Measurement of nonlinear damping in a Gr/Al metal-matrix composite

This paper is concerned with the measurement of nonlinear (i.e., strain amplitude dependent) intrinsic material damping in continuous-fiber-reinforced metal-matrix composites (MMC). The particular MMC studied is a four-ply [±θ] _s P55Gr/6061 Al composite with θ=0, 15, 30, 45, 60, 75 deg. A popular method for measuring damping is the free-decay of flexural vibrations of a cantilevered beam. However, the strain field in a cantilevered beam is inhomogeneous. Therefore, for materials whose damping is nonlinear, the measured specimen damping is not equal to the intrinsic material damping. Using an elementary algorithm develeped by Lazan, the authors extract nonlinear intrinsic material damping from the nonlinear specimen damping.     

The tensile failure modes of metal-matrix composite materials

The strengths of individual boron fibers extracted from various as-received and thermally-fatigued aluminum alloy matrix materials were measured. The results are described in terms of a Weibull distribution, and strengths of composites fabricated from these fibres are calculated in terms of lower and upper bounds. Tests conducted on composites specimens indicated that strengths approaching the upper bounds can be achieved in composites fabricated by normal diffusion bonding techniques. Cyclic temperature changes effectively reduced the strength values towards the lower bounds. It was concluded that this effect resulted from the degradation of the strength of the fiber-matrix bond.     

Experimental study of thickness-tapered unidirectional composite compression specimens

Various untabbed, thickness-tapered compression specimen geometries were studied experimentally. Both shear-loaded and end-loaded compression test methods were used to measure the strength and stiffness of carbon/epoxy and glass/epoxy unidirectional composite material systems. The compressive strength was found to be strongly dependent on specimen geometry, loading conditions, and the cracks that initiated in the taper regions and propagated to various lengths prior to catastrophic failure. A specific thickness-tapered specimen geometry that produced compressive strengths significantly higher than conventional specimen configurations resulted from the experimental optimization process. A method of determining compressive strains, and thus the compressive modulus, using this same specimen geometry and strain gages was also demonstrated.     

Multiple scattering of elastic waves in metal-matrix composite materials with high volume concentration of particles

A new approach is proposed to investigate the propagation of compressional (P) and shear (SV) waves in metal-matrix composite materials with high volume concentration of particles. The theory of quasicrystalline approximation and Waterman's T matrix formalism are employed to treat the multiple scattering resulting from the particles in composites. The addition theorem for spherical Bessel functions is used to accomplish the translation between different coordinate systems. The analytical expression of the Percus–Yevick correlation function is also given. Closed form solutions for the effective propagation constants and the dynamic effective elastic modulus of materials are obtained in the low frequency limit. At higher frequencies, only numerical results of them are presented. Numerical examples show that the phase velocities of P and SV waves in the composite materials with low volume concentration in the low frequency are in good agreement with the results in previous literatures. The effects of the incident wave number, the volume fraction of particles and the material properties of the particles and matrix on the dynamic effective elastic modulus are also examined.     

Experimental determination of damage parameters in uniaxially-loaded metal-matrix composites using the overall approach

A recently developed damage-plasticity model for metal matrix composites is examined using the overall approach to damage. In this approach, the damage mechanisms in the composite system are reflected through a single overall damage tensor assuming an elasto-plastic matrix with elastic fibers. It is shown how the model is applied to uniaxially loaded metal-matrix specimens. Each specimen is a symmetric laminate composed of four plies with different orientations. A numerical procedure is formulated and implemented to calculate the stresses, strains, and damage variables in these specimens.

In order to test the validity of the model, a series of uniaxial test experiments are conducted for the damage characterization of the metal-matrix composite. The experiments are conducted on a titanium aluminide SiC-reinforced laminate. Laminate layups of (0/90)s and (± 45)s are tested under uniaxial tension. In order to investigate and model damage evolution, each specimen is loaded to different load levels ranging from the fracture load to 70% of fracture load. Scanning electron microscopy is performed on representative cross-sections of all specimens. This information is then used to quantify damage in the composite system. Damage variables are defined based on the crack density and correlated with the theoretical model. The experimental and numerical results are then compared. Certain recommendations are finally made concerning the determination of damage parameters in metal-matrix composites.     

