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Effect of shock wave risetime on material ejection from aluminum surfaces

Description: The effect of shock wave risetime on material ejection in aluminum has been studied for loading stresses of 21 GPa. Uniform loading was accomplished with plate impact techniques by mounting specimens on a ramp wave generator. Projectile impact on one side of the wave generator produced a wave which dispersed with propagation distance. This wave was then made incident to an aluminum specimen, so that the specimen experienced non-shock loading. It was found that mass ejection from aluminum surfaces can be reduced by over two orders of magnitude relative to shock loading conditions by accelerating the surface with a wave risetime greater than about 35 ns. These results suggest an explanation for the apparent discrepancies which are sometimes observed in mass ejection measurements utilizing either plate impact or electron beam deposition to generate stress waves.
Date: September 15, 1977
Creator: Asay, J.R.
Partner: UNT Libraries Government Documents Department

Using diffusion measurements to determine pore-size distributions in porous materials

Description: A method for determining pore-size distrbutions from diffusion measurements is presented. A Wicke-Kallenbach experiment, for measuring diffusion fluxes within porous materials, was carried out over a significant portion of the transition range between Knudsen flow and bulk diffusion. If the internal porous structure of the material is modeled as myriad nonintersecting cylindrical pores, an equation may be derived for the flux as a function of pressure, in which the flux is a functional of the pore-size distribution. The equation is a Fredholm integral equation of the first kind. This is an application of the general inversion problem, and solution of the equation for the pore-size distribution is possible. It is demonstrated from calculated fluxes using postulated ideal distributions that the method works very well for both unimodal and bimodal distributions. The method is no extraordinarily sensitivire to experimental error. An example of a distribution obtained from a commercial porous catalyst is presented and compared with that obtained by mercury porosimetry.
Date: January 1, 1980
Creator: Brown, L.F. & Travis, B.J.
Partner: UNT Libraries Government Documents Department

Ductile crack initiation in the Charpy V-notch test

Description: Crack initiation and growth in the Charpy V-notch test are investigated for controlled deflections of the specimen applied both by slow bending and by impact. Charpy test specimens were deformed to various deflections, heat-tinted to mark the crack extensions, and broken apart at low temperature to allow measurement of the crack extensions. These measurements provide estimates of crack initiation as defined by various criteria. The loading point at which crack initiation occurs depends on the particular definition being used for ''initiation,'' but in all cases it is well before the maximum load is reached. When initiation is defined as the first observable micro-initiation away from the ductile blunting of the notch root, the experimental results are in good agreement with computer modeling predictions.
Date: May 25, 1978
Creator: Server, W.L.; Norris, D.M.; Jr. & Prado, M.E.
Partner: UNT Libraries Government Documents Department

Effects of heat treatment on the microstructure and mechanical properties of uranium-6 niobium alloy

Description: Aging studies on uranium-6 weight percent niobium alloy were performed at temperatures of 500 and 600/sup 0/C. Mechanical-property measurements and microstructural analyses were used to characterize the aging response of the alloy. An attractive combination of mechanical properties was produced by aging at 550 to 600/sup 0/C; namely, a 0.2% offset yield strength greater than 730 MPa with an elongation of 15 to 18%. Increasing the aging time at temperature minimized the variability in mechanical properties. Niobium homogeneity and heating rate influenced the properties and microstructure of the alloy.
Date: October 1, 1978
Creator: Snyder, W.B.
Partner: UNT Libraries Government Documents Department

Application of survival statistics to the impulsive fragmentation of ductile rings

Description: The current study addressed the problem of predicting fragment size distributions resulting from the tensile fracture of impulsive loaded bodies. Theory was restricted to one-dimensional bodies subjected to uniform loading. A rigorous treatment of the statistics of dynamic fragmentation has been attempted using concepts of survival statistics and incorporated here through the relation derived by Johnson and Mehl and Avrami. Physical concepts are introduced through the assumption of a uniform nucleation rate of fracture and the propagation of stress-relief waves governed by the tensile response of an ideally ductile material. An analytic distribution curve was derived for ductile fracture and compared with the fragmentation data of Wesenberg and Sagartz on aluminum rings. The analytic expression was found to provide a good representation of the data although further experimental work in this area is strongly needed. The present analysis and resulting fragment size distribution expressions are fairly complex even for the very simple geometry and loading conditions considered. Direct application of the method to more complicated fragmentation events would probably be difficult. Perhaps the greatest value of the present type of analysis will be the insight that it provides on the statistical nature of impulse fragmentation - hopefully a source of fresh ideas for computational models currently under development.
Date: January 1, 1980
Creator: Grady, D. E.
Partner: UNT Libraries Government Documents Department

