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 with the conventional strain-gage hole-drilling systems, the instrument measures changes in the part surface resulting from hole drilling and determines the previously existing residual stresses. However, the often time-consuming application of strain gages is avoided and the whole field of view can be analyzed. Prism measures surface distortion using electronic speckle pattern interferometry (ESPI). Only minimal sample preparation is required.
Completely new Prism software PrismS is being released in 2013. The modern interface organizes the data better and has automatic graphing, with built-in comparisons of results from different measurements and different analyses from the same data. New pixel quality evaluation helps optimizing the measurement setup and analyzing the results. An option for pixel correction has been added and the stress calculation has been sped up.
A standard Prism system comprises,
- a laser light source,
- illumination and video heads
- a high-speed drill
- a PC
- a software package for measurement and calculation of residual stress depth profiles using the Integral Method.
- Little preparation: clean measurement surface with low-reflectivity
- 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.
The hole-drilling method is considered to be a semi-destructive measurement method, because the hole drilled may be negligibly small relative to the part performance.
Electronic Speckle Pattern Interferometry (ESPI)
The sample is illuminated with coherent laser light. Due to optical surface roughness, which exists on most 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 shifts of the sample surface.
Prism uses a set of four images to describe each drilling increment. A reference beam interferes with the object beam on the CCD. Its phase is shifted by 0°, 90°, 180° and 270° for the four images. This allows calculating the phase angle for each pixel. The surface distortion due to drilling each depth increment is determined quantitatively as a phase change.
The typical procedure starts with determining the sample surface position relative to the drilling tool tip. The standard method is visual surface detection, which is independent of material properties. For electrically conductive materials, an automatic electrical contact method can be used (optional). The user then selects the desired drilling depths and starts data acquisition. Laser images of the surface are taken after each drilling increment. The stress calculation module includes tools for data review and correction.
The software integrates drilling and imaging and provides multiple variables for adjusting the process to the given measurement situation. One can activate each drilling increment separately and take laser and white-light images individually. Measurements can also be performed fully automatically for the depth list set in advance or by any combination of the two methods. Laser images are saved automatically during automatic measurement.
A planar stress state is assumed and stress depth profiles are calculated using the Integral Method. Tikhonov regularization can be applied (a form of smoothing) and poor quality pixels can be interpolated. The software provides automatic graphing for each stress calculation and makes comparisons between multiple calculations from the same data as well as from multiple measurements easy. Stresses are calculated for the sample coordinate system – horizontal and vertical directions, and shear stress – and for the principal stress directions.
The stress calculation algorithm is based on 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.
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, 1800 KB)
For more information please contact Stresstech Group offices.