The uniaxial stretching and the stress relaxation measurements were carried out for κ-carrageenan hydrogels prepared at different gelation concentration (c0). The Young’s modulus (E) for the gel with low c0 was greater than that for the gel with high c0. Significant difference also appeared in the stress relaxation behavior: the gel with low c0 showed greater and slower stress reduction. The results were explained by the specific network structure of κ-carrageenan hydrogels, where the crosslinks are composed of aggregates of κ-carrageenan chains. It was proposed that a great number of small crosslinks are generated at low c0 including imperfect fragile crosslinks and that the weak crosslink domains disappear accompanying the uniaxial stretching of the gel. It seemed that the dissociation occurs successively under the tensile stress and contributes to the stress relaxation.
Frequency f and strain rate ̇εn dependences of linear dynamic shear modulus were measured for glassy epoxy network during uniaxial stretching processes. With increasing strain εn, the storage shear modulus G´ slightly decreased to a steady value appearing at post-yield strain-hardening range of strain. The loss shear modulus G˝ markedly increased in the same strain range, where G´ decreased, and then leveled off. These variations of G´ and G˝ indicated that the glassy structure in the epoxy network changed into more unstable ones by stretching. When compared at a fixed condition of α= ̇εn/f, the functional relation between strain-induced increment of G˝ and εn was identical independently of ̇εn of stretching. Thus, frequency dispersion of the nonlinear relaxation was found to be determined only by the relative distance from the timescale of deformation and the amount of imposed strain. Whereas the decrement of G´ at a fixed α was not superposable when plotted against εn, because of (εn ) ̇ dependence of their steady values. The variation of G´ was affected not only by destabilization of glassy structure due to deformation. The observed f and ̇εn dependences of G´-εn and G˝-εn relations for glassy epoxy network during stretching were qualitatively the same as those observed for poly(methyl methacrylate) (PMMA). Thus, the dependence on εn, f and ̇εn of the nonlinear relaxation under constant-speed deformation conditions reported here is presumably universal for glassy polymers. The strain-induced variation of G´ and G˝ was smaller for epoxy network compared with PMMA stretched exactly at the identical condition. This result indicates that glassy structures in the epoxy network before stretching are more unstable because of constraint arising from crosslinked molecular structures.
The mechanism of heat resistance improvement of polystyrene injection moldings by heat treatment was studied. The increase in the Heat Distortion Temperature（HDT） by heat treatment was confirmed. Meanwhile, in dynamic viscoelasticity measurement, the shoulder of the loss tangent（tanδ） appeared at 60 to 90°C in untreated specimen. In the heat-treated specimen, the shoulder at that temperature range disappeared. Further, when the frequency dependence of tanδ was evaluated, it was found that the mobility of molecular chain in the main dispersion did not change by the heat treatment, and the mobility in the temperature range where the tanδ shoulder appeared was reduced by the heat treatment. By differential scanning calorimetry, the progress of enthalpy relaxation by heat treatment was confirmed. Also, excellent correlation existed between the progress of enthalpy relaxation and the disappearance of the tanδ shoulder by heat treatment. Since the tanδ shoulder disappeared in the heat-treated specimen in which relaxation has progressed yet, it was considered that the tanδ shoulder was caused by molecular motion accompanying the relaxation phenomenon. Addisionally, as well as the disappearance of the tanδ shoulder, the increase in HDT correlated with the progress of enthalpy relaxation by heat treatment. In conclusion, with the progress of the relaxation by the heat treatment, the tanδ shoulder disappeared, and thereby the molecular mobility at 60 to 90°C decreasesed. Since the HDT of the untreated specimen was 78°C, the HDT increased due to the decrease in the molecular mobility below 78°C by heat treatment. Thus, it is considered that the increase in HDT correlated with the progress of enthalpy relaxation by heat treatement.
Simultaneous measurements of a stress-strain curve with two-dimensional small-angle X-ray scattering (2d-SAXS) patterns were conducted at room temperature to reveal the relationship between the formation of the kink structure and necking of the specimen comprising hard and soft lamellar microdomains under the uniaxial stretching. As a result, the 2d-SAXS pattern typical of the chevron structure was observed without changing throughout the first stage of the stretching (up to the strain of 2.1) where the stress was kept constant and the specimen underwent the necking. Note that necking appeared around the strain of 0.04 ~ 0.05 and it was propagating throughout the specimen with an increase of strain up to 2.1, which is the final point of the first stage of the stretching where the stress turned into increasing linearly with strain. Upon releasing the load applied to the stretched specimen, the chevron structure was transformed into biaxially-oriented lamellar structure, suggesting that the chevron structure is only stably existing under the stretched state. To rigorously understand the relationship between the necking region and the formation of the chevron structure, we conducted 2d-SAXS measurements using the microbeam of 2-µm diameter under the stretched state of the specimen with the strain of 0.5. As a result, it was found that the lamellar structure was not suffered from necking in the band region of 160 µm width near the boundary between the necking and unstretched regions though the lamellar structure was slightly deformed. This result is significant because it was believed that the chevron structure was formed throughout the necking region. Furthermore, the lamellar structure was drastically changed into the chevron structure as a function of the distance from the boundary in the narrow necking region within 160~240 µm from the boundary.
