Crack Growth Region Research Articles

Fatigue crack growth in an aluminum alloy-fractographic study

A two-fold approach was adopted to understand the fatigue crack growth process in an Aluminum alloy; fatigue crack growth test of samples and analysis of fractured surfaces. Fatigue crack growth tests were conducted on middle tension M(T) samples prepared from an Aluminum alloy cylinder. The tests were conducted under constant amplitude loading at R ratio 0.1. The stress applied was from 20,30 and 40 per cent of the yield stress of the material. The fatigue crack growth data was recorded. After fatigue testing, the samples were subjected to detailed scanning electron microscopic (SEM) analysis. The resulting fracture surfaces were subjected to qualitative and quantitative fractographic examinations. Quantitative fracture analysis included an estimation of crack growth rate (CGR) in different regions. The effect of the microstructural features on fatigue crack growth was examined. It was observed that in stage II (crack growth region), the failure mode changes from intergranular to transgranular as the stress level increases. In the region of intergranular failure the localized brittle failure was observed and fatigue striations are difficult to reveal. However, in the region of transgranular failure the crack path is independent of the microstructural features. In this region, localized ductile failure mode was observed and well defined fatigue striations were present in the wake of fatigue crack. The effect of interaction of growing fatigue crack with microstructural features was not substantial. The final fracture (stage III) was ductile in all the cases.

Investigation of the Role of Rubber Particle Cavitation Resistance on the Toughening Mechanisms of Epoxy Resins

The occurrence of rubber particle cavitation in rubber modified epoxy resins has generated some controversy in the literature and role of cavitation resistance in toughening mechanisms has remained an enigma. In this study, hydroxyl terminated polybutadiene (HTPB) and synthesized epoxy terminated polybutadiene (ETPB) have been used to toughen an epoxy resin. FTIR and titration techniques were used to monitor the ETPB synthesis. Mean fracture toughness, KIC, of 0.65 MPa.m0.5 was measured for unmodified epoxy polymer. The measured toughness was increased by the addition of HTPB to 1.23 MPa.m0.5 and ETPB to 1.80 MPa.m0.5. The cavitation zone of the HTPB modified sample was bigger than that of ETPB modified sample which showed the higher cavitation resistance of ETPB particles. SEM studies showed volumetric expansion of rubbery phase in slow crack growth region of both samples which proved the plastic void growth. The results showed that the elevated cavitation resistance of ETPB did not suppress plastic void growth. The optical microscopy of crack tips showed shear banding which was more significant in ETPB modified sample. SEM micrographs of fracture surface of HTPB modified samples showed no tail formation while pinning tails were present in ETPB modified samples. The results showed that higher cavitation resistance activated crack pinning process and induced larger shear banding zone in ETPB modified samples. Deflection and forking of the main crack into secondary cracks was also observed in ETPB modified samples while these phenomena were not present in HTPB modified samples.

Ductilisation of tungsten (W) through cold-rolling: R-curve behaviour

Here we show that cold-rolling of tungsten (W) decreases the stable crack growth onset temperature. Furthermore, we show that stable crack growth is accompanied by crack bridging, which in turn is triggered by dislocation activity. The entire stable crack growth regime shows ductile intergranular fracture.Our ductilisation approach is the modification of microstructure through cold-rolling. In this work, we assess two different microstructures obtained from (i) cold-rolled and (ii) severely cold-rolled tungsten plates. From these plates, single-edge cracked-plate tension (SECT) specimens were cut and tested in the L-T direction. Crack growth resistance (R) curves were obtained using the direct-current-potential-drop method (DCPM). The experiments show the following results: cold-rolled plates are brittle at room temperature (RT), but show stable crack growth at 250°C (523K) and a fracture toughness, KIQ, of about 100MPa(m)1/2 at a crack extension, Δa, of 0.6mm. Severely cold-rolled tungsten plates show stable crack growth at RT and a fracture toughness, KIQ, of 100MPa(m)1/2 at a crack extension, Δa, of 0.3mm. Scanning electron microscopy (SEM) analyses of the stable crack growth region show intergranular fracture with microductile character.The question of why cold-rolling causes the stable crack growth onset temperature to decrease (or in other words, why cold-rolling causes the brittle-to-ductile transition (BDT) temperature to decrease) is discussed against the background of (i) intrinsic and extrinsic size effects, (ii) crystallographic texture, (iii) impurities and (iv) the role of dislocations. Our results suggest that the spacing between the dislocation nucleation sites (high angle grain boundaries (HAGBs) act as dislocation source) is the most important parameter responsible for the decrease of the stable crack growth onset temperature.

