Vehicles
must reach an optimum operating temperature regardless of the ambient
temperature for efficiency and emissions. However, regardless of the ambient
temperature, the engine significantly impacts the temperatures of the
components inside and surrounding it. As ramp rates are raised to attain target
temperatures as fast as feasible, this might result in substantial thermal
stress.
Thermal testing
Thermal
testing is the process of subjecting materials to analysis under thermal
control to obtain data on a wide range of properties in materials
research.
It
is a technique for determining a material's capacity to operate safely at
various temperatures. Thermal test data allows customers to understand a
product's safe working limitations and learn more about the material's general
properties and probable lifespan.
Thermal
tests measure a substance's physical, mechanical, chemical, and thermodynamic
changes. It is also possible to determine the exact temperatures and loads that
produce changes in these attributes. For example, thermal testing reveals
crystalline melt temperatures, glass transition temperatures, oxidative and
thermal stability, and material flammability.
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Thermal
testing is also used to determine the molecular structure of crystalline and polymers and identify new and unknown compounds. This can lead to improvements
in existing materials by identifying flaws and constraints.
Thermal
testing is often performed at increasing temperatures; however, isothermal
testing and analysis at lowering temperatures are also performed for the same
goals. Thermal testing is a broad and versatile analysis technique that
involves adding thermal control to unrelated analyses.
Testing
Environmental
thermal testing is critical for ensuring product quality and dependability due
to various automobile engine systems' elevated ambient and operating
temperatures. Therefore, automotive components must normally be tested at both
the cold and hot extremities and transition temperatures between those extremes,
which commonly correspond to test temperatures ranging from -40°C to +150°C.
Thermal
fatigue analysis is a supplementary test requirement. This form of testing is
designed to identify any design flaws, such as coefficients of thermal
expansion mismatches between dissimilar materials and the resulting fatigue
stresses or, in more extreme circumstances, phenomena such as structural
warpage.
Why is Thermal Testing Test Profile important?
Thermal
profiling, also known as temperature profiling, measures time and
temperature throughout a thermal process. The information gathered is critical
for understanding a product's experience as well as the overall performance of
the process, which directly impacts product quality and consistency.
It's
safe to say that most manufacturing procedures entail at least one thermal
process: monitoring food cooking, hardening metal, or sterilizing medical
devices. Thermal profiling can either validate that the process is working as
it should or highlight parts that need improvement.
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Despite
its appearance, thermal profiling is extremely straightforward when using a
temperature data logger. A multi-channel thermocouple data logger is indicated
for monitoring temperature and distribution at numerous places or points during
the process. Once installed, the thermocouple probes will consistently record
temperature measurements, resulting in a temperature profile of the product and
its surroundings.
The
data gathered will reveal how hot/cold the product becomes, when it does so,
and for how long. The recorded data can then be used to validate process
control or identify where changes can be made to improve quality, productivity,
and costs.
Important Thermal Testing Test Profiles
The following section discusses five tests used to benchmark the
component's capacity to survive some of these thermal stressors.
- WAKEUP AT LOW TEMPERATURE (According to IEC 60068-2-1, TEST AB,
COLD)
This
test verifies the component's ability to perform after long exposure to a
low-temperature extreme (Tmin).
The
test sample is examined for proper functionality at nominal supply voltage and
room temperature. The sample is soaked at Tmin for a predetermined time
(typically 24 hours), with the electrical wiring harness connected and the
supply voltage set to zero. The component is turned on and tested for proper
functionality following the soak period while still being exposed to Tmin.
- DEGRADATION AT HIGH TEMPERATURES (AS PER ISO 16750-4,
HIGH-TEMPERATURE TESTS, OPERATION)
This
test is designed to ensure that the component will continue to function after
extended exposure to high temperatures (Tmax).
The
test sample is examined for proper functionality at nominal supply voltage and
room temperature. The sample is then heated to Tmax and immersed at this
temperature for a predetermined amount of time (usually at least 500 hours).
