*Corr. Author’s Address: University of Ljubljana, Faculty of Mechanical Engineering, Laboratory for Dynamics of Machines and Structures, Aškerčeva 6, 1000 Ljubljana, Slovenia, janko.slavic@fs.uni-lj.si 631 Strojniški vestnik - Journal of Mechanical Engineering 65(2019)11-12, 631-640 Received for review: 2019-06-14 © 2019 Journal of Mechanical Engineering. All rights reserved. Received revised form: 2019-10-14 DOI:10.5545/sv-jme.2019.6197 Original Scientific Paper Accepted for publication: 2019-10-14 Harmonic Equivalence of the Impulse Loads in Vibration Fatigue Primož Ogrinec - Janko Slaviˇ c * - Miha Boltežar University of Ljubljana, Faculty of Mechanical Engineering, Slovenia In vibration fatigue, three unique types of loads are typical: random, harmonic and impulse. In an application any of these loads are possible. A fatigue-life analysis is possible in the time and frequency domains using the frequency-response function of a structure. Recent studies demonstrated that the impulse loads inﬂuence the accuracy of a fatigue-life prediction in the frequency domain. The focus of this research is a theoretical study of an equivalent harmonic load to the impulse load on a single-degree-of-freedom system in order to investigate the feasibility of impulse loads in vibration testing. This research shows that there is a relationship between the impulse and harmonic loads that is related to the underlying dynamic properties (e.g., damping, natural frequency). Based on a theoretical analysis an experimental procedure was developed for both cases of excitation, which was able to conﬁrm the theoretical analysis. Using the modal decomposition the single-degree-of-freedom approach can be generalized to multiple-degrees-of-freedom systems. Keywords: vibration fatigue, random loads, spectral methods, fatigue life, stationary and non-stationary loading, non-Gaussian loading, vibration testing Highlights • Non-stationary and non-Gaussian loads lead to signiﬁcantly shorter fatigue lives of a structure. • Impulses, that can occur in the loading signal, render the loading signal non-Stationary and non-Gaussian. • The equivalence between impulse and harmonic loads, with regard to fatigue testing is presented with an analytical derivation. • Control strategies for impulse and harmonic fatigue tests are presented. • The theoretical procedure was experimentally veriﬁed on 18 samples, with impulse and harmonic loading. • Material’s fatigue parameters were identiﬁed for G-AlSi8Cu3(226). 0 INTRODUCTION As structures are becoming lighter and loads optimised, the effects of structural dynamics [1] and random loads on the fatigue life of ﬂexible structures are becoming more important. This is known as vibration fatigue and has been the subject of various studies in recent years [2] to [7]. Vibration fatigue is focused in loads well below the yield stress (i.e., high-cycle fatigue), which is typically researched in the time domain (e.g., rainﬂow-counting algorithm [8]) or in the frequency domain (e.g., the narrow band [9], Dirlik [2] or Tovo-Bennasciutti methods [5] and [3]). When studying fatigue life in the frequency domain it is common to assume that the excitation signals and consequently the stress response of a structure are Gaussian and stationary [10] and [11]. In recent years great efforts have been made to develop the frequency-counting methods in the analysis of the fatigue life of structures excited with non-Gaussian and non-stationary excitation signals [12] to [16]. Tovo and Bennasciutti studied non-stationary switching random loads [4]. Song and Wang [17] presented a spectral-moment-equivalence lumped block method that improves the accuracy of the fatigue-life analysis for non-stationary, non-Gaussian loads and incorporates the material parameters into the equivalent spectral moments formula. Bracessi et al. [18] and Niu et al. [19] researched the inﬂuence of load Kurtosis and skewness on the damage rate in the case of non-Gaussian signals; Wolfsteiner and Sedlmair [20] and Cianetti et al. [21] presented correction factors based on those two characteristics of the loading signal. In real cases it is common to experience some forms of non-Gaussian loading [22], which can also be the result of the impulses superimposed on the random loading of a structure [23]. These impulses can be the consequence of geometric non-linearities, contact conditions, clearances, wear, etc. While the fatigue life under a combination of harmonic and random loads can be studied in the frequency domain [24], [25] and [6], the effects of these impulse loads are not well researched. The presence of impulses renders the signal non-stationary. Capponi et al. [15] and Palmieri et al. [16] noted that in the case of such signals the fatigue-life assessments with spectral methods in the frequency domain return a signiﬁcantly longer life prediction than were observed in the experimental testing. Hence, the development of new methods that can account for the presence of impulses in the stress response is of great importance.