Contour method

The Contour Method was invented by Dr Mike Prime of Los Alamos Laboratories for measuring and mapping residual stresses through the thickness of large engineering components. The method is particularly attractive in that it provides a 2-dimensional map of residual stress (one direct stress tensor component) over a cross-section of a component using a single cut. It is a strain relaxation method that is conceptually and experimentally simple, inexpensive, and uses equipment available in most engineering workshops.

At present the technique can readily measure residual stresses across sections greater than 5 mm thick and welded structures where the microstructure varies. It is increasingly being applied to map residual stresses in components to support structural integrity assessments, validate process models including weld simulation and underwrite new manufacturing routes across diverse industry sectors (nuclear, power generation, aerospace, petrochemical and transport).

The method involves making a straight cut in the sample of interest along a plane where knowledge of residual stresses is required. The created cut surfaces locally deform owing to the relaxation of residual stresses present before the cut. These deformations are measured and then applied as a boundary condition in a finite element model to determine the out-of-plane residual stress distribution at the cut surface.

Implementation
The contour method is implemented by undertaking the following steps:

  •  Specimen cutting

Cutting the structure of interest is the most critical step in the contour measurement procedure. Wire electric discharge machining (WEDM) is currently the best machining method for cutting parts. It is a non-contact technique that uses an electrically charged thin moving wire where the energy contained in a spark is used to remove material. The “finishing” or “skim” mode of WEDM cutting is preferred as the relatively low electrical energy of this mode restricts its depth of influence on material properties and associated residual stress to a few tens of microns. This is considered to be negligible in the context of contour measurements.
 

  • Surface contour measurement

After WEDM cutting, the out of plane deformation contours of the severed faces are measured using a coordinate measuring machine with either a touch trigger probe or an optical probe.
 

  • Data processing

Raw measured deformation data from the two matching surfaces are first averaged to eliminate anti-symmetric errors (e.g. a curved cut) and the effect of shear stresses. Then, extreme outliers are removed and the data smoothed, for example using 2-dimensional spline fitting, Fourier series or polynomial methods.
 

  • Stress calculation using finite element (FE) method

Finally, the smoothed displacement contour is applied as a boundary condition to the cut face of the structure in a linear elastic stress analysis giving the distribution of residual stresses in the structure which have relaxed.  The residual stresses at the cut face are equal to those in the original uncut structure.

 

Schematic of the contour method