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At least 361 records · Page 20

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NASA NTRS1977

Bayesian estimation of crack initiation times from service data

Lockheed C-130 Hercules aircraft have during their service life been periodically inspected and growing cracks around rivet holes were recorded. This record has recently been used to determine the statistical distributions of crack initiation times and the distribution of initial crack sizes. When crack initiation times are calculated from such cracks, by backward extrapolation of the growth relation, the resulting distribution of crack initiation times will indicate a preponderance of short times to crack initiation. If however, such distributions are combined with the reliability of the inspection procedure, the statistical distribution of missed initiation times can be estimated. The method used is based on Bayes theorem which permits the calculation of the 'prior' distribution (initiation times before inspection) from a knowledge of the 'posterior' distribution (initiation times obtained from the inspection) and a 'likelihood function' (reliability of the inspection) procedure. The results indicate that during an early inspection a large percentage of initiation times will be missed and that the fraction of located initiation times increases during later inspections.

Heller, R. A.↗

NASA NTRS1976

Stress-intensity factors for a thick-walled cylinder containing an annular imbedded or external or internal surface crack

The elastostatic axisymmetric problem for a long thick-walled cylinder containing a ring-shaped internal or edge crack is considered. Using the standard transform technique the problem is formulated in terms of an integral equation which has a simple Cauchy kernel for the internal crack and a generalized Cauchy kernel for the edge crack as the dominant part. As examples the uniform axial load and the steady-state thermal stress problems have been solved and the related stress intensity factors have been calculated. Among other findings the results show that in the cylinder under uniform axial stress containing an internal crack the stress intensity factor at the inner tip is always greater than that at the outer tip for equal net ligament thicknesses and in the cylinder with an edge crack which is under a state of thermal stress the stress intensity factor is a decreasing function of the crack depth, tending to zero as the crack depth approaches the wall thickness.

Erdol, R.↗

NASA NTRS1978

Finite element microscopic stress analysis of cracked composite systems

This paper considers the stress concentration problems of two types of cracked composite systems: (1) a composite system with a broken fiber (a penny-shaped crack problem), and (2) a composite system with a cracked matrix (an annular crack problem). The cracked composite systems are modeled with triangular and trapezoidal ring finite elements. Using NASTRAN (NASA Structural Analysis) finite element computer program, the stress and deformation fields in the cracked composite systems are calculated. The effect of fiber-matrix material combination on the stress concentrations and on the crack opening displacements is studied.

Ko, W. L.↗

NASA NTRS1978

A thick-walled cylinder with an axisymmetric internal or edge crack

The paper considers the elastostatic axisymmetric problem for a long thick-walled cylinder containing a ring-shaped internal or edge crack. Using the standard transform technique the problem is formulated in terms of an integral equation which has a simple Cauchy kernel for the internal crack and a generalized Cauchy kernel for the edge crack as the dominant part. As examples the uniform axial load and the steady-state thermal stress problems have been solved and the related stress-intensity factors have been calculated. Among other findings the results show that in the cylinder under uniform axial stress containing an internal crack the stress-intensity factor at the inner tip is always greater than that at the outer tip for equal net ligament thicknesses and in the cylinder with an edge crack which is under a state of thermal stress the stress-intensity factor is a decreasing function of the crack depth, tending to zero as the crack depth approaches the wall thickness.

Erdol, R.↗

NASA NTRS1979

Crack propagation in aluminum sheets reinforced with boron-epoxy

An analysis was developed to predict both the crack growth and debond growth in a reinforced system. The analysis was based on the use of complex variable Green's functions for cracked, isotropic sheets and uncracked, orthotropic sheets to calculate inplane and interlaminar stresses, stress intensities, and strain-energy-release rates. An iterative solution was developed that used the stress intensities and strain-energy-release rates to predict crack and debond growths, respectively, on a cycle-by-cycle basis. A parametric study was made of the effects of boron-epoxy composite reinforcement on crack propagation in aluminum sheets. Results show that the size of the debond area has a significant effect on the crack propagation in the aluminum. For small debond areas, the crack propagation rate is reduced significantly, but these small debonds have a strong tendency to enlarge. Debond growth is most likely to occur in reinforced systems that have a cracked metal sheet reinforced with a relatively thin composite sheet.

Roderick, G. L.↗

NASA NTRS1980

Sudden bending of cracked laminates

A dynamic approximate laminated plate theory is developed with emphasis placed on obtaining effective solution for the crack configuration where the 1/square root of r stress singularity and the condition of plane strain are preserved. The radial distance r is measured from the crack edge. The results obtained show that the crack moment intensity tends to decrease as the crack length to laminate plate thickness is increased. Hence, a laminated plate has the desirable feature of stabilizing a through crack as it increases its length at constant load. Also, the level of the average load intensity transmitted to a through crack can be reduced by making the inner layers to be stiffer than the outer layers. The present theory, although approximate, is useful for analyzing laminate failure to crack propagation under dynamic load conditions.

