Browsing by Author "Roberts, John M."
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Item A study of microcreep in magnesium single crystals in the temperature range 77 degrees to 300 degrees K(1963) Bresie, Don Alan; Roberts, John M.Single crystals of 99.97% pure magnesium were grown by the Bridgman technique. A very sensitive capacitance strain gage was employed to observe microcreep in these crystals at temperatures from 77° to 300°K. The microstrain-time results at a constant applied stress for all temperatures within this range were found to fit the empirical equation where I is the resolved shear microstrain, t is time and oc and A are constants. The constant A did not appear to be a function of temperature. Although A did seem to vary with the load increment, this dependence was not well defined by the observed results. The constant infinity was a function of several parameters but it seemed to be best described experimentally as proportional to the total strain during an incremental creep curve. The effective activation volume is defined where Ta is the resolved shear stress, k is Boltzman's constant and T is the absolute temperature in °K. The effective activation volume was found to be independent of applied stress. It was proportional to temperature such that v* = 2.38 x 10-21 T cm3 for an ultrahigh purity specimen and v* = 2.28 x 10-21 T cm3 for an alloyed crystal containing 0.046% Al. Internal stress was measured by experimentally finding the applied stress at which the forward creep balances the backward strain recovery. It was found to be 93% of the applied stress.Item Absorption of ultrasonic and hypersonic waves in LiF single crystals(1964) Goodell, Joseph P. N; Roberts, John M.; Brotzen, Franz R.Attenuation of ultrasonic-hypersonic stress waves in LiF single crystals is analyzed. As a mechanism for attenuation the vibrating pinned-dislocation model of Koehler is treated; a resonant peak in attenuation vs frequency was sought. Some other mechanisms, including temperature dependence, are considered. Various causes of modulation of the exponential decay of the acoustic wave are discussed.Item Cryogenic mechanical properties of polychlorotrifluoroethylene(1974) Ross, Michael; Armeniades, C. D.; Roberts, John M.Polychlorotrifluoroethylene (PCTFE), crystallized in two degrees of crystallinity, was subjected to tensile stress deformation experiments at several strain rates and temperatures in an attempt to relate the macroscopic deformation and failure characteristics of the polymer with its molecular organization and segmental mobility. At 3K, the higher crystallinity samples exhibited a higher tensile modulus, yield stress and a lower elongation to failure than lower crystallinity samples. At 78k and 4.2K, the lower crystallinity samples showed higher tensile moduli yield stress, stresses at failure and elongations to failure than did the higher crystalline samples. Sequential still photographs of the deforming sample were taken in the 3K experiments to determine the true-stress vs. true-strain curve for PCPFE. Fracture surfaces were examined with a scanning electron microscope to determine the mode of fracture. PCTFE appears to fail in a five-step process at 3K, in a three-step process at 78K and in a two-step process at 4.2K.Item Dislocation damping in copper single crystals(1963) Barnett, David Morton; Roberts, John M.Hysteresis loops were observed in 99.999 % pure copper single crystals under unidirectional cyclic stressing using a capacitance gage microstrain measuring technique. These loops; which represent an energy loss presumably due to dislocation damping; were measured at low frequencies as a function of stress amplitude; prestrain, and temperature. The prestrain region extended from 10-6 in/in to 10-2 in/in, and the temperature range covered was 135°K to 300°K. The decrement, or energy loss per total energy input, was found to decrease markedly with prestrain. For prestrains greater than 1 % only small decrements (less than 0.03) could be observed below stress amplitudes of 250 psi. No temperature dependence of the decrement was noted in the temperature range concerned. The microyield point, or the stress level at which an open hysteresis loop was first observed, was found to increase with prestrain, but was also temperature independent. Room temperature stress-strain curves indicated a work-hardening effect at strains less than 1%. Experimental checks revealed that the crystals tested exhibited only a limited easy glide region. The early onset of Stage II hardening is explained as a specimen size effect. The microyield point is associated with the stress necessary to operate a Frank-Read source. The prestrain dependence of the decrement and of the stress to create an open loop are thought to be due to the shortening of the average dislocation network loop length in the Stage II region. A theory describing the temperature independence of the decrement is currently unavailable.Item Dislocation damping in magnesium monocrystals(1961) Hartman, David Eugene; Roberts, John M.By employing a micro-strain gage, hysteresis loops were observed as a function of stress amplitude in magnesium single crystals from 83°K to 323°K, in the low frequency range (0.1 cps.). Broad relaxation peaks were found around 180°K. Activation energies associated with the peaks were about 0.33 ev., with a frequency factor of 108 cps. The stress amplitude dependence of the decrement was analyzed according to the Granato-Lucke and Teutonico-Granato-Lucke theories. The experimental results support the Teutonico-Granato-Lucke concept of pinned dislocations. A qualitative model is suggested to explain the symmetrical nature of the hysteresis loops, and the origin of the relaxation peak.Item Internal friction studies of halogenated hydrocarbon polymers(1975) O'Brien, Timothy; Roberts, John M.The effect of thermal history and mechanical deformation on the internal friction and dynamic shear modulus of polychlorotrifluoroethylene (PCTFE) and polytetrafluoroethylene (PTFE) was studied using a free oscillating torsional pendulum (1 Hz.) operating between 4.2° K and 3°K. Scanning electron micrographs of specimen fracture surfaces were made to correlate the mechanical properties with the observed morphology. In PCTFE, a new low temperature & relaxation was found which Increased in intensity with increasing crystallinity. This relaxation is similar to those found in several other polymers and attributed to motion of crystalline defects. The relaxation occurs around 48 K and