Fracture behavior of cross-ply graphite/ epoxy laminates

Fracture behavior of cross-ply (0/90)_4s, (0/90)_10s, (0₂/90₂)_2s and (0₄/90₈/0₄)_T laminates of T300/934 graphite/epoxy material was studied using compact tension specimens of several widths and thicknesses, center notched tension and three point bend specimens. The process of damage initiation and propagation was studied and is discussed in detail. The critical stress intensity factor was evaluated and its variation with specimen size and type is shown. On the basis of these investigations, a suitable specimen for the evaluation of meaningful fracture toughness is suggested.     

The detailed structure of delamination fracture surfaces in graphite/epoxy laminates

Delamination fracture surfaces of angle-ply graphite/epoxy specimens failed in tension were studied in the SEM. The fracture surfaces contain resin-rich and resin-poor areas, with the former showing imprints from fibres while the latter show debonded fibres. A delamination crack propagates in the outer parts of the plies adjacent to the resin-rich interlaminar region and alternates from one ply to the adjacent ply and back as it propagates. The serrations formed in between fibres in the outermost parts of plies are tilted. The sense of the tilt is in agreement with the direction of the major tensile principal stress. The delamination is suggested to proceed in steps including debonding of fibres, crack opening by the major principal tensile stress and linking together of these fractures.     

Fatigue life estimation of graphite/epoxy laminates under consideration of delamination growth

A closed form approach to the assessment of the fatigue life of graphite/epoxy laminates under cyclic tension–compression loading has been developed. The model is mechanistic and uses cyclic energy release rates for prediction of delamination growth and of critical delamination sizes which induce buckling and the final failure of the laminates. Tests performed with graphite/epoxy specimens of stacking order [0_n, ?_m]_s with severed central plies [?], and of stacking order [0₂, +45, 0₂, ?45, 0, 90]_s with a central unloaded hole, indicate good correlation between estimated values and observed delamination growth, critical buckling strength of separated plies and load cycles to failure.     

High performance estimations of delamination of graphite/epoxy laminates with electric resistance change method

Delamination of laminated composites is usually invisible or difficult to detect visually. Delamination causes low reliability for primary structures. Automatic systems for in-service delamination identifications are desired to improve low reliability. The present study employs an electric resistance change method for delamination detection. Since the method adopts reinforcement carbon fiber itself as sensors for delamination detection, this method does not reduce static or fatigue strength; also, the method is applicable to existing structures. Authors have found that the electric resistance change method with response surfaces is very effective experimentally and analytically. However, a large error of estimation remains for estimation of delamination location. In the present study, a new data processing procedure is proposed to improve performance of estimations of delamination location. The new method is applied to laminated composite beams. A delamination crack of a laminated composite beam is monitored with the new method using FEM analyses. As a result, the method reveals excellent performance of estimations of delamination location even for new data not used in regression equations.     

Nonlinear analysis of interlaminar stress in graphite/epoxy laminates with and without delamination cracks

A quasi three-dimensional yield function, which is quadratic in stresses except for σ₁₁, is proposed for graphite/epoxy composites. The elastic-plastic interlaminar stress response near a free edge in the [90/0]_s, [0/90]_s, and [45/−45]_s laminates with and without delamination cracks was investigated using the pseudo three-dimensional finite element technique. The plasticity model was evaluated by comparison with off-axis experimental data. Since shear response is the key element for nonlinear stress-strain behavior of graphite/epoxy composites, the plasticity theory predicts interlaminar stresses in the [45/−45]_s laminate significantly different from linear elasticity. Moreover, the existence of a delamination crack caused more plasticity effects on interlaminar stresses.     

Moisture absorption in graphite/epoxy laminates

This work investigates the uptake of moisture by XAS-914C and AS4/3501-6 graphite/epoxy composite laminates under a range of temperature and humidity conditions. From the experiments, the diffusion coefficients of moisture in a number of laminates have been determined, together with the equilibrium moisture levels in the matrix.

The effects of thermal spiking (thermal transients simulating supersonic heating of military aircraft) on moisture uptake were determined experimentally using a number of combinations of spiking and moisturising conditions. Thermal spiking was found to be damaging when moisture was present in the laminate, and spiking during moisturising was found to be more severe than thermal spiking of material conditioned to equilibrium moisture levels. The pronounced effects of repeated thermal spiking on moisture uptake suggest a progressive form of spiking damage, and a simple model is proposed of a damage zone progressing through the laminate during successive moisturising then spiking cycles.     

