Yield Stress Of Material Research Articles

On the elusive nature of Carbopol gels: “model”, weakly thixotropic, or time-dependent viscoplastic materials?

The question of whether Carbopol gels always behave as “model” yield stress material is addressed experimentally. Prompted by several simple hydrodynamic experiments performed with Carbopol gels that can not be fully understood within the commonly accepted “model” picture, we revisit the yielding behaviour of a Carbopol gel. When subjected to a loading/unloading process, the yielding of the Carbopol is gradual and exhibits a rheological hysteresis. By in-situ visualisation of the microstructure it is demonstrated that these features do not originate from a micro-structural damage of the micro-gel. A systematic description of the role of the rate at which the material is forced (loaded or unloaded) on the yielding scenario is given. In closing, the question of how simple a scalar model can be and yet accurately describe the experimentally observed yielding scenario of a Carbopol gel in a rheometric flow is addressed. It is concluded that simple scalar models may do such job as long as they are not too simple and include a minimal amount of physical ingredients.

Transition between solid and liquid state of yield-stress fluids under purely extensional deformations

We report experimental microfluidic measurements and theoretical modeling of elastoviscoplastic materials under steady, planar elongation. Employing a theory that allows the solid state to deform, we predict the yielding and flow dynamics of such complex materials in pure extensional flows. We find a significant deviation of the ratio of the elongational to the shear yield stress from the standard value predicted by ideal viscoplastic theory, which is attributed to the normal stresses that develop in the solid state prior to yielding. Our results show that the yield strain of the material governs the transition dynamics from the solid state to the liquid state. Finally, given the difficulties of quantifying the stress field in such materials under elongational flow conditions, we identify a simple scaling law that enables the determination of the elongational yield stress from experimentally measured velocity fields.

Local surface nanocrystallization for buckling-resistant thin-walled structures

An innovative and new technology on enhancing buckling resistance is established in this work by making use of local surface nanocrystallization to significantly improve the critical loads of thin-walled structures under axial compression loads. By capitalizing new design approaches of local nanocrystallization, critical loads of thin-walled structures can be dramatically increased owing to the marked enhancement of yield stresses of materials within the treated domain. No additional material adhering or structural modification is required in the entire procedure. An optimal design of local surface nanocrystallization is derived by evaluation of critical loads and ultimate buckling deformation of five typical nanocrystallization layouts. Theoretical results show that axial nanocrystallization stripes closed to edges produce the highest critical loads and stable ultimate buckling deformation. Furthermore, an experimental work is presented to validate the optimal design. The result concludes that welding skill is an important influencing factor in the fabrication of such nanocrystallized tubes.

Giant fluctuations in the flow of fluidised soft glassy materials: an elasto-plastic modelling approach

In this work we study the effect of an additional mechanical noise on the rheological features of yield stress materials that exhibit non-homogeneous steady flows. Using a mesoscale elasto-plastic model accounting for a viscosity bifurcation in the flow response to an external shear stress, we find that additional sources of noise can lead to a fluidisation effect. As we increase the noise intensity we evidence a transition from a non-monotonic to a monotonic rheology, associated with giant fluctuations of the macroscopic shear rate and long-time correlated dynamics. Although distinct noise models can lead to different rheological behaviours in the low stress regime, we show that the noise-induced transition from shear-localised to homogeneous flow at higher stresses appears very generic. The observed dynamical features can be interpreted as a result of an out-of-equilibrium phase transition, for which we estimate the critical exponents that appear to be independent of the specific choice of the noise implementation for the microscopic dynamics.

Modelling and parameter optimization for filament deformation in 3D cementitious material printing using support vector machine

The material flow mechanism affects the printing quality considerably in 3D cementitious material printing (3DCMP) area. A numerical model was developed to investigate the material flow mechanism during the extrusion and deposition process. To quantify the effects of flow mechanism on the filament printing quality, deformation of the printed filament was proposed. Then a Support Vector Machine (SVM) was employed to study various factors on flow mechanism, hence the deformation of the printed filament. The SVM model results showed that deformation of the printed filament is independent of plastic viscosity, however, material yield stress and relative nozzle travel speed significantly affect the deformation of the printed filament. Lastly, an empirical parametric associative model was proposed to predict the filament deformation based on material yield stress and relative nozzle travel speed.

