Tensile Fatigue Selection:

Several concepts were reviewed for the tensile loading mechanism.  Listed below are the concepts, a brief explanation, and the reason for pursuing or not pursuing.  All concepts reviewed were taken from ASTM E 466.  Many concepts were determined not feasible early in analysis and thus no design matrix was created.

Hydraulic System:  A single or dual action piston which moves the cross head up or down.

Reason for not pursuing:

-Lower than desired cycle frequency capabilities for cost.

-High maintenance.

Rotating Mass:  An eccentrically rotated mass is used to produce a tensile load on a specimen.

Reason for not pursuing:

-Extremely complex design.

-Limited loading capabilities.

-Applying bending load would have been extremely difficult.

Pneumatic solenoids:  Utilizes compressed air to produce forces. 

Reason for not pursuing:

-Noise (machine would have been operated in school machine shop) would have been a nuisance.

-Would not be able to control the shape of the load curve. 

Magnetic solenoids:  Use an electromagnetically inductive coil to move a metal slug in and out of the center of the coil. 

Reason for not pursuing:

-Solenoids available at desired load capabilities were out of price range.

-Push rods in solenoids within price range had small diameter, leading to concerns about durability.

The following two methods were looked at more closely.

Mechanical System:  Utilizes a motor driven system of lever arms to displace end of specimen.

Reasons for Pursuing:

-Desired frequencies can be achieved.

-Specimen can be forced back to original position.  No need to wait for system to relax back to original position on its own.

-Sinusoidal load curve can be produced for both tensile-only or fully reversed cycling.

Reason for not pursuing:

-Tight tolerances would make fabrication difficult.

-Load would be calculated via displacement.

Electromagnets:  Can push off against another magnet or attract a material with a high magnetic permeability.

Reasons for pursuing:

-With appropriate electronics and configurations, could produce a wide range of load curves.

-Off-the-shelf product

-No moving parts.

Reasons for not pursuing:

-Relatively low power within price range.

-Must guard against overheating (especially for a long duration test).

-Force decreases rapidly with distance.  Therefore precise positioning is more important.

Decision:

A mechanical system, similar to the one above, was initially pursued until it was determined that fabrication difficulties would not be overcome.  Electromagnets were then chosen (Figure 3-14b), along with an on/off power cycler that produces a square wave for the load curve.