Failure Modes Of Composite Materials Research Articles

Characterization on fatigue behavior and damage mechanism of C/BMI composites experienced thermal cycling

The multi-directional laminate CCF800H/AC631 bismaleimide composite material was exposed for a long time under the thermal-cycling environment (−60°C ∼+180°C), and the mass loss rate, FTIR spectra, DMA, tensile strength were tested. The fatigue stress level was determined according to the tensile strength and the fatigue performance of the before and after the thermal-cycling environment was tested. Macroscopic visual inspection and ultrasonic C-scan were used to characterize and analyze the fatigue damage of composite materials. The results show that with the increase in the number of thermal cycles, the mass loss of the composite material started with increased rapidly and then basically flat. The C/BMI composites underwent obvious thermal oxygen aging. After thermal-cycling, it would lead to changes in dynamic mechanical properties by a certain degree of post-curing, physical aging, and local interface debonding in composite materials. With the thermal cycles increased the composite material tensile strength first increased slightly and then decreased rapidly. After 300 thermal cycles, the composite materials occurred slightly damaged, and the fatigue life was apparently reduced compared with the original state. The fatigue failure modes of composite materials are mainly fiber fracture and multi-directional laminate delamination. At high stress levels, the stiffness of the specimen after thermal-cycling are lower decrease compared with original specimens, more stress levels would lead to more II stage rate of stiffness decline, and stiffness degradation curve and hysteretic energy recovery curve had enough effect to characterize damage effect of material environment induced by thermal-cycling environment factors.

Vol4 num4-7 Delamination behavior under mode I and mode II of adhesive bonded joints for different surface treatments

The automotive, aerospace and renewable energy generation industries have replaced significantly the use of traditional materials, such as steel and aluminum, by reinforced fiber composites. It has recognized that joints drilling and screwing joints damage the fibers introducing a higher concentration of stresses. That is why the application of structural adhesives, as a method of joining in reinforced composites, has increased in recent years. Delamination is the most critical failure mode of composite materials resulting in stiffness and structural strength losses. This works studies the behavior against delamination under mode I and mode II interlaminar fracture of adhesive bonded joints in carbon-epoxy composite materials, using structural adhesives of epoxy and acrylic bases. For the execution of the joints, different surface treatments have been employed: sanding, grit blasting and peel ply technique, allowing to make a fracture resistance comparision study. The double cantilever beam (DCB) and end notched flexure (ENF) tests configuration were used to determine stress-strain behavior and fracture toughness for mode I (GIC) and mode II (GIIC), respectively. Surface modifications have been evaluated by  contact angle measurements and scanning electron microscopy (SEM). The experimental results are shown in the same situation. The experimental results obtained show that, even if not showing the same delamination tendency for both fracture modes, the strain energy release rate are higher for an improved of wetting properties and an increase of surface roughness.

Impact induced dynamic response and failure behavior of aircraft structures

The impact induced dynamic response and the failure behavior of aircraft structures continues to be a critical aeronautical engineering challenge in order to improve the passenger survivability after aircraft. It is the objective of this special issue to provide a review of the work at the University of Naples Federico II on failure modes of composite materials and structures and on techniques for impact detection and damage characterization. This review consists of six papers, starting with a review of the bird impact process and the validation of the smooth particle hydrodynamics (SPH) impact model for aircraft structures, followed by a paper on the design of a passenger seat with improved safety while minimizing cost and weight in compliance with prescribed certification rules. Two papers are dedicated to the description of the impact behavior of composite laminates and of the ultrasound propagation in composite laminates. An additional paper presents an overview of experimental techniques for impact detection and of a novel method combining deep learning and wave propagation-based methods to sense impacts that induce excitation spectra at low (modal) or medium-high frequency range through piezo-patches installed on the structure. The final paper provides an overview of the analytical and numerical modelling techniques for guided wave propagation studies and strategies for the implementation of Structural Health Monitoring systems.

