Dynamic Analysis of Nonlinear Composite Structures under Pressure Wave Loading

As composite materials have been used for marine structures such as boats, ships, offshore structures, etc., the effect of FSI should be understood.In particular, FSI effect is expected to be significant for polymer composites because the water density is very comparable to the composites' densities.For example, the density of a carbon composite is approximately 50% greater than the water density.Sandwich composites consisting of very low densities of core materials have lower densities than that of water.As a result, the hydrodynamic mass associated with FSI would be very critical to composite structures under water.The growing use of composites in ship masts, superstructures, deck grates, piping, ducting, rudders, propellers, stacks, and various submarine structures requires extensive modeling and testing to help designers, builders and operators better understand composite response [Mouritz, et al., 2001].These materials are subjected to a wide spectrum of loads during manufacturing and service life.Dynamic loadings, in particular, impact type event, represent a serious design concern for use of composite.Composite structures are more susceptible to impact damage than similar metallic structures which are more ductile in nature and can absorb typically large amounts of energy without failure.Furthermore, the damage in composites from impact can go undetected even when the mechanical properties may be drastically reduced from an impact.For these reasons, numerous experimental and analytical studies have been conducted to study the dynamic response of composites subjected to impact loading [Abrate, 1994;Aslan, et al., 2003;Kwon & Wojcik, 1998].According to the review of past works, most of the research effort has been focused on low velocity impact damage, specifically, the damage predictions, and the evaluation and prediction of residual properties of damaged laminates.All of the research completed thus far has focused on damage in composites under impact loading in dry environments to support development of composites in aircraft structures.As far as dynamic response of structures under water is concerned, a great deal amount of analytical and experimental studies have been conducted on the effect of fluid force on the natural frequencies, damping ratios and mode shapes of vibrating structures in contact with fluid.This is commonly known as the Fluid Structure Interaction (FSI) problem.FSI investigations have supported many problems in submarine signaling, offshore oil structure stability, and ship structure vibrations.Through these studies, many numerical and analytical methods have been developed in order to predict the added mass and the www.intechopen.com Advances in Composites Materials -Ecodesign and Analysis 338resulting changes in natural frequency of a structure in contact with fluid.It has been determined and widely proven that the effect of fluid surrounding a structure decreases the natural frequency of a structure due to the increase in total kinetic energy of the vibrating structure and fluid from the addition of kinetic energy of the fluid.This effect can be interpreted as an added mass to the vibrating structure in the analysis of the dynamic response.Essentially as the structure vibrates, its mass is increased by the mass of the vibrating fluid with which it is in contact, consequently decreasing its natural frequency.Studies of fluid structure interaction and the added mass effect, also known as virtual mass effect, hydrodynamic mass, and hydroelastic vibration of structures, started with Lamb [Lamb, 1921] who calculated the first bending mode of a submerged circular plate.In response to a problem of submarine signaling, Lamb investigated the vibrations of a thin elastic circular plate in contact with water.In his investigation he discovered that the natural frequencies for structures in contact with fluid are lower than the frequencies in air, based on the assumption that the modes shapes are virtually the same in water as in a vacuum.The resonant frequency was determined using Rayleigh's method.Lamb's theortical results were verified experimentally [Powell & Roberts, 1923].Much later, more research was conducted for measuing and calculating natural frequencies of free vibartions of beams and plates under water [Lindholm, et al., 1965; Fu & price, 1979;Kwak, 1996].Another branch of FSI studies is underwater explosion.Both experimental and numerical studies have been conducted for metallic structures [

