Higher Crack Growth Resistance Research Articles

308 部分安定化ジルコニアの疲労特性に及ぼすアニールの影響(GS3 疲労・破壊(1),研究討論セッション)

The fatigue behavior of partially stabilized zirconia added to 3 mol % Y_2_O3 was studied with reference to the tetragonal-to-monoclinic phase transformation by static and cyclic fatigue tests for both the specimens annealed at 200℃ for 10hr in air and that at 700℃ for 10hr in vacuum, respectively. As the results, both the higher fatigue strength and the higher crack growth resistance can be obtained by the anneal at 700℃ for 10hr in vacuum, whereas the remarkable degradation in the lifetime by cyclic loading was also observed.

Review and Analysis of Powder Prior Boundary (PPB) Formation in Powder Metallurgy Processes for Nickel-based Super Alloys

Powder metallurgy of nickel-based super alloys have been developed and used for a wide range of products, owing to their excellent high temperature rupture life, creep strength and fatigue crack growth resistance. Typical processes for high performance PM super alloys include hot isostatic pressing (HIPing), hot extrusion and hot isothermal forging. Hot isostatic pressing is normally conducted at a high temperature by using metal forming standards at low pressure for a long time. However an intrinsic problem, i. e. powder prior boundary (PPB) precipitate networks, can occur in the HIPing process. In this paper, a comprehensive review of PPB formation in the powder metallurgy of super alloys is presented. Research on PPB formation process from room temperature to high temperature and during HIPing, has been reviewed and PPB formation mechanisms are discussed. Methods to reduce PPB effects on the mechanical properties of nickel-based super alloys, including heat treatment, changing the proportions of composition elements, and plastic deformation to break up the PPB networks, are reviewed.

High-temperature crack growth in a Ni-base superalloy during sustained load

The high-temperature sustained load crack growth behaviour of a Ni-base superalloy was investigated using a combination of mechanical testing in controlled atmosphere, fractographical and microanalytical investigations, and finite element modelling. The results show that the local crack front geometry is un-even on two scales – jaggedness on the scale of 100μm was observed in all specimens, whereas mm-scale waviness could occasionally be observed. The jaggedness can be explained by a percolation-type crack growth along weaker grain boundaries, whereas the large-scale waviness is presumably due to larger regions of the material having specific grain texture with high crack growth resistance. The un-even crack front is shown to potentially have considerable effects on the loading conditions at the crack tip, whereas ligaments of un-cracked material in the crack wake are deemed to have less effect on the crack tip loading due to their low area fraction. The ligaments fail intergranularly in the wake as the crack grows in the present case, as opposed to by creep fracture as previously proposed. Finally, the plastically deformed regions about the crack and crack tip are shown not to exhibit any elevated oxygen levels, implying that the damage in these regions is purely mechanical.

Mechanical strength and failure characteristics of cast Mg–9%Al–1%Zn alloys produced by a heated-mold continuous casting process: Fatigue properties

The fatigue properties and failure characteristics of a cast Mg alloy (AZ91: Mg–Al8.9–Zn0.6–Mn0.2) produced by heated-mold continuous casting (HMC) and conventional gravity casting (GC) are investigated. Excellent fatigue properties are obtained for the HMC alloy compared with the GC alloy. The high fatigue strength of the HMC alloy is a reflection of its improved microstructural characteristics, namely, tiny α-Mg grains and fine spherical eutectic structures (β-Mg17Al12). Fatigue cracks propagate mainly in the α-Mg grains and along high-hardness β-phases in both alloys. The direction of fatigue crack growth is altered as the crack reaches the eutectic phases. Because the tiny eutectic phases are distributed randomly in the HMC alloy, a meandering crack path is formed, which results in high crack growth resistance, leading to the high fatigue strength. For the HMC alloy, a striation-like failure mode can be seen in the crack growth stage, and dimple fracture is the dominant feature in the final failure stage. On the other hand, cleavage-like brittle failure with many microcracks can occur in the GC alloy. Severe lattice strain occurs during the final failure stage, especially for the HMC alloy, resulting in strong work hardening, although this is weak in the crack growth site. Severe strain is distributed uniformly around the entire cracks in the HMC alloy when the sample is fractured by monotonic loading. The lattice strains in the HMC alloy under monotonic and cyclic loading are overall higher than those in the GC alloy, owing to the high material ductility of the HMC alloy.

