High Consolidation Stresses Research Articles

Thermal rounding for shape modification of high-performance polyetherketoneketone and reinforced polyetherketoneketone-carbon fiber composite particles

This study presents shape transformation of anisotropic high-performance thermoplastic polyetherketoneketone (PEKK) and carbon fiber reinforced powder composite particles (HT-23) by thermal rounding. The shape transformation is achieved by (partial) melting of the high-temperature thermoplast microparticles. Three different process setups are presented, investigating the impact of the source of heat supply on the resulting shape modification: using a directly heated sheath gas flow, an indirect heat supply through the reactor wall and a combined approach. Regardless of the chosen setup, a modification of the particle shape was observable. The most advantageous shape transformation was observed in the indirect heating approach. In addition, the enhanced shape transformation yields an improved free flow behaviour of the powders, as quantified by ring-shear experiments. Reductions of the unconfined yield strengths of the powders for high consolidation stresses as high as 18 percent for PEKK and 30 percent for the HT-23 are achieved. Thereby, processability of the powder in laser based powder bed fusion is enhanced, extending the range of available (composite) polymer materials.

Experimental Investigation on the Strength and Deformation Aspects of Thixotropic Aging in Reconstituted Clays

This study is aimed at comprehending the effect of thixotropic aging on strength and deformation response of natural clays reconstituted in the laboratory. Three clays have been collected and samples were prepared at a moisture content of corresponding liquid limits. Prepared soil specimens were subjected to a thixotropic aging (constant moisture, constant volume and constant temperature conditions) period of 7, 14, 21, 28, 35 and 42 days. Unconfined compressive strength and direct shear tests were performed to assess thixotropic strength recovery. Thereafter, one-dimensional consolidation tests were conducted on the thixotropically aged samples to assess the compressibility characteristics. Interpretation of the responses from unconfined compression test, direct shear test coupled with Scanning Electron Microscopy imaging analyses suggest that thixotropic strength recovery is dependent on activity, percentage of clay fraction, and initial grain structure. Thixotropic strength ratio obtained from unconfined compression test and direct shear test are noticed to be different, although the trend is similar. Interpretation of 1-D consolidation test data suggest that as the thixotropic aging period increased, samples tend to provide higher yield stress compared to the unaged samples. All the void ratio-effective vertical stress responses for the clays at different thixotropic aging periods tend to converge at a same void ratio at a consolidation stress of > 1000 kPa, indicating no effect of thixotropic structuration at high consolidation stresses. In addition, it was observed that thixotropically active clays swell more than the inactive clays during the thixotropic aging period. In addition, a proportional relationship between thixotropic strength ratio and plasticity index was also noticed.

Experimental investigation of both the disturbed and undisturbed granitic saprolite soil

Granitic saprolites are abundant in many regions around the world and they are widely employed in various engineering projects. Behavior of such kinds of soils is not sufficiently understood and our knowledge about the fundamental behavior of saprolites is to a far less extent than the sedimentary soils. Owing to the limited knowledge, many problems occurred in the engineering applications that used saprolites as a construction material. This study presents results of a set of conventional drained and undrained triaxial compression shearing tests on the granitic saprolites in both the disturbed and undisturbed conditions. This study showed that the soil fabric was the paramount factor governing on the soil stress–strain response rather than the soil bonds. Under the drained shearing condition, the undisturbed soil revealed a strain-softening behavior with a dilative response at the low consolidation stress level, while a contractive response was present at the high consolidation stress levels. However, the disturbed soil showed a strain-hardening behavior with a contractive response, regardless of the consolidation stress levels during the drained shearing. The undrained response was complicated, as the soil showed both the contractive and dilative responses with phase transformation points, regardless of the soil condition (i.e., disturbed or undisturbed) and the consolidation stress levels. The bond effect of the intact soil persisted during the consolidation stage; however, its effect did not extend to the large strain-controlled shearing stage, where the tangential stiffness suddenly decreased at the axial strain of 1.5%.

