High Cycle Fatigue Region Research Articles

Subsurface crack initiation and propagation mechanism in high-strength steel in a very high cycle fatigue regime

The mechanism of subsurface crack initiation and propagation in high strength steel in a very high-cycle fatigue region was studied by a computational simulation using a fracture surface topographic analysis (FRASTA) method for specimens of a high carbon chromium bearing steel with data obtained from rotary bending fatigue testing in air. Distinctive features of the fracture surface formed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye zone, the GBF area, was observed in detail using a scanning probe microscope and a three-dimensional SEM for comparing the microstructures of the materials. The GBF area, in which a rich carbon distribution was detected by electron probe microanalysis, revealed a rough granular morphology compared with the area inside the fish eye. It was clearly simulated by the FRASTA method in which during the fatigue process multiple microcracks are initiated dispersively by decohesion of spherical carbide from the matrix around a non-metallic inclusion, and coalesce with each other into the GBF area. In the study, the mechanism for the formation of the GBF area was proposed as the ‘dispersive decohesion of spherical carbide’ model.

Fatigue Life Evaluation for a Reaping Knife of Combine

The Combine is necessary equipment in the agricultural industry. The components of the Combine are worked for a long time under inferior environmental condition. Especially reaping knife is worked in high frequency domain, and submit fatigue load while use. So, fatigue test was performed to obtain the S-N curve of real component, and load history is measured through field test. The local stresses due to these loads have been calculated by FEM. These results have been used as the input values for the multi-axial fatigue analysis of real components. For the assessment of multi-axial fatigue damage, the critical plane methods have been employed. The used parameters of critical plane methods are Morrow’s, Smith-Waston-Topper’s, and Brown-Miller’s, those were modified for the high cycle fatigue region within elastic behavior. In addition, design improvement is performed about shape of reaping knife to increase the endurance limit, and durability test for improved knife is performed. It is found that the fatigue life of improved reaping knife is increased, and durability test result show that life of reaping knife is increased enough.

A Study of Fatigue Fracture Properties of High Strength Notched Steel in High Cycle Fatigue Region with Special Focus on the Statistical Variation of the Local Fatigue Strength at the Notch Bottom

This study was conducted to examine how and why a difference of fatigue strengths between unnotched and notched specimens, in terms of maximum stress amplitude at the notch bottom, depends on the change of hardness level in steel. As a result, the local fatigue strength of the notch bottom in the notched specimen became higher than that of the un-notched one with increase of notch sharpness and this was remarkable in high strength steel. This is because the transition of the fatigue fracture mode occurs from a slip induced crack initiation mode to a defect controled one with increasing hardness in steel. In addition, using a new experimental approach combined with a simulation technique based on the concept of risk competition of defects, a computer simulation was conducted on an effect of inclusion on the local fatigue strength at the notch bottom of high strength steel. It was clarified that the difference of fatigue strength between notched and un-notched specimens in high strength steel was caused by the statistical variation of the largest inclusion size contained in the critical volume at the notch bottom.

A Study of Fatigue Fracture Properties of High Strength Steel in High and Extremely High Cycle Fatigue Regions based on Experimental and Simulation Methods.

In order to clarify experimentally the damage process for the high strength steel in high and extremely high cycle regions and to propose a simulation model based on a new approach, rotating bending fatigue tests were performed with special focus on the effect of the role of the inclusion on the fatigue properties of high strength steel. The transition of fatigue crack initiation site from surface inclusion to internal one was observed with decreasing stress amplitude. The simulation model was proposed on the basis of the concept of a risk competition of defects, in which a virtual specimen has been constructed in the computer program and all inclusions were assumed to be equivalent to a crack origin. The results of the simulation were almost in accordance with the experimental results, and it was shown that this model could make the prediction of fatigue life of high strength steel in high and extremely high cycle regions.

