Showing papers on "Deformation (engineering) published in 1989"

Mechanical properties of thin films

Abstract: The mechanical properties of thin films on substrates are described and studied. It is shown that very large stresses may be present in the thin films that comprise integrated circuits and magnetic disks and that these stresses can cause deformation and fracture to occur. It is argued that the approaches that have proven useful in the study of bulk structural materials can be used to understand the mechanical behavior of thin film materials. Understanding the mechanical properties of thin films on substrates requires an understanding of the stresses in thin film structures as well as a knowledge of the mechanisms by which thin films deform. The fundamentals of these processes are reviewed. For a crystalline film on a nondeformable substrate, a key problem involves the movement of dislocations in the film. An analysis of this problem provides insight into both the formation of misfit dislocations in epitaxial thin films and the high strengths of thin metal films on substrates. It is demonstrated that the kinetics of dislocation motion at high temperatures are expecially important to the understanding of the formation of misfit dislocations in heteroepitaxial structures. The experimental study of mechanical properties of thin films requires the development and use of nontraditional mechanical testing techniques. Some of the techniques that have been developed recently are described. The measurement of substrate curvature by laser scanning is shown to be an effective way of measuring the biaxial stresses in thin films and studying the biaxial deformation properties at elevated temperatures. Submicron indentation testing techniques, which make use of the Nanoindenter, are also reviewed. The mechanical properties that can be studied using this instrument are described, including hardness, elastic modulus, and time-dependent deformation properties. Finally, a new testing technique involving the deflection of microbeam samples of thin film materials made by integrated circuit manufacturing methods is described. It is shown that both elastic and plastic properties of thin film materials can be measured using this technique.

An explanation of the indentation size effect in ceramics

Abstract: A quantitative model is proposed to explain the indentation size effect (ISE) often observed in the hardness response of hard brittle materials, namely that hardness is observed to increase with decreasing indentation size. The model is based on a mixed elastic/plastic materials deformation response whereby plastic deformation occurs in a discrete manner progressively to relieve stresses created by elastic flexure of the surface at the edges of the indentation. During unloading of the indenter, recovery of the elastic increment of deformation, which precedes each new band of plastic deformation, results in the indentation appearing smaller than expected, particularly as the indentation sizes decrease to approach the scale of the plastic deformation band spacing. The model fits observed experimental data well and analysis of hardness/size data in this way is shown to allow both a bulk hardness value and a characteristic deformation band scale to be calculated for a given sample.

Welding of polymer interfaces

Abstract: Studies of strength development at polymer-polymer interfaces are examined and applications to welding of similar and dissimilar polymers are considered. The fracture properties of the weld, namely, fracture stress, σ, fracture energy, GIc, fatigue crack propagation rate da/dN, and microscopic aspects of the deformation process are determined using compact tension, wedge cleavage, and double cantilever beam healing experiments. The mechanical properties are related to the structure of the interface via microscopic deformation mechanisms involving disentanglement and bond rupture. The time dependent structure of the welding interface is determined in terms of the molecular dynamics of the polymer chains, the chemical compatibility, and the fractal nature of diffuse interfaces. Several experimental methods are used to probe the weld structure and compare with theoretical scaling laws, Results are given for symmetric amorphous welds, incompatible and compatible asymmetric amorphous welds, incompatible semicrystalline and polymer-metal welds. The relevance of interface healing studies to thermal, friction, solvent and ultrasonic welds is discussed.

Effects of Deformation Induced Phase Transformation and Twinning on the Mechanical Properties of Austenitic Fe–Mn–Al Alloys

Abstract: Structure and mechanical properties of austenitic Fe–(20 and 30)Mn–(0 to 7) Al alloys in the temperature range between 77 and 295 K have been investigated in relation to the occurrences of phase transformation and deformation twinning. Additions of aluminum to the 20 wt% Mn alloys significantly decreased the γ→e transformation temperature. The yield stress of these alloys increased with increasing aluminum content, whereas the strain hardening of them decreased. This tendency is prominent at low temperatures. In the 30 wt% Mn alloys the yield stress and strain hardening were almost identical regardless of aluminum contents. Additions of aluminum strongly suppress the γ→e transformation and give birth to the occurrence of deformation twinning. Calculated stacking fault energy based on a regular solution approach shows that the austenitic Fe–Mn–Al alloys which have the stacking fault energy approximately larger than 20 erg/cm2 favor the deformation twinning leading to the increase in low temperature ductility.

