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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2001

C-Coupon Studies of CMCS: Fracture Behavior and Microstructural Characterization

A curved beam 'C-coupon' was used to assess fracture behavior in a Sylramic(tm)/melt infiltration (MI) SiC matrix composite. Failure stresses and fracture mechanisms, as determined by optical and scanning electron microstructural analysis, are compared with finite element stress calculations to analyze failure modes. Material microstructure was found to have a strong influence on mechanical behavior. Fracture occurs in interlaminar tension (ILT), provided that the ratio of ILT to tensile strength for the material is less than the ratio of radial to hoop stresses for the C-coupon geometry. Utilization of 3D architectures to improve interlaminar strength requires significant development efforts to incorporate through thickness fibers in regions with high curvatures while maintaining uniform thickness, radius, and microstructure.

Hurwitz, Frances I.↗
NASA NTRS1983

Fracture behavior of hybrid composite laminates

The tensile fracture behavior of 15 center-notched hybrid laminates was studied. Three basic laminate groups were tested: (1) a baseline group with graphite/epoxy plies, (2) a group with the same stacking sequence but where the zero-deg plies were one or two plies of S-glass or Kevlar, and (3) a group with graphite plies but where the zero-deg plies were sandwiched between layers of perforated Mylar. Specimens were loaded linearly with time; load, far field strain, and crack opening displacement (COD) were monitored. The loading was stopped periodically and the notched region was radiographed to reveal the extent and type of damage (failure progression). Results of the tests showed that the hybrid laminates had higher fracture toughnesses than comparable all-graphite laminates. The higher fracture toughness was due primarily to the larger damage region at the ends of the slit; delamination and splitting lowered the stress concentration in the primary load-carrying plies. A linear elastic fracture analysis, which ignored delamination and splitting, underestimated the fracture toughness. For almost all of the laminates, the tests showed that the fracture toughness increased with crack length. The size of the damage region at the ends of the slit and COD measurements also increased with crack length.

Kennedy, J. M.↗
NASA NTRS1975

An investigation of the failure and fracture behavior of graphite/epoxy laminates

A fundamental investigation of the fracture behavior of fiber reinforced composites is discussed. It is suggested that experimental and analytical investigations should compliment each other. Further, it is suggested that in order to understand and mathematically model composite fracture it is necessary to explore, understand and characterize the stress-strain behavior of each type of laminate tested from initial loading to incipient fracture. That is, it is not only necessary to perform fracture studies but it is necessary to perform stress-strain characterization and failure studies as well. For this reason a literature survey is presented in which these three elements are discussed. An experimental program to determine tensile properties of unnotched and notched unidirectional and multi-directional laminates composed of Hercules AS Fiber and 3501 Resin is described. Experimental unnotched moduli and strengths are presented, discussed and compared to lamination theory predictions. It is shown that rate effects are small and that lamination theory gives good results for moduli but not for strengths.

Brinson, H. F.↗
NASA NTRS1979

A correlative study between analysis and experiment on the fracture behavior of graphite/epoxy composites

The paper compares the fracture behavior of a composite material by using the analytical models of Waddoups et al. (1971), Whitney and Nuismer (1974, 1975), and Snyder and Cruse (1975) with experimental results from tests performed on center-notched tensile strips. Laminate configurations of (0 deg)8s, (0 deg/90 deg)4s, (+ and -45 deg)4s, and (0 deg/+ and -45 deg/0 deg)2s from T300/934 graphite/epoxy are tested. These particular configurations are used so that the effect of various degrees of anisotropy can be studied. The procedure adopted uses the results from one test for crack size aspect ratio to predict the results of tests of other aspect ratios. For those methods that use a characteristic dimension, predictions are made by assuming the magnitude of this dimension to be constant. The validity of this assumption for a laminate is assessed by comparing predicted and experimental results. Analytical models using a characteristic dimension are compared to the model developed by Cruse (1973).

Yeow, Y. T.↗
NASA NTRS1994

The effect of various metallurgical parameters on the flow and fracture behavior of polycrystalline NiAl near the brittle-to-ductile transition

An investigation of the effect of various metallurgical parameters such as interfaces, allowing additions, test temperature, and strain rate on the flow and fracture behavior of polycrystalline NiAl is summarized. From this study, a more complete understanding of the deformation and fracture behavior of polycrystalline NiAl near the brittle-to-ductile transition temperature has been developed. A mechanism for the BDTT is proposed that is based on the operation of localized dislocation climb processes that operate within the vicinity of the grain boundaries and provide the additional deformation mechanisms necessary for grain-to-grain compatibility during plastic deformation. Finally, methods for improving the low temperature mechanical behavior of NiAl were considered and reviewed within the context of the present knowledge of NiAl-based materials and the operative deformation and fracture mechanisms determined in this study. Special emphasis was placed on the use of second phases for improving low temperature properties.