Sih, G. C.↗

NASA NTRS1980

Effect of material nonhomogeneity on crack propagation characteristics

The influence of material nonhomogeneity on the behavior of a moving crack is investigated. The model assumes a running crack in a material whose elastic properties may differ from those of the surrounding material. Theoretical calculations showed that the energy stored in elements ahead of the crack can be raised or lowered depending on the crack velocity, the crack length and the degree of material nonhomogeneity which is associated with the ratio of the shear moduli and the distance between the crack and the neighboring material with different elastic properties. Based on the strain energy density theory, predictions are made on how material nonhomogeneity can influence the initiation and/or arrest characteristics of cracks.

Sih, G. C.↗

NASA NTRS1981

Prediction of fatigue-crack growth under variableamplitude and spectrum loading using a closure model

An existing analytical crack closure model was used to study crack growth under various load histories. The model was based on a concept like the Dugdale model, but modified to leave plastically deformed material in the wake of the advancing crack tip. The model was used to correlate crack growth rates under constant amplitude loading, and to predict crack growth under variable amplitude and aircraft spectrum loading on 2219-T851 aluminum alloy sheet material. The predicted crack growth lives agreed well with experimental data. For 80 crack growth tests subjected to various load histories, the ratio of predicted-to-experimental lives (N(P)/n(T)) ranged from 0.5 to 1.8. The mean value of N(P)/N(T) was 0.97 and the standard deviation was 0.27.

Newman, J. C., Jr.↗

NASA NTRS1979

NASA-Langley Research Center's participation in a round-robin comparison between some current crack-propagation prediction methods

A round-robin study was conducted which evaluated and compared different methods currently in practice for predicting crack growth in surface-cracked specimens. This report describes the prediction methods used by the Fracture Mechanics Engineering Section, at NASA-Langley Research Center, and presents a comparison between predicted crack growth and crack growth observed in laboratory experiments. For tests at higher stress levels, the correlation between predicted and experimentally determined crack growth was generally quite good. For tests at lower stress levels, the predicted number of cycles to reach a given crack length was consistently higher than the experimentally determined number of cycles. This consistent overestimation of the number of cycles could have resulted from a lack of definition of crack-growth data at low values of the stress intensity range. Generally, the predicted critical flaw sizes were smaller than the experimentally determined critical flaw sizes. This underestimation probably resulted from using plane-strain fracture toughness values to predict failure rather than the more appropriate values based on maximum load.

Hudson, C. M.↗

NASA NTRS1981

Interlaminar crack growth in fiber reinforced composites during fatigue, part 3

Interlaminar crack growth behavior in fiber-reinforced composites subjected to fatigue loading was investigated experimentally and theoretically. In the experimental phase, inter-laminar crack propagation rates and mechanisms were determined for the cases of various geometries, laminate parameters and cyclic stress levels. A singular hybrid-stress finite element method was used in conjuction with the experimental results to examine the local crack-tip behavior and to characterize the crack propagation during fatigue. Results elucidate the basic nature of the cyclic delamination damage, and relate the interlaminar crack growth rate to the range of mixed-mode crack-tip stress intensity factors. The results show that crack growth rates are directly related to the range of the mixed-mode cyclic stress intensity factors by a power law relationship.

Wang, S. S.↗

NASA NTRS1982

A preliminary study of crack initiation and growth at stress concentration sites

Crack initiation and propagation models for notches are examined. The Dowling crack initiation model and the E1 Haddad et al. crack propagation model were chosen for additional study. Existing data was used to make a preliminary evaluation of the crack propagation model. The results indicate that for the crack sizes in the test, the elastic parameter K gave good correlation for the crack growth rate data. Additional testing, directed specifically toward the problem of small cracks initiating and propagating from notches is necessary to make a full evaluation of these initiation and propagation models.

Dawicke, D. S.↗

NASA NTRS1982

Analysis of a turbine rotor containing a transverse crack at Oak Creek Unit 17

Transient increases in one, two and three per revolution vibration characteristics of a low pressure steam turbine were observed during steam temperature reduction operations. Vibration and fracture mechanics analyses suggested the presence of a transverse shaft crack which was eventually identified by ultrasonic inspection and confirmed by destructive sectioning. Signature analyses of vibration data recorded over a two-year period prior to crack identification are correlated with fatigue crack growth, which occurred intermittently during transient temperature decreases. The apparent increased response of the rotor to vibration is due to asymmetric stiffness changes introduced by the growing transverse crack. The vibration response is predicted to increase with increasing crack depths in excess of 10% of the shaft diameter. Fracture mechanics analyses predict that fatigue crack growth occurred during periods of steam temperature decrease, when high surface tensile stresses are present. These same transient thermal stresses are shown to have retarded and prevented subsequent fatigue crack growth during steady operation.