Micromechanics of the brittle to plastic transition in Carrara marble

Abstract: Samples of Carrara marble were deformed at room temperature to varying strains at confining pressures spanning the range in mechanical behavior from brittle to plastic. Volumetric strain was measured during the experiments, and the stress-induced microstructure was characterized quantitatively using optical and transmission electron microscopy. The range of confining pressure over which transitional (or semibrittle) deformation occurs is 30–300 MPa. The macroscopic initial yield stress is constant for confining pressures greater than 85 MPa, whereas the differential stress at the onset of dilatancy increases with pressure up to 300 MPa. The dilatancy coefficient decreases rapidly with increasing pressure up to 100 MPa, and then asymptotically approaches zero for pressures up to 300 MPa. The work hardening coefficient increases with pressure up to 450 MPa; the pressure sensitivity is greatest for pressures up to 100 MPa. Active deformation mechanisms include microcracking, twinning, and dislocation glide. Transmission electron microscopy observations indicate that dislocation glide occurs, at least on a local scale, in samples deformed in the semibrittle field at pressures as low as 50 MPa and applied differential stress well below the critical resolved shear stress for glide on the easiest slip system. Cracks and voids frequently nucleate at sites of stress concentration at twin boundaries, at twin terminations, and at the intersection of twin lamallae. Geometries suggestive of crack tip shielding by dislocations are also observed. Stereological measurements indicate that at constant strain in the semibrittle field, the stress-induced crack density and anisotropy decrease with increasing pressure. Crack density and anisotropy in samples deformed to strains of 3–5% in the semibrittle field at pressures up to 120 MPa are comparable to those in the prefailure brittle sample, although an analysis of the energetics of deformation suggests that the ratio of brittle energy dissipation to total energy dissipation is at least 60% lower. We also detect a qualitative difference in the characteristic length of the cracks in the brittle and semibrittle fields. The mean dislocation density at constant differential stress increases significantly for samples deformed at pressures of 230 MPa and greater. Our results suggest that although semibrittle flow occurs over a wide range of pressure, the most marked changes in strain partitioning, and hence the style of deformation occur over a small range in pressure.

On microstructural evolution and micromechanical modelling of deformation of a whisker-reinforced metal-matrix composite

Abstract: The precipitation characteristics, the mechanisms of accelerated aging, and the variation of uniaxial tensile stress-strain behavior in response to controlled variations in matrix microstructure were investigated for a 2124 AlSiC whisker composite. The yield strength of the composite was found to be independent of matrix aging condition. However, the overall ductility decreased monotonically with an increase in aging time. A finite element analysis of the constitutive response of the composite is presented. The results of these calculations, as well as the predictions of several models for composite strengthening available in the literature, were compared with the experimental results. The presence of brittle whiskers in aluminum leads to a significant build-up of hydrostatic stresses in the matrix during plastic deformation. Void formation in the matrix of the composite as well as at the whisker-matrix interface appears to play an important role in controlling the overall failure mechanisms. Transmission electron microscopy observations of void formation a whisker ends are described for composite specimens strained in tension at room temperature and at 300°C. A detailed discussion of matrix deformation and interfacial debonding is presented in an attempt to identify the origins of low ductility in discontinuously reinforced metal-ceramic composites.

Interfacial shear strength studies using the single-filament-composite test. Part II: A probability model and Monte Carlo simulation

Abstract: A Monte Carlo simulation is developed to provide a foundation for interpreting experimental data from the single-filament-composite test. The main focus is on developing an improved procedure for arriving at a realistic value for the shear strength of the fiber-matrix interface, but it is also shown how the test may be used to characterize the strength distribution of fibers at a length scale much shorter than is achievable in standard tension tests. The simulation is based on the widely used Poisson/Weibull probability model for fiber failure that characterizes the strength in terms of the random flaws distributed along the fiber. The primary mechanical model for stress buildup at the fiber end is the same as that assumed by most authors and assumes a constant interfacial shear stress in this shear transfer zone reminiscent of a yielding plastic matrix. We also, however, consider a bilinear model that allows for a zone of debonding with a constant shear stress lower than in the primary “plastic” zone. Simulation results are cast in terms of nondimensional variables and tabulated to allow for wide applicability. Sample size and confidence interval issues are also discussed.

