Showing papers on "Composite laminates published in 1979"

On a Model for Two Phase Diffusion in Composite Materials

Abstract: N THIS NOTE we study the diffusion of moisture through a rigid matrix, and, in particular, discuss a model in which moisture is allowed to exist in two phases. We believe that a model of this type might be applicable to polymeric materials reinforced with fibers: one phase might account for moisture flowing through the matrix, the second for moisture trapped around the fibers. Two phases might also be appropriate in situations for which moisture exists in free and bound states.

Fatigue Behavior and Life Prediction of Composite Laminates

Abstract: : This report presents a survey of some of the available data on the effects of fatigue in composite laminates. Both static and fatigue behaviors of unidirectional laminae are discussed in terms of the static failure and fatigue limit strains of the constituents as well as of the loading conditions. Subcritical failure modes in multidirectional laminates are delineated and correlated with the material responses such as temperature increase, modulus change, and fatigue notch insensitivity. Also included are the effects of test frequency and compression. Methods of predicting the fatigue strengths of off-axis laminae and of angle-ply laminates are summarized. Finally, two statistical life prediction models are discussed using the material 'age'. (Author)

Effect of Curing Stresses on the First Ply-failure in Composite Laminates

Abstract: This paper describes an experimental technique of detecting the first ply-failure in laminated composites using a combination of long strain gages and acoustic emission measurement. The material sy...

Superposition of the notched strength of composite laminates

Abstract: An improved macroscopic model for predicting the strength of a composite laminate containing a circular notch is developed. Two constants are introduced which uniquely determine the notch sensitivity of a given material. A superposition method for the notched strength of composite laminates is developed which allows data for arbitrary materials and laminate configurations to be superimposed upon a single master curve. The influence of material orthotropy upon notched strength is discussed. A relative notch sensitivity parameter is introduced which allows quantification of the notch sensitivity of a given composite material system, stacking sequence, or laminate configuration.

Failure analysis of composite laminates with free edges

Abstract: Linear elastic stress distributions obtained from a refined finite element mesh are used in conjunction with the tensor polynomial failure criterion to predict the initiation of failure in symmetric, finite-width graphite-epoxy laminates under tensile loading Results are presented for a wide variety of laminates including: (+ and - theta)s angle-ply; cross-ply (0/90)s and (90/0)s; and quasi-isotropic (90/0/+ and - 45)s and (+ and -45/0/90)s It is shown that the elastic stress distributions generally compare favorably with other published results, but also indicate improved satisfaction of the stress-free boundary conditions and indicate some differences in the singular behavior of selected stress components at the free edge The tensor polynomial failure criterion is used to predict the location and mode of first failure Examination of the individual terms of the polynomial indicates different modes of failure depending upon the laminate configuration

Dynamic mechanical properties of fiber-reinforced composite materials

Abstract: This paper reviews recent efforts to characterize the internal damping and dynamic stiffness of fibre-reinforced composite materials under vibratory loading. Several trends are noted, and suggestions are offered regarding directions of future research.

Wave propagation in transversely impacted composite laminates

Abstract: An experimental study was conducted to determine the wave-propagation characteristics, transient strains and residual properties of unidirectional and angle-ply boron/epoxy and graphite/epoxy laminates impacted with silicon-rubber projectiles at velocities up to 250 m/sec. Results include the following: (1) the predominant wave is the flexural wave propagating at different velocities in different directions; (2) peak strains and strain rates in the transverse to the (outer) fiber direction are much higher than those in the direction of the fibers; (3) strain rates up to 640/sec were measured; and (4) unidirectional laminates under impact showed appreciable modulus and strength degradation in the direction transverse to fibers.

