Effect of Material Properties

7/29/2019 Effect of Material Properties

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H. T. Lee1Professor

Department of Mechanical Engineering,

National Cheng Kung University,

Tainan 701, Taiwan

e-mail: [email protected]

T. Y. TaiAssistant Professor

Department of Mechanical Engineering,Southern Taiwan University of Technology,

Tainan 701, Taiwan

C. LiuDepartment of Mechanical Engineering,


Tainan 701, Taiwan

F. C. HsuResearch Engineer

Metal Industries Research and Development

Centre,

1001 Kaonan Highway,

Kaohsiung 811, Taiwan

J. M. HsuMaster of Science

Department of Mechanical Engineering,


Tainan 701, Taiwan

Effect of Material PhysicalProperties on Residual StressMeasurement by EDM

Hole-Drilling MethodWhen measuring the residual stress within a component using the electrical dischargemachining (EDM) strain-gage method, a metallurgical transformation layer is formed onthe wall of the measurement hole. This transformation layer induces an additional re-sidual stress and therefore introduces a measurement error. In this study, it is shown thatgiven an appropriate set of machining conditions, this measurement error can be com-

pensated directly using a calibration stress factorcal computed in accordance with theproperties of the workpiece material. It is shown that for EDM machining conditions of

120 V/12 A/6 s / 30 s (discharge voltage/pulse current/pulse-on duration/pulse-off du-ration), the hole-drilling induced stress reduces with an increasing thermal conductivity

k in accordance with the relation cal325.5k0.65 MPa and increases linearly

with an increasing carbon equivalent (CE) in accordance with cal7.6 CE

22.4 MPa. Therefore, a given knowledge of the thermal conductivity coefficient orcarbon equivalent of the workpiece material, an accurate value of the true residual stress

within a component can be obtained simply by subtracting the computed value of thecalibration stress from the stress value obtained in accordance with the EDM hole-drilling strain-gage method prescribed in ASTM E837. DOI: 10.1115/1.4000219

Keywords: EDM, thermal conductivity, carbon equivalent, residual stress

1 Introduction

Engineering components invariably contain a certain degree of

residual stress induced either in their original manufacture or dur-

ing their subsequent service lives. While in some cases residual

stresses may be deliberately induced in order to obtain an im-

proved performance, e.g., prestressed concrete columns and

toughened glass, tensile residual stresses lead to a significant re-

duction in the mechanical properties of the component leading to

a loss in strength, an increased susceptibility to fatigue, creep or

environmental damage, a shortened service life, and in extreme

cases, catastrophic failure. As a result, a requirement exists for

reliable methods with which to assess the level of residual stress

within a component such that its performance can be accurately

predicted and its mechanical integrity assured. Many techniques

have been proposed to satisfy this requirement, ranging from non-

destructive methods, such as X-ray diffraction or magnetic and

ultrasonic methods, to semidestructive techniques, such as high-

speed hole-drilling or ring core and deep hole methods, to section-

ing methods, such as block removal, splitting, slicing, and so

forth.

Of these various techniques, hole-drilling methods in which the

residual stress is computed by using a strain-gage to measure, the

intensity of the strain released as a hole is drilled into the compo-

nent of interest, have a number of significant advantages. For

example, the measurement holes have very small depth and diam-

eter, and thus the damage caused to the component is highly lo-

calized, has little or no effect on the service life. Furthermore, the

analytical formulae used to establish the value of the residualstress are founded on well established, tried-and-tested engineer-ing principles. Finally, hole-drilling techniques have a simple ex-perimental setup, a straightforward operation, and a high degree

of precision. As a result, the hole-drilling strain-gage method iswidely used throughout the industrial and engineering circles for

residual stress measurement and is formally embodied in theASTM E837 standard 1 . However, traditional high-speed hole-drilling strain-gage methods have a limited ability to measure theresidual stress in components with high hardness and high tough-ness properties. When drilling such components, the tool rapidly

becomes worn, which not only degrades the quality of the mea-surement hole but also generates a significant additional residualstress within the component, and therefore produces a measure-ment error. Compared with traditional high-speed machining pro-

cesses in which the material is mechanically cut from the work-piece by a rotating tool, in the electrical discharge machining

EDM process, the workpiece material is removed via the abla-tion effect of a series of high-frequency electrical discharges gen-

erated through a thin dielectric layer separating the machiningelectrode and the workpiece surface. EDM is a noncontact ma-chining process, and thus the problem of tool wear is resolved.Furthermore, EDM is capable of machining even the hardest and

toughest of engineering materials and is widely applied through-out the modern metal-working industry as a result. Consequently,EDM represents an ideal solution for the hole-drilling strain-gageresidual stress measurement technique 2,3 .

