Large Deformation Tests Research Articles

Mechanical properties of high density polyurethane foams: II Effect of the filler size

This article presents the mechanical behaviour of rigid polyurethane reinforced by mineral fillers, for a density range ( ρ f/ ρ s) > 0.27. The influence of the particle size on the foam microstructure and its mechanical behaviour has been studied using calcium carbonate with two different sizes (with average diameter of 1 and 30 μm, respectively) and crystallised silica particles with an intermediate size. The microstructural characterisation has been performed by electron microscopy. It has shown closed spherical cells whose size is decreased when the fillers are added. The results of mechanical characterisation by large and small deformation (mechanical spectrometry) tests have been compared to different modelling approaches. It appeared that a correct description of the viscoelastic properties and of the yield stress needs to take into account the filler size compared to the wall size. Two modelling hypothesis which consider the composite foam as either a filler dispersion in a foam or a void dispersion in a filled polymer have been made. The first one is adapted to the description of PU filled with the big carbonate fillers, when the second one is better to describe the PU filled with the small carbonate fillers. The use of the crystallized silica particles gives composite foams with properties in between that of the two calcium carbonate composites, showing that the filler size in the range studied is a key parameter to take into account in the reinforcement level.

A fractal analysis approach to viscoelasticity of physically cross-linked barley β-glucan gel networks

The structure and gelation kinetics of mixed linkage barley β-glucans of varying Mw have been investigated. The fractal concept has been applied to describe the structure development of barley β-glucan gels using a scaling model and dynamic rheometry data. The model supports that the gel structure consists of fractal clusters that upon aggregation lead to a three-dimensional network. The analysis showed that with increasing Mw a denser (more packed) network is formed as indicated by the corresponding fractal dimension ( d f) values. The microelastic parameter of the model, α, showed that all gel structures were in the transition regime implying structural reordering upon ageing. The description of the microstructure as a fractal network seems to be able to explain syneresis and other observations from large deformation testing of such systems. The molecular treatment of the gelation kinetics suggests that the gelling behavior is governed by the probability of collision of chain fragments with consecutive cellotriosyl units. This is greater for small chains due to their higher diffusion rates, for chains having lower amounts of cellulose like fragments and finally for those showing smaller degree of intrachain interactions. As a result, the faster gelling systems exhibit lower fractal dimensionality (more disordered systems) something that is in accordance with current kinetic theories.

Some physical and microstructural properties of genipin-crosslinked gelatin–maltodextrin hydrogels

The physical properties and microstructure of gelatin–maltodextrin hydrogels fixed with genipin (GP) were investigated as a function of pH (3–7), maltodextrin (MD) (0–9%, w/w) and GP (0–10 mM levels), at a constant gelatin (G) concentration (10%, w/w). Network strength (elastic modulus, E) and swelling behavior were characterized by large deformation testing and by swelling index (SI). In general, network strength increased and swelling decreased at higher pH, MD and GP levels, except at pH 3, where E was independent of the GP concentration until ∼7.5 mM, above which it declined. Confocal scanning laser microscopy (CLSM) images showed phase separation to be suppressed at pH 3, whereas at pH 7, separation into a self-similar dispersed phase was apparent. Overall, the judicious use of GP to crosslink G was an appropriate means of kinetically trapping MD within the gelatin network.

Changes in apple tissue with storage time: Rheological, textural and microstructural analyses

In this study, two varieties of apples with different textural characteristics—“Granny Smith” and “Golden Delicious”—were chosen to evaluate the efficacy of rheological and textural analyses. Dynamic rheological measurements (using a controlled stress rheometer), large deformation texture analysis (uniaxial compression and penetration, using a TAXTplus) and cryo-scanning electron microscopy were used to study the macro- and microscopic changes in apple tissue structure during 14 days of storage at 20 °C. The results obtained showed that both dynamic small deformation and static large deformation tests may be used to monitor mechanical changes in apple tissues. Observation of apple flesh microstructure revealed many changes that could be related to macroscopic changes in texture and rheology. All samples with low storage modulus and Young’s modulus values had microstructural characteristics with some component of the textural unity (middle lamella, cell wall or cell membrane) affected, resulting in effects such as exudation, loss of cell material and cell separation.