[ABST RACT ] App ro ximately 70% of structural failures are due to fatigue. The fatigue mechanism in composites is much more complex than that of metals. The damage may be in one or more forms, such as a failure in a fiber -matrix interface, matrix cracking, delami nation (s), and /or fiber breakage. Matrix cracking and delamination both reduce stored energy and stiffness. Damage from these mechanisms can be detected very early in the fatigue life of a composite. This damage produces a reduction of elastic properties s uch as stiffness. There is always a correlation between damage and stiffness reduction. Fatigue in multidirectional laminates is a well understood phenomenon. It may be said that the damage process of multi -directional laminates has been clarified, at lea st qualitatively, if not quantitatively. Even though fatigue behavior is similar in woven composites, it is incorrect to apply the same concepts of the damage mechanism of multi -directional laminates to woven composites. This is because each ply in woven c omposites is itself bi -directionally reinforced by fiber bundles in the different directions. Recently, the use of woven composites for aerospace and other industrial applications has grown exponentially. These composites are typically manufactured using the low cost vacuum assisted resin transfer molding process (VARTM). With the large variations in fiber volume fraction, weave angle, thickness variations , an accurate estimation of the fatigue life prediction curve become s a challenging task. It is common knowledge that the ultimate strength of composite materials is dependent on the fiber volume fraction, thickness of the laminate and the warp weft angle. Most of the fatigue analyses today are based upon the average ultimate tensile strengths conducted p rior to fatigue testing . Variations in the se ultimate strengths of the composites can lead to extensive scatter in the fatigue data. This paper presents an innovative technique for the accurate prediction of the fatigue life in woven and braided composites . The technique uses a mu lti -variant analysis in conjunction with experimental strength data. This procedure first predicts an accurate tensile/compressive strength of a specimen and then uses these results to develop the fatigue life setup criteria . Resul ts indicate that this technique produces minimal scatter in the fatigue data, accurately predicts the tensile/compressive strengths as a function of the weave angle, the fiber volume fraction and the thickness of the specimen. This paper illustrates the ap plication of this newly developed technique in the estimation of the fatigue life in woven and braided composites.

Currently, one of the greatest challenges for the areas of Material Sciences and Structural Engineering is to perform accurate analysis for prediction of damage in composite materials, the evolution of damage and failure. Although several models and failure criterion already exist for the simulation of damage in composite materials, most models do not produce acceptable results for detailed designs. The models currently in use often under or overestimate loads required for the degradation and failure. This occurs as most of these models is based upon phenomenological or semi-empirical data, which adjust failure surfaces or failure envelopes to experiments. This approach neglects the inherent anisotropy and heterogeneity of composite materials, which cause several failure mechanisms to occur simultaneously in different materials scales and phases. One possible solution to this problem is to use and/or develop new damage and failure models based on multiscale approaches and physical failure mechanisms based on Continuum Fracture Mechanic. In this scenario, the main objective of the present work consists on studying and developing multiscale based damage models applied to composite structures manufactured with unidirectional fibers under different load cases: pure tensile, pure bending, mixed tensile-bending and multiaxial. For the development of these models, works found in the literature were critically evaluated and new formulations were studied, adapted and improved upon. The basic methodology is based on using homogenization techniques to obtain degenerated elastic properties from damaged Representative Volume Elements (RVEs); the damage profile of the RVE is defined as intralaminar cracks parallel to the fiber directions and is calculated using a multiscale approach. The multiscale approach comprehends three separate models, one in the macroscale for the calculation of accurate stress/strain states in the critical points, and two in the microscale for the prediction of intralaminar damage (matrix cracking). These models interact between themselves, as the results from each one are used as boundary conditions for the other in a computational analysis loop over load steps via an iterative process. The developed models were implemented either stand-alone Python codes or into the finite element analysis package AbaqusTM using its automatization capabilities with Python scripts, as well as subroutines in Fortran (UEL - User Element Subroutine) linked to commercial finite element package AbaqusTM

The paper presents a theoretical and experimental study of the influence of damage accumulation in composite material on the elastic fourth-rank tensor and on the strength under quasi-static, cyclic and dynamic loads. Layered polymer composites (glass or carbon fiber-reinforced plastic) are considered. This work aims to develop a mathematical model that accounts for the impact of damage in the composite material on the fourth-rank tensor of elastic properties. The constitutive relations for an orthotropic composite material are proposed, taking into account the accumulation of damage during tensile or shear loading. A numerical simulation using the finite element method is done for a periodicity cell for composite variants (unidirectional, layered, etc.) to determine the effective elastic properties. Quasi-static loading experiments were conducted on composite samples (carbon plastic) to validate the model and the degradation of elastic properties was determined by measuring the longitudinal sound speed. The full-scale experiments confirmed that damage accumulation in carbon fiber-reinforced plastic leads to the degradation of its effective elastic properties. In this connection, a technique was developed to take into account irreversible damage in an orthotropic composite material through the components of the ductility tensor, which may be helpful for the design and analysis of composite structures. The findings of the study could help with the creation of new materials with enhanced mechanical characteristics, as well as with raising the quality of already-existing materials. The developed mathematical model of orthotropic composite material can be used to enhance the structural strength properties and boost the application safety in a variety of industry.

