WO2014160134A1 - Method for determining adhesive bondline thickness - Google Patents
Method for determining adhesive bondline thickness Download PDFInfo
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- WO2014160134A1 WO2014160134A1 PCT/US2014/025891 US2014025891W WO2014160134A1 WO 2014160134 A1 WO2014160134 A1 WO 2014160134A1 US 2014025891 W US2014025891 W US 2014025891W WO 2014160134 A1 WO2014160134 A1 WO 2014160134A1
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- Prior art keywords
- adhesive
- thermal expansion
- coefficient
- strength
- stress
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- 230000001070 adhesive effect Effects 0.000 title claims abstract description 155
- 239000000853 adhesive Substances 0.000 title claims abstract description 152
- 238000000034 method Methods 0.000 title claims abstract description 41
- 239000000758 substrate Substances 0.000 claims abstract description 40
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 32
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims abstract description 32
- 229910000831 Steel Inorganic materials 0.000 claims abstract description 25
- 239000010959 steel Substances 0.000 claims abstract description 25
- 238000004458 analytical method Methods 0.000 claims abstract description 12
- 238000001816 cooling Methods 0.000 claims description 2
- 238000010438 heat treatment Methods 0.000 claims description 2
- 239000000463 material Substances 0.000 abstract description 26
- 239000003973 paint Substances 0.000 abstract description 15
- 238000013461 design Methods 0.000 abstract description 7
- 238000012360 testing method Methods 0.000 description 58
- 238000000926 separation method Methods 0.000 description 6
- 230000032798 delamination Effects 0.000 description 5
- 238000013459 approach Methods 0.000 description 4
- 229910052751 metal Inorganic materials 0.000 description 4
- 239000002184 metal Substances 0.000 description 4
- 239000012790 adhesive layer Substances 0.000 description 3
- 238000004364 calculation method Methods 0.000 description 3
- 238000005452 bending Methods 0.000 description 2
- 238000010276 construction Methods 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 238000009533 lab test Methods 0.000 description 2
- 150000002739 metals Chemical class 0.000 description 2
- 239000000047 product Substances 0.000 description 2
- 229920000049 Carbon (fiber) Polymers 0.000 description 1
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 description 1
- 239000004917 carbon fiber Substances 0.000 description 1
- 239000007795 chemical reaction product Substances 0.000 description 1
- 239000013065 commercial product Substances 0.000 description 1
- 238000005094 computer simulation Methods 0.000 description 1
- 230000002596 correlated effect Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 229910052749 magnesium Inorganic materials 0.000 description 1
- 239000011777 magnesium Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 1
- 238000004088 simulation Methods 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
- 238000010998 test method Methods 0.000 description 1
- 229920001169 thermoplastic Polymers 0.000 description 1
- 229920001187 thermosetting polymer Polymers 0.000 description 1
- 239000004416 thermosoftening plastic Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N19/00—Investigating materials by mechanical methods
- G01N19/04—Measuring adhesive force between materials, e.g. of sealing tape, of coating
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
- F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
- F16B11/00—Connecting constructional elements or machine parts by sticking or pressing them together, e.g. cold pressure welding
- F16B11/006—Connecting constructional elements or machine parts by sticking or pressing them together, e.g. cold pressure welding by gluing
-
- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F30/00—Computer-aided design [CAD]
- G06F30/20—Design optimisation, verification or simulation
- G06F30/23—Design optimisation, verification or simulation using finite element methods [FEM] or finite difference methods [FDM]
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/02—Details not specific for a particular testing method
- G01N2203/0202—Control of the test
- G01N2203/0212—Theories, calculations
- G01N2203/0216—Finite elements
Abstract
A method of designing an adhesive joint when bonding two dissimilar substrates is provided. This is accomplished by understanding the stresses and strains that at are induced on an adhesive when a hybrid bonded assembly is processed through high temperature environment, such as a paint bake oven. Stress is subjected to the adhesive when dissimilar materials with different thermal expansion properties are joined together and the bonded assembly is heated, for example, in a paint bake oven. A material such as aluminum will expand at a greater rate than a material such as steel and the adhesive will need to hold the two panels together. A finite element analysis model can be fed input criteria and employed to determine bond design or adhesive properties to retain a robust adhesive bond through the high temperature process.
Description
METHOD FOR DETERMINING ADHESIVE BONDLINE THICKNESS
FIELD OF THE INVENTION
The present invention relates to the design of a bond between two dissimilar substrates including computer-aided bond design, more particularly employing a finite element analysis method to determine the appropriate bondline thickness for a given adhesive, or determining adhesive properties based on a desired bondline thickness.
