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At least 145 records · Page 8

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NASA NTRS1991

Mechanical characterization and modeling of non-linear deformation and fracture of a fiber reinforced metal matrix composite

The nonlinear anisotropic mechanical behavior of an aluminum alloy metal matrix composite reinforced with continuous alumina fibers was determined experimentally. The mechanical behavior of the composite were modeled by assuming that the composite has a periodical microstructure. The resulting unit cell problem was solved with the finite element method. Excellent agreement was found between theoretically predicted and measured stress-strain responses for various tensile and shear loadings. The stress-strain responses for transverse and inplane shear were found to be identical and this will provide a simplification of the constitutive equations for the composite. The composite has a very low ductility in transverse tension and a limited ductility in transverse shear that was correlated to high hydrostatic stresses that develop in the matrix. The shape of the initial yield surface was calculated and good agreement was found between the calculated shape and the experimentally determined shape.

Jansson, S.↗

NASA NTRS1983

Progressive fracture of fiber composites

Refined models and procedures are described for determining progressive composite fracture in graphite/epoxy angleplied laminates. Lewis Research Center capabilities are utilized including the Real Time Ultrasonic C Scan (RUSCAN) experimental facility and the Composite Durability Structural Analysis (CODSTRAN) computer code. The CODSTRAN computer code is used to predict the fracture progression based on composite mechanics, finite element stress analysis, and fracture criteria modules. The RUSCAN facility, CODSTRAN computer code, and scanning electron microscope are used to determine durability and identify failure mechanisms in graphite/epoxy composites.

Irvin, T. B.↗

NASA NTRS1986

Progressive fracture of fiber composites

Refined models and procedures are described for determining progressive composite fracture in graphite/epoxy angleplied laminates. Unique Lewis Research Center capabilities are utilized including the Real-Time Ultrasonic C-San (RUSCAN) experimental facility and the Composite Durability Structural Analysis (CODSTRAN) computer code. CODSTRAN is used to predict the fracture progression based on composite mechanics, finite element stress analysis, and fracture criteria modules. The RUSCAN facility, CODSTRAN computer code, and scanning electron microscope are used to determine durability and identify failure mechanisms in graphite/epoxy coomposites. Results indicate that RUSCAN/CODSTRAN is an effective method of Studying progressive fracture of composites.

Irvine, T. B.↗

NASA NTRS1986

Concepts for interrelating ultrasnic attenuation, microstrucutre and fracture toughness in polycrystalline solids

Conceptual models are advanced for explaining and predicting empirical correlations found between ultrasonic measurements and fracture toughness of polycrystalline solids. The models lead to insights concerning microstructural factors governing fracture processes and associated stress wave interactions. Analysis of the empirical correlations suggested by the models indicate that, in addition to grain size and shape, grain boundary reflections, elastic anisotropy, and dislocation damping are factors that underly both fracture toughness and ultrasonic attenuation. One outcome is that ultrasonic attenuation can predict the size of crack blunting or process zones that develop in the vicinity active cracks in metals. This forms a basis for ultrasonic ranking according to variations in fracture toughness.

Vary, A.↗

NASA NTRS1988

Concepts for interrelating ultrasonic attenuation, microstructure, and fracture toughness in polycrystalline solids

Conceptual models are advanced for explaining and predicting empirical correlations found between ultrasonic measurements and fracture toughness of polycrystalline solids. The models lead to insights concerning microstructural factors governing fracture processes and associated stress wave interactions. Analysis of the empirical correlations suggested by the models indicate that, in addition to grain size and shape, grain boundary reflections, elastic anisotropy, and dislocation damping are factors that underlie both fracture toughness and ultrasonic attenuation. One outcome is that ultrasonic attenuation can predict the size of crack blunting or process zones that develop in the vicinity of active cracks in metals. This forms a basis for ultrasonic ranking according to variations in fracture toughness.

