Higher Stress Intensity Factor Research Articles

Feature size specific processing parameters for additively manufactured Ti-6Al-4V micro-strut lattices

The material properties of individual micro-struts are critical to the overall success of lattice structures. These properties can be significantly compromised by defects inherited from powder bed fusion processes. Among these defects, porous inclusions are well understood to have a detrimental effect on mechanical properties; posing a high risk to the implant under loading. While the majority of these defects can be avoided through optimisation of printing parameters, this has generally only been done for traditional bulk components with no in-designed porosity. Furthermore, a number of studies have observed changes in the frequency of such porous inclusions as feature size is reduced, indicating a size effect. This also suggests that the optimal parameters for bulk material are not necessarily translatable to the individual micro-struts which build the lattice. In this study, the relationship between parameter optimisation and feature size was investigated. Here, a higher energy density input was required for processing micro-strut lattices with an optimised relative density, than it was for bulk components. This could be attributed to faster rates of heat loss in micro-strut samples on account of their increased surface-to-volume ratio. The consequential improvement in mechanical properties was also assessed. An increase in both strength and stiffness could be largely attributed to an increase in the percentage volume of load bearing material, while improvements in failure strain were largely driven by minimisation of stress concentrations around the irregular pore morphologies. Fatigue properties did not improve beyond the effects of yielding. Rather, crack initiation was dominated by surface defects; which on account of their surface free energy, experience a much higher stress intensity factor.

Energy absorption and impact response of ballistic resistance laminate

Abstract High-speed impact performance has significantly expanded over the past few decades. The target response based on the impact conditions has been more difficult to visualise and evaluate. In this article, Ansys model analysis has been used to measure, visualise, and predict the projectile and target responses of Kevlar® (K) and Ramie® textile-reinforced unsaturated polyester resin (UP) matrix. The laminate thickness threshold was detected experimentally based on the highest stress intensity factor and energy release rates. Furthermore, tensile strength and bending of the laminate were found. The impact conditions have a significant impact on the target response; thus, an explicit dynamic analysis was used to visualise the impact response based on the number of target fixed supports (FSs). Two FS (2 FS) target absorbs 11% more energy than four FS (4 FS) target. Additionally, the target size has a major effect on the projectile and laminate responses, and a successful arrest of the projectile was detected in both cases. The smallest targets with 2 FS have the highest and wider response, where a successful change in the projectile trajectory was obtained.

Analysis of crack propagation of annular butt weld and distribution of weld-induced residual stress

The circumferential surface crack propagation and the effect of weld-induced residual stress were analyzed to comprehensively understand the mechanical behavior and fracture resistance of dissimilar girth welded joints of steel castings. The evaluation of stress intensity factors is conducted for circumferential cracks on internal surfaces, aiming to predict the fracture toughness of dissimilar welded joints. The distribution of weld-induced residual stress is obtained by thermal elastic-plastic finite element methods. The impact of residual stress on the propagation of cracks is investigated through the application of extended finite element theory. The high-stress intensity factors were found in such dissimilar welded joints due to the local bending and distortion caused by the asymmetrical weld geometry. The tensile residual stress and distortion were applied to the weld root area and its vicinity. Both the asymmetrical weld geometry and tensile residual stress contribute to the tensile distortion of the inner surface around the weld root, and enable the growth of internal surface cracks in the circumferential direction and radially from inside out of the welded joint. The fracture resistance decreases with the simultaneous influence of residual stress and welds’ asymmetry.

Fatigue Crack Growth on Several Materials under Single-Spike Overloads and Aircraft Spectra during Constraint-Loss Behavior

