民机复合材料结构冲击后压缩设计值的确定方法

冲击后压缩设计许用值是复合材料飞机结构设计的一个非常重要的参数。本文通过分析常用的目视检测方法和飞机结构冲击能量统计结果,从凹坑深度和冲击能量截止值两方面定义了目视勉强可见损伤;根据民机设计兼顾安全性和经济性的特点,作者提出压缩设计许用值中的目视勉强可见损伤应综合考虑,慎重确定。文章还研究了复合材料层压板的抗冲击性能,从工程应用角度给出了结构冲击后压缩设计许用值的确定要点,即用小试样的冲击后压缩试验结果推导的基准系数和环境系数,对元件乃至典型结构件的冲击后压缩试验结果进行修正。     

反求压电薄板智能结构荷载的有限元逆逼近方法

基于压电薄板智能结构提出一种有限元逆逼近反求结构荷载参量的方法，逼近目标函数以压电电荷响应为参量，迭代初值采用单位荷载预逼近方法；数值算例表明了该方法的有效性，可用于解决压电智能结构的荷载识别问题。     

基于提高频率灵敏度的结构损伤统计识别方法

提出了一种基于反馈控制结构动力特性的损伤统计识别方法。该方法采用基于独立模态空间的反馈控制,有目的地配置闭环系统的极点,根据损伤前后闭环系统的特征值构造损伤指标。采用假设检验判断损伤是否发生,并剔除特征量中损伤程度的影响,采用统计模式识别方法识别损伤位置。Benchmark结构的数值算例表明,本文提出的损伤识别方法能够较显著地提高模态频率对刚度变化的灵敏度,准确有效地检测在噪声环境下结构小损伤是否发生以及识别损伤位置。     

基于动力特性的结构损伤定位方法

本文推导出关于结构应变的一阶交分关系,从中揭示出结构损伤定位的合理方案．     

材料非线性对复合材料层合板热自由边界效应的影响

本文用准三维有限元法研究了材料非线性对复合材料层合板热自山边界效应的影响,给出了修正型Hahn-Tsai非线性应力-应变关系的三维形式。由本文非线性分析方法得到的层间应力与以往由线性分析方法得到的层间应力做了比较,结果表明:材料非线性能显著降低层间剪应力的集中程度,但对层间正应力影响不太明显。     

Fatigue Damage in Composite Materials

The phenomenon of fatigue is critical for designing structures including elements made of composite materials. The accurate prediction of the life and fatigue resistance of laminated composites is one of the subjects of inquiry in materials science. The ability of predicting the life of laminates is important for designing, operation, and safety analysis of a composite structure under specific conditions. To predict reliably the life of structures, it is necessary to know the mechanisms of cyclic deformation and damage. It is also necessary to develop a qualitative theory of fatigue failure that should be based on the concepts of solids mechanics. Developing such a theory requires to evaluate the microscopic parameters and the macroscopic variables of the material at the level of a laminate and the structure and to determine exactly the load modes acting on the system.     

Post-buckled propagation model for compressive fatigue of impact damaged laminates

An analytical model for prediction of compressive fatigue threshold strains in composite plates with barely visible impact damage (BVID) is presented. The model represents the complex damage morphology as a single circular delamination at a critical level and calculates the strain at which thin-film buckling of the circular delaminated region occurs. The threshold strain is defined as the strain at which the strain energy release rate for the fracture of post-buckled delaminated plies along the delamination is equal to the critical Mode I value (G_1C) for the resin. The model predicts the critical through-thickness level for delamination, the stability of delamination growth and also the sensitivity to experimental error in geometric measurements of the damage area. Results obtained using the model show an agreement of fatigue strain to within 4% of experimental values for four sets of data reported in the literature.     

