Key Engineering Materials Vols. 348-349

Abstract: The crack gauge sensor and pencil core sensor are two kinds of elements commonly used to monitor the crack propagation velocity; both of them have the advantage of easy operation, low price, etc. However, they also have the disadvantage, namely, bigger measuring error. In this paper, the work principles of these two sensors are introduced, next, these two sensors are used to measure the crack propagation velocity in the fracture mechanics experiment of gypsum plates, then according to the experimental results, the measuring errors are analyzed and the correcting methods are suggested; at last, the crack propagation velocity are discussed, and the conclusion that the starting velocity of propagation is a variable and related to the dynamic process and the restrained conditions is pointed out.

Abstract: The scattering of SH waves by a cylindrical elastic inclusion with a semicircular disconnected curve and linear cracks in an homogeneous medium is investigated and the solution of dynamic stress intensity factor is given by Green’s function, complex function method. Firstly, we can divide the space into up-and-down parts along the X axis. In the lower half space, a new suitable Green’s function for the present problem is constructed.In the upper half space, the Green’s function has been given by reference [5]. Thereby the semicircular disconnected curve can be constructed when the two parts are bonded along the interface and the linear cracks can be constructed using the method of crack-division and the integral equations can be obtained by the use of continuity conditions at the X axis. Finally, some examples and results of dynamic stress intensify factor are given and the influence of the parameters is discussed.

Abstract: The prestressed concrete girder is a new type of bridge structure. Compared with the traditional concrete girders, its crack resistance has been greatly improved. This type of new girders was used in a certain viaduct in China. In order to probe working characteristics of this type of new girders, some site tests, such as tension-camber test, concentrated load test, uniform load test and mixed load test, were carried out. The results of the tests are analyzed and summed up in this paper.

Abstract: The hysteretic energy is an important parameter to measure the plastic cumulative damage of structures during earthquake. In this paper, with a great number of ground motions collected in the 1999 Chi-Chi, Taiwan, earthquake, the main features of constant-ductility hysteretic energy spectra is analyzed. And the effects of site condition, structural period, ductility level, damping on hysteretic energy are investigated. It is concluded that for a given period, spectra decrease with ductility ratio increases. And as the ductility ratio increase, the spectra will be closer. In the short-period region (about less than 0.8s), spectra are strongly dependent on the structural period. In the long-period region, spectra tend to be constant. The limiting period depends on the ductility ratio and the local site conditions. The damping has similar effect on spectra for all the structural period regardless ductility ratio.

Abstract: Damage parameter of concrete is widely used in design and construction of concrete structures. An experimentation method has been designed, which can simulate the initial damage by mixed initiator into concrete. Damage parameter can be obtained in laboratory with standard specimens. But practical structure is very large. It is not rational to use this parameter in practical structures. So size effect on damage parameter is studied. It is concluded that initial damage and summit damage grow with the increase of structure size, but the growth rate becomes gentle with size increasing. When the size is larger than 500 mm×500 mm, the growth rate is less than 15‰, so the results obtained from specimen in 500 mm×500 mm can be used for mass concrete structures.

Abstract: The long-term goal of prediction of welding distortions in ship sections is to specify an optimal welding procedure to minimize distortions due to welding. The equivalent load method based on inherent strain for material of stainless steel is studied in this paper. A fillet welding experiment followed by infrared thermograph is done. Then simplified elastic plastic analysis model is used while combining the inherent strain theory and the experimental results for accurate and efficient analysis. The results of temperature field in FEM have been supported by the experimental data. Furthermore, the distortions of the stainless steel ship structure are calculated by FE analysis.

Abstract: The influence of corner singularity on the distribution of the stresses around the crack front for a three-dimensional structure is described in this paper. The distribution of stress singularity through the thickness of the middle tension specimen provides us an indication of crack behaviour close to a free surface. An estimation of the region where the change of singularity exponent plays an important role in fatigue crack propagation rate was carried out. A decrease in fatigue crack propagation rate close to the free surface was found. The lower fatigue crack propagation rate in the boundary layer in comparison with the interior of the specimen leads to a change of intersecting angle between crack and free surface and depends on Poisson’s ratio.

