Function Of Prestrain Research Articles

Internal friction in F.C.C. alloys due to solute drag on dislocation—II. Experimental studies on AlSi alloys

Internal friction (mechanical loss) experiments were carriedout on single crsytal and polycrystalline AlSi alloys with incoherent Si particles of varying size, spacing, and as a function of pre-strain. The resulting internal friction spectra are correlated with microstructural investigations. The loss maximum at about 450 K ( ƒ ≈ 1 Hz ), especially its “low” activation enthalpy of H = 1.14 eV, can be explained by our dislocation model described in the companion paper (Part I), which considers the enhanced transverse diffusivity of solute atoms in the vicinity of the dislocation core. The model is also shown to be in agreement with published data on AlSi, AlMg, CuSi and CuAl. It is proposed that in many instances the internal friction maxima, even when characterized by a “low” activation enthalpy, need not be due to grain boundary or particle effects, but may well be explained by core diffusion effects on solute drag acting on dislocations.

Effect of simulated sampling disturbance on creep behaviour of rock salt

This article presents the results of an experimental study of creep behaviour of a rock salt under uniaxial compression as a function of prestrain, simulating sampling disturbance. The prestrain was produced by radial compressive loading of the specimens prior to creep testing. The tests were conducted on an artifical salt to avoid excessive scattering of the results.

Fatigue of a dual- phase steel

A study has been made of the fatigue of a V containing dual-phase steel, whose tensile strength is equivalent to that of SAE 980X high strength, low-alloy (HSLA) steels, as a function of prestrain. It is found that the cyclic stress-strain curve, strain-life response and notch sensitivity are little affected by pre-strains of up to 8 pct: This is in contrast to monotonie flow strength which increases substantially with prestrain. The fatigue performance of the dual-phase steel, while different in detail from that of other HSLA steels, is intermediate between that for SAE 950X and 980X steels. However, the notch fatigue behavior is equivalent to that of 980X steels. The fatigue response of dual-phase steel can be understood in terms of its high rate of work hardening which is a consequence of its ferrite plus martensite microstructure.

Analysis of strengthening mechanisms in alloys by means of thermal-activation theory

Through a systematic measurement of the strain-rate sensitivity as a function of prestrain. theoretical models of thermally activated flow are used to identify and analyze the dominant strengthening mechanisms in a number of commercial and model alloys. Several examples of the usefulness of the technique as a diagnostic tool are given. At temperatures below the ‘plateau’ stress, measurements are in good agreement with predictions, even for relatively complex commercial materials, and it was demonstrated that changes in strengthening mechanisms (e.g. during age hardening or thermomechanical processing) can be followed directly even when the scale of the microstructure is too small for convenient observation by transmission electron microscopy. Experiments at high temperature (above the regime of jerky flow) demonstrate that, except at low stress, the material behavior at moderate strain rates is dominated by strain softening mechanisms such as dislocation rearrangement by cross-slip or climb. The major effect of solution hardening at high temperature is to increase the stress at which these processes occur.

Dynamic Poisson's ratio: A novel technique to study interfacial adhesion

AbstractA novel technique utilizing the dynamic Poisson's ratio to measure the strength of adhesion in fiber‐filled polymeric composites is described. This technique is based on the observation that glass‐filled composites exhibit a large drop in Poisson's ratio as a function of prestrain at very low prestrains. This drop, which indicates additional volume dilation, is interpreted as due to debonding or void formation at the filler‐matrix interface. This interpretation is consistent with the results obtained by using three very different glass surface treatments, with a description of the debonding process using two simplified models and with mechanical properties data. The dynamic Poisson's ratio technique is insensitive to filler orientation and has high resolution, since very small volume changes are reflected as large drops in Poisson's ratio.

Latent Hardening in Rock‐Salt Type Crystals

AbstractThe latent hardening in nine different crystals having the rock‐salt structure was studied as a function of prestrain. The latent hardening of a LiF crystal was also measured as a function of temperature. It was found that the latent hardening ratio of a crystal increases from 1.0 at zero strain to a maximum at about 0.5 to 1.5% compressive strain, then decreases to a constant value somewhat above 1.0. The maximum latent hardening ratio varies from 3.9 for LiF to 1.2 for AgCl. The difference in stress between the latent and the primary system in LiF remained essentially constant at all temperatures from 450 to 4 °K. The latent hardening ratio at 0.5% prestrain is 3.1 at 300 °K and 1.6 at 4 °K. Latent hardening was observed even above 300 °K where the flow stresses of the primary and the latent systems are essentially independent of strain rate and temperature. Dislocation distribution, revealed by etch pitting, was uniform during the primary slip, but was confined to widely separated bands during the latent hardening test. It is concluded that latent hardening in rock‐salt type crystals is caused primarily by the interaction of the glide dislocations in the latent slip system with the dipoles and dislocations in the primary slip system according to the dislocation reaction .