Study on Mechanical Bearing Strength and Failure Modes of Composite Materials for Marine Structures

With the gradual application of composite materials to ships and offshore structures, the structural strength of composites that can replace steel should be explored. In this study, the mechanical bearing strength and failure modes of a composite-to-metal joining structure connected by mechanically fastened joints were experimentally analyzed. The effects of the fiber tensile strength and stress concentration on the static bearing strength and failure modes of the composite structures were investigated. For the experiment, quasi-isotropic [45°/0°/–45°/90°]2S carbon fiber-reinforced plastic (CFRP) and glass fiber-reinforced plastic (GFRP) specimens were prepared with hole diameters of 5, 6, 8, and 10 mm. The experimental results showed that the average static bearing strength of the CFRP specimen was 30% or higher than that of the GFRP specimen. In terms of the failure mode of the mechanically fastened joint, a cleavage failure mode was observed in the GFRP specimen for hole diameters of 5 mm and 6 mm, whereas a net-tension failure mode was observed for hole diameters of 8 mm and 10 mm. Bearing failure occurred in the CFRP specimens.

Rationalized improvement of Tsai–Wu failure criterion considering different failure modes of composite materials

Different failure modes are not explicitly accounted for in the Tsai–Wu failure criterion for composite materials, and the factor F12 in the expression cannot be determined by the basic strength values of the materials. In view of these two shortcomings of the Tsai–Wu criterion, an improved criterion was proposed based on the reasonable assumption that composites exhibit infinite strength under pure hydrostatic pressure. Some coefficients in the quadratic tensor expression of the Tsai–Wu failure criterion are redetermined using the basic strength value of the material, including the coefficient F12, under four different stress states (fiber tension and compression, matrix tension and compression). The reconstructed Tsai–Wu failure criterion can distinguish the failure modes based on different coefficient values under different stress states. In the progressive damage analysis of composite materials, the stiffness can be reduced in different manners based on each failure mode. Experimental verification for different kinds of unidirectional composites under various stress states was conducted, demonstrating that the improved Tsai–Wu failure criterion has a better prediction ability and accuracy than the original criterion.

Acoustic emission characteristics of unidirectional glass/epoxy composites under mixed-mode fracture

Delamination is one of the most critical failure modes of composite materials and structures. An approach to avoid conservative designs is to use more damage tolerant composite materials. For this approach to be successful, a reliable structural health monitoring system is required to detect and identify damage during operation. In this paper, the link between mode mixity and acoustic emission characteristics is investigated. While there is a large body of research on analysis of acoustic emission for different materials and loading cases, the current work is focused on simple acoustic emission features and examine their dependence on a wide range of mode mixities. It is shown that there is a strong dependence of the acoustic emission characteristics on the mode mixity and in particular between mode I dominated and mode II dominated fracture. The results obtained could be used to develop tools to identify the type of delamination in composite structures during operation.

Mechanical behavior of composite structures subjected to constant slamming impact velocity: An experimental and numerical investigation

The interaction between deformable structures and free water surfaces can modify the fluid flow and change the estimated hydrodynamic loads in relation to rigid bodies, due to the appearance of hydroelastic effects. The flexibility and damage failure modes in composite materials introduce additional complexity for predicting hydrodynamic loads when interactive with water. This is considered to be a key challenge when using these materials in marine applications. Therefore, particular attention should be paid to this fact in the design phase and over their period of use. The aim of this work is to study the structural behavior and the effect of the flexibility of composite panels on hydrodynamic loads and the dynamic deformation response experimentally and numerically. To study these effects, composite panels with two different rigidities were subjected to various impact velocities and investigated. It should be noted that all the panels tested at a10° deadrise angle. A high velocity shock machine was used to maintain constant velocity during water entry at impact velocities of 4 m/s, 6 m/s, 8 m/s and 10 m/s. The general analysis of experimental results indicated that compared to the higher stiffness panels, the more flexible panel has a higher peak force as velocity increases. This has been attributed to the change in local velocity and local deadrise angle along the water-panel interface. The numerical model was implemented based on the Coupled Eulerian–Lagrangian Model (CEL) built-in Abaqus/Explicit finite element software. The numerical results showed a good agreement compared with experimental data for both the hydrodynamic force and the deformation response. These quantitative structural-loading data can provide a clear guide for maritime ship design.

