Showing papers in "Journal of Mechanical Strength in 2004"

Simulation of 3d residual stresses field of multi-pass welding by fem

Abstract: Temperature and stress field of 316L stainless steel is simulated by FEM using ABAQUS codes. The 3-D solid elements are used in FEM model, and temperature depended material properties are considered as well as the convection and radiation as boundary conditions. Internal heat generation loading method is applied to simulate heat source, and model change ability to simulate multi-pass weld. The temperature field and residual stress field of 3-D multi-pass weld are obtained, and the results are discusses.

STRESS-STRAIN BEHAVIORS OF 63Sn37Pb SOLDER SIMULATED BY ANAND MODEL

Abstract: A series of tensile experiments at strain rates between 10 -3 %/s and 10%/s and at elevated temperatures between 313*!K～ 398*!K were conducted on 63Sn37Pb solder. Based on the experimental data the mechanical behavior of this material was investigated and found to have strong dependence on both test temperature and strain rate. Anand model, a unified visco-plasticity model was employed to simulate the stress strain curves under different temperatures and strain rates. The simulation results showed that Anand model could effectively predict 63Sn37Pb solder's stress-strain behavior up to strain amplitude of 10%.

Reliability assessment and life prediction for zero-failure data

Abstract: A new method of reliability assessment and life prediction for zero-failure data is presented. Thus the one-sided lower confidence limits of reliability and life for zero-failure data can be calculated by the method. The reliability assessment and life prediction of the two-parameter Weibull and lognormal distributions are discussed at some length. A relational expression between the standard deviation of logarithmic life and the shape parameter of Weibull distribution is also established. The method has been used in the reliability assessment and life prediction of aircrafts and satellites successfully.

Lateral vibration of a rotor/bearing system with axial rubs

Abstract: The lateral vibration of a rotor/bearing system influenced by axial contact between disk and stator are investigated by numerical simulation. The contacting forces are equated by two parallel forces with equal magnitude, which act at two bearings laterally and form a couple. One-parametric model is then used to calculate the axial-contact-induced dry friction forces. Supposing the contact occurs at one of the two specified positions, lateral vibrations of the system are calculated at three values of speed which represent the states before and after the passage of the shaft's natural speed, and before the occurrence of the oil whirl, respectively. Results show that the sizes of the axle-necks' orbits can be enlarged and reduced as the initial compressing displacement is increased. When contacts occur at the edge of disk in horizontal plane, bifurcation diagrams reveal complicated behaviors of the axial-rubbed rotor/bearing system. These include motions with different values of period, quasi-periodic and even chaotic motion.

Method for determining functions of time series mean and variance

Abstract: A new theorem is presented, which points out that the variance of time series has the same functional form as the trend term of absolute value series of difference of the time series. Based on the discovery, a method for determining the trend term of the series is established, which obtains the periodic function of the trend term by the sample periodogram and the nonperiodic function of the trend term by multiple-point averaging, then the integral function of the trend term can be synthesized. The variance functional form of the original series can be derived from the trend function and its parameters obtained by the correlation coefficient stationary theory. This method can also be used to determine the mean function of time series.

Discussion on continuum damage model of hydraulic fracturing

Abstract: Hydraulic fracturing is a common engineering measure used in oil reservoir remolding, and numerical simulation of the process is of significant to oil recovery. A continuum damage model of hydraulic fracturing was developed by introduced Gurson damage theory. Through a simple calculation case, damage zone of a hydraulic fracturing well with a perforation was simulated. In the end, basic research method of hydraulic fracturing using damage mechanics theory.

Spectrum rectifying with negative frequency contribution eliminating

Abstract: The precisions of ratio rectifying method is very rough when it was applied to the case of low frequency component, of which the reason was that the contribution from negative frequency not taken into the model. To overcome this dilemma, the spectrum function of the single frequency signal, which contains the negative frequency contribution, was deduced out. Base on this function, the new rectifying algorithm was proposed, and the rectifying equations were presented for the cases of rectangular window and Hanning window. The new rectifying method was examined by simulation. The study shows that, the presented equations are correct, especially the precisions algorithm corresponding to the Hanning window case almost approaches to the low limit of double precision operations.

