Sags & Tensions of Span Attachments

Manufacturers of Wire and Self-supporting Cables for Power or Communication purposes are typically asked to provide fourth-order polynomial equations that describe the product's Stress/Strain behavior. These polynomials are expected to be curve-fitted to actual Stress/Strain test results for validation purposes. This mathematical approximation of the Stress/Strain curves is essential for accurate computer modeling of Sags and Tensions. They are considered valid up to a strain of 0.5%. After that point a straight-line projection (tangent) to the curve is extended, if required. This curve is called the "Initial Curve" as it represents the material's response to initial stresses when first put into service.

Most materials used in Power Conductors, Wires and Self-Supporting Cables will stretch plastically (permanently) if the stress applied exceeds a certain Yield Point. For simplicity purposes, many materials would define this point as the point where the Strain reaches 0.2%; which is cited in many metal testing criteria and ASTM tests. In reality though, this point can vary with material and is only useful for conceptual purposes. Some materials will plastically stretch a small amount under small loads and then much more significantly under heavy loads. Some plastically stretch throughout their Stress/Strain curve, such as most aluminum alloys. After the initial amount of plastic stretch occurs, no further plastic stretch will occur in this same material sample up to the same maximum extent that stress was initially applied. The straight line that occurs after maximum load on any material is called the "Final with Load" curve.