The electrical wiring harness remains connected, and the supply voltage remains
constant.
The
test sample is cycled through functional operations representative of normal
usage, with performance degradation or failure monitored. The supply voltage
can be altered from nominal to minimum and/or maximum values for defined
portions of the test.
- THERMAL CYCLING & VIBRATION (According to IEC 60068-2-64,
TEST FH, BROAD-BAND RANDOM (DIGITAL CONTROL), VIBRATION, AND GUIDANCE)
The
component's capacity to endure vibrations that would ordinarily occur under
field circumstances is verified by this test.
The
vibration of the test sample is assessed successively in all three primary axes
(X, Y, Z). A random vibration test is used to record behaviour beginning at the
low end of the frequency spectrum (i.e., road induced) and progressing to the
high end of the frequency spectrum (i.e., engine component induced). This
frequency range typically covers a 10 - 2,000 Hz frequency range at specific
nominal frequency values with a defined power spectral density (G2/Hz).
In
addition to being vibrated, the test sample is subjected to a thermal cycle.
The cycle is typically between Tmin and Tmax, with specific ramp rates (°C/min)
and dwell lengths. Each thermal cycle typically lasts 480 minutes.
- PTC - POWER TEMPERATURE CYCLE (AS PER ISO 16750-4, TEMPERATURE
CYCLING & IEC 60068-2-14, TEST NB, CHANGE OF TEMPERATURE)
The
test sample is examined for proper functionality at nominal supply voltage and
room temperature. The sample is then heated to Tmax and allowed to remain at
that temperature for a predetermined amount of time (usually one hour).
Following that, the sample is cooled to an ambient temperature of Tmin and
allowed to remain at that temperature for a predetermined time (usually similar
to the heated dwell time). The temperature change rate between Tmin and Tmax
can range from 2 to 15 °C/min.
The
supply voltage can be altered from nominal to minimum and/or maximum values for
defined portions of the test. During the temperature transition between Tmax
and Tmin, the test sample voltage can also be adjusted to zero.
- AIR-TO-AIR THERMAL SHOCK (TS) (AS PER IEC 60068-2-14, TEST NA,
CHANGE OF TEMPERATURE & ISO 16750-4, TEMPERATURE CYCLING)
This
test is performed in combination with the Power Temperature Cycle (PTC) to
validate the component's capacity to survive thermal fatigue and deterioration
caused by thermal cycling.
A
full test portfolio is often created by combining a particular number of PTC
and TS cycles. They are often carried out sequentially, with functional tests
performed simultaneously to ensure component functionality.
Solutions
A few tools are required to execute thermal testing to the given specifications efficiently. A test bench capable of performing durability and/or
performance tests, with a circuit of measurement rated for temperature
extremes; a fluid temperature conditioning system for hydraulic/fluid
components (i.e., chiller); an ambient temperature control solution (i.e.,
thermal chamber); and a PID-tuning and software solution to automate and
execute test profiles are required.
Electronic
components may typically be tested to these criteria using only a thermal
chamber. However, as hydraulic components (such as heat exchangers, valves, and
pumps) become electrified, the electronics will be subjected to thermal
stress caused by the fluid flowing through the component's body.
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Based
on the testing requirements, fluid temperature conditioning might be
complicated and costly (i.e., low-temperature wakeup or cold start versus
thermal cycling). Using liquid nitrogen, single-component testing at low
temperatures is simple. Large-scale durability testing ranging from -40°C to
+150°C may necessitate using a multi-stage chiller system with a
particular heat transfer fluid tailored for the purpose.
Outsourcing
short-term testing to a full-service laboratory may be the most cost-effective
option. Test facilities will include characterization test benches, thermal
chambers, and chillers, depending on the subsystem or component.
Conclusion
Thermal
testing is essential to practically all heat-sensitive product and material
qualification and characterization operations. Thermal testing ensures that
your products can perform in any thermal environment, whether you're prepping
components for thermal extremes or manufacturing heat-resistant materials.
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