Static Fatigue Strength Research Articles

Crack-Healing under Cyclic Stress and Improvement of the Resultant Fatigue Strength of Si<sub>3</sub>N<sub>4</sub>/SiC

Si3N4/SiC composite ceramics were hot-pressed in order to investigate their crack-healing behavior under cyclic stress and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000 °C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that when the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.

Time-dependent fracture behavior of single-walled carbon nanotubes with and without Stone-Wales defects

Based on a concept of molecular-level strain energy concentration and a kinetic fracture model, the time-dependent fracture behavior of three different types of single-walled carbon nanotubes (zigzag, armchair, chiral) with or without Stone-Wales (SW) defects were studied. It is found that the SW defects have influences on the site of crack initiation but the path of crack propagation is closely related to the chirality of the nanotube. Defect-free armchair and chiral tubes exhibit higher static fatigue strength than the zigzag tube of similar size. All of the single-walled carbon nanotubes with SW defect are predicted to have a shorter static fatigue life compared with the defect-free tubes. Moreover, geometric form of the defects is found to influence the time-dependent behavior. Predictions with the present model agree reasonably well with experimental stress-life data of single-walled carbon nanotube rope in epoxy.

2738 ムライト/炭化ケイ素マルチ複合材の応力下における自己き裂治癒挙動(S28-3 セラミックスおよびセラミックス系複合材料(3),S28 セラミックスおよびセラミックス系複合材料)

A mullite / SiC whisker / SiC particle multi-composite, having excellent crack-healing ability and mechanical properties, was hot-pressed in order to investigate the crack-healing behavior under stress and the resultant fatigue strength at the temperature of healing. A semi-elliptical surface crack 100 μm in surface length was introduced on each specimen. The pre-cracked specimens were crack-healed under cyclic or constant stress by using a three-point bending stress at 1473 K, and the resultant bending strength and cyclic fatigue strength were measured at 1473 K. The pre-crack on the surface of the specimens could be healed even under stress. The threshold stresses for crack-healing, as determined by evaluating the strengths of crack-healed specimens at a healing temperature of 1473 K, were 170 MPa for both constant and cyclic stresses, corresponding to 77% of the bending strength of the pre-cracked specimens. The static and cyclic fatigue strengths of crack-healed specimens were also investigated at a healing temperature of 1473 K.

3505 伸線強加工高強度鋼の疲労強度特性に及ぼす水素吸蔵状態の影響(S20-2 水素チャージ材の強度特性,S20 材料強度特性に及ぼす水素の影響)

Slow strain rate tests (SSRT) and fatigue tests were conducted to investigate the influence of internal hydrogen state on the quasi static SSRT strength and fatigue strength for cold drawn eutectoid steel specimens which were cathodically hydrogen charged. Internal hydrogen states were changed as follows: (a) non-charged sample, (b) the sample that contained both diffusive and non-diffusive hydrogen, and (c) the one that contained only non-diffusive hydrogen. The results of SSRT indicated that only the diffusive hydrogen caused hydrogen embrittlement under the quasi-static loading condition. S-N curves had a large scatter and the influence of non-diffusive hydrogen on the fatigue strength was unclear, because the fatigue crack was initiated at internal inclusion. Therefore, the fatigue strength was discussed based on the stress intensity factor, ΔK_<inc>, calculated from stress and inclusion size. ΔK_<inc>, giving the same N_f decreased in the order of the non-charged specimen, only non-diffusive hydrogen charged specimen and the specimen having both diffusive and non-diffusive hydrogen, indicating that non-diffusive hydrogen caused a decrease in fatigue strength.