Plastic deformation of colliding hemishells. [Steel-ASTM-A 537]

Description: The collision of two hemishells is analyzed where one hemishell is clamped and the other impinges on the inner surface of the clamped hemishell. A model of rigid-perfectly plastic behavior is proposed for the flow of material in the region of contact. An expression relating velocity and deformation is obtained and compared with the results of experiments.
Date: January 1, 1980
Creator: Moir, D.C. & Neal, T.R.
Partner: UNT Libraries Government Documents Department

The path dependence of deformation texture development

Description: It is demonstrated for the case of three different strain paths, all of which end up with the same, elongated specimen shape, that the texture developed during straining is path dependent. This is true both for experiments on aluminum polycrystals and for simulations using the LApp code.
Date: January 1, 1987
Creator: Takeshita, T.; Kocks, U.F. & Wenk, H.R.
Partner: UNT Libraries Government Documents Department

Experiments on plastic deformation at finite strains

Description: The strain hardening behavior of metals at large plastic strains is difficult to assess experimentally. Consequently, many different techniques have been used to study such behavior and no clear experimental picture has evolved. In this paper experiments are reviewed on finite plastic deformation with emphasis on work reported since the comprehensive review of Gil Sevillano, van Houtte, and Aernoudt. The macroscopic strain hardening behavior is the primary concern, but its dependence on crystal structure, purity, alloying, microstructure, stacking fault energy, grain size, and deformation mode, is also discussed.
Date: January 1, 1981
Creator: Hecker, S.S.; Stout, M.G. & Eash, D.T.
Partner: UNT Libraries Government Documents Department

Tensile cracks in creeping solids

Description: The loading parameter determining the stress and strain fields near a crack tip, and thereby the growth of the crack, under creep conditions is discussed. Relevant loading parameters considered are the stress intensity factor K/sub I/, the path-independent integral C*, and the net section stress sigma/sub net/. The material behavior is modelled as elastic-nonlinear viscous where the nonlinear term describes power law creep. At the time t = 0 load is applied to the cracked specimen, and in the first instant the stress distribution is elastic. Subsequently, creep deformation relaxes the initial stress concentration at the crack tip, and creep strains develop rapidly near the crack tip. These processes may be analytically described by self-similar solutions for short times t. Small scale yielding may be defined. In creep problems, this means that elastic strains dominate almost everywhere except in a small creep zone which grows around the crack tip. If crack growth ensues while the creep zone is still small compared with the crack length and the specimen size, the stress intensity factor governs crack growth behavior. If the calculated creep zone becomes larger than the specimen size, the stresses become finally time-independent and the elastic strain rates can be neglected. In this case, the stress field is the same as in the fully-plastic limit of power law hardening plasticity. The loading parameter which determines the near tip fields uniquely is then the path-independent integral C*.K/sub I/ and C* characterize opposite limiting cases. The case applied in a given situation is decided by comparing the creep zone size with the specimen size and the crack length. Besides several methods of estimating the creep zone size, a convenient expression for a characteristic time is derived, which characterizes the transition from small scale yielding to extensive creep of the whole specimen.
Date: February 1, 1979
Creator: Riedel, H & Rice, J R
Partner: UNT Libraries Government Documents Department

Phenomenological model for transient deformation based on state variables

Description: The state variable theory of Hart, while providing a unified description of plasticity-dominated deformation, exhibits deficiencies when it is applied to transient deformation phenomena at stresses below yield. It appears that the description of stored anelastic strain is oversimplified. Consideration of a simple physical picture based on continuum dislocation pileups suggests that the neglect of weak barriers to dislocation motion is the source of these inadequacies. An appropriately modified description incorporating such barriers then allows the construction of a macroscopic model including transient effects. Although the flow relations for the microplastic element required in the new theory are not known, tentative assignments may be made for such functions. The model then exhibits qualitatively correct behavior when tensile, loading-unloading, reverse loading, and load relaxation tests are simulated. Experimental procedures are described for determining the unknown parameters and functions in the new model.
Date: unknown
Creator: Jackson, M.S.; Cho, C.W.; Alexopoulos, P.; Mughrabi, H. & Li, C.Y.
Partner: UNT Libraries Government Documents Department