Analysis of fatigue crack propagation behaviour in SiC particulate Al2O3 whisker reinforced hybrid MMC

The fatigue crack propagation behaviour of a cast hybrid metal matrix composite (MMC) was investigated and compared with the crack propagation behaviour of MMC with Al2O3 and Al alloy in this article. Three dimensional (3D) surface analysis is carried out to analyze the crack propagation mechanism. All three materials clearly show near threshold and stable crack growth regions, but the rapid crack growth region is not clearly understood. The crack propagation resistance is found higher in hybrid MMC than that of MMC with Al2O3 whisker and the Al alloy in the low ΔK region. The crack propagation in the hybrid MMC in the near-threshold region is directed by the debonding of reinforcement-matrix followed by void nucleation in the Al alloy matrix. Besides, the crack propagation in the stable- or midcrack-growth region is controlled by the debonding of particle-matrix and whisker-matrix interface caused by the cycle-by-cycle crack growth along the interface. The transgranular fracture of the reinforcement and void formation are also observed. Due to presence of large volume of inclusions and the microstructural inhomogeneity, the area of striation formation is reduced in the hybrid MMC, caused the unstable fracture.

Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size.

Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given.

Statistical Duplex <i>S-N</i> Characteristics of Bulk Amorphous Alloy in Rotating Bending in Very High Cycle Regime

In order to investigate very high cycle fatigue properties of Zr55Al10Ni5Cu30 bulk amorphous alloy, rotating bending fatigue tests were conducted at room temperature by using 5 different series of Zr55Al10Ni5Cu30 alloy. The differences of material processing conditions are manufacturer, product year, material size and minimum diameter of specimen. Although all specimens fractured from surface, duplex S-N characteristics were observed for each series of the material. Time strength distributions at N=104 for short life region and at N=107 for long life region were well approximated by normal distribution. The entire S-N property accepting the normalized stress amplitude by the time strength at N=104 or N=107 has shown more clearly duplex S-N characteristics. In addition, P-S-N properties were also estimated from the standard deviation of time strength distributions at N=104 or N=107. Based on the observation of fracture surface by scanning electron microscope (SEM), it is confirmed that every fracture surface was consisting of typical three regions such as multi-facet region, stable crack growth region and instantaneous fracture region.

Failure Analysis of High Strength Steel Bar Used in a Wind Turbine Foundation

Screw-thread steel bars made of high strength steel fractured after exposure to a constant pre-stress of 650 MPa for more than 10 h. Visual examination, chemical analysis, mechanical tests, metallographic analysis, and fractographic examination were carried out in order to determine the causes of this failure. The fracture surfaces consisted of three regions: the crack source or fracture origin region, steady-state crack growth, and unstable or final fracture region. The crack initiated at the center of the cross section and then propagated to the periphery. The fracture exhibited a brittle appearance, accompanied with secondary cracks, tear ridges, and micropores on the grain boundary. The results revealed that this failure was due to hydrogen embrittlement of the steel.

Fretting fatigue behavior in railway axle materials

Fretting damage can be observed in automobile and railway vehicles, fossil and nuclear power plants, and aircrafts. In the present study, the railway axle material RSA1 was used to evaluate fretting fatigue life. Plain and fretting fatigue tests were conducted using a rotary bending fatigue tester with a ring and a bridge-type contact pad. The fretting fatigue limit of a standard specimen was approximately 37% lower than the plain fatigue limit. In the Shinkansen-type specimens, cracks were initiated in the contact area and finally fractured at σamp = 214 MPa. Similarly, cracks were generated early in the rounded areas of all TGV-type specimens and induced fracturing. Tire tracks and rubbed scars were also observed in the oblique crack and fatigue crack growth regions of the fractured surface. Wear debris was detected on the contact surface, and oblique cracks were initiated in the contact area at an early stage. The results of this study can be used to evaluate the structural integrity of railway axles.