Structure and deformation properties of red blood cells: concepts and quantitative methods.

Abstract: The lamellar configuration of the red cell membrane includes a (liquid) superficial bilayer of amphiphilic molecules supported by a (rigid) subsurface protein meshwork. Because of this composite structure, the red cell membrane exhibits very large resistance to changes in surface density or area with very low resistance to in-plane extension and bending deformations. The primary extrinsic factor in cell deformability is the surface area-to-volume ratio which establishes the minimum-caliber vessel into which a cell can deform (without rupture). Within the restriction provided by surface area and volume, the intrinsic properties of the membrane and cytoplasm determine the deformability characteristics of the red cell. Since the cytoplasm is liquid, the static rigidity of the cell is determined by membrane elastic constants. These include an elastic modulus for area compressibility in the range of 300-600 dyn/cm, an elastic modulus for in-plane extension or shear (at constant area) of 5-7 X 10(-3) dyn/cm, and a curvature or bending elastic modulus on the order of 10(-12) dyn.cm. Even though small, the surface rigidity of the cell membrane is sufficient to return the membrane capsule to a discoid shape after deformation by external forces. Viscous dissipation in the peripheral protein structure (cytoskeleton) dominates the dynamic response of the cell to extensional forces. Based on a time constant for recovery after extensional deformation on the order of 0.1 sec, the coefficient of surface viscosity is on the order of 10(-3) dyn.sec/cm. On the other hand, the dynamic resistance to folding of the cell appears to be limited by viscous dissipation in the cytoplasmic and external fluid phases. Dynamic rigidities for both extensional and folding deformations are important factors in the distribution of flow in the small microvessels. Although the red cell membrane normally behaves as a resilient viscoelastic shell, which recovers its conformation after deformation, structural relaxation and failure lead to break-up and fragmentation of the red cell. The levels of membrane extensional force which is two orders of magnitude less than the level of tension necessary to lyse vesicles by rapid area dilation. Each of the material properties ascribed to the red cell membrane plays an important role in the deformability and survivability of the red cell in the circulation over its several-month life span.

Martensite formation, strain rate sensitivity, and deformation behavior of type 304 stainless steel sheet

Abstract: The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed.

Deformation Characteristics of Reinforced Sand in Direct Shear

Abstract: A series of direct shear laboratory tests was performed in order to examine the mechanism of shear zone development in reinforced sands and to quantify their deformation characteristics. Six types of reinforcements were used to evaluate the effects of reinforcement concentration, stiffness, and reinforcement‐soil bond strength on the deformation and strength of reinforced sands. Tests results show that shear zones tend to be wider in reinforced soil composites than in soil alone. The width of the shear zones increases with increasing stiffness of the composite due to any combination of increased reinforcement concentration, stiffness, and reinforcement‐soil bond strength. A linear relationship between reinforcement concentration and increased strength is not observed.

Deformations caused by surface loading and tunnelling: the role of elastic anisotropy

Abstract: Elastoplastic finite element analyses were performed to determine the effects of elastic anisotropy on surface settlements caused by a uniform surface loading and by tunnelling. For typical ranges of elastic cross-anisotropic parameters, the effect of anisotropy on settlement induced by surface loading is modest; however, for the case of tunnelling, the effect of elastic anisotropy, in particular the ratio of the independent shear modulus Gvh, to vertical modulus Ev, should be considered if reasonable predictions of settlement are to be obtained. The practical implications of these find- ings are shown by considering the centrifugal model tunnel tests conducted at Cambridge University. It is found that the anisotropy has a significant effect on the shape of the settlement trough and that reasonable agreement between observa-tion and theory is obtained for Gvh/Ev, of between 0·2 and O·25, which is smaller thanthe isotropic value of Gvh/Ev, = 0·33 (for the undrained condition). The reasons for this are disc...

