Engineering topics

Morris, D. H.

Publications and source records attributed to Morris, D. H..

At least 37 records · Page 2

NASA NTRS1984

The effect of extreme temperatures on the elastic properties and fracture behavior of graphite/polyimide composites

The influence of elevated and cryogenic temperatures on the elastic moduli and fracture strengths of several C6000/PMR-15 and C6000/NR-15082 laminates was measured. Tests were conducted at -157 C, 24 C, and 316 C (-250 F, 75 F, and 600 F). Both notched and unnotched laminates were tested. The average stress failure criterion was used to predict the fracture strength of quasi-isotropic notched laminates.

Morris, D. H.↗

NASA NTRS1984

On the use of the three point bend and compact tension specimens to measure fracture toughness of composite laminates

The influence of specimen thickness on the fracture toughness of two laminates and three specimen geometries was investigated. As thickness increased the toughness decreased and approached an asymptotic value that was dependent upon the type of laminate but was practically independent of specimen geometry. Enhanced X-ray photographs and removal of an outside ply revealed that most of the delaminations were surface effects.

Harris, C. E.↗

NASA NTRS1984

A comparison of several methods of calculating fracture toughness from composite laminate test data

Critical fracture toughness was determined by two different techniques for graphite/epoxy laminates of three stacking sequences and several thicknesses. Critical fracture toughness was determined by a finite element stress analysis of a center-cracked tension specimen which yielded the stress intensity factor as a function of specimen thickness and applied load. As an alternative approach the critical strain energy release rate was determined from the compliance calibration technique. Test results from both procedures are compared and discussed.

Harris, C. E.↗

NASA NTRS1983

The effect of extreme temperatures on the elastic properties and fracture behavior of graphite/polyimide composites

Morris, D. H.↗

NASA NTRS1983

An evaluation of the effects of stacking sequence and thickness on the fatigue life of quasi-isotropic graphite/epoxy laminates

Notched and unnotched geometries at 16, 32, and 64-ply thicknesses of a 90/45/0-45 (ns) laminate and a 45/0/-45/90 (ns) laminate were tested in compression-compression fatigue. The fatigue life and the initiation, type, and progression of damage were determined. Interlaminar stresses generated at straight, free edges of axially loaded laminates were used to interpret the test results. The fatigue lives of the notched specimens did not appear to be a strong function of laminate stacking sequence or specimen thickness. The stress concentration at the hole dominated over the interlaminar stresses at the straight free edge. The unnotched specimens of the 90/45/0/-45 (ns) laminate with tensile interlaminar normal stresses delaminated more readily than did the 45/0/-45/90 (ns) laminate with compressive interlaminar normal stress. The life of the 16-ply unnotched specimens was lower than the 32- and 64-ply specimens. Delaminations were located at the interface where the maximum shear stress occurred regardless of the sense or magnitude of the interlaminar normal stress. An antibuckling fixture was effective in preventing out-of-plane motion without overconstraining the specimen.

Harris, C. E.↗

NASA NTRS1983

The effects of laminate thickness on the fracture behavior of composite laminates

The relationship between both fracture toughness and the development of crack-tip damage as a function of specimen thickness was investigated. Fracture toughness was determined experimentally using center-cracked tension specimens for (0/plus or minus 45/90)ns and (0/90)ns laminates, where ns means multiple layers with the same repeated sequence and symmetric about the midplane. Laminate thicknesses ranged from 8 plies to 96 plies. As with isotropic metals, fracture toughness was found to decrease with increasing specimen thickness and asymptotically approached a lower bound. The crack tip damage in the (0/plus or minus 45/90)ns laminate does not appear to be a function of thickness. However, there are differences in the damage of the (0/90)2s and (0/90)8s laminates and associated differences in fracture toughness.

