Temperature-dependence of acoustic fatigue life for thermal protection structures

Acoustic fatigue life evaluation is essential for thermal protection struc- tures due to the severe thermo-acoustic load in service. A study on temperature- dependence 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 ob- served 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 atten- tively performed.

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 investi- gated. 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 （Ell, E22, G12） with higher sensitivities are selected to be identified. It is shown that the elastic parameters can be accurately identified from experimental modal data.

Static analysis of elastic cable structures under mechanical load using discrete catenary theory

In this paper,the nonlinear mechanical response of elastic cable structures under mechanical load is studied based on the discrete catenary theory.A cable net is discretized into multiple nodes and edges in our numerical approach,which is followed by an analytical formulation of the elastic energy and the associated Hessian matrix to realize the dynamic simulation.A fully implicit framework is proposed based on the discrete differential geometry(DDG)theory.The equilibrium configuration of a target object is derived by adding damping force into the system,known as the dynamic relaxation method.The mechanical response of a single suspended cable is investigated and compared with the analytical solution for cross-validation.A more intricate scenario is further discussed in detail,where a structure consisting of multiple slender cables is connected through joints.Utilizing the robustness and efficiency of our discrete numerical framework,a systematic parameter sweep is performed to quantify the force displacement relationships of nets with the different number of cables and different directions of fibers.Finally,an empirical scaling law is provided to account for the rigidity of elastic cable net in terms of its geometric properties,material characteristics,component numbers,and cable orientations.Our results would provide new insight in revealing the connections between flexible structures and tensegrity structures,and could motivate innovative designs in both mechanical and civil engineered equipment.

Modified micro-mechanics based multiscale model for progressive failure prediction of 2D twill woven composites

To consider fiber random distribution at the microscale for the multiscale model based on the micro-mechanics failure(MMF)theory,clustering method is used for the extraction of amplification factors.As the clustering method is a kind of unsupervised machine learning method,the elements with similar mechanical behavior under external loading can be included in a cluster automatically at the microscale.With this modification,the fiber random distribution model can be used for multiscale damage analysis in the framework of MMF theory.To validate the modified multiscale analysis method,progressive damage analysis of a kind of 2D twill woven composites is conducted based on different microscale models.The stress values for microscale models with fiber hexagonal and random distribution patterns are compared first.Much higher stress concentration is generated in the fiber random distribution model due to the smaller inter-fiber distance especially under longitudinal shear loading.The obtained cluster distribution results exhibit the characters of the stress distribution in the two microscale models.Thereafter,tensile and compressive responses of the 2D twill woven composite are predicted with the modified multiscale analysis method and accuracy of the method is verified through comparison with published experimental results.From the simulation results,it can be found that the matrix damage initiation from the model based on the fiber random distribution model is premature compared with that from the model based on the fiber hexagonal distribution model.Besides,under tensile loading,the damage all initiates from the fill tows and propagates to the wrap tows.However,under compressive loading,the matrix damage initiates from the wrap tows in the model based on the fiber random distribution model.

Modal identification of long-span Runyang Bridge using ambient responses recorded by SHMS

Runyang Bridge is a newly built cable-supported bridge that crosses the Yangtze River in China.The bridge is composed of one suspension bridge and one cable-stayed bridge.During the construction of the bridge,a structural health monitoring system(SHMS)was installed,which was designed by the Southeast University.Since the bridge was open to traffic,quantities of structural ambient responses have been recorded by the SHMS.And,it’s really important to extract structrural information from these records for health monitoring.This paper presents the study on modal identification of the bridge.The dynamic properites including modal frequencies,mode shapes and damping ratios are extracted from the ambient responses.Two identification methods are employed including the enhanced frequency domain decomposition and the stochastic subspace identification.The identified modal parameters from the two methods are compared.Results show that modal frequencies and mode shapes from the two methods are almost the same while the damping ratios are different.

Fatigue Life of a 2.5D C/SiC Composite Under Tension–Tension Cyclic Loading:Experimental Investigation and Sensitivity Analysis

Engineering structures made of ceramic matrix composites(CMCs)usually suffer from cyclic loads during service,which could lead to disastrous failures.This work focuses on the fatigue behavior of a 2.5D C/SiC composite under tension–tension cyclic loading.Experiments of the 2.5D C/SiC composite are firstly carried out to determine the fatigue lifetime of the material at different stress levels.The fracture surfaces examined by a scanning electronic microscope indicate that the damage mechanisms under cyclic loading are closely related to crack propagation,fiber/matrix interfacial degradation,and fiber breakage.Considering the damage evolution of fibers and interfacial resistance,a micromechanical model is adopted to describe the fatigue behavior of 2.5D C/SiC composite,and the numerical results are compared with the experimental results.Further,a sensitivity analysis is performed as a function of the interfacial shear stress,fiber Weibull modulus,and fiber strength.The calculation of sensitivity factors shows that the variations of the fiber Weibull modulus and fiber strength have the most significant influence and,thereafter,the variation of interfacial shear stress.

Frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites

The panel-type structures used in aerospace engineering can be subjected to severe highfrequency acoustic loadings in service. This paper evaluates the frequency-dependent random fatigue of panel-type structures made of ceramic matrix composites（CMCs） under acoustic loadings. Firstly, the high-frequency random responses from the broadband random excitation will result in more stress cycles in a deinite period of time. The probability density distributions of stress amplitudes will be different in different frequency bandwidths, though the peak stress estimations are identical. Secondly, the fatigue properties of CMCs can be highly frequency-dependent. The fatigue evaluation method for the random vibration case is adopted to evaluate the fatigue damage of a representative stiffened panel structure. The frequency effect through S-N curves on random fatigue damage is numerically veriied. Finally, a parameter is demonstrated to characterize the mean vibration frequency of a random process, and hence this parameter can further be considered as a reasonable loading frequency in the fatigue tests of CMCs to obtain more reliable S-N curves.Therefore, the inluence of vibration frequency can be incorporated in the random fatigue model from the two perspectives.

Strain-Rate-Dependent In-Plane Compressive Properties of 3D Fine Weave Pierced C/C Composite:Failure Mechanism and Constitutive Model

In-plane compression experiments are performed on 3D fine weave pierced C/C composite at a wide strain rate range of 0.0001/s-1000/s.The in-plane compressive failure mechanism of the composite at quasi-static and high strain rates is analyzed by a scanning electron microscope.The results show that the in-plane compressive modulus,maximum stress and the corresponding strain increase with increasing strain rate.The quasi-static in-plane compressive failure mode of the 3D fine weave pierced C/C composite is characterized by the shear failure at the angle of 45°and the local buckling of the^direction fiber bundles.In comparison,the high strain rate in-plane compression failure mode of the composite is characterized by the compressive fracture of the interlaminar matrix and the progressive compression failure of the a>direction fiber bundles.A strain-rate-dependent in-plane compressive constitutive model is proposed to predict the dynamic in-plane compressive response of the composite.The proposed constitutive model is verified by experimental data.