Showing papers on "Composite laminates published in 1985"

Transverse cracking and stiffness reduction in composite laminates

Abstract: A systematic classification of the effects of transverse cracking on the stress-strain response of composite laminates is presented. Stiffness reductions resulting from transverse cracking in glass/epoxy and graphite/epoxy laminates from crack initiation to crack saturation are predicted using the stiffness-damage relationships developed by the author in a previous work. Good agreement with the experimental results is found. An assessment of the ply discount method for predicting stiffness reductions at crack saturation is also made.

Notched Strength of Composite Laminates: Predictions and Experiments—A Review:

Abstract: A self-contained review of several semiempirical fracture models for predicting notched strength of composite laminates is presented, based on notched strength data on 70 different laminate configurations of graphite/epoxy, boron/aluminum, and graphite/polyimide. Emphasis is placed on experimental results concerning such failure factors as delamination, splitting, and size of damage zone. Moreover, the fracture model parameters are correlated with the notch sensitivity of composite laminates, and the applicability of the correlations in describing the material notch sensitivity is evaluated. The predictions provided by the different models were found to be identical for all practical purposes.

Prediction of Impact Force and Duration Due to Low-Velocity Impact on Circular Composite Laminates

Abstract: Two simple and improved models--energy-balance and spring-mass--were developed to calculate impact force and duration during low velocity impact of circular composite plates. Both models include the contact deformation of the plate and the impactor as well as bending, transverse shear, and membrane deformations of the plate. The plate was transversely isotropic graphite/epoxy composite laminate and the impactor was a steel sphere. Calculated impact forces from the two analyses agreed with each other. The analyses were verified by comparing the results with reported test data.

On the impact of initially stressed composite laminates

Abstract: The impact response behavior of initially stressed composite laminates is investigated using the finite element method. An experimentally established contact law is incorporated into the finite element program. The Newmark time integration algorithm is used for solving the time dependent equations of the plate and the impactor. Numerical results, including the contact force history, deflection, and strain in the plate, are presented. Effects of impact velocity, initial stress, and the mass and size of the impactor are discussed.

Analysis of Progressive Matrix Cracking in Composite Laminates I. Thermoelastic Properties of a Ply with Cracks

Abstract: Overall stiffnesses and compliances, thermal expansion coefficients, and stress and strain averages are evaluated for a fibrous composite lamina which contains a given density of open transverse cracks and is subjected to uniform mechanical loads and thermal changes. The evaluation procedure is based on the self-consistent method and is similar, in principle, to that used in finding elastic constants of unidirectional com posites.

Delamination and Debonding of Materials

Abstract: The general topics consist of stress analysis, mechanical behavior, and fractography/NDI of composite laminates. Papers are presented on a dynamic hybrid finite-element analysis for interfacial cracks in composites, energy release rate during delamination crack growth in composite laminates, matrix deformation and fracture in graphite-reinforced epoxies, and the role of delamination and damage development on the strength of thick notched laminates. In addition, consideration is given to a new ply model for interlaminar stress analysis, a fracture mechanics approach for designing adhesively bonded joints, the analysis of local delaminations and their influence on composite laminate behavior, and moisture and temperature effects on the mixed-mode delamination fracture of unidirectional graphite/epoxy.

Prediction of tensile matrix failure in composite laminates

Abstract: A conceptually simple mixed mode fracture analysis method is presented for the prediction of tensile matrix failure in composite laminates. The analysis technique, which makes use of a mixed mode s...

Prediction of low-velocity impact damage in thin circular laminates

Abstract: Clamped circular composite plates were analyzed for static equivalent impact loads. Three plate sizes—25.4, 38.1, and 50.8 mm radii—made of quasi-isotropic graphite/epoxy laminate were analyzed. The analysis was based on the minimum total potential energy method and used the von Karman strain-displacement equations. A step-by-step incremental transverse displacement procedure was used to calculate plate load and ply stresses. The ply failure region and modes (splitting and fiber break) were calculated using the Tsai-Wu and the maximum stress criteria, respectively. Reduced moduli were then used in the failed region in subsequent increments of analyses. The analysis predicted that the failure would initiate as splitting in the bottom-most ply and then progress to other plies. Larger radii plates had a lower splitting threshold (load or energy) and a higher first-fiber failure threshold. The size and shape of the ply damage regions were different for different plies. The bottom ply damage was the largest and elongated in its ply-fiber direction. Calculated splitting damage for a 25.4 mm radius plate agreed with reported test data.

Bonded-Bolted Composite Joints

Abstract: Various aspects of the combination of adhesive bonding and mechanical fastening for fibrous composite structures are considered. The factors accounted for in the theories are explained and the source of the complete derivations is given. Analyses of undamaged structures show that, because the adhesive bond load path is so much stiffer than the load path through bolts or rivets, the combination is no stronger than a well-designed bonded joint alone. However, the combination of bonding and bolting is shown to be particularly useful for repair and to prevent damage from spreading. The issues raised are illustrated by specific analyses of large stepped-lap composite to metal joints.