Sorbitan Tristearate Layers at the Air/Water Interface Studied by Shear and Dilatational Interfacial Rheology

The shear and dilatational rheology of condensed interfacial layers of the water-insoluble surfactant sorbitan tristearate at the air/water interface is investigated. A new interfacial shear rheometer allows measurements in both stress- and strain-controlled modes, providing comprehensive interfacial rheological information such as the interfacial dynamic shear moduli, the creep response to a stress pulse, the stress relaxation response to a strain step, or steady shear curves. Our experiments show that the interfacial films are both viscoelastic and brittle in nature and subject to fracture at small deformations, as was supported by in-situ Brewster angle microscopy performed during the rheological experiments. Although any large-deformation test is destructive to the sample, it is still possible to study the linear viscoelastic regime if the deformations involved are controlled carefully. Complementary results for the dilatational rheology in area step compression/expansion experiments are reported. The dilatational behavior is predominantly elastic throughout the frequency spectrum measured, whereas the layers exhibit generalized Maxwell behavior in shear mode within a deformation frequency regime as narrow as two decades, indicating the presence of additional relaxation mechanisms in shear as opposed to expansion/compression. If the transient rheological response from stress relaxation experiments is considered, then the data can be described well with a stretched exponential model both in the shear and dilatational deformations.

The viscoelastic properties of soybean curd (tofu) as affected by soymilk concentration and type of coagulant

SummaryThe viscoelastic properties of different types of tofu were investigated. Soymilk concentrations were 5, 6, 7, 8 and 9%. Coagulants used were 30 mm CaSO4 or 30 mm glucono‐delta‐lactone (GDL). As the concentration of soymilk was increased viscosity and handling difficulties increased. A high concentration of soymilk in tofu gave a high break stress and produced hard tofu. The four‐element Burgers model fitted the creep behaviour and both viscous and elastic parameters could be acquired from model analysis, reflecting changes in elasticity and viscosity of tofu. The constant viscous parameter in the model increased with increasing soymilk concentration. The viscous parameters of viscoelastic materials like tofu gel, obtained from small deformation tests, seemed to correlate, to some extent, with the break stress obtained from large deformation tests. For hard tofu production increasing the soymilk concentration within a certain range and the partial replacement of calcium sulphate coagulant by GDL could be effective options.

New Seismic Design Criteria of Piping Systems in High-Pressure Gas Facilities

After the Great Hyogoken-nanbu Earthquake (1995), the Seismic Design Code for High-Pressure Gas Facilities of Japan was amended. This amended code requires two-step seismic assessments, that is, the evaluation of the Level 1 Required Seismic Performance for Level 1 earthquakes and that of the Level 2 Required Seismic Performance for Level 2 earthquakes. Seismic design of piping systems is newly included within the scope of the code. For Level 2 earthquakes, possible ground displacement due to liquefaction is taken into account. The evaluation method of the Level 1 Required Seismic Performance is specified in the amended code and that of the Level 2 Required Seismic Performance is proposed in the guideline. The evaluation of the former is based on elastic design and that of the latter on elastoplastic design. The propriety of the design criteria of piping systems with respect to ground displacement was confirmed by large deformation tests. In this paper, seismic design criteria of piping systems in the amended code and the evaluation method of the Level 2 Required Seismic Performance proposed in the guideline are introduced, and the results of the large deformation tests are reported.

