Engineering topics

Chan, K. S.

Publications and source records attributed to Chan, K. S..

16 records

NASA NTRS1994

Analysis of small crack behavior for airframe applications

The small fatigue crack problem is critically reviewed from the perspective of airframe applications. Different types of small cracks-microstructural, mechanical, and chemical-are carefully defined and relevant mechanisms identified. Appropriate analysis techniques, including both rigorous scientific and practical engineering treatments, are briefly described. Important materials data issues are addressed, including increased scatter in small crack data and recommended small crack test methods. Key problems requiring further study are highlighted.

Mcclung, R. C.↗

NASA NTRS1991

Evaluation of a hydrogen resistant titanium aluminide alloy

The Ti-24Al-11Nb (Ti-24-11) alloy heat treated to the fine basketweave microstructure was shown previously to be hydrogen tolerant. In order to assess its limit of hydrogen tolerance, the tensile, creep, fracture toughness, and sustained load crack growth behaviors of this alloy were studied as a function of hydrogen content. All test specimens were thermally charged with internal hydrogen and tested at 25 and 600 C. Coupon specimens were used for developing the hydrogen charging procedures and for studying compatibility of the alloy with high temperature, high pressure gaseous hydrogen. The mechanical test results indicated that the fine basketweave microstructure was tolerant to hydride embrittlement for hydrogen contents up to approximately 1500 wt. ppm, providing that the hydride formed was of the TiH2 type. On the other hand, hydrogen charging experiments indicated that the Ti-24-11 alloy was severely cracked and pulverized under zero load when the hydrogen content exceeded 3000 wt. ppm. X-ray diffraction results revealed that the dichotomous behaviors might be due to the formation of TiH(1.924) type hydrides at higher hydrogen contents. Thus, hydrogen embrittlement in the Ti-24-11 alloy with the fine basketweave microstructure depends on hydrogen content and the nature of the hydrides formed.

Chan, K. S.↗

NASA NTRS1990

High temperature inelastic deformation of the B1900 + Hf alloy under multiaxial loading - Theory and experiment

The multiaxial deformation behavior of the Ni-based alloy B1900 + Hf has been studied at elevated temperatures in the range of 649-982 C. Combined tension/torsion cyclic tests were performed on thin-wall tubular specimens under both in-phase and out-of-phase strain-controlled loading cycles. Both straining conditions resulted in stress loci of comparable magnitude, exhibiting no difference in cyclic hardening response. A phase angle was observed between the deviatoric stress and the incremental plastic strain vectors during 90-deg out-of-phase strain cycling, and nonproportional stress relaxation occurred under biaxial strain hold. The overall results have been used to assess the flow law, the hardening equations, and the applicability of the J2-based, elastic-viscoplastic model of Bodner-Partom (1979) for multiaxial loading conditions. The overall agreement between theory and experiment is good. Discrepancies are discussed in relation to micromechanical considerations.

Chan, K. S.↗

NASA NTRS1990

Inelastic deformation under nonisothermal loading

The objective of this paper is to evaluate, both experimentally and analytically, the appropriate forms of the hardening evolution equations in unified constitutive models for conditions involving nonisothermal loading. Critical experiments were performed for a cast nickel-base superalloy by using variable temperature tensile, creep, and cyclic tests in the 538-982 C temperature range. These experimental results were compared with both isothermal data and predictions of the Bodner-Partom-Bodner, 1987 elastic-viscoplastic theory to assess the effects of thermal history on constitutive behavior. The results indicate that the hardening evolution equations based on isothermal data are applicable for nonisothermal loading of these precipitation strengthened alloys. Additional thermal history effect terms in the hardening evolution equations were not required beyond those accounting for the variation of material constants with temperature. Using material constants determined solely from isothermal data, the inelastic deformation behavior of B1900 + Hf subject to thermomechanical loading were adequately predicted by the Bodner-Partom model.

Chan, K. S.↗

NASA NTRS1989

High temperature inelastic deformation under uniaxial loading - Theory and experiment

The elevated-temperature uniaxial inelastic deformation behavior of an Ni-base alloy, B1900 + Hf, is investigated by performing isothermal tensile, creep, cyclic, stress relaxation, and thermomechanical fatigue tests. The range of strain rates examined is from 10 to the -7th to 100 per sec, while the test temperatures range from 25 to 1093 C. This extensive constitutive data base has been used for evaluating the unified constitutive models of Bodner and Partom (1972) and of Walker (1972) which apply for the small-strain regime. Comparison of test results with independent model predictions indicates good agreement over a broad range of loading conditions, demonstrating the applicability of the unified-constitutive-equation approach for describing the strongly nonlinear and temperature-dependent response of meals under a wide range of deformation and thermal histories. Thus the results give confidence that the unified approach is an effective and efficient approach in which complex history-dependent thermoviscoplastic flow can be represented within a single inelastic strain-rate term.

