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A variational theorem for creep with applications to plates and columns

Description: From Summary: "A variational theorem is presented for a body undergoing creep. Solutions to problems of creep bending and creep collapse of plates, columns, beams, and shells can be obtained by means of the direct methods of the calculus of variations in conjunction with the stated theorem. The application of the theorem is illustrated for plates and columns by the solution of two sample problems."
Date: May 1957
Creator: Sanders, J. Lyell, Jr.; McComb, Harvey G., Jr. & Schlechte, Floyd R.
Partner: UNT Libraries Government Documents Department

A variational theorem for creep with applications to plates and columns

Description: A variational theorem is presented for a body undergoing creep. Solutions to problems of the creep behavior of plates, columns, beams, and shells can be obtained by means of the direct methods of the calculus of variations in conjunction with the stated theorem. The application of the theorem is illustrated for plates and columns by the solution of two sample problems.
Date: March 5, 1957
Creator: Sanders, J. Lyell, Jr.; McComb, Harvey G., Jr. & Schlechte, Floyd R.
Partner: UNT Libraries Government Documents Department

Analysis of the creep behavior of a square plate loaded in edge compression

Description: From Introduction: "In reference 1 results of creep tests and empirical method for predicting collapse times are presented for plates loaded in compression on two opposite edges and with the remaining edges unloaded and supported in V-groove fixtures. Other approximate methods for handling plates having types of edge support are suggested in reference 2; however, experimental verification for these methods is quite limited. In reference 3 an analysis based on small-deflection theory is made of the creep deflection of a simply supported plate composed of a linear viscoelastic material - that is, a material in which the stress and strain and their appropriate time derivatives are related in a linear fashion."
Date: September 1958
Creator: Mccomb, Harvey G , Jr
Partner: UNT Libraries Government Documents Department

Creep buckling of columns

Description: Report presenting formulas for the determination of the creep deflection-time characteristics of an initially curved idealized H-section column. The results were obtained from closed-form solutions of the differential equation of bending of a beam column with creep properties of a nonlinearly viscoelastic nature. Critical time is tabulated and plotted for a wide range of parameters involved.
Date: January 1954
Creator: Kempner, Joseph & Patel, Sharad A.
Partner: UNT Libraries Government Documents Department

Effects of Environment on the Low-Cycle Fatigue Behavior of Type 304 Stainless Steel

Description: The low-cycle fatigue behavior of Type 304 stainless steel has been investigated at 593 degrees C in a dynamic vacuum of better than 1.3 x 10⁻⁶ Pa (10⁻⁸ torr). The results concerning the effects of strain range, strain rate and tensile hold time on fatigue life are presented and compared with results of similar tests performed in air and sodium environments. Under continuous symmetrical cycling, fatigue life is significantly longer in vacuum than in air; in the low strain range regime, the effect of sodium on fatigue life appears to be similar to that of vacuum. Strain rate (or frequency) strongly influences fatigue life in both air and vacuum. In compressive hold-time tests, the effect of environment on life is similar to that in a continuous-cycling test. However, tensile hold times are nearly as damaging in vacuum as in air. Thus, at least for austenitic stainless steels, the influence of the environment of fatigue life appears to depend on the loading waveshape.
Date: December 1979
Creator: Argonne National Laboratory. Materials Science Division.
Partner: UNT Libraries Government Documents Department

Mechanics and Mechanisms of Creep and Ductile Fracture

Description: The main aim of this dissertation is to relate measurable and hopefully controllable features of a material's microstructure to its observed failure modes to provide a basis for designing better materials. The understanding of creep in materials used at high temperatures is of prime engineering importance. Single crystal Ni-based superalloys used in turbine aerofoils of jet engines are exposed to long dwell times at very high temperatures. In contrast to current theories, creep tests on Ni-based superalloy specimens have shown size dependent creep response termed as the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal creep tests were performed on uncoated Ni-based single crystal superalloy sheet specimens with two thicknesses and under two test conditions: a low temperature high stress condition and a high temperature low stress condition. At the high temperature, surface oxidation induced microstructural changes near the free surface forming a layered microstructure. Finite element calculations showed that this layered microstructure gave rise to local changes in the stress state. The specimens also contained nonuniform distribution of initial voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This motivated a basic mechanics study of porosity evolution in single crystals subjected to creep for a range of stress states. The study was performed using three-dimensional finite deformation finite element analysis of unit cells containing a single initially spherical void in a single crystal matrix. The materials are characterized by a rate-dependent crystal plasticity constitutive relation accounting for both primary and secondary creep. The effect of initial void spacing and creep exponent was also explored. Based on the experimental observations and results of finite element calculations a quantitative mechanistic model is proposed that can account for both bulk and surface ...
Date: August 2013
Creator: Srivastava, Ankit
Partner: UNT Libraries

