Key Engineering Materials Vols. 353-358

Abstract: The effect of undissolved ferrite amount on impact fatigue properties and failure mechanism were studied by using 42CrMo steel with subcritical quenching process The amount of undissolved ferrite were 0%, 10% and 15%, respectively. The experimental results show that the existence of undissolved ferrite can not only change the microstructure, but also increase the impact fatigue life The impact fatigue life elongates with increasing of amounts of undissolved ferrite The grain can be fined by using subcritical quenching process and the area of phase boundaries can also be greatly increased because of undissolved spheroidal carbide. The martensite and carbide form can also be changed by using subcritical quenching process The stress relaxation due to the moving of dislocations inside the ferrite and the promotion of strength due to occurring of plastic deformation and the enwinded dislocations are main reasons of improving the impact fatigue life. The impact fatigue life elongates with the increase of amounts of undissolved ferrite before the amount of undissolved ferrite reaches 10%. Under the experiment conditions, when the amount of undissolved ferrite is 10%, the impact fatigue life will be the longest.

Abstract: Thermal shock properties of Q235A steel used ternary-boride-based (TBB) cladding material has been studied. The result indicates that this cladding material has excellent resistance to thermal shock and that the cracks are not difficult to occur at the interface of cladding layer and steel substrate. The mechanism of thermal shock failure is fatigue failure brought by cycle stresses. The thermal shock has little influence on the hardness of cladding layer. The structure of cladding layer has no obvious change after the thermal shock, but the phase of steel substrate change from ferrite and pearlite to martensite.

Abstract: Considering the difference between specimens and RD2 axle of C64A freight car, the extrapolated probabilistic fatigue S-N curves of LZ50 axle steel, which include the long life regime of material, are adjusted by introducing a probabilistic fatigue affecting factor, Kaffect. Based on the adjusted curves, fatigue reliability research on the axle at different operating speeds is carried out. It utilizes the fatigue stress spectrums of the axle in two different wheel set states, namely, normal state and scratched state. The fatigue operating life with arbitrary survival probability and confidence is obtained, and the failure rate with given confidence and design fatigue life is also achieved. Analysis results indicate that the unloading groove of existent RD2 axle is the most dangerous part. When the wheel set is in normal state, fatigue life of C64A freight car RD2 axle decreases with the increase of reliability. Furthermore, scratched wheel set shortens the fatigue life strongly. Therefore, inspection and administration of scratched wheel set are significant. The application of wheel set at relatively high scratch level and high operating speed should be restricted.

Abstract: Because of the huge cost involved in data acquisition, probabilistic S-N relations should be given on a wide applicable sense to perform the reliability analysis at arbitrary survival probability-confidence (P-C) level. However, some existent fatigue databases give the material probabilistic S-N relations only with several fixed P-C levels. To realize the reliability analysis on a sense of arbitrary P-C level, a Monte Carlo simulation method is presented for reconstruction of the relations. Test data are re-gotten by a method under the new simulation policy that matching production practice according to original statistical parameters. Details are given with respect to the possible cases of known conditions. The relations are finally determined by maximum likelihood estimation on a general model to realize the analysis at arbitrary P-C level. Reconstruction of the relations for 60Si2Mn spring steel has indicated the availability and feasibility of present method.

Abstract: A complete stress-strain experiment curve, gained through exerting low confining pressure on brittle rock, reflects the deformation and destruction process of rock under different confining pressure, and reveals that after the destruction of rock, not only slip deformation but also re-destruction process will take place, which would possibly lead to further reduction of the mechanical properties of the rock mass. Through the analysis of the relation between complex failure modes, load carrying capacity after rock destruction and the volumetric strain, the paper gives us a further explanation of the complex destructive process of rock. The basis for the rock’s load-carrying capacity after destruction is an effective restraint stress, which shows that effective supporting in underground engineering is the key factor for providing the cracked surrounding rock with load-carrying capacity and guaranteeing the stability of the structure.

Abstract: Based on detailed three-dimensional finite element analyses, elastic stress and strain field of ellipse major axis end in plates with different thickness and ellipse configurations subjected to uniaxial tension have been investigated. The plate thickness and ellipse configuration have obvious effects on the stress concentration factor, which is higher in finite thickness plates than in plane stress and plane strain cases. The out-of-plane stress constraint factor tends the maximum on the mid-plane and approaches to zero on the free plane. Stress concentration factors distribute ununiformly through the plate thickness, the value and location of maximum stress concentration factor depend on the plate thickness and the ellipse configurations. Both stress concentration factor in the middle plane and the maximum stress concentration factor are greater than that under plane stress or plane strain states, so it is unsafe to suppose a tensioned plate with finite thickness as one undergone plane stress or plane strain. For the sharper notch, the influence of three-dimensional stress state on the SCF must be considered.

Abstract: Failure analysis of 4 swirls in the fuel nozzle of a gas turbine was made by macroscopic and microscopic examination, and verified test were conducted. The result showed that the swirls in the fuel nozzle subjected thermal fatigue and failed in different manner for different reasons.

Abstract: A new model is proposed for the analysis of fatigue crack growth under random loading. The fatigue rule of crack length is transformed into the monotony function rule based on types of the crack. By performing reliability analysis, the randomness of the stress, the stochastic nature of the crack growth, the fuzziness of the initial crack size and the randomness of the crack critical size are considered. The First-order-second-moment approximation method is used to obtain the solution of the probability density function. An example is given to illustrate feasibility of the proposed method.

Abstract: An effective energy criterion is presented and can be narrated as follows: only the effective release energy has an immediate effect on fatigue crack growth characteristics, and the crack extension in each load cycle is decided by the effective release energy. The effective release energy is considered as the energy released for crack extension after the fatigue crack fully opens and it is equal to the consumed energy in the same crack extension. And the consumed energy can be obtained by the cracking resistance of materials. An approach for determining cracking resistance curve is proposed and validated by experimental results. Based on effective energy criterion, a model for fatigue crack growth is presented. And fatigue life can be predicted by recourse to the model. Validation against calculations by the model and experimental data shows good agreement.

Abstract: Mechanical breakdown often comes from the fatigue in many structural parts and nuclear power plants. Among the fatigue phenomenon, especially fretting fatigue occurs in mechanical joints showing small relative movements between contact surfaces. Although the research was developed for one hundred years, occurrence mechanism is not clearly identified yet. INCOLOY alloy 800 is a iron-nickel-chromium alloy having excellent resistance to many corrosive aqueous media and high-temperature atmospheres. This alloy is used extensively in the nuclear power plants industry, the chemical industry, the heat-treating industry and the electronic industry. In this paper, the effect of fretting damage on fatigue behavior for INCOLOY alloy 800 was studied. Also, various kinds of mechanical tests such as tension and plain fatigue tests are performed. Fretting fatigue tests were carried out with flat-flat contact configuration using a bridge type contact pad and plate type specimen. Through these experiments, it is found that the fretting fatigue strength decreased about 50% compared to the plain fatigue strength. In fretting fatigue, the oblique micro-cracks at an earlier stage are initiated. These results can be used as basic data in a structural integrity evaluation of heat and corrosion resisting alloy considering fretting damages.