Stress-controlled Mode Research Articles

Fatigue Property Examinations of Conductors for Pulsed Magnets

Cu matrix composites are used as conductors for pulsed magnets that have potential to reach 100 T. The conductors are fabricated by cold drawing that introduces high densities of dislocations or interfaces and internal stress. The density of the dislocation and the interface affects the mechanical properties of the conductors, such as the tensile strength and fatigue endurance at 77 K of the composites. Understanding the performance of the conductors under cyclic loading, i.e. fatigue properties, helps one to make good use of them for pulsed magnets and to manufacture conductors to meet the requirements of the magnets, particularly when the magnetic stress reaches the limit of the mechanical strength of the conductors. The goal of this research is to understand the fatigue properties of a Cu-0.085wt%Ag conductor and to relate such properties to mechanical tensile strength, dislocation densities and interface structures. The fatigue test loading is either in stress-controlled or strain-controlled mode. This work sheds a new light on the correlation between the tensile and fatigue properties at 77 K by consideration of dislocation densities and precipitate in particle strengthened conductors.

Advanced Rheological Characterization of Non-Reversible Deformation and Fatigue Behavior in Crumb Rubber Modified Asphalt Binders

ABSTRACT Starting from the assumption that the characterization of damage behavior associates specific mechanical properties to the materials' resistance in condition of incipient failure, this paper deals with the use of different rheometrical approaches to assess the resistance to fatigue and non-reversible deformation in crumb rubber modified (CRM) asphalt binders. Damage-related properties of CRM binders at high service temperatures were studied by analyzing the mechanical response and the viscous deformation mechanism in conditions of repetitive and static creep loading. Results shown that the accumulation of non-reversible deformations and the distribution of delayed elastic and viscous components in binders delayed mechanical response strongly depend on the presence of the rubber particles. At the intermediate service temperature, the development of the complex modulus (G*) in cyclic shear tests (time sweeps) was recorded under different loading conditions and the binders fatigue resistance was evaluated according to the available failure criteria. Using both stress-controlled and strain-controlled testing mode, the relations between the stress/strain amplitude and the number of cycle to failure (S-N curves) were established. Results allowed to define some dynamics related to the contribute of the crumb rubber modification in binder damage resistance with regard to the considered loading conditions.

A CDM-based study of fatigue–creep interaction behavior

Under stress control mode, the damage evolution during fatigue, creep and their interaction behavior actually is a ductility exhaustion process in response to cyclic and static creep. Based on the ductility dissipation theory and effective stress concept of continuum damage mechanism (CDM), a new fatigue–creep interaction damage model has been developed in this paper, where the change of the inelastic strain energy density is used to define the damage variable. To assess this damage model, high temperature fatigue–creep interaction experiments have been carried out for 1.25Cr0.5Mo steel under stress control mode employing a trapezium waveform. The damage evolution laws of 1.25Cr0.5Mo steel under various combinatorial conditions with different maximum stresses and stress amplitudes are derived. Results indicate that the damage parameter and damage model presented in this paper are applicable to describe the damage evolution for fatigue–creep interaction.

Tension-Compression Fatigue Test Evaluation Using Fracture Mechanics and Field Data

ABSTRACT Fatigue cracking is one of the main distresses that affect asphalt pavements. In order to evaluate the cracking potential of several asphalt mixtures equivalent to the ones used in the sections of an accelerated loading pavement testing facility (PTF), uniaxial stress-controlled and strain-controlled direct tension-compression laboratory tests were performed. The laboratory-acquired data was analyzed using a crack growth equation with two separate techniques for estimating the dissipated pseudostrain energy. The results were compared to the observed PTF performance after subjecting the field sections to repeated load using a super-single truck tire accelerated loading device. The rankings of the mixtures based on their observed field performance and on the results of the crack growth equation were different, yielding parallel results for the strain-controlled laboratory test and opposite trends for the stress-controlled loading mode. The cause of the discrepancies could be explained by the onset and evolution of damage in each test protocol.