Dynamic behaviour and damage characteristics of loess in Xianyang, China

In engineering practice, the in situ soil layer is under different static pressures and experiences different types of dynamic loads. To determine the influence of stress state and dynamic load type on the dynamic characteristics of, and damage evolution in, saturated loess, consolidated-undrained dynamic triaxial tests were conducted on samples subjected to different confining pressures and consolidation stress ratios. During the test, rectangular wave and sine wave were selected as types of applied dynamic load. Results show that samples with good consolidation effect, and loaded by the sine wave, exhibit relatively high strength. The deformation of the samples is influenced by their initial consolidation state and the value of dynamic stress: the better the consolidation effect of samples and the lower the applied dynamic stress, the smaller the strain difference therein; with the increase of dynamic stress, greater strain occurs in samples loaded by a rectangular wave. The influence of waveforms on the initial dynamic elastic modulus changed with the stress state: this was significantly affected by application of sine wave loading (low confining pressure, low consolidation stress ratio), significantly affected by application of rectangular wave loading (low confining pressure, high consolidation stress ratio), and slightly affected by waveforms (high confining pressure). The instantaneous application of rectangular wave load can cause consolidated samples to be strengthened in the early test stage, while damage developed rapidly with the proceeding of loading process.

A new split cell for the measurement of the horizontal to vertical stress ratio of powders

According to the standardized procedures for silo design, the horizontal to vertical stress ratio is a key parameter for the correct estimate of stresses acting on silo walls. To date no standardized equipment is available for the measurement of such ratio. In this paper the state of the art is presented and discussed. A new split cell for the measurement of the horizontal to vertical stress ratio was built. It is a significant improvement of another apparatus, previously developed by the same authors, with the aim of simplifying as much as possible the design of the cell and the procedure to be applied to carry out the measurement. This new version of the cell can overcome the effects of the friction on the cell lid and on the cell bottom, without the use of the twisting action that was necessary in the former cell. The new cell was tested with three different non-compressible powders and three different compressible powders in wide range of vertical stresses between 1 and 16 kPa. Results indicate that the cell can provide repeatable and consistent results with standard deviations within few points percent for vertical stresses in the most usual range between 3 and 16 kPa. Larger standard deviations of results are found at lower stresses, in particular with the most free-flowing materials, but measurements still provide internal consistency of results. Experiments show no significant effect of the twisting action on the lid at high consolidation stresses, while some effect of the twisting is visible a low powder consolidation levels. Results of the vertical to horizontal stress ratio are compared with the most used theoretical and empirical equations to relate the ratio to the angle of internal friction. It is shown that incompressible materials are closer to the internal state of incipient shear during the tests and that, consistently, the theoretical expression relating the stress ratio to the effective angle of internal friction provides satisfactory results. Instead, compressible powders are apparently away from a condition of internal shear and, therefore, empirical and more conservative expression for the stress ratio provide better predictions.

A modified flowability classification model for moist and cohesive bulk solids

The Jenike flowability classification is designed to indicate the handleability of bulk solids in material handling processes. However, for moist, cohesive bulk solids, it may fail to represent the true flowability/handleability at high consolidation stresses. The study presented in this paper introduces a modified flowability classification model aiming to improve the flowability/handleability prediction across a wide range of consolidation pressures. The evolution of the flow function of bulk materials under increasing moistures is initially discussed, based on which the Jenike flowability classification is reviewed. The modified flowability classification is then derived by considering the flow/no flow condition for mass flow hopper designs under both low and high consolidation stress states. The influence of the bulk material type and general operational conditions are also taken in account in deriving the modified classification method. A suite of bulk solids with a wide range of material properties are selected in an experimental program to validate the modified flowability classification. Results suggested the modified flowability classification is able to more accurately indicate the overall flowability/handleability of the bulk solids independent of the applied major consolidation stress.

An experimental study of high strain-rate properties of clay under high consolidation stress

A split Hopkinson pressure bar (SHPB) combined with high pressure consolidation apparatus and high speed camera was used to obtain dynamic compressive stress-strain curves of clay whose over consolidation stress state in the process of formation was properly considered. The stress-strain relationship at various high strain rates from 60s−1 to 600s−1 was obtained. The strain rate and degree of consolidation effects on the compressive response of the consolidated clay were determined. The results show that the dynamic mechanical properties of clay in high pressure consolidation is sensitive to strain rate, and parameters like dynamic strength, failure strain and so on are significantly improved compared with unconsolidated clay which indicate that the initial stress history of the soil materials is also one of the most important factors that affect the dynamic mechanical response.