Leaky Lamb waves in fibrous composite laminates

Abstract: Results of experimental measurements and theoretical calculations on ultrasonic leaky Lamb‐wave propagation in fiber‐reinforced, unidirectional composite laminates are presented. With the Lamb wave vector oriented parallel to the fiber direction, dispersion curves of phase velocity versus frequency and plate thickness have been constructed from measurements of ultrasonic reflection from fluid‐loaded composite plates. The experimental results are supported by a theoretical model of Lamb‐wave propagation in the composite plate. The model begins with an approximate calculation of the effective, homogeneous, transversely isotropic elastic behavior of a unidirectional composite laminate in the long‐wavelength limit, using a two‐step procedure based on alternating layered media. This intermediate continuum result is then incorporated into a calculation of the ultrasonic reflection coefficient of a fluid‐loaded anisotropic plate, which is assumed to approximate the fibrous composite laminate. Good quantitative agreement with the model is found if the fiber volume fraction is taken to be an adjustable parameter. However, not all portions of the dispersion curves predicted by the model can be observed in the data. It is conjectured that relative differences in mode coupling account for this discrepancy.

Free Edge Strain Concentrations in Real Composite Laminates: Experimental-Theoretical Correlation

Abstract: The magnitude of the maximum shear strain at the free edge of axially loaded theta (2)/-theta(2)(s) and (+ or - theta(2) (s) composite laminates was investigated experimentally and numerically to ascertain the actual value of strain concentration in resin matrix laminates and to determine the accuracy of finite element results Experimental results using moire interferometry show large, but finite, shear strain concentrations at the free edge of graphite-epoxy and graphite-polyimide laminates Comparison of the experimental results with those obtained using several different finite element representations showed that a four node isoparametric finite element provided the best and most trouble free numerical results The results indicate that the ratio of maxium shear strain at the free edge to applied axial strain varies with fiber orientation and does not exceed nine for the most critical angle which is 15 deg

Analysis of Progressive Matrix Cracking in Composite Laminates.

Abstract: : This report summarizes recent results of studies on progressive ply cracking in fibrous laminates. Evaluation of stiffness changes caused by systems of aligned slit cracks which are parallel to the fiber direction in a unidirectional composite lamina is discussed. Results obtained by the self-consistent method are presented. Next, a procedure for estimating instantaneous crack densities and stiffness changes in a lamina subjected to a prescribed strain history is outlined. These results are extended to analysis of laminated composite plates under in-plane stresses. Specific examples and comparisons of analytical and experimental results are presented for two graphite-epoxy systems. Keywords: Composite materials; Cracking; Damage accumulation.

Thin, deformable composite laminate

Abstract: A thin, plastically formable composite laminate features at least one metallic outer layer (14) and a core (10) of thermoplastic plastic. The metallic outer layers (14) exhibit a flat stress-strain curve which is similar to that of the plastic core (10) and is achieved by specific types of aluminum alloys and by specific alloy treatments. The, preferably 1.5-2.5 mm thick, composite laminates are employed for the manufacture of shaped parts in light weight constructions, especially in automobile, equipment and architectural applications.

Analysis of Interlaminar Stresses in Thick Composite Laminates With and Without Edge Delamination

Abstract: The effect of laminate thickness on the interlaminar stresses in rectangular quasi-isotropic laminates under uniform axial strain was studied. Laminates from 8-ply to infinitely thick were analyzed. Thick laminates were synthesized by stacking (45/0/-45/90) ply groups, rather than grouping like plies. Laminates with and without delaminations were studied. In laminates without delaminations, the free-edge interlaminar normal stress distribution in the outer ply groups was insensitive to total laminate thickness. The interlaminar normal stress distribution for the interior ply groups was nearly the same as for an infinitely thick laminate. In contrast, the free-edge inter-laminar shear stress distribution was nearly the same for inner and outer ply groups and was insensitive to laminate thickness. In laminates with delaminations those delaminations near the top and bottom surfaces of a thick laminate have much larger total strain-energy-release rates (G sub t) and mode I-to-total (G sub t/G sub t) ratios than delaminations deep in the interior. Therefore, delaminations can be expected to grow more easily near the surfaces of a laminate than in the interior.

A theory for stress analysis of composite laminates

Abstract: Failures in laminated resin matrix composite materials often begin with matrix microcracking and delamination. These modes of damage are three-dimensional in nature and are controlled by interlaminar stresses. One important key to understanding and ultimately predicting the failures in composite materials is an analytical approach that provides reliable stress estimates in critical regions. Conventional laminate theories are inadequate for this purpose as they are based on global displacement assumptions. Moreover, the interlaminar stresses are often neglected in the initial formulations. Therefore, solutions based upon these theories cannot yield realistic stress distributions. Recent theoretical research shows that there are certain nonclassical influences that affect bending-related behavior. They include section warping and its concomitant nonclassical surface-parallel stress contributions and transverse normal strain. The stress prediction capability of a bending theory improves significantly if these nonclassical influences are incorporated. A comprehensive bending theory is developed for arbitrary composite laminates. Its effectiveness is demonstrated in examples for a cross-ply laminate and a quasiisotropic laminate.

