influence of ultrasonic stress relief on stainless steel 316 specimens_ a comparison with thermal...

Materials and Design 46 (2013) 713–723

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Materials and Design

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Influence of ultrasonic stress relief on stainless steel 316 specimens:A comparison with thermal stress relief

M. Shalvandi a, Y. Hojjat a,⇑, A. Abdullah b, H. Asadi c

a Mechanical Eng. Dep., Tech and Eng. Faculty, Tarbiat Modares University, Iranb Mechanical Eng. Dep., Amir Kabir University of Technology, Iranc Material Eng. Dep., Tech and Eng. Faculty, Tarbiat Modares University, Iran

a r t i c l e i n f o

Article history:Received 15 September 2012Accepted 10 November 2012Available online 29 November 2012

Keywords:Ultrasonic stress reliefVibration amplitudeAcoustic softeningThermal stress relief

0261-3069/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.matdes.2012.11.023

⇑ Corresponding author. Tel.: +98 21 82883364; faxE-mail address: [email protected] (Y. Hojjat).

a b s t r a c t

In conventional stress relieving by vibration, the residual stress reduces by induction of low frequency,high amplitude dynamic stress into an object. This method is confined to large pieces since the highamplitude of vibration deforms the thin or small objects. In this paper, a new stress relieving methodbased on ultrasonic vibrations is introduced which is applicable on the small or thin parts. The effective-ness of the method was verified by comparing it with thermal stress reliving. The stainless steel 316 wasselected for residual stress reduction. The effects of ultrasonic vibration amplitude, relief time and pre-load parameters are evaluated. Acoustic softening of the metal is also evaluated since this phenomenonand dislocation activation by means of the acoustic waves are the main mechanisms behind this process.Experiments show that the residual stress of the small Almen strips can be reduced about 40% by thermalstress relief, while the ultrasonic stress relief is about 36%. It means the amount of stress reduction byultrasonic method is comparable with thermal stress relieving. Statistical analyses of the experimentalresults show that the amount of stress reduction is directly proportional to the vibration amplitudeand the stress relief time. The vibration frequency and the amplitude of the experiments were24,500 Hz and 23–46 lm, respectively. Acoustic softening results showed that the tensile strength ofthe metal strongly decreases by superimposing of ultrasonic vibration in universal tensile test. Theamount of this reduction is proportional to the ultrasonic intensity. Metallographic tests showed thatthere are no changes in grain size during the ultrasonic stress relieving process, and the only effectmay be the dislocations movement.

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1. Introduction

Tensile and compressive residual stress can be induced bymechanical operations such as welding, casting, rolling and shotpeening. Residual stresses may shorten the life of the part byincreasing the fatigue crack growth rate [1,2]. To maintain the ex-pected lifetime, the residual stresses must be eliminated as muchas possible. Vibratory stress relief at resonant frequencies is awell-known technology for reduction of the residual stressescaused by welding or other manufacturing processes. The vibratorystress relief in resonant frequencies was introduced by McGoldrickand Saunders [3]. They applied mechanical vibrations with the fre-quency of 10–30 Hz and amplitude 0.375–0.4 in. to a ship struc-ture. They claimed the possibility of applying mechanicalvibration to the structures but the measurement of the residualstresses was not reported. The most related work was done byWozney and Crawmer [4]. They used shot-peened Almen strip

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(A-type) in stress relieving experiments and applied cyclic loadsto decrease the residual stress. Their outstanding result was 33%reduction in the residual stress. They mentioned that the yieldingcharacteristics of Almen material will change during the applica-tion of cyclic loads. They also showed that one can predict theresidual stress reduction knowing the initial residual stress andthe applied cyclic stress. Weiss et al. [5] relieved the residual stressof a submerged arc welded low carbon steel specimen. The speci-mens were vibrated by a laboratory shaker, and the residual stres-ses were measured by a destructive method called Sachs boring-out technique. Dawson and Moffat [6] draw out the effective vari-ables of vibratory stress relief in 1980. They reported that variousresonance frequencies (such as 33, 66, and 92 Hz) can be used forvibratory stress relieving. They claimed that the increase in appliedcyclic strain amplitude enhances the residual stress reduction. Ludand Hwang [7] evaluated the effectiveness of the vibratory stressrelief by modified hole-drilling method and showed that the firstthree resonant frequencies reduce the residual stress in the weld-ing specimen. Zhuang and Halford [8] proposed an analytical mod-el for estimation of residual stress relaxation, and showed that

