Dynamic Finite Element Modelling and Vibration Analysis of Prestressed Layered Bending–Torsion Coupled Beams

Free vibration analysis of prestressed, homogenous, Fiber-Metal Laminated (FML) and composite beams subjected to axial force and end moment is revisited. Finite Element Method (FEM) and frequency-dependent Dynamic Finite Element (DFE) models are developed and presented. The frequency results are compared with those obtained from the conventional FEM (ANSYS, Canonsburg, PA, USA) as well as the Homogenization Method (HM). Unlike the FEM, the application of the DFE formulation leads to a nonlinear eigenvalue problem, which is solved to determine the system’s natural frequencies and modes. The governing differential equations of coupled flexural–torsional vibrations, resulting from the end moment, are developed using Euler–Bernoulli bending and St. Venant torsion beam theories and assuming linear harmonic motion and linearly elastic materials. Illustrative examples of prestressed layered, FML, and unidirectional composite beam configurations, exhibiting geometric bending-torsion coupling, are studied. The presented DFE and FEM results show excellent agreement with the homogenization method and ANSYS modeling results, with the DFE’s rates of convergence surpassing all. An investigation is also carried out to examine the effects of various combined axial loads and end moments on the stiffness and fundamental frequencies of the structure. An illustrative example, demonstrating the application of the presented methods to the buckling analysis of layered beams is also presented.

Analysis of Air-Coupled Transducer-Based Elastic Waves Generation in CFRP Plates

In this paper, the analysis of non-contact elastic waves generation in carbon fiber reinforced-polymer (CFRP) plate was conducted. Full non-contact elastic waves generation and sensing methods were also analyzed. Elastic waves generation was based on an air-coupled transducer (ACT) while waves sensing was based on a laser Doppler vibrometer. The excitation frequency was equal to 40 kHz. An optimal ACT slope angle for the generation of elastic waves mode was determined with the aid of dispersion curves calculated by using a semi-analytical model. Due to the stack sequence in the composite plate (unidirectional composite), ACT slope angles were different for waves generation in the direction along and across reinforcing fibers direction. Moreover, experimental verification of the optimal ACT slope angles was conducted. It was possible to generate A0 wave mode in the direction along and across the reinforcing fibers. Optimal angles determined using ACT were equal to 16° (along fibers) and 34° (across fibers). In the case of optimal angles, elastic waves amplitudes are almost two times higher than for the case of ACT oriented perpendicularly to the plate surface. Moreover, experimental results based on ACT showed that it was possible to generate the SH0 mode in the direction across the fiber for optimal angles equal to 10°. Finally, based on the A0 wave mode propagation, the process for localization of discontinuities was performed. Discontinuities in the form of additional mass simulating damage were investigated. A simple signal processing algorithm based on elastic wave energy was used for creating damage maps. Authors compared discontinuity localization for ACT oriented perpendicularly to the plate and at the optimal slope angle. The utilization of non-contact waves excitation at optimal ACT slope angles helped to focus the wave energy in the desired direction. Moreover, in this case, elastic waves with the highest amplitudes were generated.

Method for the Microstructural Characterisation of Unidirectional Composite Tapes

The outstanding properties of carbon fibre-reinforced polymer composites are affected by the development of its microstructure during processing. This work presents a novel approach to identify microstructural features both along the tape thickness and through the thickness. Voronoi tessellation-based evaluation of the fibre volume content on cross-sectional micrographs, with consideration of the matrix boundary, is performed. The method is shown to be robust and is suitable to be automated. It has the potential to discriminate specific microstructural features and to relate them to processing behaviour removing the need for manufacturing trials.

Predictions of the axial tensile property of the unidirectional composite influenced by microfiber breakage defects

This paper primarily investigated the effect of fiber breakage defects on tensile properties of the unidirectional composite (UD) using the numerical simulation method. Different kinds of fiber breakage defects were firstly proved to exist in the UD according to the sub-micro computed tomography images at the microscale level. A strict random uniform distribution hypothesis was then proposed to introduce fiber breakage defects into the composite. Numerous microstructural models within random fiber breakage defects were created with the Monte Carlo method to analyze the fiber breakage defect effect on the UD. The results show that the tensile modulus of the UD was reduced by 17% when the fiber breakage defect volume fraction was only 1%, which indicates the effect of this kind of defect was very significant. The fiber volume fraction, defect volume fraction and property all have influences on the decrease of the UD caused by the fiber breakage defect. Finally, we derived a mathematical model to calculate the tensile modulus of the UD based on the numerical results. The proposed mathematical model has an application on the prediction of the axial modulus of the UD or the fiber tow containing large numbers of fiber breakage defects in the composites with complicated structure.

