Effect of Roughness and Residual Compression Stresses on the Ultrasonic Fatigue Endurance of Aluminum Alloy AISI 6061-T6

Materials Performance andCharacterization

Gonzalo M. Dominguez Almaraz1

DOI: 10.1520/MPC20140064

Effect of Roughness andResidual CompressionStresses on theUltrasonic FatigueEndurance of AluminumAlloy AISI 6061-T6

VOL. 4 / NO. 1 / 2015

Gonzalo M. Dominguez Almaraz1

Effect of Roughness and ResidualCompression Stresses on theUltrasonic Fatigue Endurance ofAluminum Alloy AISI 6061-T6

Reference

Almaraz, Gonzalo M. Dominguez, “Effect of Roughness and Residual Compression Stresses

on the Ultrasonic Fatigue Endurance of Aluminum Alloy AISI 6061-T6,” Materials

Performance and Characterization, Vol. 4, No. 1, 2015, pp. 45–60, doi:10.1520/

MPC20140064. ISSN 2165-3992

ABSTRACT

Ultrasonic fatigue tests were carried out on the aluminum alloy AISI 6061-T6,

presenting different values for the principal surface roughness parameters: Ra,

Rq, and Rz. For fatigue life comprised between 3� 105� 6� 106 cycles, crack

initiates at the specimen surface induced by stress-concentration and micro-

plastic deformation (micro-void coalescence); whereas for the very high cycle

fatigue (>107 cycles), the mechanism of crack initiation moves to subsurface

or inside the specimen and is associated with internal imperfections such as

micro-porosities or nonmetallic inclusions. For the first fatigue life regime,

compression residual stresses induced by the work-hardening machining

process and the associated micro-plastic deformation are the principal factors

controlling the fatigue endurance on this aluminum alloy. Experimental results

show that fatigue endurance is higher for the high surface roughness in the

3� 105� 6� 106 cycles of fatigue life, whereas this behavior is inversed in the

very high-cycle fatigue regime. These results are analyzed in terms of residual

compression stresses induced by the work-hardening machining process, the

surface roughness, and the reverse yielding or Bauschinger effect.

Manuscript received June 24,

2014; accepted for publication

April 8, 2015; published online

April 29, 2015.

1

Univ. of Michoacan (UMSNH),

Faculty of Mechanical Engineering,

Santiago Tapia No. 403, Col.

Centro, 58000 Morelia, Mexico

(Corresponding author),

e-mail: [email protected]

Copyright VC 2015 by ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959 45

Materials Performance and Characterization

doi:10.1520/MPC20140064 / Vol. 4 / No. 1 / 2015 / available online at www.astm.org

Keywords

surface roughness, compression residual stresses, ultrasonic fatigue testing, fatigue

endurance, reverse yielding

Introduction

An important number of studies has been devoted to understanding how cracks ini-

tiate and propagate in steels influenced by surface roughness. A systematic study on

machining parameters has revealed that the principal factors affecting fatigue endur-

ance on threaded Society of Automotive Engineers (SAE) 4340 steel are tool wear

(inducing compression or tension residual stresses) and cutting speed, whereas ra-

dial feed rates and cutting methods have less influence [1]. A principal motivation to

carry out this work is that no studies have been developed concerning the effect of

surface roughness on fatigue endurance of aluminum alloys, under ultrasonic fatigue

testing.

Concerning the roughness parameter Ra (the arithmetic average of absolute val-

ues of the roughness profile ordinates), in the range between 2.5 and 5lm, fatigue

endurance was mainly dependant on surface residual stress and micro-structure

rather than roughness; nevertheless, in the absence of residual stresses, an Ra higher

than 0.1 lm has an important influence on fatigue life [2].

An increase on temperature during fatigue testing induces a decrease on the re-

sidual stress and surface-roughness effect, because of stress relieving and the modifi-

cation of the crack-initiation mechanism from the surface to inside the specimen.

