Lower Crack Propagation Rate Research Articles

Enhancing fatigue crack propagation resistance of heterostructured Al composites and multistage crack mechanisms

High tensile strength and low fatigue crack propagation (FCP) rate are hard to achieve simultaneously in aluminium (Al) based materials, which has been a long-lasting topic. It is because the traditional strengthening mechanisms may lead to the increase in FCP rate. In this work, we developed dual-level heterostructures by incorporating the in-situ synthesized TiB2 particles into Al matrix, to create particle-lean zones (PLZs) and particle-rich zones (PRZs) by extrusion. Fine grains were introduced by particle-associated local recrystallization in PRZs. By means of particle and grain size distribution, a heterostructured Al composite featuring with the coarse grains in PLZs and fine grains in PRZs was fabricated. It was found that simultaneous enhancement of both the strength and FCP resistance of Al composite was achieved through the development of heterostructures. During FCP, the PRZs can retard the growth of slip bands and increase crack deflection frequency while the PLZs increase the crack deflection distance and plastic deformation capability at crack tip. The fracture behavior of composite during FCP depended on grain characteristics, particles and stress intensity range. The detailed cracking behavior for typical <100>Al and <111>Al grains in different FCP stages was identified. The associated models were developed for different FCP behaviors. Particularly, the quantitative relationship between Pairs parameters and microstructure features was established, which was critical to understand fatigue properties of Al composite reinforced by small particles. These findings can provide a strategy to design metal materials with an excellent combination of both static and dynamic mechanical properties.

The effects of Ni and Al elements on microstructure and mechanical properties of low carbon CoCrMo alloy coatings

Low-carbon cobalt-based alloy coatings with high ductility and low thermal fatigue crack propagation rates were prepared using laser cladding (LC) technology. The microstructure of the coatings was characterized using SEM, EBSD, and TEM, while the mechanical properties were tested and analyzed. The CoCrMo alloy primarily consists of γ-Co and a minor amount of ε-Co. The addition of Ni increased the stacking fault energy (SFE), resulting in the retention of γ-Co in the alloy. However, the increase of Al content reduced the SFE, leading to the precipitation of Al3Ni intermetallic compound along with a eutectic region with Al3Ni, Cr23C6, and ε-Co. More ε-Co phase appeared after tensile deformation due to the stress-induced transformation induced plasticity (TRIP) effect. Ni addition increased the content of face-centered cubic (fcc) γ-Co, enhancing the coating's ductility by approximately 85 % compared to CoCrMo. With higher Al content, the ε-Co phase increased, and the dispersion of Al3Ni improved the coatings' strength. The jagged structure of the eutectic region increased the resistance to thermal fatigue crack propagation, causing the crack propagated along the grain boundary. Optimal mechanical properties were achieved with the addition of 13 wt% Ni and 7 wt% Al, which achieved a 53 % increase in ductility and a 40 % reduction in the thermal fatigue crack propagation rate compared to CoCrMo.

Influence of Microstructure on Tensile Properties and Fatigue Crack Propagation Behavior for Lath Martensitic Steel

This paper addresses the role of multilevel microstructures on the fatigue crack propagation behavior and the tensile properties of lath martensite with different substructure sizes. Microstructure characterization of the alloy was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD), and optical microscopy (OM). Based on the classic Hall–Petch relationship, the results of tensile tests showed that martensitic block is the effective control unit of yield strength. Furthermore, the plasticity of lath martensite is not sensitive to grain size. The tensile deformation mechanisms were also discussed. Fatigue crack propagation tests revealed that the coarse grain has a higher crack propagation threshold and lower crack propagation rate than the fine grain in lath martensitic steel. The change in the plasticity zone ahead of the crack tip leads to the transitional behavior of the fatigue crack propagation rate. When plasticity zone sizes are equal to the block size, the fatigue crack propagation reverts to a stable propagation stage. Finally, an empirical model was established to predict the fatigue crack propagation rate of the stable propagation stage based on the tensile properties of the lath martensitic steel.