However, the rapid heating and cooling effects inherent in theEDM machining process prompt a change in the local microstruc-ture and result in the formation of a hard, brittle transformation

layer on the machined surface. The mechanical properties of thistransformation layer depend on the localized heating and coolingrates, and therefore vary with the distance from the machinedsurface. The differential changes in the hardness and microstruc-

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the J OURNAL OF

ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received November 5, 2008;final manuscript received June 30, 2009; published online March 21, 2011. Assoc.

Editor: Hussein Zbib.

Journal of Engineering Materials and Technology APRIL 2011, Vol. 133 / 021014-1

Copyright 2011 by ASME

Downloaded 13 Jun 2011 to 137.207.15.112. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cf


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tural characteristics of the transformation layer and underlyingheat-affected zone induce a tensile residual stress. Residual stressinduced during EDM process is due to nonhomogeneity of heatflow and metallurgical transformations or to localized inhomoge-neous plastic deformation. Investigation of the residual stress ofelectrical discharge machined surface indicated their tensile na-ture; the extremely narrow superficial zone beneath the machinedsurface. The magnitude can be easily up to several hundred MPa

4 6 . However, the formation of surface cracks has attributed tostress release within the recast layer.

In the EDM hole-drilling strain-gage method, the formation of a

transformation layer on the surface of the measurement hole leadsto a significant measurement error since the value of the releasedstrain detected by the strain-gage reflects not only the originalresidual stress within the component, but also that induced duringthe hole-drilling operation itself 7 . As a result, some form ofcalibration procedure is required to compensate for this additionalresidual stress such that an accurate assessment of the originalresidual stress within the workpiece can be obtained. In previousstudies by the current group, it was shown that the value of thehole-drilling induced stress is independent of the original stressintensity within the component, and as a result, the measuredvalue of the residual stress can be calibrated by subtracting thevalue of the residual stress induced by a hole-drilling operation ina stress-free specimen of the equivalent type 8,9 . However, thistechnique, while undeniably effective, incurs considerable time,manpower, and material costs. Moreover, the calibration factormust be re-evaluated whenever a change is made in the EDMmachining parameters. As a result, the practicality of the calibra-tion method is somewhat limited.

Accordingly, this study commences by investigating the corre-lation between the EDM machining parameters and the magnitudeof the residual stress induced during the hole-drilling operationsuch that the machining conditions, which yield a low and repro-ducible value of the residual stress can be identified. Having doneso, a further series of machining trials are performed to establishthe correlation between the magnitude of the hole-drilling inducedstress and the properties of the workpiece material such that thestress measurements obtained using the EDM hole-drilling strain-gage method can be calibrated directly without the need for anyform of additional calibration experiment.

2 Experiments

The EDM hole-drilling experiments presented in this studywere carried out using a 21-series computer numerical control

CNC die-sinking EDM machine produced by Yawjet Inc. Tai-wan and were conducted in accordance with the ASTM E837standard using an FLA-2-11 strain-gage manufactured by TokyoSokki Kenkyujo Co., Ltd. Japan and a P-3500 strain indicatorfrom Vishay Measurements Group Inc. USA . The facility is re-veals in Fig. 1. The experiments were performed using five com-mon ferrous materials, namely, AISI 4140, L6, H13, M2, and D2,respectively, and were conducted using a solid CuW electrodewith an external diameter of 1.5 mm and a kerosene dielectric.Prior to the machining trials, the workpiece specimens were heattreated to ensure a fully stress-free condition 10 .

The experiments commenced by examining the correlation be-tween the EDM machining parameters and the magnitude of thehole-drilling induced stress. Of the various EDM parameters, thepulse current and pulse-on duration are known to have the greatesteffect on the induced residual stress. Accordingly, a series ofEDM hole-drilling experiments was performed using AISI D2specimens in which the pulse-off duration was maintained at a

constant value of 30 s and the pulse current and pulse-on dura-tion were varied as shown in Table 1. In every case, the surfacemorphology of the measurement hole was observed using back-scattered image of scanning electron microscopy SEM and thevalue of the hole-drilling induced stress was computed in accor-dance with the formulae provided in ASTM E837. Having com-

pleted the experimental trials and observations, the optimal set ofmachining conditions was identified by considering the geometri-cal precision of the drilled hole, the machining efficiency, thequality of the machined surface, and so on.

