter impacts a solid surface causing intense
points of compression resulting in deformation and shear that emits stress waves from
the points of impact and dislodges debris from the damaged surface. This mechanism is
offset by protecting the surfaces of interest with energy absorbing coatings. This
mechanism affects valves, pipes, baffles, impellers, and many other electrical and
mechanical components exposed to streaming particulate matter.
7. Electrical discharge
Electrons transported as parks, partial discharges, and arcs blast target surfaces with
intense local compression causing deformation and shear that generates a wide spectrum
of mechanical and electrical energy. Electrons pass through gaps at supersonic speeds (say
30 m/s) emitting radio waves and sonic booms, generating intense local heat damage to
surfaces and producing various gaseous substances such as hydrocarbons and ozone. This
progressive mechanism ionizes proximate matter to form a discharge or plasma track.
The mechanism may be offset by maintenance of clean, dry, fit for use materials and
compartments. It is triggered by moisture, deteriorated insulation, ground faults,
looseness, and corroded contacts. This mechanism affects all statically charged and
electrically powered equipment including electrical switches, circuits, wiring harnesses,
connections, breakers, transformers, compartments, controllers, motors, DC and variable
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frequency drives, generators, filters, shaft bearings, and housings requiring an electrical
earth ground.
8. Deposition
This mechanism results from a dysfunctional and progressive accumulation of foreign
material on a critical component. Two examples of deposition are precipitated varnish
formation and accumulation on a control valve and fibrous material accumulation on a
fan. Varnish accumulation on a control valve may lead to plugging and sticking. Fibrous
material accumulations on a fan may lead to imbalance and potential fire risk. The
deposition mechanism is offset by detecting, interpreting, and addressing the specific
progressive accumulation process. Each corrective action plan is specific to its
characteristic process.
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Table 1. : Eight common failure mechanisms with equipment, contributing factors, proactive measures and
condition monitoring.
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Failure mechanism monitoring techniques
• Elemental spectroscopy
X-ray fluorescence (XRF) elemental spectroscopy of filter patch specimens is preferred for
large particle failure mechanisms including abrasion, fatigue, and severe adhesion. Optical
emission spectroscopy and XRF are both suitable for corrosion and mild adhesion
mechanisms.
• Particle count and particle shape classification
Particle counts at >4, >6, and >14 m size ranges enable condition monitoring for
contamination control. Direct imaging automatic particle shape classification or
microscopic wear particle analysis enable distinguishing of failure mechanism.
• Radio wave arc/spark detection
This new stress wave analysis technique non intrusively detects arcing, sparking, and
partial discharge events in electrical and electromechanical systems. See FIGs. 6 to 8.
• Inspect and special test
Electrical discharge for oil filled compartments may benefit from dissolved gas analysis
(DGA) looking for evidence of turn-to-turn arcing. Deposition and accumulation of matter
on flow controls, filters, screens, valves, fans, and oil compartments, is a failure
mechanism resulting from a variety of operational conditions. Inspection and testing
protocol depends on these things. For example, membrane patch colorimetry (MPC) is a
preferred testing technique to identify evidence of varnish precursors.
• Stress wave analysis
Wear mechanisms of abrasion, rubbing (associated with corrosion), fatigue, adhesion
(boundary), cavitation, erosion, and electrical discharge can be non-intrusively sensed
using a suitable analog sensor. Accelerometers, microphones, radio wave sensors, current
probes, and magnetic flux sensors are other examples of analog sensory input devices
used in stress wave analysis. Analog to digital data is oversampled and selectively
decimated to derive simultaneous sonic and ultrasonic peak-hold waveforms. A peak-hold
stress wave analysis waveform may be either maximum rectified peak or maximum peak-
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to-peak. The data shown in FIGs. 1-8 are the peak-to-peak type, and the sensors used for
these measurements were microphone or radio wave sensors as stated.
• Thermal imaging
Electrical resistance and electrical arcing and mechanical friction produce hot spots
detectable with thermal imaging.
