Analysis to Relate Data From Radial Compression Tests on Helical Springs to Tensile Material Properties

In CANDU nuclear reactors, pressure tubes reside within a calandria tube with separation maintained by helical springs installed in the annular space. Evaluation of material degradation due to the unique operating environment requires testing of ex-service spring material by compressing short spring segments by two diametrically opposed forces. The load vs. displacement results combined with the geometry allows for the stress-strain behavior to be derived. The test specimens are effectively unmodified from the as-received condition so accurate characterization of the geometry is required. Since the test response is mainly bending, error in the radial section dimension is augmented by powers of 2 and 3 when calculating bending stress and specimen stiffness respectively. Additional complications come from nonuniform loading of the coils due to end effects. Detailed analysis that accounts for end effects is applied to the linear elastic portion of the load curve to accurately quantify the specimen dimensions. With geometry determined, the nonlinear portion of the tensile curve is adjusted to reproduce the entire load curve up to the failure point. Examples are presented to demonstrate how the load corresponding to the yield point and outer fiber stress at the failure point can be determined.