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Research Papers

Numerical Simulation of Residual Stress Relaxation in Shot Peened High-Strength Aluminum Alloys Under Reverse Bending Fatigue

[+] Author and Article Information
M. Benedetti1

Department of Materials Engineering and Industrial Technologies, University of Trento, Via Mesiano 77, 38100 Trento, Italymatteo.benedetti@ing.unitn.it

V. Fontanari, B. D. Monelli

Department of Materials Engineering and Industrial Technologies, University of Trento, Via Mesiano 77, 38100 Trento, Italy

1

Contacting author.

J. Eng. Mater. Technol 132(1), 011012 (Dec 01, 2009) (9 pages) doi:10.1115/1.3184083 History: Received January 28, 2009; Revised March 28, 2009; Published December 01, 2009; Online December 01, 2009

The mechanism of the residual stress relaxation during the fatigue life of shot peened high-strength aluminum alloys was investigated. Experiments were conducted on specimens subjected to three different shot peening treatments and tested under reverse bending fatigue. x-ray diffraction (XRD) measurements were carried out to determine the initial and stabilized residual stress fields. The residual stress field created by the surface treatments has been introduced into a finite element (FE) model by means of a fictitious temperature distribution. The elastic-plastic response of the superficial layers affected by the shot peening treatments has been derived through reverse strain axial testing combined with microhardness tests and implemented in the FE model. The proposed numerical/experimental approach is able to satisfactorily predict the residual stress field evolution. Notably, relaxation has been correctly simulated in the low-cycle fatigue regime and imputed to plastic flow in compression when the superposition of compressive residual and bending stresses exceeds the local cyclic yield strength of the material. Conversely, the residual stress field remains stable at load levels corresponding to the 5×106cycles fatigue endurance.

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Figures

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Figure 6

(a) Overview of the FE model of the bending fatigue specimen illustrated in Fig. 1 with symmetry constrains; (b) detail of the surface region. The minimum thickness of the brick elements in the surface region is 12 μm.

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Figure 7

FE model with function fj(z) used to define the fictitious temperature distribution for residual stress simulation

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Figure 12

Comparison between the experimental and numerically predicted evolutions of the residual stress field in the specimen tested at several fatigue load levels: (a) CE-B120, (b) CE-Z425, and (c) CE-Comb shot peening condition. The longitudinal stress component derived from the FE simulations is plotted versus the XRD azimuth stress.

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Figure 13

Numerically predicted evolution of the longitudinal σx and transversal σy residual stress components in the specimen tested at several fatigue load levels subjected to the CE-Z425 peening treatment. Results corresponding to fatigue stress of 220 MPa omitted for the sake of clarity.

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Figure 1

Geometry of plane bending fatigue specimen. All dimensions are given in mm.

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Figure 2

Microhardness profiles of the as-received and peened variants obtained by averaging the results of three tests.

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Figure 3

Initial and relaxed residual stress profiles measured by using the XRD technique on the fatigue samples subjected to (a) CE-B120, (b) CE-Z425, and (c) CE-Comb shot peening. Stabilized residual stress profiles were measured after specimen failure about 2 mm far from the fracture surface.

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Figure 4

Comparison between the compression and tension peak stresses of the Al-7075-T651 alloy during reverse strain axial test at strain amplitude of 0.01: results of test started (a) in tension and (b) in compression

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Figure 5

Hysteresis loops for the Al-7075-T651 tested at strain amplitudes ranging from 0.006 to 0.014. Tests were started in compression to highlight the cyclic asymmetric behavior of the alloy. The cyclic yield strength corresponding to several work hardening levels was determined by assuming a kinematic hardening in order to take into account the Bauschinger effect shown by the alloy.

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Figure 8

Comparison between the experimental and numerically predicted initial residual stress profiles of the three shot peened variants

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Figure 9

Cyclic stress/strain responses obtained by FE model and reverse strain axial tests

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Figure 10

(a) Flowchart for the implementation of the cyclic mechanical response of the hardened layers of the peened material in the finite element model; (b) cyclic stress/strain curve of the base material and determination of the cyclic stress/strain curve of the hardened layers

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Figure 11

Comparison between the bending stress profile applied at the fatigue endurance and the limit stress for incipient compression yielding. The equibiaxial residual stress field and the cyclic compression yield strength (derived from the correlation between the microhardness and reverse strain axial tests) are incorporated in the limit stress according to Eq. 4.

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