A linear time-temperature relation for extrapolation of creep and stress-rupture data

Description: Report presenting a time-temperature parameter based on examination of the published stress-rupture data for a variety of materials, rather than on physical theory, is presented in equation form. An examination of published data on 40 materials representing ferritic and sustenitic steels, high-temperature alloys, and aluminum alloys is used to create conclusions.
Date: March 1953
Creator: Manson, S. S. & Haferd, A. M.
Partner: UNT Libraries Government Documents Department

Barrier and Long Term Creep Properties of Polymer Nanocomposites.

Description: The barrier properties and long term strength retention of polymers are of significant importance in a number of applications. Enhanced lifetime food packaging, substrates for OLED based flexible displays and long duration scientific balloons are among them. Higher material requirements in these applications drive the need for an accurate measurement system. Therefore, a new system was engineered with enhanced sensitivity and accuracy. Permeability of polymers is affected by permeant solubility and diffusion. One effort to decrease diffusion rates is via increasing the transport path length. We explore this through dispersion of layered silicates into polymers. Layered silicates with effective aspect ratio of 1000:1 have shown promise in improving the barrier and mechanical properties of polymers. The surface of these inorganic silicates was modified with surfactants to improve the interaction with organic polymers. The micro and nanoscale dispersion of the layered silicates was probed using optical and transmission microscopy as well as x-ray diffraction. Thermal transitions were analyzed using differential scanning calorimetry. Mechanical and permeability measurements were correlated to the dispersion and increased density. The essential structure-property relationships were established by comparing semicrystalline and amorphous polymers. Semicrystalline polymers selected were nylon-6 and polyethylene terephthalate. The amorphous polymer was polyethylene terphthalate-glycol. Densification due to the layered silicate in both semicrystalline and amorphous polymers was associated with significant impact on barrier and long term creep behavior. The inferences were confirmed by investigating a semi-crystalline polymer - polyethylene - above and below the glass transition. The results show that the layered silicate influences the amorphous segments in polymers and barrier properties are affected by synergistic influences of densification and uniform dispersion of the layered silicates.
Access: This item is restricted to the UNT Community Members at a UNT Libraries Location.
Date: December 2004
Creator: Ranade, Ajit
Partner: UNT Libraries

Biaxial Creep-Fatigue Behavior of Type 316H Stainless Steel Tube

Description: Biaxial creep-fatigue test data for Type 316 stainless steel tubes at 1100*Y are presented. The specimens were subjected to constant internal pressure and fluctuating axial strain with and without hold times in tension as well as compress ion. The results show that internal pressure significantly affects diametral ratchetting and axial stress range. Axial tensile hold is found to he more damaging than axial compressive hold even cinder a biaxial state of stress.
Date: April 1979
Creator: Majumdar, S.
Partner: UNT Libraries Government Documents Department

Characterization of Viscoelastic Properties of a Material Used for an Additive Manufacturing Method

Description: Recent development of additive manufacturing technologies has led to lack of information on the base materials being used. A need arises to know the mechanical behaviors of these base materials so that it can be linked with macroscopic mechanical behaviors of 3D network structures manufactured from the 3D printer. The main objectives of my research are to characterize properties of a material for an additive manufacturing method (commonly referred to as 3D printing). Also, to model viscoelastic properties of Procast material that is obtained from 3D printer. For this purpose, a 3D CAD model is made using ProE and 3D printed using Projet HD3500. Series of uniaxial tensile tests, creep tests, and dynamic mechanical analysis are carried out to obtained viscoelastic behavior of Procast. Test data is fitted using various linear and nonlinear viscoelastic models. Validation of model is also carried out using tensile test data and frequency sweep data. Various other mechanical characterization have also been carried out in order to find density, melting temperature, glass transition temperature, and strain rate dependent elastic modulus of Procast material. It can be concluded that melting temperature of Procast material is around 337°C, the elastic modulus is around 0.7-0.8 GPa, and yield stress is around 16-19 MPa.
Date: December 2013
Creator: Iqbal, Shaheer
Partner: UNT Libraries