Multi-scale analysis in elasto-viscoplasticity coupled with damage

The purpose of this study is to present a micromechanical approach, based on the transformation field analysis (TFA), proposed by Dvorak, which has been generalized at Onera in order to analyze the nonlinear behavior of heterogeneous materials in elasto-viscoplasticity coupled with damage. In such analysis, the macroscopic constitutive equations are not purely phenomenological but are built up from multi-scale approaches starting from the knowledge of the properties of the constituents at the microscopic or mesoscopic scales. The model can take into account some local characteristics that can evolve during the thermo-mechanical applied loads or the manufacturing process, like the grain size for metallic alloys or the fiber volume fraction for composites.The determination of some specific tensors which are present in this formulation is closely linked to the microstructure morphology of heterogeneous materials constituting the macroscopic structure. For example, an Eshelby’s based approach is more appropriate to characterize polycrystalline materials with a random microstructure, while the homogenization of periodic media technique can be used for composite materials with a sufficiently regular microstructure. The proposed methodologies allowing to perform this nonlinear analysis across the scales are illustrated with examples based on the behavior of structures reinforced with a long fiber unidirectional metal matrix composite.     

Comparison between Berkovich,Vickers and conical indentation tests: A three-dimensional numerical simulation study

Three-dimensional numerical simulations of Berkovich, Vickers and conical indenter hardness tests were carried out to investigate the influence of indenter geometry on indentation test results of bulk and composite film/substrate materials. The strain distributions obtained from the three indenters tested were studied, in order to clarify the differences in the load–indentation depth curves and hardness values of both types of materials. For bulk materials, the differentiation between the results obtained with the three indenters is material sensitive. The indenter geometry shape factor, β, for evaluating Young’s modulus for each indenter, was also estimated. Depending on the indenter geometry, distinct mechanical behaviours are observed for composite materials, which are related to the size of the indentation region in the film. The indentation depth at which the substrate starts to deform plastically is sensitive to indenter geometry.     

Development of constitutive laws for thermomechanical behaviors of composites containing multi-type ellipsoidal reinforcements

Composite materials are widely used in industrial applications because of their excellent properties and behaviors. While a composite material is defined as a mixture of two or more different materials, many research works in the literature dealt with composites of only two constituents, which are matrix and one type of particles. On the other hand, the theoretical research works that dealt with more than two constituents are rare. Using some additives affects the sintering behavior, the tribological behavior and the fracture mechanics behavior of composites. For example, a suitable amount of additives as sintering aids, in the sintering process, could lower the sintering temperature, enhance phase wettability and bonding strength and improve the interlaminar fracture resistance of a composite. Therefore, it is worthwhile to develop the constitutive laws that describe the behavior of such composite materials. Accordingly, the aim of this paper is to modify the previous paper, Shabana (2003) [Shabana, Y.M., 2003. Incremental constitutive equation for discontinuously reinforced composites considering reinforcement damage and thermoelastoplasticity. Computational Materials Science 28, 31–40], in order to propose constitutive laws that predict the thermomechanical behavior of composites containing multi-type ellipsoidal reinforcements. This includes reinforcements with different materials and/or different shapes that are represented by aspect ratios. These constitutive laws not only predict the macroscopic and microscopic thermoelastoplastic behaviors of composites containing multi-type ellipsoidal reinforcements, but also characterize their different overall effective properties such as modulus of elasticity, Poison’s ratio and thermal expansion coefficient in different directions. Beside this, they are applicable for porous materials and composites with multiple reinforcements and porosities of different shapes and distributions. In the present numerical analyses, composites with two, three and four constituents considering different materials and aspect ratios as well as reinforcement damage are discussed.     

Characterization of strain-induced damage in composites based on the dissipated energy density Part III. General material constitutive relation

An approach to characterizing failure behavior and degree of load induced internal damage in composite materials and structures is formulated in Part I of this work. It is based on a systematic experimental procedure to observe the response of composite materials subjected to multiaxial load environment. The energy dissipated by internal failure mechanisms is employed as a measure of internal damage and is characterized by an energy dissipation function, which is identified by means of a deconvolution procedure using data provided by NRL's automated in-plane loader testing machine.Part II of this work will display the dissipated energy density distributions in composite specimens that are used for the in-plane loader machine and naval structures, while Part III presents a general theory that includes the derivation for the constitutive behavior of the damaged composites.     