Quantitative Comparison of Energy Methods to Characterize Fatigue in Asphalt Materials

Different methods have been developed to assess fatigue cracking of asphalt mixtures based on dissipated energy. Most of these methods have been motivated by the need to develop a unified fatigue criterion that is independent of the mode of loading. This paper offers critical analyses of the energy methods based on their theoretical ability to: (1) unify the results from controlled-strain and controlled-stress modes of testing for the same material; and (2) accurately assess the fatigue cracking life of different materials. The efficacy of these methods is quantitatively compared using a common set of fatigue test data. The fatigue test data were obtained using the dynamic mechanical testing of three different mixtures that have been shown to exhibit different fatigue cracking resistance in the field.

固体的特性を有するスメクティック液晶のエレクトロレオロジー(<小特集>非ニュートン性,機能性を示す複雑流体の流動と応用)

The ER characteristics of a liquid crystal (8 CB) in smectic-A phase were investigated utilizing a parallel-plate rheometer under a stress control mode. The steady flow properties and solid like behavior of the liquid crystal under DC or AC electric fields were examined. Although the liquid crystal in nematic phase under steady shear flow showed the normal ER effect, i.e. increase in apparent viscosity under electric field, the viscosity in smectic phase exhibited an opposite effect in low shear rate region. Moreover, Bingham-like properties were observed in smectic phase and the yield stress obtained was affected by the electric field. When the electric field strength was low, the yield stress was almost the same as that obtained under no electric field. Above a threshold of DC electric field, the yield stress increased and then became constant. An opposite tendency was observed under AC electric fields. It appears that the smectic poly-domain structure is maintained under lower electric field conditions and has become more orderly and stronger under higher DC electric fields. By contrast, the AC electric field would generate more slippery layered structure because of the alternating electrical current on the molecules.

Exploring the transition from wall slip to bulk shearing banding in well-entangled DNA solutions

In this study we have carried out a combination of rheometric and particle-tracking velocimetric (PTV) measurements to investigate nonlinear rheological behavior of three entangled DNA solutions (with ca. 150 entanglements per chain) and, in particular, to explore a transformation from slip-dominated steady-state flow to bulk shear inhomogeneity. In the stress plateau regime, an elastic recoil-like response occurs transiently at either interfaces or sample interior after stress overshoot during a startup shear. In both startup shear and creep mode, wall slip, bulk shear banding or a combination of both have been observed in both transient and steady states. The water-based solution shows massive wall slip allowing the bulk to remain in the Newtonian flow regime. Use of glycerol as a solvent can effectively reduce interfacial slip, permitting bulk shear banding to develop in both controlled-rate and controlled-stress modes. For the glycerol based solution, a sufficiently high Weissenberg number can attain in the rheometer where PTV observations reveal homogenous shear in steady state.

Tension-Compression Fatigue Test Evaluation Using Fracture Mechanics and Field Data

ABSTRACT Fatigue cracking is one of the main distresses that affect asphalt pavements. In order to evaluate the cracking potential of several asphalt mixtures equivalent to the ones used in the sections of an accelerated loading pavement testing facility (PTF), uniaxial stress-controlled and strain-controlled direct tension-compression laboratory tests were performed. The laboratory-acquired data was analyzed using a crack growth equation with two separate techniques for estimating the dissipated pseudostrain energy. The results were compared to the observed PTF performance after subjecting the field sections to repeated load using a super-single truck tire accelerated loading device. The rankings of the mixtures based on their observed field performance and on the results of the crack growth equation were different, yielding parallel results for the strain-controlled laboratory test and opposite trends for the stress-controlled loading mode. The cause of the discrepancies could be explained by the onset and evolution of damage in each test protocol.

A unified method for the analysis of controlled-strain and controlled-stress fatigue testing

Fatigue cracking is one of the primary distresses in asphalt pavements. This study presents a method to characterize fatigue resistance of the fine portion of the asphalt mixture using the dynamic mechanical analyzer (DMA). Three mixtures were characterized in controlled-strain and controlled-stress modes of loading. The new method has several advantages as it requires reasonable testing time, uses a small amount of material, utilizes fundamental properties of the mixture, and is able to unify the results from controlled-strain and controlled-stress modes of loading. The unified method relies on identifying the different mechanisms of energy dissipation during fatigue cracking that are related to changes in the phase angle, changes in stiffness, and development of permanent deformation during the fatigue damage process. Two fatigue damage parameters are derived in this paper. The parameters are shown to have reasonable and lower coefficients of variation than conventional parameters such as number of loading cycles to failure and cumulative dissipated energy.