With temperatures higher than 600�C, crack propagation takes predominantly an

inter-granular pattern [3,4]. For metallic alloys with low surface roughness and with-

out mechanical or thermal residual stresses, the mechanism of crack initiation is

related to persistent slip bands at a crystallographic scale or grain boundary de-

cohesion [5–7].

Surface-roughness effects on fatigue endurance can be estimated for the stress-

free specimens; under this condition, fatigue cracks sometimes arise from relatively

low initial cracks even below 50 lm in depth [8]. Furthermore, roughness effects on

fatigue endurance may be eliminated by a postmachining or thermal surface treat-

ment inducing a compressive or neutral residual stress at, or near, the specimen sur-

face and, consequently, retarding crack initiation and propagation [9,10].

Concerning the geometrical parameters of surface roughness, some authors

have pointed out the predominance of amplitude parameters, such as arithmetic av-

erage of absolute values Ra and the root mean square Rq, in characterizing the rough-

ness effect on fatigue endurance [11,12]. Parameters related to the machining

process have been studied to correlate with the surface roughness and fatigue endur-

ance. Feed rate f (mm/rev) and tool nose radius rt (mm) have been mentioned as

principal machining parameters affecting surface roughness and fatigue life [13].

The theoretical maximum height roughness Rmax may be obtained by the empirical

relation:

Rmax ¼f 2

8rt(1)

ALMARAZ ON ALLOYAISI 6061-T6 46


where:

Rmax,¼ the distance between the higher peak and the deeper valley along the ref-

erence length lr on the roughness topography.

Experimental results using f¼ 0.2 (mm/rev) and rt¼ 0.2mm as high values

(Rmax � 27.5lm, real), have been compared to theoretical ones obtained with Eq 1;

these results are plotted in Fig. 1. The theoretical line is lower than the experimental

regression line and the difference increases in increasing Rmax. Experimental results

show that Rmax increases when rt decreases for a constant f (f¼ 0.2mm/rev), whereas

the axial residual stress r1 (in the longitudinal direction with regard to the

hourglass-shaped specimen) increases in compression with the feed rate f and

decreases in compression with the tool nose radius rt.

Materials and Experimental Setup

The aluminum alloy AISI 6061-T6 was used for the surface-roughness characteriza-

tion and ultrasonic fatigue testing. Standardized chemical composition in weight

and measured mechanical properties are listed in Tables 1 and 2, respectively.

FIG. 1

Effect of feed rate f and nose

radius rt on real and theoretical

maximum height roughness

Rmax [13].

TABLE 1

Standardized chemical composition in weight of aluminum alloy AISI 6061-T6.

Al 95.8 to 98.6

Cr 0.04 to 0.35

Cu 0.15 to 0.4

Fe Maximum 0.7

Mg 0.8 to 1.2

Mn Maximum 0.15

Si 0.4 to 0.8

Ti Maximum 0.15

Zn Maximum 0.25

Other Each maximum 0.05

Other Total maximum 0.15



Continuous ultrasonic fatigue testing was carried out using an ultrasonic fatigue

machine developed in our laboratory, which has been patented in 2014 (Patent No.:

323948, Mexico) (Fig. 2). The principal innovations of this machine are a Lab-View

program allowing the test starting run, the number of cycles recording, and the auto-

matic stop when the specimen fails. An additional innovation is the calibration of

specimen displacements by an inductive proximity sensor, which has a resolution of

63lm working at 1.5 KHz.

Ultrasonic fatigue testing implies the resonance condition of testing specimens;

the natural frequency in longitudinal direction must be close to the excitation source

frequency (20 KHz) to attain a stationary elastic wave along the specimen. Under

resonance, higher stress at the neck section and higher displacement at the specimen

opposite ends is verified. This condition is obtained by modal simulation using the

mechanical properties of testing specimen and its geometrical dimensions. Figure 3

shows the specimen dimensions and the finite-element modal analysis, presenting

the natural longitudinal frequency of 20029 Hz.