Effect of Processing Layer on Fatigue Strength of Extruded AZ61 Magnesium Alloy

Fatigue Property of an Extruded AZ61 Magnesium Alloy with the Processing Layer Introduced by Machining was Investigated. Rotating Bending Fatigue Tests were Carried out with the Specimen with and without the Processing Layer. According to Results of the Fatigue Tests, Fatigue Life Significantly Increased by Introducing the Processing Layer to the Specimen Surface. Fatigue Crack Initiation and Propagation Behaviors were Observed by Replication Technique during the Fatigue Test. Fatigue Crack Initiation Life of the Specimen with the Processing Layer was Slightly Longer than that of the Specimen without the Processing Layer. Higher Fatigue Crack Growth Resistance was also Observed when the Fatigue Crack was Growing in the Processing Layer in the Specimen with the Processing Layer. the Longer Fatigue Life Observed in the Fatigue Test in the Specimen with the Processing Layer could be Mainly due to the Higher Crack Growth Resistance. it is Speculated that the Fatigue Strength can be Controlled by Change in Condition of Machining Process. it could be Effective way in Industry to Improved Fatigue Strength only by the Cutting Process without Additional Surface Treatment Process.

Fatigue properties (S–N and da/dN–ΔK) of cast aluminium alloys

In this paper, the fatigue strengths of cast aluminium alloys have been examined using the S–N and da/dN–ΔK approaches on various cast samples having different microstructural characteristics. These were made by several different casting techniques, namely gravity casting, die casting and twin rolled continuous casting. Owing to the different microstructures, the fatigue properties varied. The S–N relations, e.g. the endurance limit, of the twin-rolled casting were apparently high compared to the other cast samples. The high fatigue strength was related to the high tensile strength and high ductility caused by the tiny spherical α-Al phase and fine eutectic structure. On the other hand, the low fatigue strength for the gravity and die cast samples was caused by the high stress concentration caused by the α-Al phase and needle-like eutectic silicon. Unlike the fatigue strength in the S–N approach, the da/dN–ΔK relationships for the twin rolled casting sample, i.e. the resistance to fatigue crack propagation, were low compared to the gravity casting and the die cast samples. Such a difference in the crack growth resistance was influenced by the crack growth characteristics. For example, for the gravity cast and the die cast samples, the crack growth occurs in the grain boundaries of the coarse α-Al phase and along the long eutectic silicon grains (zigzag crack path), which promotes a strong crack closure tendency, leading to high crack growth resistance. The difference in the fatigue strength determined from the S–N relation and da/dN–ΔK relation was interpreted by experimental and numerical examination of the stress–strain distribution.

Effect of domain wall characteristics on material properties of lead zirconate titanate piezoelectric ceramics

To better understand the material properties of lead zirconate titanate (PZT) ceramics, the influence of domain wall characteristics on the electrical and mechanical properties of PZT ceramics has been investigated. To obtain various domain wall characteristics, the poling process was carried out with different patterns with respect to the PZT ceramic. The domain walls were aligned in the PZT ceramic in the direction perpendicular to the poling direction. Such domain wall characteristic produced different crack growth behaviours. The crack growth occurred mainly along the domain walls of {110} for a31 (crack growth direction perpendicular to the poling direction) because of the high stress concentration between the domain walls. In contrast, the crack propagated along both domain walls and grain boundaries for a33 (crack growth direction parallel to the poling direction), leading to high crack growth resistance and good mechanical properties. The fracture toughness KIC values for the PZT ceramics in the a31 and a33 directions were about 0·5 and 2·6 MPa m1/2 respectively. It also appeared that the domain walls collapsed when a number of poling processes are conducted along different directions. These PZT ceramics had high mechanical properties due to the low stress concentrations.