Long Term Property Prediction of Polyethylene Nanocomposites

Description: The amorphous fraction of semicrystalline polymers has long been thought to be a significant contributor to creep deformation. In polyethylene (PE) nanocomposites, the semicrystalline nature of the maleated PE compatibilizer leads to a limited ability to separate the role of the PE in the nanocomposite properties. This dissertation investigates blown films of linear low-density polyethylene (LLDPE) and its nanocomposites with montmorillonite-layered silicate (MLS). Addition of an amorphous ethylene propylene copolymer grafted maleic anhydride (amEP) was utilized to enhance the interaction between the PE and the MLS. The amorphous nature of the compatibilizer was used to differentiate the effect of the different components of the nanocomposites; namely the matrix, the filler, and the compatibilizer on the overall properties. Tensile test results of the nanocomposites indicate that the addition of amEP and MLS separately and together produces a synergistic effect on the mechanical properties of the neat PE Thermal transitions were analyzed using differential scanning calorimetry (DSC) to determine if the observed improvement in mechanical properties is related to changes in crystallinity. The effect of dispersion of the MLS in the matrix was investigated by using a combination of X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Mechanical measurements were correlated to the dispersion of the layered silicate particles in the matrix. The nonlinear time dependent creep of the material was analyzed by examining creep and recovery of the films with a Burger model and the Kohlrausch-Williams-Watts (KWW) relation. The effect of stress on the nonlinear behavior of the nanocomposites was investigated by analyzing creep-recovery at different stress levels. Stress-related creep constants and shift factors were determined for the material by using the Schapery nonlinear viscoelastic equation at room temperature. The effect of temperature on the tensile and creep properties of the nanocomposites was analyzed by examining tensile and creep-recovery behavior of ...
Date: December 2008
Creator: Shaito, Ali Al-Abed
Partner: UNT Libraries

Generalized master curves for creep and rupture

Description: Report presenting the similarity of Larson-Miller master curves for rupture life and minimum creep rate for aluminum, two aluminum alloys, two steels, and two high-temperature high-strength alloys. The approximate invariance of the product of rupture life and minimum creep rate is also shown for these materials.
Date: October 1957
Creator: Heimerl, George J. & McEvily, Arthur J., Jr.
Partner: UNT Libraries Government Documents Department

Novel Experiments to Characterize Creep-Fatigue Degradation in VHTR Alloys

Description: It is well known in energy systems that the creep lifetime of high temperature alloys is significantly degraded when a cyclic load is superimposed on components operating in the creep regime. A test method has been developed in an attempt to characterize creep-fatigue behavior of alloys at high temperature. The test imposes a hold time during the tensile phase of a fully reversed strain-controlled low cycle fatigue test. Stress relaxation occurs during the strain-controlled hold period. This type of fatigue stress relaxation test tends to emphasize the fatigue portion of the total damage and does not necessarily represent the behavior of a component in-service well. Several different approaches to laboratory testing of creep-fatigue at 950°C have been investigated for Alloy 617, the primary candidate for application in VHTR heat exchangers. The potential for mode switching in a cyclic test from strain control to load control, to allow specimen extension by creep, has been investigated to further emphasize the creep damage. In addition, tests with a lower strain rate during loading have been conducted to examine the influence of creep damage occurring during loading. Very short constant strain hold time tests have also been conducted to examine the influence of the rapid stress relaxation that occurs at the beginning of strain holds.
Date: October 1, 2013
Creator: Wright, J. K.; Simpson, J. A.; Carroll, L. J.; Wright, R. N. & Sham, T.-L.
Partner: UNT Libraries Government Documents Department

Heat Treatment Effects for Improved Creep-Rupture Resistance of a Fe{sub 3}Al-Based Alloy

