Analysis
The 3.00" x .562" x 14.2" tube extrusion, through symmetry conditions, was able to be broken down to the point where the computer model was approximately 1/16 of the actual tube geometry. All the features of the extruded tube were included in the computer model with the exception of the ribs in the end radii of the internal fluid passages; thus providing for a very accurate, yet somewhat conservative analysis. Refer to the appendix to see a drawing for further description.

The three inch extrusion was chosen from the family of extruded tubes because it provided for a worse-case scenario analysis for pressure cycling. This is based on the fact that it has the widest internal fluid passage (.66" wide) of all the tubes. The length of approximately 14" is the maximum length between welds along the tube length.

The finite element analysis (FEA) model of the tube as stated above was restrained at the end where the tank and other tubes would be attached. It was also restrained for symmetry on all "sectioning" planes. These parameters reflect the actual tube as assembled into a core assembly where the tube is welded to adjacent tubes and tanks at each end, and to the adjacent tube along its length at approximately 14 inch intervals.

All computer modeling and finite element analysis was performed using SolidWorks 99 and Cosmos/Works V5, build 1999/197.

Results & Conclusions
The FEA calculated stresses and deflection at 250 psig are 19,162 psi and .0009" respectively, within the elastic range of the Aluminum material (The results of the FEA are detailed in the appendix.). The Aluminum material properties for 6063-T6 are as follows:

    • Yield Tensile Strength = 31,175 psi
    • Ultimate Tensile Strength = 34,800 psi
    • Endurance Limit (500,000,000 cycles) = 10,150 psi

(Mechanical properties furnished by "MatWeb" at www.matweb.com. Other fatigue data furnished by "Fatigue Design of Aluminum Components & Structures" by Sharp, Nordmark and Menzemer.)

Because of the pressure cycle of 0-250 psig, the stress in the tube is expected to be approximately cyclic from 0-19,162 psi. With that, it is apparent that our loading and stress level is not completely reversed and that a mean stress other than zero exists (i.e. to be completely reversed the pressure might need to cycle from —250 to 250 psig and the stress level from —19,162 to 19,162 psi).

To gain an understanding of the life of the Aluminum oil cooler tube extrusion with a non-zero mean stress, two graphs were constructed. The first is that of the tube stress level through the pressure cycle. It is labeled as "Maximum Stress @ 0-250 psig Pressure Cycling" and displays the resulting mean and alternating stress levels. The second chart is a constant-life fatigue diagram for the 6063-T6 Aluminum. This diagram is based on fatigue curves for completely reversed stresses and ductile material theory. It allows the projected life of a part with a given material to be determined based on the mean and alternating stress levels.

This analysis has been done, and it has been determined that the tube stresses (mean and alternating = 9,581 psi) are greater than the endurance stress limit at 100,000,000 cycles (mean and alternating = 8,357 psi) and less than the endurance stress limit at 10,000,000 cycles (mean and alternating = 9,726 psi). Therefore the life of tube will fall within this range. Refer to the "Oil Cooler Tube Analysis Graphical Representation…" on the following page for a visual summary.

Recommendations
Based on this analysis, at a pressure cycle of 0-250 psig, the life of the 300EXT562 extrusion will be in the 10,000,000 to 100,000,000 cycle range. All other extruded tubes for the oil cooler application would be expected to have a longer life than that of the tube analyzed.

Sincerely,

Brian Merklein
Mechanical Engineer
Schmidt Engineering, Inc.
4466 Hwy. P - Suite 204
Jackson, WI 53037
Phone: 262-677-1221
Fax: 262-677-1224
Email: info@schmidtengineering.com


Appendix