UDC 53.082, 53.088 Extension to the blind hole drilling technique for residual stress determination with airabrasive hole forming by Alan Owens, Stress Engineering Services Ltrl. Charlton Lune, Midsomer Norton, Bath BA3 4 BE For an austenitic clad ferritic steel plate it was necessary to determine the residual stress state between the austenitic surface and a region below the austeniticlferritic interface. The thickness of claddings was 6 mm and to determine the stress variation between the surface and 8mm an incremental analysis is required. Any material removal en masse in conjunction with small hole residual stress meusurements would have resulted in a stress redistribution and hence a distortion in the stress field being measured. In conjunction with strain change measurements on the other jace the sensitivity would not have been suflcient. These two options assume that a thin layer qf material oivr relatively large areas can he removed bj* (I wn-stress inducing technique. A blind hole residual stress measurement using a 13 mm diameter hole 13 mm deep will .facilitate accurate increnimtal anali~si.s to a depth of 8 mm. The end mill tnerhod of hole forming cannot be used with stainless steels because of the random damage effect which is induced in the readings. The airubrasive method which induces no damage effect was used. The standard airabrasive method of hole forming produces a 2 mm diameter and a 2 mm deep hole. The modification of the standard airabrasive system is discussed which enables 13 mm diameter and 13 mm deep holes to be formed. A deviation from the standard holelgauge layout geometry has been investigated due to non-availability of suitable standard rosettes, and a significant increase in sensitivity has been obtained. The incremental stress results for the austenitic layer and the austeniticlferritic interfbce are discussed using the results from 16mm, 3.2mm and 13mm diameterldeep holes. Key words: Residual stress measurement, airabrasive hole forming. Introduction It was required to determine the residual stress state in a weld-deposited austenitic clad ferritic steel plate. Both sides of the 150mm thick ferritic plate were clad and it was necessary to determine the residual stress state induced by the application of the cladding. Due to the different coefficients of expansion and differential temperature state during the cladding operation, residual tensile forces will be produced in the austenitic steel with balancing compressive forces in the ferritic steel. The 6mm thickness of the cladding could produce a tensile stress state in the austenitic steel which increases to a maximum value at the austenitic/ferritic interface. The actual stress state could not be satisfactorily determined by theoretical analysis and, as the 'Strain'. November 1984 working load and life of the final structure is directly dependent on the residual stress state, it was necessary to determine the stress state by experimental means. In order to obviate any edge problems the specimen size of 760mmx760mm was chosen with the same section as the final structure. The specimen is shown with the expected form of the stress distribution in Fig. 1. 65mm Fig. 1 Austenitic clad ferritic steel plate Notation &=Strain,mm/mm x lo-(' o = Stress, MN/m2 d = Nominal centre hole diameter. mm Selection of residual stress determination method Several methods of residual stress were considered: Material removal methods in conjunction with X- ray diffraction or strain gauging on the opposite face must involve the removal of a uniform layer of one face of the specimen. The method used for removal must induce no stresses. Material removal from a section of one face was modelled by finite element techniques and showed that the stress 159
Transcript
UDC 53.082, 53.088
Extension to the blind hole drilling technique for residual stress determination with airabrasive hole forming by Alan Owens, Stress Engineering Services Ltrl. Charlton Lune, Midsomer Norton, Bath BA3 4 BE
For an austenitic clad ferritic steel plate it was necessary to determine the residual stress state between the austenitic surface and a region below the austeniticlferritic interface. The thickness of claddings was 6 mm and to determine the stress variation between the surface and 8 m m an incremental analysis is required. Any material removal en masse in conjunction with small hole residual stress meusurements would have resulted in a stress redistribution and hence a distortion in the stress field being measured. In conjunction with strain change measurements on the other jace the sensitivity would not have been suflcient. These two options assume that a thin layer qf material oivr relatively large areas can he removed bj* ( I wn-stress inducing technique. A blind hole residual stress measurement using a 13 m m diameter hole 13 mm deep will .facilitate accurate increnimtal anali~si.s to a depth of 8 mm. The end mill tnerhod of hole forming cannot be used with stainless steels because of the random damage effect which is induced in the readings. The airubrasive method which induces no damage effect was used. The standard airabrasive method of hole forming produces a 2 mm diameter and a 2 mm deep hole. The modification of the standard airabrasive system is discussed which enables 13 mm diameter and 13 mm deep holes to be formed. A deviation from the standard holelgauge layout geometry has been investigated due to non-availability of suitable standard rosettes, and a significant increase in sensitivity has been obtained. The incremental stress results for the austenitic layer and the austeniticlferritic interfbce are discussed using the results from 16mm, 3.2mm and 13mm diameterldeep holes.