Fatigue crack growth behavior of TA15/TC4 dissimilar laminates fabricated by diffusion bonding

Fatigue crack growth behavior of TA15/TC4 dissimilar laminates produced by diffusion bonding is studied. A sound bonding interface and fine equiaxed microstructure are obtained to improve fatigue damage tolerance with uncompromised tensile properties. TA15 alloy in the dissimilar laminates exhibits a higher fatigue crack growth resistance. EBSD analysis confirms that more α recrystallization occurs in TA15 alloy, which contributes to the relatively random crystallographic orientation and therefore the lower crack propagation rate. Local α grains characterized by high basal or prismatic Schmid factors are activated for plastic deformation accompanied by observation of dislocation lines and tangles. This study sheds light on a simple method to improve the damage tolerance by fabricated dissimilar laminate of diffusion bonding.

Enhancing Strain Capacity by the Introduction of Pearlite in Bainite and Polygonal Ferrite Dual-Phase Pipeline Steel.

In this study the strain capacity and work-hardening behavior of bainite (B), bainite + polygonal ferrite (B + PF), and bainite + polygonal ferrite + pearlite (B + PF + P) microstructures are compared. The work hardening exponent (n), instantaneous work hardening value (ni), and differential Crussard-Jaoul (DC-J) analysis were used to analyze the deformation behavior. The best comprehensive mechanical properties were obtained by the introduction of the pearlite phase in B + PF dualphase with the tensile strength of 586 MPa and total elongation of 31.0%. The additional pearlite phase adjusted the strain distribution, which increased the initial work hardening exponent and then maintained the entire plastic deformation at a high level, thus delayed necking. The introduction of pearlite reduced the risk of micro-void initiation combined with the high frequency of high angle grain boundaries (HAGBs) in triple-phase steel, which led to a low crack propagation rate.

Microstructure-Hardness Relationships in Subzero Quenched and Aged TC21 α/β Titanium Alloy

Titanium alloys have long been recognized as outstanding strong, light and corrosion resistant alloys. TC21 alloy has recently received considerable attention due to its high strength, toughness, damage-tolerance properties and low crack propagation rate. Similar to steels, titanium alloys have demonstrated their capacity to produce martensitic microstructure upon suitable heat treatment. Their martensite and its transformations upon subsequent heat treatment proved to be an important tool to obtain controllable properties. The martensite (start) characteristic temperature (Ms) has received some attention as regards its dependence on composition. On the other hand, no similar attention was given to the dependence of the other important martensitic (finish) characteristic temperature (Mf) on composition. In view of the foregoing, this work was thus planned to fulfill this lacking information via subzero hardening treatments of TC21 α/β alloy. Detailed analysis of the so obtained microstructures via optical and scanning microscopy, and x-ray diffraction data has led to a quantitative estimation of the Mf temperature and its composition dependence. Additionally, the hardening effect of those subzero hardening treatments was studied via hardness and microhardness measurements. Significant findings were recorded which are expected to help reaching useful property levels such as strength, wear resistance and damage-tolerance.

Microstructure and micro-hardness analysis of friction stir welded bi-layered laminated aluminum sheets

Friction Stir Welding (FSW) is an economical and environment friendly solid-state welding technique. Properties such as high specific strength, low crack propagation rate and high resistance to impact and fatigue loadings enhance the use of aluminum laminates in the modern aircraft and automotive industry. In this study, FSW process was employed for butt joining of bi-layered Al-1350 laminated sheet. Preliminary experimentation, microhardness and microstructure evolution of the FSWed laminates were examined. The microscopic investigations revealed that the FSW process parameters greatly affect the dissolution of strengthening particles, material mixing and formation of defects. The Scanning Electron Microscopy (SEM) revealed that increasing in the tool rotation rate led to increase in the grain size and the size of second phase particles. However, with increasing tool traverse speed a very small rise in the grain size was observed. The increase in the density of second phase particles promoted a small rise in the hardness. Moreover, the microhardness of bi-layered aluminum weldments decreased with increase in grain size hence, following the Hall–Petch relationship. A comparative analysis of FSW of multilayered and monolithic sheet was also performed. Finally, a set of best suited parameters was suggested for the welding of bi-layered laminated sheets of aluminum.