Steady state measurements in stress plateau region of entangled polymer solutions: Controlled-rate and controlled-stress modes

Despite decades of efforts, reliable measurements of nonlinear flow behavior of well-entangled polymers in continuous shear have been challenging to obtain. The present work attempts to accomplish three important tasks: (A) overcome this challenge by adopting a strategy of decoupling rheological measurements from the outer meniscus region in a cone-partitioned plate (C/PP) setup; (B) determine whether well-entangled solutions indeed undergo a flow transformation under creep that can be taken to phenomenologically define an entanglement-disentanglement transition (EDT); (C) provide the velocity profiles of such solutions undergoing either controlled-stress or controlled-rate shear by carrying out in situ particle-tracking velocimetric (PTV) measurements. Upon removing any influence of edge fracture and sample loss, we are able to reach steady state during continual shear and elucidate more reliably the nonlinear flow behavior of well entangled polymer solutions with little ambiguity. Three well-entangled s...

Researching on Fatigue Model of Asphalt Mixtures

Firstly different fatigue testing methods of HMA were summarized, the scope of applicability of two kinds of control mode were analyzed,it is recommended that indirect tensile fatigue test with stress control mode to be used.Then effect of different testing factors were analyzed, the scope of the loading wave,stress ratio,frequency and temperature were selected.Three kinds of HMA specimens were compacted by gyratory compactor. Indirect tensile fatigue tests were carried by uniform test design method under different temperatures, frequencies and loadings, including three void ratios of asphalt mixes. Significance of the testing results was analyzed.The fatigue models were brought forward through multiple linear regression methods based on VFA, temperature, frequency, initial strain and initial modulus for general and heavy-duty loading. Comparative study of the fatigue models was made at home and abroad, it is shown that the fatigue models are comparable.They could be used for the fatigue prediction of asphalt mixtures. In order to study the mechanism of fatigue cracking and reduce the fatigue failure of the pavement, researchers of dif- ferent countries used different methods and proposed differ- ent fatigue models of asphalt mixtures . But these models were obtained by different experimental methods, different experimental conditions and different parameters, each of them has its specific application scope. In order to understand and master the fatigue law of as- phalt mixtures, three kinds of asphalt mixtures including AO 25I,AO20I and AK-16A were used,and fatigue testing was conducted under different conditions, then the fatigue model of asphalt mixtures was obtained.

Increase in the effective viscosity of nanodisperse structured systems in shear deformation at extremely low rates

In this work, it was detected for the first time that, in the range that was previously deemed to be the range of the Newtonian viscosity η 0 , systems with lyophobic interactions of particles are characterized by a nonlinear dependence of the viscosity on the shear stress with an increase in the shear rates in the range 0 ≤ ≤ 10 –3 s –1 . The viscosity increases until the yield strength P k , after which an avalanche-like breakup of the structure starts. A specific feature of the flow of the considered structured dispersions in the range P < P k is that the viscosity increases in a sawtooth manner, which is typical of breakup of the structure into aggregates of particles with subsequent restoration and strengthening. This phenomenon can be explained by an increase in the probability of contacts between particles and aggregates of particles at lyophobic parts of the surface (if there is a lyophobic‐lyophilic mosaic pattern [5]). In this work, we studied suspensions of modified (methylated) nanodisperse Aerosil in mineral oil with concentrations of 2‐7 wt %. The total flow curves of the studied suspensions under quasi-equilibrium conditions were recorded with a Haake RS1 rotational rheometer in a controlled stress mode where the shear stress was specified discretely and the shear rate was controlled. Reaching of a quasi-equilibrium state at each point was detected by the device automatically once the amplitude of oscillations of a parameter being measured reached 1% of the mean. Great attention was paid to studying the behavior of structured nanodisperse systems in the range of extremely low shear rates e ˙ (on the order of 10 –6 s –1 ). Detailed data were obtained on the structural‐rheological properties of suspensions in the highest viscosity range characterizing the virtually unbroken structure of the system.

