Advances in Fracture and Damage Mechanics VI

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Authors: Young Sang Cho
Abstract: A NDT test, spectral analysis of surface wave (SASW) method, is a relatively new technique for examining the structural integrity of structural members of building structures. The SASW method has advantages among NDT methods that it needs an access from only one side of a testing object and can measure the dispersive characteristics in layered structures. This study is to model the wave propagation of stress waves in concrete media using a finite element method. Experiments using the NDT are to be conducted to investigate the wave behavior in concrete media. This study also evaluates the surface wave velocity of single layer cement mortar systems using an experimental analysis of the SASW method, and examines the corresponding results with numerical results. Specimen models were single-layer cement mortar slab systems.
Authors: Xiao Yong Wang, Han Seung Lee, Hai Moon Jung
Abstract: Chloride penetration into concrete is the main cause of steel corrosion in concrete structures exposed to chloride-rich environments. In general, conditions on the diffusion process are dominant among various penetration mechanisms, such as ionic diffusion, capillary sorption, and so on. In recent analysis of current literature, chloride diffusion is as a simplified one-dimensional diffusion process. However, for the rebar in the corner zone of concrete beam, the diffusion belongs to a two-dimensional diffusion. Based on a galerkin finite element method, a two-dimensional diffusion differential equation is built and solved numerically and the different chloride concentration is compared to one dimensional diffusion and two-dimensional diffusion process. The service life of concrete structure members under two-dimensional chloride penetration is predicted by compared with a critical threshold chloride concentration. Compared with general one-dimensional chloride attack, the service life is considerably reduced in a corner zone due to two-dimension penetration.
Authors: Kianoush Molla-Abbasi, Henning Schütte
Abstract: Damage of materials is a progressive physical process through which the macroscopic properties of the material changes and it might end with the final failure of a part and in some cases can be disastrous. The main mechanism of brittle damage is the nucleation and growth of microcracks. A way to model the anisotropic damage is to consider its influence on the compliance/stiffness of the material at the meso-level. A numerical study of a growing mixed-mode internal crack in a unit cell was undertaken with the help of a finite element simulation. The model enables us to measure the components of the elastic compliance tensor modified by damage as the crack grows, showing the evolution of the anisotropic damage and the evolution of the type of material symmetries. The evolution of the elasticity tensor shows that the damage associated with a growing elliptical crack changes the virgin isotropic properties into orthotropic ones and by crack growth the axes of orthotropic symmetry, initially aligned with the local coordinates of the crack, rotate towards the principle loading axes. The matrix material of the unit cell is considered to be isotropic linear elastic, homogenous and the response is perfectly brittle. The characteristic crack size is small compared to the unit cell, so that the non-interacting crack assumption for the damage models is fulfilled. Crack propagation is simulated using the stepwise method, which consists of the succession of straight segments and crack growth is governed by the principle of maximum driving force which is a direct consequence of the variational principle of a cracked body in equilibrium and considers the effect of all three stress intensity factors. Without any ad hoc assumption, the crack growth rate is calculated using its thermodynamic duality with the local maximum driving force.
Authors: Young Sang Cho, Lin Xia, Seong Uk Hong, Seong B. Kim, Jun S. Bae
Abstract: Structural optimization is widely adopted in the design of structures with the development of computer aided design (CAD) and the development of computer technique recently. By applying the artificial neural network to structural optimization, designers can gain the design scheme of structures more feasibly and easily. In this paper, the genetic algorithm (GA) used in the error back-propagation (BP) network is applied to get the optimization result of the structural system. And the training pair of BP neural network is obtained from the structural analysis using a finite element program. The case study of 10 member truss structure using GA and BP will be helpful to reduce the cost of structures which is related to weight and the dynamic performance of optimization under the lateral load.
