<p>In the heating, ventilation, air
conditioning and refrigeration (HVAC&R) industry, bonding and joining play
an important role in the manufacturing and assembly process, which is critical
to the cost, safety, reliability, and design freedom of systems. The goal of
this thesis is to understand and evaluate the usage of adhesive bonds in the
manufacture of HVAC&R systems, specifically in regards to
leakage/reliability characterization and stress analysis under loading.</p>
<p>The bonding performance under
static loading is first studied using a commercial epoxy adhesive product. In
addition to the traditional surface preparation methods of mechanical and
chemical etching, a novel laser-interference surface structuring preparation
technique was utilized to improve bonding performance. Laser interference
structuring uses a ND:YAG laser beam that is split into two beams that are
re-directed to overlap on the same area of a copper alloy. A structuring pattern
near the interference structuring limits is achieved due to the phase shift
between the beams that is imparted as they are re-directed. Two different laser
structuring methods were tested: spot-by-spot and laser raster. Different
structuring parameters were varied including the laser spot size and pulses per
spot (2, 4, 6, 8, 10, 12 pulses/spot) for the spot-by-spot method, and raster
speed (2, 4, 6, 8, 10, 12 mm/s) for laser raster method. The microstructure
morphology and surface profile after processing were characterized using the
scanning electron microscopy (SEM) and profilometry for all surfaces. It was
found that the laser-interference structuring removed the surface contaminants
efficiently and formed dot- or net-shaped structures on the surface. This
indicates that melting, vaporization, and solidification of the metal happened
differently. Due to the much higher speed of the laser raster method,
considering practical industrial applications, it is selected for additional
investigation for shear strength improvement. The shear strength is measured by
a single lap shear test which pulls apart adhesively bonded single lap joint
specimen under shear loading using a mechanical tester.</p>
<p>Based on the surface profiles,
three different laser raster speeds of 2 mm/s, 6 mm/s and 12 mm/s were selected
for the manufacture of single lap joint specimens for comparison with the
traditional surface preparation methods. The shear lap strength and
displacement at maximum load were obtained for the specimens. The laser raster
at 6 mm/s increased these values by approximately 11.0% and 25.1%,
respectively, while the 12 mm/s condition had an increase of 16.8% and 43.8%,
compared with the baseline traditional surface preparation method. It is
concluded that laser structuring can enhance the single lap shear joint bonding
performance. Within the tested laser processing parameters, a higher laser
raster speed results in a larger enhancement. </p>
<p>In addition to the static loading
test with epoxy adhesive, different adhesive formulations are investigated and
developed by the collaborating adhesive manufacturer to determine their
suitability for use under the temperature and pressure conditions in HVAC&R
systems. Reliability, especially fatigue failure, is another major concern
because the strength of the adhesive joints is sufficient for HVAC&R
applications. Two primary types of fatigue may happen in practical
applications: thermal fatigue and vibration fatigue. Two test facilities were
designed and built to test the adhesive performance and understand the failure
mechanisms. For the thermal fatigue testing, a novel pressure and temperature
cyclic (PTC) test stand was designed to simulate the pressure and temperature
changes that may occur in HVAC&R systems. The test stand was designed to
switch between hot high-pressure gas and cold low-pressure gas by using a
compressor with hot gas bypass setup. For the vibration testing, a standard
industrial shaker was used to provide the required vibration at a given
displacement and frequency with a specially designed fixture for the tested
joints. In both tests, adhesive joints were tested in parallel with brazed
joints, undergoing extreme thermal and vibration loading conditions. All the
samples were leak-checked before and after the testing, which were found to be leak-free
after the testing, indicating that they pass the required qualification test
according to available standards. It is confirmed that adhesive joints can be a
potential alternative when dealing with thermal and vibration fatigue in the
common working conditions of HVAC&R systems.</p>
<p>The qualification testing is specific
to the required loading conditions, such as pressure and temperature variations,
and limited to certain tube sizes. An analytical model is developed to allow
for design and evaluation across various operating conditions. The model aims
to predict the adhesive stress and strain fields of in tube-to-tube joints
based on the geometric parameters, material properties, and the loading
conditions. In particular, the model uniquely considers the influence of
thermal expansion and contraction in the joint, which is necessary for the periodically
changing temperatures in HVAC&R systems. It is numerically solved using
Mathematica and validated against the published data in the literature. The exact
same solutions are achieved using the reported data in the literature, under
simplified conditions without any temperature change involved. The
validated model is then used in parametric studies to investigate the influence
of geometric sizes and temperature change. Several conclusions are made about
the trend of stress changes as well as the maximum stress, which provide
insight from a perspective of general design guidance. Adhesive bonding length
should be selected such that the maximum stress is smaller than the allowed
material strength for both normal and shear stress. Adhesive thickness has less
impact in the parametric range considered and is nevertheless usually dictated
by the manufacture recommendation in view of other practical considerations. In
regard to the thermal stresses, it is found that in practical HVAC&R
working environment, the temperature-induced thermal stress dominates the
stress fields and leads to significant change in the stress distribution across
the adhesive layer. If a temperature change is present, the combination of all
possible loading and temperature change should be analyzed to find the most
extreme loading condition. This work demonstrates the first stress and strain analysis of tube-to-tube
adhesive joints considering the working conditions of HVAC&R applications
involving temperature cycling. All of these results provide a detailed guidance
for use of adhesive joints across different application or locations in
HVAC&R systems. The model can be also used as a framework to evaluate and
compare the performance of different adhesives, as long as the adhesive
properties are available.</p>
<p>Lastly, it is also essential to
demonstrate the application of these joints in real HVAC&R systems. A proof-of-concept
test was done to demonstrate that the use of adhesive joints in a real system
would cause no change in operation or leakage. A commercial heat pump dryer
system was used to perform the testing at the Ray W. Herrick Laboratories. Two
adhesive joints were installed to replace the brazing joints at the compressor
inlet and outlet, where the most extreme temperature and pressure conditions
are present. Results show that the system operates without any change in
performance and experience no obvious leakage after more than 50 hours of testing
over 6 months. </p>
<p>This work explores the feasibility
and reliability of adhesive bonding of copper for HVAC&R applications. The
bonding strength of adhesive was studied and tested with both traditional
surface preparation and advanced laser-interference structuring technique. The
results show that for the tested structural epoxy adhesive, the bonding
strength is large enough considering the internal pressure in the tube and the
laser structure technique can increase the shear strength. </p>
<p>The long-term reliability with
respect to thermal, stress and vibration fatigue are then experimentally
investigated and the adhesive joints pass the qualifications tests required by
the standard. Further modeling work for predicting the stress distribution in
adhesively bonded joints is developed to understand the influence on geometric
parameters and temperature change. The adhesive length can influence the stress
distribution significantly and temperature-induced stress dominates the stress
distribution under the HVAC&R loading conditions. Further material
characterization is needed for crack propagation or detailed fatigue analysis,
which is highly dependent on the adhesive formula, working environment and
loading conditions, which can be performed with a more specific targeted
application. The experimental and modeling work in this thesis provides a foundation
for adhesives to be applied in HVAC&R applications and a framework to further
develop, optimize, and utilize adhesive joining in HVAC&R applications. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15026355 |
| Date | 21 July 2021 |
| Creators | Haotian Liu (11160378) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/EVALUATION_OF_ADHESIVE_BONDING_FOR_HVAC_R_APPLICATIONS/15026355 |
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