Hole Drilling Techniques - National Physical Laboratoryresource.npl.co.uk/.../measurement_techniques/hole_drilling.pdf · Hole Drilling Techniques Jerry Lord The NPL Materials Centre

BCA Structural Materials Workshop – 12th September 2000

Residual Stress in Context. How to Choose a Technique

Hole Drilling Techniques

Jerry LordThe NPL Materials Centre


Breakdown of PresentationBreakdown of Presentation

n Introduction and Background

n Basic Hole Drilling Technique

n Recent Developments

n Examples

n Summary

n NPL Project on Residual Stress


Survey of UK IndustrySurvey of UK Industry

n 48 replies to Questionnaire - April 1999– 31 organisations performed measurements

n Hole Drilling (22)n XRD (19)n Neutron (14)n Layer removal (12)n Other (7)


Hole Drilling TechniqueHole Drilling Technique

Taken from: UMIST Stress & Damage Characterisation Unit Brochure


Hole DrillingHole Drilling

n Measure surface strains

n Calculate original RS distribution from these

n Relieved strains decay rapidly with depth

n Limited and variable strain sensitivity

n Susceptible to errors

– Hole position

– Hole diameter and eccentricity

– Hole depth

n Induced machining stresses

– Milling cutters, end mills, drills

> 40 µε in carbon steels

> 300 µε in austenitic steels

Schematic of Relieved Strains


Development of Hole Drilling TechniqueDevelopment of Hole Drilling Technique

n Kirsch (1898) – Developed empirical solution for through hole, uniaxial stress field

n Mathar (1934) – application of a mechanical extensometer

n Soete & Voncrambrugge – first application of strain gauges

n Kelsey (1956) – variation of RS with depth using the blind hole technique

n Rendler and Vigness (1966) - Empirical analysis for blind hole, developed standard rosette geometry

n Beaney and Procter (1974) – air abrasion machining

n Bijak-Zochowski (1978) – non uniform stress calculations

n Schajer (1981) - Used FEA, tabulated calibration coefficients

n Schajer (1988) – Coefficients for non-uniform stress fields

n ASTM E837-99 and Measurements Group TN-503-3


Basic Gauge DesignBasic Gauge Design

n D is diameter of gauge circle

n Do is diameter of drilled hole


Residual Stress GaugesResidual Stress Gauges

n Limited number of designs

n 3 and 6 element rosettes

n Typical hole diameter is 1 – 4 mm

n Surface preparation is critical

n Good quality installation

n 3 wire leadwire for temperature compensation

n Instrumentation resolution ±1ìå


Typical Hole Drilling Set upTypical Hole Drilling Set up


Features of Hole Drilling TechniqueFeatures of Hole Drilling Technique

n Basic Hole Drilling– Localised RS measurements– Strains are averaged over the area of the strain gauge– Limited application

n 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

n Developments– Incremental hole drilling – Analyses for non-uniform stress fields– Analyses for steep stress gradients– Elastic – plastic behaviour


Key Attributes of the Hole Drilling TechniqueKey Attributes of the Hole Drilling Technique

n 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, ceramicscomposites, cermets, coatingssingle crystals, surface treated,crystalline and amorphous,magnetic and non-magnetic

n 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 !


Basic Hole Drilling EquationsBasic Hole Drilling Equations

σmax, σmin = ε3 – ε1 ± √ (ε3 – ε1 )2 + (ε3 + ε1 - 2ε2)2

4A 4B

p = (ε3 + ε1)/2 q = (ε3 - ε1)/2

t = (ε3 + ε1 - 2ε2)/2

where A = -a (1 + υ) / 2E and B = -b / 2E

a and b are dimensionless calibration coefficients almost independent of the specimen properties


Coefficients (ASTM E837-99)Coefficients (ASTM E837-99)

n 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


Coefficients (ASTM E837-99)Coefficients (ASTM E837-99)


Basic Hole Drilling EquationsBasic Hole Drilling Equations

σmax, σmin = P ± √ Q2 + T2

τmax = √ Q2 + T2

If…..…P = (σy + σx )/2 …. mean “hydrostatic stress”

