An analytical and experimental investigation was carried out on G40-800/R6376 graphite/epoxy laminates to evaluate the edge crack torsion (ECT) test as a candidate for a standard Mode III interlaminar fracture toughness test for laminated composites. The ECT test consists of a [90∕(±45)3∕(∓45)3∕90]s laminate with a delamination introduced by a non-adhesive film at the mid-plane along one edge and loaded in a special fixture to create torsion along the length of the laminate. Dye penetrant enhanced X-radiograph of failed specimens revealed that the delamination initiated at the middle of the specimen length and propagated in a self-similar manner along the laminate midplane. A three-dimensional finite element analysis was performed that indicated that a pure Mode III delamination exists at the middle of specimen length away from both ends. At the ends near the loading point a small Mode II component exists. However, the magnitude of this Mode II strain energy release rate at the loading point is small compared to the magnitude of Mode III component in the mid-section of the specimen. Hence, the ECT test yielded the desired Mode III delamination. The Mode III fracture toughness was obtained from a compliance calibration method that was in good agreement with the finite element results. Mode II end-notched flexure (ENF) tests and Mode I double cantilever beam (DCB) tests were also performed for the same composite material. The Mode I fracture toughness is much smaller than both the Mode II and Mode III fracture toughness. The Mode II fracture toughness is found to be 75% of the Mode III fracture toughness.

Hat stringer pull-off tests were performed to evaluate the delamination failure mechanisms in the flange region for a rod-reinforced hat stringer section. A special test fixture was used to pull the hat off the stringer while reacting the pull-off load through roller supports at both stringer flanges. Microscopic examinations of the failed specimens revealed that failure occurred at the ply termination in the flange area where the flange of the stiffener is built up by adding 451-45 tape plies on the top surface. Test results indicated that the as-manufactured microstructure in the flange region has a strong influence on the delamination initiation and the associated pull-off loads. Finite element models were created for each specimen with a detailed mesh based on micrographs of the critical location. A fracture mechanics approach and a mixed mode delamination criterion were used to predict the onset of delamination and the pull-off load. By modeling the critical local details of each specimen from micrographs, the model was able to accurately predict the hat stringer pull-off loads and replicate the variability in the test results.

Simplified expressions for the parameter controlling the load-point compliance and strain energy release rate were obtained for the edge crack torsion (ECT) specimen for Mode III interlaminar fracture toughness. Data reduction methods for Mode III toughness based on the present analysis are proposed. The effect of the transverse shear modulus G23 on Mode III interlaminar fracture toughness characterization was evaluated. Parameters influenced by the transverse shear modulus were identified. Analytical results indicate that a higher value of G23 results in a lower load point compliance and lower Mode III toughness estimation. The effect of G23 on the Mode III toughness using the ECT specimen is negligible when an appropriate initial delamination length is chosen. A conservative estimation of the Mode III toughness can be obtained by assuming G23 = G12 for any initial delamination length.

Two different families of graphite/epoxy laminates with similar layups but different stacking sequences, (0θ/-θ) s and (-θ/θ/0) s , laminates, were analyzed using three-dimensional finite element analysis for 0 = 15 and 30 degrees. Delaminations were modeled in the -θ/θ interface, bounded by a matrix crack and the stress free edge. The total strain energy release rate, G, along the delamination front was computed using three different techniques: the virtual crack closure technique (VCCI), the equivalent domain integral (EDI) technique, and a global energy balance technique. The opening fracture mode component of the strain energy release rate, G 1 , along the delamination front was also computed for various delamination lengths using VCCT. Although the finite element model did not have an orthogonal mesh, VCCT still yielded accurate results which were in agreement with the global energy balance and yielded similar G distributions across the delamination front as the EDI technique. For both layups analyzed, the matrix crack length influenced the magnitude of G for delamination. Furthermore, the opening mode, G., was greatest near the matrix crack and decreased near the free edge. The laminate stacking sequences with a matrix crack in the surface angle ply had a greater G I value than the laminate stacking sequences with an angle ply matrix crack in the interior of the specimen thickness. This is consistent with test results in the literature that show delamination occurs earlier in the fatigue life of laminates with matrix cracks in the surface plies than in the interior plies.

Symmetric tapered laminates with internally dropped plies were tested with two different layups and two materials, S2/SP250 glass/epoxy and IM6/18271 graphite/epoxy. The specimens were loaded in cyclic tension until they delaminated unstably. Each combination of material and layup had a unique failure mode. Calculated values of strain energy release rate, G, from a finite element analysis model of delamination along the taper, and for delamination from a matrix ply crack, were used with mode I fatigue characterization data from tests of the tested materials to calculate expected delamination onset loads. Calculated values were compared to the experimental results. The comparison showed that when the calculated G was chosen according to the observed delamination failures, the agreement between the calculated and measured delamination onset loads was reasonable for each combination of layup and material.