The effect of microstructures on fatigue crack growth in Q345 steel welded joint

ABSTRACTIt is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R‐ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R‐ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R‐ratio increasing. To the low R‐ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R‐ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse‐grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack‐tip stress shielding and then resulted in higher crack growth resistance.

Effects of size and spacing of uniformly distributed pearlite particles on fatigue crack growth behavior of ferrite–pearlite steels

Fatigue crack growth tests of three ferrite–pearlite steels with different size and spacing of pearlite particles, which were uniformly distributed in the ferrite matrix, were carried out to investigate the effect of microstructure on fatigue crack growth behavior in the Paris regime. The fatigue crack growth rates for the three materials did not coincide with each other, even when the crack growth curves were arranged by the effective stress intensity factor range. From in situ observations, crack tip stress shielding phenomena, such as interlocking and branching, were found on the crack wake, which enhanced fatigue crack growth resistance. A small size and spacing of pearlite particle seemed to induce small but frequent crack deflections, which resulted in crack closure phenomena. On the other hand, the large size of the pearlite particle seemed to induce stress shielding phenomena and, thus, contribute to high crack growth resistance, which was the main reason for the higher fatigue crack growth resistance of the large size and spacing of pearlite particles compared to the small size of pearlite particles.

Comparison of mechanical properties of die cast aluminium alloys: cold v. hot chamber die casting and high v. low speed filling die casting

In this paper, the mechanical properties of die cast aluminium alloys made by various die casting technologies were examined. To create high quality aluminium alloy die castings, two die casting processing technologies were employed. These were (a) ultra slow speed filling cold chamber die casting and (b) high speed hot chamber die casting. Significant improvements of the fatigue and mechanical properties were obtained for both die casting systems compared to the normal high speed cold chamber die casting technique. By comparing ultra slow die casting with hot chamber die casting, it was found that the fatigue and mechanical strengths from hot chamber die casting were higher than those for ultra slow filling die casting. The differences in material strength were attributed directly to the material properties, e.g. microstructural morphology and internal defects. Spherical fine dendritic cells in the hot chamber die casting sample gave rise to high fatigue crack growth resistance; the low crack growth resistance for cold chamber die cast aluminium is mostly due to the growth of aluminium rich α phase and the presence of eutectic silicon fibres. The fatigue strength was also related to the number of internal defects, e.g. the lower the defect rate on the fracture surface, the higher the fatigue resistance and mechanical strength. The characteristics of the principal internal defect were different depending on the die casting technology: this showed fine porosity for hot chamber die casting but solidification shrinkage and the scattered chill structure for slow and high speed cold chamber die castings. The reasons for the change of material strength were therefore influenced by the die casting process.

真空中におけるアルミニウム合金(Al2024-T4)のフレッティング疲労特性(<小特集>締結・接合・接着部の評価技術)

Plain fatigue, fatigue crack growth and fretting fatigue tests of Al alloy (Al2024-T4) in air and in vacuum were carried out using a fatigue testing machine with SEM for in-situ observation. Fretting fatigue strength in vacuum was higher than that in air, while it was lower than plain fatigue strength in vacuum. High tangential force in vacuum would result in earlier fretting fatigue crack nucleation. However, high fatigue crack growth resistance would take dominant role for higher fretting fatigue strength in vacuum.

Influence of Second Phase Particle on Fatigue Crack Growth Behavior in Ferrite Matrix Steels

Fatigue crack growth tests of two ferrite-pearlite and two ferrite-bainite steels with different size and spacing of particle, where pearlite/bainite (hard) particles were uniformly distributed in the ferrite matrix, were carried out to investigate effect of second phase particle on fatigue crack growth behavior in Paris regime. The fatigue crack growth curves for the four materials did not coincide each other, even when the crack growth curves were arranged by the effective stress intensity factor range. From the in-situ observations, crack tip stress shielding phenomena, such as interlocking, branching, etc were found on the crack wake, which enhanced fatigue crack growth resistance. Small size and spacing of pearlite/bainite particle seemed to induce small but frequent crack deflections, which resulted in crack closure phenomena. On the other hand, large size of pearlite/bainite particle seemed to induce stress shielding phenomena and then contribute to high crack growth resistance, which was the main reason for lower fatigue crack growth rate of the large size and spacing of hard particle compared the small size of hard particle.

