S. Swaddiwudhipong

Uniqueness of reverse analysis from conical indentation tests

The curvature of the loading curve, the initial slope of the unloading curve, and the ratio of the residual depth to maximum indentation depth are three main quantitiesthat can be established from an indentation load-displacement curve. A relationship among these three quantities was analytically derived. This relationship is valid for elasto-plastic material with power law strain hardening and indented by conical indenters of any geometry. The validity of this relationship is numerically verified through large strain, large deformation finite element analyses. The existence of an intrinsic relationship among the three quantities implies that only two independent quantities can be obtained from the load-displacement curve of a single conical indenter. The reverse analysis of a single load-displacement curve will yield non-unique combinations of elasto-plastic material properties due to the availability of only two independent quantities to solve for the three unknown material properties.

Direct tension test and tensile strain capacity of concrete at early age

The tensile strain capacity of concrete under uniaxial tension is investigated using the direct tension test method. The adopted method of testing improves the weak bond strength between the embedded bar and concrete and reduces the stress concentration at the end of the embedded bar. The method has overcome the difficulties in centralizing and aligning the two embedded bars in the specimens. Seven mixes of concrete were designed to study the effects of age, compressive strength and mineral admixture on the tensile strain capacity. The investigation shows that the tensile strain capacity of concrete is a relatively independent parameter. The average tensile strains at failure and at 90% failure load are 120 and 100 μe, respectively. The corresponding characteristic tensile strain values at failure and at 90% failure load are 86 and 78 μe, respectively.

ANALYTICAL SOLUTIONS OF POLYMERIC GEL STRUCTURES UNDER BUCKLING AND WRINKLE

One of the unique properties of polymeric gel is that the volume and shape of gel can dramatically change even at mild variation of external stimuli. Though a variety of instability patterns of slender and thin film gel structures due to swelling have been observed in various experimental studies, many are not well understood. This paper presents the analytical solutions of swelling-induced instability of various slender and thin film gel structures. We have adopted the well developed constitutive relation of inhomogeneous field theory of a polymeric network in equilibrium with a solvent and mechanical load or constraint with the incremental modulus concept for slender beam and thin film gel structures. The formulas of buckling and wrinkle conditions and critical stress values are derived for slender beam and thin film gel structures under swelling-induced instability using nonlinear buckling theories of beam and thin film structures. For slender beam structure, we construct the stability diagram with the...

Eulerian Finite-Element Technique for Analysis of Jack-Up Spudcan Penetration

The numerical analysis of an object penetrating deep into the seabed is a fundamentally challenging problem. This paper presents the application of a novel Eulerian-based finite-element technique to simulate the continuous penetration of a jack-up spudcan foundation into seabed of different soil profiles. The finite-element mesh is kept stationary throughout the analysis and the material is allowed to move independent of the element nodal points. Consequently, termination of computing execution from severe mesh distortion does not occur despite the material undergoing large deformation. The first part of the paper elucidates the mesh density requirement, the effect of penetration rates, and factors influencing the simulation time. The applicability of the Eulerian finite-element model is then validated through comparison with published experimental data for different soil profiles. In general, the Eulerian finite-element model is able to replicate the experimental observations well. With the Eulerian appr...

Elastic buckling analysis of ring-stiffened cylindrical shells under general pressure loading via the Ritz method

This paper presents the Ritz method for the elastic buckling analysis of shells with ring-stiffeners under general pressure loading. The stiffeners may be of any cross-sectional shape and arbitrarily distributed along the shell length. Using polynomial functions multiplied by boundary equations raised to appropriate powers as the Ritz functions, the method can accommodate any combination of end conditions. As far as it is known, the Ritz method has not been automated in this way for the buckling of ring-stiffened shells. By formulating in a nondimensional form, generic buckling solutions for shells with various end conditions, stiffener distributions and under various pressure distributions, were presented. These new buckling solutions should serve as useful reference sources for checking the validity and accuracy of other numerical methods and software for buckling of cylindrical shells. This paper also shows that the appropriate distribution of ring stiffeners can lead to a significant increase in the buckling capacity over that of a stiffened shell with evenly spaced and identical ring stiffeners.