BACKGROUND OF THE INVENTION
Hybrid bonding applications that need to be processed through high temperature environments such as paint bake ovens can be very challenging to accomplish. The Al/Steel applications evaluated in the recent past are designed essentially the as
steel/steel applications except one substrate is replaced with aluminum. However, when these bonded materials are subjected to high temperatures, such as in a paint bake oven reaching over 400° F (204° C), the differential in cofficient of thermal expansion (CTE) between the steel and aluminum cause such stress on the adhesive that delamination occurs and the bonded assembly breaks apart. Customers desire adhesives able to withstand the problem associated with the differential in CTE between these two materials.
Because of the CTE differential between aluminum and steel (or any dissimilar materials), the temperature of the bake oven has a direct effect on the amount of stress applied to the adhesive. One solution is to employ a paint bake process that uses a lower temperature to reduce the amount of stress on the adhesive during the bake. Another possible solution is to apply the adhesive in the paint shop after e-coat. An application in the paint shop is less desirable than one in the body shop but oven temperatures are significantly lower (250° F (121 ° C)).
These solutions are generally viewed as non-preferred by customers and therefore a solution to the problem of hybrid bonding application subject to high temperatures is desired.
SUMMARY OF THE INVENTION
In a first aspect of the present invention, a method for determining successful bondline thickness of an adhesive between two bonded substrates is provided, comprising: a) selecting a first substrate comprising a first coefficient of thermal expansion;
b) selecting a second substrate comprising a second coefficient of thermal expansion;
c) selecting an adhesive and determining, as a function of temperature, adhesive and cohesive strength of the adhesive as well as at least two of the following three properties of the adhesive: Elastic Modulus (Ε'), Shear Modulus (G'), Poisson's Ratio (v') ;
d) inputting into a finite element analysis model of the bonded substrates the first coefficient of thermal expansion, the second coefficient of thermal expansion, and the at least two properties of the adhesive;
e) determining, through the finite element analysis model, a maximum stress applied to the adhesive at a given temperature for a given bondline thicknesses;
f) repeating step e) for a series of bondline thicknesses; and,
g) identifying a successful bondline thickness as any bondline thickness for which the maximum stress is below the cohesive and adhesive strength of the adhesive for the given temperature.
In one embodiment of the present invention, the above-described method further comprises repeating steps e) and f) over a range of temperatures. In an additional embodiment of the present invention, range of temperatures comprises 50°F to 450°F. In another embodiment of the present invention, identifying a successful bondline thickness comprises any bondline thickness for which the maximum stress is below the adhesive and cohesive strength of the adhesive over the range of temperatures. In a further embodiment of the present invention, the successful bondline thickness comprises the minimum bondline thickness for which the maximum stress is below the adhesive and cohesive strength of the adhesive over the range of temperatures.
In yet another embodiment of the present invention, the first coefficient of thermal expansion is not equal to the second coefficient of thermal expansion. In an additional embodiment of the present invention, the first substrate comprises aluminum and the second substrate comprises steel.
In a second aspect of the present invention, a method of designing an adhesive is provided, comprising;
a) selecting a first substrate having a first coefficient of thermal expansion;
b) selecting a second substrate having a second coefficient of thermal expansion; c) selecting a desired bondline thickness for the adhesive;
d) inputting into a finite element analysis model of the bonded substrates the first coefficient of thermal expansion and the second coefficient of thermal expansion;
e) determining, through the finite element analysis model, the maximum stress the adhesive will experience as the bonded assembly undergoes a heating and cooling cycle; f) designing an adhesive comprising a cohesive strength and an adhesive strength that is higher than the maximum stress determined in step e).
In one embodiment of the present invention, step e) is repeated over a range of temperatures. In another embodiment of the present invention, the range of temperatures comprises 50°F to 450°F. In an additional embodiment of the present invention, the cohesive strength and adhesive strength are greater than the highest maximum stress determined over the range of temperatures.
In yet another embodiment of the present invention, the first coefficient of thermal expansion is not equal to the second coefficient of thermal expansion. In a further embodiment of the present invention, the first substrate comprises aluminum and the second substrate comprises steel.
Thus, there has been outlined, rather broadly, the more important features of the invention in order that the detailed description that follows may be better understood and in order that the present contribution to the art may be better appreciated. There are, obviously, additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto. In this respect, before explaining several embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details and construction and to the
arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways.
It is also to be understood that the phraseology and terminology herein are for the purposes of description and should not be regarded as limiting in any respect. Those skilled in the art will appreciate the concepts upon which this disclosure is based and that it may readily be utilized as the basis for designating other structures, methods and systems for carrying out the several purposes of this development. It is important that the claims be
regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a graph of maximum stress vs. bondline thickness, according to an
embodiment of the present invention.
FIGURE 2 is a graph of maximum stress as a function of temperature, according to an embodiment of the present invention.
FIGURE 3 is a graph of maximum stress vs. modulus, according to an embodiment of the present invention.
FIGURE 4 is a graph of T-peel stress as a function of load, according to an embodiment of the present invention.
FIGURE 5 is a graph of maximum stress vs. bondline thickness, according to an
embodiment of the present invention.