Effect of Heat Treatment on Crack Propagation and Performance of Disk Brake with Cross Drilled Holes

A vehicle disk brake, which has cross drilled hole patterns on a frictional surface for improving cooling and braking performance, has propensity to cause crack problem under the harsh braking condition. In this study, the finite element analysis by using ABAQUS was adopted to investigate a cause of crack occurrence by the thermo-mechanical phenomenon around drilled hole patterns. As a remedy of the crack generation, the disk brake rotor with stress relieving and ferritic nitrocarburizing was prepared and tested on brake dynamometer in harsh condition. The test result was examined in terms of pad wear, the braking torque variation, the braking temperature changes, the crack initiation timing, and the crack propagation speed. As a test result, the disk after heat treatment showed 3.61 % higher average braking temperature than the disk without heat treatments, to suggest decreased heat dissipation performance, however, it also showed 14 % lower pad wear amount, and 0.1 % smaller braking torque variance than the disk without heat treatments, to stand for improved contact characteristics. Also, the heat treated brake rotor showed 51.3 % lower crack propagation rate which is considered as proper countermeasure of the fracture phenomenon at the edge of cross drilled hole.

Developing the ductility and thermal fatigue cracking property of laser-deposited Stellite 6 coatings by adding titanium and nickel

A design strategy is presented for the development of a laser-deposited Stellite 6 coating with high ductility and low thermal fatigue crack propagation rate. The strategy involved adding titanium to modify carbide morphology, accompanied by an increase in the nickel content to stabilize the γ-Co phase in the matrix. The molar ratios of the titanium to the carbon in coatings were 0.5, 1.0, 1.5, and 2.0. The amount of nickel added to the titanium-modified coatings was 10 and 20 wt%. The net-like M23C6 eutectic carbides in the Stellite 6 coating transformed to isolated TiC particles with increasing molar ratio of titanium to carbon. Adding nickel to the coatings resulted in an increase in the stacking fault energy and the stabilization of the γ-Co phase. Accordingly, the ductility increased with increasing nickel concentration. The modified coating, named 1.0Ti20Ni, in which the molar ratio of titanium to carbon was 1.0 and the nickel concentration was 20 wt%, exhibited the highest elongation to fracture (~12%), along with a high strength (1230 MPa) in all coatings, Besides, the 1.0Ti20Ni coating revealed the lowest thermal fatigue crack propagation rate.

Fatigue Crack Growth in Plasma-Sprayed Refractory Materials

Fatigue crack growth in self-standing plasma-sprayed tungsten and molybdenum beams with artificially introduced notches subjected to pure bending was studied. Fatigue crack length was measured using the differential compliance method, and fatigue crack growth rate was established as a function of stress intensity factor. Crack opening under compressive stress was detected. Fractographic analysis revealed the respective crack formation mechanisms. At low crack propagation rates, the fatigue crack growth takes place by intergranular splat fracture accompanied by splat decohesion in Mo coating, eventually by void interconnection in W coating. Frequently, the crack deflected from the notch plane being attracted to stress concentrators formed by voids or favorably oriented splat interfaces. At higher values of the stress intensity factor, the intergranular cracking of splats becomes more common and the crack propagated more perpendicularly to the specimen surface.

Fatigue strength of USP treated ASTM CA6NM for hydraulic turbine runner

In order to investigate the effect of an ultrasonic shot peening (USP) treatment on the fatigue characteristics of structural materials for hydroelectric facilities, plane bending fatigue tests were carried out using USP treated ASTM CA6NM stainless cast steel. The results showed that the fatigue limit of the USP material was approximately 60% higher than that of the untreated material. Haigh’s diagram evaluation suggested that an increase in crack initiation resistance associated with the high hardness and compressive residual stress by USP treatment was the main reason for the improvement in fatigue strength. Furthermore, the simulation of fatigue crack propagation implied that compressive residual stress leads to a low crack propagation rate compared with the untreated material. Consequently, it is clear that the USP treatment is an effective process for improving the fatigue properties of CA6NM.