• Total ferrous
A magnetometer is preferred for determining total ferrous concentration (PPM) for all
ferrous oxide and ferrous metal particles from molecular to abrasive wear particle size
range. This tool is very useful for quantifying wear and severity for ferrous debris in
lubricating fluids.
• Vibration analysis
Mechanical vibrations below a maximum frequency of interest (FMAX) are monitored to
characterize proactive root causes of the failure mechanisms such as imbalance,
misalignment, looseness, resonance, and soft foot. They are also monitored in
combination with stress wave analysis techniques to characterize predictive incipient to
catastrophic stages of failure.
• Viscosity
Lubricant misapplication, e.g., wrong oil, is frequently identified by verifying correct
viscosity for in service lubricants. This directly relates to monitoring for inadequate
lubrication associated with adhesion.
• Water, coolant, and neutralization number
A convenient on-site method for monitoring water and coolant and total acid or total base
number is transmission infrared spectroscopy of in service lubricant. Laboratory titration
methods (Karl Fischer, TAN, and TBN) are also effective. Water, coolant, and acid are all
related to corrosive wear mechanisms.
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Tension, Compression and Shear “Nothing, well almost nothing, fails in compression,” is the answer from a DOE Y-12 laboratory
chief scientist, John Googin, when I asked about failure mechanisms.
John suggested that at times when I think compression is a primary cause of failure, closer
study of evidence would certainly expose either tensile or shear mechanism initiating a
progressive failure sequence. Three decades later I have not found an exception, and shear
force is nearly always a primary contributing factor from incipient to catastrophic failure
mechanisms.
Lubricated load bearing surfaces allow machines to do work by way of compression through
a lubrication film. Work is application of force through a distance or pressure through a
volume. Mechanical systems are designed to perform work and to have very long life by doing
that work through applied tension and compression. The designer intended long life gets cut
short by shear. Failure mechanisms of abrasion, corrosion, fatigue, adhesion (or boundary
lubrication regime), cavitation, erosion, and electrical discharge each have a common failure
element of shear.
The following eight figures demonstrate how compression and shear may be revealed by
simultaneous collection of sonic and ultrasonic stress waves from at least 80 kHz sampling
rate stream of digital data. These techniques apply to measurements from various sensors
including piezoelectric accelerometer, electret condenser microphone, and radio wave
antenna. An appropriate sensor is selected for the non-intrusive measurement of failure
mechanisms. Preferred stress wave analysis methods simultaneously perform peak-hold
sonic analysis of 500 Hz high pass signals and peak-hold ultrasonic analysis of 20 kHz high pass
signals from one oversampled data stream.
All graphs in Figures 1 to 8 include an orange and a blue plot. The abscissa (Y-axis) is signal
strength millivolts (mV) and the ordinate (X-axis) is time in seconds (s). The area under the
orange line is the total ultrasonic peak energy above 20 kHz. The area between the blue line
and the orange line is the total sonic peak energy between 500 Hz and 20 kHz. The ultrasonic
energy under the orange line is related to shear energy transfer mechanisms of friction and
turbulence. The sonic energy between the blue and orange lines is related to forceful
compression energy transfer reflecting work done by force through distance or pressure
through volume.
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Figure 1 displays typical adhesion and fatigue impact events. These 12 examples show forceful
high amplitude compression impact event patterns which are characteristic for these metal-
to-metal collisions. Notice in every case the ultrasonic signal (orange) remains small
compared with sonic signal (blue).
FIG. 1 : Microphone sensor collected stress waves from typical impacts involving a variety of adhesion events
and fatigue defects.
Figure 2 shows how a reciprocating compression mechanism is performing work through
compression with negligible shear. In this case the lubricant provides full hydrodynamic fluid
film minimizing shear throughout continuous reciprocating sliding contact.
FIG. 2. : Airborne stress waves from reciprocating compression with hydrodynamic fluid film separation.
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Figure 3 shows how stick-slip friction during mild adhesive rubbing wear is largely ultrasonic
without much sonic energy. The ultrasonic high pass above 20 kHz orange lines are “in front”
of the sonic high pass above 500 Hz blue lines. However, by definition, the high pass peak-
hold above 500 Hz is always equal to or greater than high pass peak-hold above 20 kHz. A
characteristic sonic wave pattern is evident in the right graph. Overall, Figure 3 represents
shear due to friction during mild adhesive rubbing wear.