Low-cycle fatigue of unidirectional composites:: Bi-linear S– N curves

Low-cycle fatigue (LCF) behavior of polymer matrix composites (PMCs) is investigated in an experimental study of unidirectional glass/epoxy composites subjected to axial tensile loading along longitudinal 0° orientation of fibers. Under high LCF loads, fatigue life of PMCs is found to be less than 10 4 loading cycles due to the high property degradation rates that are noticeably higher than those seen during high-cycle fatigue (HCF). In PMC response, unique LCF features have been identified and linked with damage accumulation patterns in unidirectional composites. At high loads near the ultimate strength of specimens, large strains and finite strain rates are found to be significant under semi-rectangular loading so the LCF behavior is affected. Lower and upper limits for the LCF life impose some restrictions on the S– N curves that are obtained for the LCF life assessment. A bi-linear S– N curve is used to approximate the data in the LCF and HCF regions. The bi-linear S– N relationship and the associated fatigue model are described by a proposed analytical formula. The concept of pre-LCF damage state is introduced.

Frictional Fatigue of a Polymer in Friction on Steel

This paper presents friction fatigue and wear-rate curves plotted using experimental data for a steel 45–polymer F4-VM friction unit. We have found low- and high-cycle fatigue regions and high-fatigue-life regions. The parameters which describe and separate these regions turn out to be stable, e.g., irrespective of which wear-rate characteristic is accepted. The paper provides a phenomenological description of wear processes in these typical regions.

A new function for fatigue curves characterization and its multiple merits

A new function is proposed for the description of fatigue curves in both low- and high-cycle fatigue regions, i.e. for the whole region of cycles from tensile strength to permanent fatigue limit (usually described by the Palmgren function). In each cycle region it can be simplified; in fact it changes into the Basquin function in the region of finite life and an analogy of the Stromeyer function for the high-cycle fatigue region can also be obtained. In comparison with the functions mentioned above, the new function has many advantages: it allows a better fit of experimental results, its parameters have unambiguous technical and geometrical meaning and they can be determined with higher accuracy. The function and its simplifications are also more suitable for extrapolation and interpolation of fatigue curves in low-cycle and very-high-cycle regions.

K-0521 超高サイクル疲労下における切欠きを有する高強度鋼の破壊モード遷移(S04-3 高強度鋼の超高サイクル疲労(2))(S04 金属材料の組織と疲労強度信頼性)

K-0521 超高サイクル疲労下における切欠きを有する高強度鋼の破壊モード遷移(S04-3 高強度鋼の超高サイクル疲労(2))(S04 金属材料の組織と疲労強度信頼性)

The interactive role of inclusions and SiC reinforcement on the high-cycle fatigue resistance of particle reinforced metal matrix composites

The effect of intermetallic inclusions on the fatigue crack initiation and growth in 2080 Al alloy and 2080/SiC p composites was investigated. Using surface replication, it was determined that, in the high-cycle fatigue region, life is dominated by the initiation process. It was also determined that the majority of initiation sites were associated with intermetallic inclusions. While 2080/SiC/20 p showed a definitive relationship between inclusion size and fatigue life, i.e., a higher inclusion size resulted in lower fatigue life, there was no correlation in 2080/SiC/30 p . This was attributed to more of the load being shared by the higher volume fraction of SiC particles and smaller average inclusion sizes in the latter composite. A conceptual model is proposed that accounts for these observations and qualitatively shows the effect of reinforcement on stress enhancement in near-surface inclusions.

Strength, Durability, and Safety of Engineering Systems

The evolution of approaches to the solution of machine dynamics and strength problems on the basis of determining static and dynamic nominal and local stresses of the operating load is discussed. It is shown how the material strength and plasticity characteristics, the cyclic strength characteristics in the high-cycle and low-cycle fatigue regions, the high-temperature long-term strength and creep characteristics, and the linear and nonlinear characteristics of fracture mechanics were sequentially used as the basic criterial parameters of the deformability and strength of structural materials. Particular emphasis was given to the results of studies on machine safety and catastrophe mechanics. Integrated approaches to the solution of strength and safety problems of potentially dangerous facilities (nuclear power stations, spacecraft complexes, aircraft, chemical plants, etc.) by analysis of all stages of their life cycle, including design, manufacture, testing, and operation, are discussed.