714 M. Shalvandi et al. / Materials and Design 46 (2013) 713–723

application of the cyclic load could lead to relaxation of residualcompressive stress in shot peened specimens. Sun et al. [9] relievedthe residual stress of marine steel shaft using a 46.4 Hz vibrationand an X-ray stress meter. They achieved a 48% reduction in macroresidual stress of the steel bar.

Concurrent ultrasonic stress reliving was introduced by Hiraiand Aoki [10]. They applied the vibration during the welding pro-cess of thin plates and measured the amount of residual stress byX-ray stress meter. Liqun and Qijia [11] reduced the internal stressin small MEMS coated wafer made of SU-8 photoresist by utiliza-tion of the ultrasonic vibrations. The coated wafer bends underthe effect of internal residual stress in the layer. The frequencyand amplitude of the ultrasonic vibration was 20 kHz and 15–20 lm, respectively. Same authors optimized the ultrasonic stressrelief parameters used in their previous publication based on thefuzzy neural network in 2010 and 2011 [12,13]. Stress relief ratescan be changed from 27.8% to 59.5% by changing the effectiveparameters. There are a lot of investigations on the effectivenessof conventional (low frequency) vibratory stress relief process,while stress relieving by ultrasonic has not been reported onmetallic parts.

In this research, the application of high power ultrasonic vibra-tions for stress reliving of metallic parts is introduced. The effectiveparameters are studied and the result is verified by experiments onshot peened Almen strips. The effectiveness of the process is esti-mated by comparing the residual stresses before and after the oper-ation. The efficiency of this process is also compared with thermalstress relieving method. The method is applicable for almost allthe thermal or mechanical residual stresses exist in thin sheets orsmall pieces, such as the welded plates in the aircraft wing.

2. Mechanism of the ultrasonic stress relieving

The acoustic coupling between the transducer and the speci-men plays an important role in transferring the vibration into thespecimen. Fig. 1 shows the schematic mechanism of the ultrasonicstress relief device designed for this research. It is generally con-sisted of a high frequency power supply capable of providinghigh-power electric pulses, an ultrasonic transducer including pie-zoelectric, matching and backing parts, a horn made of high-strength aircraft aluminum, and a special die for imposing thevibration on the work piece. Piezoelectric transfers a standingwave into the matching part. A special horn is designed to transferthe adjusted wave into the specimen. Ultrasonic vibration easesthe movement of the internal dislocations, and reliefs the residualstress by acoustic softening.

Fig. 1. Ultrasonic stress

3. Experimental steps

3.1. Preparatory works for experimental samples

Three types of Almen strip with the same length and width butdifferent thicknesses are selected. They are called N, A, and C typeswith a thickness of 0.772, 1.283 and 2.372 mm, respectively. Fig. 2shows dimensional tolerances and rolling direction of Almen strips[14,15].

Almen strips were waterjet cut from a stainless steel sheet witha thickness of 1.3 mm and sized by milling. Table 1 shows the com-position and mechanical properties of the sheet [ASTM A193/A193M-12b].

To remove the initial curvature of Almen specimen and fulfillthe required straightness and flatness mentioned in ASM Standard,heat stress relieving was performed. Before heat treatment, thestrips were fixed inside the die and fastened by four standardscrews using a manual torquemeter in order to apply equal torqueto all of the screws. According to the ASM handbook [16], the fur-nace heating rate was 5 �C/min, holding temperature 900 �C, andcooling rate 1 �C/min [17]. The furnace was purged and filled withArgon inert gas to prevent the oxidation of strips. Fig. 3 shows therelation between the heat treating temperature and stress relievingrate.