Tests were carried out at room temperature and at relative humidity comprising

between 35 % and 45 %. Three types of testing specimens were considered for this

study with regard to the surface roughness: specimens obtained from the machining

process and specimens polished with emery paper after machining—rough paper

TABLE 2

Measured mechanical properties of aluminum alloy AISI 6061-T6.

Density (kg/m3) 2740

Hardness, Brinell 96

ry (MPa) 275

ru (MPa) 315

E (GPa) 69

Poisson ratio 0.33

Elongation at break (%) 16

FIG. 2

(a) Ultrasonic fatigue testing

machine and (b) specimen of

aluminum alloy 6061-T6,

cooling tubes, and proximity

sensor to calibrate the

displacements at the

specimen-free side.



degree 80 and fine paper degree 1500. Tables 3–5 list the three surface-roughness

parameters (Ra, Rq, and Rz), measured at the neck and at the body. Each roughness

value in these tables is the average of four measures using the Mitutoyo roughness

tester, model SJ-210. In addition, the higher recorded testing temperature on the

specimen neck section was 75�C.

All ultrasonic fatigue tests were carried out under zero mean stress (loading rate

R¼�1) and uni-axial loading [14].

Concerning the cutting parameters for testing specimens, Table 6 lists the values

used in this work. A Sandvik Coromant H13A tool insert was employed for cutting

purposes, which are uncoated carbide grade cutting inserts containing primarily

tungsten carbide.

Residual stresses after machining and machining–polishing were measured

using the blind hole-drilling method, according to ASTM E837 [15]. Residual stress

accumulated before the drilling process is estimated from strain variations registered

by a three-elements stress–strain gauge rosette (EA-06-062RE-120, Measurements

FIG. 3 (a) Dimensions of ultrasonic fatigue specimen for aluminum alloy AISI 6061-T6 and (b) modal analysis and obtained

natural frequency of vibration for this specimen.

TABLE 3

Roughness parameters for specimens after machining and without polishing.

Average Value

Ra (lm) Rq (lm) Rz (lm)

No. Neck Body Neck Body Neck Body

1 0.91 1.14 1.17 1.33 6.0 5.43

2 0.96 1.66 1.23 1.97 5.39 8.10

3 1.08 1.99 1.38 2.31 5.42 8.36

4 1.16 2.38 1.48 2.91 6.44 11.80

5 1.13 0.49 1.49 0.61 6.82 3.13

6 1.21 0.66 1.54 0.86 6.69 4.28

7 0.98 3.35 1.28 3.69 5.84 12.43

8 0.95 0.98 1.20 1.21 5.25 5.49

9 0.91 0.99 1.16 1.24 5.04 5.69



http://www.astm.org/Standards/E837

Group, Toronto, ON). A high-speed hole-drilling ring (5000 rev/s) activated by air

turbine was employed for drilling; the cutter was an inverted-cone and carbide

tipped with nomination ATC-200-062 (1.6mm of diameter).

The norm ASME E837 is applicable for an isotropic elastic material in which

the residual stresses are approximately constant along the drilling hole and the val-

ues do not exceed 50 % of the corresponding yield stress of testing materials. Fur-

thermore, because the relieved strains after drilling decrease as the depth of the hole

increases and the parasite effects are higher close the edge hole, the gauge location

was fixed in the interval 0.3< r< 0.45, where r¼R0/R (R0 is the hole radius and R is

the radius of the longitudinal center of the gauge). To attain close to 100 % of strain

relieving through the drilling hole, the relation Z/D¼ 0.4 was adopted (dimension-

less hole depth) according to ASME E837, where Z is the hole depth in mm and D is

the gauge circle diameter in mm. Concerning the ratio D0/D (where D0 is the hole

diameter¼ 1.6 mm), it was: D0/D¼ 0.4. The total depth Z¼ 1.6mm was reached af-

ter 12 increments of 133.3lm each.