Effects of the powder manufacturing method on microstructure and wear performance of plasma sprayed alumina–titania coatings

Three Al 2O 3–13wt.% TiO 2 powders, with the same chemical composition but different Al 2O 3–TiO 2 distribution patterns, are plasma sprayed and the resulting coatings are compared in terms of their phase composition, microstructure, hardness, crack growth resistance, and abrasive wear performance. It is demonstrated that the degree of mixing of the Al 2O 3 and TiO 2 ingredients in the feed powder has immense impact on the phase composition, microstructure, hardness, crack growth resistance, and abrasive wear performance of the coatings. A high degree of mixing of Al 2O 3 and TiO 2 in the powder state results in more uniform microstructure, higher hardness, higher crack growth resistance, and consequently better abrasive wear resistance of the coating.

Comparison of the fatigue and fracture of Ti–6Al–2Zr–1Mo–1V with lamellar and bimodal microstructures

The present study compares the high cycle fatigue (HCF) properties of the Ti–6Al–2Zr–1Mo–1V alloy with lamellar and bimodal microstructure. This comparison study includes the high cycle fatigue (resistance against crack initiation) with different ratio of stress ( R = 0.1 and −1) and different notch concentration factor ( K t = 1 and 3), and the fatigue crack propagation behavior. The comparison principally showed much lower high cycle fatigue strength for bimodal microstructure as compared with lamellar microstructure for smooth and notched specimens at positive load ratio. Whereas, the difference of the high cycle fatigue strength result at negative load ratio is small for the two microstructures. The results of fatigue crack propagation test show that the lamellae structure also provides higher crack growth resistance than the bimodal structure, and the lamellar microstructure displayed a more tortuous and deflected crack path.

Effects of pearlite morphology and specimen thickness on fatigue crack growth resistance in ferritic–pearlitic steels

Effect of pearlite morphology and specimen thickness on fatigue crack growth (FCG) behavior in ferritic–pearlitic structural steels was investigated. Two steels with different pearlite morphologies were employed: one had islands of pearlite colonies and the other had uniformly distributed pearlite particles. Δ K-decreasing/increasing FCG test was carried out by using a standard compact tension (CT) specimen with 6 mm thickness and a single edge cracked plate tension (SECT) specimen with 1.5 mm thickness. The results revealed that the uniformly distributed pearlite microstructure had a FCG resistance higher than the islands of pearlite colonies microstructure. To understand the effect of pearlite morphology on crack growth behavior in the Paris regime in detail, constant-Δ K FCG tests with in situ SEM observation were also carried out. From in situ observations, tortuous crack path was found in the uniformly distributed pearlite microstructure. These tortuous crack paths with large crack deflection angle would promote crack closure as well as crack tip stress shielding and then resulted in higher crack growth resistance. Thin specimen indicated lower FCG rate compared to thick specimen, which resulted from the difference of crack closure behavior.

Interpretation of results of full notch creep test and comparison with notched pipe test

An investigation of two tests commonly used to determine resistance to slow crack growth in PE pipes and materials is detailed, in order to gain a greater understanding of the mechanisms involved and to resolve differences in results observed. The full notch creep test (FNCT) is carried out on small notched bars machined from sheet or pipe loaded to create high constraint at the notch tip. The notched pipe test (NPT) is a pressure test on pipe containing external machined notches. In this test, it has been observed that the use of more flexible materials allows deformation in the crack tip region and contributes to slow crack growth resistance via crack tip blunting. Good pipe performance can be achieved by selecting materials with high inherent slow crack growth resistance or by combining inherent resistance with blunting mechanisms promoted by a lower density material. It is concluded that the FNCT test, while useful for an indication of inherent slow crack growth resistance, cannot be used to predict pipe performance for a range of materials, and therefore is unsuitable as a reference test for a pipe product specification. The NPT test remains the benchmark test for pipe performance and is referenced by many specifications.