Experimental study of the strength of composite plates in bending

The strength of composite plates in bending and the effect of the angular parameters at the tips of the joint of the constituents on the strength are studied. An approximate experimental finite-stress curve (λ=1) that separates the low-stress zone from the stress-concentration zone near the angular point of the joint is obtained in the plane of the angles (α, β) by analyzing the photoelastic patterns (isochromatic curves), fracture lines, and rupture moments. A comparison with the theoretical curve obtained from the solution of the corresponding problem is performed. An experimental relation between the strength of the plate and the angular parameters of the constituents is obtained for the stress-concentration and low-stress states near the joint tips. Institute of Mechanics, Armenia National Academy of Sciences, Erevan 375019. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 4, pp. 211–215, July–August, 2000.     

Experimental modal analysis of a synthetic composite femur

In this paper we describe the experimental characterization of the modal parameters of a synthetic composite femur model widely used in biomechanical research studies. The objective of the experimental procedure was to identify the natural frequencies and mode shapes of an unconstrained (free-free) femur. The experimental data were compared with the same obtained in an analog study performed with a fresh cadaveric femur bone. Other objective of the study was to investigate modal analysis as a technique to validate a finite element model of a composite femur with isotropic material properties.     

Experimental investigation of sound absorption in a composite absorber

A composite absorber made of a polyurethane sponge and multi-layer micro-perforated plates is presented in this study. Results from an acoustic impedance tube test show that the polyurethane sponge can exhibits higher low-frequency sound absorption in front of the micro-perforated plate, while sound absorption at medium and high-frequencies remains low. The physical mechanism behind this is that the micro-perforated plate increases the denpth cavity. If the polyurethane sponge is placed behind the micro-perforated plate, the amplitude of the original absorption peak will remain constant, but the absorption peaks will shift to lower frequencies. The reason for this phenomenon is that porous materials with low flow resistance can be approximately equivalent to fluid, which not only does not affect the resonance absorption coefficient of micro-perforated plate, but also makes the peaks move to low frequency. This study has the potential applications in the sound absorption design of composite structure.     

缝合复合材料层间开裂的试验和数值研究

缝合复合材料层合板中贯穿厚度方向的缝线,能有效增强层合板的抗分层能力。本文对一种碳纤/环氧缝合复合材料层板进行了短梁三点弯试验,测得了压头的载荷-位移曲线,并观察了层间裂纹的扩展,证实了缝线对层间裂纹的阻滞作用。建立了三维有限元模型模拟了上述试验,模型中相邻的铺层之间布置了一层初始无厚度的界面单元,界面单元的失效自然模拟层间开裂,而缝线简化为面积等效的梁单元,数值结果与试验观测吻合。     

舰船舷侧复合装甲结构抗动能穿甲模拟实验

以均质钢板前置复合材料板模拟舰船舷侧复合装甲结构,结合低速弹道冲击实验,分析了结构的 破坏模式和吸能机理,比较了复合材料板与均质钢板的抗弹性能。在此基础上,根据靶板破坏模式,得到了球 头弹穿透组合靶板的剩余速度预测公式,并与实验结果进行了比较。结果表明,复合材料板的面密度吸能远 大于均质钢板的;组合靶板中前置复合装甲板的破坏模式主要为纤维拉伸断裂,而钢质背板则由于前置复合 装甲板的影响,破坏模式主要为花瓣开裂破坏;将剩余速度理论预测值与实验数据进行比较,两者吻合较好。     

Constitutive relation of discontinuous reinforced metal-matrix composites

A micromechanical model is developed to simulate the mechanical behaviors of discontinuous reinforced composites. The analysis for a representative unit cell is based on the assumption of a periodic array of aligned reinforcements. The minimum energy principle is used to determine the unknown coefficients of the displacement field of the unit cell. The constitutive behavior of composites is studied to obtain the relationship between the main variables of matrix and reinforcements. It is concluded that the flow strength of composites is strongly influenced by volume fraction, aspect ratio of reinforcement, and the strain hardening exponent of matrix. An analytical constitutive relation of composites is obtained. The predicted results are in agreement with the existing experimental and numerical results. The project supported by the National Natural Science Foundation of China (19704100) and National Science Foundation of Chinese Academy of Sciences (KJ951-1-20)     

Inelastic behavior of metal-matrix composites at elevated temperature

A set of constitutive relations are given for the prediction of the overall behavior of fiber-reinforced materials, in which both matrix and fiber constituents are thermoelastic-thermoviscoplastic materials. The temperature-dependent response of the composite to a given loading is determined from the set of equations by an incremental procedure in time. The theory is applied to investigate the overall response of unidirectional graphite thermoelastic fibers reinforcing an elastic-thermoviscoplastic aluminum matrix, to thermal and mechanical loadings. The residual microstresses and microstrains which develop in the composite when it is cooled from its curing temperature, are investigated. The effect of this residual field on the overall behavior of the composite when it is subjected to various types of mechanical cyclic loadings is studied.     