Rogers, G. W.↗

NASA NTRS1982

The crack problem in a specially orthotropic shell with double curvature

The crack problem of a shallow shell with two nonzero curvatures is considered. It is assumed that the crack lies in one of the principal planes of curvature and the shell is under Mode I loading condition. The material is assumed to be specially orthotropic. After giving the general formulation of the problem the asymptotic behavior of the stress state around the crack tip is examined. The analysis is based on Reissner's transverse shear theory. Thus, as in the bending of cracked plates, the asymptotic results are shown to be consistent with that obtained from the plane elasticity solution of crack problems. Rather extensive numerical results are obtained which show the effect of material orthotropy on the stress intensity factors in cylindrical and spherical shells and in shells with double curvature. Other results include the stress intensity factors in isotropic toroidal shells with positive or negative curvature ratio, the distribution of the membrane stress resultant outside the crack, and the influence of the material orthotropy on the angular distribution of the stresses around the crack tip.

Delale, F.↗

NASA NTRS1982

Moving cracks in layered composites

A three-layered composite with a crack spreading in the center layer has been analytically examined to evaluate the effect of material nonhomogeneity on a constant velocity crack. Two different loading characteristics are considered. In the first case, crack motion is maintained by uniform tensile stresses. In the other, crack deformation is caused by anti-plane shear stresses. Galilean transformation and Fourier sine and cosine transforms are used to determine dynamic crack tip stress fields. Standard Fredholm integral equations yield the dynamic stress intensity factors. The results show that the intensity of local dynamic stresses increases or decreases with crack length to layer thickness as a function of the relative magnitudes of the adjoining layer's material properties. Crack speed tends to increase the effect of material nonhomogeneity.

Sih, G. C.↗

NASA NTRS1983

The crack problem in a specially orthotropic shell with double curvature

Delale, F.↗

NASA NTRS1983

Fatigue crack initiation and propagation in several nickel-base superalloys at 650 C

The modes of crack initiation and propagation of several nickel-base superalloys have been examined after fatigue and creep-fatigue testing at 650 C. In fatigue, crack initiation was transgranular and frequently associated with porosity or inclusions in the higher strength alloys. These defects were usually located at the surface, except for tests at low strain ranges where larger, internal defects often initiated failure. Although fatigue crack initiation was transgranular, in those alloys with grain sizes of less than 15 microns, fatigue crack growth quickly became intergranular. This transition was environmentally assisted and did not occur for subsurface cracks until the crack broke through to the atmosphere. In the creep-fatigue cycle, which included a 900 s tensile dwell, crack initiation and propagation were both intergranular in all alloys.

Gayda, J.↗

NASA NTRS1983

A pressurized cylindrical shell with a fixed end which contains an axial part-through or through crack

A cylindrical shell having a very stiff and plate or a flange is considered. It is assumed that near the end the cylinder contains an axial flaw which may be modeled as a part through surface crack or a through crack. The effect of the end constraining on the stress intensity factor which is the main fracture mechanics parameter is studied. The applied loads acting on the cylinder are assumed to be axisymmetric. Thus the crack problem under consideration is symmetric with respect to the plane of the crack and consequently only the Mode 1 stress intensity factors are nonzero. With this limitation, the general perturbation problem for a cylinder with a built in end containing an axial crack is considered. Reissner's shell theory is used to formulate the problem. The part through crack problem is treated by using a line spring model. In the case of a crack tip terminating at the fixed end it is shown that the integral equations of the shell problem has the same generalized Cauchy kernel as the corresponding plane stress elasticity problem.

Yahsi, O. S.↗

NASA NTRS1984

Tensile strength of composite sheets with unidirectional stringers and crack-like damage

The damage tolerance characteristics of metal tension panels with riveted and bonded stringers are well known. The stringers arrest unstable cracks and retard propagation of fatigue cracks. Residual strengths and fatigue lives are considerably greater than those of unstiffened or integrally stiffened sheets. The damage tolerance of composite sheets with bonded composite stringers loaded in tension was determined. Cracks in composites do not readily propagate in fatigue, at least not through fibers. Moreover, the residual strength of notched composites is sometimes even increased by fatigue loading. Therefore, the residual strength aspect of damage tolerance, and not fatigue crack propagation, was investigated. About 50 graphite/epoxy composite panels were made with two sheet layups and several stringer configurations. Crack-like slots were cut in the middle of the panels to simulate damage. The panels were instrumented and monotonically loaded in tension to failure. The tests indicate that the composite panels have considerable damage tolerance, much like metal panels. The stringers arrested cracks that ran from the crack-like slots, and the residual strengths were considerably greater than those of unstiffened composite sheets. A stress intensity factor analysis was developed to predict the failing strains of the stiffened panels. Using the analysis, a single design curve was produced for composite sheets with bonded stringers of any configuration.

Poe, C. C., Jr.↗