FIGURE 6 is a graph of tensile and shear strength vs. temperature, according to an embodiment of the present invention.
FIGURE 7 is a graph of T-peel stress as a function of load, according to an embodiment of the present invention.
FIGURE 8 is a graph of maximum stress vs. bondline thickness, according to an
embodiment of the present invention.
FIGURE 9 is a graph of maximum stress vs. bondline thickness at various temperatures, according to an embodiment of the present invention.
FIGURE 10 is a graph of tensile strength vs. temperature, according to an embodiment of the present invention.
FIGURE 1 1 is a graph of shear strength vs. temperature, according to an embodiment of the present invention.
FIGURE 12 is a graph of elongation as a function of temperature, according to an embodiment of the present invention.
DETAILED DESCRIPTION
In a first embodiment of the present invention, a method of designing an adhesive joint when bonding two dissimilar substrates is provided. This is accomplished by understanding the stresses and strains that at are induced on an adhesive when a hybrid bonded assembly is processed through high temperature environment, such as a paint bake oven. Stress is subjected to the adhesive when dissimilar materials with different
thermal expansion properties are joined together and the bonded assembly is heated, for example, in a paint bake oven. A material such as aluminum will expand at a greater rate than a material such as steel and the adhesive will need to hold the two panels together.
In an embodiment of the present invention, Finite Element Analysis (FEA) Models of two test procedures were developed that calculates the stresses that are applied to the adhesive during the tests. The FEA models can be directly correlated to the laboratory tests and maximum stress targets can be established for the adhesive. Test variables may be easily and inexpensively evaluated to determine if they can improve the technical possibility of success. The information learned can be used to help design improved bond designs and adhesives.
In an embodiment of the present invention, the method that has been developed can be applicable to any hybrid bonding application that gets processed through a high temperature environment. If the material properties of the substrates and adhesive can be provided, a simulation of the high temperature processes can easily be run. Since the FEA models are computer simulations, a significant amount of information can be obtained, inexpensively, prior to conducting actual laboratory work. However, the particular FEA model employed is dependent upon the geometry of the bonded substrates and surrounding materials. As such, though the methods of embodiments of the present invention are generally applicable, the particular FEA model must be developed
independently for a particular geometry.
In one embodiment of the present invention, an FEA model can be developed to help narrow the lab testing that is needed to validate a potential bonding application. The material properties needed, as a function of temperature, are the coefficient of thermal expansion for each substrate and the following properties of the adhesive: Elastic Modulus (E), Shear Modulus (G), Poisson's Ratio (v), Tensile Strength*, Shear Strength**, adhesive strength, and cohesive strength. However, only two out of E, G, and v are needed since they are related by the equation E=2G(1 +v). Additionally, tensile strength and shear strength are only used to set the cohesive zone strength values in certain embodiments of the present invention.
In a further embodiment of the present invention, the method and particular FEA model can be used for any substrate and adhesive materials as long as the above material
properties are provided. In a preferred embodiment of the present invention, the substrates comprise aluminum and steel. In other embodiments of the present invention, the substrates may be selected from carbon fiber, magnesium and other metals, thermoplastics, thermosets, substrates.
In another embodiment of the present invention, it is recognized that reducing the amount of stress in the bondline will help the bonded assembly withstand the paint bake process. Increasing the bondline thickness of the bond joint significantly reduces the amount of stress on the adhesive. An example of stress as a function of bondline thickness was presented in Figure 1 . As such, in a preferred embodiment of the present invention, the output of the FEA model is employed to find a successful bondline thickness for a particular set of substrates and adhesive. In an alternate embodiment of the present invention, the output of the FEA model is employed to determine the necessary adhesive properties to successfully bond two substrates with a fixed bondline thickness.
EXAMPLE 1
Though the methods herein and the FEA can be constructed for any two substrates, in this example aluminum and steel are the two substrates modeled as they are the preferred materials for joint design in automobiles and thus are encountered regularly. Further, for these examples, unless otherwise noted, the adhesive is a commercial available Versilok® adhesive available from LORD Corporation.
The first FEA model is a one foot square aluminum/steel hybrid bonded panel test. The FEA model for the panel test assumes that the adhesive bond remains intact during the bake process. The calculations provided will correspond to the strength necessary for an adhesive to survive the bake process without failing. This FEA model can be run of several test variables to establish the strength requirements that an adhesive will need to pass.
The FEA output that represents this is the maximum principle stress for the adhesive layer. The output is a figure showing the adhesive layer with the interface on top bonded to steel and the interface on bottom bonded to aluminum. The adhesive layer is color coded to represent the amount of stress at any point in the adhesive. Successive iterations of this model will then generate data relating to the maximum stress over a range
of temperatures and a range of bond line thicknesses. Additionally, the model will predict where within the bondline thickness the maximum stress is experienced.