5402 IHジャー炊飯器の内釜開発(S72 CAE・最適化と設計,S72 CAE・最適化と設計)

Reliability technology has been developed for the IH carbon-jar rice cooker which cooks plump, tasty rice like a expert. Because this rice cooker's inner jar features a new carbon sintered material for superior thermal conductivity, the problem of ensuring the carbon jar's reliability has been an important issue. From the coupled thermoelastic analysis results, it was possible to estimate the thermal distribution at the carbon jar during rice cooking operations, and to evaluate in advance the stress produced. Moreover, static bending test and fatigue strength tests were performed to determine the variations of material characteristics variations. These tests indicated that the product offers an acceptable long-term reliability (99.9% or higher). Finally, the reliability was verified by actual product temperature measurements and testing under accelerated operating conditions, which contributes to efficient product development.

Fatigue strength of carbon fibre composites up to the gigacycle regime (gigacycle-composites)

Knowledge of the fatigue strength is necessary for design purposes also at very high numbers of load cycles. While static properties and fatigue strength curves in the low, medium and high cycle fatigue regime 1 1 Definitions of the different fatigue regimes are here used as follows: low cycle, 1−5×10 4; medium cycle, 5×10 4–5×10 6; high cycle, 5×10 6–5×10 7; gigacycle, 5×10 7–1×10 9; run out, 1×10 9. are well investigated, only few data beyond 5×10 6 cycles (gigacycle fatigue regime) are available. In a first part of this paper, the fatigue strength for various laminates of a carbon fibre reinforced thermoplastic material (AS4/APC-2) was investigated experimentally up to 10 9 cycles. The specimen shape was optimised for tensile, compression and bending load cases to get significant fatigue data. Various candidates of fatigue damage indicators have been evaluated and monitored continuously during these fatigue tests. A general trend of the fatigue strength reduction could be observed for the majority of laminates and loading conditions beyond 5×10 6 load cycles. A fatigue limit (endurance limit) was not found. Significant changes of the stiffness, resonance frequency and temperature could be measured. However, a correlation between these signals and the final failure of the specimen could not be found. In a second part, a fatigue design procedure was developed, which allows predicting the fatigue life of a typical structural detail under bending loads. Various failure criteria used in the static mode have been extended to fatigue loads. In a verification test program, the fatigue strength of a waisted beam has been assessed for bending loads and compared with the predicted fatigue life.

Fatigue Behaviour of Multilayer Braided Fabric Reinforced Laminates

In this paper, the fatigue behaviour of the laminated composites made from an epoxy matrix and four layers of flat regular braided carbon fiber fabrics of different braiding angles was investigated experimentally. Three laminate lay-ups were used. They are: [100/100/100/100], [280/100/100/280], and [280/280/280/280], where 100 within the bracket indicates that the single layer braid lamina had a braiding angle of 100 inclined with the longitudinal (loading) direction. The static tensile and tension-tension fatigue tests were conducted for those laminates, and detailed fatigue data are reported in the paper. Failure mechanisms of the tested laminates were also characterized. The stiffnesses and strengths (including static tensile and fatigue strengths) of the [100/100/100/100], [280/100/100/280], and [280/280/280/280] laminates were the highest, intermediate, and the lowest respectively. However, different failure mechanisms were recognized. It was found that under the uniaxial tensile or a high-level fatigue load, the [280/280/280/280] laminate failed catastrophically as a result of fiber breakage nearly across the cross section of the specimen. Delamination was seen only when a low-level fatigue load was applied to the laminate. In the case of the [100/100/100/100] laminate, a laminate longitudinal splitting was the main failure mode. No delamination accompanied the laminate failure. For the [280/100/100/280] laminate, significant delamination was found with any applied load (whether static or tensile fatigue of any load level). Splitting, both across the cross-sectional breakage and longitudinal, were involved in the failure of this combined laminate. These findings provide a useful insight into understanding the fatigue behaviour of braided fabric reinforced laminates.