Noebe, Ronald D.↗
NASA NTRS2001

Thermal Fatigue and Fracture Behavior of Ceramic Thermal Barrier Coatings

Thermal fatigue and fracture behavior of plasma-sprayed ceramic thermal barrier coatings has been investigated under high heat flux and thermal cyclic conditions. The coating crack propagation is studied under laser heat flux cyclic thermal loading, and is correlated with dynamic fatigue and strength test results. The coating stress response and inelasticity, fatigue and creep interactions, and interface damage mechanisms during dynamic thermal fatigue processes are emphasized.

Zhu, Dong-Ming↗
NASA NTRS1985

Nonlinear quasi-fracture behavior of polymers

This paper deals with the nonlinear quasi-fracture behavior of polymers. Using the finite element method, the quasi-fracture model composed of an isolated craze opening with a nonlinear boundary envelope forces represented by oriented fibril domains is analyzed in simply-stressed rectangular plate. The stress distribution, the displacement profile, as well as the yield or plastic or flow zone adjacent to the craze tips are obtained. It is found that the variation of the stress distribution near the craze tip is not as sharp as that for an elastic crack. The initial load required for yielding or flowing is greater for the quasifracture as compared with that for a classical fracture. Under the same load the yield or flow zone is somewhat smaller as compared with that obtained on the basis of linear elastic-plastic fracture mechanics theory.

Sun, B. N.↗
NASA NTRS2004

Sintering and Fracture Behavior of Plasma-sprayed Thermal Barrier Coatings

Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section components in order to meet future engine higher fuel efficiency and lower emission goals. There is a need to characterize the fundamental sintering and fracture behavior of the current ZrO2-(7 to 8)wt%Y2O3 coating, in order to fully take advantage of the coating capability. In addition, a thorough evaluation of the coating behavior and temperature limits will be useful for more accurately assessing the benefit gained from future advanced coating systems. In this study, the sintering behavior of plasma-sprayed ZrO2-8wt%Y2O3 coatings was systematically investigated as a function of temperature and time using a dilatometer in the temperature range of 1200-1500 C. The coating sintering kinetics obtained by dilatometry were compared with the coating thermal conductivity increase kinetics, determined by a steady-state laser heat-flux testing approach, under high temperature and thermal gradient sintering conditions. The mode I, mode II, and mixed mode I-mode II fracture behavior of as-processed and sintering-annealed coatings was determined in asymmetric flexure loading at ambient and elevated temperatures in order to evaluate the coating sintering effects on the fracture envelope of K(sub I) versus K(sub II). The coating thermal conductivity cyclic response associated with the interface delamination of the coating systems under simulated engine heat-flux conditions will be discussed in conjunction with the sintering and fracture testing results.

Zhu, Dongming↗
NASA NTRS1977

An experimental study of the fracture behavior of laminated graphite/epoxy composites

The results of an experimental investigation on the fracture behavior of unidirectional and multidirectional laminated graphite/epoxy composites is reported. Critical tensile fracture stresses for constant head rate uniaxial specimens containing single-edge notches, double-edge notches, and centrally located circular holes are presented. Results are reported for loads and notches at various angles to the fiber direction. Evidence of notch sensitivity and stable crack growth is presented. Self-similar crack growth is shown to occur for only a limited set of circumstances. Critical stresses are compared to the theories of Waddoups et al and Whitney et al. These comparisons are presented using isotropic and orthotropic stress concentration factors together with a finite width correction factor. The results tend to show that use of only isotropic correction factors allow good correlation between theory and experiment.

Brinson, H. F.↗
NASA NTRS1983

Fracture behavior of unidirectional boron/aluminum composite laminates

An experimental investigation of the fracture behavior of unidirectional boron/aluminum composite laminates was conducted in order to verify the results of mathematical models. These models predict the fiber stresses and displacements and the amount of damage growth in a center-notched lamina as a function of the applied remote stress and the matrix and fiber material properties. A brittle lacquer coating was used to detect the yielding in the matrix while X-ray techniques were used to determine the number of broken fibers in the laminate. The notched strengths and the amounts of damage found in the specimens agreed well with those predicted by the models. It was shown that for thin laminates the amount of damage and the fiber displacements do not depend strongly on the number of plies for a given notch width.

Jones, F. W.↗
NASA NTRS1987

Deformation, fatigue and fracture behavior of two cast anisotropic superalloys

Tensile and low cycle fatigue (LCF) tests were conducted on two cast anisotropic superalloys. The effects of temperature, strain rate and stress range were investigated. Deformation behavior was extensively characterized and modeled. LCF and fracture behavior were studied and correlated with deformation behavior.