Abstract The phenomenon of flat-to-slant crack growth has been studied by many in the fracture mechanics community. At low stress-intensity factors, a fatigue-crack surface is flat (tensile mode) and the crack-front region is under plane-strain conditions (high constraint). As the crack grows with higher stress-intensity factors, a 45° shear lip occurs through the thickness of the sheet or plate. This behavior is the shear mode, which is under low constraint or plane-stress conditions. In 1966, Schijve found that the transition from flat-to-slant crack growth on a 2024-T3 Alclad aluminum alloy over a wide range in stress ratios (R) occurred at a “constant” crack-growth rate. Also, Newman and Hudson showed the same behavior on 7075-Temper-6 and Ti–8Al–1Mo–1V alloys, validating Schijve’s observation that crack-growth rate was the key parameter for flat-to-slant crack-growth behavior. The materials considered herein are 2024-T3, 7075-T6, and 9310 steel. Crack-growth behavior during single-spike overloads and simulated aircraft spectrum loading are presented. The fatigue structural analysis (FASTRAN) crack-closure based life-prediction code was used to correlate the constant-amplitude (CA) crack-growth-rate data over a wide range in stress ratios (R = Smin/Smax) and rates from threshold to near fracture, and to calculate or predict the crack-growth behavior on single-spike overload tests. Crack-closure behavior is strongly dependent upon the level of constraint. The main objective was to see if the constraint-loss region is the primary reason for crack-growth delays after single-spike overloads. Also, crack-growth analyses are presented on tests that were conducted by Wanhill on 2024-T3 Alclad aluminum alloy under the transport wing standard (TWIST) spectrum. Crack-growth analyses using crack-closure theory without constraint loss was “unable” to predict crack growth under spike overloads or simulated aircraft spectra. However, predicted crack length against cycles with constraint-loss behavior compared reasonably well with all tests.

Damage Tolerance Behaviour of Stiffened Crown Panel of a Transport Aircraft Fuselage

Ever since the introduction of damage tolerance requirements in the aviation regulations, efforts continue to be made to prevent catastrophic failures due to damages present in the structure. It has also been realized that damage detection is the weakest link in the whole process of damage tolerance design to maintain continued airworthiness. The major components of the aircraft structure consist of both integral and riveted panels of sheets and stringers which are employed in fuselage skin panels, spar webs and stiffeners. In spite of all precautions, cracks or damages may arise in many of these primary structural members. These cracks cause stiffness degradation and reduce the total load-carrying capacity of the structure. In this paper, the damage tolerance behaviour of fuselage crown panel both integral and riveted stiffened panel configurations of Aluminium alloy 2024-T351 are studied using finite element based tools using crack growth analysis methods. The crack growth behaviour of both integral and riveted stiffened panels of aircraft fuselage having same geometrical configuration and subjected to uniformly distributed tensile loads is investigated. For this, a metallic stiffened panel with eight stringers, representative of crown panel of a transport aircraft fuselage is analysed with a centre skin crack propagating through the stringers. Stress intensity factors and fatigue crack propagation rates at the progressive crack tip of both types of the stiffened panels are computed by using Modified Virtual Crack Closure Integral (MVCCI) method. The stiffened panels fatigue crack growth rate was computed by using Paris law under constant amplitude fatigue loads. The analysis results show that integral stiffened panel causes higher stress intensity factor and less load bearing capability than riveted stiffened panel which has better damage tolerant capability in comparison to the integrally stiffened panel.

Experimental Study of the Effect of Wire Electrical Discharge Machining on Crack tip Opening Displacement for Compact Tension Specimens of Low Carbon Steel

Fracture mechanics approach is important for all mechanical and civil projects that might involve cracks in metallic materials the purpose of this paper is to determine a crack tip opening displacement fracture toughness experimentally, also study the effect of thickness on CTOD fracture toughness of low carbon steel and study the effect of Wire Electrical Discharge Machine (WEDM) to have a pre-crack, instead of fatigue pre-crack by using a CT specimen of low carbon steel with a thickness of (8,10, and15 mm), a width of 30mm, crack length of 15mm, and pre-crack of 1.3mm for all samples, this dimension according to ASTM-E399-13, by pulling the specimen in a 100 KN universal testing machine at a slow speed rate of 0.5 mm/min, the load applied on the specimen is generally a tension load. The crack tip plastically deforms until a critical point PC at this moment a crack is initiated. The computer-controlled universal testing machine gives the value of the load and the displacement transducer gives a crack mouth opening displacement. Critical crack tip opening displacement CTOD is found with the plastic hinge model (PHM) method. The result showed the stress intensity factor KI increases with increased loading in the elastic region and the thickness effect refers to the effect of the plastic zone at the crack tip on the stress intensity factor, In a thin specimen, a plastic zone is large at the fracture tip leads to a high-stress intensity factor at the fracture tip but in the thick specimen, on the other hand, has a small a plastic zone and a low-stress intensity factor around the crack tip. The fracture toughness is found to increase with an increase in the thickness of specimens.