The microstrain behavior of beryllium single crystals

The room temperature microstrain behavior of zone-refined single crystal beryllium has been examined as a function of prestrain. Crystals oriented for basal slip were deformed in simple tension (prestrains ≤0.09) and by low frequency cyclic compression (prestrains ≤0.40). The friction stress was determined from the energy dissipated in generating closed hysteresis loops as a function of forward plastic strain and maximum stress amplitude. The corresponding dislocation substructures were characterized by transmission electron microscopy. The friction stress T f associated with forward plastic strain amplitudes ≤5 × 10 −5 is low and in the range 0.02 – 0.13 kg·mm −2, with no significant effect of prestrain; a large fraction of T f is due to dislocation-impurity interaction so that the actual lattice friction stress on the basal plane is extremely small. Closed hysteresis loops exist at stress amplitudes up to the macroscopic flow stress level with corresponding forward plastic strain amplitudes as high as 2 × 10 −3. Under these conditions, a much higher friction stress T f(2) ~ 1 kg · mm −2 is measured. Cyclic compression eliminates the region of easy glide (Stage I) in beryllium and raises the flow stress. Differences in behavior in tension and cyclic compression, and the independence of T f with prestrain, have been related to the associated dislocation substructures.

Effect of prestrain on mechanical recovery in Pu-1 wt % Ga

The recovery of yield stress in a Pu-1 wt % Ga alloy has been studied as a function of prestrain. The specimens were prestrained by either tension or rolling and the kinetics of recovery were studied at 100 and 160° C. The total amount of recovery and the initial rate of recovery were found to be dependent on prestrain and annealing temperature.

Nonelastic strain recovery in zinc single crystals

The rapid and time dependent strain recovery which occurs in deformed zinc single crystals has been studied as a function of prestrain, specimen purity and surface condition. The total strain recovery of zinc crystals was found to increase with prestrain for a constant unloading rate or to increase when the surface is copper coated over that for an uncoated crystal at equal prestrains greater than 10 −4. Doping zinc with cadium is found to decrease the amount of total strain recovery as compared to undoped crystals at equal prestrains greater than 10 −5. The recovered strain under various conditions was found to be logarithmic in time at constant applied stress. The time dependent recovery is explainable as a creep recovery process, the inverse of the transient creep behavior discussed by Seeger. The increase of the total strain recovery with prestrain can be accounted for by an increase of dislocation density with prestrain. Only cadmium doped crystals were observed to exhibit a jerky unloading behavior at room temperature and after a certain prestrain. This jerky unloading behavior can be explained in a similar manner to that presented by Cottrell to explain the Portevin-Le Chatelier effect in iron and aluminum alloys. Interpretation of the results is not dependent upon a precise knowledge of the proportion of the dislocations partaking in strain recovery which are held up just beneath the surface of the crystal to those in the interior. The surface effect results suggest, however, that a uniform increase in dislocation density may proceed from the center to the outside of the crystal after prestrain. The internal stress associated with this type of dislocation configuration may be expected to relax almost linearly with the recovered strain.

Work-hardening and work-softening of face-centred cubic metal crystals

Abstract The present paper investigates by experiment and theory the mechanisms governing work-hardening, work-softening and slipband formation in face-centred cubic metals. The rapid hardening stage (stage II) of the stress-strain curve is investigated by flow-stress measurements at different temperatures, combined tensile and torsion experiments, and by the observation of the length of slip lines as a function of prestrain. Stage III of the stress-strain curve and the work-softening phenomena associated with it are investigated with the electron microscope, showing that the principal surface patterns (slip bands, fragmentation) are due to cross slip. It is argued that in stage II the slip distance is decreased continually by the formation of Lomer-Cottrell dislocations. This also accounts for various observations on the end of the easy glide region. The temperature dependence of work-hardening in stage III is caused by screw dislocations circumventing the sessile Lomer-Cottrell dislocations by cross dip. The mechanisms through which cross slip causes glide band formation and fragmentation are discussed in some detail.