Harris, C. E.↗

NASA NTRS1983

Interlaminar fracture toughness of composites. II - Refinement of the edge delamination test and application to thermoplastics

The mixed mode interlaminar fracture toughness, G(c), is obtained for the two thermoplastic matrices UDEL P1700 polysulfone and ULTEM polyetherimide by means of edge delamination tensile (EDT) tests on unnotched, eleven-ply graphite fiber reinforced composite specimens. A novel method is used to obtain the stiffness parameter employed in the closed form equation for the calculation of G(c), decreasing the number of stiffness measurements required and simplifying the calculations. The G(Ic) values from double cantilever beam (DCB) measurements on composites of the two thermoplastics were similar to each other, but slightly higher than the G(c) data obtained by EDT. Interfacial resin/fiber failures predominated in both the EDT and DCB tests.

Johnston, N. J.↗

NASA NTRS1983

Environmental effects and viscoelastic behavior of laminated graphite/epoxy composites

Primarily because of the polymeric matrix, graphite/epoxy composites are viscoelastic materials which exhibit creep and delayed failures. Guided by the time-temperature superposition principle, the authors are developing accelerated characterization techniques to predict the long term compliance behavior and creep rupture strength of composite materials based on short term tests at elevated temperatures. The effect of a post-cure cycle on the compliance and creep rupture strength of composite materials is discussed. The Zhurkov type failure law is applied to experimental creep rupture data of a typical laminate and is shown to correlate the data. A beneficial mechanical strengthening effect is described which significantly affects the failure strengths of specimens which have been under creep loading for a period of time. This aging effect is reversible if the specimen is allowed to recover before being loaded to failure. Other environmental aspects of composite materials are also reviewed.

Dillard, D. A.↗

NASA NTRS1983

Determination of interlaminar fracture toughness and fracture mode dependence of composites using the edge delamination test

Edge delamination testing has been conducted in order to determine the mixed mode interlaminar fracture toughness of unnotched (+, -35/0/90)s and (0/+, -35/90)s laminates with graphite fibers and epoxy matrices, with or without elastomer additives. Although the two stacking sequences were chosen so that the total strain energy release rates were equal, the percentages of crack opening and shear mode energy release rates were found to vary widely between the layups. Plots of interlaminar fracture toughness as a function of crack opening energy release rate percentage have been drawn to generate delamination failure criteria which reflect the observed fracture mode dependence.

Obrien, T. K.↗

NASA NTRS1982

A simple test for the interlaminar fracture toughness of composites

A simple test method for the measurement of delamination resistance is assessed through its application to 11-ply graphite-epoxy laminates whose matrices employ the F185 and 205 resins. The F185 resin has the same base epoxy as the 205, to which liquid and solid elastomers have been added. The critical value of the mixed mode energy release rate, calculated from a closed form equation employing nominal strain measurements from the onset of edge delamination, show that addition of the elastomers to the base epoxy increases interlaminar fracture toughness. Comparison of these results with those of width-tapered double cantilever beam (WTDCB) test data for the two materials shows a similar strain energy release rate, establishing the accuracy of the novel method. Both tests are found to be needed, however, for the quantitative characterization of interlaminar fracture toughness.

Obrien, T. K.↗

NASA NTRS1982

Predicting viscoelastic response and delayed failures in general laminated composites

Although graphite fibers behave in an essentially elastic manner, the polymeric matrix of graphite/epoxy composites is a viscoelastic material which exhibits creep and delayed failures. The creep process is quite slow at room temperature, but may be accelerated by higher temperatures, moisture absorption, and other factors. Techniques are being studied to predict long-term behavior of general laminates based on short-term observations of the unidirectional material at elevated temperatures. A preliminary numerical procedure based on lamination theory is developed for predicting creep and delayed failures in laminated composites. A modification of the Findley nonlinear power law is used to model the constitutive behavior of a lamina. An adaptation of the Tsai-Hill failure criterion is used to predict the time-dependent strength of a lamina. Predicted creep and delayed failure results are compared with typical experimental data.

