Sharif Digital Repository

The ability to determine the bearing life time is one of the main purposes in maintenance of rotating machineries. Because of reliability, cost and productivity, the bearing life time prognostic is important. In this paper, a stiffness-based prognostic model for bearing systems is discussed. According to presumed model of bearing and fundamental of damage mechanics, damage curve approach is used to relate stiffness of vibratory system and bearing running life. Furthermore, using the relation between acceleration amplitude at natural frequency and stiffness, final relation between acceleration amplitude at natural frequency and running life time according to damage curve approach can be... 

The harmonic displacement response of a beam is utilized as the input signal function in wavelet analysis. Sudden changes in the spatial variation of transformed response identify the location of damages and defects. The damage incurred causes a change in the stiffness or mass of the beam. This causes a localized singularity which can be identified by a wavelet analysis of the displacement response. In this article, it is shown that using harmonic response is superior to the static deflection response and this approach is more effective in the presence of noise and more sensitive to the versatility of the applied harmonic loads. © 2007 SAGE Publications  

Offshore jacket platforms are one of the most motivating structures for damage detection due to their importance and productivity. In this study, the application of finite element model updating in damage detection of an offshore jacket platform is investigated. The objective function of this method is based on the measured and analytical modal data, including natural frequencies and mode shapes. However, the measured data is expected to be noisy. Also, to avoid obtaining false damage results, a penalty term is added to the objective function. To update the model, genetic algorithm is utilized as a robust global searching tool. Afterward, the efficiency of this method is evaluated on several... 

The mechanical responses of biological tissues are well characterized by hyperelastic or viscoelastic models within the physiological loading range. However, during supra-physiological mechanical loading, as occurs during interventional procedures such as balloon angioplasty and arterial clamping, damage may occur in the tissue. The continuum and computational treatments of damage in soft biological tissues have attracted considerable attention over the recent years. In this chapter, we review the state of the art of this challenging area. We summarize and critically discuss various damage models, which are based on continuum damage mechanics, the theory of pseudo-elasticity, and the... 

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... 

Predicting the failure probability of composite laminates under fatigue loading has many uncertainties, such as material properties, loading, boundary conditions, etc. In this paper, the failure probability of pin-loaded composite laminates is investigated. For this purpose, the continuum damage mechanics (CDM) theory has been combined with Monte Carlo Simulation (MCS) as a physics of failure tool to predict the failure of composite laminates in different directions (fibers, matrix, and shear) to calculate the probability of failure of the composite laminates under fatigue loading. Therefore, the failure function will be formed using the CDM theory and random variables identified. Finally,... 

A novel plastic-damage constitutive model for plain concrete is developed in this paper. For this purpose, the microplane theory is proposed for overcoming the deficiency of available anisotropic continuum plastic-damage models in reproducing the true anisotropic nature of damage in multidimensional loadings. Based on the microplane theory, the degeneration process in concrete is considered along with plastic deformations. Using the principle of strain energy equivalence, a transformation between the nominal and effective states of material is achieved, that results in a decoupled formulation for damage and plasticity. A yield function with multiple internal variables and a non-associative... 

In endurance time method, seismic performance of structures under artificial intensifying accelerograms is investigated. In this method, the time duration of excitation (target time) will be assigned on the basis of linear response spectra. Damage spectra, which are the variation of a damage index versus the system period, can be used effectively to modify the target time of the excitations. In this research, various damage indices for a number of steel moment frames are estimated using the endurance time method with two types of target time, and the results are compared with those corresponding to the scaled real earthquakes. It is observed that damage estimation of the frames via the... 

In endurance time method, seismic performance of structures under artificial intensifying accelerograms is investigated. In this method, the time duration of excitation (target time) will be assigned on the basis of linear response spectra. Damage spectra, which are the variation of a damage index versus the system period, can be used effectively to modify the target time of the excitations. In this research, various damage indices for a number of steel moment frames are estimated using the endurance time method with two types of target time, and the results are compared with those corresponding to the scaled real earthquakes. It is observed that damage estimation of the frames via the... 

In this paper, the hybrid Delay-And-Sum (DAS) with Sparse Reconstruction (SR) method was further developed for damage location in composite plates. In composite materials, anisotropy leads to some challenges in using conventional damage location methods, which are developed for isotropic materials. In the hybrid DAS-SR method, the DAS and SR methods were combined as a complement of each other. To investigate the DAS-SR method for composite structures, the group velocity of the travelling wave for different directions was first measured experimentally via PZTs. The DAS and SR formulations were then modified to be compatible with the direction-dependent group velocities. The results show that... 