Vary, A.↗

NASA NTRS1989

Martian tension fractures and the formation of grabens and collapse features at Valles Marineris

Simple models of the Martian crust are summarized that predict extensional deformation style on the basis of depth, material friction and strength, and hydraulic conditions appropriate to the planet. These models indicate that tension fractures may be common features on Mars, given adequate differential stress conditions. Examples of tension fractures on Mars inferred from morphological criteria are examined based on the probable geologic conditions in which they formed and on model constraints. It is proposed that the grabens and collapse features of Valles Marineris are controlled by tension fractures in intact basement rocks that lie below impact ejecta.

Tanaka, K. L.↗

NASA NTRS2007

An Investigation of High-Cycle Fatigue Models for Metallic Structures Exhibiting Snap-Through Response

A study is undertaken to develop a methodology for determining the suitability of various high-cycle fatigue models for metallic structures subjected to combined thermal-acoustic loadings. Two features of this problem differentiate it from the fatigue of structures subject to acoustic loading alone. Potentially large mean stresses associated with the thermally pre- and post-buckled states require models capable of handling those conditions. Snap-through motion between multiple post-buckled equilibrium positions introduces very high alternating stress. The thermal-acoustic time history response of a clamped aluminum beam structure with geometric and material nonlinearities is determined via numerical simulation. A cumulative damage model is employed using a rainflow cycle counting scheme and fatigue estimates are made for 2024-T3 aluminum using various non-zero mean fatigue models, including Walker, Morrow, Morrow with true fracture strength, and MMPDS. A baseline zero-mean model is additionally considered. It is shown that for this material, the Walker model produces the most conservative fatigue estimates when the stress response has a tensile mean introduced by geometric nonlinearity, but remains in the linear elastic range. However, when the loading level is sufficiently high to produce plasticity, the response becomes more fully reversed and the baseline, Morrow, and Morrow with true fracture strength models produce the most conservative fatigue estimates.

Przekop, Adam↗

NASA NTRS1994

A methodology to predict damage initiation, damage growth and residual strength in titanium matrix composites

In this research, a methodology to predict damage initiation, damage growth, fatigue life, and residual strength in titanium matrix composites (TMC) is outlined. Emphasis was placed on micromechanics-based engineering approaches. Damage initiation was predicted using a local effective strain approach. A finite element analysis verified the prevailing assumptions made in the formulation of this model. Damage growth, namely, fiber-bridged matrix crack growth, was evaluated using a fiber bridging (FB) model which accounts for thermal residual stresses. This model combines continuum fracture mechanics and micromechanics analyses yielding stress-intensity factor solutions for fiber-bridged matrix cracks. It is assumed in the FB model that fibers in the wake of the matrix crack are idealized as a closure pressure, and an unknown constant frictional shear stress is assumed to act along the debond length of the bridging fibers. This frictional shear stress was used as a curve fitting parameter to the available experimental data. Fatigue life and post-fatigue residual strength were predicted based on the axial stress in the first intact 0 degree fiber calculated using the FB model and a three-dimensional finite element analysis.

Bakuckas, J. G., Jr.↗

NASA NTRS1982

Advanced ceramic coating development for industrial/utility gas turbine applications

The effects of ceramic coatings on the lifetimes of metal turbine components and on the performance of a utility turbine, as well as of the turbine operational cycle on the ceramic coatings were determined. When operating the turbine under conditions of constant cooling flow, the first row blades run 55K cooler, and as a result, have 10 times the creep rupture life, 10 times the low cycle fatigue life and twice the corrosion life with only slight decreases in both specific power and efficiency. When operating the turbine at constant metal temperature and reduced cooling flow, both specific power and efficiency increases, with no change in component lifetime. The most severe thermal transient of the turbine causes the coating bond stresses to approach 60% of the bond strengths. Ceramic coating failures was studied. Analytic models based on fracture mechanics theories, combined with measured properties quantitatively assessed both single and multiple thermal cycle failures which allowed the prediction of coating lifetime. Qualitative models for corrosion failures are also presented.