Authors: Charles O. Woghiren, F.P. Brennan
Abstract: This paper reports a parametric stress analysis of various configurations of rack plate stiffened multi-planar welded KK joints using the finite element method. The KK joint finds application in the leg structure of offshore Oil & Gas jack-up platforms. The rack plate is a dual purpose element of the joint because it firstly functions as a stiffener which reduces the stress concentration at the brace/chord intersection. This could be an immense contribution to the increase in fatigue life of the joint but other hot spot sites are introduced to the joint. The rack is also used for raising and lowering of the jack-up hull which gives the jack-up platform its jacking capability. Over 120 models using a combination of shell and solid elements were built and analysed within ABAQUS. Non-dimensional joint geometric parameters; β, γ and . were employed in the study with . being defined as the ratio of rack thickness to chord diameter. Stress Concentration Factors (SCFs) were calculated under applied axial and OPB (out-of-plane-bending) loading. Three critical SCF locations were identified for each load case, with each location becoming the most critical based on the combination of the non-dimensional parameters selected for the joint. This is important as careful design can shift the critical SCF from an area inaccessible to NDT to one that can be easily inspected. The SCF values extracted from the models were used to derive six parametric equations through multiple regression analysis performed using MINITAB. The equations describe the SCF at the different locations as a function of the non-dimensional ratios. The equations not only allow the rapid optimisation of multi-planar joints but also can be used to quickly identify the location of maximum stress concentration and hence the likely position of fatigue cracks. This in itself is an invaluable tool for planning NDT procedures and schedules.
Authors: M. Zappalorto, Filippo Berto, Paolo Lazzarin
Abstract: A recent approach based on the local strain energy density (SED) averaged over a given control volume is applied to well documented experimental data taken from the literature, all related to steel welded joints of complex geometry. This small size volume embraces the weld root or the weld toe, both regions modelled as sharp (zero notch radius) V-notches with different opening angles. The SED is evaluated from three-dimensional finite element models by using a circular sector with a radius equal to 0.28 mm. The data expressed in terms of the local energy fall in a scatter band recently reported in the literature, based on about 650 experimental data related to fillet welded joints made of structural steel with failures occurring at the weld toe or at the weld root.
Authors: Dong Seok Kim, Han Seung Lee, Seong Min Lee, Xiao Yong Wang
Abstract: Chloride attacks concrete structures becoming a primary factor that deteriorates the durability of concrete structures. For this reason, research has been conducted on chloride ion penetration and diffusion. This research produced an accurate durability life prediction through reliability assessments and proposes a prediction method for the chloride ion diffusion coefficient of a concrete applied assessment program for reliability. As a result, test materials were fabricated using different admixtures and chloride ion diffusion coefficient was calculated by applying an RCPT test at each equivalent age. Based on these results, reliability prediction formulas were indicated through the reliability analysis for a durability life design using a Montecarlo method. In addition, propriety was verified through comparisons and analysis using the proposed formula with the investigated data for chloride ion diffusion.
Authors: Ki Bong Park, Han Seung Lee, Xiao Yong Wang, Seung Min Lim
Abstract: This paper describes a numerical method for estimating the elastic modulus of cement paste. The cement paste is modeled as a unit cell, which consists of three parts: dehydrated cement grain, gel, and capillary pore. In the unit cell, the volume fractions of the constituents are quantified with a single kinetic function of the degree of hydration. The elastic modulus of cement paste was calculated from the total displacement of constituents when the uniform pressure was applied to the gel contact area in cement paste assumed a homogenous isotropic matrix. Numerical simulations were conducted through the finite element analysis of the three-dimensional periodic unit cell. The model predictions were compared with experimental results. The predicted trends agree with experimental observations. The approach and some of the results might also be relevant for other technical applications.
Authors: Joh Yeong Yoo, Han Seung Lee, Sung Ho Tae, Moon Byung Chul
Abstract: As concrete is a type of porous materials, water or air freely permeates concrete. Therefore the durability of concrete decreases. However, porous material with a rust inhibitor may allow permeation of water into concrete. In addition, there may be permeation of water through the rust inhibitor at the location of steel frames. The objective of the study is to investigate the penetration depth of concrete under water forced conditions with pressure.
Authors: Imke Weich, Thomas Ummenhofer
Abstract: Research has been initiated on the effects of high frequency peening methods on the fatigue strength. These methods combine an improvement of weld toe profile with an initiation of compressive residual stresses and surface hardening. The effects of two techniques, High Frequency Impact Treatment (HiFIT) and Ultrasonic Impact Treatemnt (UIT) are compared. Laser measurements of the weld seam prove that both methods increase the overall weld toe radii. Further, residual stress measurements verify the introduction of compressive residual stresses at least up to a depth of 1 mm. The values meet the yield strength combined with an increase of the surface hardness. These material mechanical effects cause an increased crack resistance. Crack detection methods prove that the material mechanical effects yield to a retarded crack initiation. Experimental results show that these effects lead to a significant increase of the fatigue strength and reduced slopes of the SN-curves.

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