Wallner lines, crack velocity and mechanisms of crack nucleation and growth in a brittle bulk metallic glass

Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed in detail to understand the underlying physical processes. Wallner lines, which result from the interaction between the propagating crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them indicates that the maximum crack velocity is ∼800ms−1, which corresponds to ∼0.32 times the shear wave speed. Fractography reveals that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-penny-shaped cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of ∼79nm. Juxtaposition of the crack velocity with this spacing suggests that the crack takes ∼10−10s for peak-to-peak propagation. This, and the estimated adiabatic temperature rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation formation. Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed.

Short fatigue crack initiation and growth modeling in aluminum 7075-T6

A series of fatigue tests is carried out to investigate the behavior of microcrack initiation and propagation in Al 7075-T6. Plate specimens with semi-circular single edge notch are tested under uniaxial loading at different load amplitudes. Optical microscopy is used to detect the initiation and growth of short cracks. Continuum damage mechanics is used to model the crack initiation. After the initiation, crack growth is studied using fracture mechanics models. For both regions of crack initiation and growth, Bayesian estimation method is used to account for uncertainties in the parameters of the model. Results of the entire fatigue life, including initiation and growth, are compared with experiments. Good agreement is observed.

Experimental characterization and micromechanical modeling of damage-induced permeability variation in Beishan granite

Triaxial compression tests with measurements of permeability were performed on core granite samples taken at 450−550m depth from the Beishan area in Gansu Province, a potential site for China׳s high-level radioactive waste (HLW) disposal. Corresponding to the distinct features in the stress–strain behaviors, the permeability of the Beishan granite was found to evolve with a clear permeability decrease in the initial microcrack closure region, a constant permeability value in the elastic region and a dramatic permeability increase in the crack growth region. The permeability increases by up to and over two orders of magnitude as deviatoric stress increases up to sample failure; but at a given deviatoric stress, the permeability reduces remarkably with the increase of confining pressure. An empirical upper bound permeability model was presented by relating the mechanisms involved in the microstructure alteration to the permeability change, and the experimental results were well simulated by the proposed model. Combined with field geological characterization and numerical simulation, the implications of the experimental results for China׳s HLW disposal were discussed.

Influence of high-pressure gaseous hydrogen on the low-cycle fatigue and fatigue crack growth properties of a cast titanium alloy

In the present study, the effect of gaseous hydrogen on the fatigue properties of a commonly used aerospace titanium alloy (Ti–6Al–4V) was studied. The low-cycle fatigue and fatigue crack growth properties were investigated at room temperature in ambient air and 15MPa gaseous hydrogen. Results showed that the low-cycle fatigue life was significantly reduced in hydrogen, and the detrimental effect was larger at higher strain amplitudes. The fatigue crack growth rate in hydrogen remained unaffected below a critical stress intensity ΔK⁎≈17MPa√m, while beyond this value, the fatigue crack growth rate fluctuated and increased with increasing ΔK. Fractography analysis clearly showed that gaseous hydrogen mainly affected the fatigue crack growth rate. On the fracture surfaces, striations were noted over the entire crack growth region in air, whereas in hydrogen striations were noted at stress intensities lower than ΔK⁎. Above ΔK⁎, secondary cracks and brittle flat surfaces with features similar to crack arrest marks were mostly observed in hydrogen. Microstructural analysis along the crack growth direction showed that the crack followed a transgranular path in air, i.e. through α colonies. In hydrogen, the crack also grew along the prior β grain boundaries and at α/β interface within the α colonies. Thereby, the detrimental effect of hydrogen in cast titanium alloy was attributed to a change in the fracture process during crack propagation.