Comparison of Small and Large Deformation Rheological Properties of Wheat Dough and Gluten

ABSTRACTThe rheological properties of dough and gluten are important for end‐use quality of flour but there is a lack of knowledge of the relationships between fundamental and empirical tests and how they relate to flour composition and gluten quality. Dough and gluten from six breadmaking wheat qualities were subjected to a range of rheological tests. Fundamental (small‐deformation) rheological characterizations (dynamic oscillatory shear and creep recovery) were performed on gluten to avoid the nonlinear influence of the starch component, whereas large deformation tests were conducted on both dough and gluten. A number of variables from the various curves were considered and subjected to a principal component analysis (PCA) to get an overview of relationships between the various variables. The first component represented variability in protein quality, associated with elasticity and tenacity in large deformation (large positive loadings for resistance to extension and initial slope of dough and gluten extension curves recorded by the SMS/Kieffer dough and gluten extensibility rig, and the tenacity and strain hardening index of dough measured by the Dobraszczyk/Roberts dough inflation system), the elastic character of the hydrated gluten proteins (large positive loading for elastic modulus [G′], large negative loadings for tan δ and steady state compliance [Je0]), the presence of high molecular weight glutenin subunits (HMW‐GS) 5+10 vs. 2+12, and a size distribution of glutenin polymers shifted toward the high‐end range. The second principal component was associated with flour protein content. Certain rheological data were influenced by protein content in addition to protein quality (area under dough extension curves and dough inflation curves [W]). The approach made it possible to bridge the gap between fundamental rheological properties, empirical measurements of physical properties, protein composition, and size distribution. The interpretation of this study gave indications of the molecular basis for differences in breadmaking performance.

Effect of pH on the Gelation Properties of Skim Milk Gels Made From Plant Coagulants and Chymosin

The effect of three milk pH values, 6.0, 6.3 and 6.7, on gelation properties was monitored by small amplitude oscillatory rheology as well as a large deformation (yield) test for gels induced by the plant coagulants, Cynara cardunculus L. and Cynara humilis L., and chymosin. Gel microstructure was studied using confocal scanning laser microscopy. For each coagulant, a decrease in pH of milk resulted in a decrease in gelation time (tg), and an increase in the rate of increase in storage modulus (G′). At pH 6.0 and 6.3, plant coagulant-induced gels reached a maximum value in G′ (G′max) followed by a decrease in G′ values during the rest of the experiment, reflecting higher proteolytic activity of plant coagulants towards caseins as determined by SDS-PAGE. Gels produced at pH 6.0 and 6.3, exhibited a minimum in loss tangent (tan δ) followed by slight increase in tan δ during gel aging, that may have been associated with faster rearrangements of the gel network structure. In gels aged for ∼6h, the values for G′, tan δ at low frequency (0.006Hz) and yield stress were higher for chymosin than for plant-induced gels. For gels with the same pH value, no major differences were observed in microstructure between coagulants. Gels made at low pH values (6.3 and 6.0) appeared to have a denser or more interconnected structure than gels made at pH 6.7. Our results suggest that, at a low pH, the type of coagulant used in gelation is likely to have a considerably impact on gel/cheese structure.

Gelation properties of salt soluble meat protein and soluble wheat protein mixtures

Salt soluble meat proteins (SSMP) and commercially available soluble wheat proteins (SWP) were characterised by SDS-polyacrylamide gel electrophoresis, differential scanning calorimetry (DSC) and small and large deformation testing. DSC scans indicated transitions similar to those of native actomyosin for the salt soluble meat extract whereas SWP did not indicate any transitions between 20 and 120 °C. Small deformation tests on SWP indicated a G′/ G″ crossover gelation temperature of 90 °C and weak gels as judged by frequency sweeps. In contrast, SSMP gelled at 40 °C and formed strong gels on heating to 90 °C. However, on autoclaving at 120 °C, 20% SWP in distilled water produced strong elastic gels with little syneresis, compared with the more brittle gels produced with 20% (w/w) SSMP as indicated by large deformation testing. Mixtures of the two proteins in the ratio SSMP/SWP (15:5) gave strong elastic gels similar to the SWP gels. Even the presence of very small amounts of SWP in the mixture, e.g. SSMP/SWP 20:1 trebled the elastic modulus compared with a SSMP gel and reduced syneresis. This was probably due to the close association of SWP with actomyosin strands as viewed by transmission electron microscopy. However, increased levels of SWP in the mixture, for example SSMP/SWP 10:10 ratio, resulted in the separation of the two protein phases as shown by phase contrast microscopy, and consequently led to lower G′ values in the mixed gels. The addition of 20 mM chloride salts showed that potassium reduced the shear modulus, sodium had no effect and calcium enhanced the shear modulus for SWP gels formed at 120 °C. In contrast, SSMP gels were stronger in the presence of potassium, followed by sodium and calcium.