Pattern formation in plants via instability theory of hydrogels

In this paper, we demonstrate how deformation patterns of leaves and fruits in growing and drying processes can be described via the inhomogeneous field theory. The distorted deformation of ribbed leaves and the ridge formation on fruit surfaces can be understood as the energy-minimizing mechanical buckling patterns. The swelling and de-swelling induced instabilities of various membrane structures or elastic sheets on elastic or gel-like substrates are simulated using the inhomogeneous field theory of a polymeric network in equilibrium with solvent and mechanical constraints. The article describes briefly the inhomogeneous field theory of hydrogel deformation and the buckling patterns of thin hydrogel films on thick substrates. The theory is then adopted to simulate the growth and drying processes of leaves and fruits through the buckling phenomena observed in the film gel of various shapes, geometric proportions, chemical potentials and mechanical constraints. The key idea is to show that the hydrogel deformation theory can capture the deformation process and various states of plant growth or drying. The study has been made in an attempt to mimic the shapes of fruits and leaves from the swelling/deswelling patterns of hydrogel films. The study provides the possibility of exploring the origin of the intriguing natural phenomena of leaves and fruits.

Equivalency of Berkovich and conical load-indentation curves

The Berkovich indenter, which is one of the most commonly used indenter tips in instrumented indentation experiments, requires a tedious 3D finite element simulation. The indenter is widely idealized as a conical indenter of 70.3° half-angle to enable a substantially less demanding 2D axisymmetric modelling. Although the approach has been commonly adopted, limited studies have been performed to investigate possible deviations due to this simplification. The present study attempts to address the equivalency of the two indenters by performing extensively both 3D and 2D finite element analyses to simulate the load-displacement response of a wide range of elasto-plastic materials obeying power law strain-hardening during indentation for both Berkovich and conical indenters, respectively. It is demonstrated that the equivalency between these two indenters in terms of curvature of the loading curve is not valid across the range of material properties under study. However, it is established that if only the ratio of the remaining work done (WR) and the total work done (WT) of the load-indentation curve is of interest, this simplification can be adopted with satisfactory results.

High Velocity Penetration/Perforation Using Coupled Smooth Particle Hydrodynamics-Finite Element Method

Finite element method (FEM) suffers from a serious mesh distortion problem when used for high velocity impact analyses. The smooth particle hydrodynamics (SPH) method is appropriate for this class of problems involving severe damages but at considerable computational cost. It is beneficial if the latter is adopted only in severely distorted regions and FEM further away. The coupled smooth particle hydrodynamics – finite element method (SFM) has been adopted in a commercial hydrocode LS-DYNA to study the perforation of Weldox 460E steel and AA5083-H116 aluminum plates with varying thicknesses and various projectile nose geometries including blunt, conical and ogival noses. Effects of the SPH domain size and particle density are studied considering the friction effect between the projectile and the target materials. The simulated residual velocities and the ballistic limit velocities from the SFM agree well with the published experimental data. The study shows that SFM is able to emulate the same failure mechanisms of the steel and aluminum plates as observed in various experimental investigations for initial impact velocity of 170 m/s and higher.

pH-Sensitive Hydrogel for Micro-Fluidic Valve

The deformation behavior of a pH-sensitive hydrogel micro-fluidic valve system is investigated using inhomogeneous gel deformation theory, in which the fluid-structure interaction (FSI) of the gel solid and fluid flow in the pipe is considered. We use a finite element method with a well adopted hydrogel constitutive equation, which is coded in commercial software, ABAQUS, to simulate the hydrogel valve swelling deformation, while FLUENT is adopted to model the fluid flow in the pipe of the hydrogel valve system. The study demonstrates that FSI significantly affects the gel swelling deformed shapes, fluid flow pressure and velocity patterns. FSI has to be considered in the study on fluid flow regulated by hydrogel microfluidic valve. The study provides a more accurate and adoptable model for future design of new pH-sensitive hydrogel valves, and also gives a useful guideline for further studies on hydrogel fluidic applications.

HIGH VELOCITY IMPACT DYNAMIC RESPONSE OF STRUCTURES USING SPH METHOD

This paper presents the dynamic response of structures under high velocity impact loading using Smooth Particle Hydrodynamics (SPH) approach. The SPH equations governing the elastic and elasto-plastic large deformation dynamic response of solid structure are derived. The proposed additional stress points are introduced in the formulation in order to mitigate the tensile instability inherent in the SPH approach. The incremental rate approach is combined with the leap-frog scheme of time integration forming solution algorithm adopted in present study. Examples on high velocity impact of the solids are presented and results from the proposed SPH approach compared with available finite element solution illustrating that the transient dynamic response under high velocity impact can be effectively solved by the proposed SPH approach.