Coming Full Circle: The Application of Microtechnology Techniques to Evaluate Bulk Materials

ABSTRACTA tensile test procedure that accommodates specimens with gage section 25 μm thick, 70 μm wide and 360 μm long was developed and demonstrated. The instrumentation and technique were adapted from those previously developed and used to test thin films, by increasing both the force capacity of the load cell and the stiffness of the pull rod. Specimens with bow-tie geometry were fabricated by photolithography from nominally 25 μm thick full hard stainless steel 302 foil. A silicon test frame fabricated by bulk micromachining techniques included tapered grips in the form of recesses in its top surface that accepted and retained the specimen grip sections. One grip was on the fixed outer portion of the frame. The other grip was on a plate suspended in the center of the frame by long slender silicon beams. Force was imposed on this plate by pin loading. The force was measured by use of a custom load cell. The displacement was measured by sub-pixel digital image correlation to surface features on the two ends of the gage section, applied to images with a resolution of approximately 0.8 μm per pixel. Yield and ultimate strengths and elongation values consistent with vendor-provided information were obtained. The values of Young’s modulus were scattered but within the range of expected behavior for the specimen material.

Fatigue of Small Helical Extension Springs Subjected to Static Torsion and Alternating Lateral Deflection

Multiaxial fatigue studies of peened helical extension springs made of music wire were performed in a specially designed testing machine. The test regime imposed static torsion and alternating laterial deflection, with the spring ends held parallel. The maximum stresses, which were primarily due to bending, occurred in the first turn out of the specimen grip at an end of the coil axis normal to the direction of lateral motion. The stresses in the middle turn ranged between 88-94 percent of those in the ends turns. The practical fatigue limit was about 100,000-300,000 cycles, with the principal tensile stress averaging 50 percent of initial wire tensile strength and maximum shear stress averaging 28 percent of initial wire tensile strength. For static stresses up to 32 percent of tensile strength, the alternating stress at the fatigue limit was essentially invariant, as predicted by Sines’ criterion for multiaxial fatigue.