Deflection (engineering)

About: Deflection (engineering) is a research topic. Over the lifetime, 30862 publications have been published within this topic receiving 298849 citations.

Asphalt pavement design--the shell method

Abstract: The method is based on a model in which the pavement structure is regarded as a linear elastic multi-layered system in which the materials are characterised by their modulus of elasticity and Poisson's ratio. The computer program BISAR is used to compute all stresses, strains and displacements at any point in the system under any number of vertical and/or horizontal surface loads. In this way, the primary design criteria have been established, i.e. the compressive strain at the top of the subgrade and the horizontal tensile strain in the asphalt. Secondary criteria such as permissible stresses in cementitious base layers, permanent deformation of the asphalt, etc., are also included. The permissible value for compressive subgrade strain has been derived from analysis of AASHO Road Test sections and structures conforming to CBR design. The permissible asphalt strain was determined from extensive laboratory measurements for various mix types at different stiffness moduli of the asphalt. In the application of the asphalt fatigue criterion allowance is made for the influence of the transverse distribution of wheel loads and for effects of healing and intermittent loading. The traffic data are converted into an equivalent number of standard design axle load applications. To introduce the influence of the ambient temperature a procedure has been developed to relate the mean annual or monthly air temperature to an effective asphalt temperature, depending on the thickness of the asphalt. The moduli of subgrade and unbound base layers should be determined at appropriate stress levels whereby the latter modulus is a function of the subgrade modulus. It is demonstrated that the modulus of a given mix, relevant for the structural design, can also be derived with sufficient accuracy using a nomograph to provide a practical system for road engineers, sets of design charts have been prepared from which combinations of thicknesses of the asphalt and unbound base layers can be derived for various mean annual temperatures, for a number of typical mixes and for various sugbrade moduli. Special attention is paid to a method of predicting the permanent deformation (rut depth) of the asphalt layers during the expected service life of the pavement. The application of the design method to the design of pavements for aircraft with multiple wheels is also dealt with the various laboratory tests and full scale road trials carried out to investigate the validity of the design method have been summarised. The practical use of the design method is illustrated by means of some examples. /Author/

Assessing micromechanical properties of cells with atomic force microscopy: importance of the contact point.

Abstract: Mechanical properties are obtainable from atomic force microscopy (AFM) indentation force-depth curves, which are calculated from relationships between tip deflection and cantilever position, i.e. deflection curves. Indentation depth is the difference between tip deflections on a rigid and a soft material for the same amount of cantilever advancement, after contact is made. Since the contact point cannot be unequivocally identified from experimental data, there is some uncertainty in estimating material properties. Using simulations, this study examines some important issues related to the influence of contact point identification on estimated material properties. Simulations for linear materials using a typical stiffness for an AFM cantilever demonstrate that certain portions of the post-contact region of deflection curves for soft and very stiff materials can be approximated by quadratic and linear functions, respectively. Based on these findings, we first develop and verify an objective, automatic method to identify the contact point for materials with linear properties. We then assess the effect of misidentifying the contact point, with and without noise. If the contact point is missed by 100 nm, however, the true material properties cannot be estimated accurately. Noise adds to uncertainty in material properties at small indentations but the combined effect of missing the contact point and noise is dominated by the former. Even though the algorithm was developed for linear materials, it is also suitable for certain nonlinear materials making it more generally applicable.

Modelling analysis of surface stress on a rectangular cantilever beam

Abstract: Three models of surface stress on a rectangular cantilever beam are presented. The surface stress is modelled as a corresponding concentrated moment at the beam free end, a corresponding concentrated moment plus a corresponding concentrated axial load at the beam free end, and a corresponding uniformly distributed axial stress plus bending moment per unit length along the beam span, respectively. The results of the three models are compared under three different loading scenarios. We also present an analysis of the error source, when using Stoney’s formula to predict the surface stress, by comparing the kinematic and loading assumptions of the three models. The surface stress effects on structure deflection are usually modelled as bending moments applied at structure free edge(s)/end(s). Modelling the surface stress effect along the beam neutral axis is presented and compared with modelling its effect at free edge(s)/end(s). The stiffening effect of tensile surface stress is also studied.