TABLE 4

Roughness parameters for specimens polished after machining with rough paper, degree 80.

Average Value



1 4.53 4.04 5.80 5.19 29.16 26.67

2 4.30 4.27 5.50 5.42 27.92 27.33

3 4.66 4.33 5.83 5.52 28.08 28.42

4 4.43 4.12 5.61 5.17 26.93 25.13

5 4.37 4.03 5.58 5.11 28.15 25.24

6 4.19 4.04 5.19 5.18 23.43 26.08

7 4.59 4.39 5.85 5.58 27.43 27.96

8 4.16 4.13 5.35 5.26 26.41 25.93

9 4.27 3.99 5.366 5.02 25.76 24.17

TABLE 5

Roughness parameters for specimens polished after machining with fine paper, degree 1500.

Average Value



1 0.80 0.76 1.05 0.99 5.00 5.29

2 0.68 0.71 0.91 0.91 4.19 5.02

3 0.70 0.66 0.94 0.84 4.07 3.85

4 0.76 0.72 1.03 0.91 4.91 4.86

5 0.54 0.63 0.711 0.86 3.51 3.99

6 0.66 0.58 0.89 0.74 3.85 3.97

7 0.75 0.75 1.01 1.00 4.59 4.51

8 0.66 0.59 0.89 0.78 4.36 3.95

9 0.70 0.65 0.93 0.82 4.39 3.77



Experimental Results

FRACTURE SURFACES

Fracture was observed systematically at the neck section of testing specimens.

Figure 4(a) shows a partial view of the fracture surface for a nonpolished specimen; a

quasi-uniform plastic layer was localized at the surface, induced by the machining

process and ranging from 6 to 10lm. A fracture path was developed systematically

at the specimen neck section (Fig. 4(b)).

Vickers micro-hardness (HV), was obtained close to the machining surface of

unpolished specimens with an indentation load of 500 g; the corresponding rake

angle for all specimens was þ20� [16]. Figure 5 shows plotting of the measured Vick-

ers indentation hardness beneath the specimen surface for both the peak and valley

localization; these values are between 365 and 530 HV. The Vickers indentation

hardness for 6061-T6 before machining is close to 107 HV; it increases to 435 HV in

the immediate zone of the specimen surface after machining. The increase of micro-

hardness is related to residual compressive stresses generated during the work-

hardening machining process, which plays an important role in the fatigue endur-

ance behavior of this aluminum alloy under ultrasonic fatigue testing.

TABLE 6

Cutting parameters.

Parameter Value

Cutting speed (m/min) 300

Nose radius (mm) 0.4

Feed ratio (mm/rev) 0.2

Depth of cut (mm) 0.6

FIG. 4 (a) Plastic layer at the specimen fracture surface and (b) fracture path through the specimen neck section.



RESIDUAL STRESSES MEASUREMENT

Residual stresses were obtained after machining using the blind hole drilling

method, allowing measuring deep distances compared to the x-ray diffraction tech-

nique (the last requiring electrolytic polishing for deep measurements). Figure 6

shows plotting of the axial or longitudinal residual stress r1, and the circumferential

residual stress r2. The high value for r1 is close to �185MPa (compression) and is

attained close to 50 lm from the specimen surface. Residual stress r1 decreases after

50lm in depth, attaining zero residual stresses at approximately 250 lm.

Circumferential residual stress r2 (Fig. 6) shows a top value of �95MPa close to

50lm from the specimen surface. Residual stresses play an important role for fa-

tigue endurance of machined metallic alloys, as it has been reported in the literature.

Initial tensile residual stresses generated by high-frequency induction heating on

annealed stainless steel AISI 304L change to compressive residual stresses during fa-

tigue testing with the corresponding increase of fatigue endurance [17]; fatigue crack

growth and fatigue life is improved in magnesium alloy AM60B by applying a tensile

overload within a constant amplitude cyclic loading, particularly for negative stress

FIG. 5

Micro-hardness measure

beneath the machined surface

of testing specimen.