Description: The iron aluminide alloy FA-180, an Fe{sub 3}Al-based alloy with a composition of Fe-28Al-5Cr-0.5Nb-0.8Mo-0.025Zr-0.05C-0.005B (at.%), is of interest because of its improved creep-rupture resistance when compared to other Fe3Al and FeAl-based alloys. Creep-rupture testing at 593 C and 207 MPa shows that FA-180 has a rupture life of approximately 100 h in the warm-rolled and stress relieved (1 h at 700-750 C) condition as compared to about 20 h for the FA-129 base alloy (Fe-28Al-5Cr-0.5Nb-0.2C). This report summarizes studies conducted in the last several years to determine the creep-rupture resistance of FA-180 as a function of the microstructures produced by varying the heat treatment temperature and cooling procedures. Solution-annealing for 1 h at 1150 C (air cooled) dramatically improved the creep-rupture life of FA-180 to about 2000 h. Transmission electron microscopy analysis showed that this strengthening was due to the precipitation of fine ZrC in the matrix and along grain boundaries. A further improvement in creep-rupture life to over 6000 h resulted from increasing the cooling rate (by quenching in oil or water) after solution annealing at 1150 C. The microstructure of the quenched specimen contained many fine dislocation loops instead of precipitates. During creep testing, these small loops evolved into a structure of larger dislocation loops and networks, resulting in significant strengthening at temperatures of 593-700 C.
Date: December 1997
Creator: McKamey, C. G. & Maziasz, P. J.
Partner: UNT Libraries Government Documents Department

Constitutive Modeling of High Temperature Uniaxial Creep-Fatigue and Creep-Ratcheting Responses of Alloy 617

Description: Inconel Alloy 617 is a high temperature creep and corrosion resistant alloy and is a leading candidate for use in Intermediate Heat Exchangers (IHX) of the Next Generation Nuclear Plants (NGNP). The IHX of the NGNP is expected to experience operating temperatures in the range of 800 degrees - 950 degrees C, which is in the creep regime of Alloy 617. A broad set of uniaxial, low-cycle fatigue, fatigue-creep, ratcheting, and ratcheting-creep experiments are conducted in order to study the fatigue and ratcheting responses, and their interactions with the creep response at high temperatures. A unified constitutive model developed at North Carolina State University is used to simulate these experimental responses. The model is developed based on the Chaboche viscoplastic model framework. It includes cyclic hardening/softening, strain rate dependence, strain range dependence, static and dynamic recovery modeling features. For simulation of the alloy 617 responses, new techniques of model parameter determination are developed for optimized simulations. This paper compares the experimental responses and model simulations for demonstrating the strengths and shortcomings of the model.
Date: July 1, 2013
Creator: Pritchard, P.G.; Carroll, L.J. & Hassan, T.
Partner: UNT Libraries Government Documents Department

In-Situ TEM Study of Interface Sliding and Migration in an Ultrafine Lamellar Structure

Description: The instability of interfaces in an ultrafine TiAl-({gamma})/Ti{sub 3}Al-({alpha}{sub 2}) lamellar structure by straining at room temperature has been investigated using in-situ straining techniques performed in a transmission electron microscope. The purpose of this study is to obtain experimental evidence to support the creep mechanisms based upon the interface sliding in association with a cooperative movement of interfacial dislocations previously proposed to interpret the nearly linear creep behavior observed from ultrafine lamellar TiAl alloys. The results have revealed that both the sliding and migration of lamellar interfaces can take place simultaneously as a result of the cooperative movement of interfacial dislocations.
Date: December 6, 2005
Creator: Hsiung, L M
Partner: UNT Libraries Government Documents Department

Behavior of Repeating Earthquake Sequences in Central California and the Implications for Subsurface Fault Creep

Description: Repeating earthquakes (REs) are sequences of events that have nearly identical waveforms and are interpreted to represent fault asperities driven to failure by loading from aseismic creep on the surrounding fault surface at depth. We investigate the occurrence of these REs along faults in central California to determine which faults exhibit creep and the spatio-temporal distribution of this creep. At the juncture of the San Andreas and southern Calaveras-Paicines faults, both faults as well as a smaller secondary fault, the Quien Sabe fault, are observed to produce REs over the observation period of March 1984-May 2005. REs in this area reflect a heterogeneous creep distribution along the fault plane with significant variations in time. Cumulative slip over the observation period at individual sequence locations is determined to range from 5.5-58.2 cm on the San Andreas fault, 4.8-14.1 cm on the southern Calaveras-Paicines fault, and 4.9-24.8 cm on the Quien Sabe fault. Creep at depth appears to mimic the behaviors seen of creep on the surface in that evidence of steady slip, triggered slip, and episodic slip phenomena are also observed in the RE sequences. For comparison, we investigate the occurrence of REs west of the San Andreas fault within the southern Coast Range. Events within these RE sequences only occurred minutes to weeks apart from each other and then did not repeat again over the observation period, suggesting that REs in this area are not produced by steady aseismic creep of the surrounding fault surface.
Date: July 9, 2007
Creator: Templeton, D C; Nadeau, R & Burgmann, R
Partner: UNT Libraries Government Documents Department