The influence of material volume on the transverse tensile strength of AS4/3501-6 graphite/epoxy composites was investigated. Tension tests of 90° laminates with three different widths and five different thicknesses were conducted. A finite element analysis was performed to determine the influence of the grip on the stress distribution in the coupons and explain the tendency for the distribution of failure locations to be skewed toward the grip. Specimens were instrumented with strain gages and extensometers to ensure good alignment and to measure failure strains. Data indicated that matrix dominated strength properties varied with the volume of material that was stressed, with the strength decreasing as volume increased. Transverse strength data were used in a volumetric scaling law based on Weibull statistics to predict the strength of 90° laminates loaded in three-point bending. Comparisons were also made between transverse strength measurements and out-of-plane interlaminar tensile strength measurements from curved beam bending tests. The significance of observed scale effects on the use of tests for material screening, quality assurance, and design allowables is discussed.

This report summarizes the results of several interlaboratory “round robin” test programs for measuring the Mode I interlaminar fracture toughness of advanced fiber-reinforced composite materials. Double cantilever beam (DCB) tests were conducted by participants in ASTM Committee D-30 on High Modulus Fibers and Their Composites and by representatives of the European Group on Fracture (EGF) and the Japanese Industrial Standards Group (JIS). DCB tests were performed on three AS4 carbon fiber-reinforced composite materials: AS4/3501-6 with a brittle epoxy matrix, AS4/BP907 with a tough epoxy matrix, and AS4/PEEK with a tough thermoplastic matrix. Difficulties encountered in manufacturing panels, as well as conducting the tests, are discussed. Critical issues that developed during the course of the testing are highlighted. Results of the round robin testing used to determine the precision of the ASTM DCB test standard are summarized.

An analysis using laminated plate theory is developed to calculate the strain energy release rate associated with local delaminations originating at off-axis, angle-ply, matrix cracks in laminates subjected to uniaxial loads. The analysis includes the contribution of residual thermal and moisture stresses to the strain energy released. Examples are calculated for the strain energy release rate associated with local delaminations originating at 90° and angle-ply (non 90°) matrix-ply cracks in glass-epoxy and graphite-epoxy laminates. The solution developed may be used to assess the relative contribution of mechanical, residual thermal, and moisture stresses on the strain energy release rate for local delamination for a variety of layups and materials.

An experimental and analytical study was conducted on the influence of matrix cracking on delamination in (+ 15/90n/-15)s glass-epoxy laminates subjected to monotonically increasing tension loads. Four different layups are examined with the number of symmetric 90° layers varying from 0 to 3 (n = 0, 1, 2, 3). Experimental results show that local delaminations form at the intersection of matrix cracks in the + 15° plies and the free edge. A quasi-three-dimensional (Q3D) finite-element analysis is used to examine free-edge stresses in undamaged laminates. Q3D results show that: (1) in-plane transverse tensile stresses exist in the +15° plies near the free edges of all of the laminates and (2) only the interlaminar shear stress is high at the + 15/θ interface. Three-dimensional (3D) finite-element analyses are used to model a + 15° ply crack near the laminate free edge. The 3D analysis results indicate that large tensile interlaminar normal, as well as shear, stresses develop at the intersection of the matrix crack and the free edge. This suggests that the interlaminar normal stress plays a significant role in the failure of these laminates and emphasizes the importance of modeling matrix cracking in delamination analyses of composite structures.

Analytical methods such as simple sublaminate analysis, as well as quasi three-dimensional finite element analysis, provide insight into delamination characteristics and reliable trend data with which to make rapid evaluations of competitive concepts in the design environment. It is concluded that free edge delamination can be managed by analysis and prevented by design. However, in order to ensure durable composite structures, more work is needed to characterize delamination that occurs from other sources, such as matrix cracking and ply drops.

