Key Engineering Materials Vols. 417-418

Abstract: The use of the damage tolerance concept is growing in the design of civil engineering structures. The aim of this paper is to provide some guides to help designing anchorages according to the damage tolerance concept. The paper shows the importance of the small defects, idealized like small cracks, in the structural integrity of these elements. The Stress Intensity Factors (SIFs) induced by small cracks in an anchor head of post-tensioned strand anchorage system are calculated. The study includes the evaluation of the influence of the shape of the anchor head on the SIF. The numerical predictions are compared with experimental results of ½ scaled specimens of Poly-methyl-methacrylate (PMMA).

Abstract: Nickel-titanium alloy are extensively used in engineering and biomedical fields for their excellent properties of shape-memory, super-elasticity and biocompatibility. Their fatigue performance has been attracted increasingly attention, because they are often used under cyclic conditions. In this paper, the ultrasonic fatigue behavior of nickel-titanium endodontic files under unconstrained condition has been studied using the self-designed ultrasonic fatigue testing equipment. The vibration and harmonious response properties of nickel-titanium endodontic files are also analyzed using finite element method. Experimental results show that the average ultrasonic fatigue life of nickel-titanium endodontic files under unconstrained conditions is more than 108. Fracture of the files always occurred at the position about 2 -3 mm near the file tip. Results of finite element analysis show that the maximum stress of the nickel-titanium endodontic files located at the position about 2 -3 mm near the file tip, where is prone to generate fatigue cracks. The results of finite element analysis are consistent with the experimental results. The scanning electronic microscope (SEM) results show that the ultrasonic fatigue cracks always initiated at the surfaces of the files, where the secondary phases or impurities existed and induced fatigue cracks under repeated stress.

Abstract: Well casing damage is a commonly existing problem in oilfield exploitation in the world. Daqing oilfield is a multiple-zone, heterogeneity sandstone oilfield, where the major influence factors which lead to casing damage are geologic factor, engineering factor, high pressure water injection and chemical factors. Among them, the high pressure water injection is the most important one. Water injection exploitation in Daqing oilfield showed that casing-damage increased with the increasing water injection pressure. However, the mechanism is not totally understood and the control method is not well developed yet. In the present work, the mechanism analysis of casing damage induced by high pressure water injection in Daqing oilfield is proposed. It is found that after high pressure water injection, the sandstone layer will expand and result in the vertical elongation of the casing. The additive tensile stress of the casing induced by vertical strains will cause casing-damage. Besides, the horizontal deformation of clay-stone increases with increasing water content of the formation layers and soaking time. The cohesion of clay-stone and inner friction angle decreased with increasing water quantity. In that case, some high obliquity formation layers which may induce high hypsography pressure difference will cause localized slip along weak formation layer interface in the area of waterishlogged clay-stone. Casing damage and well failure caused by the relative movement of the formation layer interface may therefore occur. The micro-fracture of formation induced by high pressure water injection also educed formation rupture and casing damage.

Abstract: Thermal fatigue and isothermal oxidation tests of a plasma sprayed thermal barrier coatings have been carried out. The experimental results indicated that micro-cracking as the fatigue damage occurred in each cooling stage of thermal cycles. And the spallation fracture morphology was changed by the cycle wave form; the spallation failure occurred in the ceramic top-coating under the triangle cycle wave condition, on the other hand, it occurred near the top-coating/TGO interface under the trapezoidal wave condition. The finite element analysis indicated that the local residual stress, which is the driving force of interfacial damage, increases with the thickness of the thermal grown oxide. The degradation mechanism by thermal fatigue was discussed, in addition, a simple life prediction model for the thermal fatigue of TBC systems was proposed.

Abstract: In this paper, unidirectional Manila hemp fiber-reinforced thermoset composites were fabricated and their mechanical properties and fracture behavior were also investigated. Both unsaturated polyester resin and epoxy resin were used as matrix polymer. The tensile strength and Young’s modulus of the epoxy-based composites reached the maximum of 520 MPa and 17 GPa at the fiber volume fraction of 72%, respectively. Such excellent mechanical properties are seemed to be derived both from a good permeability of resin into hemp fiber bundles and from enough stress transfer between fiber and matrix.

Abstract: In order to understand the influence of gamma-irradiation, accelerated aging and Vitamin-E addition on the fatigue crack growth properties of UHMWPE, fatigue crack growth tests of UHMWPE plates using compact tension specimens were carried out. The specimen conditioned in -irradiation and accelerated aging (the -aging specimen) has faster fatigue crack growth rate than the virgin specimens. For the Vitamin-E added specimen (the VE specimen), however, K threshold of the -aging VE specimen is smaller than that of the VE specimen. This result shows that the addition of Vitamin-E reduced the decrease of K threshold by -irradiation and accelerated aging.

Abstract: Fatigue crack growth under mixed mode loading conditions is simulated using S-version FEM. By using S-FEM technique, only local mesh should be re-meshed for new crack configuration, and it becomes easy to simulate crack growth. By combining with auto-meshing technique, local mesh easily re-meshed and curved crack path is modeled easily. Fully automatic crack growth simulation system in 3-dimensional problem is developed. Using this system, several kinds of plural surface cracks problems are simulated. It is shown that intereaction effect between two surface cracks appear in complicated manner depending on intial distances between two cracks.

Abstract: In this paper, we describe a carbody structural analysis and the result of its stress test. The purpose of this study is to evaluate the safety and functionality of the body structure operating under maximum load. The body structure was made of aluminum alloy and consisted of the side frame, under frame, roof frame, and end frame. Of these components, the side frame and under frame were the most important components in consideration of the vehicle and passenger loads. Both the structural analysis and stress test were performed under the condition that was based on the "Performance Test Standard for Electrical Multiple Units." The test results showed that the body structure is safe and stable under the condition of the designed load.

Abstract: Fracture resistance of AISI 304LN stainless steel has been examined by monotonic and cyclic J-integral tests, the latter being carried out using periodic unloading as per pre-determined R-ratio. The cyclic fracture resistance is found to be almost 1/5th of its corresponding monotonic value. The deterioration of fracture resistance under cyclic loading has been attributed to crack tip resharpening due to smearing and smashing of voids owing to rubbing of adjacent crack surfaces in the compressive load cycles.

Abstract: The study deals with the TEM characterization of samples after low-cycle fatigue (LCF) tests of austenitic stainless steels used in nuclear power plants (NPP). Microcracks are observed in the TEM thin films prepared from standard cylindrical bulk samples. In plastic zones, at the tips of microcracks, several three-dimensional crystal structure defects are detected: 1. Arched extinction contours, 2. Zone of disorientations, 3. Deformation twins of different orientations, 4. Short micro cracks deviated from the main crack direction. The above defects have been studied using TEM, including micro-diffraction; trace analysis and dark-field techniques. It is shown that the microcrack and the created defects represent the means of relaxation of elastic stresses retained in the sample after LCF. It is also shown that redistribution of stresses leads to the nucleation of microcracks and subsequent creation of all the observed defects. A dislocation scheme of the stresses, detected by the extinction contours at microcrack tip, is proposed. The comparison of the proposed scheme of stress distribution with the TEM micrographs shows a good resemblance. Consequently, a topology of stress distribution at microcrack tip - before formation of plastic zone - is determined. The shape represents a three-leaved figure. Compliances and differences between the proposed scheme and the experimentally observed defects and plastic zone size are discussed based on crystallographic analysis of the respective electron diffraction patterns.