Improvement of static fatigue strength of Si 3N 4/SiC crack-healed under cyclic stress

Si 3N 4/SiC composite ceramics were hot-pressed in order to investigate their crack-healing behaviour and the resultant static fatigue strength. Semi-elliptical surface cracks of 100 μm in surface length were made on each specimen. The pre-cracked specimens were crack-healed under a cyclic bending stress of 210 MPa in air at 900, 1000, 1100, and 1200 °C. The bending strength and static fatigue strength of the crack-healed specimens were systematically investigated at each healing temperature. The specimens which has been crack-healed and static fatigue-tested at 900 and 1000 °C showed lower static fatigue strength than those tested at 1100 and 1200 °C. Detailed investigation on the fracture surface of static fatigue-tested specimens showed that oxidation of the base material had strong effects on the static fatigue strength. It was found that if the specimens were pre-oxidized in air at 1300 °C, the surface was covered by a protective oxide layer, leading to a significant improvement of static fatigue strength at 900 and 1000 °C.

High‐Temperature Fatigue Strength of Crack‐Healed Al2O3Toughened by SiC Whiskers

Al2O3reinforced by SiC whiskers (Al2O3/SiC‐W) was hot‐pressed to investigate the fatigue strength of crack‐healed specimens at high temperature. Semielliptical surface cracks of 100 μm surface length were introduced on each specimen surface. These specimens were crack‐healed at 1300°C for 1 h in air, and static and cyclic fatigue strengths were systematically investigated at room temperature, 900° and 1100°C by three‐point bending. The static and cyclic fatigue limits of the crack‐healed specimens were more than 70% of the average bending strength at each testing temperature. Crack‐healed specimens of Al2O3/SiC‐W were not sensitive to static and cyclic fatigue at room temperature and high temperatures. Therefore, the combination of crack‐healing and whisker reinforcement can play an important role in increasing static and cyclic fatigue strengths at high temperature.

Torsional Fatigue Characteristics of Aluminum–Composite Co-Cured Shafts with Axial Compressive Preload

Power transmission shafts such as driving shafts or automotive propeller shafts should transmit static and dynamic torques with vibrational stability. Hybrid shafts made of unidirectional fiber-reinforced composite and metal have high fundamental bending natural frequency as well as high torque transmission capability: composite increases the fundamental bending natural frequency due to its high specific stiffness and metal such as aluminum or steel transmits the required torque. However, fabrication-induced thermal residual stresses due to the coefficient difference of thermal expansion of the composite and the metal are developed during manufacturing hybrid shafts so that the high residual stresses decrease fatigue resistance of the hybrid shafts, especially at low operating temperatures. In this paper, the torsional fatigue characteristics of aluminum–composite co-cure joined shafts with axial compressive preload were investigated. To change the thermal residual stresses, an axial compressive preload was given to the aluminum tube by a compressive jig during the co-cure bonding operation. In order to determine the thermal residual stresses with respect to the preload and temperature difference, stress analyses were performed by simple equations from mechanics of materials and finite element method. Then the static torque capacities and fatigue strengths of the hybrid shafts from static and fatigue torsional tests were correlated with the calculated thermal residual stresses. The fatigue strength of the hybrid shaft was much improved by the axial compressive preload, exceeding that of a pure aluminum shaft. Also, the degradation of the fatigue resistance of the hybrid shaft at subzero operating temperature was overcome by the axial compressive preload.

アルミナ分散強化銅の室温および３５０℃における疲労挙動と疲労寿命に及ぼす応力変動の影響

In order to investigate the influence of stress change on the fatigue behavior and fatigue life of an aluminum oxide-dispersion-strengthening copper alloy at elevated temperature, rotating bending fatigue tests were carried out under two-step loading at room temperature and 350°C. Both of static strength and fatigue strength decreased at 350°C. However, at the same relative stress σa, /σB, fatigue life was longer at 350°C than at room temperature. Although the cumulative ratios Σ (N/Nf) were nearly unity for both the low to high and the high to low block loadings at room temperature, Miner's rule did not hold at 350°C. These results were related to the stress dependence on the log l-N/Nf relation. That is, the crack length initiated at the same N/Nf was larger in higher stress level at 350°C, whereas there was no stress dependence in the relation at room temperature. The stress dependence on the relation at 350°C was caused by the suppression of crack initiation due to the surface oxidation.