To investigate the effect of the material properties on the mag-nitude of the hole-drilling induced stress, a second set of experi-ments was performed in which the optimal machining parameterswere used to carry out the EDM hole-drilling of all five stress-freeferrous specimens considered in the present study, namely, AISI4140 and L6 low alloy steels , H13 hot working tool steel , M2

high-speed tool steel , and D2 cold working tool steel . In eachcase, the hole-drilling induced stress was evaluated in accordancewith the ASTM E837 standard and the characteristics of the ma-chined surfaces were examined using SEM. Finally, mathematicalformulations were constructed to model the correlation between

the hole-drilling induced stress and two material parameters,namely, the thermal conductivity coefficient and the carbonequivalent in order to enable the direct calibration of the residualstress measurements obtained using the EDM hole-drilling strain-gage method.

3 Results and Discussion

3.1 Selection of Optimal EDM Conditions. As discussedbelow, the choice of suitable EDM parameters for the hole-drillingprocess is governed by five main considerations, namely, the qual-ity of the drilled hole, the size and intensity of the discharge spark,the stability of the discharge sparks, the machining duration, andthe integrity of the machined surface.

Fig. 1 The facility of EDM hole-drilling method

Table 1 Summary of EDM parameter settings used in hole-drilling experiments

Pulse-on duration s

Pulse current

4A 8A 12A 16A

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021014-2 / Vol. 133, APRIL 2011 Transactions of the ASME



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3.1.1 Quality of Drilled Hole. In the EDM hole-drilling strain-

gage method, the magnitude of the released strain is critically

dependent on the geometry of the drilled hole. In evaluating the

residual stress within a component, the formulations prescribed

within the ASTM E837 standard assume the base of the hole to be

flat and parallel to the workpiece surface. In practice, however, the

geometry of the hole depends on the rate at which the electrode is

consumed. If the electrode is consumed too rapidly, the base of the

hole has a conical rather than flat profile, and thus the computed

value of the residual stress is subject to significant error. As a

result, a correct choice of machining conditions is essential to

ensure that the electrode consumption is maintained at an accept-able level. In the current machining trials, it was found that all

seven pulse current/pulse-on duration combinations indicated in

Table 1 limited the rate at which the CuW electrode was con-

sumed and thus the geometry of the drilled hole was found to be

consistent with the constraints laid down in ASTM E837 in every

case.

3.1.2 Size and Intensity of Discharge Spark. The magnitude of

the residual stress induced during the hole-drilling operation is

directly related to the localized heating and cooling effects in-

duced by each electrical discharge. In practice, as the size and

intensity of the discharge spark increase, the amount of thermal

energy supplied to the workpiece also increases, and thus a thicker

transformation layer with a higher residual stress is formed on the

workpiece surface. Moreover, the strength of the pressure pulse

generated by each electrical spark also increases as the intensity ofthe spark increases, and thus the strain-gage is easily damaged.

Consequently, it is necessary to control the machining parametersin such a way as to achieve a compromise between an acceptablemachining rate and the need to maintain the measurement capa-bilities of the strain-gage. In the current trials, it was found that

machining conditions of 16 A / 9 s pulse current/pulse-on du-ration caused significant burn damage to the strain-gage. Thus, itwas concluded that the pulse current should be assigned a value

no higher than 12 A to ensure the reliability of the strain measure-ments.

3.1.3 Stability of Discharge Sparks. During the EDM machin-

ing process, the accumulation of carbon deposits on the workpiecesurface, or an excessive value of the duty cycle, prevents the

kerosene dielectric from restoring an insulating property betweenthe electrode and the workpiece in the interval between successive

sparks. As a consequence, a secondary discharge phenomenon isinduced in which the workpiece is bombarded by multiple electri-cal discharges within a confined region of the workpiece surface.These multiple discharges generate an intense localized heating

effect, which not only increases the magnitude of the hole-drillingresidual stress but also prompts a severe degradation in the surfaceroughness properties of the machined surface. In the current ex-periments, it was observed that excessive carbon deposits were

invariably formed on the surface of the drilled measurement holewhen the machining process was performed using machining pa-

rameters of 4 A / 9 s or 12 A / 12 s, and were occasionally

formed under machining conditions of 8 A / 9 s.

