Development Of Internal Stresses Research Articles

Dual role of deformation-induced geometrically necessary dislocations with respect to lattice plane misorientations and/or long-range internal stresses

This work is part of a continuing effort to develop a unified picture of the role of geometrically necessary dislocations (GNDs) in the evolution of the dislocation distribution of unidirectionally and cyclically deformed crystals. In particular, the dual role of the GNDs in the development of long-range internal stresses and lattice plane misorientations which arise because of the heterogeneity of the dislocation substructure is explored. Available experimental data of cyclically and tensile-deformed copper single crystals were evaluated as quantitatively as possible in the framework of the composite model. Valuable complementary information to TEM was obtained from well-designed X-ray diffraction experiments (line broadening, broadening of rocking curves, Berg–Barrett X-ray topography). The evolution of the long-range internal stresses and of the density of the GNDs with increasing deformation could be determined quantitatively as a function of deformation for cases of both single and multiple slip. In all cases studied, the GND density was found to be small and amounted only to some per cent of the total dislocation density. From the rate of evolution of the misorientations of different types of dislocation boundaries, the latter could be classified either as so-called geometrically necessary boundaries or incidental dislocation boundaries. A number of semi-empirical relationships between the microstructural parameters on a mesoscale and the parameters of deformation that were derived in this study can provide valuable guidance in future modelling.

Evolution of the coefficient of thermal expansion of a thermosetting polymer during cure reaction

The evolution of the coefficient of thermal expansion (CTE) of a thermosetting polymer during cure reaction is an important parameter for industrial applications such as composite processing since it influences the development of internal stresses in the material. The CTE being almost impossible to measure on a reacting thermoset, we propose to use an indirect method based on the modelling of ionic conductivity by a modified WLF equation, allowing to calculate the evolution of CTE from dielectric spectroscopy measurements. This method is applied to a dicyanate ester thermosetting polymer, leading to encouraging results both qualitatively and quantitatively.

Curing characteristics and internal stresses in epoxy coatings: Effect of crosslinking agent

The development of internal stresses in a series of epoxy coatings prepared using five different crosslinking agents have been studied. The crosslinking agents were: H1, 4,4′ methylenedianiline (DDM); H2, diethylentriamine (DETA); H3, cycloaliphatic polyamine; H4, polyaminoimidazoline; and H5, polyamidoamine adduct. Four different post-cure treatments were applied and the dependence of internal stress on crosslinking agent and post-cure treatment was determined. Curing was followed by monitoring the FTIR epoxy band at 916 cm− 1 and the glass transition temperature was determined using DSC to assist interpretation of the measured values of internal stress. The internal stress was tensile in all of the materials at the end of each post-cure treatment. The stress magnitudes increased monotonically with post-cure temperature. The largest stresses were recorded with H1, H2 and H3 whereas the lowest stresses were recorded with H4 and H5, which both included a flexible aliphatic chain. The effects of ageing for extended periods in dry air and in humid air (52%RH and 97%RH) were also examined. Exposure to humid air almost always caused a reduction in the tensile stress and often produced compressive stresses, attributed to swelling due to water absorption. A comparison was made of the stresses formed in coatings applied to a thin substrate that was (i) free to bend during curing, post-curing and ageing, and (ii) prevented from bending (“restrained substrate”). The general trends in behaviour were in agreement but no simple relationship could be found between the stress magnitudes obtained by the two different test configurations.

Evolution of Particle-Scale Dynamics in an Aging Clay Suspension

Multispeckle x-ray photon correlation spectroscopy was employed to characterize the slow dynamics of a suspension of highly charged, nanometer-sized disks. At wave vectors q corresponding to interparticle length scales, the dynamic structure factor follows a form f(q,t) approximately exp([-(t/tau)(beta)], where beta approximately 1.5. The relaxation time tau increases with the sample age t(a) approximately as tau approximately t(1.8)(a) and decreases with q as tau approximately q(-1). Such behavior is consistent with models that describe the dynamics in disordered elastic media in terms of strain from random, local structural rearrangements. The measured amplitude of f(q,t) varies with q in a manner that implies caged particle motion. The decrease in the range of this motion and an increase in suspension conductivity with increasing t(a) indicate a growth in interparticle repulsion as the mechanism for internal stress development implied by these models.