Chan, K. S.↗

NASA NTRS1988

Inelastic deformation and dislocation structure of a nickel alloy - Effects of deformation and thermal histories

Inelastic deformation behavior of the cast Ni-base alloy, B1900 + Hf, was investigated using data from step-temperature tensile tests and thermomechanical cyclic tests in the temperature ranges 538-760 C and 760-982 C. The deformation results were correlated with the dislocation structures of deformed specimens, identified by TEM. It was found that, in the 760-982 C temperature range, there are no thermal history effects in the inelastic deformation behavior of B1900 + Hf. In the 538-760 range, anomalous cyclic hardening and, possibly, thermal history effects were observed in thermomechanically deformed alloy, caused by sessile (010) dislocations in the gamma-prime phase.

Chan, K. S.↗

NASA NTRS1988

Constitutive modeling for isotropic materials

The third and fourth years of a 4-year research program, part of the NASA HOST Program, are described. The program goals were: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analysis of hot section components of gas turbine engines. The unified models selected for development and evaluation were those of Bodner-Partom and of Walker. The unified approach for elastic-viscoplastic constitutive equations is a viable method for representing and predicting material response characteristics in the range where strain rate and temperature dependent inelastic deformations are experienced. This conclusion is reached by extensive comparison of model calculations against the experimental results of a test program of two high temperature Ni-base alloys, B1900+Hf and Mar-M247, over a wide temperature range for a variety of deformation and thermal histories including uniaxial, multiaxial, and thermomechanical loading paths. The applicability of the Bodner-Partom and the Walker models for structural applications has been demonstrated by implementing these models into the MARC finite element code and by performing a number of analyses including thermomechanical histories on components of hot sections of gas turbine engines and benchmark notch tensile specimens. The results of the 4-year program have been published in four annual reports. The results of the base program are summarized in this report. The tasks covered include: (1) development of material test procedures, (2) thermal history effects, and (3) verification of the constitutive model for an alternative material.

Chan, K. S.↗

NASA NTRS1988

Unified constitutive models for high-temperature structural applications

Unified constitutive models are characterized by the use of a single inelastic strain rate term for treating all aspects of inelastic deformation, including plasticity, creep, and stress relaxation under monotonic or cyclic loading. The structure of this class of constitutive theory pertinent for high temperature structural applications is first outlined and discussed. The effectiveness of the unified approach for representing high temperature deformation of Ni-base alloys is then evaluated by extensive comparison of experimental data and predictions of the Bodner-Partom and the Walker models. The use of the unified approach for hot section structural component analyses is demonstrated by applying the Walker model in finite element analyses of a benchmark notch problem and a turbine blade problem.

Lindholm, U. S.↗

NASA NTRS1988

Phenomenological modeling of hardening and thermal recovery in metals

Modeling of hardening and thermal recovery in metals is considered within the context of unified elastic-viscoplastic theories. Specifically, the choices of internal variables and hardening measures, and the resulting hardening response obtained by incorporating saturation-type evolution equations into two general forms of the flow law are examined. Based on the analytical considerations, a procedure for delineating directional and isotropic hardening from uniaxial hardening data has been developed for the Bodner-Partom model and applied to a nickel-base superalloy, B1900 + Hf. Predictions based on the directional hardening properties deduced from the monotonic loading data are shown to be in good agreement with results of cyclic tests.

Chan, K. S.↗

NASA NTRS1988

The constitutive representation of high-temperature creep damage

The elastic-viscoplastic constitutive equations of Bodner-Partom were applied to modeling creep damage in a high temperature Ni-alloy, B1900 + Hf. Both tertiary creep in bulk materials and creep crack growth in flawed materials were considered. In the latter case, the energy rate line integral was used for characterizing the crack driving force, and the rate of crack extension was computed using a local damage formulation that assumed fracture was controlled by cavitation occurring within the crack-tip process zone. The results of this investigation were used to assess the evolution equation for isotropic damage utilized in the Bodner-Partom constitutive equations.