Mechano-electrochemical modeling of lithium dendrite penetration in a solid-state electrolyte: Mechanism and suppression

The mechanism of lithium dendrite penetration in solid-state electrolyte (SE) and its suppression strategies are studied based on a new phase field (PF) model involving fracture mechanics, electrodeposition processes, and mechano-electrochemical coupling (MEC) effects. Numerical results reveal the high stress-intensity factor is caused by high hydrostatic pressure in lithium, and the high stiffness of SE does not inhibit dendrite penetration. It is because the increase in Young's module of SE, ESE, makes the stress-intensity factor even more significant. That is why a stiff SE is “pierced” by the much softer lithium dendrites. Considering MEC, the increase in ESE has a competing effect on dendrite penetration causing a nonmonotonic change in dendrite length, which provides a window to mitigate dendrite penetration. Dendrite suppression by toughening SE is quantitatively evaluated. A critical fracture surface energy density of SE (γ = 3.5 J m−2) is determined. When γ > 3.5 J m−2, facture toughness becomes larger than stress-intensity factor and dendrite penetration is suppressed with ESE. However, toughening is difficult. Engineering compressive traction, Fa, in SE surfaces is a more realistic strategy, that cause a significantly inhibition in dendrite penetration with Fa from 0 to 100 MPa.

Discovering the role of the defect morphology and microstructure on the deformation behavior of additive manufactured Ti–6Al–4V

The defects remain a significant issue for additive manufactured titanium alloys, which hinders their further market penetration. In this work, we proposed a new understanding of the inhomogeneous microstructure formation affected by defects with detailed EBSD characterization, and how the defects and inhomogeneous microstructures determined the strain accumulation behavior by in-situ SEM tensile testing. Smaller prior β grains and coarser α laths were identified around the defects. In-situ tensile behavior investigations revealed that irregularly-shaped and elongated defects with high stress intensity factor (ΔKI) could induce severe strain accumulation, showing that the defect morphology was the primary factor that could determine the strain accumulation behavior. In the meantime, the inhomogeneous microstructures around the defects could further contribute to the strain accumulation, with the coarser α laths more favorable for deformation and stronger microtexture for easier slip transmission. In addition, microcracks were found initiated from a spherical pore with small loading, and arrested due to the negligible strain accumulation. Further characterization showed that oxidation, which was found more probable in spherical pores, was the cause of this microcrack initiation.

Effect of Fiber Orientation Angle on Stress Intensity Factor of Composite Plate Using Extended Finite Element Method (XFEM)

This paper presents the effect of fiber orientation angle on the stress intensity factor SIF for carbon epoxy composite plates with single-edge, center, and inclined cracks of varying lengths under tensile load. The stress intensity factor and shape factor were calculated individually for each case, with nine different fiber orientation angles computed using the extended finite element method XFEM concepts. It is found the stress intensity factor increases with increasing crack lengths while the shape factor decreases. In the case of single edge cracks, the SIF increases in the average value reached (173 %) for composite plates with different fiber orientation angles, while in the case of the center crack, the average value of SIF reaches (81 %). It was observed in this study that the increases in stress intensity factor and the decreases in the shape factor with different crack lengths were more stable in the composite plate with a fiber orientation angle of 75°. The higher values of SIF at an angle of 75° are because of the high probability of fiber slippage at 75° due to induced shear stresses in addition to the tensile stresses at the fiber-matrix interface. As a result, the crack tip has a high-stress intensity factor.

Predicting damage in aggregates due to the volume increase of the alkali-silica reaction products

Volume increase between the reactants and the products of alkali-silica reaction could reach up to 100%. Taking place inside the aggregates, ASR imposes internal pressure on the surrounding material. In the current paper, the possibility of crack growth due to such internal loading is studied. This study is done by employing a semi-analytical mechanical model comprising an elastic solution to a well-known Eshelby problem and a linear elastic fracture mechanics solution to a ring-shaped crack encircling a spheroidal inclusion. The proposed method implies the presence of pre-existing micro-fissures within the aggregate.The study reveals the dependence of the crack growing potential on the spheroid's shape: the larger the ASR pocket - the longer crack opens. The two most critical shapes, causing the highest stress intensity factor and developing the longest crack, are a sphere and a spheroid with a 1/4 aspect ratio respectively. The size analysis of the problem suggests a critical spheroid's radius below which no crack growth is expected. For a chosen material properties and expansion value, such radius lies in the range between 0.1μm and 1μm. Independently of the expansion value and the shape of the pocket of the ASR product, the developed crack length has a power-law dependence on the size of a spheroid.All the theoretical predictions are confirmed by a numerical model based on the combination of the finite element method and the cohesive zone model.

Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class of structural materials were analyzed and it was concluded that both rates coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires a high level of cyclic deformation of the metal along the entire crack front as a necessary condition for the formation of fatigue striations (FS). Based on the analysis of digital fractographic images of the fatigue fracture surfaces, a method for the quantitative assessment of the spacing of FS has been developed. The method includes the detection of FS by binarization of the image based on the principle of local minima, rotation of the highlighted fragments of the image using the Hough transform, and the calculation of the distances between continuous lines. The method was tested on 34KhN3M steel in the initial state and after long-term operation (~3 × 105 h) in the rotor disk of a steam turbine at a thermal power plant (TPP). Good agreement was confirmed between FCG rates (both macro and microscopic, determined manually or using digital imaging techniques) at high SIF ranges and their noticeable discrepancy at low SIF ranges. Possible reasons for the discrepancy between the micro- and macroscopic FCG rates at low values of the SIF are analyzed. It has also been noted that FS is easier to detect on the fracture surface of degraded steel. Hydrogen embrittlement of steel during operation promotes secondary cracking along the FS, making them easier to detect and quantify. It is shown that the invariable value of the microscopic FCG rate at a low SIF range in the operated steel is lower than observable for the steel in the initial state. Secondary cracking of the operated steel may have contributed to the formation of a typical FS pattern along the entire crack front at a lower FCG rate than in unoperated steel.

3D characterisation of hydrogen environmentally assisted cracking during static loading of AA7449-T7651

In this investigation, synchrotron X-ray microtomography was used to perform 3D in situ observations of crack initiation and growth during hydrogen environmentally assisted cracking (HEAC) in tensile samples of AA7449-T7651. Two smooth tensile samples with a 1 mm diameter gauge section were held at a fixed displacement (approx 30% of yield stress) in warm, moist air (approx 76,^circ C, 73% relative humidity). The samples were then imaged repeatedly using X-ray tomography until they fractured completely. The tomograms showing the nucleation and evolution of intergranular cracks were correlated with electron microscopy fractographs. This enabled the identification of crack initiation sites and the characterisation of the crack growth behaviour relative to the microstructure. The samples were found to fracture within an environmental exposure time of 240 min. Some cracks in both samples nucleated within an exposure time of 80 min (33–40% of the total lifetime). Many cracks were found to nucleate both internally and at the sample surface. However, only superficial cracks contributed to the final fracture surface as they grew faster owing to the direct environmental exposure and the larger crack opening. HEAC occurred prominently via brittle intergranular cracking, and cracks were found to slow down when approaching grain boundary triple junctions. Additionally, crack shielding from nearby cracks and the presence of coarse Al–Cu–Fe particles at the grain boundaries were also found to temporarily reduce the crack growth rates. After prolonged crack growth, the HEAC cracks displayed ductile striations and transgranular fracture, revealing a change in the crack growth mechanism at higher stress intensity factors.

Fatigue mechanism of medium-carbon steel welded joint: Competitive impacts of various defects

Fatigue of welded joint is a scientific issue of great significance, because welding process always causes the loss of fatigue strength up to 40–60% for welded joint of the key components, resulting in a relatively poor reliability in service. In the present work, the fatigue behavior and damage mechanism of a medium-carbon steel welded joint were systematically investigated by microstructure observation and fatigue tests. It was found that, for the medium-carbon steel welded joint, fatigue crack mainly initiated at the welding defects, e.g., welding porosity, slag inclusion and heterogeneous microstructure with a poor mechanical property (i.e., the network-block proeutectoid ferrite). The competitive impact of various defects on the fatigue damage behaviors of welded joint was discussed, i.e., the competition between the volume defects (e.g., porosity and inclusions) and the heterogeneous structure. It is revealed that the smaller surface defects are more susceptible to fatigue damage due to the low constraint of plane stress state and higher stress intensity factor at high stress amplitude, while the larger internal defects easily cause fatigue damage at low stress amplitude because the driving force for crack propagation is larger than that of the damage case from the sample surface. Generally, the fatigue lives of samples with fatigue crack initiating from the weak phase are relatively longer, about 10 times as long as those of the samples with fatigue crack starting from the sample surface. In addition, the residual fatigue crack growth life, Nf, and the initial stress intensity factor (SIF), ΔK1 at the volume defect tip exhibit a linear relation in the double logarithmic coordinate system. The present findings can provide a theoretical basis for the anti-fatigue design and fatigue life extension technology for the metallic component with welded structures.