Fault diagnosis of gearbox based on higher order cumulants

Abstract: When gearbox was in fault, the measured vibration signals were non-stationary and non-Gaussian with various Gaussianity and symmetry under different fault condition, and it usually contain strong noise. Higher-order cumulants was used to diagnose gearbox fault for it was insensitive to add-Gaussian noise and symmetry noise. Short-time analysis method can extract feature of periodicity impulsion fault signal under low signal noise ratio(SNR). Higher-order cumulants of original and its short-time energy function were calculated based on analysis of vibration with short-time processing. The normal state, medium wear state, serious wear state and broken gear vibration signals can be easily separated by the feature extraction from higher-order cumulants. The results of the research show that higher-order cumulants can quantitative analysis the offset degree of vibration signals to normal distribution, and it can quantitative analysis the wear degree after the samples were divided into short-samples.

Mathematical model for creep-rupture strength based on larson-miller parameter

Abstract: Though the differentiating and integrating process a mathematical model for creep-rupture was derived based on the attribute of state function and the general formula of Larson-Miller parameter, which correlates the creep-rupture strength with the short time tensile strength of materials at elevated temperature. Using the established model, the linear relationships between the creep-rupture stresses and the times to rupture in logarithm was proved theoretically, and reduced a long-term creep-rupture procedure of materials at elevated temperature to the measure and calculation of a short time tensile experiments at higher temperatures. Moreover, the 10 5 hours creep-rupture strength of steel Mod.9Cr-1Mo at 500℃～550℃ were calculated by this method. The predicting results show that it is consistent with actually tested values.

High cycle fatigue life prediction by local stress-strain method

Abstract: The principle of local stress-strain method and the current theory were introduced and reviewed. It pointed out that the local stress-strain method is accurate to estimate the medium and low cycle fatigue life prediction, but hard to get an accuracy result in high cycle fatigue life prediction. This is mostly because the effect of stress gradient, surface working and dimension are not taken into consideration in current method. Taking the three factors into account, a modified strain-stress equation is expounded. This method has an advantage of requiring simple calculation and is more accurate to estimate high cycle fatigue life than others. A case is used to demonstrate that the approach is suitable not only for predicting medium and low-cycle fatigue life, but also for high-cycle fatigue life prediction. It has a good application on engineering design and provides a practical reference in fatigue design.

Method of life prediction for low cycle fatigue in superalloy k403

Abstract: Low cycle fatigue tests are performed in superalloy K403 under strain controlled, at temperature 700℃. Experiment data is processed by Manson-Coffin method and results of life prediction are given. When experiment data is processed and fatigue life is predicted by Manson-Coffin method, Negative plastic strain often appears, and the results of life prediction are not as good as (designer's) request. In order to solve the problem, a new method based on Manson-Coffin method and stress fatigue(SMC) is developed, the results of life prediction are given by using new method at the same time. Results obtained by the new method showed that not only experiment data is used as possible as it can be used, but also the capability of life prediction is evidently improved.

Effect of potting on the packaging of high-g mems accelerometer

Abstract: Potting is a key step for the packaging of high-g micro-electro-mechanical system(MEMS) accelerometer According to a piezoresistive high-g MEMS accelerometer and its practical packaging structure, the modality and the response of the accelerometer were simulated using finite element method The modal frequency of the packaging structure increases with the elastic modulus of potting material But the modal frequency may be smaller than that of the packaging structure without potting if the potting material is very soft, and the output may be interfered Under the 100?000 g acceleration (1 g=98?m/s 2), the simulated output voltage of accelerometer decreases slowly with the increase of elastic modulus or density of potting material The simulated output voltages are close to the analytic results They are also linear with the accelerations applied

Strongly nonlinear thermal-mechanical coupling analysis of subway brake resistance

Abstract: Being forced by the nonlinear dynamic thermal stress, slices of the subway's brake resistor generated large elastic plastic deformation. In order to analyze the deformation mechanism, a thermal mechanical coupling model was established by FEM(finite element method). The coupling model included a nonlinear transient temperature field model which was founded by considering the influence of thermal shock and deformation work, a displacement field model which covered geometrical and material nonlinearity, and a stress field model which introduced the least square method to improve the calculation precision. Specific computation was performed by FEPG(finite element program generator) software. Final numerical results display the consistency between computational and actual deformation, which qualifies the analysis model. The regression curve obtained by temperature and displacement solutions of every iterative step provides foundation for taking measures to restrain the deformation.

Study on stress distribution at round casing thread connections

Abstract: According to the theory of elastic mechanics and plastic mechanics, a finite element method (FEM) model is developed to study the stress on round casing thread connections. Detailed analysis is given on the stress distribution of the connections under all kinds of work conditions, such as makeup process, makeup and pulling, makeup and inner pressure, by calculating the contact stress in thread with FEM. Moreover, the stress in periphery of the connections is measured under the same conditions by full scale test. The results show that load distribution is not uniform on each thread and this is the most important reason for the poor performance of the ordinary connections. On the bases of the analysis, several methods are presented to reduce the peak stress and optimize the load distribution on the threads. More clear understanding about the stress distribution at round casing thread connections can be obtained from the results presented and thus it can be taken as references in improving the design of casing thread so as to improving the quality of the connections.

