Hole Drilling Techniques

Transcription

1 Residual Stress in Context. How to Choose a Technique Hole Drilling Techniques Jerry Lord The NPL Materials Centre

2 Breakdown of Presentation Introduction and Background Basic Hole Drilling Technique Recent Developments Examples Summary NPL Project on Residual Stress

3 Survey of UK Industry 48 replies to Questionnaire - April organisations performed measurements Hole Drilling (22) XRD (19) Neutron (14) Layer removal (12) Other (7)

4 Hole Drilling Technique Taken from: UMIST Stress & Damage Characterisation Unit Brochure

5 Hole Drilling Schematic of Relieved Strains Measure surface strains Calculate original RS distribution from these Relieved strains decay rapidly with depth Limited and variable strain sensitivity Susceptible to errors Hole position Hole diameter and eccentricity Hole depth Induced machining stresses Milling cutters, end mills, drills > 40 me in carbon steels > 300 me in austenitic steels

6 Development of Hole Drilling Technique Kirsch (1898) Developed empirical solution for through hole, uniaxial stress field Mathar (1934) application of a mechanical extensometer Soete & Voncrambrugge first application of strain gauges Kelsey (1956) variation of RS with depth using the blind hole technique Rendler and Vigness (1966) - Empirical analysis for blind hole, developed standard rosette geometry Beaney and Procter (1974) air abrasion machining Bijak-Zochowski (1978) non uniform stress calculations Schajer (1981) - Used FEA, tabulated calibration coefficients Schajer (1988) Coefficients for non-uniform stress fields ASTM E and Measurements Group TN-503-3

7 Basic Gauge Design D is diameter of gauge circle D o is diameter of drilled hole

8 Residual Stress Gauges Limited number of designs 3 and 6 element rosettes Typical hole diameter is 1 4 mm Surface preparation is critical Good quality installation 3 wire leadwire for temperature compensation Instrumentation resolution 1

9 Typical Hole Drilling Set up

10 Features of Hole Drilling Technique Basic Hole Drilling Localised RS measurements Strains are averaged over the area of the strain gauge Limited application Assumptions Isotropic, linear elastic material Stresses do not vary significantly with depth Residual stresses do not exceed 0.5 yield strength Variations of stress within the boundaries of the hole are small Developments Incremental hole drilling Analyses for non-uniform stress fields Analyses for steep stress gradients Elastic plastic behaviour

11 Key Attributes of the Hole Drilling Technique Merits Large components Portable, in situ measurements Rapid and Inexpensive Little specialised equipment Semi-destructive Flat and curved shapes Wide variety of materials Metals, plastics, ceramics composites, cermets, coatings single crystals, surface treated, crystalline and amorphous, magnetic and non-magnetic Disadvantages Limited to near surface stresses Modest strain sensitivity Strain sensitivity varies with depth Limited spatial resolution Induced machining stresses Interpretation of results More qualitative than quantitative? Destructive!

12 Basic Hole Drilling Equations σ max, σ min = ε 3 ε 1 (ε 3 ε 1 ) 2 + (ε 3 + ε 1-2ε 2 ) 2 4A 4B p = (ε 3 + ε 1 )/2 q = (ε 3 - ε 1 )/2 t = (ε 3 + ε 1-2ε 2 )/2 where A = -a (1 + υ) / 2E and B = -b / 2E a and b are dimensionless calibration coefficients almost independent of the specimen properties

13 Coefficients (ASTM E837-99) Coefficients are tabulated for the 3 main gauge types Through hole and incremental drilling Typical increments 0.05D Max depth for a blind hole is ~ 0.4D Some interpolation may be required

14 Coefficients (ASTM E837-99)

15 Basic Hole Drilling Equations If.. P = (σ y + σ x )/2. mean hydrostatic stress Q = (σ y + σ x )/2. shear stress 45 o to x-y axes T = τ xy /2.. shear stress along x-y axes σ max, σ min = P Q 2 + T 2 τ max = Q 2 + T 2

16 Strain Sensitivity

17 Recent Developments and Applications Strain Measurement New rosette designs 6 gauges Full field measurements Laser speckle interferometry Moiré interferometry Holography Hole Drilling methods Taper hole drilling Optimised incremental drilling Deep hole drilling Modelling and Data Analysis Non uniform stress fields Biaxial stress distributions Steep stress gradients New Applications Thermal Barrier Coatings Thin coatings?