Stress triaxiality effect on void nucleation in ductile metals

AbstractThe stress triaxiality effect on the strain required for void nucleation by particle‐matrix debonding has been investigated by means of micromechanical modelling. A unit‐cell model considering an elastic spherical particle embedded in an elastic‐plastic matrix was developed to the purpose. Particle‐matrix decohesion was simulated through the progressive failure of a cohesive interface. It has been shown that the parameters of matrix‐particle cohesive interface are correlated with macroscopic material properties. Here, a simple relationship for the maximum cohesive opening at interface failure as a function of material fracture toughness and yield stress has been derived. Results seem to confirm that, increasing stress triaxiality, the strain at which void nucleation is predicted to occur decreases exponentially in a similar way as for fracture strain. This result has substantial implications in modelling of ductile damage because it indicates that if the stress triaxiality is high enough, ductile fracture can occur at plastic strain lower than that necessary to nucleate damage for moderate or low stress triaxiality regime.

Test of Friction Parameters in Bulk Metal Forming Based on Forward Extrusion Processes

Existing methods for examining the friction parameters in metal forming all have advantages and disadvantages. Based on the theory of plasticity, the current study established quantitative correlations among friction coefficient/factor, yield stress of the workpiece material, load and die geometry in the forward extrusion with a conical die, and then designed a procedure for testing the friction parameters in forming processes using the correlations. A series of extrusion experiments along with the numerical simulations, using AA 7050 specimens under various lubricating conditions, were carried out. The results proved that the method can obtain the friction coefficient/factor with an acceptable precision. Theoretically, since the effects of material properties, forming velocity, temperature and surficial condition, etc., on the deformation can be directly considered in the operation, this method is applicable to a wide range of material types and forming conditions. To avoid the occurrence of “barreling phenomenon” under large load which may lead to failure of the operation, it is recommended that half angle of the conical die ranges from 5 to 10 degrees.

Force Analysis of Fork Type Connecting Structure Pin Roll of Aircraft Vertical Tail in Comprehensive State

Aiming at the stress problem of pin roll connection at the vertical tail of aircraft, the thesis establishes a simplified simulation model of vertical tail. From the mechanical perspective, it takes into account the gravity, assembly force of interference fit and the influence of air resistance on the fork force. Compared with the yield stress of titanium alloy material used in the actual fork, it is concluded that actual stress is much smaller than the yield stress of the material, which verifies the rationality of pin roll design.

New Insights on the Vane Method to Evaluate Yield Stress of Cementitious Mortars

Abstract The yield stress (τ0) of cementitious materials has a direct influence on concrete casting and postcasting processes, such as stability, formwork pressure, interfacial bond strength between successive lifts, and bond to embedded steel bars. This article provides new insights for reliable τ0 measurements using the four-bladed vane method; it compares the effect of vane configuration (i.e., slotted versus nonslotted) and positioning (i.e., immersed in specimen versus placed flush with upper material surface) on τ0 responses. Test results have shown that the elastic-to-plastic transition region in τ0 profiles occurs progressively with the nonslotted vane, whereas it becomes more abrupt with the slotted vane. The presence of slots is believed to reduce the end effects and strengthen the assumption of shearing that takes place within the suspension along uniform stresses distributed at the cylindrical vane surfaces. For the given configuration, positioning the vane flush with the upper mortar surface minimized the effect of normal stresses on torque measurements, which resulted in decreased τ0 values. Useful equations with good correlation coefficients are established between τ0 measurements with respect to mortar composition, flow, and vane positioning.

Thermally neutral nano-hole and its effect on the elastic field

We use complex variable methods to rigorously design a circular nano-hole which, when introduced into a host material, does not disturb the original thermal field present in the uncut body. We refer to such a hole as ‘thermally neutral’ or ‘neutral with respect to thermal conductivity’. Our design is based on the idea that the boundary of the hole is considered to have a temperature jump attributed to surface phonon scattering. By adjusting the size of the hole, the surface phonon scattering counteracts the effect of the hole in such a way that the heat flux in the surrounding matrix remains undisturbed. Results obtained through rigorous investigation show that a uniform heat flux generates a harmonic elastic field around the neutral circular nano-hole and that the magnitude of the thermal stress is much lower than the corresponding yield stresses of most materials even in the presence of a higher temperature gradient.

