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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Fatigue response of notched graphite/epoxy laminates

Tests were performed to determine the damage states in quasi-isotropic graphite/epoxy laminates with center holes caused by cyclic tensile loading. The influence of the stacking sequence on the initiation and interaction of damage modes and the relationship between damage, strength, stiffness, and life of the laminates were also studied. X-ray radiography, moire interferometry, and stiffness change were used to monitor damage. Fatigue damage in both laminates began with matrix cracks around the holes leading to delaminations. In laminates cycled at the same percent of notched tensile strength the stacking sequence influenced the density of the matrix cracks and the modes and distribution of the damage. Ply cracking was also caused by the stacking sequence. The damage states in the two laminates produced stiffness changes of 15 to 20 percent, different rates of change in residual strength, and a factor of two to four difference in fatigue life. It was determined that continued cyclic loading produced matrix cracks which led to fatigue of the laminates.

Kress, G. R.↗
NASA NTRS1985

Interlaminar fracture toughness: Three-dimensional finite element modeling for end-notch and mixed-mode flexure

A computational procedure is described for evaluating End-Notch-Flexure (ENF) and Mixed-Mode-Flexure (MMF) interlaminar fracture toughness in unidirectional fiber composites. The procedure consists of a three-dimensional finite element analysis in conjunction with the strain energy release rate concept and with composite micromechanics. The procedure is used to analyze select cases of ENF and MMF. The strain energy release rate predicted by this procedure is in good agreement with limited experimental data. The procedure is used to identify significant parameters associated with interlaminar fracture toughness. It is also used to determine the critical strain energy release rate and its attendant crack length in ENF and/or MMF. This computational procedure has considerable versatility/generality and provides extensive information about interlaminar fracture toughness in fiber composites.

Murthy, P. L. N.↗
NASA NTRS1985

Fatigue Damage in Notched Composite Laminates Under Tension-Tension Cyclic Loads

The results are given of an investigation to determine the damage states which develop in graphite epoxy laminates with center holes due to tension-tension cyclic loads, to determine the influence of stacking sequence on the initiation and interaction of damage modes and the process of damage development, and to establish the relationships between the damage states and the strength, stiffness, and life of the laminates. Two quasi-isotropic laminates were selected to give different distributions of interlaminar stresses around the hole. The laminates were tested under cyclic loads (R=0.1, 10 Hz) at maximum stresses ranging between 60 and 95 percent of the notched tensile strength.

Stinchcomb, W. W.↗
NASA NTRS1986

Finite element analysis of end notch flexure specimen

A finite element analysis of the end notch flexure specimen for mode II interlaminar fracture toughness measurement was conducted. The effect of friction between the crack faces and large deflection on the evaluation of G sub IIc from this specimen were investigated. Results of this study are presented in this paper.

Mall, S.↗
NASA NTRS1985

Fracture analysis of notched composites

In this study the problem of part-through cracks emanating from a central notch in a composite laminate is considered. The composite laminate is modeled as an orthotropic plate and along the branches, it is assumed that the stresses are carried by the fibers only. The stresses along the uncracked portions are assumed to be proportional to the crack opening displacement at that point. After formulating the branched crack problem, the model is applied to a (4 + or - 45 deg) 2s composite laminate. The stress concentration factors for the fibers are computed for various values of the spring constant K.

Delale, F.↗
NASA NTRS1986

Dynamic stress analysis of smooth and notched fiber composite flexural specimens

A detailed analysis of the dynamic stress field in smooth and notched fiber composite (Charpy-type) specimens is reported in this paper. The analysis is performed with the aid of the direct transient response analysis solution sequence of MSC/NASTRAN. Three unidirectional composites were chosen for the study. They are S-Glass/Epoxy, Kevlar/Epoxy and T-300/Epoxy composite systems. The specimens are subjected to an impact load which is modeled as a triangular impulse with a maximum of 2000 lb and a duration of 1 ms. The results are compared with those of static analysis of the specimens subjected to a peak load of 2000 lb. For the geometry and type of materials studied, the static analysis results gave close conservative estimates for the dynamic stresses. Another interesting inference from the study is that the impact induced effects are felt by S-Glass/Epoxy specimens sooner than Kevlar/Epoxy or T-300/Epoxy specimens.

Murthy, P. L. N.↗
NASA NTRS1986

Finite-element analysis of end-notch flexure specimens

A finite-element analysis of the end-notch flexure specimen for Mode II interlaminar fracture toughness measurement was conducted. The effects of friction between the crack faces and large deflection on the evaluation of G(IIc) from this specimen were investigated. Results of this study are presented in this paper.

Mall, S.↗
NASA NTRS1987

Reducing shuttle-payload dynamic interaction with notch filters

The use of notch filters centered on the estimated bending frequencies to reduce the effect of low-frequency Shuttle-payload dynamic interaction is investigated. The Shuttle-payload dynamics that are studied are with payloads attached to the Shuttle Remote Manipulator System (RMS). Other payload-orbiter connection such as payloads pivoted out of the cargo bay on a tilt table can lead to low-frequency bending modes. Payloads attached to the RMS will, however, present greater problems in that the bending modes will be hard to predict and will tend to vary with time as the RMS changes in orientation.