We investigated an effect of carbon fiber on shape memory property of carbon fiber / shape memory polymer (epoxy resin) composites by using dynamic mechanical analysis (DMA), shape memory test, and mechanical constitutive model. As a result of DMA, the storage modulus (E') of the composite material (CF/EP) was improved by CF component, as compared with that of the pure epoxy resin (pure EP). Especially, E' at rubbery region was remarkably increased with two decades. From the results of shape memory test, CF/EP showed good shape recovery behavior as well as pure EP. Contrary, the shape fixity ratio of CF/EP was lower than that of pure EP. This is because that the difference in E' below/above the glass transition temperature (Tg ) of CF/EP became smaller (less than one decade) than pure EP (two decades). In addition, the shape recovery of CF/EP started at lower temperature than Tg, although the recovery of pure EP showed around Tg. In order to examine the shape recovery behavior of CF/EP, we compared the viscoelastic properties of three samples, pure EP, CF layer and the composite. As a result, the CF layer had higher E' than CF/EP, and no significant change in E' occurred around Tg. From the results, we speculated that the CF layer acted as a shape recovery component for the composite material. Furthermore, we proposed a mechanical constitutive model, in which the CF layer was assumed as the recover component, to qualitatively explain the shape recovery behavior of CF/EP composite. The simulated result by the constitutive model reproduced the trend of anomalous shape recovery behavior below Tg of CF/EP.
In austenitic stainless steels, the Young’s modulus ratio E100/Ehkl in single crystal is expressed as a function of the orientation parameter Γ: E100/Ehkl＝1－2.027Γ. Under the presumption that this relationship stands up on commercial stainless steels with various chemical composition, the elastic stiffness cij was calculated as follows in connection with Young’s modulus Ep of poly crystal austenitic stainless steels: c11＝1.0311Ep, c12＝0.6864Ep, c44＝0.6246Ep. As a result, the elastic stiffness ratio c12/c44 and the anisotropy parameter Ai (＝2c44/(c11－c12)), that are required in the dislocation analysis by modified Williamson-Hall method, are estimated constant at c12/c44＝1.099 and Ai＝3.624.
Fatigue life of Ti alloys is decreased by tensile stress dwell even at room temperature, which is called Cold Dwell Fatigue (CDF). CDF is similar to creep-fatigue of some metals (stainless steel, Ni-base alloy, and so on) in the point of decrease of fatigue life with stress or strain dwell at high temperature. In this study, time exhaustion rule and ductility exhaustion rule based on linear cumulative damage rule were calculated for CDF life assessments. In the case of time exhaustion rule, total damage was calculated as DF+DC< (0.03, 0.015) (DF: fatigue damage, DC: creep damage). In the case of ductility exhaustion rule, total damage was calculated as DF+DC~1. Additionally, the feature of fracture surfaces after CDF tests changed with increase of dwell time. This trend corresponded to the balance between fatigue damage and creep damage calculated by ductility exhaustion rule.
This paper describes a visualization of cavity regions in a structure by a hammering test using the level set type topology optimization based on the concept of the phase field method. In this study, the residual between the calculated displacement and the observed displacement was defined as the performance function, and the identification problem of cavity regions was formulated based on the adjoint variable and finite element methods. The numerical experiments were performed to investigate the effect of the regularization parameter. According to this result, it was found that the number of cavities and the cavity shape were affected the regularization parameter. In addition, the numerical example of a three-dimensional problem was described.
Sigma phase precipitates in duplex stainless steel during slow cooling, degrading the corrosion resistance and the notch toughness. The double loop electrochemical potentiokinetic reactivation (DL-EPR) method in a solution of 0.5 M H2SO4 + 0.01 M KSCN + 0.5 M NaCl is known to be good for detecting sigma phase. However, the influence of dissolved oxygen (DO) in solution had not been elucidated, and on-site application of DL-EPR had not been carried out. In this study, SUS329J4L stainless steel was continuously cooled at different time constants by convection using nitrogen, and its DL-EPR behavior was measured in solution with different DO concentrations. A cathodic current partially appeared at potentials nobler than the corrosion potential due to the reduction of oxygen. Elimination of DO suppressed the cathodic reaction, resulting in adequate sensitivity for detection of precipitated sigma phase. The detection sensitivity was almost the same as that for laboratory samples in 700 mL/cm2 solution and simulated small structures exposed to 20 mL/cm2 solution, and with almost the same high sensitivity as the immersion corrosion test.
Polymeric dental aligners manufactured by thermoforming are recently used in clinical treatment of misaligned teeth. In this study, a polyester sheet with 0.75 mm thickness was used and the deformability of thermoformed aligner is discussed experimentally with the help of digital image correlation method. The experimental apparatus to apply prescribed displacement to aligners mounted on a teeth model was developed focusing on the situation of treatment for misaligned left canine. The deformation mode was discussed through measured displacement field and maximum principal strain distribution. It was found that mechanical behavior of aligner was influenced more significantly by the shape of teeth, movement vector of left canine and crooked shape of aligner especially at inter-teeth borders than the thickness distribution of thermoformed aligner. The role of deformable edge part of the aligner was also revealed.