Study of yield criterion for single crystal nickel-based superalloys

Abstract: Hill's(1948) yield criterion for plastically orthotropic solids is used to predict the yield stresses of single crystal Nickel-based superalloys DD3, but the correlation to experiments at 760℃ is poor. Taking into account the effects that the components of tension and torsion stresses are coupled with each other, a new yield criterion for single crystal Nickel-based superalloys is put forward in term of the characteristics of single crystal Nickel-based superalloys. This new criterion can be applied to engineering conveniently. The method of figuring out coefficients of the new criterion is given in the paper. Equivalent stress and equivalent strain that adapt to the new criterion are defined. The new criterion, new equivalent stress and equivalent strain will degenerate to the von Mises's for isotropic material, and the coefficients that in the new criterion can be determined from uniaxial tension tests. Using the new criterion to predict the yield stresses of single crystal Nickel-based superalloys, the correlation to experiments is very good and more accurate than that of Hill's.

Micro-raman spectroscopy and its applications to measure residual stress in micro-structure

Abstract: Porous Silicon film with μm level and Silicon bulk substrate is a basic structure of MEMS (micro-electro-mechanical systems). Porous Silicon keeps the same crystal structure of bulk Silicon, but with a larger lattice parameter. The lattice mismatch with the substrate is expected to introduce a stress on the porous film, and cause cracking of the interface. MRS (micro-Raman spectroscopy) was introduced and practical information is given for the application of this technique to stress measurements in porous Silicon films, which were obtained with chemical etching technique. Tensile stress increases with porosity. Especially, Raman study on a crack of one sample reveals that higher residual stresses (0.92*!GPa) appear in the cracking area and however, fall rapidly out of the cracking area. On the other hand, using metallographic microscopy to investigate surface appearance of porous Silicon film, micro-cavity structure is expected to have a relation with the distributions of residual stress.

Life control method for pressure vessel

Abstract: Although all kinds of pressure vessels have the different predicting lives, they are inspected according to unified inspection period. When the inspection period is predetermined, the life prediction method cannot insure the structural safety, which is one of the prime reasons of pressure vessel accident. A new life control method for pressure vessel is presented. The life control equation of multiple damage and general life control formula of single damage are established. The life control equations for fatigue, stress corrosion and creep are discussed in detail. The life prediction is to estimate the life when the flaw initial size and critical size are known. On the contrary, the life control is to estimate the flaw initial size at the predetermined life. Nondestructive test would be used to detect the flaws whose sizes are bigger than the allowable initial size estimated by the method, and corresponding repair or replacement of parts or components would insure the reliability and safety of structure in its lifetime.

Experiment modal analysis on rectangular elastic fluid-filled shell

Abstract: An experimental study for a rectangular elastic fluid-filled shell system is presented. It is found that large magnitude and low frequency gravity waves will appear when the system is excited on the shell with acertain frequency. In order to study the vibration characteristics for this kind of symmetric-elastic-thin fluid-filled shell system with free surface in detail, a modal experiment is conducted and four lowest modal shapes within the analytical range are obtained firstly. The second, the curve to demonstrate the relation between the water level and system natural frequency is presented, too, which indicates that the depth of the water doesn't affect the movement form of the system, and the natural frequency of the system will reduce with the decrease of the water depth. At last, a large magnitude and low frequency gravity wave experiment is done and gravity waves are observed when the frequency of the excitation is the component of the natural frequency of shell and water. The conclusion of combined vibration is drawn and some data are obtained. All of these will offer preference for the further theoretical study.

Residual strength analysis for widespread fatigue damage in stiffened panel

Abstract: Two distinct types of damage and two residual strength criteria for widespread fatigue damage in stiffened panel are given. The residual strength criteria are the net section yield criterion and crack tip ligament yield criterion. The approximate engineering approaches for stress intensity factor(SIF) of skin with multiple site cracks and skin and stringer with cracks are given. The expression of net section yield criterion and crack tip ligament yield criterion and estimation of plastic zone size for multiple site cracks in stiffened panel are also given. The residual strengths of three types of damage in stiffened panel are tested. It is point out that the decrease of residual strength for multiple site cracks, particularly, for stringer with crack, are rather larger. Residual strength for widespread fatigue damage in stiffened panel is predicted by the two criteria, and the prediction and the testing results are in good agreement.

