Intersonic crack propagation along interfaces: Experimental observations and analysis

The isochromatic fringe patterns surrounding an intersonically propagating interface crack are developed and characterized using the recently developed stress field equations. A parametric investigation is conducted to study the influence of various parameters such as the crack-tip velocity and the contact coefficient on the isochromatic fringe patterns. It has been observed that the crack-tip velocity has a significant effect on the size and shape of isochromatic fringe patterns. The contact coefficient, on the other hand, does not affect the fringe pattern significantly. The paper also presents a numerical scheme to extract various parameters of interest such as the series coefficients of the stress field, the contact coefficient and the dissipation energy. The results show that the crack growth is highly unstable in the intersonic regime, and the energy dissipation decreases monotonically with increasing crack-tip velocity. The experimental data fit well with the recently proposed fracture criterion for intersonic interfacial fracture.     

Smart damping of laminated thin cylindrical panels using piezoelectric fiber reinforced composites

In this paper performance of a new piezoelectric fiber reinforced composite (PFRC) material has been investigated for active constrained layer damping (ACLD) of laminated thin simply supported composite cylindrical panels. The constraining layer of the ACLD treatment has been considered to be made of this PFRC material. A finite element model of smart composite panels integrated with the patches of such ACLD treatment has been developed to demonstrate the performance of these patches on enhancing the damping characteristics of thin cross-ply and angle-ply laminated composite cylindrical panels. Particular emphasis has been placed on studying the effect of variation of the piezoelectric fiber orientation in the constraining PFRC layer and the shallowness angle of the panels on the control authority of the patches.     

Sub-stripe pattern formation in liquid crystal elastomers: Experimental observations and numerical simulations

We report on some new experimental observations of pattern formation during stretching experiments of nematic liquid crystal elastomers (LCEs).     

In-plane wave motion and resonance phenomena in periodically layered composites with a crack

This paper investigates the transmission and propagation of two-dimensional (2D) time-harmonic plane waves in periodically multilayered elastic composites with a strip-like crack. The total wave field in the composite structure is represented as a sum of the incident wave field determined by the transfer matrix method and the scattered wave field described by integral representations in terms of the Green’s matrices and the crack-opening-displacements. A numerical scheme is developed to compute the wave propagation characteristics and the crack-characterizing quantities. The effects of the crack location and size as well as the angle of wave incidence are investigated using the averaged crack-opening-displacements and the stress intensity factors. Special attention of the paper is devoted to resonance wave motion and wave localization phenomena in a stack of periodical elastic layers weakened by a single strip-like crack. Numerical results are presented and discussed to reveal the usual and the resonant wave transmission by using the power-density vector and the energy streamlines in the vicinity of the crack. Wave localization due to interior and interface cracks is analyzed by considering the energy captured by a crack, and resonance induced crack growth is also discussed.     

Mode III fracture behavior of laminated composite with edge crack in torsion

A three-dimensional (3-D) finite element analysis was performed on a [90,(+45/−45)_n,(−45/+45)_n,90]_s class of laminated composites under the edge crack torsion (ECT) test configuration. Finite element delamination models were established and formulas for calculating the Mode III fracture toughness from 3-D finite element models were developed. The relations between the interlaminar fracture behavior and various configuration parameters were investigated and the effects of point loads, ends, geometry, Mode II interference, and friction were evaluated. Results showed that with proper selection of ECT specimen configuration and layup, the delamination could grow in pure Mode III in the middle region of the specimen. Specimen end effect played an important role in the ECT test. A Mode II component occurred in the end regions but it did not interfere significantly with the Mode III delamination state. Specimen dimension ratio, layup, and crack length exhibited significant effect on the interlaminar fracture behavior and the calculated strain energy release rates. However, friction between crackfaces was found to have negligible effect on the interlaminar properties.     

High-rate deformation and fracture of fiber reinforced concrete

This paper presents the results of dynamic compression and splitting-tensile tests of cardiff fiber reinforced concrete (CARDIFRC) composite using the Kolsky technique and its modification. The strength and deformation characteristics of fiber-reinforced concrete were determined experimentally at high strain rates. The mechanical characteristics were found to depend on the strain rate and stress rate. A uniform interpretation of the rate effects of fracture of the tested fiber-reinforced concrete is given on the basis of a structural-temporal approach. It is shown that the time dependences of both the compressive and tensile strengths of fiber reinforced concrete are well calculated using the incubation time criterion.     

Phenomenological and numerical modelling of short fibre reinforced cementitious composites

In this paper, a constitutive model for short fibre reinforced cementitious composites will be presented. This model is based on the St. Venant–Kirchhoff model, which is a special case of a hyperelastic material. This model is refined to include the fibre orientation distribution. Numerical FEM simulations with the developed constitutive model and fracture simulations using the discrete element method are presented. The outcomes of these numerical methods demonstrate how important it is to monitor and further to control the fibre orientation distribution during the manufacturing process. As the manufacturing process might involve casting, as, e.g., in the case of steel fibre reinforced concrete, an outlook on simulations of the manufacturing process in order to predict and to control the fibre orientation distribution is given.     