Appleby, Brent D.↗
NASA NTRS1987

Strain energy release rates of composite interlaminar end-notch and mixed-mode fracture: A sublaminate/ply level analysis and a computer code

A computer code is presented for the sublaminate/ply level analysis of composite structures. This code is useful for obtaining stresses in regions affected by delaminations, transverse cracks, and discontinuities related to inherent fabrication anomalies, geometric configurations, and loading conditions. Particular attention is focussed on those layers or groups of layers (sublaminates) which are immediately affected by the inherent flaws. These layers are analyzed as homogeneous bodies in equilibrium and in isolation from the rest of the laminate. The theoretical model used to analyze the individual layers allows the relevant stresses and displacements near discontinuities to be represented in the form of pure exponential-decay-type functions which are selected to eliminate the exponential-precision-related difficulties in sublaminate/ply level analysis. Thus, sublaminate analysis can be conducted without any restriction on the maximum number of layers, delaminations, transverse cracks, or other types of discontinuities. In conjunction with the strain energy release rate (SERR) concept and composite micromechanics, this computational procedure is used to model select cases of end-notch and mixed-mode fracture specimens. The computed stresses are in good agreement with those from a three-dimensional finite element analysis. Also, SERRs compare well with limited available experimental data.

Valisetty, R. R.↗
NASA NTRS1986

KI-solutions for single edge notch specimens under fixed end displacements

The KI solution for a finite length single-edge notch specimen loaded under fixed-end displacements is derived using a crack compliance analysis. Numerical and experimental checks of the KI solution are provided. Good agreement between the experimental and numerical solutions is observed. The applicability of conventional fracture mechanics to correlate crack growth data generated under displacement control is discussed.

Marchand, N.↗
NASA NTRS1986

Finite element analysis of the end notched flexure specimen for measuring Mode II fracture toughness

The paper presents a finite element analysis of the end-notched flexure (ENF) test specimen for Mode II interlaminar fracture testing of composite materials. Virtual crack closure and compliance techniques employed to calculate strain energy release rates from linear elastic two-dimensional analysis indicate that the ENF specimen is a pure Mode II fracture test within the constraints of small deflection theory. Furthermore, the ENF fracture specimen is shown to be relatively insensitive to process-induced cracks, offset from the laminate midplane. Frictional effects are investigated by including the contact problem in the finite element model. A parametric study investigating the influence of delamination length, span, thickness, and material properties assessed the accuracy of beam theory expressions for compliance and strain energy release rate, GII. Finite element results indicate that data reduction schemes based upon beam theory underestimate GII by approximately 20-40 percent for typical unidirectional graphite fiber composite test specimen geometries. Consequently, an improved data reduction scheme is proposed.

Gillespie, J. W., Jr.↗
NASA NTRS1986

On the analysis and design of the end notched flexure (ENF) specimen for Mode II testing

The end notched flexure (ENF) specimen is examined as a candidate for measuring interlaminar fracture toughness in skew symmetric loading. A simple design study for sizing the ENF specimen to minimize geometric nonlinear response and to avoid nonlinear material behavior or flexural failure is presented. Results indicate that interlaminar shear effects may be significant for tough resin systems requiring large thickness-to-length geometries. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.

Carlsson, L. A.↗
NASA NTRS1986

Short fatigue cracks in notched aluminum specimens

The short-crack fatigue behavior in notched specimens of Al 2024-T3 was studied at values of the stress ratio, R, equal to 0.5, 0.0, -1.0, and -2.0. Crack growth was monitored using a replica technique for cracks ranging in length from a few tens of a micron to the specimen's thickness (2.3 mm). Crack opening displacement was measured for surface cracks as short as 0.035 mm using the interferometric strain/displacement gage, a laser-based optical technique described by Sharpe (1982). It was found that the growth rates of short cracks were faster than the rates of long cracks for R values of -1.0 and -2.0. No significant difference was observed for R = 0.0, and at R = 0.5, the short cracks actually grew more slowly that the long ones. At all R values, the closure stresses measured for short cracks were smaller than those predicted for large cracks.

Lee, J. J.↗
NASA NTRS1987

Strain-energy-release rate analysis of the end-notched flexure specimen using the finite-element method

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, G sub II. The G sub II values were computed using two different techniques: the virtural 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-semi-span (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 G sub II values calculated using the compliance derivative method compared extremely well with those calculated using the VCCT. The G sub II 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 G sub II 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.

Salpekar, S. A.↗
NASA NTRS1988

Strain energy release rate analysis of the end-notched flexure specimen using the finite-element method

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, G sub II. The G sub II 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-semi-span (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 G sub II values calculated using the compliance derivative method compared extremely well with those calculated using the VCCT. The G sub II 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 G sub II values for one material system calculated from the finte-element analysis agreed with one solution in the literature and disagreed with the other solution in the literature.

Salpekar, S. A.↗
NASA NTRS1989

Response of notched AS4/PEEK laminates to tension/compression loading

Fatigue life, damage-initiation and propagation, and residual strength data are presently examined to ascertain the response of notched AS4/PEEK specimens to fully reversed tension/compression loading. Stiffness measurements made during the low-level fatigue history show that compression stiffness and tension stiffness degrade throughout the fatigue life. Damage to specimens fatigued at higher cyclic stresses developed primarily in the direction perpendicular to the loading. As in the case of specimens fatigued at lower stress levels, residual compressive stress decreased with damage development.

Simonds, Robert A.↗
NASA NTRS1990

Matrix toughness, long-term behavior, and damage tolerance of notched graphite fiber-reinforced composite materials

The long-term behavior of notched graphite-fiber-reinforced composite laminates with brittle or tough matrix materials and different fiber architectures was investigated using damage measurements and stiffness change, residual strength, and life data. The fiber/matrix materials included T300/5208, AS4/3501-6, AS4/1808, AS4/PEEK, and C3000/PMR-15 matrices and unidirectional tape and woven cloth fiber architectures. Results of damage evaluation and of residual strength measurements during the fatigue damage development showed that the long-term behavior and damage tolerance are controlled by a number of interacting factors such as the matrix toughness, fiber architecture, loading levels, and damage types and distributions.

Bakis, C. E.↗