Low Deformation Levels Research Articles

X-ray peak profile analysis of crystallite size distribution and dislocation type and density evolution in nano-structured Cu obtained by deformation at liquid nitrogen temperature

X-ray peak profile analysis was employed to determine the crystallite size distribution and the evolution of dislocation type and density in pure Cu deformed by rolling at liquid nitrogen temperature for the following rolling reduction levels: 67, 74, 87, and 97%. The results show that as the deformation level increases, the variance and the median of the crystallite size distribution decreases. It was also found that the dislocation density decreases in the first segment of the deformation, and increases slightly after reaching a minimum. This can be explained by the fact that the mobility of the dislocations was limited by the low deformation temperature. In order to reduce the strain energy, the dislocations reorganize themselves into dislocation cell structure, which leads to the reduction of the X-ray coherent domain length. Furthermore, it was established that full dislocations dominate the deformation process at lower deformation levels, when the median of the crystallite size distribution is greater then 50 nm. At higher deformation levels and smaller crystallite size, the fraction of partial dislocations become significant, while the overall dislocation density increases slightly and the population of the full dislocation decreases. It is concluded that the increase in dislocation density is due to the emission of a new generation of partial dislocations, which become the leading deformation mechanism.

Intergranular strain correlation with magnetic behavior in steel

The correlation between the magnetic behavior and intergranular residual strain was investigated on several mild steel samples which were plastically deformed in the range 0.5%–20% engineering strain followed by unloading. At low deformation levels (0.5%–∼2%) the samples exhibited Luders banding, but after 2% strain the samples appeared uniformly deformed. Magnetic Barkhausen noise (MBN) measurements were used to characterize magnetic behavior and neutron diffraction measurements were used to determine the inter-granular strain. MBN results indicated a high MBN value in the direction transverse to the applied stress. This corresponded to a high strain in the (100) crystallographic direction (the easy magnetization direction), as observed using neutron diffraction. Little correlation was obtained between the MBN result and the (112) strain, which is generally assumed to reflect the macroscopic residual stress. Angular MBN and neutron diffraction results further confirmed the strong correlation between the (100) strain and the MBN result.

A rapid-curing alginate gel system: utility in periosteum-derived cartilage tissue engineering

In this study, we have developed a rapid-curing alginate gel system and demonstrated its utility as a scaffold for periosteum-derived chondrogenesis for articular cartilage tissue engineering applications. A homogeneous mechanically stable gel was formulated by inducing gelation of a 2% (w/v) solution of a high G content alginate (65–75% G) with a 75 m m solution of CaCl 2. The gel exhibited near-elastic behavior at low levels of deformation (15%, R 2=0.996), Young's modulus of 0.17±0.01 MPa, and rapid gelation kinetics (<1 min to completion). The in vitro cell culture of chondrocytes in the gel yielded alginate/cell constructs that lacked the continuous, interconnected collagen/proteoglycan network of hyaline cartilage. In addition, we have demonstrated that this gel system is capable of supporting periosteum-derived chondrogenesis. We observed that when whole-tissue explants of periosteum were cultured in vitro within the gel, after 6 weeks, significant quantities (>50%) of the total area of the periosteal explants was composed of cartilage that was hyaline-like in appearance and contained cartilage-specific proteoglycans and type-II collagen. It is envisioned that such explants could be transplanted or regenerated in vivo within the biodegradable alginate matrix for the treatment of partial or full-thickness defects in articular cartilage. Importantly, the injectable delivery of the gel could be used in filling complex defects in the articular surface via minimally invasive procedures.

