Issue 35

L. C. H. Ricardo et alii, Frattura ed Integrità Strutturale, 35 (2016) 456-471; DOI: 10.3221/IGF-ESIS.35.52 465 variable amplitude loading. Basic research is conducted under use of simplified load sequences such as single overload or underload or block loading with alternate mean loads. Phenomena like crack growth retardation or acceleration are described making reference to base-line constant amplitude data. It is generally agreed, however, that real life load spectra also need to be applied in order to get a realistic picture of the relevance and significance of the mechanisms involved. S tandardized load sequences or L oad–time H istories (SLH’s) presently available provide an appropriate selection of load sequences to be used in the development of components, but they can also advantageously be used for other tasks. In this section it will be presented an overview on and a summarizing description of standardized load–time histories. With the need for optimum light-weight design, originally the aircraft industry was the main driver for these efforts. Two of the most well known SLH’s are the TWIST [91] and FALSTAFF [92] sequences for transport and fighter aircraft, respectively, which have been and are still being applied for numerous studies on materials, joints and other structural elements. For automotive applications, the CARLOS [93] series of SLH’s have been presented including the very recent load sequence for car trailer couplings, CARLOS-TC. In the US, activities were mainly centered on the derivation of test load sequences to be used for evaluation and development of fatigue life prediction methodology. Bodies like the SAE Fatigue and Evaluation Committee took a pragmatic approach by selecting test load sequences from existing strain measurements, which were felt to be typical for the ground vehicle industry. Altamura & Straub [94] presents a work where discuss different ways to work with variable amplitude loading and the strategies to conduct fatigue analysis in structures. It is shown the methodology for discretization of random loads in blocks to be used in the development of components. And, also, it is presented the procedure to evaluate crack growth under constant and variable amplitude loading. Probabilistic fatigue crack growth is discussed as well the mathematics models available to use like Monte Carlos simulation. It is generally agreed that the structural load variations should be characterized in the time domain since in most cases the range (or amplitude) of a load, stress or strain cycle and its respective max or mean value can be considered as fatigue- relevant. Furthermore, the sequence or mixing of load cycles of different ranges and mean values must not be neglected. Analyses in the frequency domain give insight into the frequency content of a load signal which is particularly useful for flexible structures, but do not deliver the above-mentioned values. Many structural loading environments can be described as sequences of different modes [95] which may be a particular flight, driving a car on certain road types, a sea state of a given severity, etc. These modes of operation contain load cycles of different, but typical magnitudes and frequencies. Often distinct patterns of grouped load cycles can be distinguished, they are called a loading event or element, such as braking or cornering of a car, different flight phases or maneuvers of an aircraft. Zheng [96] provides a criterion for omitting small loads. In past, the underload (or subload) was defined as the nominal stress amplitude lower than or equal to the endurance limit, and the underload effect on fatigue life was investigated experimentally by using smooth specimens. Test results showed that underload cycles applied to smooth specimens increased the fatigue life or the endurance limit of low-carbon steel [96] and cast iron [97], which was called “coaxing”. However, past research on the underload effect was not associated with the omission of small load cycles in life prediction [98,99]. The omission of small load cycles is necessary and important in compilation of the load spectrum [100,101], once the accumulated damage will not affect the prediction of the fatigue life and the assessment of the fatigue reliability of structures [102,103], and it is most cost effective in fatigue tests of components and structures under long-term variable- amplitude or random loading histories [104]. Up to date, some empirical criteria have been proposed and used [105,106]. However, how to omit the small loads in life prediction by using the local strain approach was not clearly set forth [106]. In the discussion of the importance of crack growth under variable amplitude loading, Youb & Song [106], using results obtained from single edge crack bending (SEB), mentioned that Schijve [101] was one of the first works covering this topic. Kikukawa et al. [107] have extensively measured crack opening behavior under various random loadings and reported that crack opening point is controlled by the maximum range-pair load cycle (which we call hereafter “the largest load cycle”) in a random load history and is identical to the crack opening result of constant amplitude loading corresponding to the largest load cycle. Based on this crack opening behavior, they proposed a simple prediction procedure for crack growth under random loading. The phenomenon of plasticity-induced fatigue crack closure under plane strain conditions is one of the most controversial topics concerning the mechanics of crack propagation. No general consensus exists among the scientific community concerning the physical mechanism for crack closure under plane strain conditions. One of the problems is on how to prepare the mesh and the procedure used in crack propagation. With three-dimensional models it becomes necessary to use normal contact approach to node release; in plane stress, spring is normally used to help the crack propagation, using contact resources for crack propagation and considering material nonlinear analysis it will result in a big result file and will spend a considerable time processing to end the simulation. According to Fleck [108] the source of discontinuous closure appears to be a residual wedge of material on the crack flanks, located just ahead of the initial position of the crack tip.