Mode I fatigue delamination of Zanchor-reinforced CF/epoxy laminates

The Zanchor process is a novel through-thickness reinforcement technique in which in-plane yarns are entangled with each other using special needles. Mode I interlaminar fatigue crack growth behavior was investigated in carbon fiber (CF)/epoxy cross-ply laminates with Zanchor reinforcement. The laminates were molded with a Zanchor-reinforced CF dry fabric through resin film infusion (RFI). Delamination fatigue tests were carried out using double cantilever beam (DCB) specimens. The threshold values of the maximum energy release rates, G_Imaxth, under R = 0.1 were 70 J/m² for Zanchor 0 (base laminate without Zanchor reinforcement) and 240 J/m² for Zancor 2 (the density of Zanchor reinforcement is twice as high as the unit density), respectively; those under R = 0.5 were 80 J/m² for Zanchor 0 and 400 J/m² for Zancor 2, respectively. Thus, the threshold values for Zanchor 2 were about 3.4–5 times higher than those without Zanchor reinforcement. This increase induced by Zanchor reinforcement is almost the same or higher than that obtained under static loading (3.5 times). It is common that the increase in the fracture toughness, G_Ic, induced by replacing the matrix resin with a tougher system only partially contributes to the increase in the fatigue threshold, G_Imaxth. On the other hand, the increase in G_Ic induced by Zanchor reinforcement was fully translated to the increase in G_Imaxth. This is why Zanchor 2 gives one of the highest fatigue threshold values among existing toughened composite material systems. The difference between the reinforcing effects under static and fatigue loadings was discussed in conjunction with the microscopic fracture mechanism.     

Ballistic impact behaviour of stitched graphite/epoxy laminates

Four stitched graphite/epoxy laminates of different thicknesses were subjected to high-velocity impact tests. Two steel spheres, 12.7 and 20 mm in diameter, were used as bullets during the tests, carried out at two different speeds (65 and 129 m/s). Perforation occurred only under some of the experimental conditions adopted, whereas rebound was verified in other cases. As expected, the perforation energy increased with increasing the panel thickness and projectile diameter.In order to predict the perforation energy as a function of target thickness and bullet diameter, the Reid and Wen model and the Cantwell and Morton model were used. The Cantwell and Morton model was suitably modified: a new hypothesis was made, and an easier formulation, needing a single constant to be experimentally determined, was obtained.The dependence of perforation energy on laminate thickness was well described by the models considered, which provided very similar predictions. The same happened for the rebound conditions. However, only the modified Cantwell and Morton formula was effective in modelling the influence of the impactor diameter, while the Reid and Wen model provided a theoretical value about 50% higher than the measured one.The delaminated area was measured by ultrasonic C-scan in pulse-echo mode. It was found that a linear relationship links the delamination extent and the maximum energy absorbed by the panel, whichever the specimen thickness and projectile diameter. This correlation becomes ineffective for thick laminates, probably because of a change in failure modes occurring at sufficiently high thicknesses.     

Hygrothermal influence on mode I edge delamination in composites

A shear deformation theory is developed to calculate the interlaminar stresses and energy release rate associated with mid-plane edge delamination growth in graphite/epoxy laminates. The analysis includes the effect of residual thermal and moisture stresses. Results indicate that residual thermal stress increases the interlaminar stresses and strain energy release rate, while moisture content tends to alleviate thermal effects. Moreover, the value of moisture content at which the interlaminar stresses and strain energy release rate are totally alleviated is not affected by the stacking sequence.     

Intralaminar and interlaminar fracture in graphite/epoxy laminates

Various matrix failure modes (intralaminar and interlaminar) in T300/934 graphite/ epoxy laminates are studied. The intralaminar mode is considered by using centre-notchedtension. surface-notched-tension, three-point-bend and compact-tension specimens where transverse fracture toughness and 0° split initiation are investigated. The interlaminar fracture is studied by using double-cantilever-beam and cracked-lap-shear specimens for mode 1 and mode 2 respectively. A simple method for the prediction of split initiation is given and it is seen that the predicted and experimental results agree well. In addition to testing the dry specimens. a few hygrothermal conditions are also used to assess the influence of environment on various failure modes. The effect of environment shows a mixed trend on fracture toughness depending on damage mechanisms involved in the failure modes. The moisture and temperature show a deleterious effect on interface-controlled failure modes, but a beneficial effect on the modes controlled by matrix cleavage.     