Fracture Modes and Acoustic Emission of Composite Materials

Abstract: Unidirectional fibrous composite material laminae are the construction material of a laminate. Four failure modes can occur in this material, and the identification of these modes is as important as the identification of onset of fracture. Acoustic emission (AE) pulses, generated by the change in the stress field resulting from cracking in a material, have a unique pattern of amplitude distribution that depends only on the mode of fracture and the type of material. By using a tapered double-cantilever beam as grips for the unidirectional lamina, a crack with a constant stress intensity factor was induced, thus eliminating the factor of the stress level. Placing the lamina in different directions relative to the grips imposed a different failure mode. By plotting the log of the relative number of AE pulses above some level versus the log of the relative level, a single constant coefficient can be measured for each material and mode of fracture. The AE energy is proportional to the energy released by the cracking; this proportionality depends on the medium where the AE waves have to travel, and not on the cracking mode.

Metal-thermoplastic-metal laminates and a process for the preparation of such laminates

Abstract: This invention relates to construction panels comprising metal-thermoplastic-metal laminates and a process for preparing same, said laminates possessing in combination critical service properties of high heat distortion temperature, low coefficient of linear thermal expansion, low inelastic ratio, and lightness in weight.

Fracture of composite laminates by three-point bend

Abstract: An experimental investigation has been carried out on the fracture behavior of T300/5208 graphite/epoxy laminates using three-point bend and tension tests. One laminate, a [0/±45/90] ns orientation, was used withn=1 for tension specimens andn=4 and 8 for bend specimens. The crack-growth-resistance curve was constructed by compliance matching from crack-opening displacement and load data obtained from three-point bend tests. The value of effective crack length and corresponding crack resistance at onset of instability was nearly constant and independent of the initial crack length and laminate thickness. Experimental data from both bend and tension tests correlated reasonably well to the average-stress-failure criterion. Data were also compared with results found in the literature.

Boundary layer thermal stresses in angle-ply composite laminates

Abstract: Boundary-layer thermal stress singularities and distributions of angle-ply composite laminates under uniform thermal loading are investigated through a system of sixth-order governing partial differential equations developed with the aid of the anisotropic elasticity field equations and Lekhnitskii's complex stress functions Results are presented for cases of various angle-ply graphite/epoxy laminates, and it is shown that the boundary-layer thickness depends on the degree of anisotropy of each individual lamina, thermomechanical properties of each ply, and the relative thickness of adjacent layers

Characterization of impact damage in composite laminates

Abstract: The potential application of the Holscan 400 as a laboratory tool to monitor and characterize impact damage in graphite/epoxy composite material was evaluated. Impact damage in 16- to 32-ply-thick material was examined. Anumber of modifications were made to the basic Holscan system and the unit evaluated in terms of impact damage characterization ability and ease of operation in a laboratory environment. The results showed several of the modifications to be needed to provide a system with the desired capability and ease of operation.

Application of the theory of viscoelasticity to composite materials

Abstract: Reinforced polymers and, particularly, glass-plastics are outstanding and rather widely used representatives of the class of composite materials. A characteristic feature of composite polymers is that some of their components have viscoelastic properties. Determining the state of stress and strain of such composites involves great mathematical difficulties and the procedure depends on the structural characteristics of a given material. Here a method of calculations is proposed applicable to viscoelastic composites with a regular structure, i.e., with a definitely ordered spatial distribution of mechanical inhomogeneities.

Evaluation of Flawed Composite Structural Components Under Static and Cyclic Loading

Abstract: The effects of initial defects on the fatigue and fracture response of graphite-epoxy composite laminates are presented The structural laminates investigated were a typical angle ply laminate, a polar/hoop wound pressure vessel laminate, and a typical engine fan blade laminate Defects investigated were full and half penetration circular holes, full and half penetration slits, and countersink holes The effects of the defect size and type on the static fracture strength, fatigue performance, and residual static strength are shown as well as the results of loadings on damage propagation in composite laminates The data obtained were used to define proof test levels as a qualification procedure in composite structure subjected to cyclic loading

Nonlinear temperature dependent failure analysis of finite width composite laminates

