Qingguo Fei

Temperature-dependence of acoustic fatigue life for thermal protection structures

Acoustic fatigue life evaluation is essential for thermal protection structures due to the severe thermo-acoustic load in service. A study on temperaturedependence of acoustic fatigue life for a C/SiC panel is presented in this paper. Effects of temperature on both the structural responses and the S-N curves are investigated. The Dirlik method is adopted to predict the fatigue life of a C/SiC panel at three different temperatures respectively. Significant differences are observed from the results of numerical simulations between the fatigue lives of the panel in the three cases. The temperature-dependence of acoustic fatigue life of a C/SiC panel is verified, and fatigue test of the material needs to be more attentively performed.

Utilization of modal stress approach in random-vibration fatigue evaluation:

Random-vibration fatigue evaluation can be of considerable importance in the design phase of aerospace structures due to the severe dynamic loads in service. This paper presents the utilization of modal stress approach to the issue of structural random-vibration fatigue evaluation. Prognosis of random fatigue hotspots by using stress mode shapes is theoretically demonstrated. A two-step procedure is proposed for computational efficiency. Firstly, modal stress analysis is conducted to locate the fatigue hotspots in a dynamic structure. Secondly, the frequency domain-based approach for random fatigue evaluation is performed at these hotspots, as opposed to the computation of the entire structure as before. The capability of stress mode shapes to locate fatigue hotspots is verified by numerical investigations. The finite element model of a single-lap plate structure containing various opening holes was constructed for case study. Six elements were identified as hotspots by using modal stress distributions. Then, random responses and fatigue evaluation of the entire structure were carried out for verification. Good agreement was observed between the fatigue damage contour and the modal stress distributions, which can indicate that the critical positions predicted by stress mode shapes have good accuracy. The calculation time and storage space can be significantly reduced by means of the proposed evaluation procedure. Therefore, the accuracy and efficiency of utilization of modal stress approach in random fatigue evaluation can be ensured.

Using Sherman–Morrison theory to remove the coupled effects of multi-transducers in vibration test

Modal parameter identification is adversely affected by the mass loading of the transducer in experiments, especially when multi-transducers are arranged on the lightweight structure. In order to remove the coupling effects of transducers on each measurement point, a hierarchical multi-transducers eliminations method based on Sherman–Morrison theory is investigated. The method consists of two steps: (1) Decomposition: multiple elimination is decomposed into multi-levels, the relationship of the frequency response functions between each level is illustrated in the tree diagram; (2) Elimination: according to the relationship between each level, the measured frequency response functions are modified level by level. Numerical simulation is conducted by employing a three-degrees-of-freedom spring-mass system and the robustness is verified in the noise case. Experimental investigations are undertaken by employing a lightweight cantilever beam: Laser Doppler vibrometer is adopted to obtain measured frequency response functions without transducer mass loading effect, which are regarded as the target data. The initial frequency response functions are obtained in the case, in which multi-accelerometers are arranged and the effects should be removed. The result shows that the method can effectively decouple the frequency response functions due to transducers. In the elimination process, it is necessary to delete duplicate information (frequency response functions), which can greatly reduce the amount of calculation. And the effects of multi-transducers mass can be removed and the corrected frequency response functions are in quite good agreement with the target values.

Prediction of Statistical Energy Analysis Parameters in Thermal Environment

An algorithm is presented to predict the statistical energy analysis (SEA) parameters of structures in a thermal environment. An energy flow model considering thermal effects is established by the ...

Nonstationary Random Vibration Analysis of Wing with Geometric Nonlinearity Under Correlated Excitation

An algorithm that integrates Karhunen-Loeve expansion (KLE), nonlinear finite element method (NFEM), and a sampling technique to quantify the uncertainty is proposed to carry out random vibration a...