3-D observations of short fatigue crack interaction with la2mellar and duplex microstructures in a two-phase titanium alloy

In situ observations of short crack growth in powder-processed Ti-6246 have been undertaken using synchrotron X-ray microtomography to investigate crack tip interaction with microstructure. Together with post-mortem analysis using electron backscatter diffraction (EBSD), it was possible to identify a number of microstructural features that affect crack propagation rates by causing crack bifurcation, crack bridging and crack deflection. Three samples with different microstructures were tested in this way: lamellar, duplex and a heterogeneous microstructure that showed regions of lamellar and duplex microstructure. The in situ fatigue experiments were carried out with a maximum stress of 410MPa and R=0.1. The three microstructures showed significantly different short crack propagation rates, with the lamellar microstructure displaying the fastest and the duplex microstructure the slowest rate. It was found that the lamellar microstructure develops a deeper crack than the duplex microstructure that is related to significant crack bifurcation taking place near the surface region in the lamellar but not duplex microstructure. It was also found that a columnar lamellar microstructure creates a relatively smooth crack front while a basket-weave-type microstructure forces the crack tip to undulate on the lath width scale. Crack bridging of the fine lamellar region of the duplex microstructure was observed, which seems to hinder significant crack bifurcation to occur, but still provides improved crack growth resistance that explains the low crack propagation rate. In the third microstructure the crack tended to grow slightly asymmetrically due to the heterogeneous nature of the microstructure, resulting in the intermediate growth rate. EBSD grain orientation and Schmid factor analysis of regions including the crack revealed that the crack path is strongly influenced by the crystallographic orientation of the α lamellae and grains. While in the lamellar microstructure the crack tends to grow across lamellae favourably orientated for basal slip, in the duplex microstructure the crack path follows primary α grains favourably orientated for prismatic slip. It is suggested that the change to basal orientation in the lamellar microstructure is related to the requirement of α/β slip transfer in the lamellar microstructure and the Burgers relationship favouring basal slip in such a case. In addition, the crack growing in primary α tends to get diverted within the α grain and is not seen in the lamellar microstructure. This crack deflection in primary α grains might further slow down crack propagation and could be related to the availability of different (11–20) planes while basal planes only display one orientation in a hexagonal close-packed crystal.

On the mechanism of fatigue crack propagation in ductile metallic materials

An overview of our research performed during the last 15 years is presented to improve the understanding of fatigue crack propagation mechanisms. The focus is devoted to ductile metals and the material separation process at low and intermedial crack propagation rates. The effect of environment, short cracks, small-scale yielding as well as large-scale yielding are considered. It will be shown that the dominant intrinsic propagation mechanism in ductile metallic materials is the formation of new surface due to blunting and the re-sharpening during unloading. This process is affected by the environment, however, not by the length of the crack and it is independent of large- or small-scale yielding.

Discrete dislocation modelling of near threshold fatigue crack propagation

At low crack propagation rate in metals and alloys the discrete nature of plasticity is essential to understand the fatigue phenomena. A short overview of the different types of performed discrete dislocation simulations of cyclically loaded cracks and their essential results are presented. The discrete dislocation mechanics deliver the changes of the stresses and displacement during cyclic loading. However, it does not give directly the crack propagation rate. In the simulations one has to assume a propagation mechanism. A comparison implies two things: the different simulations, the experimentally observed crack growth behaviour and crystallographic features will be used to show, which crack propagation mechanism is more appropriate in which case.

Research on Titanium Alloys with High Toughness and High Damage Tolerance

The recent shift in the design focus for aeroplanes from strength to damage tolerance has led to a subsequent shift in the focus of Ti alloy research. China first started to research Ti alloys with damage tolerance from the year 2000. The first product stemming from this research is a Ti alloy with high strength, high toughness and damage tolerance (TC21). TC21 exhibits high strength (UTS  1100MPa), high fracture toughness (K1c  70MPa.m1/2) and a low crack propagation rate (da/dN being similar to Ti-6-4 with  annealing). Another Ti alloy, named TC4-DT, has also been produced. It has moderate strength, along with high toughness and damage tolerance (UTS  900MPa, K1c  70MPa.m1/2, da/dN being similar to Ti-6-4 with  annealing). Both TC21 and TC4-DT are now undergoing rapid development, with the former alloy also being applied to a full scale aeronautical application. Both TC21 and TC4-DT have promising futures in the industry. They will be the main Ti alloys with damage tolerance utilised in the Chinese market.

