Experimental Evidence of Specimen-Size Effects on EN-AW6082 Aluminum Alloy in VHCF Regime

Stefano Invernizzi,Alberto Carpinteri,Francesco Montagnoli

doi:10.3390/app11094272

Stefano Invernizzi, Alberto Carpinteri + Show 1 more

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https://doi.org/10.3390/app11094272

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Abstract

The present paper investigates the influence of the specimen size of EN-AW6082 wrought aluminium alloy subjected to very high cycle fatigue (VHCF) tests. The hourglass specimens were tested under fully reversed loading condition, up to 109 cycles, by means of the ultrasonic fatigue testing machine developed by Italsigma® (Italy). Three specimens groups were considered, with a diameter in the middle cross-section ranging from 3 mm up to 12 mm. The stress field in the specimens was determined numerically and by strain gauge measurements in correspondence of the cross-section surface. The dispersion of experimental results has been accounted for, and data are reported in P-S-N diagrams. The decrease in fatigue resistance with increasing specimen size is evident. Theoretical explanation for the observed specimen-size effect is provided, based on Fractal Geometry concepts, allowing to obtain scale independent P-S*-N curves. The fatigue life expectation in the VHCF regime of the EN-AW6082 aluminium alloy full-scale components is rather overestimated if it is assessed only from standard small specimens of 3 mm in diameter. Experimental tests carried out on larger specimens, and a proper extrapolation, are required to assure safe structural design.

Highlights

Since the introduction of the ultrasonic fatigue testing machine by Mason in 1950 [1,2], the interest of the scientific community in the long-lifetime of metallic materials in the very-high cycle fatigue (VHCF) regime has continuously increased [3]
VHCF tests are commonly performed on specimens with a standard diameter of 3 mm, so that the fatigue resistance of full-scale components has to be extrapolated through theoretical models, which have not yet been fully validated experimentally by using ultrasonic fatigue testing machines
In order to increase the range of tested risk volume, the same authors carried out VHCF experiments on AISI H13 steel hourglass and gaussian samples with risk volumes of 55 mm3 and 5000 mm3, respectively [19]

Summary

Introduction

Since the introduction of the ultrasonic fatigue testing machine by Mason in 1950 [1,2], the interest of the scientific community in the long-lifetime of metallic materials in the very-high cycle fatigue (VHCF) regime has continuously increased [3]. In order to reduce the testing time, suitable fatigue testing devices working at high frequencies have been designed, so that it is possible nowadays to perform tests up to 1010 cycles in a very short time in comparison with traditional testing techniques, such as servo-hydraulic or rotating bending machines [8] Another important issue regarding the VHCF field is the specimen-size effect on the fatigue resistance in the ultralong life regime. VHCF tests are commonly performed on specimens with a standard diameter of 3 mm, so that the fatigue resistance of full-scale components has to be extrapolated through theoretical models, which have not yet been fully validated experimentally by using ultrasonic fatigue testing machines To this aim, Furuya [9] performed VHCF tests on high-strength steel specimens with a diameter in the middle cross-section ranging from 3 mm up to 8 mm. By exploiting lacunar fractality concepts [23], the specimen-size effect on fatigue resistance was theoretically assessed, and scale invariant generalized P-S∗-N curves provided

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