3.2. Shot peening process

Shot peening of specimens was carried out by Reymehr Co., ma-chine, according to the standard instructions [14]. The air pressurewas 8 MPa, nozzle distance to the strip was set at 100 mm, angle ofimpact was set to be 83�, nozzle diameter was 5 mm and the aver-age measured shot diameter was 0.45 mm. Fig. 4a shows the pre-pared setup on the shot peening machine. The achieved Almenarc height is shown in Fig. 4b. For measuring the Almen arc height,Almen gage was made with respect to the shot peening standard(Fig. 4c).

3.3. Ultrasonic stress relieving

After shot peening of the specimens and measuring the ob-tained Almen arc height, ultrasonic stress relief operation was per-formed on the specimens. High power MPI Ultrasonic generatorwas used to create vibration in the transducer. The resonant fre-quency of the transducer was about 24,500 Hz. Hence modal anal-yses were carried out by ANSYS, and a horn with the same resonantfrequency of the transducer was designed for this purpose which

relief mechanism.

Fig. 2. Dimension of a type Almen strip [14,15].

M. Shalvandi et al. / Materials and Design 46 (2013) 713–723 715

was made up of Al7075-T6. Ultrasonic vibrations transferred to thespecimens via the horn. The effect of vibration amplitude, relieftime and pre-load force were investigated. Relieving time was 2,5 and 8 min and vibration amplitude 23, 34.5 and 46 lm. Fig. 5shows the experimental setup of ultrasonic stress relief operation.

3.4. Acoustic softening evaluation

Acoustic softening is a phenomenon that occurs during theultrasonic stress relieving, and extremely decreases the yield andultimate tensile stresses of the metal. An experimental setup wasprepared to experimentally investigate the phenomenon. The ten-sile specimen was designed using ANSYS modal solution, applying3D solid95 element. Natural frequencies of 15–30 kHz were ob-tained. The length of altering specimen was determined for naturalfrequency of 20 kHz.

Points 2 and 4 in Fig. 6 are the vibration nodes, and points 1, 3and 5 are the vibration anti-nodes. As the cross section areas aredifferent, the vibration amplitude at point 3 is greater than theone at point 1.

Am3Am1¼ A1

A3

A1 ¼ 5:76 � A3

Am3 ¼ 5:76 � Am1

ð1Þ

where A is the cross section area of the sample and Am the vibrationamplitude at specified points. Fig. 7 shows the sample which is de-signed according to ANSYS output, and the standard tension testdimensions [based on ASTM E8/E8M-11]. To prevent the vibrationof other components, the sample is fixed at vibration anti-nodes(points 2 and 4).

To study the acoustic softening effect, a high power ultrasonictransducer was connected directly to the tensile sample and vi-brated it with approximate power amplitude of 300 and 600 Wduring the tensile test (Fig. 8).

Measurements with Eddy current sensor showed that the vibra-tion amplitude at point 1 with 300 and 600 W is equal to 4 and8 lm, respectively. Thus, according to the Eq. (1), the vibrationamplitude at the point 3 is equal to 23 and 46 lm, respectively.

3.5. Thermal stress relieving process

Three specimens were chosen to be stress relieved with thethermal process. The specimens were heat treated in the furnace

Table 1Mechanical properties and chemical composition of stainless steel 316 [ASTM A193/A193

Chemical compositionMaterial number 1.4401Symbol X5CrNiMo17122Trade name Stainless steel 316Element (wt.%) C Cr Mn

Max 0.08 Max 18 Max 2.0

Mechanical propertiesYield strength (Min MPa) Tensile strength (Min MPa) Density (kg/m3

290 580 800

without clamping. Thermal stress relieving conditions were thesame as before (see Section 3.1).