In the service of composite materials, scenarios involving repeated impacts are frequently encountered. In our study, the time‐frequency characteristics of acoustic emission signals during repeated impact processes were investigated using wavelet packet transform. Principal Component Analysis was utilized to identify the frequency bands containing the most information. Shannon entropy was employed to select the optimal wavelet basis function. The terms DDD3, ADD3, and DAA3 correspond to matrix cracking, delamination, and fiber failure, respectively. The initial impact, due to the presence of numerous internal voids in the specimen, often leads to stress concentration, thus resulting in a prolonged duration of matrix cracking during the initial impact. Delamination becomes more active when matrix cracking initially occurs. Fiber failure typically accompanies unloading when it becomes active. Matrix cracking dissipates the most energy, followed by delamination and then fiber failure. When the final impact penetration occurs, the characteristic frequency band proportions for the quasi‐isotropic matrix cracking, delamination, and fiber failure are 48.99%, 22.39%, and 7.16%, respectively. For the cross‐ply laminate, the corresponding proportions of the characteristic frequency bands are 56.62%, 14.46%, and 6.98%, respectively. Highlights Multiple impact tests were conducted, and acoustic emission signals were recorded. Principal component analysis was performed to select the characteristic frequency bands. The time‐frequency characteristics of the two types of laminated composites during the impact process were analyzed. The trends of the characteristic frequency band proportions under multiple impacts on the composites were investigated.

Finite element analysis of composite structures with smart constrained layer damping

Subsurface damage in composite materials is difficult to detect using visual techniques, and other current inspection methods lack the ability to perform quick, wide-area inspections without the need for reference signatures or baseline measurements. This paper presents a method for detecting and locating subsurface damage in composite materials without historical reference measurements by considering the nonlinear behavior of the material in the vicinity of damage. Nonlinear behavior is identified by comparing frequency response functions measured at different input amplitudes. It will be shown that the nonlinear behavior of the material is most evident in the areas nearest to the damage. The proposed inspection method is demonstrated both analytically and experimentally. First, a finite element model of a sandwich beam is developed using Bernoulli–Euler beam elements to represent each layer of the beam and springs to represent the interface between the layers. A bilinear stiffness nonlinearity is simulated to represent disbond damage between the top and core layers of the beam. The simulated disbond damage is localized by identifying degrees of freedom which indicate significant nonlinear response through a comparison of frequency response functions measured at various input amplitudes. Next, the method is demonstrated experimentally by identifying disbond damage in a fiberglass sandwich panel. A three-dimensional scanning laser vibrometer is used to measure the forced frequency response of the panel in its damaged state as it is excited at two or more amplitudes of excitation by a piezoelectric actuator. Comparisons of the frequency response functions measured at different input amplitudes show that the subsurface damage introduces nonlinear behavior which resembles a bilinear stiffness nonlinearity, and the differences in the frequency response functions are largest in the vicinity of the damage location. In addition, it was found that improved localization of the damage is achieved by investigating the response at higher frequencies. This work has application as a nondestructive method for detecting and locating subsurface damage in composite materials and, by using a laser vibrometer for noncontact measurement, allows for quick, wide-area inspection of composite materials without the need for reference signatures or baseline measurements.

Damage and failure of the fiber reinforced composites remain as a challenging research subject in the area of material science and engineering. In this study a novel particle assembly model is developed using two dimensional Discrete Element Method (DEM) for the purpose of simulating the damage and failure process of the single-fiber composite (SFC) under axial tension. Fiber (SiC) and matrix (Epoxy) are represented by particles bonded together through elastic parallel bonds which are calibrated by a series of numerical tests. The contacts between the fiber particles and matrix particles are directly accounted for the fiber/matrix interface which is represented by the contact softening model similar to the cohesive zone model (CZM) in the continuum mechanics. The single-fiber composite tensile test is carried out using the developed DEM model in order to evaluate the interactions between fiber breakage, interfacial debonding and matrix cracking. The numerical results have demonstrated the capability of the developed DEM model in simulating the entire failure process of each individual constituent of the single fiber composite. This study has also confirmed that the DEM model has unique advantages over the conventionally numerical models in terms of dealing with the evolution of microscopic damages in composite materials.