FIG. 3. Airborne stress waves from mild adhesive rubbing wear.
Figure 4 is similar to Figure 3 with abrasive cutting wear doing physical work evidence by the
separation between ultrasonic orange and blue lines.
FIG. 4. Airborne stress waves from abrasive wear.
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Figure 5 shows the process of erosion with fluid and particulate impacts delivering
compression and shear events in these time domain plots. These typical plots were recorded
at a waterfall. Coincidentally, water action shows up in the blue color, particle impacts are
sand color.
FIG. 5. : Airborne stress waves from erosion.
Figure 6 shows radio wave energy during partial discharge events. Note that electrical
discharge events tend to be very, very fast ultrasonic events. However, in partial discharge
the radio wave signals appear slow and attenuated as seen in this figure. Notice sonic energy
is several times stronger than ultrasonic radio wave energy. Perhaps this is evidence that the
electrons departing from conductors are not completely escaping the surrounding insulating
matter.
FIG. 6. Radio stress waves from electrical partial discharge.
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Figure 7 shows plots from microphone and radio wave sensors producing sonic and ultrasonic
stress waves tracking events occurring during continuous plasma arcing electrical discharge
with 10 kV arcing over a ~10 mm gap. Typically arcing produces fast ultrasonic events.
Therefore, the orange line tends to overlay and obscure the blue line in the plots. Arcing
events are more common than sparking events for electrical equipment 2 kV and above..
FIG. 7. Sonic and ultrasonic event stress waves using microphone and radio wave sensor to monitor a
continuous plasma and arcing electrical discharge with 10 kV
Figure 8 shows plots from microphone and radio wave sensors producing sonic and ultrasonic
stress waves tracking events occurring during spark events from 120 V source. Sparking
events are more common than arcing events for electrical equipment 480 V and below.
FIG. 8. Microphone and radio wave sensors detect stress waves from sparking electrical discharge originating
from a 120 V source.
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Conclusion This article attempts to describe how equipment fails and what may be done to improve
overall reliability. The article identifies common mechanisms of failure: abrasion, corrosion,
fatigue, adhesion, erosion, cavitation, electrical discharge, and deposition.
Each mechanism has contributing factors, proactive measures, and affects various kinds of
equipment. Several preferred non-intrusive monitoring techniques are identified to enable
proactive and preventive efforts for improved reliability.
Simultaneous sonic and ultrasonic stress wave analysis using microphone and radio wave
sensors which advance state of the art are featured from the broad list of valuable condition
monitoring techniques.
Source of this article :
Ray Garvey - R&D Engineer, I-care Reliability Inc. Ray is an engineer and inventor named on two dozen US patents associated
with oil analyzers, infrared thermography, machine monitoring, and
composite structures. Ray is known for his participation in developing CSI
5100 and CSI 5200 minilabs. Ray received his BS Degree from West Point
and MS degree from the University of Tennessee. His professional
certifications have included Professional Engineer (PE), Certified
Lubrication Specialist (CLS), and US Army Engineer (LTC retired). Ray
worked for the US Army Corps of Engineers, US DOE Uranium Gas
Centrifuge Program, CSI Emerson Process Management, I-care Reliability
Inc., and Spectro Scientific
i “Identifying Root Causes of Failure with Condition Monitoring”, Ray Garvey and Pat Henning, Machinery Lubrication Magazine, December 2012 ii “How Machinery Wear Rates Impact Maintenance Priorities”, Ray Garvey, Machinery Lubrication Magazine, March 2003 iii “Intelligent Decimation: Closing the Gaps Between Vibration and Oil Analyses”, Ray Garvey, Machinery Lubrication Magazine, April 2019 iv “Composite Hull for Full-Ocean Depth”, R. E. Garvey, Marine Technology Journal, Volume 24, Number 2, June 1990 v “Converting Tribology Based Condition Monitoring into Measurable Maintenance Results”, by Ray Garvey and Grahame Fogel, Computational Systems Inc., 1998