Documentation for Adhesive Selection used for Aero-Chassis Integration
- Tommy Tran
This page will document the work and progress involved in selecting and validating an adhesive option for aero-chassis integration.
Final Adhesive Selected is: EA E-120HP (TDS attached to page)
Document used to compile all the work is here:
Things to look for when selecting adhesives
When deciding on what adhesive to use, one of the first things that should come to mind is identifying what substrates need to be bonded together. Most adhesives are engineered for a certain application or a limited set of materials, e.g. only wood or only steels.
For this project, the main substrates involve 4130 chromoly steel and carbon fiber reinforced plastic (CFRP). One can begin researching adhesive suppliers that have these types of adhesives and compiling a list of potential options. It is also very useful to contact adhesive suppliers directly to seek their technical expertise on what options they recommend, which can greatly reduce your research time. Some companies may not be in a position to sponsor large quantities of adhesives for free, but they may still try to offer their technical expertise as a substitute for qualifying for our sponsorship rewards.
There are many types of categories of adhesives that are meant for larger or smaller applications. By researching adhesives for your specific application, you will naturally find a common adhesive category that often shows up. Research can also be done on the adhesive categories if one is obliged to specialize in this area.
Our specific application includes large loading and large impact which calls for a stronger adhesive category. Research has found that structural epoxy adhesive are the primary options for this application.
Afterwards, there are many other adhesive factors such as working time, curing time, etc. that will be factored into the final adhesive choice. The primary focus for this project was to find an adhesive that was appropriately specified based on its adhesive strength.
How the adhesive strength required was calculated
The adhesive must sustain all appropriate loading that individual components of the car are expected to withstand. For example, aerobody panels are not expected to be structural as per regulation therefore it only needs to withstand its own weight. The bonding between the chassis and the bottom panel is expected to be structural as per our design requirements for the car to function safely therefore it needs to withstand a 2G bump and vehicle collisions (based on regulation).
The analysis for adhesive strength focused on the larger loading scenarios. If an adhesive was selected based on the strength requirement of a larger load scenario, then applications with lesser load conditions will also be accounted for.
The approach for calculating adhesive strength involved hand calculations of simplified force models and simulations using Ansys Static Structural. A simplified force model involves creating a more simplified and worst case scenario. This was to be done because there exists multiple adhesive joints between the bonding of two components, each at different angles. This made it difficult to accurately calculate the force distribution using solely hand calculations. Simulations were used to get a better approximation of the expected force the adhesive would endure. Figure 1 shows how the bonding in MS XIV can become more complex when more adhesive joints are involved which brings the need for simulations.
Figure 1: Images to show complex bonding in MS XIV compared to more basic bonding scenarios
There are two main loading scenarios that the vehicle should be designed towards:
2G bump (ASC2020, Appendix F, F.2)
This basically features a loading scenario consisting of twice the weight of the car’s mass
Vehicle collision scenarios. (ASC2020, Appendix F, F.3)
There are a total of 12 loading scenarios featuring a 5G impact at different directions and angles
Not all carbon fibre parts are expected to be structural therefore selected crash scenarios are placed at higher priority for analysis
Figure 2: Vehicle Collision Scenarios (ASC 2020, Appendix F, F.3.3)
Bonding scenarios in MS XIV that were considered in this analysis
The bonding between the Chassis and the Bottom Panel must be structural.
This structure contributes to the foundation of the car.
The bonding between the bulkheads and the Chassis.
Bulkheads A and C cover the front and back of the Chassis.
They are used for cover and may be used for mounting.
They are not designed to be structural; the Chassis is primarily responsible for withstanding vehicle impacts
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Figure 3: Shows the bonding area and force diagram for the bonding between the Chassis and Bottom Panel. Bulkhead A can be seen at the front (to the left) and Bulkhead C can be seen at the back (to the right). | Figure 4: Bonding areas for Bulkhead A | Figure 5: Bonding areas for Bulkhead C |
Table #1: Shows the bonding scenarios to be analyzed and the related design and stress elements associated with each bonding scenario.
Bonding Scenario | Design Requirement | Loading Scenario | Expected Stress | Notes |
|---|---|---|---|---|
Chassis and Bottom Panel | Structural | 2G Bump | Tensile | This analysis laid the foundation for adhesive selection |
Vehicle Crash Scenarios | Shear | |||
Bulkheads to Chassis | Strong enough to allow mounting of potential dynamic assembly parts | 2G Bump | Shear | Later in the analysis, bulkheads were concluded to not be structural which led to ceasing its analysis. |
Vehicle Crash Scenarios | Tensile |
Tips for Creating a Simplified Model or Worst Case Scenario
The importance of creating a simplified model is to create a more feasible way of understanding the problem so that basic hand calculations can be performed. It is strongly preferred that the physics model is modified in a way that makes it weaker meaning it will result in larger stresses hence the name worst case scenario. It is better to design towards an approximated higher standard than an approximated lower standard for safety reasons.
In terms of the analysis of the adhesives, creating a worst case scenario involves assuming all stresses in tensile (since adhesives are weaker in tensile and stronger in shear) and reducing bonding area (decreasing area increases stress).
Hand calculations are required to compare with simulation results. If the simulation results are far from your hand calculations, it strongly suggests that a miscalculation or a misinterpretation of the physics model has occurred. It is important to revisit the assumptions made.