The amount of stress in the adhesive varies significantly with bondline thickness. For example, in a standard Versilok® adhesive available from LORD Corporation, a thickness of 0.010 inches (0.254mm) at 428 ° F (220 °Q, which corresponds to the standard thickness used for closure panels at the maximum e-coat bake temperature, results in a maximum stress of 399 psi (2.75MPa) and this maximum stress is located at the interface of the aluminum and the adhesive. Comparatively, when the bondline thickness is increased by a factor of 2, to 0.020 inches (0.508mm) at 428° F (220° C), the maximum stress is reduced to 201 psi (1 .39 MPa). If the bondline thickness is increased again by a factor of 2, to 0.040 inches (1 .02mm) at 428 ° F (220° C), the maximum stress is reduced to 103psi (0.710 MPa) and the location of the maximum stress moves away from the interface with the aluminum toward the center of the bondline. The stress at the interface with the aluminum is reduced to 84psi (0.58 MPa).
Figure 1 is a graph of maximum stress as a function of bondline thickness, ranging from 0.004 to 0.120 inches (0.102mm to 3.05mm). At a thickness of 0.035 inches
(0.889mm) or greater, the maximum stress moves away from the interface with the aluminum.
The stress that is applied to the adhesive during the paint bake oven is directly proportional to the temperature of the oven. Processes that use lower temperature bake ovens will reduce the amount of stress during the paint bake oven process. Additionally the strength of adhesives is greater at lower temperatures and can withstand more stress. Figure 1 shows the maximum stress is 399 psi (2.75 MPa) for a bondline thickness of 0.010 inches (0.254mm) at 428 ° F (220 ° C) which is tie highest e-coat bake temperature for any automotive OEM specification.
Maximum stress is also a function of temperature, and for example, for a bondline thickness of 0.010 inches (0.254mm) at 383° F (195°Q, the maximum stress is 320 psi (2.21 MPa). For a thickness of 0.010 inches (0.254mm) at 325° F (163°C), the maximum stress is 264 psi (1 .82 MPa). For a thickness of 0.010 inches (0.254mm) at 250° F (121 °C), the maximum stress is 213 psi (1 .47 MPa).
Figure 2 is a graph of maximum stress as a function of bake temperature, ranging from 428 ° F (220 °C ) down to 78 ° F (26 °C) (or no bakeven). There is an overall of reduction of stress when lowering the temperature bake oven down from 428 ° F (220 °C) down to 200 ° F (93.3 °C).
The stress increases in some cases as the temperature continues to go down from 200° F (93.3°C ) to 100° F (37.8° C). The reason fd iis is the modulus properties are increasing as the temperature approaches the Tg of the adhesive. The adhesive is carrying more of the stress load and is strong enough to withstand the higher stress levels.
If no bake ovens are used the application will need to be conducted in final assembly which is more expensive than bonding applications in the body shop. Today this process would be used for hybrid bonding applications such as aluminum to steel because adhesives that can withstand the high stress during a paint bake have not been
developed. Commodity type adhesives can be chosen for a room temperature curing process. The goal of this project is to help in the development of hybrid bonding
applications that can be painted afterwards.
The stress and strain that is applied to the adhesive during the paint bake oven is directly proportional to the modulus of the adhesive. Since this model assumes the adhesive will not fail, the elongation of the adhesive will need to increase as the modulus properties are decreased. The modulus properties of adhesives vary significantly from adhesive to adhesive and are also highly variable with the temperature. There is a sharp drop in the modulus values when the temperature goes above the Tg of the material. The Dynamic Mechanical Analyzer (DMA) is the easiest way to measure and obtain the modulus as a function of temperature. The material property needed for use in the model is the Shear Modulus (G'), which along with Poisson's Ratio (ν') can be used to calculate Elastic Modulus (Ε').
The Shear Modulus for an adhesive drops as the temperature increases and the sharpest drop occurs when the temperature approaches the Tg of the adhesive. For example, in an e-coat oven, the modulus of Versilok is lower by a factor of 100 as compared to room temperature. To use the FEA model, it is critical that the modulus properties are provided as a function of temperature.
For comparison purposes regarding modulus, the data used to run the model will be based on the data that is available on fully cured Versilok 253/254. The values for modulus used in these comparisons range from 1 % of Versilok to 10000% of Versilok, 36 psi (0.25 MPa) and 2379 psi (16.40 MPa), respectively. These two extreme values will make some general trends more obvious.
When the modulus is very low, the relative movement of the panels due to dissimilar expansion rates is mainly absorbed by the adhesive. There is very little bending of the hybrid bonded assembly and the aluminum panel is free to expand longer in length. The results of a low modulus provide a very low amount of stress in the adhesive (only 36 psi (0.25 MPa)) but a correspondingly high elongation is needed to allow relative movement of the panels.
