Failure Criteria Based On The Characteristic Legnth Of Materials

The fatigue life predictions of engineering structures and mechanicalcomponents are always affected by defects and micro structures (such as cracksand voids). In the existing theories for defects with different sizes and shapes,different theories and criteria are adopted, and these theories are independent.Furthermore, some common problems still can not be solved (such as the microvoids problems). The aim of this research is to establish a unified theory, which issuitable for various types of defects. The motion of this research is from aneglected contradiction in engineering mechanics: on the one hand, the strengthproperty of material is regarded as the intrinsic property of material,unchangeable with the geometry shape; on the other hand, for defects withdifferent shapes, different theories are developed with the different assessmentparameters. Even for the same type of defects, the criteria change with the defectsize. For example, the small crack and long crack require different theories. Whatis more, for the micro voids in the precise machines, there is not an effectivestrength criterion.The method of this research is to propose a theory firstly, then to verify thefeasibility of the theory. The theoretic basis comes from the theory of materialstrength, and the validation techniques are from two aspects: one is theuniformity between the proposed theories degenerated and the existing theories;another is the effectiveness of the proposed theory for the problems which cannot be solved by the existing theories.It has the following innovative points:Firstly, the characteristic fracture length concept is proposed and is used toexpansion the range of continuum mechanics for the polycrystalline materials.The characteristic fracture length is a strength parameter extracted from the material intrinsic properties, so it is not affected by the geometry shape or size.This means that it possibly can be applied to the various types of defects, nomatter what the size or shape is. Meanwhile, the theoretical basis is discussed andthe validation of the characteristic fracture length is examined by experiments.Secondly, based on the analysis of yield mechanism and local plasticdeformation of polycrystalline materials, the characteristic length concept iscontributed in the yield strength, which is similar to the case of brittle fracture.The expression of characteristic yield length of polycrystalline materials isobtained through the dislocation pile-up theory. The phenomenon of increasingyield stress with decreasing diameter size of a center hole in a plate is explainedreasonably with a yield criterion that is constructed on the concept ofcharacteristic yield length. In order to verify the yield criterion, experiments arecarried out with plate specimens drilled variously under room temperature, andthe results agree well with that predicted by the yield criterion.Thirdly, the characteristic fatigue length of the brittle rupture of material isconcerned and investigated as an intrinsic material property, both theoreticallyand experimentally. Based on the concept of the fatigue failure determined by adamaged zone instead of a point, it is proposed that the material failure occurswhen the average stress amplitude over the characteristic fatigue length is largerthan the material fatigue strength. Plate specimens made of SUS630H900stainless steel are conducted in fatigue experiments at room temperature to verifythe characteristic fatigue length-based theory. Circular holes of various diametersare introduced in the center of specimens to generate the stress concentration andthe size effect. The sizes of holes affect the fatigue limits of the specimens. Andpredictions of nominal fatigue limits agree well with experiment results.