3.1.4 Machining Duration. When using the EDM hole-drillingstrain-gage method, it is desirable to increase the machining speed

in order to reduce the machining time. Furthermore, it is generallyaccepted that the hole-drilling operation should last no longer than4060 min since the strain-gage tends to become damaged if ma-chining is continued beyond this point. In practice, the machining

rate increases with the intensity of the electrical discharges. How-ever, as discussed above, the strain-gage is easily damaged if thesize and intensity of the electrical discharges are not carefullycontrolled. Therefore, in specifying the machining parameters a

compromise is inevitably required. In the hole-drilling experi-ments performed in this study, it was found that the time requiredto drill the hole to the maximum depth specified in the ASTM

E837 standard exceeded 1 h when the machining parameters were

specified as 4 A / 9 s o r 8 A / 9 s see Fig. 2 . Accordingly,these particular machining conditions were deemed to be infea-sible for the current ferrous specimens.

3.1.5 Integrity of Machined Surface. In the EDM machiningprocess, the molten metal, which is not swept away from themachining area by the dielectric solidifies to form a brittle recast

layer. Since this recast layer cools more rapidly than the underly-ing base material, an intense thermally induced stress is generatedwithin the workpiece. If the magnitude of this stress exceeds themaximum tensile strength of the workpiece material, surfacecracks are formed as the recast layer cools. In a previous study

11 , the current group reported that the surface cracking phenom-enon resulted in smaller values of the detected strain since theresidual stress induced in the hole-drilling process was partiallyreleased each time a new crack was initiated in the workpiecesurface. Figure 3 presents backscattered scanning images showingthe surface morphologies of AISI D2 specimens machined usingall of the parameter combinations summarized in Table 1. It isseen that each specimen is affected to a greater or lesser extent bysurface cracking. From a close inspection of the size and densityof the cracks shown in the various images in Fig. 3, it can be

inferred that the seven machining conditions indicated in Table 1can be ranked in terms of their surface cracking effects as follows:

12 A/12 s 16 A/9 s 12 A/9 s 4 A/9 s

8 A/9 s 12 A/3 s 12 A/6 s

Reviewing the results and discussions presented above, it isinferred that the optimal EDM hole-drilling parameters for AISID2 steel are as follows:

1 a discharge voltage of 120 V 2 a pulse current of 12 A 3 a pulse-on duration of 6 s 4 a pulse-off duration of 30 s

Accordingly, these parameter settings were utilized in a second

series of EDM machining tests designed to evaluate the effects ofthe workpiece material properties on the magnitude of the residualstress induced during the hole-drilling process.

3.2 Correlation Between Thermal Conductivity Coeffi-cient and Hole-Drilling Induced Stress. Table 2 summarizes thethermal conductivity coefficients of the five ferrous materials con-sidered in the present study and indicates the corresponding valueof the hole-drilling induced stress as evaluated using the optimalmachining conditions described above. Note that the thermal con-ductivity coefficient of AISI 1045 and the corresponding hole-drilling induced stress reproduced from Ref. 12 are also pre-sented for comparison purposes. The data presented in Table 2 are

Fig. 2 Influence of pulse current and pulse-on duration param-eters on hole-drilling induced stress and time required toachieved specified machining depth




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plotted in a graphical form in Fig. 4. In general, it is observed that

the magnitude of the hole-drilling induced stress referred to here-

after as the calibration stress decreases as the thermal conductiv-

ity coefficient increases. However, it can be seen that the AISI D2

specimen is a notable exception to this trend, i.e., it has a lowcalibration stress despite having the lowest thermal conductivity

of all the specimens shown in the figure. As discussed below, the

distinctive behavior of this particular specimen suggests that the

material properties of AISI D2 steel are different in some way

from those of the other ferrous specimens and therefore result in a

different mechanical response when processed using the same ma-

chining conditions.