A four-year study of shear-wave splitting in Iceland: 2. Temporal changes before earthquakes and volcanic eruptions

Abstract This chapter reports temporal variations in the time-delays between split shear-waves before both earthquakes and volcanic eruptions in Iceland. The hypothesis is that during a build-up of stress, crack distributions in a large volume surrounding the immediate source zone are modified until the level of cracking reaches fracture criticality, when shear strength is lost, rocks fracture, and earthquakes, or some types of eruptions occur. In one two-year period, when volcanic and magmatic activity appeared to be low, changes in shear-wave splitting in SW Iceland were observed routinely before earthquakes with magnitudes between M 3.5 and M 5.1. Assuming a linear relationship between earthquake magnitude and the rate of increasing crack aspect-ratio in this comparatively narrow amplitude range, the time and magnitude of a M 5 earthquake was successfully stress-forecast. These results confirm a new understanding of pre-fracturing deformation of in situ rock that has implications over a wide range of situations where the crust undergoes changes at low levels of deformation below those at which rocks fracture. Potential applications of this new understanding include monitoring hydrocarbon production, as well as stress-forecasting earthquakes and some volcanic eruptions.

Separation of Static Recrystallization and Reverse Transformation of Deformation-induced Martensite in an Austenitic Stainless Steel by Calorimetric Measurements

During plastic deformation of metals and alloys a small fraction, between 1 and 5%, of the energy employed in cold working, is stored inside the material in the form of defects, mainly dislocations. This stored energy derived from cold working is the driving force for recrystallization and is released during annealing. Despite recrystallization presents one of the smallest driving forces amongst the solid state transformations, for highly deformed metals, measuring stored energy due to deformation can be assessed by calorimetric methods. For austenitic stainless steels (ASSs), depending on steel composition and processing variables, plastic deformation apart from introducing crystal defects, can also cause the appearance of deformation-induced martensites, DIM, which has an important influence on the strain-hardening coefficient and on the formability. Two types of martensite may occur in the ASSs, namely: a -(bcc, ferromagnetic) and e-(hcp, paramagnetic). Some researches show that for low deformation levels the amount of e-martensite is predominant in relation to a . When the quantity of a grows continuously with the degree of deformation, the amount of e-martensite goes through a maximum and lowers thereafter, suggesting the following transformation sequence: g→e→a . Reversion of a -martensite of the DIM in the ASS is less studied than its formation. A good judgment about the thermal stabilities of the eand a martensites, as well as the deformation bands introduced by cold working, can be obtained from the work of Singh. He detected that during annealing (with duration of 1 h) of a cold rolled AISI 304, e-martensite remained stable up to 200°C, that a was stable up to 400°C, whereas deformation bands stayed in nonrecrystallized regions up to 800°C. Guy and co-authors studied the reversion of a induced by cooling and by deforming in two different steels (18%Cr–8%Ni and 18%Cr–12%Ni). In both cases, after 2 min at 600°C, practically all a -martensite reverted to austenite. Martins and co-authors studied reversion of a -martensite in the 304 and 304L steels and concluded that the reversion temperature was little sensitive to the type of steel, strain and the effect of precipitation treatments introduced prior to cold working. For the 12 studied conditions (2 steels 2 pretreatments and 3 strain levels), the temperature (annealing for 1 h) for 50% a (T50%RM) reversion was always within the 550 20°C range. Tavares and co-authors studied reversion of a -martensite and observed that the T50%RM depended only slightly on strain and heating rate. The temperatures, measured for 3 levels of strain and 3 heating rates, stayed between 549 and 573°C. Analysis of the above mentioned works shows that reversion of the strain induced martensites in the ASS occurs at temperatures much lower than the recrystallization temperature, despite the complete reversion of a -martensite, may reach temperatures of the order of 750°C. The main objective of this work is to verify if it is possible to separate, with the help of calorimetry, the phenomena of reversion of deformation-induced martensite and recrystallization in a AISI 304 cold worked austenitic stainless steel.