Gelation properties of soya and whey protein isolate mixtures

Gelation properties of commercially available soya and whey protein isolates (15–18% w/w) were tested in isolation and when mixed in ratios ranging from 17:1 to 1:17 soya/whey to give a final protein concentration of 18% protein (w/w) in distilled water. The isolates and mixtures were investigated by both small and large deformation rheological analysis, which indicated phase separation of the two proteins systems. Phase inversion occurred at 5:1 ratio (soya/whey) where the lowest shear modulus value, by large deformation testing, was 3.5 compared to 5.5 for 18% (w/w) soya and 10.7 for 18% (w/w) whey proteins in distilled water. The G′ value obtained by small deformation testing, also showed the lowest value at the 5:1 ratio (soya/whey proteins). In addition, gels exhibited maximum syneresis at this ratio indicating instability at the point of inversion. Electron microscopy in combination with immunocytochemistry using anti-soya or anti-whey antibodies also indicated phase separation of the two types of proteins. Phase separation was considered to occur due to differences in the molecular sizes of the proteins and physical–chemical differences including hydrophobicity as verified by the cis-parinaric fluorescent probe technique.

Effect of Antioxidants, Citrate, and Cryoprotectants on Protein Denaturation and Texture of Frozen Cod (Gadus morhua)

To investigate the role of antioxidants and cryoprotectants in minimizing protein denaturation in frozen lean fish, cod fillets were treated with either antioxidants (vitamin C (500 mg kg(-1)) or vitamin C (250 mg kg(-1)) + vitamin E (250 mg kg(-1))), antioxidants (vitamins C + E 250 mg kg(-1)each) with citrate (100 mg kg(-1)), cryoprotectants (4% (w/w) sucrose + 4% (w/w) sorbitol), or a mixture of antioxidants (vitamins C + E 250 mg kg(1)), citrate (100 mg kg(-1)), and cryoprotectants (sucrose 40 g kg(-1) + sorbitol 40 g kg(-1)). Untreated and treated fish samples were stored at -10 degrees C; cod fillets stored at -30 degrees C were used as a control. Stored frozen samples were analyzed at intervals for up to 210 days for changes in protein extractability, thermodynamic parameters (transition temperature T(m) and enthalpy DeltaH), structure by FT-Raman spectroscopy, and rheological properties by large and small deformation tests. Results indicated that protein denaturation and texture changes were minimized in the presence of cryoprotectants, as well as in the presence of antioxidants with citrate, antioxidants alone, or the mixture of antioxidants, citrate, and cryoprotectants. In the presence of increased formaldehyde levels in fish treated with vitamin C, toughening was still lower compared to that of the -10 degrees C control due to the antioxidant property of vitamin C. Thus, ice crystal formation and lipid oxidation products are the major factors that cause protein denaturation in lean frozen fish, and antioxidants in addition to cryoprotectants can be used to minimize toughness.

Effects of Preparation Temperature on Gelation Properties and Molecular Structure of High‐Amylose Maize Starch