Internal stresses in cold-deformed Cu–Ag and Cu–Nb wires

The co-deformation of Cu–Ag or Cu–Nb composite wires used for high-field magnets has a number of important microstructural consequences, including the production of very-fine-scale structures, the development of very high internal surface-area-to-volume ratios during the drawing, and the storage of defects at interphase interfaces. In addition, the fabrication and co-deformation of the Cu and Ag or Nb, which differ in crystal structure, thermal expansion, elastic modulus and lattice parameter, lead to the development of short-wavelength internal stresses in both composites. In this paper, these internal stresses are characterized by neutron diffraction and transmission electron microscopy as a function of the imposed drawing strain. The internal stresses lead to important changes in the elastic–plastic response, which is related to both magnet design and service life. The second derivative ∂2 σ/∂2 ε of the stresses with respect to strain is used to describe the low-strain anelasticity of the composites. The internal stresses in Cu–Nb are higher than in Cu–Ag and, consequently, the absolute values of (∂2 σ/∂2 ε)Cu–Nb are higher than those of (∂2 σ/∂2 ε)Cu–Ag at low strains.

The influence of a martensitic phase transformation on stress development in thermal barrier coating systems

Thermal barrier coatings (TBCs) provide thermal insulation and oxidation protection of Ni-base superalloys in elevated temperature turbine applications. Thermal barrier coating failure is caused by spallation, which is related to the development of internal stresses during thermal cycling. Recent microstructural observations have highlighted the occurrence of a martensitic bond coat transformation, and this finite-element analysis was conducted to clarify the influence of the martensite on the development of stresses and strains in the multilayered system during thermal cycling. Simulations incorporating the volume change associated with the transformation and experimentally measured coating properties indicate that out-of-plane top coat stresses are greatly influenced by the presence of the martensitic transformation, the temperature at which it occurs relative to the strength of the bond coat and attendant bond coat plasticity. Intermediate values of bond coat strength and transformation temperatures are shown to result in the highest top coat stresses.

Microstructure, mechanical properties and oxidation behavior of a multiphase (Mo,Cr)(Si,Al)2 intermetallic alloy–SiC composite processed by reaction hot pressing

A multiphase (Mo,Cr)(Si,Al) 2 intermetallic alloy matrix-composite comprising body-centered tetragonal, C11 b (tI6) structured (Mo 0.23 Cr 0.10 )(Si 0.67− x Al x ) and hexagonal, C40 (hP9) structured (Mo 0.16 Cr 0.17 )(Si 0.67− x Al x ) (where x ≤ 0.0008) phases as well as a dispersion of α-Al 2 O 3 and β-SiC phases has been processed by reaction hot pressing of Mo, Si, Al, MoSi 2 and Cr 3 C 2 powders. A noticeable increase could be observed in the hardness of the matrix as well as that of the composite. The indentation cracks follow a more tortuous path and have shorter lengths compared to those in MoSi 2 , although the testing of Single Edge Notch Bend specimens has not shown any improvement in fracture toughness. Oxidation at 1873 K has shown the formation of a scale, containing the crystalline SiO 2 identified as crystoballite, Cr 2 O 3 , α-Al 2 O 3 and Cr 2 MoO 6 phases. With increase in the time of oxidation, a relatively smooth and thick SiO 2 film appears to overlap the other crystalline phases, and cracks develop in the scale due to development of internal stresses during growth of different oxides or cooling to room temperature.

Recent developments on Smart Composites with embedded SMA wires and FBG sensors(International Workshop on Smart Materials and Structural Systems, W03 Jointly organized by Material & Processing Division, Material & Mechanics Division, Dynamics & Control Division and Space Engineering Division.)

Shape memory alloy wires are nowadays available in small diameters, allowing their direct integration into polymer composite materials without altering the integrity of the structure. These hybrid materials offer the potential to adapt their shape, thermal or vibration response, when they are heated above the transformation temperature of the SMA. This requires to master both their design and processing to reliably produce the desired level of activation. Additionally, strain and temperature sensing should be integrated as well in the composite, to allow for a closed-loop control. This is achieved by embedding Fiber Bragg Grating optical fibers. Recent results will be reviewed, with particular emphasis on the processing issues including the choice of materials and of a manufacturing route for the embedment of pre-strained wires, the development of internal stresses in the host material during post-cure and activation, and the evaluation of the interfacial strength required to sustain the activation induced stresses. The use of coupled FBG sensors to simultaneously measure temperature and strain will be presented, with results from on-going research.

Microstructure Evolution in Ferritic Stainless Steels during Large Strain Deformation

Deformation microstructures were studied in ferritic stainless steels during cold bar rolling and swaging to total true strains about 7. Two steels, i.e. Fe-22Cr-3Ni and Fe-18Cr-7Ni with coarse-grained ferritic and fine-grained martensitic initial microstructures, respectively, were selected as starting materials. Microstructure evolution in the both steels was characterized by the development of highly elongated (sub)grains aligned along the rolling/swaging axis. The transverse size of these (sub)grains in the Fe-22Cr-3Ni steel gradually decreased to about 0.1 μm with increasing the strain. On the other hand, the transverse (sub)grain size in the Fe-18Cr-7Ni steel decreased to its minimal value of 0.07 μm with straining to about 3 followed by a little coarsening under further working. The strengthening of worked steels that revealed by hardness tests correlated with the microstructure evolution. The hardness of the Fe-22Cr-3Ni steel increased with cold working within the studied strain range, while that of the Fe-18Cr-7Ni approached a saturation after fast work hardening at strains below 3, leading to an apparent steady-state behaviour. Development of strain-induced (sub)grain boundaries and internal stresses in the steels with different initial microstructures during severe deformation is discussed in some detail.

