Abstract: This paper presents an outline of the development, verification and application of a new model of crack tip stress fields in the presence of a plastic enclave around a growing fatigue crack. The approach taken rests on capturing the effects of this ‘plastic inclusion’, comprising the crack tip and crack wake plastic zones, via elastic stress distributions applied at the elastic-plastic boundary. The model is therefore independent of the mechanisms of plastic deformation and potentially applicable to a variety of materials. A Muskhelishvili complex potential extension to the Williams crack tip stress field is found for four stress parameters representing a K-stress, a T-stress, a crack retardation stress and a compatibility-induced shear stress at the elastic-plastic boundary. This model is validated via full field fitting to photoelastic stress fringe patterns, obtained from epoxy resin and polycarbonate specimens. It has also been extended to the strain fields measured in digital image correlation techniques, which allows its application to metallic alloys.
Abstract: Reliability is an important engineering requirement for consistently delivering acceptable product performance through time. As time progresses, the product may fail due to time phenomena such as time-dependent operating conditions, component degradation, etc. The degradation of reliability with time may increase the lifecycle cost due to potential warranty costs, repairs and loss of market share. In design for lifecycle cost and preventive maintenance, we must account for product quality, and time-dependent reliability. Quality is a measure of our confidence that the product conforms to specifications as it leaves the factory. Reliability depends on 1) the probability that the system will perform its intended function successfully for a specified interval of time, and 2) on the probability that the system response will not exceed an objectionable by the customer or operator, threshold for a certain time period. Quality is time-independent, and reliability is time-dependent. This paper presents a methodology to determine the optimal design and preventive maintenance of time-dependent, multi-response systems, by minimizing the cost during the life of the product. The lifecycle cost includes a production, an inspection, and an expected variable cost. All costs depend on quality and/or reliability. A roller clutch example highlights the design methodology for lifecycle cost.
Abstract: For an engineering structure with an actual fatigue life over that corresponding to a so-called fatigue limit, appropriate reliability assessment and fatigue life prediction are essential for developing the structure and sustaining its high quality in service. Basic clues are explored. A competition fatigue initial mechanism is shown to provide a requirement of material primary quality management. Affordable deduced material and structural probabilistic S-N curves are presented by fitting into material mid-and-long life S-N data and fatigue limits and, then, comparing to structural fatigue limits. Random cyclic stress-strain relations are depicted for constructing random stressing history of structures. Reliability assessment and fatigue life prediction are established to synthetically consider the interference of applied stresses deduced from the random cyclic stress-strain relations and capacity strengths derived from the structural S-N relations with an expected life. Affordable and appropriate method has been then developed to realize the reliability assessment and fatigue life prediction including the super long life regime. Availability of the present method has been indicated through a reliability analysis to the velocity related reliabilities and fatigue lives of a railway axle.
Abstract: . In this study, fatigue tests were carried out using a diecast Mg alloy AZ91 to study the distribution of fatigue lives under constant stress amplitudes. During the fatigue process of the diecast Mg alloy, cracks initiated from the casting defect within inside of the specimen, and then propagated prior to final failure of the specimen. Distributions of fatigue lives at the constant stress amplitudes can be represented by the Weibull distributions. The scatter in the distribution of fatigue lives becomes larger at the lower stress amplitude of 80 MPa as compared with those at the higher stress amplitudes. There is a common relationship regardless of the stress amplitudes between the initial maximum stress intensity factors Kimax and fatigue lives Nf. The lower Kimax, the longer Nf becomes. Integrating the fatigue crack propagation law from the initial maximum stress intensity factor Kimax to the fatigue fracture toughness Kfc, the relation Kimax vs. Nf can be successfully evaluated.
Abstract: The objective of this study is to investigate the effect of the low or high strain rate on the impact fatigue properties of the nickel foam material and to understand the lifetime of this material which is subjected to the repeated impacts at different energy levels. Failures of foam materials under single and repeated impacts analogous to fatigue are essential to designers and users in military and aerospace structures. The material failure induced by repeated impact loading becomes a critical issue because of significant loss of stiffness and compressive strength in the foam material. Testing methods to study impact(that is, high strain rate) fatigue are quite numerous; no single standard testing procedure is defined for studying the impact fatigue property of a material. The increasing application of foam material in aerospace structures, owing to high specific stiffness and strength has attracted a great concern about the high sensitivity to impact damage introduced during manufacture or in service, and the effects of such damage on structural degradation. To investigate this issue, this study sets up an experimental procedure to determine the impact fatigue properties of nickel foam material. This study performs both experimental and numerical investigations to catch the impact fatigue behavior of nickel foam with open type. Design life and probability of failure or survival at specified life can be calculated so that the fatigue life of nickel core material subjected to repeated impact loading is predicted.
Abstract: Based on the concept of multilevel statistics, mixed-effect fatigue reliability models are presented, by which fatigue reliability can be directly calculated according to stress distribution and fatigue life distribution function condition to stress. Mathematically, the fatigue reliability is estimated as the expectation of a conditional survival probability function to the stochastic stress history. Especially, such models are capable of estimating the fatigue reliability of a component with competing failure mechanisms such as conventional fatigue and giga-cycle fatigue, where two groups of P-S-N curves are involved.
Abstract: An amorphous styrene-acrylonitrile (SAN) copolymer has been modified by various concentrations of metal oxide (MeO) nano-particles up to 0.50 vol.%. Atomic force microscopy of the modified thermoplastics showed that the nano-particles were well dispersed in the matrix. The incorporation of the nano-particles had a marginal effect on the glass transition temperature and yield stress. However, the Young’s modulus increased with the volume fraction of the nano-particles. The fracture and fatigue properties also had a marked increase with the addition of the nano-particles. The fracture energy was increased from 316±10 J/m2 to 445±27 J/m2, and the maximum fracture energy threshold was increased from 17±1 J/m2 to 34±2 J/m2 at 23 °C. Scanning electron microscopy (SEM) studies showed that debonding of nano-partilces, subesequent plastic void growth and large scale fibril deformation initiated by mulitiple crazing were observed in the process zone of the nano-modified composites.
Abstract: Compressor disk is a key rotor part of aeroengines. Alone with the increase of thrust weight ratio of aeroengines, the work conditions of compressor disk are more and more rigor. So the stress level of compressor is very high nowadays. As we know that the stress analysis is a basic of age analysis and reliability analysis, Do a finite element analysis (FEA) is necessary before and after the compressor disks have been made. In this paper, a detail analysis was done with finite element method (FEM), and the stress distribution of disk we got was consistent with the real stress distribution. Also the FEA results show that the location of maximum stress is the failure site of the disk. It is useful work for further work.
Abstract: , was experimentally investigated by a replica technique to seven smooth hourglass shaped specimens for railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack (MSC) stage and the physical short crack (PSC) stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in MSC stage. This behavior was corresponding to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. The banded structure appeared a barrier because each band was rich in hard layered pearlites for the crack to cross. In PSC stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. The two barriers are inherent in the material and the crack initiation and growth are subjected to an evolutionary process under competition between the inherent resistances from the barriers and the increasing driving force from the growing crack size. This provides a prehensive understanding of the crack initiation and growth.