ABSTRACTIn this study, 3% aqueous high‐amylose maize starch (Hylon VII) dispersions were heated to temperatures of 140–165°C. The onset and rate of gel formation was observed using a small‐strain oscillation rheometer as a function of temperature from 90 to 25°C. The gel formation clearly began earlier in high‐amylose starch paste preheated at lower temperatures, but the rate of gelation was slower and the resulting gel was weaker in comparison with starch pastes preheated at higher temperatures. In addition, the structure of the final gels was studied using large deformation compression measurements. The most rigid gel structure on the basis of small and large deformation tests was obtained for high‐amylose starch gel preheated to 150–152°C, depending on the type of measurement. The rate of gelation was also fastest in that temperature range. High‐amylose gels heated to higher temperatures lost their rigidity. The molecular weight distribution of starch molecules was measured by size‐exclusion chromatography. Heating caused extensive degradation of amylopectin, which had a great effect on amylose gel formation and the final gel properties of high‐amylose maize starch. Micrographs of Hylon VII gels showed that phase separation of starch components visible in light microscopy occurred on heating to higher temperatures.

Rheological Properties of Dough During Mechanical Dough Development

During mechanical development dough is subjected to both shear and extensional deformations. Thus, it is expected that both flow conditions contribute to the development of dough. In order to monitor rheological changes, occurring during mixing, shear and extensional properties of dough prepared with two flours of different strength and various levels of mixing energy were determined using fundamental rheological methods. Rheological measurements included: small deformation and large deformation (shear test), planar extensional flow and a combined shear/extensional flow test, namely extrusion test. Results obtained in this research showed that, during mixing, dough develops with an increase in both apparent shear and extensional viscosities. For all the tests, plots of the measured rheological properties as a function of the mixing energy resembled typical mixing curves. This indicated that the increase in the power drawn by the mixer motor is due to the increase in both apparent shear and extensional viscosities. After peak dough development these properties decreased synchronously with the mixing curves. Results from small deformation shear tests exhibited large variability, particularly when non-mixed and underdeveloped doughs were tested. This variability was associated with poor water distribution in the sample due to insufficient mixing. Results of large deformation tests, including shear, planar extensional flow and the extrusion test, were less variable and showed that mixing and type of flour affect the rheological properties of dough.

Rheological influence of non-gelling amylopectins on β-lactoglobulin gel structures

The effect of non-gelling potato amylopectin on the gel properties of a particulate β-lactoglobulin gel was studied by small and large deformation rheology and by light microscopy. Two different techniques using small deformations, one measuring the modulus in shear and the other measuring the modulus in compression, have been compared. The fracture tests of the gels were performed in tension. The concentration of β-lactoglobulin was kept constant at 6 wt%, and the amount of potato amylopectin was varied from 0 to 2 wt%. Two preparations of potato amylopectin, with different rheological behaviour, were used. The results illustrate the importance of a full rheological characterisation, since small and large deformation tests responded different to the structure. Both the viscoelastic and the failure properties of β-lactoglobulin gels changed on addition of potato amylopectin even at low concentrations. The effect of the potato amylopectin concentration on the rheological properties of the mixed system varied with the properties of the potato amylopectin. The higher viscosity (HV) potato amylopectin had a shear-thinning behaviour and a small yield stress, while the lower viscosity (LV) potato amylopectin had a lower viscosity with a Newtonian behaviour. The gels containing the LV potato amylopectin increased in the modulus of small deformations with increasing potato amylopectin concentration, while the stress at fracture was constant up to a concentration of 0.5 wt% potato amylopectin and then increased with increasing potato amylopectin concentration. For the gels containing the HV potato amylopectin the modulus of small deformations reached a maximum at 0.25 wt% potato amylopectin and then decreased with increasing potato amylopectin concentration. The stress at fracture was constant up to 0.5 wt% potato amylopectin and decreased at higher concentrations. The state of aggregation of β-lactoglobulin was influenced both by concentration and properties of potato amylopectin. The higher the potato amylopectin concentration the larger the pores in the β-lactoglobulin gel.