4. Residual stress in Almen strips

To develop the mathematical explanation of deflection, the sim-ple beam theory is used (Fig. 9a). The basic formula for the follow-ing loading at any point (x) along a beam is [18,19]:

MI¼ r

c¼ E

Rð2Þ

where M is the bending moment at x, I the moment of inertia of thebeam which is equal to:

I ¼ bt3

12ð3Þ

where b and t are the length and width of the rectangle cross sec-tion, respectively. Neutral axis is parallel to the length of the rectan-gle and c is the distance from the neutral axis. In symmetricalsections this is the distance from the midpoint or center of gravityof the section. E is the Young’s modulus. R is the radius of curvatureof the beam when it bends under load. The stress in a simple beamcan be calculated as:

d ¼ McI

ð4Þ

For a rectangular beam the greatest stress is at the surface:

C ¼ t2; rmax ¼

6M

bt2 ð5Þ

Although the distribution of residual stress through the thick-ness is unlikely linear, it is assumed to be linear. As the sheet isfishtailed or cut in a central planar direction or shot peened, thebending moment created by the residual stress will released(Fig. 9c). The bending moment may be expressed as:

M ¼ EIR

ð6Þ

The maximum longitudinal stress at the surface is obtained bysubstituting Eqs. (3), (6) and c = t/4 in Eq. (5):

rmax ¼Et4R

ð7Þ

If the deflection (d) is small compared to the radius of curvature(R), R may be expressed in terms of the deflection (d) and thelength of the curved surface (L), by:

R ¼ L2

2dð8Þ

Eq. (8) is derived from Fig. 10, where isosceles triangles RLK andmdn are similar, and therefore R/L = m/d or R = Lm/d. For small an-gles, L is equal to the arc 1–2 and m is about half of arc 1–2 (Fig. 9c).Substituting Eq (8) in Eq. (7) gives:

M-12b].

Ni P Si S FeMax 14 Max 0.045 Max 1.0 Max 0.03 61.8–72.0

) Hardness (RB) Elongation in 2 in or 50 mm (Min %)79 50

Fig. 3. The effect of duration and temperature on thermal stress relief, according toASM [17].


rres ¼Etd

2L2 ð9Þ

The residual stress as a function of d0 becomes:

rres ¼2Etd0

L2 ð10Þ

where d0 is the Almen curvature and is measured by an accurate Al-men gage at the middle of the Almen strip.

5. Results and discussion

5.1. Shot peening results

After shot peening process, Almen arc was measured by Almengage and residual stress of the strips was calculated by Eq. (10). Ta-

Fig. 4. Almen test procedure. (a) Shot peening of Alme

Fig. 5. Experimental setup of ultra

ble 2 shows the results of shot peening tests and the amount ofresidual stress.

5.2. Thermal stress relief results

Experimental results showed that residual stress of Almen stripscan be removed considerably by thermal stress relief process(Fig. 11). In this figure, the black columns show the initial residualstresses caused by shot peening, and the white columns show theresidual stresses after thermal stress relieving. According to thisdata, the average reduction of residual stress by the thermal processis approximately 40%. As a result, a large amount of stress-reduc-tion happens when the initial residual stress is relatively high.

5.3. Ultrasonic stress relief results

5.3.1. Overall effects of ultrasonic stress relievingFig. 12 shows the residual stress in 13 samples before and after

ultrasonic stress relieving. Difference between initial values ofresidual stress in specimens is the consequence of difference inshot peening processes. Based on these data the average stressreduction rate is about 36%. The amount of stress relieving byultrasonic is not as much as thermal treatment, but it can be usedfor large components that could not go under thermal process.

5.3.2. Effect of vibration amplitudeAccording to experimental results, when the ultrasonic ampli-

tude increases from 23 to 46 lm, the amount of stress reduction

n strip, (b) Alemn arc height and (c) Almen gage.

sonic stress relief treatment.

Fig. 6. Modal simulation of tension specimen designed for acoustic softening evaluation.


increases respectively (Fig. 13). When the ultrasonic amplitude is46 lm, Almen residual stresses decrease from 237 to 120 MPa.Thus, the vibration amplitude has a positive effect on stress reduc-tion rate.

