Hygrothermal influence on delamination behavior of graphite/epoxy laminates

The hygrothermal effect on the fracture behavior of graphite/epoxy laminates has been investigated as part of an overall effort to develop a methodology for damage-tolerance predictions in advanced composite materials. Several T300/934 laminates were tested using a number of specimen configurations (double cantilever, compact tension, and cracked lap shear) in order to evaluate the effects of temperature and humidity on delamination fracture toughness under Mode I and Mode II loading. The specimens were exposed to different humidity levels and temperatures for varying periods of time prior to testing. The preexposed specimens were tested under room conditions, and fracture energies during initiation and propagation were estimated. Acoustic emission was used to detect crack initiation. It was indicated that moisture has a slightly beneficial influence on fracture toughness or critical strain energy release rate during Mode I delamination but a slightly deleterious effect on Mode II delamination and Mode I transverse cracking. The failed specimens were examined by scanning electron miscroscope (SEM) and topographical differences due to fracture modes were identified. The effect of moisture on fracture topography could not be distinguished.     

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.     

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.     

Interlaminar and intralaminar fracture surface morphology in graphite/epoxy laminates

Fracture surface morphological studies of graphite/epoxy (T300/934) laminates have been performed to distinguish between mode I and mode II interlaminar and intralaminar fractures. Additionally, the effect of a few hygrothermal conditions on mode I fracture surface is also assessed. Centre notched tension, compact tension, double cantilever beam and cracked lap shear specimens have been used to study these fracture modes. While mode I fracture is characterized by branched cracks between fibers, river pattern and chevron markings on resin rich zones, the epoxy hackles dominate mode II fracture surface. The interlaminar and intralaminar fractures can be distinguished by fiber spacings and some matrix features. The effect of increase in temperature and/or moisture is seen to cause reduction in matrix microcracking and increase in fiber pullout in mode I.     

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.     

Dynamic delamination fracture toughness of a graphite/epoxy laminate under impact

Dynamic delamination fracture toughness in a [90/0]_5s T300/934 graphite/epoxy laminate was investigated using impact loading. Delamination cracks of three different sizes were embedded at the mid-plane of the composite specimen. The threshold impact velocity that causes propagation of the delamination crack was used in the dynamic analysis with the finite element method. From the finite element solution, the time-history of the strain energy release rate was calculated. The critical strain energy release rate was taken to equal the maximum value of the response history.     

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.     

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.     

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 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.     

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.     

The role of microdamage in tensile failure of graphite/epoxy laminates

Damage development during quasistatic tensile loading of several laminates of graphite/epoxy material is examined and compared to damage development in laminates of a similar graphite/epoxy material subjected to tension-tension fatigue loading. Emphasis is placed upon following damage development at the microstructural level. Evidence of the important role of off-axis ply cracks in localizing and controlling fiber fracture in adjacent load-bearing plies for both loading modes is resented. The relationship between fiber fracture density and static load level is presented for tensile loading of unidirectional and cross-ply laminates by direct observation of fiber fracture in situ. The frequencies of occurrence of multiple adjacent fiber fractures are also reported. The cross-ply laminate results are compared with those from fatigue testing. Significant differences are described and discussed.     

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.     

Dynamic mixed-mode I/II delamination fracture and energy release rate of unidirectional graphite/epoxy composites

Mixed-mode open-notch flexure (MONF), anti-symmetric loaded end-notched flexure (MENF) and center-notched flexure (MCNF) specimens were used to investigate dynamic mixed I/II mode delamination fracture using a fracturing split Hopkinson pressure bar (F-SHPB). An expression for dynamic energy release rate G_d is formulated and evaluated. The experimental results show that dynamic delamination increases linearly with mode mixing. At low input energy E_i ? 4.0 J, the dynamic (G_d) and total (G_T) energy rates are independent of mixed-mode ratio. At higher impact energy of 4.0 ? E_i ? 9.3 J, G_d decreases slowly with mixed I/II mode ratio while G_T is observed to increase more rapidly. In general, G_d increases more rapidly with increasing delamination than with increasing energy absorbed. The results show that for the impact energy of 9.3 J before fragmentation of the plate, the effect of kinetic energy is not significant and should be neglected. For the same energy-absorption level, the delamination is greatest at low mixed-mode ratios corresponding to highest Mode II contribution. The results of energy release rates from MONF were compared with mixed-mode bending (MMB) formulation and show some agreement in Mode II but differences in prediction for Mode I. Hackle (Mode II) features on SEM photographs decrease as the impact energy is increased but increase as the Mode I/II ratio decreases. For the same loading conditions, more pure Mode II features are generated on the MCNF specimen fractured surfaces than the MENF and MONF specimens.     

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.     

Analysis of mode II fractured surfaces of graphite/epoxy laminates using SEM and optical reflectivity methods

This paper presents a study of scanning electron microscope surface morphology and the reflectivity of the mode II fractured surface of graphite/epoxy laminates. A discrete number average method was developed for the quantitative characterization of hackles formation. The variation of hackle marks linear density with impact energy for end‐notch flexure and centre‐notch flexure for pure mode II loading were determined. The results indicated that hackles formation decreased with impact energy and the maximum hackles formation were observed at the centre area. Reflectivity was observed to be increased with the angle of reflection and decreased with impact energy, which is in agreement with the scanning electron microscope analysis. With the incident light scanned in the transverse direction, the intensity of the reflected light increases much more exponentially compared with along the fibre direction. Reflectivity in the fibre direction was clearly lower than in the transverse direction in the damaged area. The greater the damaged area, the less the intensity (reflectivity) of light reflected.