Shear Failure of Adhered Polystyrene (to Aluminum)

• banfillb
The stress will only be a function of the thermal expansion of the aluminum and the shear strength of the foam.In summary, the previous engineer did not consider the thermal expansion difference between the foam and the metal panels, and the foam has begun to fail in shear. I am looking for a temperature gradient which is "allowable" before the styrofoam will shear. I have calculated the shear strength of the foam using the elastic strain at which it will fail. If σ>F_v, then shear failure will occur.

banfillb

Hi All,

So here's the problem. A project I am working on has a composite metal panel assembly, which a previous engineer designed. The metal panels are simply glued to the polystyrene (styrofoam sm). The problem is that the previous engineer did not consider the difference in thermal expansion between the foam and the aluminum, and the foam has begun to fail in shear, and detach from the metal panels.

I'm putting together a report basically laying out the problem, and what I want to find is a temperature gradient which is "allowable" before the styrofoam will shear.

Basically what I have done is set the thermal stress due to restricting thermal expansion (σ=EαdT) equal to the shear strength of the styrofoam (452kPa) and solved for the dT...which ended up coming out to 2.39degC...which seems extremely low to me.

Any suggestions on where I have gone wrong? The only thing I can think of is that I can't directly use the thermal stress due to restriction of thermal expansion directly as a shear force...not sure.

Thanks,

Thermal stress (σ) created by thermal expansion resistance will be calculated using the following formula:
σ=E∙α∙dT
Where: σ= Thermal Stress (kPa)
E= Youngs Modulus (GPa)
α= Thermal Expansion Coefficient (m/(m℃))
dT= Temperature Differential (℃)
The thermal stress (σ) value will then be compared to the shear strength of the Styrofoam SM. If σ>F_v, then shear failure will occur.

The thermal stress (σ) will then be set to the shear strength (F_v) of Styrofoam SM in order to find the minimum temperature differential which shearing in the Styrofoam SM will occur.
CALCULATIONS:

σ=E∙α∙dT
σ=(3x10^6 kn/m^2 )(63x10^(-6) m/(m℃))(100℃)
σ=18,900 kPa
σ>F_v
∴, shearing WILL occur

452kPa=(3x10^6 kn/m^2 )(63x10^(-6) m/(m℃))∙dT
dT=2.39℃
∴, shearing will occur at any temperature differential greater than 2.39℃

You seem to have calculated the stress in the steel assuming it cannot expand, and then applied all that stress across the interface to the styrofoam.

That is the wrong thing to do (and your temperature difference is obviously much too small).

It would make more sense to find the difference in thermal strain between the metal and the foam (caused by the different expansion coefficients) and compare that with the elastic strain at which the foam will fail.

But the measured data here http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070008201_2007006834.pdf seems to imply the thermal expansion of the foam is very nonlinear (and large, and irreversible) above about 100 C.

You can probably simplify the analysis in that PDF, since I would guess Young's modulus for the foam is negligible compared with aluminum.

1. What is shear failure of adhered polystyrene?

Shear failure of adhered polystyrene refers to the failure of the bond between polystyrene and aluminum due to shear stress. This can occur when the applied shear stress exceeds the strength of the adhesive bond, causing the two materials to separate.

2. What causes shear failure of adhered polystyrene?

Shear failure of adhered polystyrene can be caused by a variety of factors, including improper surface preparation, inadequate adhesive selection, or excessive shear stress from external forces. It can also be influenced by environmental factors such as temperature and humidity.

3. How can shear failure of adhered polystyrene be prevented?

To prevent shear failure of adhered polystyrene, it is important to carefully select the appropriate adhesive and ensure proper surface preparation. The adhesive should have sufficient strength to withstand the expected shear stress, and the surfaces should be clean and free of any contaminants. Additionally, it may be beneficial to use mechanical or physical fasteners to provide additional strength to the bond.

4. What are the consequences of shear failure of adhered polystyrene?

The consequences of shear failure of adhered polystyrene can vary depending on the specific application. In some cases, it may cause structural damage or compromise the integrity of the material. It can also result in additional costs for repairs or replacements.

5. Are there any industry standards or guidelines for testing shear failure of adhered polystyrene?

Yes, there are industry standards and guidelines for testing shear failure of adhered polystyrene, such as ASTM D1002 or ISO 4587. These standards outline specific procedures and methods for testing the shear strength of bonded materials, providing a standardized approach to evaluating shear failure.