Abstract: Top beam flange fractures were observed in the post-earthquake steel frame building inspections. Both seismic loads and gravity loads have effect on top beam flange fractures. The objective was to quantify the variation of stress intensity factor to weld root flaw sizes on top of the beam flange at column face in steel frame connections under gravity loads. Finite-element analyses were used to study fracture toughness in welded beam-column connections. Investigations of fracture behavior mainly focused on the standard pre-Northridge connection geometry. Finite element analysis was performed using the ANSYS computer program. Stress intensity factor was calculated through a J-integral approach. The parametric study was conducted to quantify elastic fracture demands as a function of beam geometries. Results show that the distributions of stress intensity factor tend to be even across the flange width under gravity loads. The thickness of beam web has little effect on stress intensity factors. Stress intensity factor increases nearly linear with the increase of flaw size, and it increases with the increase of depth of beam and width of beam. Stress intensity factor reduces with the increase of thickness of beam flange.

Abstract: This study deals with the SEM and optical microscopic characterization of fatigue plastic deformation process during fatigue crack initiation to understand where, why and how cracks initiate under conditions of low cycle fatigue. Samples were prepared from the 13Х11Н2В2МФ high-chromium stainless steel used for fusion power applications. The low-cycle tests were conducted at room temperature with the standard V-notched samples prepared from conventional stainless steel. The following characteristics were studied during fatigue tests: 1 macrocrack propagation, 2. interaction between macrocrack and isolated microcracks, 3. interaction between macrocrack and slip bands, 4. interaction between macrocrack and microstructure elements of the steel. The above experiments show that during macrocrack propagation a plastic zone is formed around it, where isolated microcracks and slip bands of 2-3 different directions are observed. Measurement of plastic zone dimensions after different number of cycles of deformation show that plastic zone size increases during the first stage of cyclic deformation (until definite number of cycles are completed), and then remains unchanged. The observations show that main crack is composed of individual micro-components, the lengths of which are in a good correlation with the dimensions of microstructure elements of the steel (former austenite grains, martensite crystals). It was revealed that during growth, as a rule, macrocrack rarely propagates along isolated microcracks and slip bands. Direction of macrocrack propagation changes while passing from one microstructure element to another, so that main direction is the same. No preferable transcrystalline or intercrystalline propagation of macrocrack has been observed in the investigated steel. It is shown that after subsequent fatigue tests, dimensions of the previously created slip bands increase, and additional new slip band are also formed. The sites and frequency of slip bands’ formation in plastic zone are also studied. It was observed that the boundaries and mainly the sites of intersection of martensite crystals are the sites of isolated (rough) microcracks’ formation. The dimensions of slip bands are comparable with those of martensite crystals. The angles between the main crack propagation direction and slip bands varied from 30o to 60o, however, most of the slip bands were oriented at 45o to the main crack. Based on the obtained results a conclusion is made that plastic deformation in samples go inhomogeneously. In plastic zones, along with the heavily deformed areas, almost non-deformed areas are also observed. The speed of fatigue fracture increases with the increase in frequency and amplitude of deformations. Generally, the annealed samples are destructed prematurely in comparison with non-annealed ones of the investigated steel.

Abstract: The impact echo method has been widely used to evaluate the integrity of concrete structures. This paper is to study damage detection technique of concrete members using impact echo method, one of the nondestructive tests based on stress waves. Stress wave propagation behavior is to be studied based on wave mechanics in concrete media. Based on the theoretical study, nondestructive tests using stress waves will be conducted to observe the wave behavior on structural damages. For the damage detection of concrete structures, a mock-up specimen was prepared, and various tests were conducted for the study of the behavior of stress waves. The concrete slab of the flaw detection consist of 5 types; the first group for the thickness presumption, the second group for the position of the voids, the third group for the position of the re-bars, the fourth group for the position conduit, and the fifth group for the position of the crack. Test results are evaluated and summarized to verify if the nondestructive tests can survey damages in the concrete structures in this paper.