Environmental Failure Modes
Presented by Ralph Harris
Senior Consultant, TÜV Product Service
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Covered in this presentation
What types of failures to expect during environmental tests
In some cases I will look at how the environment is simulated in a
laboratory so it can be seen where there are difficulties.
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August 2010
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Temperature
Expected failure modes of thermal tests.- Low temperature
•
Effect on fluids
•
Effect on gasses
•
Effect on electronics
•
Effects on moving parts
•
Effect on materials
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August 2010
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Temperature
Expected failure modes of thermal tests - High Temperature
• Misfit of parts due to expansion
• Loss of lubricants grease due to flow from point of application.
• Overheating Electronics work out of specification /burn out
• Permanent loss of flexibility of plastics
• Changes in colour
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August 2010
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Temperature
Thermal shock
Failure modes are all those included for low and high temperature, plus
differential expansion and contraction failure modes.
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Temperature Testing Issues Airflow
Re-circulate air to maintain control.
Environment is still air,
is it valid to test in
moving air?
Approaches taken to counter this
are:
Modify the test temperature to
take account of the cooling effect
Baffle the test item
Switch fans off and after stabilisation do a quick
functional test.
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August 2010
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Temperature
Many tests have test durations measured from “Thermal Stabilisation”
From Def Stan 00-35
Unless otherwise specified, thermal stabilisation is deemed to have occurred when that part of the test
specimen considered to have the longest thermal lag:
a) For heat-dissipating specimens - is changing at no more than 2°C per hour.
b) For non heat-dissipating specimens - reaches the nominal test temperature within specified test tolerances.
Individual test specifications may identify closer tolerances, for example ±1°C, for critical components.
For non heat-dissipating specimens, the final temperature will be the mean temperature of the test chamber.
For heat-dissipating specimens, it is necessary to make repeated temperature measurements to establish
when stabilisation has occurred.
To reduce the time required for thermal stabilisation during testing, the chamber temperature may be adjusted
beyond the specified test limits; but only where such temperature overrun will not adversely affect critical
components or parts of the test specimen, or take it past the test temperature limit.
There is often a relaxation of the requirement so that only important parts of the test specimen
are considered.
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Temperature
Things that affect thermal stabilisation
•
Flow (airflow) heat capacity of medium
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Thermal mass
•
Insulation
•
Drying
•
External finish (radiation effects)
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Solar Radiation
There are three types of
solar radiation testing
Full spectrum
(including UV) testing
Infrared
(heating effect) testing
Simulation by heating only.
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August 2010
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Solar Radiation
Full Spectrum testing
This test addresses UV degradation and heating effects.
Infrared testing
This test only addresses the Infrared heating effects of solar radiation. It
produces temperature gradients in the test sample from “exposed side”
to “shade side”
Simulation by heating only
Where it is the intention to assess the effect of heating by the sun and
temperature gradients are not important an elevated dry heat test can be
used.
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Solar Radiation
Test difficulties
The test is often considered to be a simulation of a real life environment but
it is difficult to perform this simulation.
Solar power
Since we are so far from the sun the solar intensity measured at the top and
bottom of an equipment in the field is almost identical. However in a
laboratory simulation, the distance from the lamps has a significant effect on
the wattage received, consequently in the laboratory the position of the
sample relative to the lamp affects the severity of the test.
There are intensity distribution errors due to lamp placement and design
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Solar Radiation Test Difficulties Light Spectrum.
Measure spectrum
Purchase full spectrum or IR lamps
Lamp aging.
If the lamp is not compliant the options are.
1) Filter the lamp.
2) Replace the lamp.
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August 2010
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Solar Radiation
Test difficulties
Airflow
The solar radiation test is performed in an environmental chamber
which requires forced air circulation to control the temperature.
If the test has the objective of simulating heating effects the airflow
can have a significant effect and this should be specified.
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August 2010
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Humidity
Failure modes of humidity tests
• Absorption into and adsorption onto, materials
• Reaction with materials (often corrosion)
• Shorting electrical circuits
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Humidity
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Condensation
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Reaction with vapour in the air is slow.
•
Reaction with condensate is faster.
•
When conditions exist which allow condensation to occur
corrosion tends to occur more rapidly as does water
absorption. Additionally there is the potential to short out
electrical circuits and for galvanic corrosion to occur.