The opposite effect occurs with an adhesive with a very high modulus. There is very little stretching of the adhesive so the hybrid bonded assembly bends to
accommodate the extra length of the aluminum panel compared to the steel panels. An adhesive of this type will not need a lot of elongation but it will need to be exceptionally strong (7400 psi (5102 MPa) at 428° F (220° C)). Thse extreme values for modulus are not practical but the modulus is a key variable for hybrid bonding of dissimilar metals.
Recalling Figure 1 , the maximum stress for the baseline standard Versilok with a bondline thickness of 0.010 inches (0.254mm) and a bake temperature of 428 ° F (220 ° C), is 399 psi (2.75 MPa). Modest changes in modulus produce a notable change in maximum stress. For example, the maximum stress for an adhesive with a modulus half that of Versilok is 307 psi (2.12 MPa) as compared to 503 psi (3.47 MPa) for an adhesive with a modulus 2 times greater than Versilok. This represents a change in stress of
approximately ±100 psi (0.689 MPa) from the baseline.
A balanced approach on material properties may be needed to optimize an adhesive for hybrid bonding. An adhesive with a higher modulus can be acceptable but it must be complimented with higher strength values at elevated temperatures. An adhesive with a lower modulus can be acceptable but it must be complimented with higher elongation properties. To achieve a significant reduction is stress a reduction by a factor of 10 may be needed in the modulus properties. Figure 3 is a graph of maximum stress as a function of modulus properties that range from 10% to 200% of standard Versilok.
The Second FEA test is the T-Peel Dead Load Creep Test. The purpose of the T- Peel dead load test is to apply stress to the adhesive bond joint in a controlled manner that can be varied by the amount of weight applied to the test sample. For this reason it is best to use the same substrate for each coupon in this test. An Al/Steel hybrid bonded sample will have additional stress applied to the adhesive that is not controlled by the amount of weight. Any substrate of interest can be chosen and aluminum will be used because delamination typically occurs at the Al / adhesive interface. The FEA model for the T-peel test with cohesive zones turned off assumes that the adhesive bond remains intact during the bake process. The calculations provided will correspond to the strength necessary for an adhesive to survive the bake process without failing. This FEA model can be run as a function of several test variables to establish the strength requirements that an adhesive will need to pass.
The amount of stress that is applied to the adhesive is directly controlled by the weight of the load applied to the sample. The maximum stress for no weight (0.0 Kg) applied to a sample with a bondline thickness of 0.010 inches (0.254mm) at 428° F
(220° C). The maximum stress is 70 psi (0.48 MPa), but is located at the backside end of the sample, away from the area of interest. This stress is due to the CTE difference between the adhesive and aluminum. The stress in the area of interest is approximately 43 psi (0.27 MPa) based on the green color coded area of the figure.
The maximum stress is 104 psi (0.717 MPa) for a 5 lb (2.27Kg) load and 280 psi (1 .93 MPa) for a 20 lb (9.1 Kg) load. The location of the maximum stress is located where the metal panels first begin bending and the initial thickness is 0.010 inches (0.254mm). Comparing the 2 figures also shows the adhesive will need higher elongation for increasing loads.
Figure 4 shows the graph of maximum stress as a function of applied load, ranging from 0 to 20 lbs (0 to 9.1 Kg). There is a linear increase in stress as a function of applied load.
A sample with 20 lbs (9.1 Kg) applied to a sample with a very large bond gap of 0.100 inches (0.254mm). The maximum stress is 138 psi (0.951 MPa) which is a
reduction of stress as compared to a bond gap of 0.010 inches (0.254) which is 280 psi (1 .93 MPa).
Figure 5 shows the graph of maximum stress as a function of bondline thickness, ranging from 0.010 to 0.100 inches (0.254mm to 2.54mm). There is a reduction is stress as a function of increasing bondline thickness, however stress reduction is fairly
insignificant compared the large changes in bond gap. For this reason the T-peel test will only be evaluated with the standard bond gap of 0.010 inches (0.254mm) which is typically used for this test.
FEA models described above assume that the adhesive bond remains intact during the bake process. We know from actual laboratory testing that the adhesive will fail when enough stress is applied to the adhesive bond joint. Adhesive strength properties need to be added to enable the model to predict whether a failure will occur. Figure 6 shows a graph of tensile strength and shear strength as a function of temperature for fully cured Versilok.
When the cohesive zones are turned on, it becomes possible to compare the FEA model predictions with actual test results. There is a significant amount of available data for the T-peel dead load creep test on standard Versilok. Historically this test has been conducted at 383 ° F (195°C ) so for comparison purpoes the FEA model will be conducted at the same temperature.
Modeling a T-peel test with an exaggerated load of 40 lbs (18.2) applied to the sample results in separation occurring at the interface of the adhesive and the substrates (both top and bottom). Additionally, the first debonding occurs at the point of highest stress which is at the point of where the metal begins to bend and the bond thickness is 0.010 inches (0.254mm), and is predicted to occur between 1 17° F (47.2° C ) and 157° F (69.4° C).