Particulate size effects in the particle-reinforced metal-matrix composites

The influences of particle size on the mechanical properties of the particulate metal matrix composite are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted. The project supported by the National Natural Science Foundation of China (19891180, 19925211) and by the Chinese Academy of Sciences (KJ951-1-201) and “Bai Ren” plan     

Experimental simulation of matrix cracking and debonding in a model brittle matrix composite

A model composite material system was designed to simulate typical damage mechanisms in unidirectional fiber reinforced brittle matrix composites. Experiments were performed at low to high quasistatic, macroscopic loading rates . At all loading rates reversal of the transverse strain was observed and was correlated to matrix cracking and debonding. The optical method of coherent gradient sensing (CGS) was used to obtain qualitative information regarding the stress fields and to observe the progression of damage. It was found that the sequence of damage formation (damage path) depended on the macroscopic loading rate. At lower loading rates periodic matrix cracks developed; minimal debonding of the reinforcement-matrix interface occurred only much later in the experiment. At higher loading rates extensive debonding followed propagation of the initial matrix crack, and periodic cracking was not observed. Several features of the material response of the model material system were also observed in a previously studied unidirectional ceramic matrix composite.     

Experimental test and modeling of hollow-core composite insulators

Hollow-core composite post insulators, used in high voltage electrical equipment, are important parts of the power substation systems. In many applications the composite insulators are considered as slender cantilever columns, fixed at one end and connecting to a conductor at the other end. During earthquakes the post insulators are damaged and sometimes fail near their base connection. When the post is pulled laterally, the tube dislocates from the walls in the end flange, and slips in and out the flange. Subsequently the composite tube sticks in the flange and slips again if the load is reversed, as it is occurring during earthquakes. In this study, an analytical model is developed using a combination of linear and nonlinear springs, viscous and frictional dampers and inertial masses. The developed macroscopic model is governed by a third-order differential equation which is derived in a state-space and solved by using Runge?CKutta integration in MATLAB. Several prototype insulators have been tested at the University at Buffalo??s Structural Engineering and Earthquake Simulation Laboratory (SEESL). Through a methodical identification of the stiffness, mass, friction, and damping properties, the analytical model is verified to produce reliable estimates of strength, damping and global behavior.     

Experimental study on a separate type thermosyphon

The passages of vapor flow and the returning liquid flow, are perfectly partitioned in a separate thermosyphon. Therefore the flooding limit can be eliminated, and practicability based on its construction is highly evaluated. In the present work, a container tube made of heat resisting glass, in which an electric heater is inserted, is selected as the heating section of the experimental equipment. Distilled water is used as the working fluid. The influence of the heater type, the diameter of evaporating section and the liquid fill charge on the heat transfer performance have been studied. The larger heat transfer coefficient is achieved in the case of the U type heater, the larger tube diameter and the less liquid fill charge of the evaporating section. The useful correlation equations of the heat transfer coefficient in the evaporator have been derived.

---

Experimentelle Untersuchung an einem Trennstrom-Thermosiphon

Zusammenfassung In einem Trennstrom-Thermosiphon sind die Strömungskanäle für die Dampfströmung und den Kondensatrücklauf vollkommen separiert. Die Flutgrenze kann deshalb unberücksichtigt bleiben. In dieser Untersuchung dient ein temperaturbeständiges Glasrohr mit innenliegender Heizung als Wärmequelle. Arbeitsmedium ist destilliertes Wasser. Untersucht wurden der Einfluß des Heizelementtyps, des Durchmessers der Verdampferstrecke und der Füllmenge auf das Wärmeüber-gangsverhalten. Das U-Typ Heizelement liefert bei größerem Rohrdurchmesser und geringerer Füllmenge in der Verdampferstrecke die höchsten Wärmeübergangskoeffizienten. Für diese wurden Korrelationsgleichungen aufgestellt.

     

Experimental study of HE-II boiling on a sphere

Regimes of superfluid-helium boiling on structural-steel spheres 4.8 and 6.0 mm in diameter, with heaters installed inside, are examined. Experimental data on the evolution of vapor films formed on the spherical surfaces are obtained. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 6, pp. 78–84, November–December, 2006.