Fatigue Behaviors of a Ni-free ZrCuFeAlAg Bulk Metallic Glass in Simulated Body Fluid

Fatigue behaviors of a biocompatible Ni-free Zr60.14Cu22.31Fe4.85Al9.7Ag3 Zr-based bulk metallic glass (BMG) have been studied under three-point-bending test in a simulated body fluid (SBF) at 37 °C and compared with those in air at room temperature (RT). The BMG shows a high fatigue limit of approximately 366 MPa in SBF, which was slightly lower than that in air (400 MPa). The fatigue cracks tended to initiate from the defects such as cast-pores, inclusions and corners of the samples and propagate in a similar path in SBF and in air. Three distinct regions, i.e. a crack-initiation region, a stable crack-growth region and an unstable fast-fracture region were clearly observed on the fatigue-fractured surface. Although pitting occurred at the defects where crack initiated, it does not affect significantly the fatigue life of the BMG, because the lifetime in the present BMG is mainly determined by crack propagation. The high corrosion-fatigue limit of the studied BMG results from its excellent corrosion resistance in SBF and intrinsically good toughness.

Strain ratio effects on low-cycle fatigue behavior and deformation microstructure of 2124-T851 aluminum alloy

The low-cycle fatigue tests of 2124-T851 aluminum alloy with strain ratios of −1, −0.06, 0.06 and 0.5 were conducted under constant amplitude at room temperature. Microstructural and fractographic examinations of the material after fatigue tests were performed by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. Firstly, the results showed that the material exhibited cyclic softening characteristic as a whole. The degree of softening decreased linearly with the increasing strain amplitude and the decreasing strain ratio. The lower fatigue life and ductility of the material corresponded to the larger strain ratios. Secondly, microstructure observations revealed that the density and length of slip bands increased with the increasing strain ratio at the given strain amplitude, and so did the volume fraction and size of coarse constituents, which were responsible for the reduction of fatigue life and ductility of the material. Finally, the SEM micrographs revealed that multiple crack initiation sites took place on the fracture surfaces at different strain ratios. The reduction of stable crack growth area with the increasing strain ratio was observed. Unstable crack growth region was only observed under R≠−1.

Fatigue Short Crack Growth and Mechanism of GH4133B Superalloy Used in Turbine Disk of Aero-Engine

The short fatigue crack growth tests for circular notched compact tension specimens of GH4133B superalloy used in turbine disk of aero-engine are carried out at ambient temperature and atmospheric pressure. The stress intensity factor ranges and the fatigue crack growth rates at various stress ratios are measured, and the corresponding effective stress intensity factor ranges considering the crack closure effect are calculated. It is shown that the effective stress intensity factor range ΔKeff, can be applied to describe the deceleration and acceleration of crack growth rate during the short crack propagation. The fatigue fracture surface morphologies in the short crack growth region are investigated using a scanning electron microscopy. It is found that there is a cleavage step between two adjacent radial striations, a series of early fatigue striations exist on the cleavage step, and some secondary cracks perpendicular to the direction of main crack propagation emerge on the fracture surface, the superalloy exhibits a mixed fracture mode in the short crack growth region, which reveals the microscopic mechanism of short crack propagation that the fatigue crack growth rate is primarily higher, and then gradually decreases with the propagation of short crack.

Research on the microstructure, fatigue and corrosion behavior of permanent mold and die cast aluminum alloy

Permanent mold (PM) and high pressure die cast (HPDC) AlMg5Si2Mn are employed to investigate the microstructure, fatigue strength and corrosion resistance. Results indicated that the mechanical properties (Rm, R0.2 and δ) of HPDC specimens (314MPa, 189MPa and 7.3%) are significantly better than those of PM specimens (160MPa, 111MPa and 2.5%) due to the finer grain size and less cast defects. Fatigue cracks of PM samples dominantly initiated from shrinkage pores and obscure fatigue striations are observed in crack growth region. Corrosion and pitting potentials of PM and HPDC AlMg5Si2Mn alloy are around −1250mV, −760mV and −1220mV, −690mV respectively. Numerous pits are observed around the grain boundaries because the corrosion potential of Mg2Si is more anodic than that of α-Al matrix. In addition, the superior corrosion resistance of HPDC samples can be attributed to the fine grain size and the high boundary density which improved the formation of oxide layer on the surface and prevented further corrosion.