FIG. 6

Residual stress after machining

in the longitudinal or axial

direction r1 and in the

circumferential direction r2,

beneath the specimen surface.



ratios [18]; and predominant compression residual stresses improve the fatigue en-

durance of hard-turned AISI 52100 high-strength steel [19].

ULTRASONIC FATIGUE RESULTS

Ultrasonic fatigue endurance results are plotted in Fig. 7 for the three testing speci-

mens: no polished specimens and polished with rough and fine emery paper after

machining. Fatigue endurance for the rough-paper-polished specimens is revealed to

be higher when fatigue life is comprised of �3� 105� 6� 106 cycles, with regard to

the fine-paper-polished and nonpolished specimens. Furthermore, beyond the

10� 106 s of cycles, this tendency is modified and fatigue endurance of rough-paper-

polished specimens becomes lower with regard to the two other specimens. Fatigue

endurance of fine-paper-polished and nonpolished specimens in the vicinity of

5� 105 cycles is close to 158MPa (green and blue logarithmic interpolation lines),

whereas at this fatigue life the rough-polished specimens show fatigue endurance

close to 167MPa (red logarithmic interpolation line). Concerning the very long

FIG. 7

Ultrasonic fatigue endurance

for specimens from machining

and polished with rough and

fine emery paper.

FIG. 8

Axial residual compression

stresses after polishing.



fatigue life (5� 108 cycles), fatigue endurance of rough-polished specimens is close

to 115MPa, whereas this mechanical property is about 125MPa for the fine-

polished and nonpolished specimens. These results are discussed in the next section

with the aid of compressive residual stresses, ultrasonic vibration loading, surface

roughness, reverse yielding (Bauschinger effect), and crack-initiation sites.

Compression residual stresses for axial direction have been measured after pol-

ishing with emery paper grade 80 and 1500. Figure 8 shows plotting of the compres-

sion residual stresses (each point representing the average of three measured points)

from the specimen surface to 200lm in depth after polishing. A reduction of these

stresses is observed in both cases, and the most important reduction corresponds to

specimens polished with emery paper grade 1500.

Discussion

Some authors have reported an important increase on fatigue limit (10 %) for

electro-polished steels with regard to the nonpolished specimens and a fatigue limit

improvement of about 36 % related to shot peening residual compression stress,

which transfers the crack imitation from the surface to inside the specimen [20].

These results show that residual stresses induce a high effect on fatigue endurance,

sometimes higher with regard to the surface roughness. In addition, residual stresses

in metallic alloys generated during machining are affected (in magnitude and in

sign) by machining parameters [21], such as cutting speed, nose radius, feed ratio,

rake angle, coolant, etc., and by the nature of metallic alloys. Aluminum alloys after

machining frequently show low compressive residual stresses at the surface, increas-

ing to the interior and reaching the high compressive stress at 40–50 lm [21–23].

These residual stresses decrease to zero or attain tensile residual stresses after reach-

ing the high compressive value when the depth is about 150–250lm.

From a modeling point of view, surface roughness at a microscopic scale is asso-

ciated with the concept of stress-concentration factor Kt, assuming the roughness as

FIG. 9 Fracture surfaces: (a) crack initiation at the surface, specimen polished with rough paper, r¼ 170 MPa, 5� 105 cycles and

(b) crack initiation inside, specimen polished with fine paper, r¼ 128 MPa, 3� 108 cycles.



micro-notches [24]. For a single notch, the corresponding stress-concentration

factor in a panel subjected to uni-axial tension is:

Kt ¼ 1þ 2ffiffiffiffiffiffia=q

q

where:

a¼ the notch height, and

q¼ the notch root radius.

Because the average notch height of the surface is not currently measured in

practice, Neuber has proposed an empirical relation [25]:

Kt ¼ 1þ n

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffik Rz =q

� �r

where:

Rz¼ the 10-point surface height,

k¼ the ratio between spacing b and the notch height a, and

n¼ a coefficient with the values: n¼ 1 for shear and n¼ 2 for tension.