Internal stress control in epoxy resins and their composites by material and process tailoring

AbstractThe development of internal stress during cure of epoxy and hyperbranched polymer‐modified epoxy resins was characterized, taking into account the evolving viscoelastic properties, the volumetric shrinkage due to the chemical reaction, and the thermal expansion. A criterion for void formation during cure in a constrained mold was proposed, providing guidelines for the construction of a process window for manufacturing of void‐free composites. It was shown that the internal stress development in epoxy resins during cure is strongly influenced by the presence of hyperbranched polymer modifiers. The role of these modifiers was illustrated for the case of autoclave processing of glass fiber/epoxy composites. This study showed that higher fiber volume fractions could be used with hyperbranched polymer‐modified resins than with unmodified resins, for producing void‐free laminates. It also appeared that by suitable tailoring of the process cycle, a fully stress‐free laminate could be obtained after cure, using the modified resin.

Characterizations at very high temperature of electric arc-induced long-period fiber gratings

Electric arc-induced long-period fiber gratings, with low insertion loss (<0.2 dB) and high isolation peaks (−24 dB), have been fabricated in standard single-mode fiber (Corning SMF28). High temperature characterizations of the fabricated gratings in the range of 30 to 1200 °C have been performed. The sensitivity are in good agreement with recent published works. Specific spectral shift of the resonance peaks with temperature have been shown. Temperature annealing of the gratings, at 200, 400 and 600 °C show high thermal stability compared with UV-induced gratings. More particularly, modal dependence of the resonance shift and time dependence during annealing cycles, have been investigated at high temperature. The evolutions of the spectra have been related to the development of internal stresses in the fiber. These temperature characterizations help to explain the writing mechanism of these gratings.

Micromechanical analysis of internal stress development during single-fibre fragmentation testing of Ti/SiC f

An axisymmetric finite element model of a single fibre composite tensile specimen having a Ti–6Al–4V matrix reinforced with one SCS6 SiC fibre has been constructed. The sliding of the fibre is modelled using a constant critical shear stress and fibre fragmentation introduced using the element removal technique. In this way, the fibre and matrix strains have been computed as the fibre fractures progressively. The results are compared to synchrotron X-ray strain scanning measurements made in-situ during the test and critical interfacial parameters are extracted. Good agreement is found between numerical and experimental results based on a critical shear stress of 200 MPa for loading and unloading including both forward and reverse slip.

Mechanical response of wood perpendicular to grain when subjected to changes of humidity

This paper describes the deformations caused by stress and humidity interaction, mechano-sorption, in the cross grain directions of wood and the relaxation or accumulation of internal stresses caused by these deformations. Long-term tests on small clear specimens in cyclic climates with both tensile and compressive loads were carried out. The development of internal stresses in timber was measured indirectly at different times during the adsorption and desorption processes. Released deformations were measured from cross-sections after cutting them to small slices. These deformations were used to estimate the internal stresses caused by the humidity variations. Tests with constant loads and multiple humidity cycles show a mechano-sorptive strain that is ten times higher than the elastic strain. It is shown that existing models for describing mechano-sorption perpendicular to grain are inaccurate when applied to multiple humidity cycles. The present results demonstrate that if the mechano-sorptive behaviour and the moisture gradients in wood can be accurately described, it is possible to predict the stress distribution in a timber cross-section by knowing the climate history.

The use of co-solvents in supercritical debinding of ceramics

Abstract Extraction of organic binders (paraffin waxes) from injected green ceramic parts, by using supercritical fluids, has proven to be very efficient (short time of extraction, no defects in the green part). The extraction is more rapid when the binder is in the liquid state due to capillary migration but the capillary pressure could damage the ceramic green body because of development of internal stresses. In turn, extraction of the binder in solid state avoid formation of defects but the kinetics of extraction by solubilisation in supercritical carbon dioxide is reduced. An organic co-solvent can be added to supercritical CO 2 in order to increase the solid paraffin molecules solubility. Polar and non polar organic co-solvents were tested and their efficiency on the solubility of the paraffin molecules were compared. The influence of the length of the chains and of the number of side groups of co-solvent molecules were examined. The extraction was also performed by adding mixed co-solvent molecules. Mixtures of polar and non polar co-solvents were introduced in supercritical CO 2 . Improved extraction by a factor 5 was obtained with n-hexadecane molecule as co-solvent, which represents a good compromise between destruction of the paraffin binder molecular cohesion, which requires, on one hand, large molecules, and mobility, which requires, on the other hand, small molecules.

