Showing papers on "Composite laminates published in 1975"

Uniaxial Failure of Composite Laminates Containing Stress Concentrations

Abstract: Two previously developed failure criteria for predicting the uniaxial tensile strength of a laminated composite containing through-the-thickness material discontinuities (notches) are subjected to further experimental scrutinization. In particular, the two-parameter (unnotched tensile strength of the laminate and a characteristic length) models, which are capable of predicting observed discontinuity size effects without resorting to the concepts of linear elastic fracture mechanics, are based on limited experimental verification. In the present paper, and experimental program is presented which examines the effect of changes in the material system, the laminate fiber orientations, and the notch shape and size (stress gradient), on the model predictions. This is accomplished by obtaining experimental data on two material systems, glass/epoxy and graphite/ epoxy, in conjunction with two orientations of fiber-dominated laminates containing through-the-thickness circular holes and sharp tipped cracks of several sizes. In addition to the test results, two observations based on the models are presented. First, the statistical failure distribution for a composite containing a circular hole is predicted using the models and shown to agree well with experimental observations. Second, an Irwin type correction factor applied to the stress intensity factor is shown to result in nearly constant values of the critical stress intensity factor for all values of crack length. The correction factor is shown to be related to the characteristic length of the present models.

Curing Stresses in Composite Laminates

Abstract: Analysis of curing stresses in resin matrix composite laminates involves a temperature range over which variation of elastic moduli is appreciable. A method based on total stress-strain-temperature relations is formulated and applied to determine the curing stresses in boron/epoxy composite laminates. This method is shown to be preferable to the incremental method because the former requires the thermal strains and the stress-strain relations only at the final temperature of interest. The use of incremental constitutive equations is also discussed, and it is shown that proper care must be exercised to insure inclusion of interaction terms that have no counterparts in linear theories.

Hybrid composite laminate structures

Abstract: The invention relates to laminate structures and specifically to essentially anisotropic fiber composite laminates wherein metal foils are selectively disposed within the laminate to produce increased resistance to high velocity impact, fracture, surface erosion, and other stresses within the laminate.

Application of a Special X-Ray Nondestructive Testing Technique for Monitoring Damage Zone Growth in Composite Laminates

Abstract: A modified X-ray nondestructive testing technique was used to study matrix cracks parallel to fibers and delaminations between plies in graphite-epoxy composite material. A tetrabromoethane (TBE) opaque additive applied at the source of the damage zones enhanced the flaw image. Center-slit specimens were fabricated from Modmor II/Narmco 5208 graphite-epoxy laminates with three different ply orientations. Tensile ramp and sawtooth cyclic loadings at different levels were applied to these specimens. Periodic X-ray monitoring was conducted to observe the initiation and growth of cracks and delaminations at the slit tips. The initiation of damage zones appeared as fiber separation in the ′45-deg directions tangent to the semicircular periphery of the cutout tips. As the load level was increased, this fiber separation continued while fiber separation at other locations also appeared. Delamination first occurred at approximately the same time as fiber separation. The delaminated area was found to surround the most severe fiber separation region and grew in size as the load level was increased. Limited test results indicated a slow and an accelerated damage growth rate during ramp loading. For cyclic loading the majority of growth occurred on the first load cycle. Subsequent cyclic loading to the same level contributed little additional growth in the majority cases.

Determination of Fracture Strength in Orthotropic Graphite-Epoxy Laminates

Abstract: A fracture test program performed on orthotropic graphite-epoxy laminates is reported. The test data are used to evaluate the effects of specimen size and specimen configuration on the measured value of laminate fracture strength. It is shown that apparent fracture strength is independent of specimen configuration and specimen thickness, but varies significantly with respect to crack length. As crack length increases, however, the apparent fracture strength is found to asymptotically approach a limiting value, taken to be the valid laminate fracture toughness. The test data are also used to evaluate two analytical models for predicting laminate fracture toughness. The first model links basic ply properties and the fracture toughness of angle-ply laminates, while the second model relates the fracture toughness of an arbitrary orthotropic laminate to the fracture properties of its angle-ply components. Both models show good agreement with the test data. It is concluded that linear elastic fracture mechanics does provide a meaningful characterization of crack growth in orthotropic composite laminates, if some specified conditions are met. Study of cases which do not meet these conditions is recommended for future work.

Mode I Stress Intensity Factors for Symmetrically-Cracked Orthotropic Strips

Abstract: The effects of material anisotropy on the stress intensity factor are investigated numerically for both double-edge-cracked and center-cracked orthotropic tensile strips. The material properties employed correspond to homogeneous material models of mid-plane symmetric fiber composite laminates of varying ply orientation and ply properties. The effects of anisotropy are found to be dependent on both specimen geometry and material properties. Complete decoupling of these geometric and material influences does not appear possible. However, the principal geometric parameter seems to be the distance from the crack tip to the free edge of the specimen; the effects of material properties appear to correlate with the in-plane elastic shear modulus. It is suggested that anisotropic effects can be sufficiently controlled, by means of specimen geometry, to justify the use of the known isotropic stress intensity factor as a reasonable (and usually conservative) estimate of the anisotropic value in most materials.

