Post-Yield Material Characterization for Strain-Based Design

Summary Conventional material specifications and test methods were developed to support load-based designs in which inelastic deformations are relatively small and yield strength is the primary material factor governing design. However, in strain-based designs where substantial portions of the structure soften under post-yield deformation, more detailed characterization of the post-yield material behavior is required. This paper presents a framework for describing the post-yield properties of metals (including strain-rate dependence of yield strength) a testing method for measuring post-yield strength in terms of strain and strain rate, and an analytical basis for extrapolating measured properties to static conditions for strain-based design and quality assurance (QA). Introduction Typical test specifications for determining the mechanical properties of oil-country tubular goods (OCTG) were developed to provide an index of mechanical strength to support common load-based design methods. Advancing recovery techniques impose conditions on many well structures that exceed the limits of these methods and the material characterizations on which they are founded. Among these new techniques are those used to recover heavy oil. While typical conditions in heavy-oil reservoirs appear benign, enhanced-oil-recovery (EOR) methods such as thermal stimulation and ultrahigh sand production create some of the most challenging conditions for well structures. Imposed deformations commonly exceed the yield limit of the material, therefore post-yield material characteristics govern much of the structural response. Industry-standard material tests provide only limited characterization of post-yield behavior, particularly at strain levels near the yield point (both pre- and post-yield). Furthermore, test strain rates can affect the measured material strength significantly. Field loading usually occurs at much lower rates and is then sustained for extended periods. A method for characterizing post-yield material properties is, therefore, desired to adequately support designs for such applications. This paper proposes a new basis for characterizing mechanical steel properties that provides the static strength and stiffness over the post-yield strain range. Relaxation characteristics are interpreted from testing, and local stiffness properties are provided. Although static properties are inferred, the test and interpretation basis allows the tests to be executed in a relatively brief time frame, making it possible to apply the method in QA programs to confirm post-yield properties for strain-based designs. A test apparatus built to implement the material-characterization protocol is presented, and sample results are provided to demonstrate the method.