5.3.3. Effect of stress relieving timeAccording to experimental results, when the relief time in-

creases from 2 to 8 min, the residual stresses decrease from 185to 105 MPa (Fig. 14).

It may be concluded from Fig. 14 that the stress reduction rateincreases by increasing the relief time, but it is true for a shortduration, according to other investigation results [11], after a longrelief time such as 20 min, it may decrease.

Fig. 7. Tension specimen dimension and ultrasonic v

Local heating is one of the mechanisms that can be used forexplanation of how acoustic stress relief works. By increasing therelief time, there would be enough time to heat the localized zonesin the grain boundaries.

5.3.4. Effect of pre-loadFig. 15 shows the variation of stress reduction rate as a function

of preload. Pre-loading force varies from 3.2 kN to the 8 kN bysteps of 1.6 kN. The residual stress decreases 20 and 58 MPa byapplying 3.2 kN and 8 kN pre-load respectively.

For comparing the effectiveness of parameters and finding theeffectiveness of the parameters statistically, one-way ANOVA anal-ysis was performed. As shown in Table 3, vibration amplitude has

ibration amplitude [based on ASTM E8/E8M-11].

Fig. 8. Experimental setup for acoustic softening evaluation.

Fig. 9. Schematic of (a) simple beam theory, (b) distribution of longitudinal residual stress and (c) compressive and tensional residual stress at the surface of specimen [19].

Fig. 10. Schematic representation of the deflection formula derivation [19].


the maximum standardized beta coefficient (0.143) hence the mosteffective parameter is vibration amplitude. Standardized beta coef-ficient value for stress relief time is 0.083, so the effect of relieftime on stress reduction rate is less than vibration amplitude. Fi-nally, the least effective parameter is the pre-loading force.

5.4. Acoustic softening evaluation

Acoustic softening experiments were performed at two levels ofultrasonic intensity; 300 W and 600 W. Tensile testing was startedwith superimposing ultrasonic vibration, and then the stress–strain curve was plotted.

According to Fig. 16, yield stress of the metal is 263 MPa and theultimate tensile strength is 543 MPa, by applying 300 W ultrasonicvibration, the yield point and tensile strength decrease to 178 MPa,and 388 MPa, respectively. Using a 600 W ultrasonic vibration, theyield and tensile strength decrease to 146 and 305 MPa, respec-tively. These results show that superimposing of ultrasonic vibra-tion on tensile testing has softening effects, and decrease theshear stress of the material.

For investigation of ultrasonic effects on the plastic behavior ofthe metal, the ultrasonic generator switched on and off during thetensile test. Fig. 17 shows the results of experiments. When ultra-sonic generator switched on, the required stress for the same straindecreased immediately. This sharp reduction is considered to bethe consequence of acoustic softening. When the ultrasonic gener-ator is switched off again, tensile stress returns to the value a littlebit higher than its original value; this increase is assigned as acous-tic hardening.

5.5. Metallography of specimens

To study the effect of the ultrasonic stress relief process on grainboundaries, metallographies of the Almen specimens were per-formed before and after the ultrasonic process. To reduce the ef-fects of cutting process on the metallurgical properties ofspecimens, fine electro-discharge wire cutting process was usedto cut the specimens, and then specimens mounting were done

Table 2Almen arc results and obtained residual stress of Almen strips.

Sample no. SS1 SS2 SS3 SS4 SS5 SS6 SS7Almen arc (mm) 0.246 0.415 0.273 0.420 0.435 0.535 0.315Residual stress (MPa) 109.40 184.56 121.40 186.78 193.45 237.92 140.08Sample no. SS8 SS9 SS10 SS11 SS12 SS13Almen arc (mm) 0.195 0.250 0.41 0.396 0.22 0.234Residual stress (MPa) 86.72 111.18 182.33 176.10 97.83 104.06


according to the ASM directions [20]. After mounting the speci-mens, mechanical grinding and polishing processes were used toprepare the samples. Etching process was carried out with(50 mL HCl, 10 g CuSO4, and 50 mL H2O). Specimens were etchedwith this etchant for 45 s then metallography photos were takenand analyzed. Figs. 18 and 19 show the microstructure of austeniticstainless steel 316 before and after ultrasonic stress relieving,respectively.