Effect of edge and internal cracks in 8YSZ thermal barrier coatings subjected to mechanical loading: An numerical approach

Abstract The paper presents fracture parameters for edge and internal cracks subjected to mechanical load by the finite element method. The influence of crack length and crack position on edge and internal crack in TBCs on the strain energy release rate (SERR) and stress intensity factor (SIF) are discussed in detail. The numerical results show that the edge crack near the top surface results in high SERR and SIF mode I. In edge crack, the decreasing trend in the SERR was observed with a decrease in crack length and crack position change towards the TC/BC interface. About 60.06% reduction in the SERR was observed for 100 µm edge crack positioned near the top surface when shifted towards TC/BC interface. Similarly, 60 µm and 80 µm edge crack show a 30% and 56% reduction in SERR. Whereas, 20 µm and 40 µm edge crack shows 1.76 and 1.48 times more value of SERR. The internal crack shows a small value of SERR and SIF mode I irrespective of crack length and position compared to edge crack. For internal crack, as the crack length was increased from 20 µm to 100 µm, the reduction in SERR value increases from 59.23% to 90.23%. Similarly, the edge crack shows high SIF mode I compared to internal cracks irrespective of crack length and position. The edge crack appears to be more unstable than the internal crack and propagates early to cause the failure. The crack length and position strongly influence the crack initiation and propagation, resulting in the failure mechanism.

Improving the bending toughness of Al-Si coated press-hardened steel by tailoring coating thickness

A ferrite layer is formed between the outer intermetallics layers and the martensite substrate during the hot stamping process of Al-Si coated press-hardened steel (PHS). Unexpectedly, it is found here that the ferrite layer does not effectively prevent brittle cracks propagating continuously from the intermetallics to the martensite substrate. This leads to a high stress intensity factor (SIF) at the crack tip, therefore initiating highly localized shear deformation in the martensite substrate, degrading the bendability of the steel. A thinner Al-Si coating produces thinner brittle intermetallics and ferrite layers, and therefore shorter coating cracks and smaller SIF at the crack tip, thus improving the bendability. In addition, a thinner Al-Si coating also has a lower material cost while keeping similar paintability and corrosion resistance. The thin Al-Si coating could potentially change the current practice of Al-Si coating, impacting the global automotive industry.

Mode-I, Mode-II, and Mode-III Stress Intensity Factor Estimation of Regular- and Irregular-Shaped Surface Cracks in Circular Pipes

Stress intensity factor (SIF) of surface cracks (semi-circular, semi-elliptical, and irregular shapes) located on a circular pipe (hollow) has been determined using numerical method. The irregular-shaped crack geometry was considered in the present work in order to simulate the cracks that are generally developed during service condition. The interaction behavior of multiple cracks on SIF calculation has also been investigated. Crack aspect ratios [a/c; “a” is the crack depth and where “c” is semi-major axis of elliptical crack] ranging between 0.6 and 1.0 and crack depth ratios [(a/d); where “d” diameter of the pipe] ranging between 0.1 and 0.4 were considered. Irregular-shaped cracks were modeled by keeping the crack depth (a) constant. For a regular profile semi-elliptic crack [(a/c) = 0.6], higher SIF was observed at crack middle region [(S/S0) = 0]. In contrast, higher SIF was observed at crack surface region [(S/S0) = ± 1] for semi-circular crack. Asymmetric spreading of SIF was noticed because of added effect of mode-II and mode-III fracture. In addition, the leading mode of fracture at short crack depth [(a/d) ≤ 0.1] was mode-II (in-plane shear) and mode-III (out of plane shear) fractures whereas at deep crack depths [(a/d) ≥ 0.4], the leading mode of fracture was mode-I fracture which is crack opening mode. Marginal influence of crack aspect ratio was noticed at crack surface region [(S/S0) = ± 1] and the effect is significant at crack middle region [(S/S0) = 0]. For a constant crack depth, higher SIF was obtained at the middle region [(S/S0) = 0] compared to regular profile crack. When comparing the observations of regular semi-elliptic crack, higher influence of mode-II and mode-III fracture was observed for irregular-shaped semi-circular crack which may leads to additional crack growth. Thus, approximating the standard crack geometry may over/under predict the SIF values. It is also inferred that the presence of adjacent cracks significantly affects the SIF of middle crack due to interaction effect. The interaction behavior was observed to be more pronounced when multiple cracks are located at closer interval. The interaction effect reduces gradually when the distance between crack centers increases. The improved SIF solutions obtained with consideration of additional fracture modes will be useful during the residual life determination.