Numerical homogenization of elastic properties and plastic yield stress of rock-like materials with voids and inclusions at same scale

Mechanical properties of rock-like materials are affected by voids and mineral inclusions. Different kinds of analytical solutions have been established for the determination of effective elastic properties and plastic yield stress or failure strength of those materials. However, voids and inclusions are generally considered at different scales. By adopting the Fast Fourier Transform technique, we propose in this paper a numerical homogenization method for modeling the effective elastic properties and plastic yield stress of rock-like materials having voids and mineral inclusions at the same scale. Both are embedded in an elastic-perfectly plastic matrix. The overall elastic-plastic properties are estimated by taking into account the morphology and spatial distribution of pores and inclusions. It is found that the macroscopic elastic modulus is sensitive to both void and inclusion geometry and orientation. The overall yield stress is mainly controlled by the porosity, the inclusion volumetric fraction, the void shape, distribution and orientation. Due to the presence of voids, the influences of inclusion geometry and orientation on the overall yield stress are significantly reduced. This is an important difference with inclusion-reinforced materials without voids.

Yield stress of cementitious materials under different squeezing rates

In the present work, the squeeze flow techniques were used to investigate the influence of squeezing rates to the yield stress of different cementitious materials, including mortars and cement pastes. Tested samples were prepared under similar conditions of room temperature and atmospheric pressure. Compositions of motars and cement pastes were tested at three squeezing rates (20 and 200 mm/s) 15 min after mixing. The results indicate an increasing in the material’s yield stress with the rising of the squeeze velocity. This increase is evident at low tensile speeds (2 mm/s) and fast pulling speeds (200 mm/s). At an average speed of 20 mm/s, this increase is not obvious. It shows that the pressing speed in construction period has a significant impact on the ability to remove the mortar and the upper surface (masonry bricks, tiles adhesive, etc). High squeezing speed will increase the ability to resist peeling of materials when sticking to mortar.

Yield stress criteria to assess the buildability of 3D concrete printing

3D concrete printing (3DCP) using the extrusion method is a commonly used additive manufacturing technique to construct the structures layer by layer without using formwork. Therefore, the green (wet or fresh) strength of the printable concrete determines the possible printable height of the structure and the rate of printing to avoid collapse during the printing process. This paper aims to identify the buildability criteria based on the green strength of concrete and the effect of early age material properties on the stability of printed structures. The commonly available strength-based failure models based on the material yield stress and the vertical stresses induced were developed considering higher aspect ratios (height to width ratio). This may be not suitable for lower aspect ratios used in 3DCP layer geometries. Also, the frictional behaviour due to the applied vertical stresses should be considered in the material used for 3DCP applications. In this work, the Mohr-Coulomb based buildability criterion was developed and validated with laboratory experiments and numerical simulations. The laboratory experiments were carried out to establish the time-dependent material and rheological properties of 3D printable concrete mixes. Further, 3D printing of hollow circular sections was carried out to study the failure modes and to obtain the build height at the time of failure. The nonlinear finite difference model of the 3D printed sections was developed with the user-defined time-dependent material models to assess the accuracy of the proposed buildability criterion. Additionally, the proposed buildability criterion was further validated with the 3DCP experimental and numerical data presented in literature and all confirm that the proposed criterion shows higher accuracy over existing methods in assessing the buildability.

Is the von Mises criterion generally applicable to soft solids?

The purely viscoplastic paradigm has forged the understanding of yield stress materials, leading to misconceptions associated to the set of quantities that are needed to characterize yielding transition and also to the significance of the measured quantities (Thompson et al., 2018). The following assertions, usually taken for granted, are actually simplistic constitutive assumptions: (i) there exists a scalar that represents the yielding condition, (ii) there is a universal rule that maps the yield stress tensor onto that scalar, (iii) the von Mises yielding criterion holds (Thompson et al., 2018). That is, these statements are not necessarily true for a general yield stress material. In the present study we investigate the yielding of seven materials in two extensional loading conditions: traction and compression. We show that the von Mises criterion does not perform well for the yielding of these materials. A new criterion that includes the third invariant of the (deviatoric) yield stress tensor is proposed to handle the differences between traction and compression values of the stress at the yielding point. Even this criterion is not able to accommodate the yielding obtained in shear loading for these materials. In this regard, the most likely scenario is that no universal criterion exists for all kinds of yield stress materials.

Rapunzel! Rapunzel! How Strong Is Your Hair?