An alternative relation is proposed for the effective stress-concentration factor

K 0f in the function of roughness parameters [12]:

K 0t ¼ 1þ nRa

q0

� �Ry

Rz

� �(4)

where:

n¼ the same parameter as in Eq 3,

Ra¼ the average roughness,

q0 ¼ the average radius obtained from the dominant profile valleys, and

Ry¼ the peak-to-valley high roughness.

For fatigue endurance purposes, the effective fatigue stress–concentration factor

K 0f , is evaluated from Eqs 3 or 4 by:

K 0f ¼ 1þ qðK 0t � 1Þ(5)

where:

q¼ the notch sensitivity,

which is:

q ¼ 1

�1þ b

q0

� �(6)

In Eq 6, b is a material property, related to ultimate tensile stress ru for steels [26]:

b ¼ 0:0252070 MPa

ru

� 1:8; for ru � 550 MPa(7)

Fatigue endurance associated with surface roughness has been obtained for the

AISI 4130 CR steel, considering a constant compressive residual stress regardless the

cutting conditions at constant tension stress ratio R¼ 0.1 and frequency of 12 Hz [12].

The results of this study relating roughness parameters and fatigue endurance were as



follows: under a high-cycle-fatigue regime or low-stress loading, fatigue endurance

decreases with an increase of average roughness, whereas, at low-cycle-fatigue regime

or high-stress loading, fatigue strength increases with the roughness parameter Ra. No

available data was found by the author concerning aluminum alloys.

The obtained fatigue endurance results for the aluminum alloy 6061-T6 under

ultrasonic fatigue testing and R¼�1 (Fig. 7) present a similar behavior and an

attempt to assess the nature of this phenomenon is as follows:

1. The magnitude of plastic deformation on the fracture surface is considerablylower in the very high-cycle fatigue regime compared to fatigue life compris-ing between 3� 105 and 6� 106 cycles. The crack initiation on the first men-tioned regime is predominantly localized at the subsurface or inside thespecimen, associated with internal imperfections such as nonmetallic inclu-sions or micro-porosities (Fig. 9(b)). Furthermore, in this case, residualstresses on the surface contribute to internal crack initiation [27].

2. Crack initiation between 3� 105 and 6� 106 cycles is localized frequently atthe specimen surface, accompanied by shear stress and plastic deformation.The origin of surface crack initiation is associated with plastic deformation,which is related to micro-voids coalescence traits observed in this ductile alu-minum alloy (Fig. 10).

3. Fatigue strength under ultrasonic testing of this material is affected by threeprincipal factors: (a) the machined surface integrity, (b) the residual compres-sive stresses, and (c) the reverse yielding or Bauschinger effect (for the veryhigh-cycle fatigue regime) [28–30].

4. Under ultrasonic fatigue testing, the reverse yielding effect is expected to below because of the high-frequency loading and the increase in testing temper-ature (all tests between 3� 105 and 6� 106 cycles were carried out at the spec-imen neck temperature close to 75�C; temperature for tests >107 cycles waslower).

5. In this work, we observed the combined effect of roughness, compressivestresses, and a low effect of reverse yielding on the >107 cycles regime; no par-ticular analysis for each factor and the corresponding nonlinear effects on fa-tigue endurance was carried out. From the experimental results, it seems thatresidual compressive stresses combined with high micro-plastic deformationat the surface represent a barrier for the surface crack initiation and propaga-tion in the 3� 105 and 6� 106 cycles of fatigue life, increasing fatigue endur-ance for high-roughness specimens in this regime. On the other hand, for the>107 cycles of fatigue life, crack initiation moves to subsurface or inside the

FIG. 10

Micro-voids coalescence for

fatigue life 3� 105 – 6� 106

cycles.



specimen. The compressive residual stresses after polishing are higher for thehigh-roughness specimens, combined with a decrease of residual stresses dur-ing the test (not measured in this study), which is associated with the high-fre-quency/low-loading regime [31]. The maximum value of residual stresses islocalized close to 50lm from the specimen surface (Fig. 8), and this fatiguelife regime is accompanied by a low Bauschinger effect.