Effect of degree of die upset on magnetic behavior in Nd13.9(Fe0.92Co0.08)80.3B5.3Ga0.5

DC demagnetization (DCD), isothermal remanent magnetization (IRM), and reversible magnetization measurements were used to examine the magnetic reversal behavior in melt-quenched samples of composition Nd13.9(Fe0.92Co0.08)80.3B5.3Ga0.5 subjected to different levels of thermomechanical deformation (die upsetting). The reversible magnetization measurements indicate that in the thermally demagnetized condition domain walls are present within the material for all levels of deformation. During demagnetization from a saturated state, domain walls are present in a minority of grains at low levels of deformation but these domain walls disappear at higher levels of deformation. This behavior is explained in terms of changes in the relative strengths of nucleation and pinning fields. Comparison of these results to previous results obtained on similar materials indicate that changes in the magnetization behavior with degree of deformation are more marked in samples which are free of Ga, indicating that Ga additions act to suppress the nucleation of new domain walls, presumably in the grain boundaries.

Estimating the plastic collapse of pressure vessels using plasticity contours

This investigation concerns the use of non-linear finite element analysis (FEA) to predict the plastic collapse of pressure vessels with and without defects. Computational non-linear plastic analysis of pressure vessels to estimate the collapse pressure is an iterative process. As a consequence, if the models are large and complex, considerable computer resources and time are required to generate solutions. As normal materials tend not to be ideally elastic–plastic (no sudden plastic collapse), a number of techniques are used to estimate the collapse pressure based on the FEA solution. These techniques often rely on plotting the pressure–strain curve; however, other techniques are also possible and are considered here. The aim of this paper is to assess the different methodologies in conjunction with FEA. Firstly, to consider their accuracy and secondly, to ascertain whether a particular technique would result in a reduction in the analysis time necessary to recover the plastic collapse pressure. Of the methods analysed, one based on the measurement (as a percentage) of the leading edge of the effective plastic strain (EPS) contour band showed promise. Under increasing pressures, regardless of geometry or material yield curve, the %EPS contour area coverage displays linear behaviour before through-wall plasticity is achieved. Based on this it is possible to extrapolate from low levels of plastic deformation up to plastic collapse pressures without requiring additional non-linear analyses, resulting in significant cost reductions in computational analysis times.

Mechanics of Concrete Participation in Cyclic Shear Resistance of RC

In conventional design of shear-dominated reinforced concrete elements shear resistance is assumed to comprise reinforcement and concrete contributions. Because the design code assumes these contributions to be uncoupled and independent of the load history, the estimated nominal shear strength of reinforced concrete is insensitive to the number and intensity of imposed load cycles. However, in accordance with recent experimental evidence, the concrete contribution to shear resistance degrades with increasing ductility demand, thereby rendering the estimated nominal shear strength conservative for low levels of deformation, and increasingly unconservative for larger imposed displacements. This paper attempts to establish the parametric relationship between shear strength and deformation demand through a nonlinear analytical model of cyclic plane-stress states in reinforced concrete. The formulation establishes tensorial, constitutive, and equilibrium relations, all expressed in terms of average stresses and strains. The model was used to conduct a parametric investigation of several design variables. The rate of strength degradation and ductility capacity were used to gauge the parametric sensitivity of the problem. It was found that the calculated concrete shear contribution degraded with increasing deformation demand and was enhanced by an increase in the level of axial load and the amount of transverse reinforcement. A simplified design equation was developed from the analytical results to express the dependence of the concrete contribution term on the intensity of imposed deformation demand.

Dynamic deformation of Etna Volcano observed by satellite radar interferometry

Satellite radar interferometry of Mt. Etna volcano, Sicily, Italy, reveals a sequence of deformation characterized by deflation during the end of the 1993 eruption, inflation from 1993–1995 with an increase in the inflation rate immediately before its resumed eruptive activity in late 1995. This was followed by very low deformation levels during the following year. The source of the deformation changed from a depth of 9 km during deflation to more than 11–14 km during the subsequent inflation, consistent with a model in which deflation at shallower levels is followed by inflation at greater depth as the volcano system recharges from below before its next eruption. This study demonstrates that radar interferometry provides an important contribution towards understanding the dynamic deformation of volcanoes. By revealing large scale changes in their pre‐eruption deformation rates, radar interferometry could play an important role in volcano eruption monitoring.