A tapered composite laminate subjected to tension load was analyzed using the finite element method. The {[07/(±45)]/1[(±45)3]/[0/(±45)/0]}5 glass/epoxy laminate has a (±45)3 group of plies dropped in three distinct steps, each 20 ply-thicknesses apart, thus forming a taper angle of 5.71 degrees. Steep gradients of interlaminar normal and shear stress on a potential delamination interface suggest the existence of stress singularities at the points of material and geometric discontinuities created by the internal plydrops. The delamination was assumed to initiate at the thin end of the taper on the-45/+45 interface indicated by the arrow in the laminate layup and the delamination growth was simulated in both directions, i.e., along the taper and into the thin region. The total strain-energy-release rate, G, and the mode I and mode II components of G, were computed at the delamination tips using the virtual crack closure technique. In addition, G was calculated from a global energy balance method. The strain-energy-release rate for a delamination growing into the thin laminate consisted predominantly of mode I (opening) component. For a delamination growing along the tapered region, the strain-energy-release rate was initially all mode I, but the proportion of mode I decreased with increase in delamination size until eventually total G was all mode II. The total G for both delamination tips increased with increase in delamination size, indicating that a delamination initiating at the end of the taper will grow unstably along the taper and into the thin laminate simultaneously.

Thin-skinned composite honeycomb sandwich structures from the trailing edge of the U.S. Army's Apache and Chinook helicopters have been tested to ascertain their susceptibility to water intrusion as well as such intrusions' effects on impact damage and cyclic loading. Minimum-impact and fatigue conditions were determined which would create microcracks sufficiently large to allow the passage of water through the skins; damage sufficient for this to occur was for some skins undetectable under a 40X-magnification optical microscope. Flow rate was a function of moisture content, damage, applied strain, and pressure differences.

A tension fatigue life prediction methodology for composite laminates is presented. Tension fatigue tests were conducted on quasi-isotropic and orthotropic glass epoxy, graphite epoxy, and glass/graphite epoxy hybrid laminates. Edge delamination onset data were used to generate plots of strain energy release rate as a function of cycles to delamination onset. These plots were then used along with strain energy release rate analyses of delaminations initiating at matrix cracks to predict local delamination onset. Stiffness loss was measured experimentally to account for the accumulation of matrix cracks and for delimination growth. Fatigue failure was predicted by comparing the increase in global strain resulting from stiffness loss to the decrease in laminate failure strain resulting from delaminations forming at matrix cracks through the laminate thickness. Good agreement between measured and predicted lives indicated that the through-thickness damage accumulation model can accurately describe fatigue failure for laminates where the delamination onset behaviour in fatigue is well characterized, and stiffness loss can be monitored in real time to account for damage growth.

A simple analysis method based upon a transverse shear deformation theory and a sublaminate approach is utilized to analyze a mixed-mode edge delamination specimen. The analysis provides closed form expressions for the interlaminar shear stresses ahead of the crack, the total strain energy release rate, and the strain energy release rate components. The parameters controlling the behavior are identified. The effect of specimen stacking sequence and delamination interface on the strain energy release rate components is investigated. Results are compared with a finite element simulation for reference. The simple nature of the method makes it suitable for preliminary design analyses which require a large number of configurations to be evaluated quickly and economically.

Two-dimensional finite-element analysis of the end-notched flexure specimen was performed using 8-node isoparametric, parabolic elements to evaluate compliance and Mode II strain energy release rates GII. The GII values were computed using two different techniques: the virtual crack-closure technique (VCCT) and the rate of change of compliance with crack length (compliance derivative method). The analysis was performed for various crack-length-to-semispan (a/L) ratios ranging from 0.2 to 0.9. Three material systems representing a wide range of material properties were analyzed. The compliance and strain energy release rates of the specimen calculated with the present finite-element analysis agree very well with beam theory equations including transverse shear. The GII values calculated using the compliance derivative method compared extremely well with those calculated using the virtual crack-closure technique. The GII values obtained by the compliance derivative method using the top or bottom beam deflections agreed closely with each other. The strain energy release rates from a plane-stress analysis were higher than the plane-strain values by only a small percentage, indicating that either assumption may be used in the analysis. The GII values for one material system calculated from the finite-element analysis agreed with one solution in the literature and disagreed with the other solution in the literature.

This study focused on characterizing matrix cracking and delamination behavior in multidirectional laminates. Static tension and tension-tension fatigue tests were conducted on two different layups. Damage onset, accumulation, and residual properties were measured. Matrix cracking was shown to have a considerable influence on residual stiffness of glass epoxy laminates and could be predicted reasonably well for cracks in 90° plies using a simple shear lag analysis. A fracture mechanics analysis for the strain energy release rate associated with 90° ply matrix crack formation was developed and was shown to correlate the onset of 90° ply cracks in different laminates. The linear degradation of laminate modulus with delamination area, previously observed for graphite epoxy laminates, was predicted for glass epoxy laminates using a simple rule of mixtures analysis. The strain energy release rate associated with edge delamination formation under static and cyclic loading was difficult to analyze because of the presence of several contemporary damage phenomena.