Showing papers on "Deformation (engineering) published in 1980"

Void Nucleation Effects in Biaxially Stretched Sheets

Abstract: The effects of void nucleation occurring during the deformation history on forming limit curves are considered for both in-plane and punch stretching employing a constitutive model of a porous plastic solid. Both plastic strain controlled and stress controlled nucleation processes are simulated by a two parameter void nucleation criterion. For in-plane stretching, plastic strain controlled nucleation can have, in certain circumstances, a significantly destabilizing effect on the forming limit curve. However, within the framework of plane stress theory which neglects the enhance­ ment of the hydrostatic stress due to necking, a stress controlled nucleation process is not found to be significantly destabilizing. In punch stretching a ductile rupture criterion, which limits the maximum volume fraction of voids, as well as the ap­ pearance of a well defined thickness trough, is adopted as a localized necking criterion. Only plastic strain controlled void nucleation is considered here in out-ofplane stretching. The resulting forming limit curves have the same shape as those obtained previously with void nucleation neglected.

Plastic Deformation of Fine‐Grained Alumina (Al2O3): I, Interface‐Controlled Diffusional Creep

Abstract: Plastic deformation in fine-grained alumina polycrystals (grain size 1 to 15 μm) was studied. At least three distinct deformation mechanisms are important: diffusional creep, basal slip, and unaccommodated grain-boundary sliding. The first and most important of these processes is addressed in this paper. Analysis of the deformation dynamics suggests that both lattice and grain-boundary diffusion are important in the diffusional creep. Aluminum, rather than oxygen, lattice and grain-boundary diffusion are rate-controlling because oxygen diffusion is very slow in the lattice but very rapid in grain boundaries. Significantly, the diffusional creep can become interfacecontrolled at low stresses, causing the often-reported non-Newtonian creep behavior of fine-grained alumina.

Motion of a spherical microcapsule freely suspended in a linear shear flow

Abstract: The motion of a spherical microcapsule freely suspended in a simple shear flow is studied. The particle consists of a thin elastic spherical membrane enclosing an incompressible Newtonian viscous fluid. The motions of the internal liquid and of the suspending fluid are both described by Stokes equations. On the deformed surface of the membrane, continuity of velocities is imposed together with dynamic equilibrium of viscous and elastic forces. Since this problem is highly nonlinear, a regular perturbation solution is sought in the limiting case where the deviation from sphericity is small. In particular, the nonlinear theory of large deformation of membrane shells is expanded up to second-order terms. The deformation and orientation of the microcapsule are obtained explicitly in terms of the magnitude of the shear rate, the elastic coefficients of the membrane, the ratio of internal to external viscosities. It appears that the very viscous capsules are tilted towards the streamlines, whereas the less viscous particles are oriented at nearly 45° to the streamlines. The tank-treading motion of the membrane around the liquid contents is predicted by the model and appears as the consequence of a solid-body rotation superimposed upon a constant elastic deformation.

Adhesion and Adsorption of Polymers

Abstract: One: Polymer Surface Interactions- Introductory Remarks- Statistical Mechanics of Surface Tension and Adsorption- Surface and Interfacial Tensions of Polymer Melts and Solutions- Donor-Acceptor Interactions at Interfaces- Surface Tension of Solids: Generalization and Reinterpretation of Critical Surface Tension- A Role of Molecular Forces in Adhesive Interactions of Polymers- Discussion- Two: Characterization of Adhesive Interfaces- Introductory Remarks- Photoacoustic Spectroscopy for the Study of Adhesion and Adsorption of Dyes and Polymers- Adhesive-Adherend Bond Joint Characterization by Auger Electron Spectroscopy and Photoelectron Spectroscopy- Techniques for Measuring Adhesive vs Cohesive Failure- Failure Characterization of a Structural Adhesive- Dielectric Relaxation Gradients in an Adhesive Bond- New Approach to the Understanding of Adhesive Interface Phenomena and Bond Strength- Discussion- Three: Polymeric Structural Adhesives- Introductory Remarks- Fracture Mechanics and Adherence of Viscoelastic Solids- The Viscoelastic Shear Behavior of a Structural Adhesive- Surface Energetics and Structural Reliability of Adhesive Bonded Metal Structures- Composition and Ageing of a Structural, Epoxy Based Film Adhesive- Viscoelastic Characterization of Structural Adhesive via Force Oscillation Experiments- Crack Healing in Semicrystalline Polymers, Block Copolymers and Filled Elastomers- Discussion- Four: Fracture Strengths in Polymeric Systems- Introductory Remarks- Criterion of Interfacial Fracture on Tensile Adhesive Joint- Deformation and Shear Strength of FRP Adhesive Joints- Stress Distribution and Strength in Shear on the Adhesive Lap Joint Loading Bending Moment- Fracture Criteria on Peeling- Superposition of Peel Rate, Temperature and Molecular Weight for T Peel Strength of Polyisobutylene- Discussion- Author Index

Plastic Deformation of Fine‐Grained Alumina (Al2O3): II, Basal Slip and Nonaccommodated Grain‐Boundary Sliding

Abstract: Plastic deformation was studied in fine-grained alumina polycrystals (grain sizes 1 to 5 μm); several deformation mechanisms occur simultaneously. This paper is concerned with basal slip and unaccommodated grain-boundary sliding (GBS). The basal slip can give rise to a deformation texture. Cavitation occurs due to the occurrence of both unaccommodated GBS and basal slip, because of the marked plastic anisotropy of alumina.