Low-cycle fatigue properties of ultrafine-grained zinc–22 wt.% aluminum alloy during room-temperature superplastic flow

Low-cycle fatigue behavior during room-temperature superplastic flow was investigated for ultrafine-grained Zn–22 wt.%Al alloy. This alloy exhibited a longer fatigue life and a lower crack propagation rate than other materials at high strain amplitudes above ±5%. In addition, many cavities and secondary cracks were observed near the tips of cracks. These are probably responsible for the apparent reduction in the crack propagation rate and the long fatigue lifetime of this alloy.

Increasing the fatigue limit of a bearing steel by dynamic strain ageing

Dynamic strain ageing processes may increase the fatigue limit of steels. To activate these processes, thermomechanical treatments with cyclic loadings at the temperature of the highest effectiveness of dynamic strain ageing can be applied. The treatments increase the dislocation density and change the dislocation structure to a more stable state by immobilisation of mobile dislocations due to locking processes by dissolved carbon atoms and small carbides. The generated dislocation structure requires higher stress amplitudes to induce dislocation movements, and consequently a longer crack initiation period, a lower crack propagation rate and a higher fatigue limit at room temperature occur.

フェライト/ベイナイト]ハイブリッド材の疲労き裂進展挙動に及ぼす硬度の影響

We have examined fatigue crack propagation behavior in the [ferrite/bainite]-Hybrid materials with different hardness under a constant stress intensity factor range. It was found that crack propagation in the ferrite phase decelerated with reaching to the [ferrite/bainite]-interface, probably due to supression of plastic deformation at the crack tip. This suppression tendency of the crack propagation became higher when hardness difference between ferrite and bainite was larger. In the case with the highest hardness difference, fatigue crack did not pass through the [ferrite/bainite]-interface. Lower crack propagation rate was also found in the bainite which showed higher work softening due to crack propagation.

Crack initiation and propagation behavior of zirconium cladding under an environment of iodine-induced stress corrosion

Tests of iodine-induced stress corrosion cracking (ISCC) were carried out to elucidate the initiation and propagation of cracks in the claddings of zirconium alloys. Zircaloy-4 cladding and Nb-contained zirconium cladding were pressurized with and without a pre-cracked state at 350°C in an iodine environment. The results show that pitting nucleation and growth play an important role in initiating ISCC. Pits preferentially grow and agglomerate around the grain boundary, where the number of pits increases with the iodine concentration and the hoop stress of the claddings. A model of grain-boundary pitting coalescence and a model of pitting-assisted slip cleavage, which were proposed to clearly elucidate the crack initiation and propagation process under ISCC, produce reasonable results. The Nb-contained zirconium cladding exhibits higher ISCC resistance than Zircaloy-4 from the standpoint of a higher threshold stress-intensity factor and a lower crack propagation rate.

OS5-1-6 The effect of film thickness on crystal rotation behavior with fatigue crack propagation in copper films

Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover an elliptical through-hole in a base plate subjected to push-pull cyclic loads, annealed copper films with the thickness of 100μm and those reduced the thickness from the 100μm to 50μm by an electro-polishing were fatigued under a constant stress amplitude with a stress ratio of zero. The crystal rotation behavior with the fatigue crack propagation was investigated by measuring the crystal orientation around the fatigue crack initiated from the notch root before and after fatigue testing, using EBSD (Electron Back-scatter Diffraction) method. Then, the change of crystal orientation with fatigue testing was evaluated quantitatively from the misorientation between the crystal orientation matrix on the same point obtained before and after fatigue testing. As a result, the angle of the crystal rotation obtained from the region showing the high fatigue crack propagation rate was larger than that obtained from the region showing the low fatigue crack propagation rate for the film with the thickness of 100μm, while the fatigue crack propagated faster in the film with the thickness of 50μm than that with the thickness of 100μm regardless of the small crystal rotation angles with the fatigue testing for the film with the thickness of 50μm.