Multiaxial fatigue life prediction of composite materials

In order to analyze the stress and strain fields in the fibers and the matrix in composite materials, a fiber-scale unit cell model is established and the corresponding periodical boundary conditions are introduced. Assuming matrix cracking as the failure mode of composite materials, an energy-based fatigue damage parameter and a multiaxial fatigue life prediction method are established. This method only needs the material properties of the fibers and the matrix to be known. After the relationship between the fatigue damage parameter and the fatigue life under any arbitrary test condition is established, the multiaxial fatigue life under any other load condition can be predicted. The proposed method has been verified using two different kinds of load forms. One is unidirectional laminates subjected to cyclic off-axis loading, and the other is filament wound composites subjected to cyclic tension-torsion loading. The fatigue lives predicted using the proposed model are in good agreements with the experimental results for both kinds of load forms.

Failure Analysis of Fibre-Reinforced Composite Laminates

Experimentalinvestigation of failure modes of composite materials requirescorrelating the fundamentals of composite materials, their mechanicalproperties as well as their failure characteristics in the presence of defects. In this paper, three formats of defects of hole (single, double & quadruple) as a discontinuity were incorporated in modal analysisusing mode shape frequency. Unique failure modes of these specimens provided overview regarding mechanical behaviour of composite materials containing defects. Certain correlations were observed between defects and resulting properties.

Stress analysis in FRP composites

Summary A composite material, in mechanics sense, is a structure with the ingredients as element transferring forces to adjacent members. The issue of defects and their effect on the mechanical properties of composites is of great concern among high end users. Experimental investigation of failure modes of composite materials requires correlating the fundamentals of composite materials, their mechanical properties as well as their failure characteristics in the presence of defects. In this paper, three formats of defects of hole (single, double and quadruple) as a discontinuity were incorporated along with tensile testing. Unique failure modes of these specimens provided overview regarding mechanical behaviour of composite materials containing defects. Certain correlations were observed between defects and resulting properties. Results are in agreement with general behaviour of FRP composite laminates and it can be concluded that for low deformation in composite laminates, number of layers must be increased, which at the same time results in increase of von-Mises stress. Fibres are the main constituents which are responsible for strength of a composite laminate and they along with fibre orientation, play an important role on its load bearing capacity. It can be inferred based on the analysis that cross-ply configuration [0°/90°] has good load bearing capacity as well as least deflection emphasizing more strength.

Nonconvex bundle method with application to a delamination problem

Delamination is a typical failure mode of composite materials caused by weak bonding. It arises when a crack initiates and propagates under a destructive loading. Given the physical law characterizing the properties of the interlayer adhesive between the bonded bodies, we consider the problem of computing the propagation of the crack front and the stress field along the contact boundary. This leads to a hemivariational inequality, which after discretization by finite elements we solve by a nonconvex bundle method, where upper-$C^1$ criteria have to be minimized. As this is in contrast with other classes of mechanical problems with non-monotone friction laws and in other applied fields, where criteria are typically lower-$C^1$, we propose a bundle method suited for both types of nonsmoothness. We prove its global convergence in the sense of subsequences and test it on a typical delamination problem of material sciences.

The behavior of delaminations in composite materials - experimental results

Delamination is one of the most common failure modes of composite materials. It may occur as a consequence of imperfections in the production process or the effects of external factors during the operational life of the composite laminates, such as the impact by foreign objects. This paper presents the results of mechanical tests and the optimum degrees of filling the composite materials (CM) with hydrophobic powder (Tarkosil T-20) depending on the latter mass concentration. The results present test samples of the CM with the underlying interlayer defects. The samples were fabricated of twenty-ply pre-preg (fiberglass or carbon fiber). The industrial grade glass is T-25 (VM) specification 6-11-380-76. The composite materials have nanosized additives in structure. The volume concentration of nanopowders is varying from 0.1% to 0.5%. This kind of research has been done for the first time.

