Inverse Autofrettage for Titanium Hydraulic Tubing

“Inverse Autofrettage” is an adaptation of the century old autofrettage method used to generate residual compressive stresses at the inner surface of thick walled tubes like cannon barrels. It relies upon briefly exposing formed hydraulic tubing (which is thin wall, in this case) to very high hydrostatic pressure in a specific manner that results in large residual hoop compression strain. With inverse autofrettage the entire wall thickness of the tube is prestrained, not just the inner bore surface. This prestrain reduces the operating hoop tensile stress in the tube under hydraulic pressure, significantly increasing fatigue life and reducing propagation rate of surface flaws such as scratches. In Phase I the process is optimized both analytically and through process trials on ¼” tubing. Tube sections are treated to various residual strain levels, and these strains measured by X-ray diffraction. Tube specimens are then impulse pressure fatigue tested at 5,000 psi, with and without a series of carefully controlled, laser generated surface notches to generate fatigue life curves. In the Option, similar work is performed with formed (curved) tubing sections. Substantial improvements in cycle life and flaw tolerance are anticipated, and several commercialization partners have expressed interest in the process.