Hand Calculations and Simulations for Finding Adhesive Strength Requirement for Bonding the Chassis and the Bottom Panel
Loading Scenario: 2G Bump
A 2G Bump is a frequent loading condition that a car will undergo. By selecting an adhesive to withstand this load, it can be certain that the bonded adhesive joints will remain intact during regular travel.
Finding Sum of Forces:
Finding Bonding Area:
Bonding Area was found by reviewing the SolidWorks Assembly and using the measuring tool. The bonding area is where two parts are expected to interface with each other and have adhesives applied.
Initial bonding area was determined to be all flat tubes at that touch the bottom flaps of the Bottom Panel. After multiple analysis reviews, the bonding area has been reduced to create a new worst case scenario.
The analysis consisted of finding the force reactions in the Chassis tubes that interfaced with the Bottom Panel. If all tubes were selected, then an average value of the forces would be given. This is easily calculated by hand. Simulations allow for more in-depth analysis by allowing probes to measure specific tubes. This means we can know the approximate force reaction of each tube. Contact probes were used to remove the need to add unnecessary fixed boundaries that may invalidate the physics model.
Figure 6: The final bonding areas between the Chassis and Bottom Panel
Debrief of Results:
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Figure 7: Hand Calculations and Simulation Results
Loading Scenario: Vehicle Collisions
Vehicle Collisions include the absolute worst case scenarios that a vehicle is designed to withstand in the case of emergency. In terms of selecting an adhesive, only vehicle components that are required to be structural will need to have sufficient adhesive strength. In MS XIV, there are many bonded joints between carbon fibre parts and steel tubes, but not all joints are designed to be structural. For example, the purpose of an aerobody is to enhance our vehicle’s aerodynamics; it is not meant to sustain a load as large as a vehicle collision. On the contrary, the purpose of the Chassis is to withstand large impacts and large loads, i.e. vehicle collisions, therefore it is very much a structural part.
When analyzing MS XIV, the most concerning structural joint being bonded by adhesives is between the Chassis and the Bottom Panel. This connection revolves around very frequent loads and large range of loads during vehicle use. Other potential areas of concern include the Bulkheads A, B, and C. Research and analysis has been done on the stresses in the Bulkheads, but it was concluded that the Bulkheads did not serve a structural role. The largest area of debate was whether the Bulkheads served a role in distributing the force during a vehicle collision as they spanned the entire width of the car. As the Chassis has already been designed to independently withstand vehicle collisions, the analysis of the Bulkheads was no longer significant to selecting a sufficient adhesive.
In face of the vehicle collisions, the main stress induced in the adhesive joint between the Chassis and the Bottom Panel is a shear stress.
Figure 8: Hand Calculations and Simulation Results
Safety Factor of Adhesives can be found on this page:
Adhesive Research (Types of Adhesives, Adhesive Behaviour, Adhesive Design Tips)
In conclusion, our minimum safety should be 6.
Summary of Results
From extracting the information from the hand calculations and simulation results, the peak stresses are as outlined in the table below.
Table #2: Strength Requirements Derived from Simulations and Hand Calculations
Loading | Hand Calc Stress (MPa) | Simulated Stress (MPa) | Types of Stress |
2G Bump | 0.17 | 0.47 | Mostly Tensile |
B Pillar Impact (5G) | 0.93 | 2 | 1.5 MPa in Tensile and 1.3 MPa in Shear |
Minimum Safety Factor Required: 6
Final Decision on Adhesive and Quantity
The process of making a final decision on an adhesive involves finding suppliers and adhesive options for your application. As mentioned before, this process can be shortened through asking companies for their recommendations for our application. They might even provide free samples. Once adhesive options have been gathered, it is valuable to collect information from each adhesive’s technical data sheets to compare critical factors such as tensile strength, working time, curing time, curing temperature, and any other factor relevant to your application.
In the case of MS XIV, many discussions have been had with Henkel and they were generous enough to already provide us samples. They have also given a list of their recommendations for our specific application. From their list of recommendations, there were a few that met the design requirements. Here is a list of things that should be considered:
Adhesive Strength
Components
If it’s two component then it requires mixing
Cure Temperature
If the curing process requires high temperatures then it would increase the difficulty and inconvenience
Working Life
After the adhesive is applied, the adhesive joint must be held with pressure for a duration up to 24 hours. The process to put the bonding parts into a jig may be difficult or time-consuming depending on the complexity of the adhesive joint
Bond line thickness
This is the thickness of the adhesive layer that will produce the adhesive’s maximum strength. It is better to have one that is slightly larger because in-house manufacturing of carbon fibre parts can result in gaps up to 3 mm.
Although, it was found that adhesives generally have similar bond line thickness of between 0.2-0.3 mm.
Figure 9: Example of a Comparison Table
The final adhesive decision was E-120HP because it had a significantly longer working life. It has a tensile strength of 41 MPa therefore it will have a safety factor of 20.5. It is reassuring to have a larger than necessary safety factor because as of right now it is uncertain whether the manufacturing of carbon fibre parts and the process of applying adhesives will yield a bond line thickness within the adhesive’s ideal range. This larger adhesive strength will hopefully compensate for any causes for concern.