Elastic-plastic fracture mechanics assessment of low constraint aluminium test specimens

Recent studies have shown that the near crack-tip stress field at a given J value is dependent on geometry. This dependence has been linked to the degree of constraint in the geometry, with low constraint geometries losing J dominance at very low deformation levels. New approaches centred on the use of a two-parameter description (e.g. J-T and J-Q) of the crack-tip stress-strain state have emerged. However, there is a serious lack of experimental and numerical results for low constraint geometries to quantify the T-stress and Q-value in the literature. This paper describes details of an experimental and numerical program carried out on low and high constraint geometries (CCT and TPB) fabricated from an aluminium alloy. The results show that the experimental and numerical fracture toughness values (J c ) agree within ±10 percent. The T-stress and Q-value two-parameter methodologies are successful at indexing the fracture toughness, ordering the data into a systematic trend of decreasing fracture toughness with increasing T or Q, albeit with some scatter. This allows the use of practical two-parameter failure criteria, in the form of J-T and J-Q loci, to predict the behaviour of cracked components, without the conservatism associated with the use of high constraint test geometries.

Effect of phase transformations on the creep rupture properties of two type 316 weld metals

Many of the factors which give rise to the low and variable creep rupture properties of type 316 weld metals are not yet fully understood. In the present study two weldments fabricated using the same welding parameters, but with compositional differences which give rise to different weight fractions of sigma phase and M23C6 carbides have been examined after creep testing. It was found that lower creep rupture strength and ductility was associated with higher weight fractions of sigma phase, which give rise to a higher number density of cavities at low deformation levels and facilitate crack propagation along sigma-austenite interfaces. The observed microstructural differences have been explained with reference to the predictions which can be made using thermodynamic analysis.

The microstructure of rolled and annealed tungsten rod

This paper reports a study of the microstructural changes that occur when potassium-doped tungsten ingots are rolled at elevated temperatures. The effect of annealing on the microstructure of the rolled material is also considered. All samples were rolled on a Kocks mill. At low levels of deformation, the grain boundaries are primarily high-angle boundaries, and many grains are dislocation free. Both of these features probably result from dynamic recrystallization during rolling. As deformation increases, the grains become more elongated, and more low-angle boundaries are found within the material. Also, the potassium gets drawn into narrower and longer tubes. When these rolled rods are annealed at temperatures between 1275 ‡C and 1950 ‡C, several changes occur in the microstructure. The material undergoes abnormal grain growth. The temperature at which this occurs depends on the length of the anneal, the amount of de-formation the rod has received, and the spatial location in the rod. This spatial distribution most likely results from strain gradients that exist in the rolled rod. The abnormal grain growth is accompanied by a decrease in hardness. The potassium-containing tubes in the rod also break up into bubbles during annealing. The temperature at which this breakup occurs again depends on the length of the anneal and the amount of deformation.

Mass transfer in unmetamorphosed carbonates and during low-grade metamorphism of arenites

Summary Study of stylolites in Silurian carbonates of southern Ontario, and of spaced cleavage in Lower Palaeozoic meta-arenites of Nova Scotia, shows that diffusional or infiltrational mass transfer has been the dominant mechanism for dimensional change. The carbonates show a quite specific pattern of stylolite growth in, and normal to, the mean plane of the stylolite, and there is always a low-porosity halo about each stylolite. Implied shortening usually exceeds the amplitude of any teeth in stylolites and may reach 5% of the ‘local’ sequence. In the meta-arenites, increased inequancy, decreased long-axis fluctuation of quartz grains, lithon to cleavage, coupled with virtually constant grain dimension parallel to cleavage, suggest gross loss of volume. Modal and chemical analyses suggest that an average cleavage zone represents a dilation of −50%, expressed as shortening normal to cleavage. Bulk loss is about 10%, almost all SiO 2 . In both cases, mass removal begins at zero or low levels of overall deformation. For the carbonates, total deformation is near zero; the process shuts down by self-suffocation. In the meta-arenites the process was probably partly driven by the conditions associated with greenschist-level metamorphism. In both cases, collapse across the surfaces of dissolution is nicely adjusted to keep the gross shortening approximately homogeneous.