Rheological characterisation of ‘weak gel’ carrageenan stabilised milks

The use of a controlled stress rheometer to characterise the rheology of weak gel behaviour of milk containing carrageenan is described. The interaction of carrageenan with the protein in milk produces a long-range network structure where the physical properties are dependent on the type of carrageenan used. With the use of a sensitive controlled stress rheometer, it was possible to characterise the rheological properties of these weak gels and differentiate between samples using both large deformation techniques, and small amplitude oscillatory measurements. Large deformation tests enabled the gels to be qualitatively compared based on an apparent yield stress, ‘structure point’, apparent shear viscosity and degree of hysteresis. Small amplitude oscillatory rheometry was applied to make comparisons from measurements that did not exceed the linear viscoelasticity of the system. Both the frequency and time dependence of the rheological properties were measured to characterise final gel properties and the kinetics of network development. Both large deformation rheology and small amplitude oscillatory rheology have been effective in providing quantitative and qualitative comparisons between the rheological properties of samples containing weak gel networks.

Large-deformation tests and total-Lagrangian finite-element analyses of flexible beams

Presented here is a total-Lagrangian displacement-based finite-element formulation for general anisotropic beams undergoing large displacements and rotations. The theory fully accounts for geometric nonlinearities (large rotations), general initial curvatures, and extensionality by using Jaumann stress and strain measures, an exact coordinate transformation, and a new concept of orthogonal virtual rotations. Moreover, transverse shear deformations are accounted for by using a first-order shear-deformation theory. To verify the accuracy of the finite-element model, two test fixtures have been built for bending and twisting experiments. Large static deformation tests of beams with different loading conditions have been performed. The finite-element results agree closely with the experimental results and numerically exact solutions obtained by using a multiple shooting method to solve for post-buckling deformations of highly flexible beams undergoing large static rotations and displacements in three-dimensional space.

EFECT OF pH BUFFERS ON MECHANICAL PROPERTIES OF GELLAN GELS

ABSTRACTThe strength and deformability of calcium cross‐linked gellan gels as affected by pH 3.5 and 5.0 citrate and acetate buffers were measured by large compressive deformation test until failure. The trend of dependence of gel strength on polymer and calcium concentrations was similar to gels formed in distilled water without pH adjustment. A critical calcium concentration was observed for each gellan concentration. Gels formed at the critical calcium concentration exhibited the maximum strength. The chelating effect of pH 5.0 citrate buffer greatly increased the critical calcium concentration. The failure strain, representing the deformability, of gellan gels formed in buffers behaved differently from gels formed in distilled water. In the pH 3.5 buffer systems, gellan gels were brittle regardless of gellan and calcium concentrations. In the pH 5.0 buffer systems, gellan gels were brittle at high calcium concentrations and ductile at calcium concentrations less than 24 mM in citric buffer and less than 6 mM in acetate buffer.

THE EFFECTS OF pH AND TIME ON RHEOLOGICAL CHANGES DURING EARLY CHEESE MATURATION

ABSTRACTChanges in rheological properties of cheese curd during the initial stages of ripening, with pH (5.45–6.05) and with time (2–14 days), were evaluated using a small strain oscillatory test and a newly designed large strain deformation test. The new test method developed for evaluation of grated cheese employed extrusion flow technique and was carried out using the Instron Universal Testing Machine. The values for the storage modulus (G') measured using the small strain test increased with an increase in the pH of cheese curds from 5.45 to 5.90. The results from the large strain test showed essentially similar trends. The samples of curd also showed an increased solid‐like behaviour as the pH increased and appeared to become more elastic with time during the two‐week period of analysis.

Nonlinear viscoelasticity of cheese

Cheese viscoelasticity is commonly measured using steady uniaxial compression, steady uniaxial extension, shear and compressive creep, and stress relaxation in shear and compression. Viscoelastic properties for many cheeses have also been studied using small amplitude oscillatory shear (SAOS). However, there is little on the measurement of nonlinear viscoelastic properties. The large deformation test usually conducted on cheese to study nonlinear viscoelasticity is uniaxial compression, but this test hardly departs from linear behavior. In this work, the nonlinear viscoelasticity of four cheese varieties was studied using large amplitude oscillatory shear (LAOS). A sliding plate rheometer incorporating a shear stress transducer was used. The data were evaluated using spectral analysis, and results are presented mainly in the form of shear stress versus shear rate loops.