On the normalized behavior of naturally and artificially structured clays

The influence of soil structure on the volume change behavior of clay soils has been often studied with reference to intrinsic properties and normalization of reconstituted clays. However, to date, there has been relatively little research conducted on the normalizing behavior of naturally and artificially structured clays. In this paper, the yielding modes and normalization, by introducing the void index, are clarified for the one-dimensional compression of natural Finnish clays. Based on 24 samples with different cement contents and curing times, two novel approaches are proposed to normalize the post-yielding range of artificially structured clays. Laboratory results show that approach I, i.e., linear extrapolation of the post-yielding line in a log-log graph, can normalize well the compression behavior of artificially structured clays. However, the normalized curves of the void index vs. the effective stress in the semi-log plot deviate from Burland's intrinsic compression line (ICL) at very high consolidation stresses. To address this issue, a novel formulation of the ICL is deduced from laboratory data for artificially structured clays. Approach II, i.e., redefinition of the constants of compressibility to modify the void index, results in closer convergence of the normalized compression curves. The normalized curves by the modified void index agree well with the proposed intrinsic compression line for artificially structured clays (SICL). Finally, the generalization of Burland's ICL can redefine the intrinsic formulations corresponding to different levels of the yielding stress by modifying the void index.

Cyclic behaviour of loose anisotropically consolidated Ottawa sand under undrained torsional loading

The behaviour of loose, anisotropically consolidated Ottawa sand was examined under undrained cyclic loading in the hollow cylinder apparatus. The stress level, the cyclic stress level and the consolidation stress ratio were varied. The instability line of anisotropically consolidated loose Ottawa sand, defined under monotonic loading, is shown to form a boundary in the vicinity of which a sudden increase in the rate of excess pore water pressure and strain accumulation is observed, and the sand strain-softens. However, this unstable response is arrested at phase transformation, and the sand specimens show a stable or temporarily stable effective stress cycle at low and high consolidation stress ratios respectively. Similar behaviour is observed for isotropically consolidated sand, which exhibits initial liquefaction after strain-softening. The terminal excess pore water pressure associated with this final stage is described as a function of consolidation stress ratio, while the development of excess pore water pressure during cyclic loading is uniquely defined for all tests as a function of normalised shear work. The cyclic resistance of the sand decreases with decreasing consolidation stress ratio. Anisotropic consolidation significantly affects the stiffness and damping ratio values of loose Ottawa sand.

Behavior of Clay-Tire Mixtures Subjected to Undrained Monotonic Loading

Soil - tire mixtures have been recently used as construction materials in civil engineering projects. In this research, a number of undrained triaxial tests were carried out on the compacted clay-tire mixtures and their behaviors were compared with behavior of pure clays. The results of the tests indicate that adding more than about 20%-30% tire to the low plasticity clay doesn’t reduce the shear strength in comparison with the associated values of pure clay. In the mixtures made of high plasticity clay, the shear strength decreases by increasing tire content. Moreover, friction angle and cohesion values vary with increasing tire-chips, which is completely dependent on the clay plasticity. The results also demonstrated that, for the mixtures of low plasticity clay, maximum excess pore water pressure occurs in the specimens with tire content of about 10%-20%. For the mixtures consist of high plasticity clay, at low consolidation stress, the pore water pressure decreases slightly with an increase in tire content. At high consolidation stress, excess pore water pressure induced within these mixtures increases slightly with an increase in tire-chips content. Adding tire-chips to the clay also lead to increase deformability of mixtures considerably.

Role of consolidation state in the measurement of bulk density and cohesion

Characterization of powder consolidation and flow is important to a large number of industries that process solids. A variety of powder testers exist today, however most are application specific and results they provide are mainly qualitative. Comparison studies have been attempted previously; their main focus was whether various methods correlate, while the reasons for disagreement in the results have seldom been investigated. Among the different methods used to characterize powder flow behavior, density-based methods are some of the simplest and most popular. They are also some of the least sensitive and least reproducible. A method comparison study was performed to characterize the variability in bulk density as measured by various powder flow characterization methods. It was found that the density of the powders tested in most unconsolidated methods had a near-perfect linear correlation with one another, suggesting that there exists a critical “dilated” density value that is independent of the measurement method and can be considered a material property. The densities of the powders in consolidated states correlated poorly, especially for the methods where consolidation mechanisms were not tightly controlled (i.e. tapping). In addition, the effect of the consolidation state of the powder bed during testing on cohesion measurements was studied. The cohesion measured by shear cells at differing consolidation states and by an avalanching method, during which the powder bed is dilated, were compared. It was found that the differences in cohesion results from the shear cells tested can be attributed to differences in the powder consolidation state. Cohesion results acquired from a dilated powder bed correlate more linearly with the shear cell cohesion results at low consolidation stresses than at high consolidation stresses; further supporting the impact of the powder bed packing state on the measurement of cohesion.

