Papers by Keyword: Damage Accumulation

Abstract: Experimental investigation on the influence of load interaction on the life of Single Crystal Nickel-based Superalloy is conducted. Three kinds of load spectrums considering single and coupled failure mode are designed. Life tests are carried out under creep, thermo-mechanical fatigue (TMF), and creep-TMF interaction loading. The test results show that test lives, under the creep-TMF interaction loading, are lower than the life predictions given by the linear damage accumulation (LDA) rule, indicating that the load coupling can accelerate the damage evolution process. The microstructure of fractured specimens shows that under the creep-TMF interaction loading, rafting cause more dislocations to accumulate in the at γ/γ′ phase boundary, which could be the evidence of life decrease.

Abstract: Cyclic delamination experiments with multilayered structures were performed in four point bending mode using a central notch for crack initiation. In a preceding study, the propagation of the delaminating cracks could be interpreted on the basis of the Paris law. In order to obtain more insight into the mechanisms of fatigue and crack propagation, FEM simulations of the experiments were conducted. The material models used in the simulation involve strain gradient elasticity, kinematic hardening plasticity and creep. Following the concept of damage mechanics, the crack propagation rate of delamination was related to the inelastic strain accumulated per loading cycle. Thereby, singularities of stresses and strain at the crack tip were suppressed by the regularizing effect of strain gradient elasticity.

Abstract: A method of fatigue damage accumulation based upon application of energy parameters of the fatigue process is proposed in the paper. Using this model is simple, it has no parameter to be determined, it requires only the knowledge of the curve W–N (W: strain energy density N: number of cycles at failure) determined from the experimental Wöhler curve. To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This work contains the results of uni-axial random load fatigue tests with different mean and amplitude values performed on 6082 T6 aluminium alloy specimens. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.

Abstract: Failure of weld joints under single and cyclic pulsating loading conditions is under consideration. A weld joint is modeled by three-layer composite. Stepwise propagation of the internal I mode crack under cyclic loading is investigated. Delamination of bimaterial composed of two structured materials is considered when a crack is located at the interface between two media. Loads under pulsating loading conditions are studied for elastic-plastic material. For analysis of this process, diagrams of quasi-brittle fracture of solids under cyclic loading conditions are proposed to be used. One of curves of the proposed diagram bears resemblance to the Kitagawa-Takahashi diagram. Estimates of average dimensionless velocity of stepwise crack propagation per loading cycle have been obtained in an explicit form for plain specimens of finite width. The relations derived for the average crack growth rate can be considered as structural expressions for plotting Paris’ curves.

Abstract: Fatigue crack growth rate at the plane stress is predicted by Paris equation, associating it with the stress intensity factor (SIF), generalized parameter of the elastic stress field near the crack tip. The finite element method allows modelling of the incremental crack growth in the plates, where the finite element grid should be re-meshed on each crack growth step fitting the first principal stress planes. However, the linear fracture mechanics format (LEFM) based evaluation of the two-dimensional fatigue cracks does not provide always the correct crack front assessment and appropriate life-predictions. It is shown that approach using the damage accumulation simulation and strain-life criterion may be promising in analysis of the two-dimensional cracks. Application of the approach is illustrated. The simulation results are in good agreement with the experimental data.

Abstract: Finite-element modeling of reinforced concrete structure fracture process was carried out using models of nonlinear concrete with damage accumulation. Mechanism of cracks initiation in reinforced concrete structures was ascertained and kinetics of cracks growing in concrete due to corrosion of rebars was obtained. Fracture processes of road bridge fragment and railway bridge ballastless bed under corrosion of rebars was simulated.

Abstract: In this research, new progressive fatigue damage models are established to calculate the fatigue life and simulate damage process of composite pin joints. The proposed models based on residual strength and residual stiffness of unidirectional laminates, have three parameters to present the different damage state, which can accurately describe the growth process of fatigue damage propagation by the mathematical method. The fatigue damage models combining with stress analysis, failure analysis, and material property degradation process, can predict the fatigue life, damage state and residual material properties of composite structures under arbitrary loading conditions. Using the models, composite pin joints with different stacking sequence are analyzed, fatigue life and damage quantification are concluded simultaneously. The proposed models and the process of analysis provide a way to solve the fatigue durability of composite structures.

Abstract: The paper is aimed at an assessment of influence of damage accumulation in a thermoplastic/carbon laminate during a tensile cycling loading on durability, Young modulus and both instant and time-dependent local mechanical characteristics of a thermoplastic matrix, analysed by means of instrumented microindentation. Standard mechanical data from three-point bending are compared with results of the local microindentation testing.

Abstract: In this paper a study of damage accumulation evaluation of highly stressed composite layer using acoustic emission method and local strain measurements by optical fiber based sensors in form of Fiber Bragg Grating (FBG) is presented. The study was carried out using composite NOL-ring specimens. These kind of specimens can simulate behavior of the cylindrical part of the pressure vessel. Quasistatic and creep tests of CFRC made NOL specimens were performed. It has been shown that there is a correlation between changes in the strain field of composite material, detected by FBG optical sensors, and changes in acoustic emission signals. The possibility of a kind of calibration of composite material strain state monitoring system by AE measurements and at the same time evaluation of damage accumulation was confirmed.

Abstract: Experimental verification of the damage accumulation model of low cycle fatigue crack initiation and growth is executed. Loading wave form typical for discs of gas turbine engines is studied. Linear damage accumulation model is found to give acceptable accuracy for crack initiation and growth in powder nickel alloys. This model is acceptable for disc cyclic durability prediction.