Abstract: A quasi-three dimensional, nonlinear elastic finite element stress analysis of finite width composite laminates including curing stresses is presented. Cross-ply, angle-ply, and two quasi-isotropic graphite/epoxy laminates are studied. Curing stresses are calculated using temperature dependent elastic properties that are input as percent retention curves, and stresses due to mechanical loading in the form of an axial strain are calculated using tangent modulii obtained by Ramberg-Osgood parameters. It is shown that curing stresses and stresses due to tensile loading are significant as edge effects in all types of laminate studies. The tensor polynomial failure criterion is used to predict the initiation of failure. The mode of failure is predicted by examining individual stress contributions to the tensor polynomial.

Mechanical and thermophysical properties of graphite/polyimide composite materials

Abstract: An on-going program to characterize advanced composites for up to 50,000 hours of exposure to simulated supersonic cruise environments is summarized. Results are presented for up to 25,000 hours of thermal exposure and 10,000 hours of flight simulation at temperatures up to 560K (550 F) with emphasis on HTS/710 graphite/polyimide composite material. Results to date indicate that the maximum use temperature for HTS/710 may be reduced to 505K (450 F) for long-time (1000 hours) application such as the supersonic transport. Preliminary thermophysical properties data for HTS/PMR15 graphite/polyimide were generated. These data include thermal conductivity, thermal expansion, and specific heat from 115K (-252 F) to 590K (600 F) and emittance at room temperature and 590K (600 F). The purpose in generating these data was to validate use of state-of-the-art property measurement methods for advanced graphite fiber reinforced resin matrix composites. Based on results to this point, thermal expansion measurements for composites are most difficult to perform. A high degree of caution in conducting thermal expansion tests and analyzing results is required to produce reliable data.

Development of a realistic stress analysis for fatigue analysis of notched composite laminates

Abstract: A finite element stress analysis which consists of a membrane and interlaminar shear spring analysis was developed. This approach was utilized in order to model physically realistic failure mechanisms while maintaining a high degree of computational economy. The accuracy of the stress analysis predictions is verified through comparisons with other solutions to the composite laminate edge effect problem. The stress analysis model was incorporated into an existing fatigue analysis methodology and the entire procedure computerized. A fatigue analysis is performed upon a square laminated composite plate with a circular central hole. A complete description and users guide for the computer code FLAC (Fatigue of Laminated Composites) is included as an appendix.

Stationary transfer in fibrous composite materials

Abstract: The effective heat-conduction coefficients are calculated for fibrous materials of different structure.

A fracture mechanics approach to the analysis of graphite/epoxy laminated precracked tension panels

Abstract: A methodology for the prediction of fracture in fiber composite laminates under monotonically increasing load is presented. In the model, laminate behavior is based on damage growth in individual plies with a fracture mechanics approach being used to continually assess the damage in each ply. Within each ply, damage may occur by crack extension parallel to or perpendicular to the fiber orientation in the ply. Critical values of strain energy release rate for both modes of damage are inferred from experimental data on center-cracked, zero-degree unidirectional laminates. At present, interply interactions are neglected. Despite this severe oversimplification, a comparison of strength predictions from the laminate model shows reasonable agreement with experimental results, both quantitatively and qualitatively, for [0/90] 4 s , [′45] 4 s , and [0/′45/0] 2 s graphite/epoxy laminates. The features of the laminate model are contrasted with the models of a number of other researchers.

Preliminary bolted-joint data

Abstract: Bolted-joint strength and failure modes for advanced graphite/polyimide composite laminates at 116K (-250 F), room temperature, and 589K (600 F) were determined to provide preliminary design data. The bolted-joint test setup for the low and elevated temperature tests is described. Test results are reported on a quasi-isotropic laminate of Celion 6000/PMR-15. Single bolt, double lap shear specimens were tested to obtain maximum joint strength and failure mode. The effect of joint geometry and temperature on joint strength and failure mode is presented.