Mechanical properties of Si- and C-doped directionally solidified TiAl–Nb alloys

The microstructure and mechanical properties of directionally solidified Ti–47Al–3Nb, Ti–47Al–3Nb–0.2Si–0.1C, Ti–47Al–3Nb–0.3Si–0.2C alloys were evaluated. The lamellar thickness of Si- and C-doped alloys was fine and uniform, and little or no precipitates were found within the lamellae or at inter-lamellar interfaces. Room temperature yield stress increased rapidly with increasing Si and C contents, likely due to solid solution hardening as well as due to a reduction in lamellar spacing distribution. However, fracture toughness of “type-I” specimens of the alloys decreased marginally as Si and C were introduced and as their content increased. It is argued that the α 2 volume fraction decreases with increasing Si and C contents so that the frequency of delamination-type separation decreases, and this feature is thought to principally contribute to the high crack growth resistance of “type-I” specimens. Furthermore, fracture toughness of Ti–47Al–3Nb–0.2Si–0.1C alloy specimens with “type-III” orientation, polycrystalline form, and with “type-I” orientation increased in order of “type-III” → polycrystal → “type-I”. These results are discussed and the ability to obtain a respectable combination of strength and toughness through Si and C addition to a TiAlNb alloy produced by directional solidification is demonstrated.

Cyclic fatigue crack propagation behavior of Ti 3SiC 2 synthesized by pulse discharge sintering (PDS) technique

Cyclic fatigue properties of Ti 3SiC 2 were investigated. When the crack was shorter, it was always arrested due to rapidly increasing crack-tip shielding. Multiple energy dissipative mechanisms including crack deflecting, branching, delamination and grain bridging are responsible for the observed stable crack propagation and high crack growth resistance in this material.

The effect of microstructures on the fatigue crack growth in Ti6Al4V laser welds

The effect of microstructures on fatigue crack growth rates (FCGRs) in Ti6Al4V plates and laser welds was investigated. Experimental results revealed that the location of tensile fracture of laser-welded specimens was always in the base metal, owing to the presence of martensite in the narrow fusion zone (FZ). For welds aged at various temperatures, only minor differences in tensile properties were observed. The various heat treatments of such plate did not affect FCGRs significantly. The fracture surface was characterized by fatigue striation at high ΔK values. When the R ratio was increased from 0.1 to 0.4, higher FCGRs would be resulted. The decomposition of martensite in the aged FZ causes retardation of FCGRs in a weld, which is characterized by a mixed mode fracture. The more tortuous crack path within the FZ and HAZ was also indicative of high crack growth resistance. Roughness-induced crack closure caused by crystallographic fracture was responsible for decreased FCGRs.

Fatigue crack growth resistance in SiC particulate and whisker reinforced P/M 2124 aluminum matrix composites

Fatigue crack growth is examined in P/M 2124 aluminum alloys reinforced with SiC particles (SiC p) and whiskers (SiC w) over a wide spectrum of growth rates from 10 −12 to 10 −4 m per cycle. Effects of aging treatment, orientation of crack growth direction with respect to the rolling direction, mean stress (or stress ratio), and reinforcement volume percent on the fatigue crack growth threshold are investigated in terms of crack tip shielding mechanisms. Comparison to fatigue crack growth in the unreinforced alloy indicates that crack growth resistance in the composites is superior to the monolithic alloy at low stress intensity ranges, Δ K. Specifically, at the lower growth rates the superior crack growth resistance of the composites is due to the formation of tortuous crack paths and a consequently enhanced roughness induce crack closure. Fatigue response near threshold is found to be relatively insensitive to changes in aging treatment with observed variations reflecting the associated changes in plastic behavior. In whisker reinforced composites, the orientation of the crack with respect to the rolling direction had a significant effect on fatigue crack growth rate while in the particulate reinforced composites orientational effects were not significant. Increasing the volume fraction resulted in higher crack growth resistance at low growth rates in the particulate reinforced materials. At high stress ratios, resistance in both whisker and particulate reinforced composites was lowered, and the measured fatigue crack propagation threshold, Δ K th, was found to be independent of the reinforcement morphology and volume percent.