Andersson, C. A.↗

NASA NTRSDate not supplied

Interlaminar R-Curve Effects in Thermoplastic Composites: Experiments and Model Correlations

The interlaminar fracture behavior of thermoplastic composites under Mode-I and Mode-II static loading is investigated in this research. The material system under consideration is a carbon fiber reinforced low melt semi-crystalline resin TC1225 LMPAEK T700G (T700/LMPAEK)†. Characterization experiments were conducted to measure the load-displacement response, peak load, and propagation of delamination from a pre-implanted Kapton® insert. The Mode-I response was characterized using a double cantilever beam (DCB) test according to the ASTM D5528-22 standard. It was observed that the propagation exhibited significant resistance to delamination growth, i.e., an R-Curve effect. The test data indicated that the steady-state fracture toughness, G SS I , was approximately twice the mean initiation fracture toughness, G Ic . The Mode-II response of an end notch flexure (ENF) specimen tested according to the ASTM D7905-19 standard exhibited an unexpected stable behavior upon attaining peak load. Prior to attaining peak load, the Mode-II response exhibited some non-linearity, which may be attributed to nonlinear bulk material response, e.g., hyper-elasticity, viscoelasticity, plasticity, etc., and potentially friction at the load/support rollers and R-Curve effects. Finite element models based on the Cohesive Zone Method (CZM) and the Virtual Crack Closure Technique (VCCT) were used to predict the DCB and ENF specimen responses. DCB analysis results obtained using G SS I predicted propagation consistent with experimental data. However, results obtained with the as characterized G Ic were inaccurate in predicting delamination growth onset and peak load. ENF analysis results obtained with the as-characterized Mode-II fracture toughness, G IIc , were inaccurate in predicting the peak load and propagation. Hence, R-Curve effects were included in both DCB and ENF simulations, which enabled improved predictions. The experimentally determined Mode-I R-Curve was used for the DCB simulations, whereas an R-Curve was derived for Mode-II assuming that the change in compliance of the specimen was associated with crack growth only.

Prabhakar M Rao↗

NASA NTRSDate not supplied

Interlaminar R-Curve Effects in Thermoplastic Composites: Experiments and Model Correlations

Prabhakar Rao↗

NASA NTRS2011

Examination of Relationship Between Photonic Signatures and Fracture Strength of Fused Silica Used in Orbiter Windows

A commercially available grey-field polariscope (GFP) instrument for photoelastic examination is used to assess impact damage inflicted upon the outermost pane of the orbiter windows. Four categories of damage: hyper-velocity impacts that occur during space-flight (HVI); hypervelocity impacts artificially made at the Hypervelocity Impact Technology Facility (HIT-F); impacts made by larger objects falling onto the pane surface to simulate dropped items on the window during service/storage of vehicle (Bruises); and light scratches from dull objects designed to mimic those that might occur by dragging a dull object across the glass surface (Chatter Checks) are examined. The damage sites are cored from fused silica window carcasses, examined with the GFP and other methodologies, and broken using the ASTM Standard C1499-09 to measure the fracture strength. A correlation is made between the fracture strength and damage-site measurements including geometrical measurements and GFP measurements of photoelastic retardation (stress patterns) surrounding the damage sites. An analytical damage model to predict fracture strength from photoelastic retardation measurements is presented and compared with experimental results.

Yost, William T.↗

NASA NTRS1982

Minimum silicon wafer thickness for ID wafering

An analytical model, based on fracture mechanics analysis, is proposed for estimating the minimum wafer thickness as a function of the diameter requirement for solar cells. The conditions under which the model can be applied are discussed with reference to the critical flaw size, the applied force, and the width of the side support. It is shown that the equivalent cantilever force applied during ID slicing can be estimated from the wafering mechanical yield data. The width of the wafer side support was found to be a significant factor in controlling the minimum allowable wafer thickness during slicing. Wafer side support width requirements were found to increase with decreasing wafer thickness.

