Prism - Residual stress measurement based on hole-drilling and ESPI
Prism is an advanced hole-drilling system that dramatically improves the ability to make quick and accurate residual stress measurements. Just like the conventional strain gage hole-drilling systems, the instrument measures changes in the part surface caused by the hole drilling and determines the previously existing residual stresses. However, the often time-consuming application of strain gages is avoided and a large area around the hole is analyzed, not averages from just a few areas. Prism measures surface distortion using electronic speckle pattern interferometry (ESPI). Only minimal sample preparation is required.
The update of the coefficient data set in 2015 has significantly improved the quality of the stress results. This also increases the relative depth from which reasonable results can be expected.
A standard Prism system comprises
- the main unit with controllers and power supplies, also housing the laser light source and electronic shutters,
- an illumination stand,
- the video head,
- a high-precision, high-speed spindle system (air-cooled electric drill),
- a pneumatic air controller and a chip air block
- a PC and
- the software package for the measurement and calculation of residual stress depth profiles using the Integral Method.
- Little preparation: clean measurement surface with low-reflectivity (may be spray painted)
- Non-contact: only requires direct visual path to part; no strain gages to be applied
- Fast: extensive depth profile within ½ hour
- User friendly: easy-to-use Windows based software
- Easy: computer controlled, automatic measurement
- Materials: includes materials difficult for XRD like titanium and plastics; limited mainly by the ability to drill holes of good quality
Hole-drilling is a residual stress measurement technique, where stressed material is removed by drilling a small blind hole in the area of interest. The remaining material around the hole spontaneously finds a new stress equilibrium. This re-arrangement of stresses leads to a slight distortion of the surface near the hole. Though the displacements are small, they are measurable with ESPI and allow the calculation of the stresses that were present in the part prior to drilling.
Electronic Speckle Pattern Interferometry (ESPI)
The sample is illuminated with coherent laser light. Due to optical surface roughness, which exists on many sample surfaces naturally, the image the camera receives is not sharp but shows a speckle pattern. Diffuse light scattering lets each camera pixel receive light from multiple locations, which interferes constructively or destructively, creating bright and dark spots in the image. This speckle pattern is characteristic of the surface topography and changes with surface shifts.
The camera image is created by the interference of the object beam with the reference beam. The latter is phase shifted in ¼-wavelength increments and an image is taken for each shift, so that the surface condition is characterized by four images, before and after each drilling increment. The four images allow calculating a phase angle for each pixel, which then is translated into a surface displacement.
The typical procedure starts with determining the sample surface position relative to the drilling tool tip. In the standard method, the user moves the drill towards the sample using software commands and assesses the live camera images. Alternatively, zero may be determined for electrically conducting materials with an optional attachment. The user then selects the desired drilling depths and other drilling parameters and starts data acquisition.
The software’s user interface makes it easy-to-use while providing a variety of options for adjusting measurement and calculation settings. The user can activate each drilling increment separately and take laser and white-light images manually. But the measurement can also be performed fully automatically for the whole depth list or by any combination of the two methods. The measurement can be interrupted at any time. Each drilling increment can be subdivided into multiple steps to control contact time between tool and part. Data analysis is performed after completion of a measurement. A measurement can be appended to as long as the sample hasn’t moved.
The basis for the stress calculation is a set of coefficients for a cylindrical hole in a semi-infinite body and a planar stress state. The stress depth profiles are calculated using the Integral Method. Tikhonov regularization (a form of smoothing) is optional. The stress calculation is equivalent to that described in ASTM E837 for strain gage hole-drilling, yet correlates displacements directly with stresses. Strains are not evaluated. The software allows the user to set the ring-shaped analysis area and apply an optional pixel correction that replaces values of poor-quality pixels through interpolation. The stress calculation assesses the changes occurring in each individual drilling step – rather than changes relative to the starting condition. This means that errors do not accumulate and that the measurement is less sensitive to disturbances.
The software is designed to calculate stress depth profiles for multiple scenarios, such as different analysis areas, depth increment and image set selections, and regularization factors. It provides automatic graphing for comparing the results. Multiple measurements also can be compared easily. Graphs are generated for stresses in the sample coordinate system – horizontal and vertical directions, and shear stress – and for the principal stress directions. Data from any and all calculations and measurements can be copied with a single command for use in spreadsheets.
References related to Prism
- M. Steinzig and E. Ponslet, "Residual Stress Measurement using the hole drilling method and laser speckle interferometry, Parts I-IV”, Experimental Techniques, Vol.27, Issues 3,4,5,&6, 2003
- G. S. Schajer and M. Steinzig, “Full-Field Calculation of Hole-Drilling Residual Stresses from ESPI Data”, Experimental Mechanics, Vol.45, No.6, pp.526-532, 2005
- G.S. Schajer and M.B. Prime, “Use of Inverse Solutions for Residual Stress Measurements”, J. Eng. Mater. Technol., 125(3), pp.375-382, 2006
- Y. An and G. S. Schajer, “Pixel Quality Evaluation and Correction Procedures in ESPI”, Experimental Techniques, Vol.105, pp.106-112, 2010
- G.S. Schajer and T.J. Rickert, “Incremental Computation Technique for Residual Stress Calculations Using the Integral Method”, Experimental Mechanics, Vol.51, No.7, pp.1217-1222, 2011
References for hole-drilling in general
- ASTM E837 - 08e2 Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method
- Good Practice Guide No. 53 - The Measurement of Residual Stresses by the Incremental Hole-Drilling Technique, P V Grant, J D Lord and P S Whitehead, The National Physical Laboratory, UK
Prism brochure in English (pdf, 1500 KB)
For more information please contact Stresstech Group offices.