MEASURING ELASTIC MODULUS AND POISSON RATIO FOR HIGH-TEMPERATURE MATERIALS BY Moiré INTERFEROMETRY

Abstract: It is mainly about using the wave front interference principle of Moire interferometry analyzed the possibility of non-contact measuring elastic modulus and Poisson ratio for high temperature materials by Moire interferometry and the types of grating specimers of Moire interferometry in high temperatures. The means of elastic modulus and Poisson ratio of the materials measured in high temperatures, and application for a large number of aviation materials are completed. A new perfect measuring way has been developed.

Influence of crack on structure vibration of gear tooth

Abstract: The dynamic model of a cracked gear tooth is established and, the dynamic response and dynamic characteristic, i.e., the natural frequency, the mode shape and so on, due to crack of gear tooth are analyzed. The influence of crack position and crack depth etc. on dynamic characteristic of gear are deep investigated by using theoretic modelling analysis and the finite element method (FEM) simulation (single tooth model and four tooth model). It shows that the influence of crack depth and crack position on both natural frequency and mode shape of gear are great, the influence of crack position is larger than the influence of crack depth, the natural frequency is drop and mode shape is changed when a crack started, this natural frequency drops with the increase of crack depth, and low order natural frequency is notably drop and high order natural frequency is slow drop, the natural frequency are biger drop when crack locate tooth root and that these natural frequency are little drop when crack locate tooth tip. The mode shape of gear tooth structure very different from uncrack when a crack started, this amplitude is augmented in crack neighborhood. This results show that the influence of crack on gear structure vibration is salient with the increase of crack depth, These have important worth for damage detection and fault diagnosis of gear system.

Engineering analogy method for calculating structural fatigue damage of aircraft

Abstract: It is well know that the best way to manage the life of aircraft is to monitor fatigue consumption of each aircraft, which is very popular among western countries However, it has not been applied in China, due to the shortage of reliable fatigue meters With more and more new types of aircraft going to service, the situation has changed It is possible now to manage the life of aircraft by monitoring its fatigue damage consumption, as each of new type of aircraft is equipped with flight parameter recorder One of the most important aspects for monitoring aircraft fatigue consumption is to find a suitable method for calculating fatigue damage by using recorded flight parameters Although many methods are available for calculating fatigue damage, none of those has been accepted As the Miner theory is expressed in a very simple mathematics formula, it choice material constant m because there are big difference among the calculated results by using different value of the mA method based on the relative Miner theory for calculating fatigue damage of aircraft is presented A systematical study is carried out by calculating more than 30?000 hours of flight data recorded from 60 aircrafts of two types, from which a conclusion is drawn that the order of fatigue damage values calculated by this method for all aircrafts in one fleet are same no matter what value of material constant m is chosen Therefore it is proven that this method is a valuable tool for monitoring aircraft fatigue consumptions

Damage mechanism of 3d-c/sic composite

Abstract: CMCs have the potential applications in high temperature structure, but the damage at high temperature of 3D-C/SiC is not very clearly. The preforms used in present work fabricated with T300 carbon fibers which woven into 3-dimension 4-direction structure with 22° weaving angle. 3D-C/SiC was processed by chemical-vapor infiltration(CVI) of SiC into preforms ,before CVI processing fibers were coated with carbon by CVD. Micro-cracks are produced from thermo-stress or external stress. Crack computing formula in unidirectional brittle matrix composites can be used to coarsely estimate matrix crack stress and crack spacing of 3D-C/SiC. Pores inter fiber yarn tows are deformation in creep. On pores surface more likely generated matrix cracks and angles of inter fiber yarn tows were changed, the creep damage concentrated in cross points of fiber yarn tows. Creep is caused by damage, and it can explain by damage creep mechanism. In the bend testing, fracture toughness, creep and fatigue tests, the fiber bundles attempt to extend in load direction. Main mesoscopic damage mechanism of 3D-C/SiC is sliding process inter fiber yarn tows and damage generated by it. Fracture morphology is more rough in room temperature, low fatigue stress and more fatigue cycles, but the length of fiber pullout is longer. The fracture morphology is less rough in the case of high temperature, high stress and less fatigue cycles and the length of fiber pullout is shorter. Debonding and sliding between fiber tows and matrix or between fiber and matrix generate damage, but the main damage caused by wearing between fiber tows and matrix. So the large damage in weaving crossing-point of fiber tows appears.

