Characterization of Elastic-Plastic Fracture Behavior in Thin Sheet Aluminum

Composite overwrapped pressure vessels (COPVs) are used in space flight vehicles to contain pressurized gases. A COPV consists of a metallic liner to contain the gas or fluid and a composite overwrap for strength. The metallic liner must survive four lifetimes with the largest crack that could be missed during an inspection. Current light weight designs are considering liners that experience elastic-plastic behavior that violates the assumptions of current linear elastic fracture mechanics (LEFM) theory. Tests are being conducted to characterize the behavior of small cracks in thin aluminum coupons to both validate elastic-plastic fracture mechanics (EPFM) models and to develop test methods for experimental certification. The tests are conducted under strain control to replicate the conditions in a COPV liner. A digital image correlation (DIC) system is used to measure the applied far field strain and uses that strain to control a servo-hydraulic load frame. Small fatigue cracks (0.5–2.5 mm long) are cycled under a strain range that yields the material in both tension and compression. Two additional DIC systems are used to make high magnification measurements (about 100 pixels per mm) of the crack opening displacement on one side and the back face strain on the other side of the coupon.