Fluid pressures in deforming porous rocks

Abstract: The constitutive relations describing the fluid pressure response of a porous medium to changes in stress and temperature must reflect the microscopic processes that are operative over the time scale allowed for the deformation. Short-duration deformations are readily described by undrained moduli, and intermediate duration deformations by drained moduli, both of which are formulated through linear elastic theory. Long-term deformations that operate over geologic time are normally dominated by irreversible processes and result in considerably larger deformations, for the same applied stress conditions, than would be expected from their elastic counterparts. Model constitutive equations are developed for both elastic and irreversible processes and the magnitude and interpretation of the relevant material properties examined. Although the theory is presented in general terms, a sample calculation shows that for sandstone the inelastic deformation is one and one half orders of magnitude greater than the elastic deformation at the same applied stress. This difference in magnitude has a significant effect on the effective hydraulic diffusivity, various pore pressure coefficients, and the prospective fluid pressure development of the sediment.

Adhesive failure and deformation behaviour of polymers

Abstract: An instrument has been developed to determine the adhesive fracture energy as a function of the most important parameters such as temperature, contact time etc. and to study the stress–strain behaviour during bond separation. Additionally, the deformation processes during debonding were observed by high speed photography. Investigations of two high molecular weight polymers, polyisobutylene (PIB) and polyethylhexylacrylate (PEHA), showed two different types of bond separation: “brittle” behaviour with low adhesive failure energy for PIB and the formation and deformation of fibrillar structures for PEHA leading to much higher strains at break and adhesive failure energies. It follows from mechanical measurements that both polymers differ mainly by their entanglement networks. The much longer entanglement spacing for PEHA leads to the formation of fibrillar structures which, in accordance with a theory of Good, seem to be the reason for strong adhesion.

Carbide precipitation during stage I tempering of Fe-Ni-C martensites

Abstract: Microstructural changes which accompany the first stage of tempering have been studied by transmission electron microscopy (TEM) and electrical resistometry in two Fe-Ni-C alloys that form platelike martensite. The e-carbide transition phase in these alloys adopts a platelike shape with a habit plane near {012=α. Electron diffraction data indicate that the carbide may be partially ordered, resulting in orthorhombic symmetry, and therefore, this phase is designated as e′- carbide. The carbide particles contain a fine internal substructure which appears to represent an internal accommodation deformation (faulting) on the carbide basal plane. Detailed analysis of the kinematics of carbide precipitation suggests that the observed habit plane and accommodation deformation permit an invariant-plane strain transformation which minimizes elastic strain energy. The apparent selection of only a limited number of possible orientation variants is explained in terms of the symmetry of the parent martensitic phase, which is known to undergo spinodal decomposition prior to the nucleation of the transition carbide. The martensitic substructure is not found to exert any significant influence on this overall precipitation behavior.

The effects of pressure and porosity on the micromechanics of the brittle‐ductile transition in quartzite

Abstract: Room temperature experiments on a quartzite with 7% porosity show that with increasing pressure there is a transition from brittle faulting to ductile cataclastic flow at about 600 MPa. In contrast, experiments on nonporous quartzite show only faulting up to 1000 MPa. The transition to cataclastic flow in the porous quartzite is due to the changing influence of the pores. Microstructural observations and elastic theory suggest that the transition occurs at the pressure where the differential stress producing a compressive stress concentration sufficient to initiate pore collapse becomes less than that producing a tensile stress concentration sufficient to nucleate axial microcracks at the tops and bottoms of pores. Cataclastic flow of the porous quartzite is only transient. Deformation remains distributed and accompanied by work hardening only as long as pore collapse continues (to ∼16% strain); at higher strains, microcracking produces a net dilatancy, which causes strain weakening that leads to faulting (at ∼26% strain). At a confining pressure of 750 MPa the faults are stable, whereas at 1000 MPa they are unstable. The results indicate that porous rocks may undergo limited ductile deformation at shallow levels in the crust, but higher strains will cause deformation to localize.