Analysis of temperature and impact damage of fiber reinforced composites

Fracture resistance of fiber reinforced composites with polymer matrix is examined owing to impact loading and heat treatment over a wide range of temperatures. Test data are presented for the nonhomogeneous peeling behavior of layered composite specimen damaged by impact. Analytical results are also given for the peeling growth rate under the action of edge peel off and transverse shear.     

广布损伤裂纹扩展试验与数值模拟

对三种开裂模式无钉载平板的广布损伤裂纹扩展试验进行了研究,发现裂纹间的干涉作用明显促进了裂纹的扩展,多裂纹情况下的裂纹扩展速率远大于单裂纹.同时,利用FRANC2D/L计算和分析了单裂纹、多孔相邻裂纹、多孔不相邻裂纹、多孔等长裂纹等四种开裂模式的裂纹扩展行为及应力强度因子分布.结果表明:裂纹扩展初始阶段主要受孔自身应力集中影响,裂纹间的干涉作用弱;随着裂纹扩展到一定长度时,裂纹间的干涉作用显著增大,且随着裂纹间韧带长度的减小而快速增加,与试验结果一致.     

Intermittency and multiscale dynamics in milling of fiber reinforced composites

We have analyzed the variations in cutting force during milling of a fiber-reinforced composite material. In particular, we have investigated the multiscale dynamics of the cutting force measured at different spindle speeds using multifractals and wavelets. The multifractal analysis revealed the changes in complexity with varying spindle speeds. The wavelet analysis identified the coexistence of important periodicities related to the natural frequency of the system and its multiple harmonics. Their nonlinear superposition leads to the specific intermittent behavior. The workpiece used in the experiment was prepared from an epoxy-polymer matrix composite reinforced by carbon fibers.     

Continuum modeling of twinning,amorphization, and fracture: theory and numerical simulations

A continuum mechanical theory is used to model physical mechanisms of twinning, solid-solid phase transformations, and failure by cavitation and shear fracture. Such a sequence of mechanisms has been observed in atomic simulations and/or experiments on the ceramic boron carbide. In the present modeling approach, geometric quantities such as the metric tensor and connection coefficients can depend on one or more director vectors, also called internal state vectors. After development of the general nonlinear theory, a first problem class considers simple shear deformation of a single crystal of this material. For homogeneous fields or stress-free states, algebraic systems or ordinary differential equations are obtained that can be solved by numerical iteration. Results are in general agreement with atomic simulation, without introduction of fitted parameters. The second class of problems addresses the more complex mechanics of heterogeneous deformation and stress states involved in deformation and failure of polycrystals. Finite element calculations, in which individual grains in a three-dimensional polycrystal are fully resolved, invoke a partially linearized version of the theory. Results provide new insight into effects of crystal morphology, activity or inactivity of different inelasticity mechanisms, and imposed deformation histories on strength and failure of the aggregate under compression and shear. The importance of incorporation of inelastic shear deformation in realistic models of amorphization of boron carbide is noted, as is a greater reduction in overall strength of polycrystals containing one or a few dominant flaws rather than many diffusely distributed microcracks.     

Stability of crack propagation associated with fracture energy determined by wedge splitting specimen

The wedge splitting test is performed on notched shaped specimens that enables the determination of energies for large fracture surfaces and material exhibiting brittle behaviour. A stability condition is deduced and found to depend on the Young's modulus and the R-curve behaviour. The latter is defined by the fracture toughness K_R and fracture energy R_c both of which depend on the crack length. A stable crack propagation is enhanced by high ratios of K′_R/K_R and R_c/K_R². The wedge loading tends to behave like raising the rigidity of the testing machine. The results are applied on an example with a special geometry.     

Non-linear analysis of functionally graded fiber reinforced composite laminated plates,Part I: Theory and solutions

This paper deals with the large amplitude vibration, non-linear bending and postbuckling of fiber reinforced composite laminated plates resting on an elastic foundation in hygrothermal environments. Two kinds of fiber reinforced laminated plates, namely, uniformly distributed and functionally graded reinforcements, are considered. The material properties of fiber reinforced laminated plates are estimated through a micromechanical model and are assumed to be temperature-dependent and moisture-dependent. The motion equations are based on a higher order shear deformation plate theory that includes plate-foundation interaction and the hygrothermal effect. A two-step perturbation technique is employed to determine the non-linear to linear frequency ratios of plate vibration, the load-deflection and load-bending moment curves of plate bending, and postbuckling equilibrium paths of laminated plates.     

