Combined cycle fatigue of 7075 aluminum alloy – Fracture surface characterization and short crack propagation S.E. Stanzl-Tschegg a,⇑ , M. Meischel a , A. Arcari b , N. Iyyer b , N. Apetre b , N. Phan c a University of Natural Resources and Life Sciences, BOKU, Institute of Physics and Materials Science, Peter-Jordan-Straße 82, 1190 Vienna, Austria b Technical Data Analysis, Inc. (TDA), 3190 Fairview Park Drive, Suite 650, Falls Church, VA 22042, USA c US Naval Air System Command, Patuxent River, MD, USA article info Article history: Received 19 June 2015 Received in revised form 21 October 2015 Accepted 26 October 2015 Available online xxxx Keywords: High frequency testing Combined loading Variable amplitude fatigue Life prediction Small cracks abstract Aim of this study is an interpretation of the influence of variable-amplitude (VA) cycles superimposed to low-frequency loads on fatigue life of 7075-T651 Al-alloys. Constant-amplitude (CA) 20 kHz stress/ strain-life (S–N) and (e–N)-curves with and without superimposed mean loads serve as basis. For combined fatigue loading, life-time measurements were performed. Life-time estimations based on the S–N results reveal a damaging effect of the superimposed ultrasonic vibrations in the high cycle fatigue (HCF) and the very high cycle fatigue (VHCF) regimes. The CA and VA-life time results are correlated with fractographic observations. An interpretation of fatigue lives under combined low and high-frequency VA-loading is proposed considering small/short-crack propagation and arrest mechanisms. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The loading histories for many rotorcraft and fixed-wing aircraft components contain a large percentage of low amplitude cycles. The spectrum characteristic is usually such that, small-amplitude cycles are superimposed on top of steady, maneuver, carrier or duty cycles. Usually, these small-amplitude cycles arise from several discrete sources such as environment and structural vibra- tions. Examples of these loadings are very common: buffet, gust and flutter loadings on fixed-wing aircraft and vibratory loadings resulting from primary and secondary modes on rotorcraft. These loads are also typical on aircraft engine components. The small amplitude loading cycles alone fall under the traditional definition of high-cycle fatigue (HCF, life > 10 5 cycles), while the main cycles usually fall under the low-cycle fatigue (LCF, life < 10 5 cycles) categories. Moreover, the small-amplitude cycles usually occur at higher frequency than the main-cycle loadings [1–7]. Although these combined cyclic-fatigue (CCF) loadings, i.e., combination of VHCF loadings superposed on slowly varying LCF loadings, are commonly encountered, no reliable methods yet exist to predict damage and failure modes/mechanisms for these types of service histories. The standard practice is to either ignore the damage contribution from small cycles or assign a predefined damage without any qualifying data. To characterize fatigue crack initiation and crack growth lives from service spectrum loadings of this nature, critical fatigues tests are being performed at University of Natural Resources and Life Sciences, Vienna, BOKU. The goal of these tests was to evaluate the influence of combined effects of mean stress and superimposed small cycles on fatigue life. This paper discusses experimental work and results from one set of critical tests that have been carried out. These tests focused on perturbing the slowly varying carrier/duty cycle with smaller high frequency cycles which are sinusoidal or random in nature. The carrier/duty cycles are of constant and variable amplitude, of different wave profiles and at different frequencies [7]. The paper is organized as follows: following this introduction, a description of the experimental set-up and loading process are given in Section 2. Details of both CA and VA test results are presented in Section 3. Discussion and analysis of the CA and VA results are provided in Section 4 followed by concluding remarks and a short outlook in Section 5. 2. Experimental details 2.1. Tests overview Fatigue tests were performed at University of Natural Resources and Life Sciences, Vienna, BOKU with testing equipment consisting of an ultrasonic-fatigue device and a servo-hydraulic machine [8,9]. Experimental life-time results were analyzed and correlated http://dx.doi.org/10.1016/j.ijfatigue.2015.10.022 0142-1123/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +43 1 47654 5160; fax: +43 1 47654 5159. E-mail address: stefanie.tschegg@boku.ac.at (S.E. Stanzl-Tschegg). International Journal of Fatigue xxx (2015) xxx–xxx Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue Please cite this article in press as: Stanzl-Tschegg SE et al. Combined cycle fatigue of 7075 aluminum alloy – Fracture surface characterization and short crack propagation. Int J Fatigue (2015), http://dx.doi.org/10.1016/j.ijfatigue.2015.10.022