In order to compare the FEA model to actual lab tests, the model needs to be run with a series of loads applied. The model can be run by trial and error until a small range of load values separates a test that passes with a test that fails. The FEA model was run several times to determine that failure will occur between 7 and 8 lbs (3.18 to 3.63 Kg) of load applied to the sample. More runs can easily be done to narrow the range further.
Table 1 is (actual) experimental data for the T-peel dead load creep test. Two differences become obvious when comparing these results to the results of the FEA model.
1 . The actual results have a significant amount of test variability that is not accounted for in the model.
2. The actual results are significantly lower than predicted by the model.
Table 1
A significant reason for why the actual results are lower is that Versilok is not fully cured when used for normal automotive production. Versilok is cured at room temperature and the bonded assembly is baked in the e-coat oven at 383 ° F (195°C ) in this case. It has been well established that the adhesion properties of Versilok are not fully developed
until after the e-coat oven bake is completed. When the tests in Table 1 are conducted, if a failure occurs, the aluminum coupon is delaminated (or debonded) from the adhesive at the interface and there is an adhesive separation of the aluminum from the adhesive. When Versilok is room temperature cured, the adhesion strength of Versilok to the surface of aluminum is significantly lower than the cohesive strength.
If the adhesion strength is equal to or greater than the cohesive strength, the failure mode becomes cohesive failure of the adhesive. If this can be achieved than the actual test results will significantly improve and theoretically match the predicted results of the FEA model («7 lbs (3.18 Kg)).
It is not practical to actually measure the adhesion properties when the failure mode is adhesive separation. It is difficult to generate this data for tensile adhesion and shear adhesion results are highly variable. It is however possible to estimate the adhesion properties using the FEA model, the known cohesive properties, and the actual test results of the T-peel test.
We know that if the failure mode of an actual test is cohesive, the adhesion strength is equal or greater than the cohesive strength. We also know that if the failure mode is adhesive separation, the adhesion strength is less than the cohesive strength. When this is the case the FEA model of the T-peel test can be run with a reduced value of the cohesive properties, such as 80%, 60%, etc. An estimation of the adhesion strength can be made when the FEA model matches the actual lab results.
In a test with 2 lbs (0.91 Kg) and a test with 3 lbs (1 .36Kg) results obtained when the cohesive zone properties used in the model are set at 50% of the actual cohesive strength values. The actual test results have at least one failure occurring slightly above 3 lbs (1 .36Kg) and no failures occur at 2.86 lbs (1 .30Kg) or below. The estimation of setting the adhesion properties at approximately 50% of the cohesive properties is valid as long as no actual test results exhibit a failure at 2 lbs (0.91 Kg) or less.
If desired the FEA model can be run again and again to narrow the window between pass and fail with different inputs for the load applied and cohesive zone properties.
By knowing Versilok will pass the T-peel dead load test at 383 ° F (195 ° C) with 2 lbs (0.91 Kg) of applied load, we need to determine the amount of stress in the adhesive.
Figure 7 is a graph of maximum stress as a function of applied load, ranging from 0 to 20 lbs (0 to 9.1 Kg) at 383 ° F (195 ° C) and at 428 ° F (220C). There is only a small difference due to temperature.
With a 2 lb (0.91 Kg) load applied the stress in the adhesive is 62 psi (0.43 MPa). This is the target for stress that can be provided to a customer that represents the maximum level of stress that Versilok can withstand at 383 ° F (195°C). A better target might be 50 psi (0.34 MPa), maximum which includes a 20% safety factor.
This target can also be used for current applications on aluminum where
delamination problems have occurred when too much stress in the adhesive is caused by a poor fit between the inner and outer panels. This target also correlates to the
requirements for creep tests in some automotive specifications. For example, a common requirement for the creep test in shear is 0 creep with a 10 Kg load (or .030 MPa) applied to the sample during an e-coat bake at 383 ° F (195°Q.
For hybrid bonding applications, the strength values obtained from the T-peel test can also be compared to the Panel test. Figure 8 is a graph of the peak stress calculations of the panel test when conducted at 383 ° F (195°C). Using the results of 62 psi (0.43 MPa) from the T-peel test, the model for the panel test predicts a failure at 0.080 inches
(2.03mm) bondline thickness or below and passing results at 0.090 inches (2.29mm) or above. Further testing will be needed to determine if the predicted result matches the actual results. Figure 9 illustrates the maximum stress in the adhesive for various bondline thicknesses at different temperature conditions.
Standard Versilok can be used for Al/Steel Hybrid Bonding under the right conditions. Using the FEA model, we have established an upper limit for the amount of stress allowed, during the e-coat bake, of 50 psi (0.34 MPa). It should be possible to design an Al/Steel hybrid bonded assembly that meets this requirement. However, the end product may not be practical or it may not meet other requirements that are important to the hybrid assembly. For this reason material property improvements of the adhesive can also be part of the solution for hybrid bonding.
