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International Journal of Fatigue 25 (2003) 999–1005 www.elsevier.com/locate/ijfatigue Lifetime predictions for real loading situations—concepts and experimental results of fatigue crack growth M. Sander, H.A. Richard ∗ Institute of Applied Mechanics (FAM), University of Paderborn, Pohlweg 47-49, D-33098 Paderborn, Germany Abstract In this paper experimental results for real fatigue loading situations and loading changes are presented. It can be shown that the retardation effect depends on many factors, e.g. the overload ratio or the loading direction. Also, the lifetime is affected by the reconstruction of counting methods. In addition the results of simulations with the program code NASGRO are presented and verified by appropriate experiments. © 2003 Elsevier Ltd. All rights reserved. Keywords: Fatigue crack growth; Interaction effects; Mixed mode; Simulation; NASGRO 1. Introd uction The quality of a product is of increasing importance for indu strial applicat ions, because cost s arising from missi ng quali ty are enormous. Especially the missing quality, which is caused in the early phase of the devel- opment of a product is involved with high consequential costs. Not only economic but first of all the safety-engin- eering factors in this phase are important for a break- proof development of components and struc tures . For this purpose, it is very important to obtain a meaningful statement about the lifetime. But a component or a struc- ture is exposed to varying loading situations during their app lic ati on. These var yin g loa din g cha nge s pro duc e retardation and acceleration of the fatigue crack growth, the so called interaction effects, which enormously com- plicate reliable lifetime predictions. In the first part of this paper, experimental results of loading changes are presented. Afterwards, the results of simulations with the program code NASGRO are presented and verified by appropriate experiments. ∗ Corre spon ding auth or. Tel.: +49-5251-602203 ; fax: +49-5251- 603719. E-mail address: [email protected] (H.A. Richard). 0142-1123/$ - see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0142-1123(03)00112-9 2. Experi mental investi gations and results 2.1. Syste matis ation of fatig ue crack growth experiments Genera lly fat igu e cra ck gro wth can be divide d int o two main groups: single stage loading (constant ampli- tud e loa ding) and service load ing (variable amplitud e loading) [1]. With experiments under single-stage load- ings, statements can be made about the crack initiation, the crack path and starting of unstable crack growth, but it is difficult to get any information about the lifetime of structures and components under real loading conditions. Therefore, experiments must be performed with single over- and underloads, over- and unde rload sequenc es, block loadings and combination loadings. Furthermore, all of these groups have to be investigated under mode I as well as under mixed mode loading conditions. 2.2. Mater ial, specimen and exper imenta l techn iques The exper iment al inves tigati ons are perfor med with two different types of specimen. For pure mode I fatigue loadings the CT specimen, w = 72 mm, which is stan- dardised by the ASTM sta nda rd E647, is use d. Both mode I and mixed mode experiments are carried out with the compact tension s hear (CTS) specimen, which was developed by Richard [3]. All specimen are taken from