Failure and fatigue mechanisms in composite materials

Abstract: A phenomenological description of microfailure under monotonic and cyclic loading is presented, emphasizing the significance of material inhomogeneity for the analysis. Failure in unnotched unidirectional laminates is reviewed for the cases of tension, compression, shear, transverse normal, and combined loads. The failure of notched composite laminates is then studied, with particular attention paid to the effect of material heterogeneity on load concentration factors in circular holes in such laminates, and a 'materials engineering' shear-lay type model is presented. The fatigue of notched composites is discussed with the application of 'mechanistic wearout' model for determining crack propagation as a function of the number of fatigue cycles.-

Analytical displacements and vibrations of cantilevered unsymmetric fiber composite laminates

Abstract: A fiber composite flat cantilever plate that has symmetric and nonsymmetric laminate configurations is theoretically investigated to determine its static and dynamic structural response. The finite element analysis method used includes a unique triangular finite element developed at Lewis for the analysis of fiber composite airfoils. The various responses investigated include tip displacements, natural frequencies, and fundamental mode shapes. The results show that the displacements and the natural frequencies can be in considerable error for nonsymmetric laminate configurations if the membrane-bending coupling is not taken into account. Structural response results obtained from ten laminate configurations are presented in tabular forms and may be used as an aid in selecting laminate configurations for composite airfoils.

Fatigue of notched fiber composite laminates. Part 1: Analytical model

Abstract: A description is given of a semi-empirical, deterministic analysis for prediction and correlation of fatigue crack growth, residual strength, and fatigue lifetime for fiber composite laminates containing notches (holes). The failure model used for the analysis is based upon composite heterogeneous behavior and experimentally observed failure modes under both static and fatigue loading. The analysis is consistent with the wearout philosophy. Axial cracking and transverse cracking failure modes are treated together in the analysis. Cracking off-axis is handled by making a modification to the axial cracking analysis. The analysis predicts notched laminate failure from unidirectional material fatique properties using constant strain laminate analysis techniques. For multidirectional laminates, it is necessary to know lamina fatique behavior under axial normal stress, transverse normal stress and axial shear stress. Examples of the analysis method are given.

Tensile Creep of Angle-Plied Boron/Epoxy Laminates

Abstract: Tensile creep tests were performed at 75°F (24°C) and 300°F (149°C) on 26 specimens cut from ±30-deg and ±45-deg laminates of boron/epoxy composite material to characterize the creep behavior of these materials. A linear viscoelastic laminate theory proposed by McQuillen was compared with the experimental data for the tests run at 75°F (24°C). The predicted creep strains differed greatly from the measured values, indicating that the linear theory is not applicable to laminates with these filament orientations. A general creep equation of the form e = kσmtn was fitted to the experimental data with excellent correlation. The stress exponents was fitted to the experimental data with excellent correlation. The stress exponents m varied from 1.79 to 5.17 for the four series of tests, thus characterizing the nonlinear behavior of boron/epoxy laminates with these filament orientations.

Investigation of Failure Mechanisms in Fiber Composite Laminates.

Abstract: : An analytical investigation is made into the behavior of fiber composite laminates after initial lamina failure has occurred in top (outside) layer. The analysis investigates the relative importance of delamination of the failed layer and of stress concentrations in the adjacent lamina to the ultimate failure mechanisms of laminates. A general interlaminar stress analysis is developed which takes into account interface nonlinear behavior and delamination effects. The analysis is specialized to a laminate whose top layer contains a long crack. Laminated plate theory is used in conjunction with the interlaminar stress analysis to analyze first and subsequent failure of graphite/epoxy (0/90), (plus or minus 45), and (0/plus or minus 45) laminates under several stress states.

Application of PATCHES-III to the Three-Dimensional Response of Laminated Composite Structures,

Abstract: : A recently developed program for the stress analysis of general solids of composite material is applied in this study to a sandwich panel impact problem and a partially cracked laminate problem Objectives of the study are: (1) to determine the load distribution, the stresses and the deformations in individual plies of these composite laminates and (2) to demonstrate the modeling capabilities and limitations of the PATCHES-III program Preparatory to the analyses, the program was updated to automate the superposition of symmetry - asymmetry subcases for more efficient modeling The present application of the PATCHES-III system to laminated composites was successful in predicting the three-dimensional response of individual plies at very reasonable modeling and computational costs Engineering checks of the overall response with hand calculations using beam and plate models were in good agreement for both the sandwich panel and the partially cracked laminate A modeling error in the boundary conditions for the latter problem introduced fictitious normal forces for the asymmetry subcase These forces were evaluated when the error was discovered and found to be small, on the order of 0015 lbs maximum, and judged not to have a significant effect on the redistribution of load around the crack The constraint force calculations were made possible by a postprocessor data file system developed as one of the objectives of the effort The Experience gained from the present study also indicates the need for improvement in certain areas