Q = (σy + σx )/2 …. shear stress 45o to x-y axes”

T = τxy/2 ….. shear stress along x-y axes


Strain SensitivityStrain Sensitivity


Recent Developments and ApplicationsRecent Developments and Applications

n Strain Measurement– New rosette designs – 6 gauges– Full field measurements

– Laser speckle interferometry– Moiré interferometry– Holography

n Hole Drilling methods– Taper hole drilling– Optimised incremental drilling– Deep hole drilling

n Modelling and Data Analysis– Non uniform stress fields– Biaxial stress distributions– Steep stress gradients

n New Applications– Thermal Barrier Coatings– Thin coatings?


n ASTM E837-99 does not apply

n 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

n Meticulous Measurement Practice is Required

Analysis Methods – Non Uniform Stress FieldsAnalysis Methods – Non Uniform Stress Fields



n 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

– Stresses then back calculated

– Max 4/5 depth increments

n 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



n 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 !

n Average Stress– Approximate stress calculation

– Useful only when experimental calibration available

– Uses concept of “Equivalent Uniform Stress”

n 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!


Analysis Methods – Non uniform Stress FieldsAnalysis Methods – Non uniform Stress Fields

Taken from: SEM Handbook of Measurement of Residual Stresses, Ed. J. Lu, 1996

n 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


Potential Errors and UncertaintiesPotential Errors and Uncertainties

n Hole dimensions – diameter, concentricity, profile– Centre of drilled hole to coincide with centre of gauge circle to ± 0.025 mm

n Hole depth (measure and control to < 1 µm !)n Surface roughness and flatnessn Specimen preparationn Induced stresses from machining the holen Material propertiesn Incorrect gauge selection

eg use small size where steep stress gradients

n Calibration coefficients and Data Analysisn Equipment and measurement resolution, systematic errors

n UNCERT Code of Practice No. 15


Typical Hole ProfilesTypical Hole Profiles

PCD Diamond Drill Air Abrasion


Hole Drilling Technique - DevelopmentsHole Drilling Technique - Developments

Taken from: UMIST Stress & Damage Characterisation Unit Brochure

Laser Speckle InterferometryHolography

Moiré Interferometry

Deep Hole DrillingTaper Hole Drilling

Optimised Incremental Drilling


Enhanced Strain SensitivityEnhanced Strain SensitivityTaper Hole DrillingOptimised Step Increments


Moiré InterferometryMoiré Interferometry


Application – Quenching Stresses 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


Application – Machining StressesApplication – Machining Stresses

Grinding and sanding Machining stresses - steep stress gradient


Application - WeldsApplication - Welds

Ref: Flavenot and Lu, Proc. Of Surface Eng, current trends and future prospects, Ed. S A Meguid, Elsevier, June 1990

Steep stress gradients

Ref: Lu and Flavenot, Experimental Techniques 21, Nov 1989


Application – Shot Peened SurfaceApplication – Shot Peened Surface


Application - CoatingsApplication - Coatings

Residual Stress Distribution in plasma sprayed HAp coatings

Coating thickness 80 µm Coating thickness 200 µm

Ref: Han, Nan et al, J. Mats. Sci. Letters,18 (1999) 1087-1089


SummarySummary

n Hole Drilling is rapid and versatile

n Basic Hole Drilling Technique has only limited application

n Incremental Hole Drilling should be used

n Interpretation of the data is very important (and often very difficult)

n Select a data analysis method appropriate to the stress field

n Significant research effort at present


NPL Project on Residual StressNPL Project on Residual Stress

n Focus on XRD and Hole Drilling

n Intercomparison – to compare techniques

n Input into draft standard for XRD

n Good Practice Guide for Hole Drilling

n Aluminium, shot peened steel, TiN coating

n Intercomparison - starts September 2000

Participation is still open !

Documents

Hole Drilling Techniques - National Physical Laboratoryresource.npl.co.uk/.../measurement_techniques/hole_drilling.pdf · Hole Drilling Techniques Jerry Lord The NPL Materials Centre