Acoustic emission-based study to characterize the initiation of delamination in composite materials

Among the failure modes of composite materials, delamination is the most striking one. It may result in the reduction of stiffness and long-term performance of composite materials. The initiation stage is very crucial in delamination. This is because initiation stage requires higher amount of force, and after this stage, there is a stable propagation of delamination that requires little force and causes the failure of the structure. Initiation of delamination consists of two main stages including minor initiation and major initiation. These two stages are vital stages in the delamination process. Understanding the delamination behavior and their internal micro events in these stages may lead to better design and result in enhanced strength against crack initiation. This article proposes a combined method to discriminate various fracture mechanisms in the initiation of delamination. The combination of sentry function analysis and frequency analysis of acoustic emission (AE) waveforms are used to analyze crack initiation and to discriminate its internal micro failures during mode I delamination. Discrimination process is derived from the power spectrum density of AE waveform in distinct frequency intervals. Scanning electron microscopy (SEM) observation was used to determine different fracture mechanisms and associate these observed fracture mechanisms and their equivalent AE signal frequency range. It is shown that this combined method is a powerful device to study the behavior of the initiation of delamination.

Damage Categorization of Glass/Epoxy Composite Material Under Mode II Delamination Using Acoustic Emission Data: A Clustering Approach to Elucidate Wavelet Transformation Analysis

Delamination is the most common failure mode in composite materials, since it will result in the reduction of stiffness and can grow throughout other layers. Delamination consists of two main stages: initiation and propagation. Understanding the behavior of the material in these zones is imperative, hence the identification of this mode of failure is of great importance. There are several methods to identify damage in materials, one of which is using acoustic emission (AE) signals. Most of the pervious works have used statistical methods based on the energy of AE signals, but in this study, the normalized form of shock wave is used. The aim of this study is to extract a general pattern for specific damage from AE signals including all of the other damage signals. The method consists of a discrete wavelet packet decomposition of AE signals accompanied with a clustering algorithm, which gives the distribution of the normalized AE signal energy on the frequency band. Test set-up involved End Notched Flexure (ENF) test to detect mode II delamination on glass/epoxy composite material. The data obtained from ENF test specimens is used for the wavelet packet decomposition, and the energy of different levels of decomposition for each shock wave is clustered using different clustering algorithms including K-means and Fuzzy C-mean. Scanning Electron Microscope was used to validate the results.

Acoustic Emission based on sentry function to monitor the initiation of delamination in composite materials

Delamination is the most common failure mode in composite materials, since it will result in the reduction of stiffness and can grow throughout other layers. Delamination is consisted of two main stages including initiation and propagation. Understanding the behavior of the material in these zones is very important, hence it has been thoroughly studied by different methods such as numerical methods, Acoustic Emission (AE), and modeling. Between these two regions initiation is a more vital stage in the delamination of the material. Once initiation occurs, which normally requires greater amount of force, cracks can easily propagate through the structure with little force and cause the failure of the structure. A better knowledge of initiation can lead to better design and production of stronger materials. Additionally, more knowledge about crack initiation and its internal microevents would help improve other parameters and result in higher strength against crack initiation. AE is a suitable method for in situ monitoring of damage in composite materials. In this study, AE was applied to test different glass/epoxy specimens which were loaded under mode I delamination. A function that combines AE and mechanical information is employed to investigate the initiation of delamination. Scanning electron microscope (SEM) was used to verify the results of this function. It is shown that this method is an appropriate technique to monitor the behavior of the initiation of delamination.

Computationally optimized formulation for the simulation of composite materials and delamination failures

The numerical simulation of complex failure modes of composite materials, such as delamination, can be computationally very demanding, as it requires special elements and/or numerical strategies to characterize damage onset and propagation. This work presents several formulations developed to optimize the computational performance of an explicit finite element code designed specifically for the simulation of large scale composite structures. The composite mechanical performance is obtained with the matrix-reinforced mixing theory, a simplified version of the serial/parallel mixing theory that does not require an iterative procedure or the calculation of the tangent stiffness matrix. The number of elements required to perform the simulation is reduced by stacking several layers inside a single finite element. This work also proposes a modification of the isotropic damage law, capable of taking into account the residual strength provided by friction in type II fracture modes. The ability of these formulations to successfully predict the mechanical performance of composite materials is assessed with the ply drop-off test. In this test a laminate with a change of thickness in its mid-span is loaded until it breaks due to a delamination process. The formulation proposed obtains a very accurate prediction of the experimental response of the test, as it provides a very good characterization of the initial laminate stiffness, the delamination onset, and its propagation along the specimen.