Finite-element analysis and experimental study on tensile fatigue behaviour of bitumen?stone adhesion

Tensile fatigue behaviours of bitumen-stone adhesion were investigated using a dynamic mechanics analyser under stress-controlled mode at two temperatures of 5 and 25°C and various controlled-stress levels. Failure characteristics including interfacial failure and cohesive failure were examined using image analysis of fracture surfaces. Finite-element analysis on stress distributions was conducted under different temperatures, film thickness and interfacial bonding conditions. A Coulomb-Mohr like criterion in combination with shear and normal stresses is proposed to deal with the extreme thin adhesive layer, which can be further simplified into an adhesive zone without significant loss of accuracy for stress analysis.

Instability of entangled polymers in cone and plate rheometry

Flow instability in three entangled polymer systems including a 10 wt% 1,4-polybutadiene (PBD) solution, an 11.4 wt% polyisobutylene (PIB) solution, and a long chain branched polyethylene melt (LD 146) was investigated in both stress-controlled and rate-controlled experiments in the cone–plate geometry. It was found that flow instability occurred for experiments in both rate- and stress-controlled modes. The effects of cone angle or rim gap and shearing time on flow instability were studied. The smaller cone angle and shorter shearing time delay (in terms of stress or shear rate) the occurrence of severe instability and mass loss of the PBD solution but not for the PIB. Our data are consistent with the dramatic shear rate jump for the flow curve constructed from the stress-controlled experiments being associated with mass loss after the severe instabilities. We also find that the Cox–Merz representation gives a powerful tool for investigation of flow instability. Finally, another interesting result in this work is that it seems that the stress overshoot can be related to the onset of flow instability in the present system.

Ratcheting behavior of an epoxy polymer and its effect on fatigue life

Ratcheting behavior of an epoxy resin and its effect on fatigue life has been experimentally investigated. All the fatigue tests were carried out under stress-controlled mode, and stress–strain data were recorded during the entire fatigue life period by a newly established non-contact real-time strain measurement and control system. Quantitative results of evolutions of ratcheting strain, strain range and strain energy density in tests with various mean stresses and stress amplitudes have been retrieved from the stress–strain data. The amount of ratcheting strain and accumulation rate was sensitive to the stress amplitude and mean stress. Fatigue lives of these tests were compared with previous test results under strain-range-controlled mode and no obvious ratcheting strain effect on fatigue life was observed for this epoxy polymer. Additional fatigue tests with resting periods were further performed to investigate the mechanism of the experimental observations on this epoxy polymer.

Damage accumulation model for monotonic and dynamic shear fracture of asphalt-stone adhesion

A damage accumulation model for shear fatigue of asphalt-stone adhesion was developed with the aid of experimental data obtained from monotonic and dynamic force-controlled tests. Constant static- and stress-rate monotonic tests and cyclic fatigue tests were performed on the special designed specimens of double-layered-sandwich using dynamic thermal mechanics analyzer. Experimental results showed that it was feasible to describe monotonic and cyclic tests of stress-controlled mode using one single damage accumulation model based on stress–time path, which makes it possible to reduce the number of specimens tested and time-consuming fatigue tests are no longer necessary, since the same information can be obtained from monotonic test data without loss of accuracy. It also found that the bonding fracture in shear mainly attributes to cohesive failure through binder interlayer due to extensive creep deformation.

Yield stress behavior for crude oil–polymer emulsions

Since all industrial applications of crude oil–polymer emulsions demand emulsions transportation, it is important to study the flow characteristic in term of apparent yield stress. The yield stress measurements of crude oil–polymer emulsions were carried out using RheoStress RS100 under controlled stress mode. Controlled stress rheometers provide the most direct technique for the measurement of yield stress. The yield stress measurements were carried out for crude oil–Alcoflood polymer emulsion over the range of 0–75% by volume of crude oil concentration and 0–10 4 ppm of polymer concentration. Three different Alcoflood polymers were employed in this investigation. These are AF1235, AF1275, and AF1285. The yield stress measurements of polymer aqueous solutions and crude oil–polymer emulsions are extensively investigated. Casson Model can be used to predict the apparent yield stress for either Alcoflood polymer aqueous solutions or crude oil–Alcoflood polymer emulsions. The increase of polymer concentration improves the initial flow resistance for the three tested polymer materials. At a polymer concentration beyond 10 3 ppm, the apparent yield stress raises strongly with polymer concentration. For polymer concentration below 10 3 ppm, the three polymer materials show close values of the apparent yield stress. For polymer concentration higher than 10 3 ppm, the polymer material of AF1275 causes higher apparent yield stress than AF1285 and AF1235 in order. Non-linear three dimensional model is provided to predict the apparent yield stress of crude oil–Alcoflood polymer for a wide range of crude oil and Alcoflood polymer concentrations.