Short-wavelength buckling and shear failures for compression-loaded composite laminates

Abstract: The short-wavelength buckling (or the microbuckling) and the interlaminar and inplane shear failures of multi-directional composite laminates loaded in uniaxial compression are investigated. A laminate model is presented that idealizes each lamina. The fibers in the lamina are modeled as a plate, and the matrix in the lamina is modeled as an elastic foundation. The out-of-plane w displacement for each plate is expressed as a trigonometric series in the half-wavelength of the mode shape for laminate short-wavelength buckling. Nonlinear strain-displacement relations are used. The model is applied to symmetric laminates having linear material behavior. The laminates are loaded in uniform end shortening and are simply supported. A linear analysis is used to determine the laminate stress, strain, and mode shape when short-wavelength buckling occurs. The equations for the laminate compressive stress at short-wavelength buckling are dominated by matrix contributions.

Nonlinear Stress-Strain Behavior of Graphite/Epoxy Laminates

Abstract: The nonlinear stress-strain behavior of graphite/epox y was investigated. One hundred and eighty-one coupon specimens of the [ ±0] s family, with 0 equal to 0°, 5°, 10°, 15°, 20°, 25°, 30°, 35°, 40°, 45°, 50°, 55°, 60°, 75°, and 90°, were tested in uniaxial tension. All specimens were unnotched. Three types of tests were conducted: monotonic-to-failure; load, unload, reload-to-failure; and 100 load-unload cycles followed by a load-to-failure. In the latter two cases, the load cycle entered the nonlinear stress-strain region of the specimen. Two basic loadings were found to cause nonlinear stress-strain behavior in a unidirectional ply: longitudinal tension, which causes "stiffening," and shear, which causes "softening." These effects combined to cause overall stiffening in the [ ±8] s laminates for 0 less than 20°, and softening for 6 greater than 20°. The load, unload, reload tests showed three important results. First, there was a "permanent strain" left at zero load for those laminates that softened. Second, the reload path has the same modulus as the original load path and rejoins the basic stressstrain curve at the point to which the laminate was previously loaded. Third, the laminates show no memory of one previous loading after being reloaded to the original load point as further loading produces a stress-strain curve unaltered by one load, unload, reload cycle including no change in fracture stress. Implications for general laminates are discussed.

Accelerated viscoelastic characterization of T300-5208 graphite-epoxy laminates

Abstract: A viscoelastic response scheme for the accelerated characterization of polymer-based composite laminates in applied to T300/5208 graphite/epoxy. The response of uni-directional specimens is modeled. The transient component of the viscoelastic creep compliance is assumed to follow a power law approximation. A recursive relationship is developed, based upon the Schapery single-integral equation, which allows approximation of a continuous time-varying uniaxial load using discrete steps in stress. The viscoelastic response of T300/5208 to transverse normal and shear stresses is determined unsing 90 deg and 10 deg off-axis tensile specimens. In each case the seven viscoelastic material parameters required in the analysis are determined experimentally using short-term creep and creep recovery tests. It is shown that an accurate measure of the power law exponent is crucial for accurate long-term prediction. A short term test cycle selection procedure is proposed, which should provide useful guidelines for the evaluation of other viscoelastic materials.

Iosipescu shear properties of graphite fabric/epoxy composite laminates

Abstract: The Iosipescu shear test method is used to measure the in-plane and interlaminar shear properties of four T300 graphite fabric/934 epoxy composite materials. Different weave geometries tested include an Oxford weave, a 5-harness satin weave, an 8-harness satin weave, and a plain weave with auxiliary warp yarns. Both orthogonal and quasi-isotropic layup laminates were tested. In-plane and interlaminar shear properties are obtained for laminates of all four fabric types. Overall, little difference in shear properties attributable to the fabric weave pattern is observed. The auxiliary warp material is significantly weaker and less stiff in interlaminar shear parallel to its fill direction. A conventional strain gage extensometer is modified to measure shear strains for use with the Iosipescu shear test. While preliminary results are encouraging, several design iterations failed to produce a reliable shear transducer prototype. Strain gages are still the most reliable shear strain transducers for use with this test method.

Low-velocity impact response of laminated beams subjected to initialstresses

Abstract: Finite element procedures are used in conjunction with a numerical algorithm to compute the impact response of a graphite-epoxy laminated beam subjected to tensile initial stresses. The effect of initial stresses on the contact duration, impact force, coefficient of restitution, and bending and shear stresses are discussed. The analytically computed contact force history and strain response are compared with some experimental results.

The fundamental constants of orthotropic affine slab/plate equations

Abstract: The global constants associated with orthotropic slab/plate equations are discussed, and the rotational behavior of the modulus/compliance components associated with orthotropic slabs/plates are addressed. It is concluded that one cluster constant is less than or equal to unity for all physically possible materials. Rotationally anomalous behavior is found in two materials, and a simple inequality which can be used to identify regular or anomalous behavior is presented and discussed in detail.