Introduction It was required to determine the residual stress state in a weld-deposited austenitic clad ferritic steel plate. Both sides of the 150mm thick ferritic plate were clad and it was necessary to determine the residual stress state induced by the application of the cladding. Due to the different coefficients of expansion and differential temperature state during the cladding operation, residual tensile forces will be produced in the austenitic steel with balancing compressive forces in the ferritic steel. The 6mm thickness of the cladding could produce a tensile stress state in the austenitic steel which increases to a maximum value at the austenitic/ferritic interface. The actual stress state could not be satisfactorily determined by theoretical analysis and, as the
'Strain'. November 1984
working load and life of the final structure is directly dependent on the residual stress state, it was necessary to determine the stress state by experimental means. In order to obviate any edge problems the specimen size of 760mmx760mm was chosen with the same section as the final structure. The specimen is shown with the expected form of the stress distribution in Fig. 1.
65mm
Fig. 1 Austenitic clad ferritic steel plate
Notation &=Strain, mm/mm x lo-(' o = Stress, MN/m2 d = Nominal centre hole diameter. mm
Selection of residual stress determination method Several methods of residual stress were considered: Material removal methods in conjunction with X- ray diffraction or strain gauging on the opposite face must involve the removal of a uniform layer of one face of the specimen. The method used for removal must induce no stresses. Material removal from a section of one face was modelled by finite element techniques and showed that the stress
159
redistribution produces significant stress concentration on, and immediately below, the new surface. The concentration can only be quantified by a knowledge of the residual stress field. The removal of a uniform layer by etching or electro-chemical methods is not practicable for a surface of 0.6m2. The semi-destructive technique of hole drilling was considered. For ease of operation the blind centre hole technique appeared most suitable. In order that the hole forming does not induce any strain, methods such as air abrasion, electrochemical machining (ECM) and electro-discharge machining (EDM) must be considered. Only the former method of airabrasion has been accepted as a standard method of blind hole forming in conjunction with measurement of strain relief due to the difficult environment associated with the ECM and EDM methods. The method of the centre hole drilling is well documented' and with particular reference to airabrasive hole forming2 However, the standard eqLipment which is commercially available has the limitation of hole diameter 2.4 mm and hole depth 2.5 mm. For maximum sensitivity of the strain measurement due to stress relief, it is necessary for the hole depth to be approximately equal to the hole diameter. For the measured strain changes to be used with any accuracy the final hole size must be parallel sided. An incremental analysis has been developed3 based upon the results of calibrations carried out by finite element analysis and verified by experimental work. This analysis enables the variation of residual stress to be determined with respect to depth for a hole depth range of approximately 7% to 65% of hole diameter. Outside this hole depth range the technique is less accurate due to the small strain response and hence low sensitivity. In order to determine the residual stress state through the interface, it was specified that the residual stress state to a depth of 8mm was required. With the blind hole method and incremental analysis, a hole diameter of 13 mm was required. A hole depth of 13mm was also preferred in order that the data could be used in curve smoothing processes. With the ability to extend the blind hole technique to 13mm diameter holes the stress variation between depths of 0.6 and 8.3mm could be determined. The stress variation more adjacent to the surface could be obtained by the conventional 2 mm hole size.
Equipment The standard airabrasive equipment produces the parallel sided hole shown in Fig. 2A by a rotating nozzle-the central nipple does not affect the measured strain relief. The operating parameters are also given in this figure. The conventional equipment, Fig. 3, comprises compressor, dust extractor, controller, drilling head and optical head; the latter being used for depth and diameter measurement.
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TYPKAL PROFILE FOR ANOMINAL 1-6mm AIRABRASIVE FORMED HOLE
I --I 2 2mm
OPERATING PARAMETERS
AIR SUPPLY PRESSURE 5 4 bar AIR SUPPLY RATE 0 01 rnl/mtn P O W E R FLOW 10 gmslmin DRlLLlNj TIME 15 mins NOZZLE T Y P E %mire MZZLE DIAMETER 0 45mm POWDER TYPE 53 micron Aluminn
Oxide
TYPICAL PROFILE FOR A NOMINAL12 5mm AIRABRASIVE FORMED HOLE
125mm I OPERATING PARAMETERS
fl AIR SUPPLY PmSSURE AIRSUPPLY RATE 019-025m'lmin FOWOER FLOW 25 qmslmln DRILLING TIME 200 mins NOZZLE TYPE torbide M Z L E DIAMETER 13mm POWDER TYPE 53 muon Aluminium
71-8 8 bar
ox Ide
Fig. 2 Air abrasive hole forming and hole profile; A-standard, B-uprated.