Softening by void nucleation and growth in tension and shear

Abstract: The effect of void nucleation and growth on overall stress-strain behavior is investigated for solids undergoing plastic straining under axisymmetric and shearing conditions. Contact between the void surface and the nucleating particle is taken into account and is found to be important under shear and under axisymmetric straining when the stress triaxiality is low. The notion of the macroscopic stress drop due to nucleation of a void is defined and computed, both for isolated voids and for voids in periodic arrays. The stress drop for an isolated void in an infinite matrix can be used to predict softening due to void nucleation when the void concentration is dilute. Interaction between voids in shear during nucleation is analysed numerically and softening effects are calculated along with large strain aspects of void deformation during subsequent growth.

Effect of residual strain on the substructure development and mechanical response of shock-loaded copper

Abstract: Shock recovery experiments have been performed to assess the role of residual strain on the substructure evolution and mechanical response of oxygen-free electronic copper. Alterations in momentum trapping design and assembly tolerances produced samples which, while having been all similarly impacted at 518 m s−1 (10 GPa for symmetrical copper impact) for a pulse duration of 1 μs, possessed residual plastic strains ϵres ranging from 1.5% to 26%. Increasing the amount of residual strain is observed to correlate with a change in the deformation substructure from dislocation cells to microbands and deformation twins; the substructure at ϵres = 1.5% consists of dislocation cells while that of ϵres = 26% includes a high density of deformation twins. Coincident with this structure alteration, increasing the ϵres from 1.5% to 26% increases the reload yield strength of shock-loaded copper by 40%. The role of residual strain on metallurgical investigation of the structure-property relationships in shock-deformed materials and the occurrence of deformation twinning in f.c.c. metals is discussed.

Plastic deformation and mobility in glassy polymers

Abstract: Deformation of several linear glassy polymers such as: a-PS, a-PMMA, a-PET, PC and epoxy-aromatic amine networks was investigated by deformation calorimetry, DSC, IR-Fourier spectroscopy and irreversible deformation recovery at heating. All polymers deformed in the glassy state have shown number of common features. In all cases irreversible deformation is followed by marked sample internal energy increase which reaches up to 20–50% of total mechanical input. This energy excess stored in a quite local plastic shear defects (PSD) which carry with them local irreversible deformation. The formation and growth of these PSD is the rate controlling step of macroscopic plasticity process in polymeric glasses. During of process development some PSD are converted due to defect interaction to new local structures with conformational changes of macromolecular backbone.

Plastic deformation and solid-phase transformation in β-phase polypropylene

Abstract: Tensile specimens of isotactic polypropylene, initially crystallized in the β-form with high purity, were tested at various temperatures. It was observed by WAXD that the most important phenomenon occurring in the course of a tensile test is the β-smectic transformation for specimens drawn at lower temperature and β-α transformation for specimens drawn at higher temperature, which takes place in the crystalline lamellae and is induced by plastic deformation. c-Axis oriented β-crystals have never been found in the drawn specimens. It was also shown that the β-smectic transformation propagates inside the spherulites from the equatorial regions, in which the chain axes are nearly parallel to the drawing direction, towards the diagonal and polar regions. Finally, it was noted that microvoids are formed simultaneously during plastic deformation, which may be due to the volume contraction induced by the β-α or β-smectic transformation.

Composition and temperature dependence of tensile properties of austenitic FeMnAlC alloys

Abstract: The effects of aluminium addition (0–4 wt%) to Fe-26MnAl alloys and carbon addition (01 – 03 wt%) to Fe-30Mn-1AlC alloys on tensile properties, especially tensile elongation, were investigated from room temperature to 4 K The transformation of austenite to deformation twins during the tests was very beneficial in enhancing tensile elongation at cryogenic temperatures The amount of deformation twins formed during plastic deformation was not the major factor for maximum elongation, but the optimum work hardening rate by the gradual formation of deformation twins played an important role The maximum elongation peak shifted to lower temperatures with increased aluminium (5 aluminium wt%), but moved to higher temperatures with increased carbon content (03 wt%) The new flow equation σ = Kϵ N exp ( Mϵ ) was applied to calculate uniform elongations The calculated values were in reasonable agreement with the measured values