Evaluation of Fatigue Crack Growth Rates and Threshold Conditions in an Al 2124-T851 Alloy at Different Load Asymmetry Considering Microstructure Orientation

Results of a fairly comprehensive experimental programme aimed at evaluation of fatigue crack growth (FCG) rates in an aircraft Al 2124 alloy in T851 state, after hot rolling, solution annealing, small plastic strain and artificial ageing are presented and discussed. Measurement was performed not only in the region of stable crack growth, but also in the threshold region enabling to estimate threshold values of stress intensity factor range using method of regression analyses. FCG rates were evaluated in both L-T and T-S directions to evaluate sensitivity of FCG resistance on different microstructure orientation. Different load asymmetry conditions R = Fmin / Fmax were used, namely R = 0.1 and 0.6. Repeated measurements in more than one specimen at each condition enabled to evaluate reproducibility, scatter of measurement and to perform eventually probabilistic assessment. The reproducibility of measurement was particularly good for L-T orientation. In this case, threshold values were somewhat higher and FCG rates lower in comparison with the T-S orientation. In the contrary, cyclic fracture toughness in T-S orientation was slightly higher. The results are discussed form the viewpoint of residual fatigue life in aircraft components considering probabilistic aspects.

Load and temperature interaction modeling of fatigue crack growth in a Ni-base superalloy

An approach was developed to predict the thermo-mechanical fatigue crack growth rates under typical gas turbine engine spectrum loading conditions. The material studied in the development of this model was a polycrystalline superalloy, Inconel 100. Load interaction effects were determined to have a major effect on the crack growth life. A yield zone load interaction life prediction model was modified to include temperature dependent properties. Multiple overload effects were included in the model to incorporate enhanced retardation compared to single overload retardation behavior. Temperature interaction effects were included and proved to be very important because of the wide temperature ranges to which turbine engine components are subjected. The effects of oxidation and temperature changes were accounted for in the model by accelerating crack growth in regions that had been previously affected by elevated temperatures. Experimental data of isolated, first order effects were used to calibrate and verify the model. Temperature dependent mechanical properties were determined and were essential in the model’s development. Parametric studies were performed using this model to assess the sensitivity of specific crack growth variables on life predictions.

A new data analysis technique to examine crack tip opening angle values tested in API X65 pipe steel

ABSTRACTThis paper presents a new data analysis technique to rapidly identify the region of stable crack growth in crack tip opening angle (CTOA) testing of a modified double cantilever beam. The technique basically used the load–displacement data from CTOA testing on API X65 steel, which demonstrated a region of constant slope after the peak load. In total, 22 data points (out of 90) fell in this region, for which the variation of CTOA versus crack length remained almost constant. The CTOA measurement was conducted from crack edges and from a fine reference grid, using photographs from two cameras in front and rear sides of the CTOA specimen. As the visual analysis of the individual photographs is tedious and rather lengthy, the presented technique can be easily used for rapid and precise identification of the mean CTOA from those data in the constant slope region of the load–displacement plot.

Monotonic tension and creep behavior of single crystal CMSX-486 under combustion environment

The monotonic tension and creep loading behavior of single crystal nickel-base superalloy, CMSX-486 was investigated at 1093°C under ambient laboratory and simulated combustion environments. There was degradation in tension and creep performance under combustion environment. A reduction of about 30% in yield strength and ultimate tensile strength and 75% reduction in strain-to-failure were observed in combustion environment relative to laboratory environment. The creep rupture life reduced in combustion environment in comparison to laboratory environment. The creep rupture occurred at a lower strain level in the combustion environment when compared to laboratory environment. The laboratory environment tested specimens experienced higher % neck area reduction, exhibiting more ductility relative to the combustion tested specimens. The mass loss in combustion environment was about 3 times larger than in laboratory environment. The microstructure showed different distribution patterns of γ/γ′ phases. The combustion environment resulted in relatively more contiguous regions of γ phase which presented favorable condition for faster propagation of defects resulting in creep rupture over shorter time span. The subcritical crack growth regions on the fracture surface of the laboratory environment tested specimens were relatively much rougher with more number of craters/voids resulting in longer creep rupture life.