This paper investigates different methods of time domain and frequency domain on assessment of Fatigue life. The most suitable method of calculating stress for components that their natural frequencies have some degrees of interaction with the frequency range of the applied forcing functions are considered and modal effects are discussed. In practice, the validation of static analysis in evaluating of Fatigue life used in the most engineering soft wares is studied.

Fatigue behavior of fiber reinforced composites is still difficult to analyze or understand to be determined. It is due to various parameters affecting and their complicated interactions which come from the constituents’ physical and mechanical behavior. Hence, conducting experiments and developing Fatigue models are necessary in determination of Fatigue behavior in many cases. On the other hand, complicated behaviors lead the application of composite materials to be accomplished with a number of experiments and/or including high safety factors in design calculations in both the process may not be cost effective. This paper introduces a new algorithm and model to determine Fatigue response of damaged circular composite beam. The results are evaluated by experimental results. By using the proposed model, the number of experiments and the time needed to determine Fatigue behavior of damaged circular beams are significantly reduced. To determine the constants introduced in the local Fatigue damage model, cyclic tests are performed up to limited load cycles. The predicted results by the model and obtained from the experiments represent satisfactorily good agreements.

One of the main advantages for the increasing engineering application of composites in weight-critical structural applications is their high specific stiffness and strength. Composite materials behavior is complicated more than metallic material because of different mechanisms, damage growth rate and effect of them in each other. In this paper, a continuum damage mechanics based model is developed to simulate stiffness degradation and Fatigue life prediction of laminated composites under Fatigue loading conditions.Damage parameters are used to estimate the degradation of elastic properties in matrix, fiber and shear direction. The material properties of the damage evolution equations are derived by testing on 0o and 90o unidirectional plies and [0/90]s cross-ply laminate. To evaluate the model under multiaxial Fatigue loading, arbitrary states of stress and stress ratio available results of experiments on unidirectional 90, 0 and 30 plies under Fatigue loading conditions are used. Also, to evaluate the model for composite laminates with stress concentration results of experiments of pin-loaded cross-ply [04/904]s laminate are used. The obtained results show the capability of proposed model in Fatigue life prediction of unidirectional and crossply laminates under uniaxial and multiaxial Fatigue loading with different states of stress.

Echium amoenum Fisch. & C.A.Mey. has been used for the management of the common cold, inflammation, depression, anxiety, and Fatigue. This study aimed to explore the anti-Fatigue and antioxidant effects of aqueous extract of E. amoenum (AEEA) on a rat model of acute Fatigue. After preparing for AEEA, 30 Wistar rats were divided into five groups of six animals. Group 1 received distilled water; Group 2 was treated with distilled water and underwent a forced swimming test (FST); Groups 3-5 rats were administered AEEA (250, 500, and 1000 mg/kg), and subsequently underwent FST. Then, the levels of some biochemical parameters and oxidative stress markers were measured in the serum and liver tissues of rats. AEEA treatment significantly augmented the swimming time of rats compared to the control group. AEEA-treated animals had increased serum glucose and decreased serum urea levels. AEEA diminished serum activities of creatine phosphokinase (CPK) and lactate dehydrogenase (LDH). Treating the rats with AEEA activated superoxide dismutase (SOD) and catalase (CAT) and decreased total oxidant status (TOS) in serum. AEEA increased the levels of thiols and glutathione (GSH) as well as the activity of glutathione peroxidase (GPx) and CAT in the liver tissues. In the liver tissue, AEEA also induced the activity of SOD and reduced TOS. These results may suggest AEEA as a potential anti-Fatigue natural product to ameliorate the adverse effects of acute Fatigue on the body.