Figure 5 presents backscattered electron images showing the

surface morphologies of the measurement holes drilled in AISI

4140, L6, H13, M2, and D2 specimens under the optimal EDM

conditions of 120 V/12 A/6 s / 30 s. It can be seen that under

a 300X magnification; surface cracks are evident only in the AISI

D2 specimen. As commented above, AISI D2 has the lowest ther-

mal conductivity of the present specimens 20.9 W m1 K1 , and

the images presented in Fig. 5 suggest that this causes the AISI D2specimen to be more prone to surface cracking. As discussed in

Ref. 11 , the initiation and propagation of cracks within the trans-

formation layer results in a partial release of the hole-drilling in-

duced residual stress. As a result, the AISI D2 specimen has a

relatively low calibration stress even though it has a low thermal

conductivity see Fig. 4 . Applying a curve fitting technique to theexperimental data presented in Fig. 4, it is found that the calibra-

tion stress varies with the thermal conductivity coefficient k in

accordance with the relation cal=150.7k0.44 MPa. However, the

correlation coefficient is found to be just R2 =0.6712 due to the

outlier effect of the AISI D2 specimen, and thus this mathematical

correlation is inappropriate for calibrating the value of the residual

stress measured in stressed components using the EDM hole-

Table 2 Thermal conductivity and calibration stress values forferrous materials considered in present study and AISI 1045considered in Ref. 7

Materials

Thermalconductivity

W m1 K1Calibration stress value

MPaAverage MPa

4140 42.7 27.5 28.7 - - 28.1L6 36.4 31.1 31.5 - - 31.3H13 28.6 34.4 36.7 37.9 - 36.3M2 21.3 42.9 44.9 45.5 46.6 45.0D2 20.9 34.1 33.9 31.4 30.0 32.41045 50.2 25.8 7 - - - 25.8

Fig. 3 Backscattered electron images showing AISI D2 surface morphology followingmachining under various EDM conditions note that the arrows indicate surface cracks

Fig. 4 Variation in calibration stress with thermal conductivityof workpiece material note that the EDM conditions are 120V/12 A/6 s/30 s in every case

Fig. 5 Backscattered electron images showing surface morphologies of current ferrousspecimens machined under EDM conditions of 120V/12A/6 s/30 s note that the ar-rows indicate surface cracks




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drilling strain-gage method.When the AISI D2 specimen is omitted and the data presented

in Table 2 are replotted, Fig. 6 shows that the calibration stressvaries with the thermal conductivity coefficient in accordance

with the power law relation cal=325.52k0.65 MPa. In this case,

the correlation coefficient is found to have a value very close to 1 i.e., R2 =0.9976 , and therefore the power law relation providesthe means to obtain an accurate prediction of the hole-drillinginduced stress directly from the thermal conductivity coefficientof the workpiece. Consequently, in the EDM hole-drilling strain-gage method, the true value of the residual stress within a com-ponent can be obtained simply via the formulation

act = mes + cal = mes + 325.52k0.65 1

where mes is the value of residual stress computed in accordancewith the guidelines laid down in the ASTM E837 standard. Byapplying the calibration scheme presented in Eq. 1 , the need toconduct additional experiments to determine a suitable compensa-tion factor is avoided and all the attendant time, material, andlabor costs are therefore saved.

3.3 Correlation Between Carbon Equivalent and Hole-Drilling Induced Stress. According to the International Instituteof Welding IIW 13 , the carbon equivalent CE of a material isgiven by

CE = C +1

5Cr +

1

5Mo +

1

5V +

1

6Mn +

1

15Ni +

1

15Cu 2

Figure 7 illustrates the variation in the calibration stress with thecarbon equivalent properties of the current AISI 4140, L6, H13,and M2 specimens. Note that the AISI D2 specimen is again omit-ted while the AISI 1045 specimen is once again included. It isobserved that the magnitude of the calibration stress increaseswith an increasing value of the carbon equivalent. From inspec-tion, it is found that the two parameters are related via the power

law cal

=7.6

CE

+ 22.4 MPa. Furthermore, the correlation co-

efficient is found to have a high value of 0.9909. Therefore, thevalues of the hole-drilling induced stress obtained from this powerlaw are in excellent agreement with the experimental results andtherefore provide a suitable value with which to calibrate the mea-surement results obtained from the EDM hole-drilling strain-gagemethod.

The carbon equivalent property was originally developed as ameans of determining the weldability of carbon steels. Due to therapid heating and cooling effects inherent in welding processes,the microstructure of the weldment commonly undergoes an aus-tenite to martensite transformation as its cools. This microstruc-tural transformation induces a significant stress within the heat-

affected zone and causes the weldment to crack if its magnitudeexceeds the maximum tensile strength of the workpiece material.As shown in Eq. 2 , the carbon equivalent scales the concentra-tion of each element of the steel in accordance with its ability towithstand this austenite to martensite transformation. In general,carbon steels with low carbon equivalents have excellent weld-