It is important to remember that the material properties need to be evaluated for the whole temperature range of the paint oven process. Some properties such as strength are more important to evaluate at elevated temperatures which are above the Tg of the adhesive.
Additionally, as discussed above stress in the bondline is a function of adhesive modulus. A lower modulus can help reduce the amount of stress, but it will require the adhesive to stretch more to accommodate the difference in thermal expansion properties between the aluminum and steel.
An adhesive with a higher modulus will restrict the relative movement of the aluminum and steel panels, but the adhesive will need to be stronger to handle the higher stress values that will be in adhesive bondline.
One approach is not necessarily better than the other in regards to high or low modulus targets. The important thing is to match material properties such as adhesive strength and elongation with the target modulus properties.
If adhesion strength is improved to a level equal to or greater than the cohesive strength, the failure mode will be cohesive failure of the adhesive in actual tests. If this is achieved, further improvements can be made by increasing the cohesive strength of the adhesive. Again it is the hot strength of the adhesive that is critical when the adhesive is most likely above the Tg. The adhesive will be in the rubbery range of its material state. High strength at room temperature will not necessarily be an improvement.
EXAMPLE 2
Several adhesives were evaluated in order to choose the best candidate for a proposed aluminum/steel roof buck for an automobile. Commercial product Versilok 253/254 was trialed on an actual vehicle and did not pass because the aluminum delaminated from the steel during the e-coat bake oven. Several prototype adhesives were screened using the 1 Foot Square AL/Steel Hybrid Bonded Panel Test and the T- Peel Dead Load Creep Test. All of the prototypes evaluated had significant improvements compared to standard Versilok. An experimental Versilok, VE, was chosen as the best candidate for future trials.
Table 2 shows the test results of the prototype VE compared to standard Versilok. The test is conducted with various bondline thicknesses to determine when a passing result is achieved. The Versilok and VE adhesives were applied at 25mm bond width and room temperature cured prior to a bake at 220°C, representing a standard process.
Comparisons between each adhesive can be made as follows:
1 . Versilok exhibits significantly more delamination compared to the prototype for each thickness tested.
2. In the areas where delamination occurs, Versilok exhibits adhesive separation from the aluminum where the prototype exhibits predominantly cohesive failure mode.
TABLE 2: Failure Mode Summary for 1 Foot Square Test
1 . Standard Versilok can only withstand approximately 1 lb (0.45Kg) of load as
compared to the prototype which can withstand up to 9 lbs (4.1 Kg) of load during the e-coat bake.
2. When failure occurs, Versilok exhibits adhesive separation of the adhesive from the aluminum where the prototype exhibits cohesive failure of the adhesive.
TABLE 3: T-Peel Dead Load Test
By comparing the failure modes of the actual tests, we can conclude that after room temperature curing is completed, the prototype has significantly better adhesion strength compared to standard Versilok, as evidenced by the adhesion failure (AF) of the Versilok. As discussed above, adhesion strength is a significant material property that is important for hybrid bonding. Since the failure modes of the prototype are cohesive (CF), we can also conclude that adhesion strength of the adhesive is equal to or greater than the cohesive strength of the adhesive. By using the T-Peel FEA model, a direct comparison of the adhesion strength of the prototype can be made to standard Versilok.
The adhesion strength properties of Versilok were established above to be approximately 50% of the cohesive strength properties. This was established by running the FEA model with various values for tensile strength and shear strength and matching
the results of the model with the actual test results. The same procedure can be used to compare the strength properties of the prototype to Versilok. The prototype passed the test with a 9 lb (4.1 Kg) load applied and failed the test with a 10 lb (4.5Kg) load applied.
After running the FEA model with various values for tensile strength and shear strength the adhesion strength of the prototype is 10% greater than the cohesive strength of Versilok. This represents an adhesion strength that is 120% greater than the adhesion strength of Versilok.
Using the chart in Figure 7 we can estimate the amount of stress that the prototype can withstand during an e-coat bake oven at 383° F (195 °C). By passing the test with a 9 lb (4.1 Kg)load, the adhesive is withstanding 145 psi (1 .OOMPa) of stress which is also greater than 2 times of the amount of stress that Versilok can handle. If including a safety factor a good recommendation for the amount of stress that is acceptable is around 100 psi (0.689 MPa).
The cohesive strength properties can be measured directly with tensile test and lap shear tests, conducted as a function of temperature. Since the adhesion strength is equal to or greater than the cohesive strength for VE, the values measured should correlate with the adhesion strength calculated by the FEA model. Figure 10 compares the tensile strength of VE and standard Versilok. Figure 1 1 compares the shear strength of VE and standard Versilok. As illustrated, the tensile strength and shear strength properties of VE are slightly stronger at elevated temperature compared to standard Versilok.
