Hole – drilling
The hole drilling method is considered a semi-destructive mechanical method to measure residual stresses using strain gages (residual stress measurement). It allows accurate experimental stress analysis in different materials and locations at moderate budget. Indeed, in terms of cost, accuracy and versatility, this is one of the most efficient, well reputed and widely used in industrial field, semi-destructive methods for the measurement of residual stresses.
The hole-drilling method consists in drilling a small hole using a dedicated high speed drilling technology in the centre of a special strain gage rosette with a minimum of 3 grids. The drilling process is realized step-by-step: at the end of each drilling step, the released strains are acquired by a dedicated digital strain gage amplifier. Then, all the acquired strains are processed in order to evaluate the residual stresses that originally were present in the workpiece.
The hole-drilling strain-gage method has been standardized at world level by the ASTM E837-20 “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gage Method”.
The ASTM E837-20 specifies the number of the required drilling increments, the numerical coefficients to find out the value of residual stresses, the data processing method and the measurement-related uncertainty. ASTM E837-20 standard specifies also the complete hole-drilling procedure to determine residual stress profiles near the surface of an isotropic linearly elastic material: it includes the requirements in term of instrument specifications, practical step-by-step information about the drilling and acquisition process and final calculation algorithms with related coefficients for the evaluation of residual stresses.
Since 2010 SINT Technology has been accredited to standard ISO/IEC 17025:2017 by the Italian accreditation body ACCREDIA with accreditation number 910 for the execution of laboratory and in-field hole drilling measurements according to ASTM E837
Depending on the testing conditions, SINT Technology has developed a dedicated evaluation software, called EVAL, used also for its measurement service that can overcame some typical limitations of the hole drilling method according to the ASTM standard (intermediate thickness), for the correction of the main sources of experimental errors in that measurement (errors generated by eccentricity and bottom hole chamfer) and specific algorithms for the evaluation of the measurement uncertainty.
The hole-drilling measurements can be carried out in different testing configurations depending on the dimensions of the strain gage rosette and the related endmill used to perform the hole: by changing these parameters it is possible to increase the accuracy close to the surface or, in alternative, the total depth of analysis.
According to the ASTM standard there are 3 possible different hole-drilling configurations, depending on the sizes of the strain gage diameter and the related endmill:
- Strain gage rosette with a diameter of 1/32’ (approximately 2.56mm) using an endmill of approximately 0.8mm/1.0mm can be used for the evaluation of the residual stress up to the total depth of 0.5mm with an optimal step resolution closed to the surface of the workpiece.
- Strain gage rosette with a diameter of 1/16’ (approximately 5.10mm) using an endmill of approximately 1.8mm/2.0mm is the most universal solution because it offers a good depth of analysis of 1.0mm with a good step resolution close to the surface of the workpiece. Several different strain gage rosettes available in the market belong to this category.
- Strain gage rosette with a diameter of 1/8’ (approximately 10.26mm) using an endmill of approximately 3.8mm/4.0mm can be used for the evaluation of the residual stress up to the maximum total depth of 2.0mm with a lower step resolution close to the surface of the workpiece.
SINT Technology is able to offer a dedicated measurement service using the hole drilling method both in laboratory or directly on-field. This method can be applied in different materials as standard metals, advanced engineering alloys, polymeric and composite materials.
Furthermore, for special or non-standard applications, SINT Technology is able to customize the hole method by using rosettes specially designed with larger dimensions than the commercial ones (Custom Hole Drilling). In this way it is possible to obtain the calculation of the residual stresses up to a maximum depth of about 10.0 mm by realizing a hole with a larger diameter: this method allows to obtain a complete stress profile up to higher depths without applying more complex and much more expensive methods.
Depending on the dimensions of the customized rosette used in the measurement, SINT Technology is able to calculate the matrices of the most suitable coefficients for carrying out the calculation of the uniform or not-uniform residual stresses in these non-standard conditions.
Further readings & Scientific Papers
- ASTM E837-20. Standard Test Method for Determining Residual Stresses by the Hole-Drilling StrainGage Method. West Conshohocken, PA: ASTM International, West Conshohocken; 2020. Available from: www.astm.org
- Emilio Valentini, Lorenzo Bertelli, Alessio Benincasa and Simone Gulisano (December 13th 2019). Recent Advancements in the Hole-Drilling Strain-Gage Method for Determining Residual Stresses, IntechOpen, DOI: 10.5772/intechopen.90392. Available from: https://www.intechopen.com/online-first/recent-advancements-in-the-hole-drilling-strain-gage-method-for-determining-residual-stresses
- Valentini E, Bertelli L, Benincasa A. Improvements in the hole-drilling test method for determining residual stresses in polymeric materials. Materials Performance and Characterization. 2018;7(4):446-464. DOI: 10.1520/MPC20170123 ISSN 2379-1365
- Schajer GS, Whitehead PS. Hole drilling and ring coring. In: Schajer GS, editor. Chapter 2 in Practical Residual Stress Measurement Methods. Chichester, UK: Wiley; 2013. pp. 29-64
- Ajovalasit, M.Scafidi, B.Zuccarello, M.Beghini, L.Bertini, C.Santus E.Valentini, A.Benincasa, L.Bertelli, The hole-drilling strain gauge method for the measurement of uniform or non-uniform residual stresses AIAS TR-01:2010 AIAS Working Group on Residual, 2010, https://www.mts3000.com/pdf/AIAS_TR_01_2010_2.pdf