A macroscopic-level hybrid lattice particle modeling of mode-I crack propagation in inelastic materials with varying ductility

This paper presents a numerical method, known as hybrid lattice particle modeling (HLPM), for the study of mode-I crack formation and propagation in two-dimensional geometry subject to a fixed-grip condition. The HLPM combines the strength of two numerical techniques, particle model (PM) and lattice model (LM), for the purpose of solving dynamic fragmentation of solids within a various Poisson’s ratio range. A Lennard-Jones-type potential is employed to describe the nonlinear dynamic interaction of each macroscopic-size particle with its nearest-neighbors. Crack initiation and propagation is investigated for materials with different Young’s modulus, tensile strength and varying ductility. It is demonstrated that crack patterns and propagation closely match the anticipated physical behavior of inelastic materials. Finally, the HLPM is applied to the investigation of a functionally designed composite material of an elastic–brittle infrastructure material coated with a ductile layer for the protection of fracture propagation. The ultimate application is aimed at the retrofitting of failing infrastructure.     

Non-linear analysis of functionally graded fiber reinforced composite laminated plates,Part II: Numerical results

In this Part, the extensive parametric studies performed are reported and numerical results are presented for the non-linear vibration, non-linear bending and compressive postbuckling of uniformly distributed and functionally graded fiber reinforced unsymmetric cross-ply and/or antisymmetric angle-ply laminated plates resting on Pasternak elastic foundations under different hygrothermal environmental conditions. The numerical results show that the functionally graded fiber reinforcement has a significant effect on the postbuckling response and load-bending moment curves of plate bending, whereas this effect is less pronounced on the load-deflection curves of plate bending and the linear and non-linear frequencies of the same plate.     

Micromechanical modelling and simulation of chopped random fiber reinforced polymer composites with progressive debonding damage

The aim of the present paper is to provide a quantitative prediction of the elastic-damage behaviour of randomly oriented fiber polymer composites. A constitutive model based on micromechanical considerations is presented. The nucleation and growth of voids induced by progressive fiber debonding is combined with the constitutive relationship. Failure resulting of excessive damage accumulation is captured by a critical void volume criterion and a vanishing element technique. Experimentally, damage accumulation in random glass fiber–polyester composites was monitored by a videoextensometry technique able to control the local strain rate. Good agreement of model predictions with experimental data is pointed out. The model was implemented into a finite element program and numerical applications on composite structures (a tensile specimen and a plate containing a central hole) are presented to illustrate the capability of the approach. Digital image correlation method was also used to measure the full-field strain in a notched specimen under tensile loading. The simulated results compared favourably with those obtained from experiments.     

Fatigue behavior of E-glass fiber reinforced phenolic composites: effect of temperature, mean stress and fiber surface treatment

Phenolic matrix is reinforced by unidirectional E-glass fibers with volume fractions of 0.30 and 0.45. Three different surface treatments are applied to the E-glass fibers. The composite specimens are tested at ambient condition and temperatures of 100°C 150° and 200°C with stress levels of R(σ_min/σ_max) equal to 0 and 0.4 for load frequencies of 1.5, 10 and 25 Hz. Data are presented in terms of S/N curves and assessed by degradation of modulus based on compliance. For a particular fiber glass surface treatment and volume fraction, the composite specimen is notched and tested at room temperature and 200°C. A fatigue strength reduction factor K_f is defined and obtained such that the results could be compared with those of the unnotched specimens. Notch effect is small if the hole diameter is equal to the specimen thickness; it would be important for larger hole sizes. Fractured surfaces are examined by the scanning electron microscope.     

Dynamic fracture of concrete compact tension specimen: Experimental and numerical study

Compared to quasi-static loading concrete loaded by higher loading rates acts in a different way. There is an influence of strain-rate and inertia on resistance, failure mode and crack pattern. With increase of loading rate failure mode changes from mode-I to mixed mode. Moreover, theoretical and numerical investigations indicate that after the crack reaches critical velocity there is progressive increase of resistance and crack branching. These phenomena have recently been demonstrated and discussed by O?bolt et al. (2011) on numerical study of compact tension specimen (CTS) loaded by different loading rates. The aim of the present paper is to experimentally verify the results obtained numerically. Therefore, the tests and additional numerical studies on CTS are carried out. The experiments fully confirm the results of numerical prediction discussed in O?bolt et al. (2011). The same as in the numerical study it is shown that for strain rates lower than approximately 50/s the structural response is controlled by the rate dependent constitutive law, however, for higher strain rates crack branching and progressive increase of resistance is observed. This is attributed to structural inertia and not the rate dependent strength of concrete. Maximum crack velocity of approximately 800 m/s is measured before initiation of crack branching. The comparison between numerical and experimental results shows that relatively simple modeling approach based on continuum mechanics, rate dependent microplane model and standard finite elements is capable to realistically predict complex phenomena related to dynamic fracture of concrete.     

Modelling of fibre pull-out in cracked fibre reinforced composites with linear elastic fracture mechanics

A method of finite element modelling the fracture mechanics of a fibre reinforced cement composite is presented. It embodies the use of beam elements with negative extensional stiffness. The method compares favourably with experimental work. Whilst fibre reinforced cement composites are used in this work, the techniques used are applicable to any type of fibre composite material.