Plastic relaxation of the transformation strain energy of a misfitting spherical precipitate: Ideal plastic behavior

Abstract: Assuming ideal plastic behavior for an isotropic matrix containing a misfitting spherical precipitate, the total amount of work expended during elasto-plastic deformation is calculated and compared with the total strain energy in the corresponding pure elastic state. For precipitates larger than one micron (µm), the effective yield stress is taken as the macroscopic yield stress while for smaller precipitates, size-dependent yield stresses are obtained from the Ashby-Johnson model. In the case of coherent submicron precipitates, the effective yield stress becomes the theoretical yield strength and thus plastic relaxation is not possible unless the transformation stress is extremely large. For incoherent submicron precipitates, the effective yield stress is approximately inversely proportional to the precipitate radius,r. Hence plastic relaxation again is not possible whenr < 10 nm, but whenr ≃100 nm the strain energy can decrease by 10 ∼ 40 pct at a misfit of 3 pct. For supra-micron particles, however, the ratio of the effective yield stress to the shear modulus becomes 10−3 or less, and plastic relaxation can reduce the strain energy by factors of 3 to 15 at misfits of 1 to 3 pct. Under this circumstance, the plastic zone becomes wide, its radius ranging from 3 to 5r.

Experimental studies of scale effects on shear strength, and deformation of rock joints

Abstract: The effect of scale on shear strength is studied by performing direct shear tests on different sized samples of large joints. A rubber moulding system is used to obtain impressions of the roughness from a variety of natural joint surfaces. A brittle model material is developed and used to cast several sets of identical interlocking specimens, which are in turn subdivided into sets of equidimensional joint block samples, each of the sets representing a different average block size or joint length. All sample sizes are tested in the same relative direction of shearing and under precisely the same level of normal stress. A total of eleven joints displaying a large variety of surface roughness is included in the experimental programme. The results from those tests show that peak shear strength is a strongly scale-dependent variable. Scale effects are most pronounced in cases of rough undulating joints, whereas they are virtually absent for relatively planar joints. The key factor behind the changing behaviour and strength with increasing scale is the involvement of different sizes of asperities in controlling the peak behaviour of different lengths of joints. An important consequence is a decrease in the value of roughness coefficient (JRC) and wall strength (JCS) operating at larger scale. Use of those indices may enable realistic allowance to be made. for the scale effect in peak shear strength. The normal deformability of rock joints is investigated by conducting loading/unloading and repeated load cycling tests on a wide ranging variety of fresh and weathered joints in five different rocks. The shear deformability is studied by performing direct shear tests at different levels of normal stress in a portable shear box. Several mismatched joints are also tested under compression. In the present work the analytical representation of the stress deformation curves is considered in detail. Quantitative relationships between maximum closure, aperture, wall strength and roughness are defined.

A quantitative metallographic assessment of structural degradation of type 316 stainless steel during creep‐fatigue

Abstract: — Optical and scanning electron microscopy have revealed the existence of grain boundary cavities in a series of specimens which had been cyclically deformed in the strain range ±0.25% with hold times ranging from 0 to 1000 min. The way in which these defects increase in size and number has been measured and found to correlate with the creep strain accumulated during the hold periods. A further observation is that a critical amount of deformation is required before either fatigue or creep type damage is nucleated. These observations lead to an alternative method to the currently accepted linear damage summation rule for estimating a lower bound of the creep-fatigue endurance.

The noncontinuum crack tip deformation behavior of surface microcracks

Abstract: The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.

Nucleation and growth of creep cavities in a Type 347 steel

Abstract: The nucleation and growth of intergranular creep cavities has been studied in a Type 347 austenitic steel tested at 550 and 650°C over a limited range of stress. It was found that the number of cavities per unit volume is linearly related to the time-dependent creep strain, being independent of stress or temperature over the conditions tested. The results indicate that cavity nucleation is controlled by a deformation process. Studies on cavity growth show that the cavity diameter increases approximately linearly with time, with a stress dependence for growth similar to that for secondary creep. These results are consistent with a diffusion growth model when a simple deformation-controlled nucleation law is incorporated. Using this growth model the observed stress and time dependence for the total void volume can be predicted, and the rupture life has been estimated reasonably well using a simple fracture criterion.