18 Analysis Methods Non Uniform Stress Fields ASTM E does not apply 4 Stress calculation methods have been developed Integral Method - Best for highly non uniform stress fields Power Series - For smoothly varying non-uniform stress fields Incremental Stress least accurate Average Stress least accurate Meticulous Measurement Practice is Required

19 Analysis Methods Non Uniform Stress Fields Integral Method Uses FE calibrations Use with highly non uniform stress fields RS assumed constant within each depth increment Individual contribution to measured strain identified at each depth increment Power Series Uses FE calibrations Use when RS vary smoothly with depth Uses weighted averages of strain data to improve numerical stability Permits many small increments Limited spatial resolution Stresses then back calculated Max 4/5 depth increments

20 Analysis Methods Non Uniform Stress Fields Incremental Strain Approximate stress calculation Useful only when experimental calibration available Only considers strain relief due to new increment Ignores contributions from previous increments Errors increase significantly with depth from surface Use with care! Average Stress Approximate stress calculation Useful only when experimental calibration available Uses concept of Equivalent Uniform Stress EUS. is the uniform stress within the total hole depth that produces the same total strain relief as the actual non-uniform stress distribution Difficult to interpret, use with care!

21 Analysis Methods Non uniform Stress Fields Comparison Integral Method gives good stepped approximation Power Series Method gives a close straight line fit Incremental and Average Stress Methods significantly underestimate the stress field remote from the surface Taken from: SEM Handbook of Measurement of Residual Stresses, Ed. J. Lu, 1996

22 Potential Errors and Uncertainties Hole dimensions diameter, concentricity, profile Centre of drilled hole to coincide with centre of gauge circle to ± mm Hole depth (measure and control to < 1 µm!) Surface roughness and flatness Specimen preparation Induced stresses from machining the hole Material properties Incorrect gauge selection eg use small size where steep stress gradients Calibration coefficients and Data Analysis Equipment and measurement resolution, systematic errors UNCERT Code of Practice No. 15

23 Typical Hole Profiles PCD Diamond Drill Air Abrasion

24 Hole Drilling Technique - Developments Optimised Incremental Drilling Laser Speckle Interferometry Holography Moiré Interferometry Deep Hole Drilling Taper Hole Drilling Taken from: UMIST Stress & Damage Characterisation Unit Brochure

25 Enhanced Strain Sensitivity Optimised Step Increments Taper Hole Drilling

26 Moiré Interferometry

27 Application Quenching Stresses Repeatability of measurements - water quenched carbon steel Quench Stresses in pearlitic steel Taken from: SEM Handbook of Measurement of Residual Stresses, Ed. J. Lu, 1996

28 Application Machining Stresses Grinding and sanding Machining stresses - steep stress gradient

29 Application - Welds Steep stress gradients Ref: Flavenot and Lu, Proc. Of Surface Eng, current trends and future prospects, Ed. S A Meguid, Elsevier, June 1990 Ref: Lu and Flavenot, Experimental Techniques 21, Nov 1989

30 Application Shot Peened Surface

31 Application - Coatings Residual Stress Distribution in plasma sprayed HAp coatings Coating thickness 80 mm Coating thickness 200 mm Ref: Han, Nan et al, J. Mats. Sci. Letters,18 (1999)

32 Summary Hole Drilling is rapid and versatile Basic Hole Drilling Technique has only limited application Incremental Hole Drilling should be used Interpretation of the data is very important (and often very difficult) Select a data analysis method appropriate to the stress field Significant research effort at present

33 NPL Project on Residual Stress Focus on XRD and Hole Drilling Intercomparison to compare techniques Input into draft standard for XRD Good Practice Guide for Hole Drilling Aluminium, shot peened steel, TiN coating Intercomparison - starts September 2000 Participation is still open!