Key Engineering Materials Vols. 297-300

Abstract: Resistance spot welding is used extensively to fasten sheet for automotive applications. In many components, these welds should maintain their integrity under severe loading conditions. However fatigue strength of the spot welded joint is considerably lower than base metal due to stress concentration at the nugget edge, and is influenced by its geometrical and mechanical factors such as welding condition and etc. In this paper, it is estimated that effect of strain rate variation on fatigue life of spot welded joint. The analytical method proposed to overcome above difficult using lethargy coefficient concept for evaluating the fatigue life cycle of spot welded joint. The reliability of the life cycle is completed by comparing with the life cycle obtained by fatigue test for the specimen with the welding current. And the above procedure is numerically extended to get the life of dynamic strain rate region.

Abstract: Effect of mixing process on the fracture behavior of HA/PLLA Composites were investigated. Fracture toughness values of HA/PLLA composites prepared under different mixing time and rotor speed were measured. The fracture surface morphology was also examined by scanning electron microscopy. It was found that the fracture toughness of HA/PLLA composite decreases due to decrease of ductile deformation of PLLA matrix and debonding of interfaces with increase of the rotor speed and mixing time. Effect of mixing process on neat PLLA was also assessed, and it was found that the fracture toughness of PLLA decreases due to such pocess. Disappearance of multiple craze formation and thermal degradation were found to be the primary mechanisms of the toughness degradation.

Abstract: An aluminum borate whisker (Al18B4O33w) was recognized as an attractive reinforcement for the aluminum alloy, It is good strength, high wear resistance, good thermal stability and low cost, comparable with those of other whiskers such as SiC and Al2O3. In general, aluminum borate whisker reinforced AC4CH alloy composites have been produced by squeeze casting process, which is capable of promoting the higher specific mechanical properties. α+Al18B4O33w/AC4CH (α: SiO2, Al2O3, TiO2sol) composites were fabricated by squeezecasting method in which the molten aluminum infiltrated into the whisker preform. The primary objective of this study was to establish the influence of the fracture toughness of α+Al18B4O33w reinforced composite by different containing inorganic binder. In addition, the fracture mechanism was evaluated through the scanning electron micrographs. As results, fracture toughness the composite containing TiO2 sol was shows plane about 9.28Mpa-m0.5. TiO2 inorganic binder can enhance fracture toughness of the Al18B4O33w reinforced composite. In order to obtain Al18B4O33w reinforced aluminum composites to have good fracture toughness use of TiO2 inorganic binder is recommend.

Abstract: When huge energy transfer systems like nuclear power plants and steam power plants are operated for long times at high temperatures, mechanical properties change and ductile-brittle transition temperature increases by degradation. So we must estimate the degradation in order to assess safety, life expectancy, and other operation parameters. The sub-sized specimen test method using the surveillance specimen, and BI (Ball Indentation) method were developed for evaluating the integrity of metallic components. In this study, we will present the evaluation of the ductile-brittle transition temperature using the BI test and the sub-sized specimen test. The four classes of the thermally aged 1Cr-1Mo-0.25V specimens were prepared using an artificially accelerated aging method. The tensile test, the fracture toughness test, and the BI test were performed. The results of the fracture toughness tests using the sub-sized specimens were compared with those of the BI test. The evaluation technique of the ductile-brittle transition temperature using the BI test was also discussed. Our results show that the ductile-brittle transition temperatures rose as the aging time increased. We suggested that the fracture toughness results of the sub-sized specimen test and the IEF results of the BI test could be used in the estimation of the ductile-brittle transition temperature as material degrades.

Abstract: The fatigue characteristic of a material or a structure is derived from fatigue tests of standard specimens. However, many researches have reported that test results of standard specimens are very different from those of real structures or components. One reason for this difference is the constraint effects according to the geometrical difference. Therefore, to calculate more accurate fatigue life, the constraint effect must be considered by comparing test results of standard specimens with those of real structures or components. Another reason for this difference is the surface condition. All surfaces of a standard specimen are polished to obtain similar conditions in a fatigue test. However, in a piping system, surface conditions of components are different from each other and very different from that of a standard specimen. Because fatigue life is effected by a surface condition, to evaluate the fatigue life of a piping system, fatigue tests must be conducted with a specimen extracted from a pipe with the same surface condition. The objective of this paper is to evaluate the fatigue characteristic of a real waterworks pipe by conducting fatigue tests with standard specimens and non-standard specimens of base metal and weld metal. Standard fatigue specimens and non-standard specimens were extracted from a steel pipe used in waterworks. Also, fatigue tests of pipes used in water service were carried out and then compared with those of standard specimens and non-standard specimens. From these results the relation between the S-N diagram of a specimen and that of a pipe specimen was evaluated.

