Key Engineering Materials Vols. 417-418

Abstract: This paper presents the simple method to determine the complex stress intensity factor of interface crack in bi-material plate under bending. In the present method, the stress values at the crack tip calculated by FEM are used and the stress intensity factors of interface crack are evaluated from the ratio of stress values between a given and a reference problems. A single interface crack in an infinite bi-material plate subjected to tension and shear is selected as the reference problem in this study. The accuracy of the present analysis is discussed through the results obtained by body force method. As the result, it is confirmed that the present method is useful for analyzing the interface crack under bending.

Abstract: The methods of diagnosis of damage through measuring the changes in dynamic structural properties have been intensively studied for many years now. On the contrary to mechanical or aeronautical engineering, however, the practical application of the methods in the case of civil engineering structures meets a number of difficulties indicating the need for further research. This concerns in particular the need for the shaking table investigations since the results of such tests are very limited. Therefore, the aim of the present paper is to show the results of the shaking table study conduced on a model of two-storey steel frame building. The 1.56 m high model structure was constructed of columns with cross section of mm. Damage was defined as a reduction in stiffness and introduced by replacing the columns with elements having mm cross section. Seven different structural damage cases described by different number of columns with the reduced cross section were considered in the study. During the shaking table experiment the base of the structure was excited under the sweep-sine loading. The results of the study show a characteristic decrease in the natural frequencies with the progression of damage. This trend has been especially observed for the first natural frequency, for which the decrease in the frequency value has been found to be uniform. The results of further analysis show the scattered values of the structural damping ratios without showing any relation with the level of structural damage.

Abstract: In this study, the effect of water absorption on mechanical properties of hemp fiber reinforced green composite(HGC) was examined. Plain woven hemp fabric was used as reinforcement. Emulsion type biodegradable resin was used as matrix. The composite was made by compression molding method. Water absorption rate and the effect of the ratio on the mechanical properties were examined. Following results are obtained. The water absorption rate increased with an increase of absorption time. The water absorption rate had an equilibrium state within 30 days. The duration was not dependent on fiber content. In the case of that specimen was kept in water 182 days, the strength decreased 70% of the ultimate one. But in the case that the specimen has dry process after water absorption process, the strength recovered 57%. The recovery ratio is not dependent on water absorption time. The strength reduction rate after dry process is not dependent on fiber content. The strength of resin decreased 62%, and the strength of fiber decreased 13% by water absorption. So the strength reduction of the composite is due to the effect of matrix.

Abstract: Two important factors in damage tolerance engineering are the damage size that can be tolerated and the use of applied nondestructive testing (NDT) technique for detection of defects. Generally, NDT of large mechanical structures, such as vessels and pipes in nuclear power plants, is carried out according to national codes, and the damaged mechanical parts detected during periodic NDT are repaired or replaced after the evaluation. However, unexpected accidents can occur if cracks in the structure grow and exceed the tolerance limit during operation. Therefore, there is a need to develop NDT techniques that can detect micro-damage in the vessels and pipes. In this paper, we propose a cylinder-type magnetic camera to fill this need. We use a cylindrically integrated Hall sensor array (CIHaS) as a magnetic sensor to create magnetic images, and we use a solenoid coil as the source of the magnetic field detected by the magnetic camera in order to achieve high crack-detection ability. The proposed method is tested with an examination of the cracks on a prepared pipe sample.

Abstract: Wheels are essential parts of an express train, as they support the train’s weight, but because of the limited time available for testing and repair in the maintenance factory, the 1000 wheels of each high-speed train must be tested within 1 h. Therefore it is essential to develop a new NDT system that is able to (a) detect cracks quickly and (b) provide high crack-detection ability regardless of the state of the materials. In this paper we propose a scan-type magnetic camera to satisfy this need. We use a linearly integrated Hall sensor array (LIHaS) as a magnetic sensor to make high-speed testing possible, and we use a small, yoke-type magnetizer as the magnetic source for the magnetic camera to assure high crack-detection ability. We tested the method by examining the cracks on a sample wheel traveling at 27.5km/h.