Evolution of internal stresses in rolled Zr702α

Internal stresses due to anisotropic thermal and plastic properties were investigated in a rolled Zirconium-α. The thermal stresses induced by a cooling process were predicted using a self-consistent model and compared with experimental results obtained by X-ray diffraction. The study of the elastoplastic response during uniaxial loading was performed along the rolling and the transverse direction of the sheet, considering the influence of the texture and the thermal stresses on the mechanical behaviour. We used an elastoplastic self-consistent formulation and the predicted results are compared with mechanical tests. The role of twinning and slip on the development of internal stresses is also discussed.

Influence of relative humidity on the development of internal stresses in epoxy resin based coatings

Solvent-free epoxy resin based coatings have been applied and left for 24 hours at room temperature for initial setting under dry conditions. The curing process of the coatings was then completed at different relative humidity conditions. Meanwhile internal stress development was monitored as a function of time using the restrained beam deflection technique. Internal stress build up in solvent-free epoxy coating during the post-curing process was found to be highly dependent on the degree of relative humidity (RH) and the type of the curing agent. The development of tensile internal stress during the curing process of the different epoxy coatings took place only under low relative humidity atmospheres (<10–30%, depending on the curing agent), whereas compressive internal stress advanced at higher relative humidity conditions.

Effects of boric acid on hydrogen evolution and internal stress in films deposited from a nickel sulfamate bath

The effects of boric acid additions on the pH close to the electrode surface, on the hydrogen evolution reaction and on the internal stress in the plated films were studied for the high speed electroplating of nickel from a nickel sulfamate bath at a current density close to the nickel ion limiting current density. The study was carried out at 50 °C and pH 4.0 using a 1.55 M nickel sulfamate plating bath containing boric acid at concentrations ranging from 0 to 0.81 mol L−1. The variation of the internal strain in the plated nickel films was determined in situ using a resistance wire-type strain gauge fitted to the reverse side of the copper electrode substrate. The solution pH at a distance of 0.1 mm from the depositing nickel film was measured in situ using a miniature pH sensor assembly consisting of a thin wire-type antimony electrode and a Ag/AgCl/sat. KCl electrode housed in a thin Luggin capillary. The addition of boric acid was shown to effectively suppress the hydrogen evolution reaction at nickel electrodeposition rates (18.0 A dm−2) close to the limiting current density (~20 A dm−2). Consequently, the solution pH adjacent to the plating metal surface was maintained at a value close to that in the bulk solution and the development of high internal stresses in the deposited nickel films was avoided.

Adaptive composites incorporating shape memory alloy wires. Part 2: development of internal recovery stresses as a function of activation temperature

Shape memory alloy (SMA) wires have been integrated in composite laminates consisting of an epoxy resin reinforced by aramid fibres. In the first part of this series, a new methodology was proposed for the simultaneous measurement of stress and temperature in the reinforcing fibres during shape memory wire activation. Those tests were conducted at three activation temperatures (40, 60 and 100°C) and the internal compressive stress distribution was extracted for low wire volume fraction composites. It was observed that for higher wire volume fractions the high compressive recovery stresses generated during electrical resistive heating of the wires led to the geometric failure of the composite coupons. In this work, measurements are conducted on hybrid laminate coupons of dimensions that prevent geometric (global) specimen failure. The internal stress distribution is measured at relatively high activation temperatures of 80 and 100°C and a filtering technique based on fast Fourier transform (FFT) is employed to simulate the stress and temperature variability and to filter the associated experimental noise. The results show that the higher stresses appear in the middle of the mid-wire distance and that the values at 100°C activation are not significantly different than those at 80°C indicating the presence of an upper limit in the transmission of wire recovery stresses in these systems.

Autogenous shrinkage and induced restraining stresses in high-strength concretes

Development of internal stresses induced by restrained autogenous shrinkage in high-strength concretes at early ages was investigated. Effects of water/binder ratio and the presence of silica fume on the stress developed were evaluated and considered in conjunction with the creep behavior of the concretes. The restrained autogenous shrinkage resulted in a relatively high stress that sometimes caused premature cracking in the high-strength concrete. This occurred mainly when the ratio between the restraining stress and the tensile strength approached 50%. The stresses were not as high as expected from the autogenous shrinkage, since considerable relaxation took place due to creep. Thus, the viscoelastic nature of the concrete at early age has a considerable influence on the stresses generated.

Effect of Solidification Segregations on Phase Transformation Kinetics and on the Development of Internal Stresses during Cooling of Steels

The prediction of residual stresses during cooling has been widely developed for homogeneous materials. Here a coupled thermal-metallurgical-mechanical model that takes into account carbon content heterogeneities is used in order to analyse the possible effects of solidification segregations on the development of microstructures and residual stresses in heavy pieces during cooling.