The Bauschinger effect refers to a decrease on yield stress for reversed loading

and involves not only the initial yield stress of testing material, but also the entire

stress–strain behavior after pre-straining. Bauschinger effect coefficient Bef decreases

with the previous plastic strain, and a lower value for Bef promotes the reverse yield-

ing affecting the residual stress distribution. A thin plastic strain layer is developed

on testing specimens’ surface during machining and a value of Bef¼ 0.4 affects the

ultrasonic fatigue testing for the >107 cycles of fatigue life. Figure 11 shows plotting

of the variation of residual axial stresses with the Bauschinger effect coefficient Beffor this aluminum alloy.

Conclusions

The following conclusions are drawn from the ultrasonic fatigue endurance results

on this aluminum alloy and the described machining and roughness parameters, as

well as residual stresses:

• For the 3� 105 – 6� 106 cycles of fatigue life, residual compressive stressescombined with high micro-plastic deformation at the surface are predominanton fatigue behavior, and fatigue endurance of rough-polished specimens isrevealed to be higher with regard to the nonpolished and fine-paper-polishedspecimens. On the other hand, the >107 cycles of fatigue life stress concentra-tion related to surface roughness affects the fatigue strength; nonpolished andfine-paper-polished specimens present higher fatigue endurance with regardto the rough-polished specimens. This behavior is accompanied with a

FIG. 11

Axial residual stresses

associated with the

Bauschinger effect coefficient

Bef, for >107 cycles of fatigue

life.



decrease of the work-hardening residual stresses during the high-frequency/low-loading testing and a low increase on the reverse yielding effect.

• A transition point of this fatigue endurance behavior is localized close to10� 106 s of cycles,

• Crack initiation for the 3� 105 – 6� 106 cycles frequently occurs at the sur-face, and it is associated with an important plastic deformation generated bymicro-voids that coalescence on the ductile aluminum alloy (Fig. 10). For the>107 cycle’s regime, crack initiates at the specimen subsurface or inside thespecimen and is related to imperfections, such as nonmetallic inclusions andmicro-porosities.

• Reverse yielding effect is nonexistent in the 3� 105 – 6� 106 cycles because ofhigh loading and temperature; it presents a low effect for the >107 cycles offatigue life.

• For fatigue life comprised of between 6� 106 and <107 cycles, no clear differ-ence is observed concerning fatigue life and crack initiation on the three typesof testing specimens.

• Machining parameters, such as tool nose radius, feed rate, cutting speed, andrake angle are determinants for surface roughness and residual stress valueand sign. The two first parameters strongly influence the ultrasonic fatigue en-durance of this aluminum alloy.

• Vickers micro-hardness of this aluminum alloy is close to 107 HV beforemachining and cutting; it increases to 435 HV after these processes accompa-nied by compressive residual stresses.

• Residual compressive stresses generated during work-hardening machiningcombined with high micro-plastic deformation at the surface seem to be atthe origin of higher fatigue endurance of rough-polished specimens in the3� 105 – 6� 106 cycles.

• Additional investigations are needed to identify the individual effects of sur-face integrity, machining residual stresses, and reverse yielding (Bauschingereffect) on the ultrasonic fatigue endurance of this aluminum alloy. The non-linear interaction of these factors on the ultrasonic fatigue strength is anotherprincipal subject for future investigations.

ACKNOWLEDGMENTS

The writer gratefully acknowledges the University of Michoacan (UMSNH) in Mex-

ico for use of the facilities in developing this study. The writer is also grateful to

CONACYT (The National Council for Science and Technology, Mexico) for finan-

cial support.

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