Chen, C. P.↗

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 NTRS1982

Micromechanical predictions of crack propagation and fracture energy in a single fiber boron/aluminum model composite

The axisymmetric finite element model and associated computer program developed for the analysis of crack propagation in a composite consisting of a single broken fiber in an annular sheath of matrix material was extended to include a constant displacement boundary condition during an increment of crack propagation. The constant displacement condition permits the growth of a stable crack, as opposed to the catastropic failure in an earlier version. The finite element model was refined to respond more accurately to the high stresses and steep stress gradients near the broken fiber end. The accuracy and effectiveness of the conventional constant strain axisymmetric element for crack problems was established by solving the classical problem of a penny-shaped crack in a thick cylindrical rod under axial tension. The stress intensity factors predicted by the present finite element model are compared with existing continuum results.

Adams, D. F.↗

NASA NTRS2003

Modeling of High-Strain-Rate Deformation, Fracture, and Impact Behavior of Advanced Gas Turbine Engine Materials at Low and Elevated Temperatures

Gamma titanium aluminides have received considerable attention over the last decade. These alloys are known to have low density, good high temperature strength retention, and good oxidation and corrosion resistance. However, poor ductility and low fracture toughness have been the key limiting factors in the full utilization of these alloys. More recently, Gamma-met PX has been developed by GKSS, Germany. These alloys have been observed to have superior strengths at elevated temperatures and quasi-static deformation rates and good oxidation resistance at elevated temperatures when compared with other gamma titanium aluminides. The present paper discusses results of a study to understand dynamic response of gamma-met PX in uniaxial compression. The experiments were conducted by using a modified split Hopkinson pressure bar between room temperature and 900 C and strain rates of up to 3500 per second. The Gamma met PX alloy showed superior strength when compared to nickel based superalloys and other gamma titanium aluminides at all test temperatures. It also showed strain and strain-rate hardening at all levels of strain rates and temperatures and without yield anomaly up to 900 C. After approximately 600 C, thermal softening is observed at all strain rates with the rate of thermal softening increasing dramatically between 800 and 900 C. However, these flow stress levels are comparatively higher in Gamma met PX than those observed for other TiAl alloys.

Shazly, Mostafa↗

NASA NTRS2018

Reactive Transport and Mass Balance Modeling of the Stimson Sedimentary Formation and Altered Fracture Zones Constrain Diagenetic Conditions at Gale Crater, Mars

On a planet as cold and dry as present-day Mars, evidence of multiple aqueous episodes offers an intriguing view into very different past environments. Fluvial, lacustrine, and eolian depositional environments are being investigated by the Mars Science Laboratory Curiosityin Gale crater, Mars. Geochemical and mineralogical observations of these sedimentary rocks suggest diagenetic processes affected the sediments. Here, we analyze diagenesis of the Stimson formation eolian parent material, which caused loss of olivine and formation of magnetite. Additional, later alteration in fracture zones resulted in preferential dissolution of pyroxene and precipitation of secondary amorphous silica and Ca sulfate. The ability to compare the unaltered parent material with the reacted material allows constraints to be placed on the characteristics of the altering solutions. In this work we use a combination of a mass balance approach calculating the fraction of a mobile element lost or gained, τ, with fundamental geochemical kinetics and thermodynamics in the reactive transport code CrunchFlow to examine the characteristics of multiple stages of aqueous alteration at Gale crater, Mars. Our model results indicate that early diagenesis of the Stimson sedimentary formation is consistent with leaching of an eolian deposit by a near-neutral solution, and that formation of the altered fracture zones is consistent with a very acidic, high sulfate solution containing Ca, P and Si. These results indicate a range of past aqueous conditions occurring at Gale crater, Mars, with important implications for past Martian climate and environments.

Gale crater↗

NASA NTRS1974

The surface geometry of inherited joint and fracture trace patterns resulting from active and passive deformation

Hypothetical models are considered for detecting subsurface structure from the fracture or joint pattern, which may be influenced by the structure and propagated to the surface. Various patterns of an initially orthogonal fracture grid are modeled according to active and passive deformation mechanisms. In the active periclinal structure with a vertical axis, fracture frequency increased both over the dome and basin, and remained constant with decreasing depth to the structure. For passive periclinal features such as a reef or sand body, fracture frequency is determined by the arc of curvature and showed a reduction over the reefmound and increased over the basin.

Podwysocki, M. H.↗