EXPERIMENTAL INVESTIGATION ON VERY-HIGH-CYCLE FATIGUE OF GCr15 STEEL

Abstract: Attempts are made on revealing the very-high cycle fatigue behavior and fatigue mechanism of high strength steels. GCr15 steel was used as testing material and rotating bending fatigue test was adopted. The fractography of fatigue failure was observed by optical microscopy and scanning electron microscopy. The observations shows that, for the number of cycles to fatigue failure between 10~6 and 4×10~8 cycles, fatigue cracks mostly initiated in the interior of specimen and originated at non-metallic inclusions. The results indicate that the fatigue life of specimens with crack origin at the interior of specimen is longer than that with crack origin at specimen surface. The experimental results and the fatigue mechanism were further analyzed in terms of fracture mechanics and fracture physics.

Method for determining parametric functions of generalized time-varying arma model

Abstract: A method for determining parametric functions of generalized time-varying ARMA model is presented. It firstly determines the mean and variance functions of the time series, and then changes the generalized time-varying ARMA model into time-varying ARMA model. The function forms of time-varying parameters are determined by the sample periodogram and multiple-point average. The parametric functions of time-varying ARMA model are obtained by least squares algorithm and maximum likelihood method respectively. Thus a complicated time-varying problem is changed into a simple time-invariant problem for further processing.

Contact stress analysis with large deformation of sealingring under operation conditions for solid motor

Abstract: Based on the structure characteristic and sealing reliability of the solid engine, the sealing structure is disassembled and simplified at first. By using ANSYS engineering analysis system, the penalty contact element method is used to set up the non linear finite element analysis for 3D model of the axial symmetric hyper elastic contact problem of the O rubber sealing ring. The distributions of contact stress at the sealing contact areas are analyzed and the sufficient and necessary conditions of sealing under the operation condition are given for solid rocket motor. Considering the hyper elasticity and contact case, it researches the effect on sealing and contact by different pressures of the sealed structure. As the influence patterns are analyzed, some possible factors resulted in sealing failure are found. The analysis method in this paper is in structure to the correct selection of O ring seals especially in the case of important applications. The computational results are well consistent with experiment, showing that the technique is valid and applicable in Engineering.

Calculation and analysis of two dimension fracture parameters using ansys

Abstract: Mostly, the three fracture parameters were calculated by finite element method. Both two-dimension specimen and three-dimension specimen were studied by using the software of ANSYS. Quadratic element was applied to simulate singularity around the crack tip, and the model was meshed freely. It proved that the results of ANSYS are approximate to that of theoretical estimation formula and there are some internal connections among the three fracture parameters .It is a useful tool in calculation of fracture parameters, and provides an accurate and convenient method in fracture design.

Crack propagation mechanism under far field cyclic compression

Abstract: It was demonstrated that crack could grow under far field cyclic compression load in experiments. However, this kind of crack growth phenomena is difficult to be observed and described accurately by mathematic methods because the tension residual stress area is too small and crack face contact exists. Finite element method was used to calculate the residual stress fields of three crack tip geometries. It was found that tension residual stress is a key factor for crack growth under cyclic compression. The rationality of different crack tip geometries is discussed. Meanwhile, this work shows that crack propagation is limited and stable under cyclic compression.

Damage mechanics model for the initiation life of stress corrosion cracking

Abstract: The problem of stress corrosion cracking initiation has been studied on the base of continuum damage mechanics. The attack of load together with environment will weaken the materials, and the degree of the weakening can be described well by the damage variable used in damage mechanics. Just as the use of the damage mechanics in predicting the initiation life for creep and fatigue cracks, a stress-corrosion damage evolvement model, which has considered the roles of the threshold stress, effective stress and the existed damage, is established. And the effects of the environment corrosion are presented with the parameters in the evolvement model. Then a stress-corrosion crack initiation life model is deduced from the damage evolvement model with the concept of the damage accumulation. The initiation lives of the stress corrosion cracking have been predicted for two kinds of materials at different stress levels. The predicted results show a good agreement with the testing results.

Robust design of laminated composite plates based on the last-ply failure criterion

Abstract: A last-ply failure (LPF) analysis method for laminated composite plates is incorporated in the finite element method, based on this, optimization and robust optimum problems are presented. The composite structure is analyzed considering both in-plane and out-of-plane loads. For a lamina, two major failure modes are considered: matrix failure and fiber breakage. When a lamina has failed, the laminate stiffness is modified to reflect the damage, and stresses in the structure are re-analyzed. This procedure is repeatedly performed until the whole structure fails and thus the ultimate strength is determined. A structural optimization problem is solved with the fiber orientation and the lamina thickness as the design variables and the LPF load as the objective. Finally, robust optimum design method for laminates is presented and discussed.