Abstract: Thermal barrier coatings (TBCs) have been employed for the insulation of substrates from high temperature in gas turbine plants. The TBC system consists of ceramic top coating, metallic bond coating and substrate. Delamination of the ceramic coating is important problem in TBC systems. In this paper, the delamination mechanism was studied by residual stress history under thermal aging and thermal cycle conditions. In-plane residual stress histories of ceramic coating and bond coating after thermal aging and cycling were measured by X-ray diffraction method. The residual stress under thermal cycling was also calculated by FEM analysis. The results obtained were as follows: (1) in-plane surface residual stresses of the coatings scarcely changed regardless of the increase of thermally grown oxidation (TGO). (2) high compressive thermal stress, residual stress at room temperature, in ceramic coating induced by thermal stress did not occur. It was found that stress of ceramic top coating was relaxed by micro cracks and driving stress of delamination was in-plane high compressive stress.

Abstract: The purpose of this paper is to discuss the plastic deformation and pressure reversal of pipe during hydrostatic testing. The higher the test pressure, the smaller the number of defects that remain undetected. In order to study the effect of hydrostatic testing, full scale burst experiments were carried out with API 5L X65 SSAW pipes of 660mm outside diameter and 12.5 mm wall thickness. Nine spiral oriented deflects had been machined in the weld. Deformations of deflects were measured. The pressure reversal was computed. It can be concluded that if a hydrostatic test can be successfully accomplished without the failure of any defect, the likelihood of a pressure reversal will be extremely small.

Abstract: To check the structural strength of the body of the newly–made container freight car, load was added to the underframe of the container freight car. The objective of this study is to evaluate whether or not the underframe of a freight car under the maximal strength is safe. A freight car for transporting railroad freight is usually designed and made of SM490YA steel and SM490Y steel. The carbody of rolling stock is a principal structure that supports major equipment of the underframe and the container freight. Therefore, the strength evaluation of this structure is important. Both structural analysis and loading test were performed under the loading condition. Prior to the evaluation of structural strength, finite element method software was used for structural analyses on stress distribution in a carbody of a freight car. The strain gauges were attached on the carbody based on FEM results. The actual vertical loading test and horizontal compression loading test were conducted

Abstract: An experimental investigation was performed on mode I delamination of z-pinned double-cantilever-beams (DCB) and associate z-pin bridging mechanisms. Tests were performed with ten types of samples: (1) big-pin reinforced DCB (double-cantilever-beams) with three areal densities D=2.01%, 5.15%, 8.04%, respectively; (2) median-pin reinforced DCB with three areal densities D=0.85%, 2.17%, 3.40%; (3) small-pin reinforced DCB with three areal densities D=0.25%, 0.63%, 0.90% and (4) without pin reinforced DCB specimens. Delamination tests samples were prepared from unidirectional continuous carbon fibre/epoxy prepreg (T300/TDE86), made into 3 mm thick unidirectional laminates with and without a block of Z-pins in the crack path. Fracture testing was carried out under Mode I (standard DCB test). Experiments have shown that increases in debond resistance and ultimate strength depend on the material, size, density, location of the pins and the mechanisms of pin deformation. A finite element (FE) model is developed to investigate mode I delamination toughness of z-pin reinforced composite laminates. The z-pin pullout process is simulated by the deformation of a set of non-linear springs. A critical crack opening displacement (COD) criterion is used to simulate crack growth in a DCB made of z-pinned laminates. The toughness of the structure is quantified by the energy release rate, which is calculated using the contour integral method. The FE model is verified for both unpinned and z-pinned laminates. Predicted loading forces from FE analysis are compared to available test data. Good agreement is achieved. The numerical results indicate that z-pins can greatly increase the mode I delamination toughness of the composite laminates.

Abstract: In this study, we propose an interesting scheme for analyzing micro-pillar compression test results based on finite element method. It uses only load and displacement data measured by micro-pillar compression test, a framework of conventional incremental metal plasticity, and an iterative scheme for updating the material parameters. This is an inverse approach to determine the constitutive relation of materials based on experimentally measured load and displacement. As a demonstrative example, Ni-Co micro-pillars with a diameter of 10 ㎛ and a height of 20 ㎛ were fabricated by micro-machining process, and their load-displacement data were measured by a micro-pillar compression tester. Using the proposed scheme and the measured load-displacement data, the stress-strain curves for the Ni-Co micro-pillars were estimated.