Elastic Load Transfer Research Articles

Load transfer model for vertically loaded non-circular piles

Load transfer model for vertically loaded non-circular piles

Size effect on elastic stress concentrations in unidirectional fiber reinforced soft composites

We revisit the classic problem of determining stress concentrations on neighboring fibers to multiple, transversely-aligned fiber breaks in a planar, unidirectional fiber–matrix composite. Fibers are assumed to be perfectly bonded to the elastic matrix. Finite size effects on stress concentration are studied by varying the overall length of the composite relative to the characteristic load transfer length between broken and intact fibers. As an alternative to the discrete fiber and matrix framework in the classic analysis of Hedgepeth, and its extension by Hikami and Chou, the fiber stress distribution in the composite is obtained through continuum modeling of the composite as a highly anisotropic elastic plate, whereby the stresses and stress concentration factors at fiber locations in the discrete model are extracted in a closed form. For composites of finite length, the stress concentration factors determined using the continuum model compare favorably with numerical solution of the discrete shear-lag model. In the limit of a plate with infinitely long fibers, our stress concentration factors also agree well with the exact results of Hikami and Chou. For composites having a length less than the characteristic elastic load transfer length, and loaded under displacement boundary conditions, we show that local stress concentrations vanish irrespective of the size of the crack or the number of fiber breaks. This behavior becomes important when modeling and interpreting laboratory experiments on the mechanical behavior of recent soft composite specimens consisting of stiff fibers in an extremely compliant elastic matrix.

Effect of grout strength on the stress distribution (tensile) of fully-grouted rockbolts

Abstract The paper deals with and axial stress distribution (ASD) obtained from pull-out tests of strain gauged rockbolts in high strength rock media. For this purpose, an experimental research was conducted considering the only axial forces acting on fully grouted rockbolts (FGR). Strain gauges were used to observe the behavior of rockbolts and to determine the load transfer mechanism of the bolts to rock mass. In the test, five grout mixture having different properties was prepared and stress–strain mechanism of the bolt-grout interface was investigated for these conditions. ASD was demonstrated elastic load transfer behavior from the rockbolt to the outlying rock under the applied load (for each grout type) and stress concentration was rather a high level at 70 mm distance from rock surface. Axial stress curves drawing with the aid of strain readings were showed the regular distribution with increasing of grout stiffness. The test results of strain gauged rockbolts show that the shear stress distribution (SSD) is uniform and, the style of the distributions along the rockbolt was similar under various loading levels for all grout types. Shear stress was rather a small amount at the pulling end of rockbolt but it reached the peak after a short distance away from the pulling end of rockbolt and subsequent to peak value decayed rapidly with the load increase. The results were showed that strength properties of grout played an important role in SSD and ASD along the rockbolt. The increase in grout strength and rigidity made the stress distributions regular along the rockbolt.

Behaviour of piles in consolidating soil

ABSTRACTThe development of negative skin friction on circular piles in a consolidating layered soil is investigated by an elasto-plastic load transfer theory which accounts for slippage of the soil. The elastic load transfer theory is premised on the compatibility condition that the vertical displacement of the ‘pile’ is equal to the summation of the vertical displacement of the layered soil due to the consolidation of the upper soil layer and the vertical displacement in the ‘soil layers’ along the pile’s centroidal axis caused by a system of pile-soil interactive forces. Slippage of the soil is accounted for by imposing the shear strength of the clay layer as the limit of the pile-soil interface shear stress. The saturated upper clay is consolidating under a uniform surcharge in accordance with Terzaghi’s one-dimensional consolidation theory. The validity of the proposed solution is confirmed by comparison with field measurements. Extensive parametric studies with regard to the effect of pile-soil slip on pile behaviour are presented.

Deformation behaviour of an advanced nickel-based superalloy studied by neutron diffraction and electron microscopy

Deformation mechanisms under tensile loading at room temperature have been studied in a polycrystalline nickel-based superalloy containing close to 50 vol.% γ′. In order to identify the effect of γ′ particle size on deformation mechanisms, model microstructures with unimodal γ′ size distributions were developed. The investigations were carried out by combining in situ loading experiments using neutron diffraction and two-site elasto-plastic self-consistent plasticity modelling with detailed post-mortem electron microscopy. The microscopy work also includes results for samples strained at 500 °C. During early plastic deformation, the diffraction data demonstrate that γ and γ′ display the same elastic strain response, indicating that at this stage γ′ is cut by dislocations regardless of the γ′ particle size. Scanning electron microscopy studies showed an abundance of shearing processes in all three microstructures, hence supporting the conclusions drawn from the diffraction experiment. As the material is further deformed, elastic load transfer from γ to γ′ was observed in the medium (130 nm) and coarse (230 nm) γ′ microstructures but not in the fine (90 nm) γ′ microstructure. The load transfer can be explained by assuming that Orowan looping becomes an additional operative deformation mode. Transmission electron microscopy confirmed that in the fine γ′ microstructure deformation takes place by strongly coupled dislocations cutting the γ′, while the medium and coarse γ′ microstructures showed additional signs of Orowan looping.