Comparing a DMA analysis of VE to standard Versilok results in the prototype having a higher Tg and therefore has a higher modulus in the temperature range of 75°F to 200°F. At high temperatures experienced in an e-coat oven the modulus of the adhesives are similar. This is the best data available and works well when comparing one adhesive to another. Further investigation of modulus properties will be conducted for actual hybrid bonding opportunities.
The last material property needed to run the FEA model is the Coefficient of Thermal Expansion (CTE). The CTE of the 2 adhesives are similar to each other. Each adhesive has one value below the Tg and one value above the Tg. The CTE values are as follows:
1 . Standard Versilok
b. Above Tg = 99 μ in / (in- ° F)
2. E1005994/E1006083
a. Below Tg = 71 μ in / (in- ° F)
b. Above Tg = 104 μ in / (in- ° F)
One additional material property that is important for hybrid bonding is elongation, although the material property is not used for FEA models. Figure 12 illustrates the elongation of the prototype VE compared to standard Versilok as a function of
temperature. The prototype has more elongation than Versilok. The elongation of both products goes up slightly with temperature until the Tg of the adhesive is reached. As the temperature increase above the Tg the elongation of both products drops significantly. The drop in elongation can potentially have a negative impact for hybrid bonding applications.
Claims
1 . A method for determining successful bondline thickness of an adhesive between two bonded substrates, comprising:
a) selecting a first substrate comprising a first coefficient of thermal expansion; b) selecting a second substrate comprising a second coefficient of thermal expansion;
c) selecting an adhesive and determining, as a function of temperature, adhesive and cohesive strength of the adhesive as well as at least two of the following three properties of the adhesive: Elastic Modulus (Ε'), Shear Modulus (G'), Poisson's Ratio (v') ;
d) inputting into a finite element analysis model of the bonded substrates the first coefficient of thermal expansion, the second coefficient of thermal expansion, and the at least two properties of the adhesive;
e) determining, through the finite element analysis model, a maximum stress applied to the adhesive at a given temperature for a given bondline thicknesses;
f) repeating step e) for a series of bondline thicknesses; and,
g) identifying a successful bondline thickness as any bondline thickness for which the maximum stress is below the cohesive and adhesive strength of the adhesive for the given temperature.
2. The method of claim 1 , further comprising repeating steps e) and f) over a range of temperatures.
3. The method of claim 2, wherein identifying a successful bondline thickness comprises any bondline thickness for which the maximum stress is below the adhesive and cohesive strength of the adhesive over the range of temperatures.
4. The method of claim 3, wherein the successful bondline thickness comprises the minimum bondline thickness for which the maximum stress is below the adhesive and cohesive strength of the adhesive over the range of temperatures.
5. The method of claim 2, wherein the range of temperatures comprises 50°F to 450°F.
6. The method of claim 1 , wherein the first coefficient of thermal expansion is not equal to the second coefficient of thermal expansion.
7. The method of claim 1 , wherein the first substrate comprises aluminum and the second substrate comprises steel.
8. A method of designing an adhesive, comprising;
a) selecting a first substrate having a first coefficient of thermal expansion;
b) selecting a second substrate having a second coefficient of thermal expansion; c) selecting a desired bondline thickness for the adhesive;
d) inputting into a finite element analysis model of the bonded substrates the first coefficient of thermal expansion and the second coefficient of thermal expansion;
e) determining, through the finite element analysis model, the maximum stress the adhesive will experience as the bonded assembly undergoes a heating and cooling cycle; f) designing an adhesive comprising a cohesive strength and an adhesive strength that is higher than the maximum stress determined in step e).
9. The method of claim 8, wherein step e) is repeated over a range of temperatures.
10. The method of claim 9, wherein the cohesive strength and adhesive strength are greater than the highest maximum stress determined over the range of temperatures.
1 1 . The method of claim 9, wherein the range of temperatures comprises 50°F to 450°F.
12. The method of claim 8, wherein the first coefficient of thermal expansion is not equal to the second coefficient of thermal expansion.
13. The method of claim 8, wherein the first substrate comprises aluminum and the second substrate comprises steel.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US201361778631P | 2013-03-13 | 2013-03-13 | |
| US61/778,631 | 2013-03-13 |
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| WO2014160134A1 true WO2014160134A1 (en) | 2014-10-02 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/US2014/025891 WO2014160134A1 (en) | 2013-03-13 | 2014-03-13 | Method for determining adhesive bondline thickness |
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| CN112765734A (en) * | 2021-01-27 | 2021-05-07 | 同济大学 | Method for predicting curing deformation and residual internal stress of door cover part of adhesive heterogeneous vehicle body |
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| US10489525B2 (en) | 2016-02-17 | 2019-11-26 | Toyota Motor Engineering & Manufacturing North America, Inc. | Systems and methods for composite thermal interface material microstructure property prediction |
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| CN112765734A (en) * | 2021-01-27 | 2021-05-07 | 同济大学 | Method for predicting curing deformation and residual internal stress of door cover part of adhesive heterogeneous vehicle body |
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