ability characteristics and can be welded without the need for anyparticular precautions. In the EDM process, the intense thermalenergy supplied by the electrical sparks prompts similar heatingand cooling effects to those observed in the welding process, andthus the recast layer formed on the upper surface of the machinedspecimen is also prone to cracking as the result of a metallurgicaltransformation. As discussed earlier, surface cracking of the recastlayer i.e., the upper strata in the transformation layer results in apartial release of the hole-drilling induced stress, and thus thecalibration stress cannot be reliably determined using the generalmathematical correlation given above. As a consequence, whenperforming the EDM hole-drilling strain-gage method, it is essen-tial to specify the machining parameters in such a way that thethermal input to the workpiece is maintained at a sufficiently lowlevel to suppress surface cracking. The power law given above

relating the calibration stress and the carbon equivalent i.e.,cal=7.6 CE +22.4 MPa is based on the hole-drilling stress

measurements obtained in crack-free specimens i.e., AISI 4140,L6, H13, and M2 machined using the optimal parameter settings

of 120 V/12 A/6 s /30 s. The excellent linearity of this powerlaw relationship confirms the feasibility of using the carbonequivalent property of the workpiece material as a means of com-puting a suitable calibration stress value provided that the machin-ing conditions are specified in such a way that surface cracking isprevented.

3.4 Calibration Procedure for EDM Hole-Drilling Strain-Gage Measurement Process. The experimental results presentedin Secs. 3.2 and 3.3 have shown that the hole-drilling inducedstress in AISI 4140, L6, H13, and M2 specimens can be predictedwith a high degree of accuracy given suitable machining condi-

tions i.e., 120 V/12 A/6 s / 30 s and a knowledge of eitherthe thermal conductivity or carbon equivalent of the workpiecematerial. The properties of most engineering materials are readilyavailable in literature, and thus the mathematical relationshipspresented in the preceding sections provide a quick and reliablemeans of calibrating the residual stress value obtained using theEDM hole-drilling strain-gage method. However, the results havealso shown that when surface cracks are formed in the recastlayer, the magnitude of the hole-drilling induced stress cannot bereliably predicted using these mathematical correlations. In suchan event, a suitable value of the calibration stress can only beobtained by performing calibration trials using unstressed samples

Fig. 6 Variation in calibration stress with thermal conductivitynote that the EDM conditions are 120V/12A/6 s/30 s in ev-ery case; also, the results are presented only for those speci-mens in which a crack-free recast layer is obtained, i.e., thecalibration stress for the AISI D2 specimen is deliberatelyomitted

Fig. 7 Variation in calibration stress with carbon equivalentnote that the EDM conditions are 120V/12A/6 s/30 s in ev-ery case; also the results are presented only for those speci-mens in which a crack-free recast layer is obtained, i.e., thecalibration stress for the AISI D2 specimen is deliberatelyomitted




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of an equivalent material. Figure 8 presents a flowchart illustratingthe calibration procedure for the EDM hole-drilling strain-gagemeasurement of the residual stress in components fabricated fromferrous materials such as AISI 4140, L6, H13, and M2 for whicha set of machining conditions can be found, which yield a crack-free recast layer. Figure 9 presents the equivalent procedure forferrous materials in which no such conditions can be found e.g.,AISI D2 .

As shown in Fig. 8, the calibration procedure commences byidentifying the machining parameters, which satisfy four basiccriteria, namely, 1 a measurement hole whose geometry is con-sistent with that prescribed in ASTM E837, 2 a small dischargespark, 3 a stable discharge process, and 4 a machining time ofless than 1 h. Having done so, the recast layer is inspected tocheck for the presence of surface cracks. If surface cracking isobserved, the machining parameters are adjusted and the hole-drilling operation is repeated. However, if the recast layer iscrack-free, the calibration procedure moves to the next step inwhich a search is made in literature for the thermal conductivitycoefficient of the workpiece material. Assuming that this propertycan be found, the value of the calibration stress is computed di-

rectly in accordance with the power law relation cal=325.5k0.65 MPa, where the thermal conductivity coefficient has

units of W m1 K1 and the calibration stress has units of MPa. Ifthe value of the thermal conductivity coefficient cannot be found,a search is made for the carbon equivalent of the workpiece ma-terial. The carbon equivalent values of most ferrous materials are

readily available in literature. However, in the event that the valuecannot be found, it can be computed directly by analyzing thechemical composition of the workpiece and then applying the for-mula given in Eq. 2 . Having determined the carbon equivalentvalue, the calibration stress is computed using the correlation

