Damage and Fracture of Composite Materials and Structures

Fiber-reinforced composites cover wide range of composite materials and structures including laminated panels of unidirectional plies, sandwich structures with fiber-reinforced composite skins and flexible core, and fiber-metal laminates. A typical advanced composite structure will likely experience quasi-static and dynamic loading. Under such damage-induced operating load conditions, reliability of the structure largely depends on the continual process of damage initiation and subsequent damage propagation to catastrophic fracture. The development of validated damage and failure models and availability of vast computing power would enable establishment of a validated framework for addressing structural reliability and safety issues of these composite structures.

Crack propagation in solid members is an important reason for structure failure. In recent years, many research interests are focused on intelligent control of crack propagation. With the rise in temperature, contraction of prestrained SMA fiber embedded in matrix makes retardation of crack propagation possible. However, with the rise in temperature, separation of SMA fiber from matrix is inevitable. This kind of separation weakens effect of SMA fiber on crack tip. To overcome de-bonding of SMA fiber from matrix, a knot is made on the fiber in this paper. By shape memory effect with the rise in temperature, the knotted SMA fiber generates a couple of recovery forces acting on the matrix at the two knots. This couple of recovery forces may restrain opening of the mode-I crack. Based on Tanaka constitutive law on SMA fiber and complex stress function near an elliptic hole under a point load, a theoretical model on mode-I crack is proposed. An analytical expression of relation between stress intensity factor (SIF) of mode-I crack closure and temperature is got. Simulation results show that stress intensity factor of mode-I crack closure decreases obviously with the rise in temperature higher than the austenite start temperature of SMA fiber, and that there is an optimal position for SMA fiber to restrain crack opening, which is behind the crack tip. Therefore the theoretical model supports that prestrained SMA fiber with knots in martensite can be used to control mode-I crack opening effectively because de-bonding between fiber and matrix is eliminated. Specimen of epoxy resin embedded with knotted SMA fiber can be made in experiment and is useful to an analytical study. However, in practical point of view, SMA fiber should be embedded in engineering structure material such as steel, aluminum, etc. The embedding process in these matrix materials should be studied systematically in the future.

Computational micromechanics models offer the possibility of analysis and quantification of the failure mechanisms that take place at the micro-scale level and are the responsible of the damage in the composite with high accuracy and with the need for very few hypotheses. Although these kind of analyses are common in current scientific literature, the analysis performed are generally limited to the stress/strain fields. This work makes use of a micromechanical model to analyze the crack tip and the cohesive zone of an interlaminar crack loaded in mode I for a carbon fiber reinforced polymer (CFRP). A periodic square fibre distribution is assumed and modelled in a FE environment and a degradation law is used to simulate damage in the matrix. This simulation allows both stress and strain quantification during crack opening and fracture mechanics analysis, such as the estimation of the critical value of the energy release rate and the quantification of the length of the cohesive zone, which is a parameter required for the application of cohesive elements.

According to statics and geometry of a random fiber crossing the cracked matrix surfaces, this work derived, at first, linear distribution of pressure provoked by the inclined fiber in the matrix and related the pressure to fiber axial force, bending moment and shear. To determine this pressure, self-consistent deformation model was proposed. With presuppose of constant interface fiber/matrix stress, the fiber and matrix displacements near the intersection of the fiber and the crack surfaces were calculated by means of integrating the Kelvin’s fundamental solution of and the Mindlin’s complementary solution, over a crack open of 0.001 mm. Then the stresses at points underneath of the inclined fiber were obtained using the same approach. The maxima normal stress in the fiber was calculated and compared with the fiber strength. Using failure criterion of five parameters for brittle materials, the spalling extent was determined. In the simulation, mechanical parameters of steel fiber and concrete were utilized and the percentages of active fibers were obtained with varying fiber strength and interface resistance. The results show that the percentage of active fibers increases with the fiber strength enhancement but decreases with the interface resistance increment. It draws a conclusion that the higher fiber strength and the lower interface resistance, more benefit to the debonding process. This work demonstrates that spalling effect is of great importance for fiber toughening in brittle matrix and the presented model allows for optimization of the parameters involved in toughening analysis.