For incremental analysis with the standard hole size, it is necessary that strain readings are made as the hole is continuously drilled. Interchanging the optical head and the drilling head could cause slight differences in the set-up of the drilling head for each depth increment and hence produce a non-parallel sided hole. For ten depth increments the drilling time for each increment would be of the order of 1 minute. The starting and stopping of the operation is associated with unknown and variable drilling rates compared to the uniform and known drilling rate for continuous operation. It is not possible, therefore, to stop the drilling operation after time intervals to obtain strain readings only, without any estimates of hole depth being in error.
'Strain', November I984
Fig. 3 Standard equipment.
For the larger size hole, depth readings can be made by a dial gauge method at the hole position remote from the nozzle. The present equipment is designed to enable parallel sided holes to be continuously drilled. A problem associated with drilling to depth by airabrasion is the inability for powder to be removed from the hole bottom and hence, the hole becomes blocked with powder. Parallelity of the hole size must be expressed as taper in the hole diameter with depth and as the hole diameter increases the same depth taper has less effect on the results. So parallelity has not caused any problems with the larger holes. Due to the ability to stop the drilling operation to take strain readings, it is also possible at this time to blow air into the hole and eliminate any possibility of the hole being blocked. The same principle is used to produce the larger holes but all equipment has been replaced for uprated models with the exception of the optical head which is not now required for depth/diameter measurements. The compressor, dust extractor, regulator, cooling unit, controller and drilling head with separate handpiece, are shown in Fig. 4. A section through a 12.5mm hole is given in Fig. 2B with the operating parameters. ‘Srrain ’, November 1984
Choice of gauge geometry
Strain gauge rosettes to be used,with the centre hole technique are commercially available for 0.8, 1.6 and 3.2mm hole sizes. The typical geometry is shown in Fig. 5A. The length and width of each element in the rosette is equal to the nominal hole diameter. For hole sizes in excess of 3.2mm it is necessary that rosettes are constructed of individual gauges and equality of hole diameter, gauge length and gauge width must be deviated from if commercially available strain gauges are to be utilised. The use of the finite element technique in the calibration of the incremental analysis’ involves the construction and loading of a numerical model for each hole depth with a sufficient mesh density around the hole to give twenty nodes at the position of each strain gauge element. An averaging technique is used on the strain surface at the gauge element position to determine the average strain for each load case and each hole depth. The variation of strain with depth can, therefore, be determined for different stress/depth profiles. Experimental calibrations in easily applied stress fields such as uniform stress (axial tension) and linearly varying stress (bending) are very difficult to carry out, time consuming and expensive. To generate known stress fields of different forms with
161
C
Fig. 4 Uprated equipment.
I+- d"OMINAL HOCE DIAMETER
AFTER WILLING
FOR MMINAL HOE DIAMETER'- REslWAL SRAlN
STRAIN RELIEF @EFFICIENT= MEASURED nRnlN 6'3%
12.5mrn
%
LEVEL DRILLING
STRAIN LEVEL AFTER DRILLING
r FOR NOMINAL HOLE DIAMETER -
STRAIN RELIEF COEFFICIENT = M ~ & ~ [ ~ 5 ! ~ , ~ , E F = 2.319
Fig. 5 Strain gauge layout for centre hole technique; A- conventional, B-modified.