In this paper, the evolution of elastic–plastic damage in the composite laminates under fatigue conditions is modeled. Continuum damage mechanics (CDM) has been coupled with the bridge micromechanics model to estimate the fatigue damage and life for laminated composite structures. Based on the elastic–plastic bridging model, three damage variables are defined. These variables estimate the fiber, matrix, and fiber/matrix damage response at the ply scale. To model the beginning of plastic deformation, a yield function is utilized, and evolution equations of the damage variables are obtained. Then the developed deformation plastic model is calculated. The model parameters are calibrated by... 

This investigation employed an interrupted torsion fatigue test using the linear amplitude sweep procedure to assess the intrinsic self-healing potential of unaged, aged, rejuvenated, and re-aged asphalt binders. Continuous oscillatory shear strain loading at different damage levels were interrupted with a single rest period of different durations. An index based on the decrease in crack length following the rest period (Ha) has been defined to accurately reflect the occurrence of healing. The results indicate that Ha is higher at lower damage levels and for longer rest-period durations. The amount of irreversible crack growth increased with an increase in crack propagation and aging level.... 

Permeability reduction in porous media as a result of frail and tenuous fine particles migration would decrease the productivity index in the subterranean reservoirs. During reservoir stimulation by injecting fluids into the reservoir, as the salinity condition of the formation brine changes, fine particles initiate the triggering process. In this study, MgO-based nanofluid as a fines fixation agent was stably prepared based on the particle size distribution and characterized through transmission electron microscopy analysis. Afterward, several core flooding tests were performed using Berea sandstone cores to study the effect of nanofluid injection on fines fixation in the water shock... 

Trapezoidal Cemented Sand and Gravel Dam, namely Trapezoid CSG, is a new type of dam. Due to lack of dynamic studies in the field of CSG dam, this research was performed to analyze Trapezoidal CSG dam using dynamic Finite element method with ABAQUS Software. To investigate possible earthquake-induced damages, fragility curves are plotted based on damage index, the length of the cracks created at the dam base and the area of cracked elements in the dam. The seismic analysis indicated that minimum and maximum tensions are generated in the heel and toe of the dam, respectively. According to the fragility curves, with increase in PGA, the possibility of the exceeding the defined limit state is... 

The tensile behaviour of Saffil short fibre reinforced aluminium composites has been investigated in terms of Young's modulus, yield strength, ultimate tensile strength, and strain to failure. A rationale is given for the unusually high strain to failure found in this material by applying a failure criterion based on damage accumulation, recently developed for particulate reinforced MMCs  

In this paper, the continuum damage mechanics model originally proposed by Lemaitre (Journal of Engineering Materials and Technology. 1985; 107: 83-89) is presented through an adaptive finite element method for three-dimensional ductile materials. The macro-crack initiation-propagation criterion is used based on the distribution of damage variable in the continuum damage model. The microcrack closure effect is incorporated to simulate the damage evolution more realistic. The Zienkiewicz-Zhu posteriori error estimator is employed in conjunction with a weighted Superconvergence Patch Recovery (SPR) technique at each patch to improve the accuracy of error estimation and data transfer process.... 

In this paper, the continuum damage mechanics model originally proposed by Lemaitre [1] is presented through an adaptive finite element method for three-dimensional ductile materials. The macro-crack initiation-propagation criterion is used based on the distribution of damage variable in the continuum damage model. The micro-crack closure effect is incorporated to simulate the damage evolution more realistic. The Zienkiewicz-Zhu posteriori error estimator is employed in conjunction with a weighted superconvergence patch recovery (SPR) technique at each patch to improve the accuracy of error estimation and data transfer process. Finally, the robustness and accuracy of proposed computational... 

Various damage indexes have been introduced in recent years incorporating different parameters for estimating structural damage. Amongst these indexes, Plastic Ductility and Drift have been the center of attention of standards and building codes, like FEMA-356, because of their straightforward physical interpretation and ease of calculation. In this paper, several steel moment frames have been considered and their responses have been evaluated under a set of scaled earthquakes. A group of various damage indexes, which included cumulative and non cumulative, cyclic fatigue based, deformation based and modal parameter based, has been considered and the damage to structures has been evaluated...