Chan, K. S.↗

NASA NTRS1985

Constitutive modeling for isotropic materials (HOST)

This report presents the results of the second year of work on a problem which is part of the NASA HOST Program. Its goals are: (1) to develop and validate unified constitutive models for isotropic materials, and (2) to demonstrate their usefulness for structural analyses of hot section components of gas turbine engines. The unified models selected for development and evaluation are that of Bodner-Partom and Walker. For model evaluation purposes, a large constitutive data base is generated for a B1900 + Hf alloy by performing uniaxial tensile, creep, cyclic, stress relation, and thermomechanical fatigue (TMF) tests as well as biaxial (tension/torsion) tests under proportional and nonproportional loading over a wide range of strain rates and temperatures. Systematic approaches for evaluating material constants from a small subset of the data base are developed. Correlations of the uniaxial and biaxial tests data with the theories of Bodner-Partom and Walker are performed to establish the accuracy, range of applicability, and integability of the models. Both models are implemented in the MARC finite element computer code and used for TMF analyses. Benchmark notch round experiments are conducted and the results compared with finite-element analyses using the MARC code and the Walker model.

Lindholm, U. S.↗

NASA NTRS1985

A survey of unified constitutive theories

The state of the art of time temperature dependent elastic viscoplastic constitutive theories which are based on the unified approach werre assessed. This class of constitutive theories is characterized by the use of kinetic equations and internal variables with appropriate evolutionary equations for treating all aspects of inelastic deformation including plasticity, creep, and stress relaxation. More than 10 such unified theories which are shown to satisfy the uniqueness and stability criteria imposed by Drucker's postulate and Ponter's inequalities are identified. The theories are compared for the types of flow law, kinetic equation, evolutionary equation of the internal variables, and treatment of temperature dependence. The similarities and differences of these theories are outlined in terms of mathematical formulations and illustrated by comparisons of theoretical calculations with experimental results which include monotonic stress-strain curves, cyclic hysteresis loops, creep and stress relaxation rates, and thermomechanical loops. Numerical methods used for integrating these stiff time temperature dependent constitutive equations are reviewed.

Chan, K. S.↗

NASA NTRS1983

Measurement and analysis of critical crack tip processes associated with variable amplitude fatigue crack growth

Crack growth retardation following overloads can result in overly conservative life predictions in structures subjected to variable amplitude fatigue loading when linear damage accumulation procedures are employed. Crack closure is believed to control the crack growth retardation, although the specific closure mechanism is debatable. Information on the relative contributions to crack closure from: (1) plasticity left in the wake of the advancing crack and (2) crack tip residual stresses is provided. The delay period and corresponding crack growth rate transients following overloads are systematically measured as a function of load ratio (R) and overload magnitude. These responses are correlated in terms of the local 'driving force' for crack growth as measured by crack tip opening loads and delta K sub eff. The latter measurements are obtained using a scanning electron microscope equipped with a cyclic loading stage; measurements are quantified using a relatively new stereoimaging technique. Combining experimental results with analytical predictions suggests that both plastic wake and residual stress mechanism are operative, the latter becoming predominate as R increases.

Hudak, S. J., Jr.↗

NASA NTRS1982

Pressure concentrations due to plastic deformation of thin films or gaskets between anvils

Plastic deformation of either a sample or a gasket between diamond anvils leads to large pressure concentrations, i.e., the maximum pressure can be many times the average pressure. This behavior is discussed using elementary plasticity theory for the case where the pressures are sufficiently low that the yield stress can be assumed not to vary with pressure. It is then shown that the pressure concentration factor can be even much larger when the yield stress of the sample at the highest pressure is much greater than the yield stress at the lowest pressure. This is illustrated with solid xenon where it is shown that the assumption that Nelson and Ruoff made about the pressure distribution in their xenon samples is incorrect. The pressure distribution is shown to be much steeper than assumed. Thus, the pressure they observed electrical conduction in xenon was above 1 Mbar.

Chan, K. S.↗

NASA NTRS1979

Transformation pressure of ZnS by a new primary pressure technique

An experiment designed to measure the transformation pressure of zinc sulfide by a primary pressure method using an indentor-anvil system made of single-crystal diamond is discussed. A nonsorting electrode-grid which can be applied directly to the anvil using optical lithography is described, together with the procedures used for positioning.

Ruoff, A. L.↗

NASA NTRS1976

Analysis of contact pressures attainable using indentors which are bodies of revolution

The elastic behavior of a conical indentor with a rounded tip pressed against a flat anvil has been analyzed. It is found that when the radius of contact extends beyond the rounded tip, the contact radius increases much more rapidly as the pressure increases than it does for the case of a fully spherical or parabolic tip. Consequently, the pressure achieved for a rounded conical tip for a given applied force is considerably less than for a spherical or parabolic tip.

Ruoff, A. L.↗