cal=7.6 CE +22.4 MPa, where the carbon equivalent is ex-

pressed in percentage terms and the calibration stress has units ofMPa. Having computed a suitable value of the calibration stress inaccordance with either the thermal conductivity or the carbonequivalent of the workpiece, the residual stress within the compo-nent of interest is measured using the EDM hole-drilling strain-gage method in accordance with the guidelines laid down in theASTM E837 standard. Finally, the actual value of the residual

stress within the component act is calculated by applying the

calibration stress

cal to the measured stress

mes in accor-dance with

act = mes + cal = mes + 325.5k0.65

or

act = mes + 7.6CE + 22.4 3

As discussed previously, surface cracks are formed in the recastlayer of the AISI D2 specimens irrespective of the machiningconditions applied. Consequently, the calibration equations pre-sented in Eq. 3 cannot be applied since they are both based onexperimental data obtained from specimens in which a crack-free

Fig. 8 Calibration procedure for ferrous materials in which crack-free recast layer can be obtained




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recast layer was obtained. However, in the hole-drilling experi-

ments involving the AISI D2 specimens, it was observed that for

a given set of machining parameters, the magnitude of the hole-

drilling induced stress remained approximately constant. For ex-

ample, under the optimal machining conditions of 120 V/12

A/6 s /30 s, the hole-drilling induced stress was found to have

a value ofcal= 322.4 MPa. The variance of2.4 MPa is suf-

ficiently small that a calibration stress ofcal= 32 MPa represents

a reasonable value with which to calibrate the measured stress

mes in AISI D2 specimens provided that the stressed component

is hole-drilled using the same set of EDM machining parameters

simultaneously minimizing the cracking density of the recast

layer. Having identified these machining parameters, a stress-freespecimen is prepared by performing an annealing operation. Fol-

lowing the annealing process, the oxidation layer is removed from

the specimen surface by performing a polishing operation and the

strain-gage is adhered to the specimen surface. The hole-drilling

induced stress cal is then measured in accordance with ASTM

E837 using the optimal machining parameters. The value of the

residual stress within the component of interest is obtained by

repeating the EDM hole-drilling process using the same set of

machining parameters. Finally, the actual value of the residual

stress within the component is computed by subtracting the cali-

bration stress cal from the measured stress mes .

4 Conclusions

This study has performed a series of EDM hole-drilling strain-gage experiments using AISI 4140, L6, H13, M2, and D2 work-pieces in order to investigate the effects of the physical properties

of the workpiece material on the magnitude of the hole-drillinginduced stress. The experimental results have then been used toestablish suitable calibration formulae to compensate the residualstress measurements obtained in accordance with the ASTM E837

standard. The major findings and contributions of this study canbe summarized as follows.

In the EDM hole-drilling strain-gage method for measuring theresidual stress within a component, the hole-drilling operation

generates an additional residual stress as the result of thermallyinduced microstructural changes in the local machining area. Thisstudy has demonstrated the feasibility of predicting the magnitudeof this hole-drilling induced stress from the thermal conductivityor carbon equivalent properties of the workpiece material. In thisway, the residual stress measurement obtained using the methodlaid down in ASTM E837 can be compensated directly withoutthe need to perform additional calibration trials. As a conse-quence, both the practicality and the cost of the EDM hole-drillingstrain-gage method are considerably improved.

1. Given EDM machining parameters of 120 V/12 A/6 s /

30 s discharge voltage/pulse current/pulse-on duration/

Fig. 9 Calibration procedure for ferrous materials in which crack-free recast layer can-not be obtained




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pulse-off duration , the magnitude of the hole-drilling stressinduced in ferrous materials with a crack-free recast layer is

related to the thermal conductivity k of the workpiece ma-

terial via the relation cal=325.5k0.65 MPa. The corre-

sponding correlation coefficient is R2 =0.9976, and thus it

can be inferred that an excellent agreement exists betweenthe predicted value of the hole-drilling induced stress andthe experimental value. As a result, the power law relation

provides an accurate and reliable means of compensating theresidual stress obtained using the method prescribed in theASTM E837 standard.

2. Given EDM machining parameters of 120 V/12 A/6 s /30 s, it has been shown that the magnitude of the hole-

drilling stress induced in ferrous materials with a crack-freerecast layer is related to the CE of the workpiece material

via the relation cal=7.6 CE +22.4 MPa. The corre-

sponding correlation coefficient is R2 = 0.9909, and thus thepower law relation provides a suitable means of compensat-ing the residual stress value obtained in accordance with theASTM E837 standard.