In this issue, the strength of a well-known biological material, hair, is assessed across a number of mammalian species, from humans to elephants. The multiscale keratin-based structure of hair is of interest, representing a possible archetype for biomimetic high-strength threads.

Yielding of a quartz sand from saturated to dry state

The paper presents the results of an experimental work where we analyse the behaviour of an unsaturated quartz sand in a wide range of degree of saturation (from saturated to dry state). The possibility of anticipating the hydro-mechanical behaviour of the soils when they approach the dry state is fundamental in many areas. An extensive experimental program, including controlled-suction and constant water content oedometric tests, was carried out to deeply analyse the water retention behaviour and the relationship between the yield stress and suction (Loading-Collapse curve). All elasto-plastic models provide a monotonically increase of the yield stress with suction. This assumption implies that the yield stress in the dry state is larger than the one relative to the saturated state, in contrast with the classical geotechnical points of view, which suggest that the yield stress of dry granular material must be approximately the same as that of the saturated one. The obtained results show that the yield stress of the sand does not increase monotonically with the suction, as predicted by commons models. In fact, the Loading - Collapse curve showed in this work presents a maximum point, and the yield stress for saturated condition is almost the same of the dried one.

Evaluation of the Yield Stress of Solids upon Unloading from a Shock-Compressed State

A simple method for evaluating the yield stress of shock-compressed material from the measured unit free-surface velocity history is proposed, verified, and approbated. For the first time, the dynamic yield stress of shock-compressed metal (aluminum) is experimentally evaluated at an elevated temperature. It is found that its value is lower than that at the room temperature and lower than the initial value of the dynamic yield stress at high temperatures.

Crystallization Kinetics of Composite Materials Based on Polyethylene Mixture with High and Low Density

The influence of the ratio of high-density polyethylene and low-density polyethylene on the regular changes in breaking stress, tensile yield stress, elongation, and chilling temperature of composite materials are considered. In accordance with the Avrami equation, the formation and development of crystallization centers in a zone of the first-order phase transition are investigated.

Bilinear Load–Displacement Curve of Semi-supported Steel Shear Walls

Elastic stiffness and ultimate shear capacity are two main parameters of a structural system to obtain its ideal bilinear load–displacement. In the previous studies, the ultimate shear capacity of semi-supported steel shear walls (SSSW) which are a new lateral resisting system, has been determined. In this system, wall plates do not have any direct connection to the main columns of structure and they are connected to secondary columns which do not carry the gravity loads. The used thin plate in the SSSW elastically buckles at low levels of lateral loads and the wall plate stays on a fairly vast region with elastic post-buckling behavior (elastic stiffness). In this study, the Von-Karman plate equations are solved by the Galerkin method to find displacement field of the wall plate in the elastic post-buckling region as well as the maximum shear load after which the plasticity expand in the wall plate. Thus, the elastic stiffness of system is calculated. As the analytical procedure is complicated, the method is applied on 144 examples with different material and geometrical properties. Using linear regression technique, a concise formula is proposed to predict the elastic stiffness of system. The dimensions of wall plate are only the effective parameters in the suggested formula and the elastic stiffness is independent of the overturning moment, section of secondary columns and yield stress of material. Using the ultimate shear capacity and elastic stiffness, an ideal bilinear curve is obtained for the lateral load versus the horizontal displacement. The shear capacity at the end of elastic post-buckling region and out of plane displacement are acceptably validated with those of FE analysis for some examples.

Temperature-time large strain mechanical model for Poly(vinylidene fluoride)

The temperature-dependent visco-hyperelastic-viscoplastic behavior of poly(vinylidene fluoride) is investigated and modeled. The proposed mathematical formulation of the visco-hyperelastic element is constructed using a generalized Maxwell model consisting of non-linear springs and viscous dampers in series with a viscoplastic element. The Yamashita and Kawabata strain energy function is considered for modeling the hyperelastic response of the non-linear springs’ elements. The viscoelastic response is represented by the Prony series invoking the time-temperature correspondence principle and employing the Arrhenius equation to define the shift factors. The viscoplastic flow rate is defined by a phenomenological model activated when the Frobenius norm of the deviatoric portion of the Cauchy stress exceeds the material yield stress. The material parameters are calibrated from stress relaxation and monotonic tensile tests performed at different temperatures. Model predictions showed good agreement with the experimental results.