Modal Strain Based Method for Dynamic Design of Plate-Like Structures

Design optimization of dynamic properties, for example, modal frequencies, can be of much importance when structures are exposed to the shock and/or vibration environments. A modal strain based method is proposed for fast design of natural frequencies of plate-like structures. The basic theory of modal strains of thin plates is reviewed. The capability of determining the highly sensitive elements by means of modal strain analysis is theoretically demonstrated. Finite element models were constructed in numerical simulations. Firstly, the application of the proposed method is conducted on a central-massed flat plate which was topologically optimized by the Reference. The results of modal strain analysis at the first mode have good agreement with the results from the topology optimization. Furthermore, some features of the strain mode shapes (SMSs) of the flat plate are investigated. Finally, the SMSs are applied to the optimization of a stiffened plate. Attention is focused on the distributions of the SMSs of the stiffeners, which also shows good agreement with the results from the topology optimization in the previous study. Several higher orders of SMSs are extracted, which can visualize the most sensitive elements to the corresponding modal frequency. In summary, both the theory and simulations validate the correctness and convenience of applying SMSs to dynamic design of plate-like structures.

Demarcation for the Coupling Strength in the MODENA Approach

MODal ENergy Analysis (MODENA) is an energy-based approach which is proposed recently to provide a pure tone analysis of power flow. The net exchanged power between two coupled oscillators is proportional to the weighted difference of total energies of oscillators. Contrary to Statistical Energy Analysis (SEA) or Statistical modal Energy distribution Analysis (SmEdA), the MODENA approach can deal with strong coupling case where the power can flow from the oscillator with lower energy to the one with higher energy. The level of coupling strength between oscillators may affect the accuracy of the MODENA approach. This research work aims to propose a criterion to determine the level of coupling in the MODENA approach. A non-dimensional parameter named coupling strength factor is defined to clearly demonstrate the level of coupling strength. Two numerical examples: (a) a two-oscillators coupling case and (b) a multi-modal coupling case are conducted to show the effectiveness of the proposed criterion.

Non-Stationary Random Vibration Analysis Using Multi-Correlated Random Processes Excitations

An algorithm that integrates Karhunen-Loeve expansion (KLE) and the finite element method (FEM) is proposed to perform non-stationary random vibration analysis of structures under excitations, represented by multi random processes and that are correlated in both time and spatial domains. In KLE, the auto-covariance functions of random processes are discretized using orthogonal basis functions. The KLE for multi-correlated random processes relies on expansions in terms of correlated sets of random variables reflecting the cross-covariance of the random processes. During the response calculations, the eigenfuntions of KLE used to represent excitations are applied as forcing functions to the structure. The proposed algorithm is applied to a two degree of freedom system and a stiffened panel for both stationary and non-stationary correlated excitations. Two methods are proposed to obtain the structural responses: 1) the modal superposition method, and 2) the direct method. Both the effectiveness and the computational efficiency of the proposed methods are studied through numerical simulations. Results show that both the modal superposition method and the direct method can describe the statistics of the dynamic response with sufficient accuracy. However, the modal superposition method is applicable using finite element programs whereas the direct method is more efficient for complex systems. The structural responses due to same type of correlated random processes are bounded by the response obtained by both perfectly correlated and uncorrelated random process excitations. If the impact of cross-correlation is ignored, the response will be larger for the perfectly correlated case and smaller for the uncorrelated case than that for the actual conditions. The structural response increases with a decrease in the correlation length and with an increase in the correlation magnitude. The proposed methodology can be applied for the analysis of any complex structures and any type of random excitations.

Numerical-experimental method for elastic parameters identification of a composite panel

A hybrid numerical-experimental approach to identify elastic modulus of a textile composite panel using vibration test data is proposed and investigated. Homogenization method is adopted to predict the initial values of elastic parameters of the composite, and parameter identification is transformed to an optimization problem in which the objective function is the minimization of the discrepancies between the experimental and numerical modal data. Case study is conducted employing a woven fabric reinforced composite panel. Three parameters ( E 11 , E 22 , G 12 ) with higher sensitivities are selected to be identified. It is shown that the elastic parameters can be accurately identified from experimental modal data.