Hygrothermal effects on the dynamic compressive properties of graphite/epoxy composite material

The effects of temperature and moisture on the response of graphite/epoxy laminated composites to high strain rate penetration loading using the Split Hopkinson Pressure Bar apparatus was investigated. The results show that in the thickness direction loading under extreme temperature, moisture and combined moisture and temperature conditions, the compressive strength, elastic modulus, and energy absorbed decrease exponentially. Failure strain and displacement increase linearly with temperature and moisture with particle velocity increasing linearly with temperature but independent of moisture content. The combined effect of temperature and moisture on the damage process was more apparent than the effect of temperature or moisture acting alone. At the same impact energy, the results show the failure properties to be sensitive to the strain rate, with energy absorbed increasing linearly with strain rate at low temperature and remaining relatively constant at high temperature. The compressive yield strength increases as the strain rate increases both at low and high temperatures while the ultimate strength (maximum strength) decreases slightly with strain rate.     

Thermal damage effects on delamination toughness of a graphite/epoxy composite

The effects of overheating AS/3501-6 graphite/epoxy composite material have been examined after exposures in air up to 350°C. Degradation was assessed from measurements of the interlaminar fracture toughness in both Mode I tension and Mode II shear as well as the interlaminar shear strength and hardness. Exposures up to 30 minutes at 225°C and up to 15 minutes at 300°C did not significantly reduce the interlaminar toughness. However, longer exposure at 300°C and short exposures at 350°C produced catastrophic decreases in toughness values, shear strength and hardness. The primary origin of embrittlement was attributed to deterioration of the epoxy as opposed to any fibre or fibre-matrix interfacial weakening.     

The micromechanics of fatigue-induced delamination of a graphite/epoxy composite

Fatigue crack growth in the resin layer between 0 and 90 plies of an AS/3501-5A graphite fibre/epoxy composite is discontinuous. Regularly spaced extensions of the crack front occur after periods of arrest. Crack compliance and tip strain fields have been measured to determine how the local minimum (K _min ^l) and maximum (K _max ^l) crack tip stress intensities affect growth. Contact of the fracture surfaces and swelling of the 90° ply modify these local stress intensities by an amount sensitive to load ratio (R), and the resulting propagation rate depends strongly onR. A model capable of describing thisR effect relates the distance of each individual crack advance to K_max ^l and the duration of each arrest toK _mnax ^l -K _min ^l, i.e., to K _eff. We discuss the genesis of this model, and its explanation of the large Paris law coefficient which results if growth rates are instead expressed against the applied cyclic stress intensity.     

Characterization of delamination behavior of unidirectional graphite/PEEK laminates using cracked lap shear (CLS) specimens

Delamination failure criterion is an important tool for characterizing the fracture behavior of laminated composites under mixed loading. In this paper, a fracture envelope was built based on the energy release rate as a fracture criterion of graphite/PEEK laminates. Unidirectional cracked lap shear (CLS) specimens were employed to calculate mode I and mode II energy release rates (G_I, G_II. Static fracture tests were conducted using the specimens with two different lap to strap thickness ratios in order to obtain a wide range of G_I/G_II values. The G_I/G_II values for each thickness ratio were calculated numerically using finite element analysis. The results showed that as a delamination length changes, the G_I/G_II varies from 0.13 to 0.48 depending on the lap to strap thickness ratio. It was also found that a linear fracture envelope may be appropriate for a CLS composite specimen.     

Compression failures of damaged graphite epoxy laminates

This paper describes the results of a joint numerical and experimental investigation into effects of delamination and impact damage on the compressive strength of graphite epoxy laminates. The numerical analysis predicts that as the size of the damage increases a stage is reached after which any further significant increase in the damage results in only a relatively small decrease in the residual compressive strength.     

Strength prediction of bolted joints in graphite/epoxy composite laminates

A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.