Elastic plastic analysis of growing cracks

Description: The elastic--plastic stress and deformation fields at the tip of a crack which grows in an ideally plastic solid under plane strain, shows small scale yielding conditions. Results of an asymptotic analysis suggests the existence of a crack tip stress state similar to that of the classical Prandtl field, but containing a zone of elastic unloading between the centered fan region and the trailing constant stress plastic region. The near tip expression for the rate of opening displacement delta at distance r from the growing tip is found to have the form delta-. = ..cap alpha.. J-./sigma/sub o/ + ..beta..(sigma/sub o//E) a-. ln(R/r) but the presence of the elastic wedge causes ..beta.. to have the revised value of 5.08 (for Poisson ratio ..nu.. = 0.3); also, (a = crack length, sigma/sub o/ = yield strength, E = elastic modulus, and J denotes the far-field value), and (1-..nu../sup 2/)K/sup 2//E for the small scale yielding conditions considered. The parameters ..cap alpha.. and R cannot be determined from the asymptotic analysis, but comparisons with finite element solutions suggest that, for small amounts of growth, ..cap alpha.. is approximately the same for stationary and growing cracks, and R scales approximately with the size of the plastic zone, being about 15% to 30% larger. For large scale yielding, a similar form applies with possible variations in ..cap alpha.. and ..beta.., in cases which maintain triaxial constraint at the crack tip. In the fully yielded case R is expected to be proportional to the dimension of the uncracked ligament. Model crack growth criterion requiring a critical delta at some fixed r from the tip, is re-examined in light of the more accurate solution. Results suggest that the J versus ..delta..a relation describing growth is dependent on the extent of yielding. It is suggested that this ...
Date: May 1, 1979
Creator: Rice, J R; Drugan, W J & Sham, T L
Partner: UNT Libraries Government Documents Department

Recommendation of a triaxial failure theory for graphite

Description: An extensive literature search has been conducted on the application of triaxial failure theories to an anisotropic brittle solid. This study leads to the conclusion that the strength tensor theory of Tsai and Wu appears most promising for HTGR structural graphites. The reduction of this theory to model the transversely isotropic nature of HTGR structural graphites is presented. The extensions to the ongoing biaxial stress experimental program, needed to provide the additional data required to verify this theory, are defined.
Date: May 1, 1979
Creator: Tang, P.Y.
Partner: UNT Libraries Government Documents Department

Statistical treatment of fatigue test data

Description: This report discussed several aspects of fatigue data analysis in order to provide a basis for the development of statistically sound design curves. Included is a discussion on the choice of the dependent variable, the assumptions associated with least squares regression models, the variability of fatigue data, the treatment of data from suspended tests and outlying observations, and various strain-life relations.
Date: January 1, 1980
Creator: Raske, D.T.
Partner: UNT Libraries Government Documents Department

Solution hardening and strain hardening at elevated temperatures

Description: Solutes can significantly increase the rate of strain hardening; as a consequence, the saturation stress, at which strain hardening tends to cease for a given temperature and strain rate, is increased more than the yield stress: this is the major effect of solutes on strength at elevated temperatures, especially in the regime where dynamic strain-aging occurs. It is shown that local solute mobility can affect both the rate of dynamic recovery and the dislocation/dislocation interaction strength. The latter effect leads to multiplicative solution strengthening. It is explained by a new model based on repeated dislocation unlocking, in a high-temperature limit, which also rationalizes the stress dependence of static and dynamic strain-aging, and may help explain the plateau of the yield stress at elevated temperatures. 15 figures.
Date: October 1, 1982
Creator: Kocks, U.F.
Partner: UNT Libraries Government Documents Department

Effect of quench rate and refrigeration on the mechanical properties of HP9-4-20 steel

Description: Difficulties in obtaining adequate mechanical properties in the HP9-4-20 portion of the B83 structural midcase have raised some questions concerning the effect of quench rate and post-quench refrigeration on the strength and toughness of the material. As a result, a program was initiated to determine if either of these parameters contributed to the observed degradation in properties or whether some other step in the processing history of the material was responsible. Initially, the nominal cooling rate of a full size mid-case was calculated numerically using a finite difference thermal conduction code. A series of HP9-4-20 steel samples was then processed with quench rates which bracketed this calculated cooling rate. It was found that neither quench rate nor refrigeration had a significant effect on the mechanical properties at cooling rates that might reasonably be encountered in production hardware. Consequently, it was concluded that the inadequate mechanical properties exhibited by early B83 mid-case components were related to an improper tempering heat treatment, and not an inadequate quench.
Date: February 1, 1982
Creator: Klimowicz, T.F. & Lippold, J.C.
Partner: UNT Libraries Government Documents Department