Cold deformation of a nickel-base superalloy

An analysis of the low deformation level (less than 5% strain) characteristics of polycrystalline samples of an engineering alloy is presented. The material used was a wrought nickel-base superalloy, Nimonic 80 A, the microstructure of which had been modified in a controlled way to give a related series of phase distributions. This allowed the roles of the various microstructural constituents to be assessed. Selected mechanical property data were correlated with the deformation microstructures, as determined by transmission electron microscopy. The presence of carbide particles on the grain boundaries was found to affect both the observed deformation patterns and the work-hardening rate. However, although the majority of the dislocation nucleation events occurred at the grain boundaries, the stress-strain characteristics were more sensitive to intragranular than to intergranular microstructures.

Effect of aqueous operating media on the low-cycle fatigue of constructional steels

The action of aqueous corrosive media on the strength of steels under cyclic loading is determined to a significant degree by the influence of two active elements, oxygen and hydrogen, participating in the corrosion process. The authors have investigated the influence of these active elements on the strength of constructional steels under cyclic loading under conditions simulating the operating conditions of heat-transfer equipment. The metal of the samples was hydrogen impregnated with cathodic polarization. The deaerated media have an insignificant effect on the low-cycle life of 20, 15Kh2MFA, 15Kh2NMFA AND 08Kh 18N10T steels at a temperature of 120-270/sup 0/C and under the quite long influence of corrosive media at low levels of elastoplastic deformation. The action of hydrogen occluded by the plastically loaded steel with cathodic polarization led to a sharp decrease in low cycle life. The tested steels revealed different tendencies toward hydrogen cyclic fatigue. The influence of the test conditions may be determined in an analysis of the adequateness of the Lanjer-Manson model. The results of determination of the adequateness according to the Fisher criterion are shown. The norm of the logarithms of lives in the low-cycle area is practically independent of the level of deformation.

Mechanical properties of chemical vapor deposited coatings for fusion reactor application

Chemical vapor deposited coatings of TiB2, TiC and boron on graphite substrates are being developed for application as limiter materials in magnetic confinement fusion reactors. In this application severe thermal shock conditions exist and to do effective thermo‐mechanical modelling of the material response it is necessary to acquire elastic moduli, fracture strength, and strain to fracture data for the coatings. Four point flexure tests have been conducted from room temperature to 2000 °C on TiB2 and boron coated graphite with coatings in tension and compression and the mechanical properties extracted from the load‐deflection data. In addition, stress relaxation tests from 500 ° to 1150 °C were performed on TiB2 and TiC coated graphite beams to assess the low levels of plastic deformation which occur in these coatings. Significant differences have been observed between the effective mechanical properties of the coatings and literature values of the bulk properties.

Experimental investigation of plastic properties of lead over a wide range of strain rates

The results of experiments on strain-rate behaviour for technically pure lead are given. Experiments were performed in compression using cylindrical specimens of different slenderness ratios over a wide range of strain-rate 2 × 10 −3 s −1 < ϵ ̇ < 2 × 10 3 s −1 . For high strain-rates the SHPB technique was employed. Small instabilities in the flow stress at low deformation levels both in static and dynamic compression, were observed. Numerical values of strain-rate sensitivity for the lead investigated were found and compared with the results from other sources.

Effect of shock stressing on diffusion mobility of reactants in a Cu−Al alloy

The stressing of materials with shock pressures of about i00 kbar and more is accompanied by efficacious changes in their structure and mechanical properties, in particular, by considerablehardening at low deformation levels. It is characteristic that under certain shock-stressing conditions softening under subsequent heating sets in at higher temperatures than after static loading [i]. This probably shows the stability of the structural changes caused by the passing of shock waves. A study of the degree of stability and the nature of these changes is of interest from the standpoint of the possibility of controlling the mechanical properties of materials with high-temperature working after shock stressing. In the present work it is shown that explosion working has an effect on the nature of the deposits, the penetration depth, and the reaction zone structure with internal oxidation of an alloy carried out at 950~