A comparison is made between conventional (i.e. uniform) and chirped optical fibre Bragg gratings (FBGs) for the detection of matrix cracking damage in composite materials. Matrix cracking damage is generally the first type of visible damage to develop under load in the off-axis plies of laminated composites and is generally the precursor of more serious damage mechanisms, particularly delamination. The detection of this type of damage is thus important, particularly in aerospace applications. Using a uniform FBG, characteristic changes develop in the reflected spectrum which can be used to identify crack development in the composite. The additional advantage of using a chirped grating is that the crack position can also be located.

The main purpose of the present investigation was to determine the damages generated by the low velocities with the help of the experimental method (Impact by a drop test) and the finite element method. The commercial transient finite element package LS-dyna used to model the effect of slug impactor and circular notch induced damage in composite material subjected to low velocity impact. The studied composite material was T800S/M21 made of carbon/epoxy. The effect of circular notch was examined. The composite material was subjected to stainless steel slug impactor in the transverse direction dropping the composite laminate at the center with three different velocities (2.85 m/s, 3.47 m/s and 4.17 m/s). The analysis was carried out using the model MAT59 for laminate and a rigid body model MAT20 for the slug impactor. The automatic_surface to_surface has been used to model the contact between the impactor and the laminate and the contact_automatic_surface_to_surface_tiebreak to simulate delamination between layers. A good agreement had been shown between the finite element results and the experimental values obtained from the drop tower test. The impact load, impact energy, displacements and the maximal were reported as function of the impact time. The delamination area and the layer impact energy were represented as function of layer orientations. By increasing the impact velocity, the displacement, the delamination area, the contact load and the impact energy increases respectively. The circular notch had an effect on the displacement values; this values increases in respect of the impact velocities. At the reverse, the contact load decreases respectively in function of the velocities. The main purpose of the present investigation was to determine the damages generated by the low velocities with the help of the experimental method (Impact by a drop test) and the finite element method. The commercial transient finite element package LS-dyna used to model the effect of slug impactor and circular notch induced damage in composite material subjected to low velocity impact. The studied composite material was T800S/M21 made of carbon/epoxy. The effect of circular notch was examined. The composite material was subjected to stainless steel slug impactor in the transverse direction dropping the composite laminate at the center with three different velocities (2.85 m/s, 3.47 m/s and 4.17 m/s). The analysis was carried out using the model MAT59 for laminate and a rigid body model MAT20 for the slug impactor. The automatic_surface to_surface has been used to model the contact between the impactor and the laminate and the contact_automatic_surface_to_surface_tiebreak to simulate delamination between layers. A good agreement had been shown between the finite element results and the experimental values obtained from the drop tower test. The impact load, impact energy, displacements and the maximal were reported as function of the impact time. The delamination area and the layer impact energy were represented as function of layer orientations. By increasing the impact velocity, the displacement, the delamination area, the contact load and the impact energy increases respectively. The circular notch had an effect on the displacement values; this values increases in respect of the impact velocities. At the reverse, the contact load decreases respectively in function of the velocities.

An efficient computational framework for hailstone impacts on composite plates utilizing a semi-empirical viscoplastic contact law

Nonlinear damage in the composite materials is developed with the growth of damages in the material under fatigue loading. Nonlinear ultrasonic techniques are sensitive to early stage damages such as, fiber breakages, matrix micro-cracking, and deboning etc. Here, in this work, early stage damages are detected in Unidirectional (UD) carbon fiber composite under fatigue loading. Specimens are prepared according to American Society for Testing and Materials (ASTM) standard. Specimens are subjected to low cycle high load (LCHL) fatigue loading until 150,000 cycles. Sensors are mounted on the specimen used for actuation and sensing. A five count tone burst with low frequency (fc =375 kHz) followed by high frequency (fc =770 kHz) signal, was used as actuation signal. Pitch-catch experiments are collected at the interval of 5,000 cycles. Sensor signals are collected for various excitation voltage (from 5V to 20V, with 5V interval). First Fourier Transform (FFT) of the sensor signals are performed and side band frequencies are observed at around 770 kHz. Severity of damages in the material is quantified from the ratio of amplitude of side band frequencies with the central frequency. Nonlinearity in the material due to damage development is also investigated from the damage growth curve obtained at various excitation amplitude. Optical Microcopy imaging were also performed at the interval of 5,000 to examine developments of damages inside the material. This study has a good potential in detection of early stage damages in composite materials.

7 - Time-dependent nonlinear damage behavior of ceramic–matrix composites

10 - The fatigue behavior of composite materials for high-temperature applications

Matrix damage of fibrous composites: Effects of thermal residual stresses and layer sequences