Milligan, Walter W.↗
NASA NTRS1984

Fracture behavior of thick, laminated graphite/epoxy composites

The effect of laminate thickness on the fracture behavior of laminated graphite epoxy (T300/5208) composites was studied. The predominantly experimental research program included the study of the 0/+ or - 45/90 sub ns and 0/90 sub ns laminates with thickness of 8, 32, 64, 96 and 120 plies and the 0/+ or - 45 sub ns laminate with thickness of 6, 30, 60, 90 and 120 plies. The research concentrated on the measurement of fracture toughness utilizing the center-cracked tension, compact tension and three point bend specimen configurations. The development of subcritical damage at the crack tip was studied nondestructively using enhanced X-ray radiography and destructively using the laminate deply technique. The test results showed fracture toughness to be a function of laminate thickness. The fracture toughness of the 0 + or - 45/90 sub ns and 0/90 sub ns laminates decreased with increasing thickness and asymptotically approached lower bound values of 30 ksi square root of in. (1043 MPa square root of mm and 25 ksi square root of in (869 MPa square root of mm respectively. In contrast to the other two laminates, the fracture toughness of the 0/+ or - 45 sub ns laminate increased sharply with increasing thickness but reached an upper plateau value of 40 ksi square root of in (1390 MPa square root of mm) at 30 plies. Fracture toughness was independent of crack size for both thin and thick laminates for all three laminate types except for the 0/90 sub 2s laminate which spilt extensively. The center cracked tension, three point bend and compact tension specimens gave comparable results.

Harris, C. E.↗
NASA NTRS1984

Fracture behavior in silicon

The fracture mechanics of crystalline Si are reviewed, together with known techniques for minimizing the occurences of fracture and/or their effects. The fracture toughness (Kic) of Si varies only 10 percent from cell-to-cell and standard values have been established for different types of crystalline Si cells. A critical flaw size of 10-100 microns has been identified, and also pertains to polycrystalline materials. Chemical polishing is known to double the value of Kic, while edge rounding has no effect. Internal stresses, particularly those caused during ribbon growth, do not exceed 10 percent of Kic. External stresses are imposed by the module hardware and the ambient environment. Multiple contacts reduce the effects of cell fracture and series-parallel wiring in modules in arrays ameliorates the effects of single-cell failures. During manufacturing, maintenance of quality control and removal of sheets displaying aberrations can, depending on the costs and the implementation of the array reliability features, result in arrays delivering any desired level of reliability.

Leipold, M. H.↗
NASA NTRS1980

Mode I fracture behavior of nonlinear materials

A finite element analysis is presented for the Mode I fracture behavior of cracked plates (stationary crack) made of different nonlinear materials (elastoplastic or elastic locking). The assumed stress-strain behavior of the nonlinear materials was piecewise linear. For each given 'stationary' crack size, the corresponding critical remote tensile stress was calculated based on the maximum crack-tip stress failure criterion. It was found that in the log-log plots of the critical remote stress versus critical crack length, the fracture data of the piecewise linear materials obey a 'stepwise-linear-inverse-square-root' fracture law. It is also shown how the fracture data of the piecewise linear materials can be fitted by proper piecewise graphical shifting of the classical 'inverse-square-root' fracture curve for the linearly elastic materials.

Ko, W. L.↗
NASA NTRS1982

Fracture behavior of low-density fibrous ceramics

A study has been carried out to characterize the fracture behavior of a particular class of silica tiles used in the thermal protection system of the Space Shuttle with emphasis on the strength distribution and time dependence of strength. The approach adopted is based on fracture mechanics concepts developed in recent years for the design of brittle materials. The study confirms that proof-testing and tile densification should lead to improved reliability of the thermal protection system. The research also suggests that the subcritical crack growth behavior of the fibrous tiles is similar to that observed in bulk glass of similar composition and that failure occurs by the tensile fracture and fragmentation of individual fibers.

Green, D. J.↗
NASA NTRS1984

Deformation and fracture behavior of Ni-Mo-Al(gamma/gamma prime-alpha) in situ composite

Tensile properties of a directionally solidified (DS) eutectic alloy of the nominal composition Ni-33 Mo-5.7 Al (weight percent) have been investigated both at room temperature and elevated temperatures. The microstructure-mechanical property relationship has been studied for the alloy both in the as-DS and heat-treated conditions. Changes in the yield strength, the work hardening behavior, and the fracture morphology have been explained in terms of the microstructural changes due to the heat treatment. The yield drops observed have been attributed to the microdebonding due to the possible segregation of impurities at the fiber-matrix interface, and partly to the strain aging. The deformation mechanism has been identified to be the cutting of gamma prime particles.

Sriramamurthy, A. M.↗