The deformation of commercial aluminum-magnesium alloys

Abstract: The deformation and fracture of Al-Mg alloys have been investigated over a range of strain rates and temperatures. The rate and temperature-dependence of the deformation can be understood on the basis of the mobility of Mg atoms. When the Mg atoms can diffuse at a sufficient rate they interact with dislocations to produce serrated yielding and a zero or negative strain rate sensitivity. This greatly restricts the tensile ductility. Outside of this serrated flow regime enhanced ductilities are achieved due to the positive strain rate sensitivity stabilizing any inhomogeneities which develop. However, the final fracture is also influenced by the constituent particle concentration. Voiding occurs at particles and if the concentration of particles is sufficiently large the voids link up by local shear. Decreasing the constituent particle concentration, together with a reduction in the amplitude of serrations, results in larger tensile ductilities. At high temperatures, quite large strain rate sensitivities can be achieved but the ductility is finally limited by the propensity for voiding in these alloys.

Creep and strain recovery in hot-pressed silicon nitride

Abstract: It is observed that creep response in hot-pressed silicon is characterized by two parallel phenomena; one accounts for a persistent non-recoverable plastic deformation and the other for a transient viscoelastic recoverable deformation. The persistent creep component is time-dependent, and apparently follows parabolic time kinetics. It is further observed that creep is characterized by a power law stress exponent of about 4 and an activation energy of 848 kJ mol−1. The viscoelastic recoverable component of strain is found to be independent of the total plastic strain in the material. The recovery rate at any given time is directly proportional to the preceding creep stress and therefore can be considered linear viscoelastic. The creep compliance of the viscoelastic transient is temperature-dependent with an activation energy of about 722 kJ mol−1. It is further observed that the viscoelastic recovery is characterized by a spectrum of retardation times and can be modelled by a series of Kelvin analogue models. Finally, the viscoelastic recovery and the viscoelastic component of subsequent creep appear to be inversely related and apparently obey Boltzman superposition. A model is developed for the creep and recovery behaviour of hot-pressed silicon nitride consistent with all experimental observations and based in relative grain motion accommodated by the fluid grain-boundary glass liquid flow, cavitation and wedge opening.

Mechanical behaviour and structure of snow under uniaxial tensile stress

Abstract: The mechanical behaviour of snow was studied at — 10°C under uniaxial tensile stress in a range of cross-head speed 6.8 × 10–8 to 3.1 × 10–4 ms–1 and snow density 240-470 kg m–3.It was found from the resisting force-deformation curves that the snow was deformed in two different ways: namely, brittle and ductile deformation at high and low strain-rates, respectively. The critical strain-rate dividing the two deformation modes was found to depend on the density of snow. In ductile deformation, many small cracks appeared throughout the entire specimen. Their features were observed by making thin sections and they were compared with small cracks formed in natural snow on a mountain slope. The maximum strength of snow was found to depend on strain-rate: at strain-rates above about 10–5 s –1, the maximum strength increased with decreasing strain-rate but below 10–5 s–1 it decreased with decreasing strain-rate.

XII – Compression of Spherical Cells

Abstract: A number of results from the analysis of the deformation of spherical nonlinear elastic membranes compressed between two parallel plates are presented. The motivation for the investigation was to clarify certain results obtained from experiments on the compression of spherical cells, in particular, sea-urchin eggs in cell biology. Of interest is the relationship between the nature of the deformation of the cell, the magnitude of the applied compressive force, and the description of the physical properties of the cell membrane. We present results for two different assumptions on the nature of the material behavior of the membrane, first, to compare results with model experiments on fluid-filled rubber balloons, and second, to compare results with experimental data on cells. We show how the results from nonlinear membrane theory can be used to estimate elastic constants of membranes of compressed spherical cells.

Network disentanglement and time‐dependent flow behaviour of polymer melts

Abstract: For small strains Lodge's rubberlike-liquid theory is a valid description of the rheological behaviour of polymer melts, but at higher strains the theory fails: The phenomenon of shear thinning which is characteristic for the shear flow of nearly all polymer liquids is not explained, and in elongation the deviation of the experimental data from the predictions of Lodge's theory reflect also a flow thinning, and not a strain hardening in spite of the pronounced S-shape of the stress-strain diagrams. Comparing the measured stress growth and stress relaxation data with the predictions of the theory, it must be concluded that the temporary physical network structure of the polymer melt is destroyed increasingly with the magnitude of the deformation. Hence, the number of entanglements decreases with increasing strain. For simple shear and uniaxial extension of a well-defined polyethylene melt the strain dependence of the relative entanglement density is discussed. The irreversibility of the disentanglement process of the network is considered.