The initiation and subsequent progression of delamination in CFRP composite laminates is examined using finite element method. A 12-ply CFRP composite, with a total thickness of 2.4 mm and anti-symmetric ply sequence is simulated under three-point bend test setup. Each unidirectional composite lamina is treated as an equivalent elastic and orthotropic panel. Interface behavior is defined using cohesive damage model. Complementary three-point bend test on the specimen is performed at crosshead speed of 2 mm/min. The measured load–deflection response at mid-span location compares well with predicted values. Interface delamination accounts for up to 46.7% reduction in flexural stiffness from the undamaged state. Delamination initiated at the center mid-span region for interfaces in the compressive laminates while edge delamination started in interfaces with tensile flexural stress in the laminates. Anti-symmetric distribution of the delaminated region is derived from the corresponding anti-symmetric ply sequence in the CFRP composite. The dissipation energy for edge delamination is greater than that for internal center delamination.

Improved high-order sandwich beam theory is used to model the local deformation under the central indenter for sandwich beams with Aluminum/Alumina FG skins loaded under three-point bending. First shear deformation theory (FSDT) is used for the FG skins while three-dimensional elasticity is used for the flexible core. By using the model to consider the way in which different wavelengths of sinusoidal pressure loading on the top FG skin are transmitted to the core and to the bottom FG skin, two spreading length scales λ_t and λ_b are introduced and calculated. λ_t and λ_b, which are two functions of the beam material and geometric properties, characterize the length over which a load on the top surface of a beam is spread out by the skins and the core. When semi-wavelength is greater than λ_t (or λ_b), the contact load at the top FG skin is transmitted relatively unchanged to the core (or to the bottom FG skin). Conversely, when L/m < λ_t (or λ_b), the applied load is spread out by the top FG skin (or by the top FG skin and the core) over a length of the order of λ_t (or λ_b). Reasonable agreement is found between theoretical predictions of the displacement field under the indentation loading and FEM results of ANSYS using a sandwich beam with functionally graded skins and transversely flexible core.

Injection-molded in situ anisotropic liquid crystalline polymeric (LCP) micro-fibres reinforcing polycarbonate (PC) composite are light-weight materials with ‘tailorable’ mechanical properties. Using a novel fibre-recruitment (FR) computer model, we have investigated the effects of compatibilization on the microstructure–property relationship in LCP-PC composites, focusing on the recruitment, pull-out and rupture of LCP fibres and LCP-PC interfacial failure. The model represents a parallel array of LCP fibres, of differing lengths and diameters to account for the natural variation, embedded in PC matrix. When an increasing external load acts on the composite, the fibres are recruited in tension. Initially, these fibres undergo linear elastic deformation. At a certain higher applied load, a fraction of the fibres yields and undergoes plastic deformation; eventually, a fraction of these fibres fractures. The FR model was used to evaluate the macroscopic structural and material properties of the fibre, such as fibre diameter, elastic modulus, yield and rupture (σ _r ) stresses.

The influence of moisture absorption on the interlaminar fracture behaviour of 8/8 harness satin weave glass/epoxy composite was investigated. Two series of specimens with 0°/0° and 90°/90° predominant interfaces immersed in water for different duration were tested under double cantilever beam (DCB mode I), single leg bending (SLB mode I + II) and end notched flexural (ENF mode II) loadings. In general, the apparent flexural modulus: E, and the fracture toughness: G _C, decrease with increasing moisture content. This effect is more remarkable if mode II participation is bigger. The value of G _C measured on 90°/90° specimens reveals higher than that on 0°/0° ones, but the variation in G _C is inversed under ENF loading. The experimental results have been correlated with a criterion previously proposed by the first author expressed by: \( G_{TC} = G_{IC} + (G_{IIC} - G_{IC} )\left( {\frac{{G_{II} }}{{G_{I} + G_{II} }}} \right)^{m}.\) A good agreement is shown with m = 2/3 at all moisture contents and interfaces. Regarding the R-curves under DCB loading, water absorption leads to a higher rate of increment in the resistance in the early crack growth. However, the maximum of the resistance to the crack growth decreases with the moisture content.