The purpose of this study is to define a piecewise Fatigue damage model (PFDM) for the prediction of damage in composite laminates under cyclic loading based on the continuum damage mechanics (CDM) model. Assuming that damage in fiber-reinforced plastic structures accumulates nonlinearly, a piecewise degradation growth function is defined and coupled with CDM and micromechanics approaches. The model divides the damage behavior of fiber, matrix, and fiber/matrix debonding at the ply scale, into three different stages. For generality, a fully multi-stage damage formulation on a single-ply level is employed. The unknown parameters of the PFDM are estimated according to obtained experimental data of damage mechanisms associated with the composites laminate under cyclic loading. To predict multidirectional composite laminates' Fatigue life, the proposed model was implemented in Abaqus software by the subroutine. In a validation against experimental data on carbon fiber reinforced material, the model proves to provide a good numerical approximation of the damage during the Fatigue loading. The results reveal that by considering the multi-stage process in stiffness reduction, the proposed model can estimate the Fatigue life of composite laminate under multiaxial cyclic loading conditions more accurately than the similar model in the literature.

Echium amoenum Fisch. & C.A.Mey. has been used for the management of the common cold, inflammation, depression, anxiety, and Fatigue. This study aimed to explore the anti-Fatigue and antioxidant effects of aqueous extract of E. amoenum (AEEA) on a rat model of acute Fatigue. After preparing for AEEA, 30 Wistar rats were divided into five groups of six animals. Group 1 received distilled water; Group 2 was treated with distilled water and underwent a forced swimming test (FST); Groups 3-5 rats were administered AEEA (250, 500, and 1000 mg/kg), and subsequently underwent FST. Then, the levels of some biochemical parameters and oxidative stress markers were measured in the serum and liver tissues of rats. AEEA treatment significantly augmented the swimming time of rats compared to the control group. AEEA-treated animals had increased serum glucose and decreased serum urea levels. AEEA diminished serum activities of creatine phosphokinase (CPK) and lactate dehydrogenase (LDH). Treating the rats with AEEA activated superoxide dismutase (SOD) and catalase (CAT) and decreased total oxidant status (TOS) in serum. AEEA increased the levels of thiols and glutathione (GSH) as well as the activity of glutathione peroxidase (GPx) and CAT in the liver tissues. In the liver tissue, AEEA also induced the activity of SOD and reduced TOS. These results may suggest AEEA as a potential anti-Fatigue natural product to ameliorate the adverse effects of acute Fatigue on the body.

An analytical model for the prediction of drillpipe Fatigue life has been proposed. The model utilizes a physically based critical plane approach. This approach uses two damage parameters: one is based on normal strain amplitude and the other is based on shear strain amplitude. Included in both parameters, the effect of the normal stress on a specific critical plane has been considered. Meanwhile, stress concentration and Fatigue strength reduction factors are also taken into account in this analysis. A drillstring is used to transmit rotation to the drill bit and provide fluid circulation in hole bottom while drilling. As these functions are being performed, the drillpipe experiences a combination of cyclic loads that include bending, torque, axial force and pressure. This situation is very common in directional drilling operations.Axial force and pressure can be assumed to be non-cyclic for short intervals. However, a torque load, like bending, is cyclic in nature. Due to common slip/stick effect, drill pipes experience small torque fluctuations with high frequency. Also, torque fluctuations occur due to making connections. The torque changes from zero to a relatively constant value soon after rotation resumes. In a directional well with high friction, torque can be significant factor and may cause long term Fatigue damage. Since drillpipes are generally thin-walled tubulars, it is assumed that they are in a plane stress condition. An analytical model is developed in this paper to calculate the stresses and strains that drillpipes experience during drilling operations. Then a proposed analytical model has been applied to predict drillpipe Fatigue life. Two computer programs were also accomplished and used. The first one allows the calculation of the loads, and stresses along the drill string. The second one allows the prediction of the Fatigue damage in several planes along the drill string for a certain drilled interval. It has been shown that there is a good agreement between test results and the Fatigue model predictions. It is also shown that torque and stress concentrations are very important and must be considered in drill pipe Fatigue analysis. The drillpipe Fatigue model is compared to the present API approach. It is observed that the drillpipe Fatigue model is more conservative than the present API approach. This is because the API model does not account for torque.