Postbuckling behavior of composite plates with through-the-width delaminations

Fiber-reinforced composite materials have been increasingly used over the past few decades in a variety of applications in which a fairly high ratio of stiffness/strength to weight is required. However, these materials are prone to a wide range of defects and damage that can cause significant reductions in stiffness and strength. Delamination is one of the most common failure modes in composite materials which affect the overall stiffness of the structure. In this paper, the compressive behavior of composite laminates with through-the-width delaminations is investigated analytically. The analytical method is based on the CLPT theory, and its formulation is developed on the basis of the Rayleigh–Ritz approximation technique to analyze the bucking and postbuckling behavior of delaminated laminates. The method can handle both local buckling of the delaminated sublaminate and global buckling of the whole plate. Also the three-dimensional finite element analysis is performed by using ANSYS5.4 general purpose commercial software, and the results are compared with those obtained by the analytical model. The agreement between the results is very good.

Initiation and growth of delamination in glass/epoxy composites subjected to static and dynamic loading by acoustic emission monitoring

Delamination is one of the most common failure modes of composite materials. It may result from imperfections in the production process or may be caused by service life conditions, such as impact by foreign objects. The presence of delamination in the composite material may reduce the overall stiffness. The geometrical parameters, material proprieties, and loading conditions are important factors which affect the initiation, development and growth of delamination (Chen HP, David L. Comp. Sci. and Technol. 1993; 46:325–33). Experimental results obtained from monotonic tests and instrumented drop tests have shown good agreement between monotonic and dynamic critical load and crack-growth data. Previous studies made on different composite materials (Meraghni F, Benmedakhane S, Benzeggagh ML. In: Poursartip A, Street K, editors. Proc. ICCM 10, vol. I Vancouver, Canada: Woodhead Publishing, 1995:359–66; Barre S, Benzeggagh ML. Comp. Sci. and Technol. 1994; 54:369–76; Benzeggagh ML, Benmedakhene S. Comp. Sci. and Technol. 1995; 55:1–11) have allowed us to set up a schematic model of acoustic emission. In this model the different levels of amplitude signals emitted by materials under different types of loading are assigned to different damage mechanisms. This experimental methodology has been applied to this work and has allowed continuous monitoring of damage growth through delamination tests. This type of analysis has also allowed us to highlight the complementary aspect of acoustic-emission amplitude distributions with microscopic observations.

An acoustic-emission characterization of the failure modes in polymer-composite materials

A new methodology for the analysis of failure modes in composite materials by means of acoustic emission techniques has been developed. A single-carbon-fiber composite based on a polyester matrix, has been used as a simple model. The occurrence of fiber-breakage during tensile loading tests has been observed by a polarized light microscope and concurrently detected by a resonant acoustic probe. The resonant probe has been used as a trigger for the reading of fiber failure events. Single acoustic emission events from a wide-band probe has been recorded for FFT Analysis. The single-fiber specimen, having a unique failure mode, has advantages for the standardization of AE techniques for the quantitative analysis of failures in polymer-composite materials. The same procedure can be exploited to investigate other failure modes namely, fiber matrix solidus debonding and matrix cracking.

Void/Bridged-Crack Interactions and Their Implications for Defect Coalescence

Void-crack interactions with crack bridging play an important role in the determination of rupture failure modes of composite materials. In this paper, an integral equation approach is developed to study the behavior of systems containing multiple in teracting voids and cracks with a general form of crack bridging. The formulation is based on a superposition technique that decompose the interacting voids and cracks into a number of subproblems, each involving only a single void or single crack. The integral equations are fully non-singular and thus can be solved effectively with a Gauss integra tion scheme. Numerical examples are given in order to study the effects of bridging, bridg ing anisotropy, void size, and void spacing, among others, on the crack-tip behavior. The influence of void-crack interactions on the fracture path are also investigated. For a material containing doubly periodic collinear holes arranged with offsets in subsequent rows, it is found that the fracture path can be formed either by the coalescence of the holes in a row or by the coalescence of the holes across neighboring rows, depending on the ratio of vertical to horizontal void spacing.