3. Of the five ferrous materials considered in the present ex-periments, a crack-free recast layer was obtained in the AISI4140, L6, H13, and M2 workpieces under 21 machining

conditions of 120 V/12 A/6 s /30 s. However, surfacecracks were observed in the AISI D2 workpiece under allvalues of the machining parameters. The propensity of the

AISI D2 workpiece to surface cracking is thought to be theresult of its low thermal conductivity. The experimental re-sults have indicated that the surface cracking phenomenonleads to a partial release of the hole-drilling induced stress,and thus the power law correlations given in points 2 and 3above can no longer be applied to compensate the residualstress measurements obtained using the EDM hole-drillingstrain-gage method.

4. Although the hole-drilling induced stress in AISI D2 speci-mens cannot be predicted directly from the thermal conduc-tivity or carbon equivalent of the workpiece, the present ex-perimental results have shown that under machining

conditions of 120 V/12 A/6 s / 30 s, the hole-drilling in-

duced stress has a value of cal= 322.4 MPa. The vari-

ance of 2.4 MPa is sufficiently small that a calibration

stress ofcal=32 MPa can be regarded as a suitable value

with which to calibrate the measured stress mes providedthat the EDM hole-drilling operation is performed under thesame set of EDM conditions.

References 1 ASTM Standards, 2008, ASTM E837-08e1, Standard Test Method for Deter-

mining Residual Stresses by the Hole Drilling Strain-Gage Method, http://

www.astm.org/Standards/E837.htm. 2 Lee, H. T., Hsu, F. C., and Tai, T. Y., 2004, Study of Surface Integrity Using

the Small Area EDM Process With a CopperTungsten Electrode, Mater. Sci.Eng., A, 364, pp. 346356.

3 Lee, H. T., Rehbach, W. P., Tai, T. Y., and Hsu, F. C., 2004, RelationshipBetween Electrode Size and Surface Cracking in the EDM Machining Pro-cess, J. Mater. Sci., 39, pp. 69816986.

4 Navas, V. G., Ferreres, I., Maran, J. A., Garcia-Rosales, C., and Sevillano, J.G., 2008, Electro-Discharge Machining EDM Versus Hard Turning andGrindingComparison of Residual Stresses and Surface Integrity Generated

in AISI O1 Tool Steel, J. Mater. Process. Technol., 195 13 , pp. 186194. 5 Ekmekci, B., 2007, Residual Stresses and White Layer in Electric Discharge

Machining EDM , Appl. Surf. Sci., 253, pp. 92349240. 6 Ekmekci, B., Tekkaya, A. E., and Erden, A., 2006, A Semi-Empirical Ap-

proach for Residual Stresses in Electric Discharge Machining EDM , Int. J.Mach. Tools Manuf., 46, pp. 858868.

7 Lee, H. T., Mayer, J., Hsu, F. C., Rehbach, W. P., Weirich, T., Dimyati, A., andTai, T. Y., 2006, Application of EDM Hole-Drilling Method to the Measure-ment of Residual Stress in Tool and Carbon Steels, ASME J. Eng. Mater.

Technol., 128, pp. 468475. 8 Lee, H. T., Liu, C., Hsu, F. C., and Hsu, J. M., 2008, An Enhanced Calibra-

tion Scheme for the EDM Hole-Drilling Strain Gage Method for the Measure-ment of Residual Stress in Ferrous Materials, Mater. Trans., 49 8 , pp. 1905

1910. 9 Lee, H. T., and Liu, C., 2008, Calibration of Residual Stress Measurements

Obtained From EDM Hole Drilling Method Using Physical Material Proper-

ties, Mater. Sci. Technol., 24 12 , pp. 14621469. 10 ASM, 2000, Heat Treatments GuideStandard Practices and Procedures for

Steel. 11 Tai, T. Y., 1999, Measurement of Surface Residual Stress in Steel with Hard

Layer by Using EDM Hole-Drilling Method, MA thesis, National Cheng-

Kung University. 12 Hsu, F. C., 2005, The Calibration Study of EDM Hole Drilling Method for

Residual Stress Measurement, Ph.D. dissertation, National Cheng-Kung Uni-

versity. 13 Failure Analysis and Prevention, ASM Metals Handbook, 9th ed., 1986,

American Society for Metals, Metals Park, OH, Vol. 11, p. 8.


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