The austenitic base stainless steel grain boundaries are clear inFig. 18. Fig. 19 shows no considerable changes in the grain size ofthe specimen. It may be resulted that the stress relieving processhas not changed the grain size.

To explain the stress relief mechanism, it is necessary to studythe special effects of high power ultrasonic treatment. This treat-

Fig. 11. Thermal stress

Fig. 12. The amount of Almen strip residual stresses

ment has three major effects on the specimens, namely acousticsoftening, acoustic hardening and stress reduction. There is a lotof work on the plastic framework of the acoustic softening that ex-plains the mechanism of ultrasonic effects. Applying high powerultrasonic vibrations to tensile samples during a constant straintest reduced the tensile strength and yield stress of the metalimmediately [21,22]. The percentage of this stress reduction is pro-portional to the ultrasonic intensity imposed to the specimen dur-ing the tensile test and is independent of vibration frequency overthe range of 15 Hz–1.5 MHz [21]. Vibration amplitude influenceson the tensile properties of the metal too [23].

The effects of ultrasonic vibration on the tensile properties ofmaterials are presented by crystal plasticity theory. In this theory,dislocation flow on a slip system of metal is presented in a contin-uum sense as c (plastic shear strain). In a special slip system, a, theplastic strain rate is given by [24]:

_ca ¼ ca0sgnðsaÞ sa

ga

��

� �m

ð11Þ

where ca0 is the reference strain rate, m is the rate sensitivity expo-

nent (material properties) and ga is the strength of a slip system.Ultrasonic process causes some internal changes in the materialthat leads to decrease the tensile strength so the plastic strain rateincreases. When applying the high power ultrasonic, ga is smaller.

relieving results.

before and after ultrasonic stress relief process.

Fig. 13. The effect of ultrasonic amplitude on stress reduction rate (stress relief time: 5 min and pre-load: 4.8 kN).


Hence the modified formula for plastic slip with acoustic softeningis given by

_ca ¼ ca0sgnðsaÞ sa

ga � Us

��

� �m

ð12Þ

where US is a coefficient showing the effect of the ultrasonic soften-ing and its value is less than 1. According to the other investigations,ultrasonic energy is absorbed in some regions such as grain bound-aries, dislocations and voids. It means that by increasing the ultra-sonic intensity, US, is getting close to zero, and _ca increasesextremely.

Eq. (12) explains the effect of ultrasonic in plastic range. Someinvestigations show that the ultrasonic effect takes place just inplastic range [25], while the others claim in both elastic and plasticrang [22]. This research shows that the ultrasonic softening effectstake place in both elastic and plastic range which realizes the ultra-sonic stress relieving. The prospect is that, when the ultrasonicvibrations are removed, the tensile strength returns to its initial va-lue, but this does not happen and tensile strength reaches a valuehigher than its initial value. Acoustic hardening is similar to work

Fig. 14. The effect of stress relief time on stress reduction

hardening, and it is independent of ultrasonic frequency and is di-rectly proportional to the applied ultrasonic intensity (IU).

5.6. Acoustic movement of dislocations

From dislocation theory, the shear stress that is needed to causea plastic flow is the stress required for dislocation movement.Ultrasonic treatment decreases the amount of shear stress. If theshear stress decreases to zero, then da = s, where da is the stressproduced by ultrasonic waves when passing through a solid metal.da can be determined by Eq. (13) [22]:

da ¼ qmc ¼ffiffiffiffiffiffiffiffiffiffi2IUEm

rð13Þ

where q is the density of material, c the velocity of sound, m themetal particle displacement and IU the ultrasonic intensity. Dislo-cations absorb the acoustic stress and move easily. There are threemechanisms of ultrasonic energy absorption in dislocation; reso-nance, relaxation and hysteresis [23]. The natural frequency ofoscillation of dislocation is about 108 Hz, so the ultrasonic vibra-

rate (vibration amplitude: 6 lm and pre-load: 4.8 kN).

