Papers by Keyword: Crack Nucleation

Abstract: In order to ensure the long term durability of mechanical structures, the fatigue property of structural components should be clarified in the long life region such as the gigacycle regime. The rotating bending fatigue tests in very high cycle regime were carried out for a nickel chromium molybdenum steel for structural use of machines (JIS Material Code: SNCM439) in this study. Based on the initiation site of the fatigue crack, fracture modes were classified into the following typical three modes: (1) usual surface fracture, (2) surface defect-initiated fracture and (3) interior inclusion-initiated fracture, respectively. In S-N diagram, experimental data in the usual surface fracture mode appeared at higher stress levels with fewer loading cycles, whereas the data in the other two fracture modes appeared at lower stress levels with more loading cycles. Thus, the duplex S-N property was confirmed for this steel in the very long life regime. In order to clarify the fatigue mechanism of the interior inclusion-initiated fracture, the quantitative evaluations were made by applying the stress intensity factor range. The fatigue crack initiation and propagation processes in the interior inclusion-initiated fracture were divided into four stages: formation of the fine granular area (FGA) due to initiation and coalescence of micro-debondings, formation of the fish-eye due to penny-shape crack propagation, crack propagation as surface crack and final catastrophic fracture.

Abstract: It is now possible to formulate the relation of the linear size of cracking arising on welding the cylindrical homogeneous-metal. Mathematical simulation of metal crystallization on welding and micro structural analysis give an insight into the fact that there begins crack nucleation in a weld root. Experimentally, by means of Acoustic Emission (AE), one can study welding and obtain the amplitude distribution of AE signals from cracking against the background of the hindrance accompanying this process. The conditions were found making impossible cracking.

Abstract: This research uses a non-destructive method of neutron diffraction to measure the tri-axial residual stresses in a friction stir welded aerospace fuselage component: a stringer-to-skin lap joint. Two different specimens were examined. Fatigue testing was performed on both specimens to determine their fatigue lives. Effects of the different components of residual stresses were examines and related to fatigue performance. A combination of fractography, hardness testing, and residual stress measurement was used to predict areas of high probability of structural failure in the friction stir welded lap joints.

Abstract: This research focuses on high cycle fretting fatigue crack nucleation prediction. A plastic steel/steel cylinder/plane contact was investigated keeping constant the normal force and the maximum fatigue stress ratio but varying the fatigue stress ratio (R_F=0.6 to 1). The evolution of the crack length as function of the applied fretting tangential force amplitude at 10⁶ cycles allows us to formalize the crack nucleation condition. It shows that the threshold tangential force marking the crack nucleation (i.e. b_{p_th}=0μm) is not affected by the fatigue stress ratio. However an increase of the fatigue stress amplitude sharply increases the crack extension. To model the experiments, a 2D plastic plain strain FEM modeling is performed. As expected the computed stress field description is mesh dependent. However we demonstrate that a representative stress description is obtained at the 3^rd node (i.e. 2^nd node below the surface). By coupling this mesh condition and non-local critical distance approach, reliable prediction of the crack nucleation risk can be achieved either considering a Crossland multi-axial fatigue analysis or using a basic uni-axial Haighs description.

Abstract: Study on tensile fracture behavior of TiAl alloy by means of the macro fracture theory and micro dislocation block theory. A quantitative analysis method of micro crack nucleation and crack mechanism for TiAl alloy is performed with the help of the dislocation distribution model, and is based on the strain energy density theory and criterion, a crack criterion of TiAl alloy instability is established. The experimental results confirmed that the dislocation model and S criterion on tensile fracture behavior of TiAl alloys are effective.

Abstract: This paper summarizes the rubber component's fatigue life research methods, mainly includes the crack nucleation method, the crack propagation method and the S-N curve method, sums up the these methods from different angels of fatigue life, and various methods based on the choices are given; Based on the working conditions and fatigue failure evaluation criteria to choose the appropriate design calculation method, in order to achieve relatively accurate results of fatigue life prediction and simplify the calculation, and separately summarized FEM used in the rubber fatigue life on the application and progress in the recent stage.

Abstract: Fatigue cracks in polycrystalline copper may originate from PSBs or grain boundaries. They usually form at the specimen surfaces, but also internal small stage I (shear) cracks have been observed with the ECC/SEM technique. They are formed together with a strongly elongated dislocation cell structure, which is reflecting in many cases localized deformation in “slip lamellae” with eventual ladder-like features, being typical of PSBs. Both, PSBs and small non-propagating cracks are initiated at cyclic stress/plastic strain amplitudes below the conventionally reported PSB threshold values, if the number of cycles exceeds a minimum, e.g. approximately 5x108 in the VHCF range. The internal small cracks are formed not only in polycrystalline electrolytic copper of 99.98% purity but also in high purity (99.999%) material.

Abstract: Wear and rolling contact fatigue interaction in rolling/sliding contact is an important topic in the research on structural integrity of rails and railway wheels. Wear is in competition with rolling contact fatigue, as it removes surface material layers, reducing cracks length and hindering their propagation. Cracks nucleate by accumulation of cyclic unidirectional plastic strain (ratcheting). In this paper a model for ratcheting assessment is discussed and applied to the UIC 900A steel, after a calibration based on experimental results. The experimental tests allow also a characterization of the crack formation condition for this material. By this model, a computer program is developed in order to simulate in a very short time the effect of a large number of load cycles, providing a tool for predicting crack formation and propagation rates.

Abstract: In this paper, the characterization of fretting damage on press-fitted specimens is proposed by experimental methods. A series of fatigue tests and interrupted fatigue tests on pressfitted specimens were carried out by using a rotate bending fatigue test machine. Macroscopic and microscopic characteristics were observed to identify fretting damage mechanism with a scanning electron microscope (SEM) and profilometer. The mechanism of fretting fatigue damage on pressfitted structure is discussed from experimental results. It is found that small cracks of 30~40
m in depth are initiated when the specimen reached about 10% of the total life, and thus almost 90% of the fretting fatigue life of press fits can be considered to be in the crack propagation phase. Most of fatigue cracks are initiated at 1050
m inner side of contact edge, and multiple cracks are nucleated and interconnected in the fretted surface. The crack nucleation angle in the near contact edge region is larger than that in the inside of the contact edge region. The fretting wear increased with increasing fatigue cycle. Since the fretting wear is relevant to the evolution of surface profile, the fretting fatigue is observed to be closely related with the fretting wear.

Abstract: Interstitial free (IF) steels are widely used as thin sheet in the automobile industries because of their many favorable properties. Although, fatigue properties of IF steels do not have significant importance to auto body makers, however, they are very concerned about the tensile and fatigue strength of the steels used for structural purposes to ensure safety of passengers. So, fatigue results of this steel might help researchers to understand the behaviors of high strength steels. In this study cyclic and static properties have been studied at room temperature in the air. Initiative has also been taken to observe the fatigue fracture morphology of this steel. Experimental results show that the fatigue limit is corresponding to about 40% of tensile strength and 80% of the yield strength of this steel. Fractographic observations reveal a mixed type of fracture mode (intergranular and transgranular cracking) fractures.