Improvement of Static Fatigue Strength of Silicon Nitride Composite Crack-Healed under Cyclic Stress(International Workshop on Smart Materials and Structural Systems, W03 Jointly organized by Material &amp; Processing Division, Material &amp; Mechanics Division, Dynamics &amp; Control Division and Space Engineering Division.)

Improvement of Static Fatigue Strength of Silicon Nitride Composite Crack-Healed under Cyclic Stress(International Workshop on Smart Materials and Structural Systems, W03 Jointly organized by Material &amp; Processing Division, Material &amp; Mechanics Division, Dynamics &amp; Control Division and Space Engineering Division.)

Crack-healing behavior and static fatigue strength of Si3N4/SiC ceramics held under stress at temperature (800, 900, 1000 °C)

Abstract Si3N4/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100 μm in surface length was introduced on each specimen. The pre-crack was healed under constant bending stress of 210 MPa at 800, 900 and 1000 °C. Applied stress of 210 MPa is ∼70% of the bending strength of pre-cracked specimen. Bending strength and static fatigue strength of crack-healed specimens were systematically investigated at each crack-healing temperature. The bending strength of crack healed specimen showed almost the same value as smooth specimen. Thus, Si3N4/SiC composite ceramics could heal a crack even under constant bending stress of 210 MPa at 800, 900 and 1000 °C. Moreover, crack-healed zone had quite high static fatigue limit at each crack-healing temperature. These conclusions indicate that Si3N4/SiC composite ceramics has an ability to heal a crack under service condition, i.e. high temperature and applied stress.

911 人工関節用アルミナの擬似生体環境中静疲労強度の評価

911 人工関節用アルミナの擬似生体環境中静疲労強度の評価

Crack-healing behavior of Si3N4/SiC ceramics under stress and fatigue strength at the temperature of healing (1000 °C)

Si 3 N 4 /SiC composite ceramics were sintered and subjected to three-point bending specimens made according to the appropriate JIS standard. A semi-circular surface crack of 100 μm in diameter was made on each specimen. We systematically studied crack-healing behavior, and cyclic and static fatigue strengths at the service temperature (1000 °C) by using three kinds of specimens (smooth, cracked and crack-healed). The main conclusions are as follows: (1) Si 3 N 4 /SiC composite ceramics have the excellent ability to heal a crack at 1000 °C; (2) this sample could heal a crack even under cyclic stress at 1000 °C; (3) a new crack-healing process was proposed. The sample crack-healed at 1000 °C by the process exhibited a sufficient static and cyclic fatigue strength at 1000 °C.

Measurement of orthotropic electric conductance of CFRP laminates and analysis of the effect on delamination monitoring with an electric resistance change method

Since delaminations of composite laminates are usually invisible or difficult to detect by visual inspections, delamination causes low reliability for primary structures. Automatic systems for delamination identifications in-service are desired in order to improve this low reliability. The present study employs an electric resistance change method for detection of delaminations. Since the method adopts reinforcement carbon fibre itself as sensors for delamination detections, this method does not cause reduction of static strength or fatigue strength; also, this method is applicable to existing structures. In the present study, a relationship between fibre volume fraction and orthotropic electric conductivities is confirmed by experimentation and the effect of measured orthotropic electric conductance on delamination monitoring is discussed analytically with FEM analyses. Two types of cross-ply laminates are prepared for delamination monitoring analyses: [0/90]s and [90/0]s. Electric resistance changes due to delamination creation are discussed for both specimen types with results of electric current density diagrams. As a result, it can be concluded that the fibre volume fraction has a large effect on electric conductance of the transverse and thickness directions, and electric conductance of the thickness direction has significant effects on delamination detection with the electric resistance change method.