162 'Srrain'. November I984
any repeatability is almost impossible. The finite element technique is capable of being used in calibration work and has been shown to agree with experimental calibrations made in axial tension and bending. At the hole edge the strain relief is complete but at the gauge element only a fraction of the strain is relieved. As a result all measured strains must be factored by a strain relief coefficient. The coefficient is typically 4.4 for the conventional strain gauge geometry and is a function of the gauge geometry and hole diameter. In uniaxial tension the strain distribution at each gauge element is shown in Figs. 6, 7 and 8. It is seen that the part of the gauge closest to the hole edge is effecting a large proportion of the measured strain change and large areas of the gauge surface contribute very little to the measured strain change. If the gauge element was less wide and less long, the measured strain changes would be increased with an increase in measurement accuracy and corresponding reduction in the strain relief coefficient. The gauge layout used with the larger holes is shown in Fig. 5B. The same proportional distance between hole edge and the start of the gauge element has been utilised in order that existing finite element results can be used even though a further increase in sensitivity would be achieved. The gauge element dimension has not been reduced further with greater increase in sensitivity due to any difficulties in alignment. The
increased sensitivity with a typical strain relief coefficient of 2.3 enables the depth range for incremental analysis to be increased to between 5% and 75% of hole depth.
Test procedure and results At six positions on the steel plate, the variation of residual stress with depth was determined by a series of hole sizes. A 2.1 mm diameter, 2.1 mm deep hole was drilled with conventional equipment and a 1.6 mm standard strain gauge rosette to determine the residual stresses between 0.1 mm and 1.3 mm. A 3.4mm diameter, 3.4mm deep hole was drilled with the modified equipment and a 3.2 mm standard strain gauge rosette to determine the residual stress state between 0.25 mm and 2.5 mm. A 12.7mm diameter, 12.7mm deep hole was drilled with the modified equipment and a made-up strain gauge rosette to determine the residual stress state between 1 mm and 9 mm. Each position was towards the centre of a face and the three holes were spaced outside their respective zones of influence. The results from three measurement positions are shown combined in Fig.9. It is noted that there is good agreement in the overlap regions of the analysis. The results show there is a significant reduction in stress between the interface and the surface with a low stress state at the surface.
\, _/----- -._ I,</’
, - -_ _-- -’ .
ELEMENT 1 W I A T O N OF STRAIN INDUCED INTO BY PROOUCING HOLE IN A UNlAXlAL
GAUGE ELEMENT STRESS FIELD
Fig. 6 Variation of strain induced into gauge element by producing hole in a uniaxial stress field+lement 1 .
‘Strain’, November 1984 163
\ g)UETO DRILLING
ELEMENT 2 WIATIDN OF STRAN INDUCED INTO GAUGE ELEMENT BY PROWClNG HOLE IN A UNIAXIAL STRESS FIELD
Fig. 7 Variation of strain induced into gauge element by producing hole in a uniaxial stress field-element 2.
,.
IN
pEMICROSTRAIN
,
ELEMENT 3 VARIATION OF STRAIN INDUCED INTO GAUGE ELEMENT BV PRODUCING HOLE IN A UNIAXIAL STRESS FIELD
Fin. 8 Variation of strain induced into gauge element by producing hole in a uniaxial stress field-element 3.
164 'Strain'. November 1984
HOLES NO2 HOCES NO3 - HOLES WI 400T 4001-
- I I
_ - _-I-
I 1 1 1 I
5 200 ”€ I i
GAUGE EDUIF’MEM U K D
HOLE NOMlNALd ACTUAL No DIAMETER DIAMETER LAx)UT
L. , 2 5 5 rnrn 2 5 5 rnm 2 5 5 rnrn
INTEPFAE I
4OOT I
ACCURATE RNGE
1 16mrn 22rnrn STANDARD STANMD 013-1.27rnrn
I J
I 2 1 3.2mrn 1 34mrn I STANDARD I UPRATED I 0 2 5 - 2 5 4 m m
12 5rnrn 12 5rnm MOOlFlED UPRATD 102-8 89mT
Conclusions References The work has shown that modified gauge geometry can double the strain sensitivity of the centre hole drilling technique, and hence improve the accuracy. The popular airabrasive technique can be extended to enable holes up to 13mm diameter, 13mm deep, to be produced. For the correct component geometry, this extension to the technique can be most useful. (3)
(1)
(2)
Acknowledgements The author wishes to thank Stress Engineering Services Limited, for their permission to prepare this Paper.
Beaney, E. M. and Procter E., ‘A critical evaluation of the centre hole technique for the measurement of residual stresses’, Strain, 10
Beaney, E. M., ‘Accurate measurement of residual stress on any steel using the centre hole method’, Strain, 12 (1976), 99-106. Owens A. ‘Calibration of the centre hole technique of residual stress measurement’, Proc. Joint B. S. S. M ./I. Prod. E. Conference on ‘Product liability and reliability’, University of Aston, 1-5 September, 1980, B.S.S.M., Newcastle upon Tyne, (1 980).