Effect of calcium on mechanical properties of recycled aluminiumcasting alloys

The effect of the addition of calcium on tensile properties, fracture toughness, and fatigue characteristics was investigated in JIS AC4C-T6, JIS AC2B-T6, and JIS AC4D–T6 aluminium casting alloys with various iron contents. The number of coarse platelike Al-Fe based intermetallic compounds increased with increasing iron content. The modification of the eutectic silicon morphology is successfully achieved by the addition of 40 ppm calcium. The addition of calcium makes the shape of the intermetallic compound, as well as the eutectic silicon, finer and round, and this seems to contribute to the improvement of mechanical properties, i.e. elongation, Charpy absorbed energy, both impact and conventional fatigue strengths, dynamic fracture toughness, and impact fatigue fracture toughness. Impact fatigue strength, especially in the high cycle region, increases with the addition of calcium. In the lower stress high cycle fatigue region, a higher fatigue strength with increasing iron content is observed owing to the deflected crack path along Al-Fe based intermetallic compounds.

Effect of calcium on mechanical properties of recycled aluminiumcasting alloys

The effect of the addition of calcium on tensile properties, fracture toughness, and fatigue characteristics was investigated in JIS AC4C-T6, JIS AC2B-T6, and JIS AC4D–T6 aluminium casting alloys with various iron contents. The number of coarse platelike Al-Fe based intermetallic compounds increased with increasing iron content. The modification of the eutectic silicon morphology is successfully achieved by the addition of 40 ppm calcium. The addition of calcium makes the shape of the intermetallic compound, as well as the eutectic silicon, finer and round, and this seems to contribute to the improvement of mechanical properties, i.e. elongation, Charpy absorbed energy, both impact and conventional fatigue strengths, dynamic fracture toughness, and impact fatigue fracture toughness. Impact fatigue strength, especially in the high cycle region, increases with the addition of calcium. In the lower stress high cycle fatigue region, a higher fatigue strength with increasing iron content is observed owing to the deflected crack path along Al-Fe based intermetallic compounds.

Static Fatigue Strength and Fracture Mechanism of Ceramic/Metal Joints.

Fatigue strength and fracture mechanism of ceramic/metal (Si3N4/Cu/SUS 304) joints were investigated under cyclic bending loading. Results showed clearly the characteristic of the cyclic fatigue, and the fatigue strength at l07 cycles was about one-half of the static bending strength. Different types of failure were observed for low-cycle and high-cycle fatigue regions. In the low-cycle fatigue region, the crack initiates at the copper interlayer near the interface of the ceramic side, while in the high cycle fatigue region the crack initiates throughout the entire copper interlayer. Fracture surfaces were examined by SEM and EPMA. Fatigue striations were clearly identified. 0n the basis of these results, a model is suggested for the fracture mechanism. The crack initiates and grows at the copper interlayer and finally kinks to the ceramic side. It is concluded that the copper interlayer dominates the fatigue strength of the joint.

Development of Design Margin Decision System for Advanced Materials Based on Reliability.

The design concept based on reliability is effective for advanced materials, because advanced materials such as fiber-reinforced composites have variable mechanical properties. In addition, as the failure mode in the low cycle fatigue region differs from that in the high cycle fatigue region, it is difficult to predict the fatigue life of advanced materials. So far, we have developed a design margin decision system based on reliability by evaluating fatigue life. In this study, the system is improved for application to advanced materials and a method to divide whole sets of data into the above two regions and to calculate the design margin in each region is presented. From numerical examples for CF/PEEK laminates and epoxy resin, it is revealed that a more reasonable S-N curve and design margin are obtained by the proposed system.

Cyclic Fatigue Strength and Fracture Mechanism of Ceramic/Metal Joints.

Fatigue strength and fracture mechanism of ceramic/metal (Si3N4/Cu/SUS 304) joints were investigated under cyclic bending loading. Results showed clearly the characteristic of the cyclic fatigue, and the fatigue strength at l07 cycles was about one-half of the static bending strength. Different types of failure were observed for low-cycle and high-cycle fatigue regions. In the low-cycle fatigue region, the crack initiates at the copper interlayer near the interface of the ceramic side, while in the high cycle fatigue region the crack initiates throughout the entire copper interlayer. Fracture surfaces were examined by SEM and EPMA. Fatigue striations were clearly identified. 0n the basis of these results, a model is suggested for the fracture mechanism. The crack initiates and grows at the copper interlayer and finally kinks to the ceramic side. It is concluded that the copper interlayer dominates the fatigue strength of the joint.

