Issue 38

A. Eberlein et alii, Frattura ed Integrità Strutturale, 38 (2016) 351-358; DOI: 10.3221/IGF-ESIS.38.45 351 The effect of varying loading directions and loading levels on crack growth at 2D- and 3D-mixed-mode-loadings A. Eberlein, H.A. Richard Institue of Applied Mechanics, University of Paderborn, Pohlweg 47-49, 33098 Paderborn, Germany eberlein@fam.upb.de A BSTRACT . While product’s operation the loading situation commonly changes. The local loading situation on an existing crack then can shift to a combined loading, composed of mode I, mode II and mode III, and consequently influence the product’s durability significantly. This influence on further fatigue crack growth and structures’ failure can be positive or negative. Present article describes and discusses the effect of varying loading directions from mode I- to 2D-mixed-mode-loading as well as from mode I- to mixed- mode I + III-loading. Moreover, experiments on varying loading levels are performed by interspersing mixed-mode block loads in cyclic mode I base load, cyclic mode II base load as well as in cyclic mode III base load. K EYWORDS . 3D-mixed-mode; Loading directions; Loading levels; CTSR- specimen; 3D-fatigue crack growth. Citation: Eberlein, A., Richard H. A., The effect of varying loading directions and loading levels on crack growth at 2D- and 3D-mixed-mode-loadings, Frattura ed Integrità Strutturale, 38 (2016) 351-358. Received: 01.06.2016 Accepted: 30.06.2016 Published: 01.10.2016 Copyright: © 2016 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION arts in many technical applications often are subjected to variable cyclic loading. While service life these products due to transportation, assemblage, site of operating as well as in use experience time-dependent loadings, so-called service loadings, which result from single over-/underloads, variable loading levels as well as changing loading directions. Thereby the crack growth can both accelerate and retard. Anyway, the fatigue crack growth is not only controlled by the current loading parameters Δ K and R -ratio, but also by the loading history. Due to the fact that different loading changes interact with each other during a service loading, the characterisation of the whole interaction effects on fatigue crack growth under variable loading amplitude are generally investigated separately by means of four different categories. Sander [1] classify these in single over-/underloads, over-/underload sequences, block loading and service loading. The experimental investigations in this contribution study the fatigue crack growth under variable loading amplitude by interspersing block loading into a constant baseline-level loading. Block loadings represent several succeeding overloads. Generally, block loadings are distinguished between high-low, low-high or the appropriate combination low-high-low sequences [2]. Within the block loadings the R-ratio can be different. Therefore block loadings can be classified in three types of low-high-low sequences, whereby always one characteristic parameter during the transition from one to the next block load is constant. The characteristic parameters are K Bl,min = const., R = const. and Δ K Bl = const. In this paper the block loading tests were performed with a constant R -ratio, as Fig. 1 shows. P