•
Condensation will occur on a surface if the temperature of a
surface is below the dew point of the environment that
surrounds it. The most commonly noticed example of this is
water condensing on glasses when someone walks from a
cold environment into a warm kitchen.
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Humidity
Relative humidity what is it relative to?
When water evaporates into the air the vapour exerts a pressure (the partial
pressure of the vapour)
The partial pressure of the vapour is proportional to the amount of water vapour
present in the air and the vapour pressure is a maximum when the air is
saturated with water.
The pressure exerted by water vapour when the air is saturated is known as
the saturation vapour pressure.
The saturation vapour pressure is proportional to temperature.
Relative humidity is defined as the ratio of the actual vapour pressure to the
saturation vapour pressure over a plane liquid water surface at the same
temperature, expressed as a percentage.
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Humidity
Specifying humidity performance
It is common to see claims for equipment performance as a temperature
range followed by a humidity range.
e.g. 50 to -5 °C, 10 to 95%rh. “Non condensing”
What does this mean?
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Humidity
Types of humidity test
• Steady state humidity
• Cyclic humidity tests
• Constant vapour pressure tests
• Accelerated testing HAST
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Cyclic Humidity Test Difficulties
Cyclic humidity - How many cycles?
•
•
•
How much wet air goes back in in one cycle?
Very little not enough to increase the internal humidity to a point which
would stimulate a failure.
If possible reduce the length of each cycle
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August 2010
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Pressure
Failure modes of pressure/altitude tests
Deformation of sealed vessels
Deformation of semi sealed vessels during
rapid pressure changes
Over heating due to loss of air convection
cooling
Electrical breakdown due to loss of air
insulators
Changes in speed due to loss of air viscosity
Vacuum drying including boiling of fluids loss
of solvents
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Pressure
Types of pressure test
Static vacuum tests.
Cyclic vacuum tests.
Explosive or rapid decompression tests.
Over pressure tests.
Over pressure tests in water. not required for most avionics but common on
products which are expected to function after immersion e.g. emergency
beacons.
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Pressure Test Issues
Issues with performing pressure tests
The following are some of the issues which may be encountered when
performing pressure tests.
Cables
Functional testing
Viewing functional responses
Temperature changes
Recording /control issues
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Sand and Dust
Causes of failures
Primary effects
Dust and sand cause failures by obscuration, abrasion, clogging of filters
and general impairment of mechanical systems
Secondary effects
Accumulations support growth
Conductive dusts
Corrosive dust
Moisture retentive dust
Combustible dust
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Dust and Sand Test Difficulties
Measurement and control of Temperature, Airflow , Dust concentration,
Humidity
Requires a source of dust
conforming to the standards.
Hazards Silicosis, Explosion
Photo courtesy of Particle Technology Ltd
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Icing
Failure modes
Unable to operate test sample
Test sample breaks when ice is
chipped off
Ice adheres to “protected” surfaces.
Damage due to expansion of ice as
it freezes.
Breakage due to weight of ice
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Icing
Ice types
Frost
Glazed ice
(or hard rime)
Soft Rime
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Icing Test Difficulties
Getting the right type of ice
The damage potential of ice is related to the type of ice itself. Glazed
ice is the most severe form this is a hard clear ice which is difficult to
remove. This is the ice type most commonly required.
Weaknesses in the ice can occur during application
Maintaining the ice can also be a problem.
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Failure Modes of Mechanical Tests
Deformation
Fatigue
Loosening
Electrical Shorts
Wear /Fretting
Excessive resonance
Excessive Noise
Compaction
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Mechanical Test Difficulties
•
Specialist test equipment
•
Representatively mounted
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Rain / Wet tests
Rain/wet tests
These tests include driving rain, drip tests, immersion and various hose and
spray tests.
The wet tests are fairly unsophisticated tests which give an indication of the
water resistance of a product.
Despite the obvious failures of functional tests the only other failure mode
would be water ingress.
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Ralph Harris
Senior Consultant, TÜV Product Service
Tel: +44(0) 1489 558202
rharris@tuvps.co.uk
www.tuvps.co.uk
TÜV Product Service Ltd
August 2010