Efficient numerical method for time independent plasticity

Description: A new numerical method for the solution of plasticity equations is presented. The plasticity model is a commonly used elastic-plastic strain hardening model for combined isotropic-kinematic hardening with Ziegler's modification. Thermal strains and temperature variable properties are included. The method is also specialized to the non-thermal linear strain hardening case where only two additional vector and two scalar algebraic equations, including one square root, are needed over that needed for the elastic case. The method has roughly the same accuracy as conventional numerical treatments of the plasticity equations but requires roughly one-third as much computational time.
Date: November 1, 1977
Creator: Krieg, R.D.
Partner: UNT Libraries Government Documents Department

Effect of chemistry variations on the weldability of commercially pure vanadium sheet

Description: Tensile tests and metallographic examinations were done on electron beam welds on six different chemistries of vanadium. The welds were found to maintain base metal strengths even though large grains were present in the fusion zone. Nitrogen and oxygen were found to be the most effective strengtheners. The weld and surrounding area in material containing high nitrogen and oxygen exhibited higher hardness than the base metal.
Date: February 12, 1982
Creator: Glenn, T.G.; Elliston, G.W.; Edstrom, C.M. & Johns, W.L.
Partner: UNT Libraries Government Documents Department

Estimate of the contribution of load transfer to the yield strength and hardness of a dual-phase steel

Description: The ratio of the average stresses in ferrite and martensite at yield of a dual-phase steel is calculated from a shear-lag model of load transfer, based on a shape parameter of the martensite particles. For a specific steel, a stress ratio of 2.2 is determined. This value is compared to the stress ratio obtained from hardness and microhardness determinations. The results qualitatively and tentatively support the strengthening role of load transfer in dual-phase steels.
Date: June 1, 1981
Creator: Gurland, J
Partner: UNT Libraries Government Documents Department

Theory for laminated plates with a through-the-thickeness crack

Description: The flexural response of a laminated plate with a through-the-thickness crack was considered in this investigation. A laminated plate theory of the Reissner type has been developed to treat this problem. For demonstration purposes, the theory was applied to a balanced symmetric laminate which contains a through-the-thickness central crack. The stress intensity factor for the crack was determined and the influences of the various material and geremetric parameters on the stress intensity factor were discussed.
Date: January 1, 1980
Creator: Chen, E. P.
Partner: UNT Libraries Government Documents Department

Effects of high mean stress on the high-cycle fatigue behavior of PWA 1480

Description: PWA 1480 is a potential candidate material for use in the high-pressure fuel turbine blade of the Space Shuttle Main Engine. As an engine material it will be subjected to high-cycle fatigue loading superimposed on a high mean stress due to combined centrifugal and thermal loadings. This paper describes results obtained in an ongoing program to determine the effects of a high mean stress on the high-cycle fatigue behavior of this material.
Date: March 1, 1985
Creator: Majumdar, S.; Antolovich, S. & Milligan, W.
Partner: UNT Libraries Government Documents Department

Work hardening correlation for monotonic loading based on state variables

Description: An absolute work hardening correlation in terms of the hardness parameter and the internal stress based on the state variable approach was developed. It was found applicable to a variety of metals and alloys. This correlation predicts strain rate insensitive work hardening properties at low homologous temperatures and produces strain rate effects at higher homologous temperatures without involving thermally induced recovery processes.
Date: January 1, 1977
Creator: Huang, F.H. & Li, C.Y.
Partner: UNT Libraries Government Documents Department

Consistent stress-strain ductile fracture model as applied to two grades of beryllium

Description: Published yield and ultimate biaxial stress and strain data for two grades of beryllium are correlated with a more complete method of characterizing macroscopic strain at fracture initiation in ductile materials. Results are compared with those obtained from an exponential, mean stress dependent, model. Simple statistical methods are employed to illustrate the degree of correlation for each method with the experimental data.
Date: January 1, 1980
Creator: Priddy, T.G.; Benzley, S.E. & Ford, L.M.
Partner: UNT Libraries Government Documents Department

Fatigue crack initiation and propagation in steels exposed to inert and corrosive environments. Final report, May 1, 1977--December 31, 1977

Description: The fatigue crack initiation life of AISI 1018 steel was investigated using compact tension specimens having sharp notch root radii. The data were analyzed using two methods for predicting initiation in strain cycling experiments. Also, another approach in which initiation is related to the stress intensity factor was developed. The next phase, that of propagation, was studied using AISI 1018 steel and a new high strength steel HY-180. The crack propagation data obtained for both steels tested in air can be described accurately by the power law first suggested by Paris, da/dN = C(..delta..K)/sup n/, where a is the crack length, N the number of cycles, and C and n are material constants. However, the exponent n was found to be two times larger for AISI 1018 steel than HY-180 steel.
Date: February 1, 1978
Creator: Youseffi, K. & Finnie, I.
Partner: UNT Libraries Government Documents Department