Fatigue limit

About: Fatigue limit is a research topic. Over the lifetime, 20489 publications have been published within this topic receiving 305744 citations. The topic is also known as: endurance limit & fatigue strength.

Fatigue studies of nanoscale structures for MEMS/NEMS applications using nanoindentation techniques

Abstract: Mechanical properties of nanoscale structures are needed to design reliable micro/nanoelectromechanical systems (MEMS/NEMS). Most material properties are known to be size-dependent and such properties of the nanoscale structures have not been well characterized. Bending strength and fatigue properties of nanoscale silicon beams with a 6-μm length, a 255 nm height and widths ranging from 400 to 800 nm were evaluated using a depth-sensing nanoindenter with a harmonic force. In the bending tests, the beams failed in a brittle manner with a flat fracture surface. Load cycles used in continuous stiffness measurement were used to perform fatigue tests. The contact stiffness was monitored continuously throughout the fatigue test. The abrupt decrease in the contact stiffness indicates fatigue damage has occurred. Cleavage steps were found on the fatigue fracture surface. Failure mechanisms of the beams during bending and fatigue are also discussed in conjunction with the surface to volume ratio, surface defects, and cleavage planes. The dynamic nanoindentation fatigue test used in this study can be satisfactorily used to evaluate the fatigue behavior of nanoscale structures for use in MEMS/NEMS.

Fatigue Performance Prediction of North Carolina Mixtures Using the Simplified Viscoelastic Continuum Damage Model

Abstract: Fatigue performance modeling is a major topic in the field of asphalt concrete modeling work. Currently, the only standard fatigue test available for asphalt concrete mixtures is the flexural bending fatigue test, AASHTO T-321. Several issues are associated with flexural fatigue testing, the most important being that the stress state is not uniform but varies with the depth of the specimen, and that the beam specimen fabrication equipment is not widely available. Viscoelastic continuum damage (VECD) fatigue testing is a promising alternative to flexural fatigue testing. Different researchers have successfully applied the VECD model to asphalt concrete mixtures using constant crosshead rate direct tension tests. However, due to the load level limitation of the newly released Asphalt Mixture Performance Tester (AMPT) testing equipment, there is an immediate need to develop a model that can characterize fatigue performance quickly using cyclic test data. In this study, a simplified VECD model, developed at North Carolina State University, is applied to various North Carolina mixtures that are part of the NCDOT project, Local Calibration of the MEPDG for Flexible Pavement Design. A failure criterion that is based on pseudo stiffness is developed from the test data. The application of the VECD model using this failure criterion results in very good agreement between the measured and predicted fatigue life for the eleven mixtures. In addition, a completely independent verification study is conducted for the FHWA ALF mixtures, including both unmodified and modified mixtures. Again, it is found that the simplified VECD model predicts the fatigue life of the ALF mixtures well. It is shown that the simplified VECD model based on the data from a single temperature and a single strain level can predict fatigue test results fairly accurately under various temperature conditions and at various strain levels. It is also shown that the model can be utilized further to simulate both controlled strain and controlled stress direct tension fatigue testing and gain insight into the impact of various mixture design variables, such as asphalt content, binder grade, NMAS, and the inclusion of RAP materials, on the fatigue performance.

Theory of the Fatigue of Metals

Abstract: The most striking features of fatigue phenomena are: (1) The quasi-brittle nature of fatigue fracture. A metal, however ductile, can break in a fatigue test without any appreciable external deformation, like a brittle material. A further similarity to brittle fracture is that fine cracks and other faults, which would not influence noticeably the static strength of a ductile material, substantially impair its fatigue endurance. (2) Internal distortions . In spite of the possible absence of any external deformation, heavy local distortions can be observed microscopically on a material subjected to a fatigue test (Ewing and Humfrey 1903). The evidence for internal distortions has been extended by the X-ray work of Gough and his collaborators (Gough 1933; Gough and Wood 1936, 1938 a , 1938 b ) who found that X-ray photographs of fatigue-fractured metals show, in the immediate neighbourhood of the fatigue crack, qualitatively and quantitatively the same alterations as those of metals fractured in static tests.