Fatigue strength and fracture mechanism of ceramic-metal joints under cyclic bending

Abstract The cyclic fatigue strength and fracture mechanism of the ceramic-metal joint Si3N4-SS304 were investigated under cyclic bending loading. The results show that the fatigue strength at 107 cycles was about half the static bending strength. Different types of failure were observed for the low-cycle fatigue and high-cycle fatigue regions. Fracture surfaces were examined with SEM and EPMA. Three distinctive failure zones, including a fatigue striation zone, were clearly identified. The fatigue crack initiates and grows at the copper interlayer and finally kinks to the ceramic side. It is concluded that the copper interlayer dominates the fatigue strength of the joint.

Influence of composition and distribution of the reinforcing particles on fatigue properties of metal matrix composites

Fatigue-life-time behaviour has been examined of extruded 6061 aluminium alloy composites reinforced 15 vol.% SiC and 10 vol.% Al 2 O 3 particles. The peak particle sizes are at about 4.5 and 6 μm. Whithin measured S-N curves the fatigue life-time at given stress amplitudes of SiC p /AA6061 is superiour to that of Al 2 O 3p /AA6061 in the low-cycle fatigue region as well as in the high-cycle fatigue region. The discussion of these results has been done by means of theoretical evaluated crack propagation curves. Interfacial bonding has been studied by means of TEM investigations

Fatigue properties of particle-reinforced metal-matrix composites

Fatigue-lifetime behaviour has been examined for extruded 6061 aluminium alloy composites reinforced with 15 vol% SiC and 10 vol% Al 2O 3 particles. The peak particle sizes are at about 4.5 and 6 μm. Within measured S- N curves the fatigue lifetime at given stress amplitudes of SiC p/AA6061 is superior to that of Al 2O 3p/AA6061 in the low-cycle fatigue region as well as in the high-cycle fatigue region. These results are discussed by consideration of theoretically evaluated crack propagation curves.

PREDICTING FATIGUE CRACK INITIATION LIFE OF AN ALUMINIUM ALLOY AT LOW TEMPERATURES

Abstract— A simple approach to predict the fatigue crack initiation (FCI) life of notches at low temperatures is developed. The stress cycle to initiate a 0.25 mm crack at a notch tip is presented as a function of an equivalent stress amplitude. The results of the study indicate that the FCI life of an aluminium alloy increases with decrease in temperature and the effect of temperature on the FCI resistance can be predict in the form of an FCI coefficient C and the FCI threshold. The coefficient C is the parameter controlling FCI resistance in the low cycle fatigue region and it can be calculated from the elasticity modulus and the strain hardening exponent. The threshold is the parameter governing FCI resistance in the high cycle fatigue region and its value depends mainly upon the endurance limit. The temperature- dependence of the endurance limit can be predicted by a thermal activation model. By means of this approach, the fatigue crack initiation life at low temperatures can be predicted from the material tensile properties without any additional low temperature fatigue tests and without any empirical modification so long as the endurance limit of the metal at room temperature is predetermined. The results suggest that this approach is applicable to other aluminium alloys.

Cyclic hardening and substructure of AlMg alloys

The dislocation substructure and macroscopic regularities of cyclic hardening of aluminium alloy AMg6 are studied in the low and high cycle fatigue regions. The density of dislocations and dislocation loops as well as the microhardnesses of the surface and the near-surface layer are measured as functions of the strain amplitude, the number of cycles, the environment and the temperature. It is found that, as the number of cycles increases, the quantitative evolution of the initial dislocation substructure occurs mainly in the near-surface layer and the cyclic hardening develops in two (the high amplitude region) or three (the low amplitude region) stages. The importance of each structural component is estimated in relation to hardening in these regions. On the basis of our results and reports (on alloys with lower contents of magnesium) in the literature the effects of the stacking fault energy upon the substructures due to cyclic hardening of AlMg alloys are analysed in a wide range of magnesium contents (0–6.5 at.%) together with the difference between the substructures of these alloys and those of pure f.c.c. metals under similar conditions.