Probing the nature of the contact between fine particles by using ultrasound propagation

The propagation velocity ( v s ) of an ultrasonic signal through a granular material depends on the type of interparticle contact. For noncohesive glass beads, a power law behavior v s ∝ σ c 1 / 6 for consolidation stresses applied ( σ c ) above 1 MPa has been measured in previous work. This equation is compatible with Hertz's interaction law between elastic solids. In the present work, we have tested the propagation velocity of ultrasound signals through a sample of fine powder. The tensile strength and compactivity of the powder were previously measured by means of the Seville powder tester (SPT), indicating plastic deformation of the surface asperities in contact for small to moderate consolidation stresses. However, the measurements of ultrasound propagation at high consolidations presented here are compatible with Hertz's law. This finding suggests that for high consolidation stresses, surface asperities are flattened, and it is therefore the elastic deformation of the bulk of the particles that determines the transmission of ultrasonic pulses.

Effect of compaction history on the fluidization behavior of fine cohesive powders

Fine particles agglomerate in the fluidized state due to the strength of interparticle attractive forces as compared to particle weight. Interparticle adhesion can be largely increased by consolidation stresses applied during powder handling. As a consequence, fragments of the consolidated powder may persist when the powder is fluidized, which gives rise to large agglomerates of strongly adhered particles in fluidization. This history-dependent effect can be minimized by coating the particles with surface additives such as silica nanoparticles. In this paper, we investigate the effect of high consolidation stresses sigma(c) previously applied to samples of silica-coated fine particles on their fluidization behavior. Our experimental measurements show that, even though homogeneous fluidization is still observed, the average agglomerate size and fractal dimension of the agglomerates increase as sigma(c) is increased. Bed expansion in the fluidized state is hindered by previously applied high consolidations, which we attribute to an increase of the largest stable size of mesoscopic fluid pockets. As a consequence, we observe that the initiation of macroscopic bubbling is delayed up to larger values of the fluid velocity.

The tensile strength and free volume of cohesive powders compressed by gas flow

We report measurements of tensile strength and average free volume for a set of cohesive powders as a function of consolidation stress. Powders with average particle size between 7.8 and 19.2 μm were made from a styrene/butadiene copolymer, and were subsequently surface-treated with different concentrations of a submicron fumed silica. This silica acts as a flow control additive by controlling interparticle forces. The measurement technique consists of initialization of the sample by fluidization and subsequent consolidation by compression under a given gas flow while continuously monitoring the sample volume. By reversing the gas flow, a tensile stress is applied to the sample. For each consolidation state, we determine the tensile strength and the average free volume of the powder. We find that the relation between the free volume and the consolidation stress follows a logarithmic form. The magnitude of the interparticle forces is estimated from bulk measurements. At high consolidation stresses, the average tensile force per contact increases proportionally to the square root of the consolidation force per contact. Physical implications of these results are discussed.

State Diagram of fine cohesive powders at zero shear

AbstractWe present measurements of the tensile strength for a set of model toners, with a typical diameter of 10 microns, as a function of the consolidation stress and void fraction. The technique used to change the consolidation state is to centrifuge the powder. For each consolidation state the tensile strength is measured by subjecting the powder to a tensile stress by means of a direct gas flow. The average free volume is calculated by measuring the powder bed height by means of an ultrasonic device. All these processes allow us to draw the State Diagram of the powder at zero shear. The influence of the hardness, and size of the asperities, both controlled by flow control additives, as well as the influence of the radius of the particle, is determined, We find that, contrary to the case of low consolidations where the tensile strength was a linear function of the consolidation stress, at high consolidation stresses the tensile strength is a sublinear function of the compressive stresses.

Fabric evolution during the deformation of chalk

The consolidation and shear deformation behaviour of a single outcrop chalk is described in phenomenological terms based on the interpretation of laboratory deformation experiments. Studies of the fabrics of the deformed materials are then used to validate the interpretations. Chalk behaviour is strongly influenced by the progressive destruction of its intact fabric: showing at shear failure, a transition from discrete fractures at low consolidation stres ses where the fabric remains intact, to a pervasive cataclasis at high consolidation stresses where the fabric has become substantially disaggregated. This bulk response to deformation can be described using a rheological framework based on conventional soil mechanics.

One-dimensional consolidation curves of a medium sand

The note sets out a method for the reliable measurement of sand compressibility. The sand is consolidated under ko condition in a triaxial apparatus. A specimen of 100 mm diameter is used. This overcomes significant errors of measurement associated with apparatus compression and specimen preparation in the standard oedometer test. One dimensional consolidation curves are obtained for brasted sand at various densities from a series of triaxial extension tests by applying exceptionally high consolidation stresses to the specimen. It is concluded that since the gradation of brasted sand is fairly typical of a new sand, the curves can be used as a first estimate of the probable consolidation properties of similar sand deposits. /TRRL/