Effects of stress waveform and water absorption on the fatigue strength of angle-ply aramid fiber/epoxy composites

The influences of stress waveform and water absorption on the tension–tension fatigue fracture behavior were investigated in ±45° angle-ply laminates of aramid fiber reinforced epoxy matrix composite. For dry specimens, the fatigue strength under negative pulse waveform was higher than that under the positive pulse waveform. Rotation of fibers to the longitudinal direction, which resulted from creep deformation caused by the cyclic loading superimposed on the maximum stress hold time, decreased the compliance, thereby increasing the fatigue life under the negative pulse waveform. Water absorption degraded the fiber/matrix interfacial strength and caused the swelling of the matrix, which resulted in decreases in the static tensile strength and fatigue strength. Although the strength under the negative pulse waveform was slightly higher than that under the positive one, the influence of stress waveform on fatigue strength was smaller in wet specimens.

Self Crack-Healing Behavior under Stress of Silicon Nitride Ceramics and Resultant Strength at the Crack-Healed Temperature.

Si3N4/SiC composite ceramics were sintered and subjected to three-point bending. A semi-elliptical surface crack of 100μpm in surface length was made on each specimen. A crack was healed under static bending stress of 210 MPa at 800, 900 and 1000°C. Applied stress of 210 MPa is ∼70% of the bending strength of as-cracked specimen. Bending strength and static fatigue strengths of crack-healed specimens were systematically investigated at each crack-healed temperature. The bending strength of crack healed specimen showed almost the same value as the smooth specimen. Thus, Si3N4/SiC composite ceramics could heal a crack even under static bending stress of 210 MPa at 800, 900 and 1000°C. Moreover, crack-healed zone had sufficient static fatigue strength at each crack-healed temperature. These conclusions indicate that Si3N4/SiC composite ceramics has the ability to heal a crack under service condition, i.e. high temperature and applied stress.

Static and fatigue strength of a die-cast aluminium alloy under different feeding conditions

This paper reports investigations of the influence of porosity and casting defects on the static and constant-amplitude fatigue strength of a die-cast aluminium alloy. Three batches of specimens, differing in the sprue-runner design and consequently in content and types of defects, were tested in ‘as-cast’ conditions. Defects consisted of gas and shrinkage pores as well as cold fills, dross and alumina skins. Casting defects were observed to reduce significantly the static and fatigue properties of the material. Whereas for the static characteristics the decrease was progressive with the porosity range, for the fatigue strength the decrease was most significant from the lowest to the middle porosity range. The batches were classified with regard to the porosity level, as the metallurgical defects were not detectable through X-ray examination. The content and size of metallurgical defects were observed to increase together with the porosity level. Scanning electron microscopy (SEM) observation of the fracture surfaces demonstrated the important role played by dross, alumina skins and, above all, cold fills on the fatigue fracture.

Fatigue Strength and Fracture Mechanisms of Porous Ceramics

Static and cyclic fatigue strength in porous SiC and Cordierite ceramics were evaluated using quasi-static and cyclic tests, i.e. applied force, P, increasing and applied force range, ΔP, increasing tests, respectively. The evaluation method used Weibull distribution, in which the fracture parameters are stress at failure, σf, for quasi-static loading and maximum stress at failure, σmax, f, for cyclic loading. The results revealed that the static fatigue behavior had an important role on the degradation of fracture strength and that cyclic component of loading do not influence on the fracture strength. Statistical characteristics of the fatigue strength were expressed by P-S-N curve estimated by the results of ΔP-increasing tests. During the fatigue experiments, crack nucleation was detected using acoustic emission (AE), and the fracture surfaces were observed by SEM. Based on these results, fracture mechanisms of the porous SiC and Cordierite ceramics were controlled by particle binders, and the binder fractures have a great variety of magnitude.