Observations of instability, hysteresis, and oscillation in low-Reynolds-number flow past polymer gels.

We perform a set of experiments to study the nonlinear nature of an instability that arises in low-Reynolds-number flow past polymer gels. A layer of a viscous liquid is placed on a polydimethylsiloxane (PDMS) gel in a parallel-plate rheometer which is operated in stress-controlled mode. As the shear stress on the top plate increases, the apparent viscosity stays relatively constant until a transition stress where it sharply increases. If the stress is held at a level slightly above the transition stress, the apparent viscosity oscillates with time. If the stress is increased to a value above the transition stress and then decreased back to zero, the apparent viscosity shows hysteretic behavior. If the stress is instead decreased to a constant value and held there, the apparent viscosity is different from its pretransition value and exhibits sustained oscillations. This can happen even if the stress is held at values below the transition stress. Our observations suggest that the instability studied here is subcritical and leads to a flow that is oscillatory and far from viscometric. The phenomena reported here may be useful in applications such as microfluidics, membrane separations, and polymer processing. They may also provide insight into the rheological behavior of complex fluids that undergo flow-induced gelation.

Rheological Behavior of Greases: Part I—Effects of Composition and Structure

The aim of this work is to show the correlation between grease rheological behavior and its composition and structure (base oil viscosity, soap nature, and concentration). A rheological investigation was carried out under the controlled stress mode. The results, obtained over different time intervals, show that grease behaves like a non-Newtonian viscoelastic material, with a narrow linear domain. The soap nature and concentration are the dominant parameters when grease is submitted to low shearing. This was confirmed by TEM observations: the closer networks gave the highest rheological properties. Base oil viscosity influenced grease behavior only under high strain rates. Some properties are discussed with respect to their influence on the contact replenishment, which is a critical point for mechanism lifetime prediction. Finally, the authors suggest using rheometry as an alternative technique to standardized methods. Yield stress varies in the same way as penetration does, but is further influenced by the sample macrostructure. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in Orlando, Florida, October 11–13, 1999

Structural damage accumulation and stable postcritical deformation of composite materials

The macroscopic failure of composite materials is preceded by complex multilevel processes accompanied by accumulation and localization of damaged centers and formation of a failure cluster. Therefore, the study of these mechanisms is one of the basic problems for the mechanics of modern composite materials used in aerospace engineering. The formation of a theory of the stable postcritical deformation of the work-softening media is considered. The pseudo-plastic deformation affected by structural damage of granular composites is investigated within the framework of the considered two-level structurally phenomenological model of heterogeneous media. The stable evolution of the interconnected processes is accompanied by stress redistributions, partial or complete unloading, and strain or damage localization that are one of the main causes of implementation of the postcritical deformation stage. The numerical calculation results of inelastic deformation and failure of the periodic unidirectional fiber-reinforced composites are presented under conditions of the displacement-controlled transverse proportional loading mode. The main mechanisms of the work-softening behavior for the indicated type of materials are described in the macro-homogeneous stress-strain states. Macroscopically, the failure of heterogeneous media as a result of postcritical deformation and the loss of stability of damage accumulation depends on the stiffness of the loading system. When a deformable body is fixed on the closed surface with sufficiently but not infinitely large coefficients of stiffness, it is possible to observe the equilibrium development of the localized volumes of work-softening and damage. The constitutive equations for the work-softening isotropic, transverse isotropic, and orthotropic media are presented. The effect of the loading system on the stability of deformation, damage accumulation, and failure under monotone and nonmonotone triaxial loading was studied. The growth of failure strains with increase in stiffness of the loading system and unequal resistance of heterogeneous body are registered and investigated. A preventive unloading method is offered for the mathematical modeling of the damage accumulation during the testing of the materials on the servo-controlled systems. The displacement-controlled mode is simulated by a series of soft loading and unloading cycles. The detected phenomenon of failure where the unloading leads to stress-strain diagrams with a negative slope of the descending branch was not found either in the displacement or stress-controlled monotone loading mode.