There are numerous applications for high impact resistant fiber-reinforced composite especially in aerospace, automotive, marine, and military fields. Impact damages of foreign objects on composite structures is of great importance, because may severely reduce their strength and stability. These impacts are ranging from low speed impacts like dropping of a tool to high speed impacts when small sands or debris throw away by aircraft tires. This research focuses on fiber reinforced composites, impact tests, failure modes and damage propagation through the composite during destructive impact load.

The Dynamic fracture toughness K_Id, is determined for unidirectional carbon-epoxy and glass-epoxy composite materials, by means of an experimental–numerical method. An instrumented Hopkinson bar is used to make the tests with pre-cracked specimens loaded on a three point bending configuration. Specimen receives a sudden impact load that generates the opening of the crack faces. Dynamic pulses registered on the incident and transmitted bars are used to determine the load history applied on the specimen. A strain gage is placed on the specimen to register the wave propagation and therefore to determine the onset of the crack growth. This load history is then used in a numerical analysis done by the ABAQUS software to determine the Dynamic Stress Intensity Factor time evolution. Knowing the time to fracture it is possible to estimate the Dynamic Fracture Toughness K_Id. For the composite material specimens, the tests were made for different impact velocities.

This paper reviews the advancement of study of composite laminate subjected to impact loading for aerospace application such as Glass fiber-epoxy and Carbon fiber-epoxy unidirectional laminate plate. The impact testing set-up and equipments are overviewed in this paper including the calculation to obtain the force, velocity and energy absorbed by the laminate during impact. The initial damage can be explained by the threshold of energy absorption while the successive damage mechanism can be described by the drop of force obtained from the history of force during impact. The corresponding evidence of damage related to force drop shows the scenario of damage mechanism; matrix cracking, delamination and fiber failure. The influence of projectile and laminate parameters to the impact behavior is reviewed as well. The failure criteria for each phase of damage mechanism are described in detail. The proposed elements model to simulate the damage are overviewed as well, which can be divided into two categories; element without interface layer and with interface layer integrating degradation of mechanical properties. The exposed failure criteria and elements model are the basis of today’s development of FEA simulation for impact purpose.

This project studied the response of sandwich panel subjected to blast loading. The panels were based on two different face sheets (aluminium and woven glass-fibre/epoxy) and an aluminium honeycomb core. Experimental studies were carried out to analyse the effect of skin and core thickness on the blast response of the panels. The sandwich panels with glass-fibre/epoxy face sheets exhibited delamination in the face skin and core crushing, whereas failure in the sandwich panels with aluminium skins involved permanent visible indentation and core crushing. It was concluded that the composite-skinned sandwich structures offered a superior blast resistance to the aluminium-skinned system.

The high velocity impact response of a range of polypropylene-based fibre-metal laminate (FML) structures has been investigated. Initial tests were conducted on simple FML sandwich structures based on 2024-O and 2024-T3 aluminium alloy skins and a Self-Reinforced Polypropylene (SRPP) composite core. Here, it was shown that laminates based on the stronger 2024-T3 alloy offered a superior perforation resistance to those based on the 2024-O system. Tests were also conducted on multi-layered materials in which the composite plies were dispersed between more than two aluminium sheets. For a given target thickness, the multi-layered laminates offered a superior perforation resistance to the sandwich laminates. The perforation resistances of the various laminates investigated here were compared by determining the specific perforation energy (s.p.e) of each system. Here, the sandwich FMLs based on the low density SRPP core out-performed the multi-layer systems, offering s.p.e.’s roughly double that exhibited by a similar Kevlar-based laminate. A closer examination of the panels highlighted a number of failure mechanisms such as ductile tearing, delamination and fibre failure in the composite plies as well as permanent plastic deformation, thinning and shear fracture in the metal layers. Finally, the perforation threshold of all of the FML structures was predicted using the Reid–Wen perforation model. Here, it was found that the predictions offered by this simple model were in good agreement with the experimental data.