Fig. 15. The influence of pre-loading force on stress reduction rate (vibration amplitude: 6 lm and relief time: 5 min).

Table 3Coefficients of ANOVA analyses.

Model Unstandardizedcoefficients

Standardized Coefficients t

B Std. error Beta

1 (constant) 24.818 30.509 .813Preload �.202 2.882 �.028 �.070Vibration amplitude 1.375 3.858 .143 .356Relief time .533 2.572 .083 .207


tion at the frequency of 20 kHz cannot have a visible effect on theresonance mechanism. According to the other investigations[22,23] relaxation and hysteresis cannot be suitable mechanismsfor the ultrasonic energy absorption in the dislocations. It seemsthat local heating theory can describe the acoustic softening. It issuggested that since dislocations absorb the ultrasonic energy, lo-cal heating occurs in the circumference of the dislocations whenultrasonic stress waves effluence through the specimen. Thisheated area of the dislocations is the weak points in the metaland need the least shear stress to move around and to cause plasticslip. This research investigated this mechanism by applying twoamounts of intensity; IU = 42 W/cm2 and IU = 84 W/cm2.

Fig. 16. Strain–stress curves of typical tensile spe

6. Conclusion

In this paper, the high power ultrasonic stress relieving methodis introduced and studied for thin metallic strips. It is shown thatup to 36% reduction in the residual stress is possible. Vibrationamplitude, process time and pre-load are the main parameterswhich have significant influence on the ultrasonic stress relief rate.It is shown that, the vibration amplitude is directly proportional tothe stress reduction rate and has the most significant effect on it.Maximum relief happens at the highest amplitude. In this research,the highest amplitude was about 46 lm and residual stresses of Al-men strip dropped from 237 to 120 MPa. Process time has also adirect effect on the residual stress reduction. When process timeis 2 min, the residual stresses decrease by 34 MPa, while whenthe process time increase to 8 min the residual stresses decreaseapproximately 75 MPa.

Pre-loading force has a little effect on the stress reduction.When it increased from 3.2 to 8 kN, the stress relief increased from20 to 58 MPa. Statistical analyses by using of SPSS showed that thevibration amplitude has the highest effect and the pre-load has theleast effect on stress reduction. Metallography tests did not showany change in the size of grains, due to ultrasonic stress relieving.Therefore, the local heating in the grain boundaries might be themain mechanism of stress relieving.

cimen with and without ultrasonic vibration.

Fig. 17. Strain–stress curves of typical tensile specimen with and without ultrasonic vibration.

Fig. 18. Light micrograph of austenitic stainless steel 316 before ultrasonic stressrelieving (500X).

Fig. 19. Light micrograph of austenitic stainless steel 316 after ultrasonic stressrelieving (500X).


Thermal treatment showed 40%reduction in the residual stress,while the reduction achieved by ultrasonic was 36%. Therefore, thenew method is comparable with thermal treatment, and could beused in the cases that thermal stress relief has limitations.

The results of acoustic softening showed that ultrasonic inten-sity directly affects the material characteristics. Applying ultra-sonic vibration with 300 W (IU = 42W/cm2), decreases the yieldstrength from 263 to 178 MPa and reduces the tensile strengthfrom 543 to 388.3 MPa. Applying ultrasonic vibration with 600 W(IU = 84W/cm2), causes the sharp drop of yield strength and tensilestrength from 263 to 146.4 MPa and from 543 to 304.4 MPa,respectively. These results prove that by increasing the ultrasonicintensity from 42 to 84 W/cm2, ultrasonic softening coefficient de-creases sharply, weakens the slip strength between the grains, andcauses the material to be softened.

In the tensile test, when the ultrasonic generator switches off,the stress goes slightly higher than the initial value (without ultra-sonic), this effect is acoustic hardening, and the amount of thisincrement depends on ultrasonic intensity (IU) applied during thetensile testing.

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