Identification of Impact Damage in S-2 Glass Composite Missile Casings Using Complementary Vibration and Wave Propagation Approaches

The future United States military arsenal is transitioning from homogeneous metallic materials to stronger and lighter heterogeneous composite materials. Although these composites offer numerous advantages such as the ability to tune strength to weight ratios for each particular application, composites are susceptible to numerous damage mechanisms and environmental factors. Accidental in-field impacts resulting from mishandling or transportation loads have been identified by the U.S. Army as the primary cause of damage in composite weapon systems. This paper presents a hybrid approach to detect, locate, and quantify damage in filament wound canisters using a complementary set of vibration-based (transmissibility and embedded sensitivity) and wave propagation-based (phased array beamforming) methods. It is shown that this hybrid approach accurately detects, locates, and quantifies the damage imposed by 1.36 and 6.78 N-m impact energies. By measuring the static stiffness of the baseline canister at the mid-point, it was determined that the 1.36 N-m impact resulted in an overall 6% reduction of the tubes bending stiffness, while the 6.78 N-m impact resulted in a 28% reduction.     

功能梯度材料构件三维分析的细观元模型

提出一种新颖的功能梯度构件分析的细观元法,给出了方法模型、基本算式及特点与功能。细观元法对构件的常规有限单元内部设置密集细观单元以反映材料特性梯度变化,又通过协调条件将各细观元结点自由度转换为同一常规有限元自由度,再上机计算。这种细观元法既能充分反映材料功能梯度及组分变化特性,而其计算单元与自由度又与常规有限元一样,是一种针对功能梯度构件分析的有效数值方法。算例表明了细观元法对不同情况下功能梯度构件分析的适应性与精度。     

The application of interferometric techniques to the nondestructive inspection of fiber-reinforced materials

A method for increasing the sensitivity of dynamic materials evaluation (DME) to localized damage in fiber-reinforced composites was examined. To obtain this improved sensitivity, different aspects of DME were examined. These included an increase in the frequency used to evaluate the dynamic properties, utilization of mode-shape information and different procedures for evaluating the experimental data.The extent of the internal damage was determined using measured changes in the dynamic properties of the system (loss factor, dynamic stiffness and mode shape). To obtain the response information at higher frequencies a modalanalysis system was built around the performance characteristics of a laser doppler vibrometer (LDV) and an electronic speckle pattern interferometer (ESPI). These two devices provided complementary information for the determination of the dynamic characteristics of each vibration mode. With this system, damage-induced changes in the dynamic characteristics of composite materials were measured at frequencies up to 10 kHz.The results of this study showed the following. (1) Torsion modes provide the most sensitivity to localized internal damage. (2) The evaluation of higher frequency NDI data requires the ability to correlate the measured loss factor and resonant frequencies with the actual mode shape. (3) The data obtained over the frequency range of the test could be reduced to a series of slopes that provide a sensitive indication of the material condition. (4) The sensitivity of the dynamic method to localized damage is limited by the measurement of the loss factor.     

THREE-DIMENSIONAL CELLULAR AUTOMATA MODEL FOR PREDICTING LOCAL CORROSION

针对环境作用下金属表面容易形成坑蚀而致使结构性能退化这一问题. 采用元胞自动机方法，建立描述腐蚀环境中金属表面坑蚀形成及演化的三维模型，对不同腐蚀溶液浓度和温度条件下结构表面蚀坑随时间的变化过程进行了模拟，为定量评估服役环境中结构的剩余强度及寿命提供可能.     

Thermomechanical assessment of damage in composites

This paper examines, experimentally and numerically, the use of thermal emission measurements to determine the surface stresses for fibre composite materials. Particular attention is paid to the use of this method for problems associated with damage assessment and repair. In contrast to most traditional methods, the thermal emission profile reflects the interaction of load, geometry, material and damage in a non-destructive fashion. It represents a possible method for the scaling of test data obtained from coupon tests to tests on full scale structures.     