Electrical and Rheological Percolation in Polystyrene/MWCNT Nanocomposites

A systematic electrical and rheological characterization of percolation in commercial polydisperse polystyrene (PS) nanocomposites containing multiwall carbon nanotubes (MWCNTs) is presented. The MWCNTs confer appreciable electrical conductivities (up to ca. 1 S/m) to these nanocomposites at a concentration of 8 vol %. In addition to enhancing the electrical properties, even at small concentrations (ca. 2 vol %), MWCNTs significantly enhance the rheological properties of PS melts. At concentrations exceeding 2 vol %, a plateau appears in the storage modulus G‘ at low frequencies, indicating the formation of a percolated MWCNT network that responds elastically over long timescales. Network formation, in turn, implies a diverging complex viscosity vs complex modulus curve. A focus of this study is on the correlation between electrical and rheological properties at the onset of percolation. The experimental results indicate that the elastic load transfer and electrical conductivity are far more sensitive to ...

Micromechanics of fibre fragmentation in model epoxy composites reinforced with α-alumina fibres

The micromechanics of fibre fragmentation in α-alumina/epoxy model composites have been investigated. Both sized and desized Nextel 610 α-alumina fibres embedded in room- and high-temperature (80°C) cured epoxy resin matrices have been used. The technique of luminescence spectroscopy has been used to map the strain along the fibres during tensile loading of the matrices, and the distribution of interfacial shear stress has been derived by using a force-balance consideration. The experimental data were modelled with shear-lag analyses that account for both the elastic load transfer and friction at a debonded interface. The effects of fibre sizing, matrix curing temperature and fibre surface roughness upon the interfacial shear strength have also been determined.

In situ phase strain monitoring during isothermal creep of metal matrix composites

Abstract Phase strains were measured during constant load isothermal creep tests of Al/SiC p/w composites using neutron Bragg diffraction. The results were interpreted using an Eshelby-type model allowing for inelastic and elastic load transfer contributions. The experimental results show that factors increasing the matrix constraint gave improved load transfer to the stiffer reinforcement, and that for some experimental conditions, the benefit of the reinforcement presence was completely lost.

NiTi and NiTi-TiC composites: Part IV. Neutron diffraction study of twinning and shape-memory recovery

Neutron diffraction measurements of internal elastic strains and crystallographic orientation were performed during compressive deformation of martensitic NiTi containing 0 vol pct and 20 vol pct TiC particles. For bulk NiTi, some twinning takes place upon initial loading below the apparent yield stress, resulting in a low apparent Young's modulus; for reinforced NiTi, the elastic mismatch from the stiff particles enhances this effect. However, elastic load transfer between matrix and reinforcement takes place above and below the composite apparent yield stress, in good agreement with continuum mechanics predictions. Macroscopic plastic deformation occurs by matrix twinning, whereby (1 0 0) planes tend to align perpendicular to the stress axis. The elastic TiC particles do not alter the overall twinning behavior, indicating that the mismatch stresses associated with NiTi plastic deformation are fully relaxed by localized twinning at the interface between the matrix and the reinforcement. For both bulk and reinforced NiTi, partial reverse twinning takes place upon unloading, as indicated by a Bauschinger effect followed by rubberlike behavior, resulting in very low residual stresses in the unloaded condition. Shape-memory heat treatment leads to further recovery of the preferred orientation and very low residual stresses, as a result of self-accommodation during the phase transformations. It is concluded that, except for elastic load transfer, the thermal, transformation, and plastic mismatches resulting from the TiC particles are efficiently canceled by matrix twinning, in contrast to metal matrix composites deforming by slip.

A new data reduction scheme for the fragmentation testing of polymer composites

A new reduction scheme of fragmentation data for the derivation of interfacial mechanical properties in polymer composites is proposed. The scheme is based on a theoretical model that accounts for elastic load transfer and friction at the interface, as well as for the statistical nature of fibre strength. Interface mechanical behaviour is characterized by two independent parameters, namely the interface bond strength and interface frictional resistance. Derived values of the two interface properties are computed, such that they yield the best possible agreement between experimental and theoretical results for the evolution of fibre fragment aspect ratio and debonding ratio as a function of applied strain. Results are reported for carbon fibres embedded in an epoxy matrix, with different levels of fibre surface treatment.

Elastic load transfer from partially embedded axially loaded fibre to matrix

Elastic load transfer from partially embedded axially loaded fibre to matrix

Axially Loaded Piles in Layered Soil

The behavior of axially loaded piles in layered soil is investigated in terms of effective stresses, using a rigorous elastic load transfer theory. Following the technique of Muki and Sternberg, the problem is decomposed into extended soil layers with elastic properties of the layered soil and a fictitious pile characterized by Young's moduli equal to the difference between the Young's modulus of the real pile and the respective Young's moduli of the soil layers. The unknown fictitious pile force and the fictitious pile top settlement are determined by equating the vertical displacement of the fictitious pile to the vertical displacement caused by a system of interactive forces in the extended soil layers along the centroidal axes of the original pile location. The real pile force and displacement can then be calculated once the fictitious pile force is known. Extensive parametric studies with regard to the base load, bearing layer load, and pile top settlement are presented graphically for the design and analysis of axially loaded piles in layered soil.