has an activation energy of roughly 3 kcal/mole. The crystallinity of PTFE was not changed, and evidenoe supporting a relaxation in this polymer is not conclusive. Large uniaxial deformation resulted in the reduction of the low temperature internal friction in both polymers (disappearance of the S relaxation in PCTFE), and the enhancement of the higher temperature if relaxations in both polymers. This made the speotra appear to be those of very low crystallinity specimens. This fact combined with the morphology seen in the micrographs suggests that the deforming stress acts through the lnterorystalllne links to destroy the order in the crystallites.Item Investigation of a polymer concrete composite material for runway applications(1983) Zeilenga, Jay Howard; Armeniades, C. D.; Hightower, Joe W.; Roberts, John M.The use of fiberglass cloth layers as flexural reinforcement for polymer concrete was investigated. The influences of resin systems, cloth, aggregate, fine fillers, moisture content of aggregate, and specimen depth on the properties of the composite material were studied. The laboratory investigation was oriented towards the composite’s considerable potential for panel applications and possible application as a pavement material. The concept of quickly installed runways made of this polymer concrete composite material is discussed. The composite with two bottom layers of a light-weight glass cloth exhibited twice the flexural strength and three times the elongation limit of polymer concrete alone. A silane coupling agent was shown to improve the retention of strength of polyester concrete made with wet aggregate for moisture contents below three percent. The thickness of the composite, and of the polymer concrete alone, only slightly affected the flexural strength. This lack of sensitivity to specimen depth is important when considering production of panels of varying thicknesses. The ability of the composite to deform considerably, and its higher strength in excess of that for polymer concrete and ordinary concrete was noted as being desirable for constructing thin, durable, fast-setting pavements over weak and non-uniform subgrades.Item Slip line studies in deformed ordered and disordered Cu3Au single crystals(1970) Shaikh, Fasih Ahmed; Roberts, John M.A detailed study of the slip lines in deformed ordered and disordered Cu3Au single crystals has been carried out. The slip lines have been observed via optical metallography and by the two-step replica technique and subsequent examination in the electron microscope. Typical macroscopic stress-strain curves for each type of crystal have been noted as well as the change of specimen axis orientation with deformation. The crystals were deformed in strain increments of about 5/0: After each strain increment, characteristic slip lines and specimen axis rotation were evaluated and studied. The current results are discussed in the light of the quantitative work hardening theories presented for ordered L12 type superlattices by Vidoz26 and Schoeck.25 The results do not lend total support to either theory. In a qualitative discussion of the experimental data, we find the theory of Schoeck25 to define the pertinent obstacles for dislocation glide in this type superlattice. For clarity, however, it appears further analysis of the details of the breakdown of attractive superdislocation junctions in ordered type L12 super-lattices is required.Item Temperature dependence of the activation volume and the flow stress of Cadmium single crystals(1961) Coplin, Donald Herbert; Roberts, John M.Cadmium single crystals of 99.994% purity were tested by the strain rate change method to determine the activation volume over the temperature range from 77°K to 508°K. The temperature dependence of the flow stress and of the work hardening coefficient over the same range was also investigated. All three parameters exhibited an anomalous behavior at a temperature of about 275°K from that predicted by a single mechanism model as discussed by Seeger for the same temperature range. Theories are discussed to explain the observed results for cadmium as veil as to be consistent with previously reported results on magnesium and zinc. It is concluded that the Seeger approach explaining these deformation parameters is correct with some extension and modification. It is believed that the deformation, behavior is of a different nature below the temperature of about 275°K from that above. Specific models for these differences are discussed.Item The effect of hydrogen on the elastic constants of tantalum single crystals(1976) Stewart, William LeRoy; Roberts, John M.The elastic constants of tantalum were measured in the pure state and at several concentrations of hydrogen doping. These measurements were carried out in the temperature range between 23°C and -127°C on four different crystals, two each of the (llO and LOO orientations. This study set out to determine whether or not a significant Snoek effect existed, i.e., if C* (where C* = 1/2(^ - C12)) decreased significantly with increasing hydrogen content. In carrying out this study a more accurate and extensive check could be made on data previously determined by the inverted torsion pendulum method; ultrasonic measurements at room temperature. McSkimin’s technique^ was used for measuring the acoustic wave velocity in solids. This velocity was then used with the respective densities to calculate the elastic constants. This study yielded a decrease in C of .11 percent per atomic percent hydrogen in the alpha phase region. In addition, C-Q was found to increase by .16 percent per atomic percent hydrogen, increased .16 percent per atomic percent hydrogen, and (where = 1/2 (C^ + C^2 + 2C^) increased by .68 percent per atomic percent hydrogen at room temperature. The Voigt averaged Young's modulus, E, was found to increase by .69 percent per atomic percent hydrogen and the bulk modulus, K, was found to increase by .72 percent per atomic percent hydrogen. The temperature coefficients of the elastic constants exhibited no appreciable change due to hydrogen addition Analysis of the data shows that the tetragonal distortion due to hydrogen in the tantalum lattice is very slight and the distortion parameter X^ - X^ is evaluated as 2.74 x 1 per atomic percent hydrogen. Correlation of the current results with lattice parameter and dilation changes of tantalum due to hydrogen reveals hydrogen to produce a dipole distortion of strength A = 3.88 eV and B = 3.5 eV 3 based upon the Seeger, Mann and Jan Notation.