Damage localization using transient non-linear elastic wave spectroscopy on composite structures

To reduce the costs related to maintenance of aircraft structures, there is the need to develop new robust, accurate and reliable damage detection methods. A possible answer to this problem is offered by newly developed non-linear acoustic/ultrasonic techniques, which monitor the non-linear elastic wave propagation behaviour introduced by damage, to detect its presence and location.In this paper, a new transient non-linear elastic wave spectroscopy (TNEWS) is presented for the detection and localization of a scattered zone (damage) in a composite plate. The TNEWS analyses the uncorrelations between two structural dynamic responses generated by two different pulse excitation amplitudes by using a time-frequency coherence function. A numerical validation of the proposed method is presented. Damage was introduced and modelled using a multi-scale material constitutive model (Preisach-Mayergoyz space).The developed technique identified in a clear manner the faulted zone, showing its robustness to locate and characterize non-linear sources in composite materials     

Investigation of Damage in Composites Using Nondestructive Nonlinear Acoustic Spectroscopy

The presented experimental work describes the nondestructive damage examination of polymer-matrix composites using acoustic methods under the consideration of nonlinear effects. The aim is to analyze these nonlinear effects in order to provide a quantification of the nonlinear acoustic transmission which is related to the damage state and its severity in the composite material. The first objective was to study the effectiveness of the distortion evaluation method and its related parameter: the “Total Difference Frequency Distortion” (TDFD) parameter. The TDFD was utilized as a new damage indicator to quantify the progressive damage state in composite materials. The TDFD method had initially been proposed to characterize the distortion of audio amplifiers. A custom-made setup was developed that imposes acoustic signals to the structure. The samples’ vibrations were afterwards analyzed by a laser vibrometer and further spectrum evaluations. The developed method was applied to two composite materials, both reinforced with taffeta woven glass-fibers, but having different thermoset polymer matrix, i.e. vinylester and epoxy. The damage was introduced in the specimen by tensile tests with a stepwise increase of the tension loading. It was observed that damage influences the intensity of nonlinear intermodulation after having introduced two harmonic and constant signals of different and randomly chosen frequencies in the specimen. The nonlinear intermodulation was then quantified by computing the TDFD parameter. In the specific case of epoxy based composites, high frequency peaks were noted for the high tensile loading levels only. The TDFD parameter was then modified in order to take into account this effect. For both studied composites, the modified TDFD parameter increases with the damage accumulation caused by the applied stepwise tensile loading.     

Closed-loop based detection of debonding of piezoelectric actuator patches in controlled beams

A practical closed loop control based damage detection scheme is presented aiming at detecting small damage in controlled structures. In this detection method, a deliberately designed sensitive control system is used to augment small frequency shifts caused by small structural damage. Since a small frequency change can destabilize such a sensitive control system, it can be easily observed and thus the small damage can be detected. To perform active control of structures, a modal velocity observer (MVO) is designed by combining two observers, which can be used for multi-mode control. By properly choosing the parameters in the MVO, it can be made very sensitive to the frequency shift suitable for small damage detection. To demonstrate this method in detecting small debonding of piezoelectric patches on a smart beam, a detailed model of beam with partly debonded piezoelectric patches is established based on the Timoshenko’s beam theory, in which both transverse and longitudinal vibrations are modeled, and a characteristic equation is also derived to examine the effect of the debonding on the control performance. Both the model and the control law are validated by an active vibration control experiment. Finally, an example is given to illustrate application of the method in piezoelectric actuator debonding detection. The results show that even a small edge debonding in a piezoelectric actuator patch can make the sensitive control system unstable, and therefore can be detected.     

A Novel Method for Dynamic Short-Beam Shear Testing of 3D Woven Composites

A new test method for performing dynamic short-beam shear tests using a momentum trapped Hopkinson pressure bar is proposed. Angle-interlock 3D woven composite specimens were tested under quasi-static and dynamic loading conditions to determine the effect of loading rate on damage evolution. The equilibrium condition in the composite specimen under dynamic loads was verified using finite element analysis of the experiment. A high speed camera was used to capture delamination initiation and propagation during both quasi-static and dynamic experiments. Analysis of the load-deflection curves and the high speed images revealed a good correlation between the modes of damage initiation and propagation with the features in the loading response. The apparent inter-laminar shear strength and the bending stiffness increased with rate of loading. While the damage was observed to propagate at a relative steady rate during quasi-static loading, the high rate of energy input during dynamic loading resulted in a rapid propagation of damage and a subsequent loss of stiffness in the composite as noted in the load-deflection curve.