MICROMECHANICAL FINITE ELEMENT PREDICTION OF INTERLAMINAR TRACTION-SEPARATION LAWS USING J-INTEGRAL APPROACH

Dynamic impact loading of woven composites leads to mesoscale damage such as interlaminar transverse cracks and intralaminar tow-tow delamination cracks. At the microscale, this damage may be modeled as fracture between [90/90] and [0/90] unidirectional composite laminates. Microscale finite element model (FEM) resolution of dynamic impact at structural length scale is intractable, but mesoscale FEM resolution is possible with current computational resources. However, mesoscale cohesive zone modeling of this damage requires appropriate tractionseparation laws. These laws are predicted in this work with fiber length-scaleresolved FEMs, which include residual stress, experimentally measured, ratedependent, nonlinear matrix behavior, and experimentally measured, computationally validated, rate-dependent fiber-matrix interface properties. The J-integral from elastoplastic fracture mechanics is computed under mode I and mode II loading and differentiated to determine the traction-separation laws.

Framework for Predicting Failure in Polymeric Unidirectional Composites through Combined Experimental and Computational Mesoscale Modeling Techniques

As composites continue to be increasingly used, finite element material models that homogenize the composite response become the only logical choice as not only modeling the entire composite microstructure is computationally expensive but obtaining the entire suite of experimental data to characterize deformation and failure may not be possible. The focus of this paper is the development of a modeling framework where plasticity, damage, and failure-related experimental data are obtained for each composite constituent. Mesoscale finite elements models consisting of multiple repeating unit cells are then generated and used to represent a typical carbon fiber/epoxy resin unidirectional composite to generate the complete principal direction stress-strain curves. These models are subjected to various uniaxial states of stress and compared with experimental data. They demonstrate a reasonable match and provide the basic framework to completely define the composite homogenized material model that can be used as a vehicle for failure predictions.

Modelling Anisotropy Influence on Guided Wave Scattering at Composite Delaminations

Carbon fibre reinforced composite laminates are widely used in aerospace structures but are prone to barely visible impact damage (BVID). Depending on impact severity, delaminations can form below the surface of the laminate, reducing the load bearing capacity. Efficient structural health monitoring (SHM) of composite panels can be achieved using guided waves propagating along the structure. Propagation and scattering of the A0 Lamb wave mode in a quasi-isotropic composite laminate was modelled using full three-dimensional (3D) Finite Element (FE) simulations. Individual ply layers were modelled using homogeneous unidirectional composite material properties to accurately capture the anisotropy effects. FE predictions for scattering and energy trapping at delaminations were compared to experimental measurements. Noncontact, full-wavefield guided wave measurements were obtained using a laser vibrometer. Good agreement was found between experiments and FE predictions. The effect of delamination shape and depth was investigated through a numerical parameter study. The angular dependency of the amplitude of the scattered wave was calculated. The influence of ply layer anisotropy on wave propagation in an undamaged laminate was investigated numerically. The sensitivity of guided waves for the detection of delaminations due to barely visible impact damage (BVID) in composite panels has been verified.

ANALYSIS OF STRENGTH CRITERIA IN THE DESIGN OF PRODUCTS FROM COMPOSITE MATERIALS

Technological progress gives rise to the continuous expansion of the class of structural materials and the improvement of their properties. The appearance of new materials is due to the natural desire to increase the efficiency of the structures under development. One of the most striking manifestations of progress in the development of materials, structures and technology is associated with the development and application of composite materials. Composites have a number of obvious advantages over other materials, in particular over metals. Such advantages are high specific strength and rigidity, high corrosion resistance, good ability to withstand alternating loads and others. It should be noted another, perhaps the most important feature of composites - is the ability to change the properties of the material in accordance with the purpose of the structure and the nature of its load during operation. Under the influence of loads on the structure, its strength is estimated by the ultimate state of the materials of the structural elements. When a boundary state arises in a material, its transition to another mechanical state - elastic, plastic, or fracture state - occurs. This article aims to determine the optimal criterion for the strength of composite material that takes into account different values of ultimate stresses not only in different directions of the coordinate axes, but also to stretch and compress and further calculate the maximum allowable load for single-layer unidirectional composite material During the research the main properties of composite materials, methods of manufacturing parts from composite material, their main properties and methods of destruction were considered. The characteristics of the strength criteria of composite materials are given, the most suitable for calculating the maximum value of the allowable load for a single-layer unidirectional composite material is determined. The proposed approach to the optimal design of elements of single-layer composite structures may be of interest to developers of numerous and analytical methods for solving problems of optimal design of more complex structures.