Abstract: Low-cycle fatigue tests were carried out in air in a wide temperature range from room temperature to 650oC to investigate the role of temperature on the low-cycle fatigue behavior of two types of stainless steels, cold-worked (CW) 316L austenitic stainless steel and 429 EM ferritic stainless steel. CW 316L stainless steel underwent additional hardening at room temperature and in 250-600oC: plasticity-induced martensite transformation at room temperature and dynamic strain aging in 250-600oC. As for 429 EM stainless steel, it underwent remarkable hardening in 200-400oC due to dynamic strain aging, resulting in a continuous increase in cyclic peak stress until failure. Three fatigue parameters, such as stress amplitude, plastic strain amplitude and plastic strain energy density, were evaluated. The results revealed that plastic strain energy density is nearly invariant through a whole life and, thus, recommended as a proper fatigue parameter for cyclically non-stabilized materials.

Abstract: The high cycle fatigue properties of two kinds of wrought Al 2519 alloys without and with scandium of 0.10% were investigated. The fatigue strength was determined at R = 0.1 under constant amplitude loading conditions in air. The alloy with scandium of 0.10% showed a little lower tensile yield strength and higher fatigue strength values. The fine grained Al-0.10Sc alloy exhibited a higher resistance against fatigue crack nucleation despite the lower yield stress in comparison to the coarse grained Al 2519 alloy. The results can be explained mainly with the microstructural differences between both alloys. This results are due to the presence of coherent Al3 (Sc, Zr) precipitates and a very fine subgrain structure.

Abstract: A series of test for the fatigue crack growth rate da/dN and the threshold ΔKth values were performed with CT specimens on a ship-condenser material titanium alloy plate and rolled ring. Base metal, welded joint and HAZ (heat affected zone) materials were used to make different test specimens. Specimens made from the titanium plate were cut along L-T direction, those made from rolled ring were obtained along C-R and L-R direction respectively. Results show that the fatigue crack growth rate value of welded joint is much higher than those of base metal and HAZ material. The da/dN values of C-R direction specimens are much higher than those of L-R direction specimens, whereas the ΔKth values are lower. It means that welding process may lead to a great reduction in the fatigue property for titanium alloy and the effect of crack orientation on fatigue property is not negligible for titanium alloy. To select a proper orientation of titanium material is therefore very important in engineering practice. Results also indicate that a simplified method can be used to calculate the ΔKth values for titanium material, that is, ΔKth values may be calculated directly from the da/dN expression in a zone near the threshold and the laborious measurements of ΔKth may therefore be saved.

Abstract: For many fatigue-critical parts of machines and structures, the load history under operating conditions generally involves variable amplitude loading rather than constant amplitude loading. An accurate prediction of fatigue crack propagation life under variable amplitude loading requires a thorough evaluation of the load interaction effects. In this study, fatigue tests under both constant and variable amplitude loading were carried out to investigate the overload effects on fatigue crack propagation of the notched specimens. Strain distributions around the crack tip before and after a tensile overloading were measured using the ESPI (Electronic Speckle Pattern Interferometry) system. The size of the plastic zone was determined from the measured strain distributions. The study proposes a crack propagation prediction model that incorporates the overload ratio effect. A comparative work for the overload ratio effect demonstrated that the prediction by the proposed model was in good agreement with the experimental results. The prediction of fatigue crack propagation including multiple overloads with the proposed model show also a good agreement with test results.

Abstract: Crack propagation of X-70 pipeline steel in near-neutral pH solution was studied under different modes of cyclic loading. A revised equation of crack growth rate vs. Δ K was obtained. Average crack growth rate increased with cycles under conditions of different R values. Linear shape notch specimen made cracks much easier to initiate and propagate than V-shaped notch specimen did. For different R values, the curves of crack growth rate with cycles were similar, but the platform propagation period and quick propagation period were different obviously. Crack growth rate at both periods increased and thus failure time decreased markedly with decrease of R value. The propagation directions of cracks were different under different cyclic loading conditions. Under mode I (single tensile stress) cyclic loading, cracks were straight and perpendicular to the tensile stress axis, while under mixed-mode I/III (tensile/shear stress) cyclic loading, cracks were sinuous and did not propagate in the direction perpendicular to the main tensile stress axis.