Dillard, D. A.↗

NASA NTRS1981

Creep rupture of polymer-matrix composites

The time-dependent creep-rupture process in graphite-epoxy laminates is examined as a function of temperature and stress level. Moisture effects are not considered. An accelerated characterization method of composite-laminate viscoelastic modulus and strength properties is reviewed. It is shown that lamina-modulus master curves can be obtained using a minimum of normally performed quality-control-type testing. Lamina-strength master curves, obtained by assuming a constant-strain-failure criterion, are presented along with experimental data, and reasonably good agreement is shown to exist between the two. Various phenomenological delayed failure models are reviewed and two (the modified rate equation and the Larson-Miller parameter method) are compared to creep-rupture data with poor results.

Brinson, H. F.↗

NASA NTRS1981

Creep and creep rupture of laminated graphite/epoxy composites

An incremental numerical procedure based on lamination theory is developed to predict creep and creep rupture of general laminates. Existing unidirectional creep compliance and delayed failure data is used to develop analytical models for lamina response. The compliance model is based on a procedure proposed by Findley which incorporates the power law for creep into a nonlinear constitutive relationship. The matrix octahedral shear stress is assumed to control the stress interaction effect. A modified superposition principle is used to account for the varying stress level effect on the creep strain. The lamina failure model is based on a modification of the Tsai-Hill theory which includes the time dependent creep rupture strength. A linear cumulative damage law is used to monitor the remaining lifetime in each ply.

Dillard, D. A.↗

NASA NTRS1981

The viscoelastic response of a graphite/epoxy laminate

An accelerated characterization procedure has been developed for time-dependent compliances and strengths of resin-matrix composites. The procedure, which is based on a time-temperature-stress superposition principle, is applied to a graphite/epoxy laminate T300/934. Creep rupture analytical models are discussed, and model predictions are compared with experimental data. Master curves for the creep compliances are given.

Brinson, H. F.↗

NASA NTRS1981

The influence of environmental effects on the mechanical properties of graphite/epoxy laminates

The time dependent deformational and creep rupture processes in graphite/epoxy laminates are examined as a function of temperature and stress level. The role of accelerated characterization of design materials is discussed, and the use of a nonlinear time-temperature-stress-superposition principle in the determination of compliance master curves as a function of stress and temperature is considered. Creep rupture analytical models are discussed as applied to polymer-matrix composites, and the data is shown to be in general agreement with a logarithmic stress and temperature-dependent delayed fracture model. Comparisons between creep and rupture experiments and phenomenological models are presented.

Brinson, H. F.↗

NASA NTRS1980

Accelerated characterization of graphite/epoxy composites

A method to predict the long term compliance of unidirectional off-axis laminates from short term laboratory tests is presented. The method uses an orthotropic transformation equation and the time-stress-temperature superposition principle. Short term tests are used to construct master curves for two off-axis unidirectional laminates with fiber angles of 10 and 90 degrees. Analytical predictions of long term compliance for 30 and 60 degrees laminates are made. Comparisons with experimental data are also given.

Griffith, W. I.↗

NASA NTRS1980

The viscoelastic behavior of the principal compliance matrix of a unidirectional graphite/epoxy composite

A testing program was conducted to determine the time-temperature response of the principal compliances of a unidirectional graphite/epoxy composite. It was shown that the two components of the compliance matrix are time and temperature independent. In addition, the compliance matrix is symmetric for the viscoelastic composite. The time-temperature superposition principle is used to determine shift factors. It was shown that the shift factors are independent of fiber orientation for fiber angles that vary from 10 to 90 deg with respect to the load direction.

Morris, D. H.↗

NASA NTRS1980

Creep-rupture of polymer-matrix composites

An accelerated characterization method for resin matrix composites is reviewed. Methods for determining modulus and strength master curves are given. Creep rupture analytical models are discussed as applied to polymers and polymer matrix composites. Comparisons between creep rupture experiments and analytical models are presented. The time dependent creep rupture process in graphite epoxy laminates is examined as a function of temperature and stress level.

Brinson, H. F.↗