The numerical simulation of fatigue crack propagation in Inconel 718 alloy at different temperatures

Inconel 718 alloy is the most commonly used material of aero-engine turbine today. However, Inconel 718 alloy is known to suffer from the fatigue crack propagation during engine operation, which will lead to unstable fracture and seriously threatens the safety of the engine. In this paper, we research the fatigue crack propagation process of Inconel 718 alloy unilateral notched standard CT specimen at room temperature (298.15 K), 573.15 K, and 823.15 K under the type I cyclic fatigue load. The ABAQUS is used for numerical simulation. First, parametric models are established. Second, the extended finite-element method (XFEM) is used to in the calculation process for describing the crack state. Third, the stress intensity factor at the crack tip under different temperatures is calculated, and the extended finite-element crack length is solved to obtain the fatigue crack growth rate da/dN curve. The innovation is we use the XEFM method to predict the fatigue crack growth process of Inconel 718 alloy, and compare it with the test results to verify the reliability of the XEFM method under low stress intensity factor. We also provide some possible reasons for the error of XEFM results under high stress intensity factors, and the development of simulation methods should be emphasized.

Waveform and frequency effects on corrosion-fatigue crack growth behaviour in modern marine steels

The primary focus of this work is to investigate the sensitivity of cyclic waveform, frequency (f), load level and microstructure on the corrosion-fatigue crack growth rate (CFCGR) in modern normalised-rolled (NR) and thermomechanical control process (TMCP) ferrite-pearlite steels in the Paris Region of the da/dN vs. ΔK log-log plot. Constant amplitude sinewave (si) and trapezoid waveform (generally referred to here as hold-time (h-t)) were used under frequencies of 0.2 Hz, 0.3 Hz and 0.5 Hz and stress ratio of 0.1. Comparison is also made between the crack path in the S355 TMCP steel under si and h-t in seawater (SW). The role of microstructure in retarding or accelerating fatigue crack growth in SW is also discussed. Experimental results showed that the CFCGR corresponding to the si is higher than that of the h-t for all the load levels and frequencies examined. It was observed that reduction in the f and fatigue load level increased the CFCGR for the h-t but had little effect on the si. Generally, f in the range 0.2–0.5 Hz had little effect; and for a given f an increase in load led to a reduction in the CFCGR, in the Paris Region (PR) for both si and h-t in SW. Under both si and h-t, the CFCGR in the TMCP steels (e.g. S355G8 + M, S355G10 + M) is lower than that of the normalised steels (e.g. S355J2 + N). Metallurgical analyses on the fractured surface of corrosion-fatigue specimens show that the main active crack tip blunting process is the primary factor controlling the CFCGR of steel at high stress intensity factor range (SIFR) and low f in SW. The results obtained from this study have been discussed in terms of the potential impact on the structural design and integrity of offshore wind turbine foundations.

Evaluation of fatigue crack propagation in steel ESET specimens subjected to variable load spectra

This paper reports on an experimental study of fatigue crack growth in steel specimens. First, block loading tests (sequences of low and high stress intensity factor ranges ΔK) are discussed. Limited crack growth retardation occurs at transitions from low to high load ranges; significant retardation or crack arrest are observed in high-low transitions. Next, semi-random load spectra are created, processed using a peak-and-valley analysis and further reduced by removing the load ranges below the stress intensity factor threshold ΔKth. Rainflow counting is performed to obtain load profiles consisting of a sequence of blocks with constant ΔK. For the semi-random and the (reduced) peak-and-valley spectra no significant load interaction is observed. Pronounced crack growth retardation is observed in an ordered spectrum obtained by rainflow counting. The strong reduction in number of cycles of the (reduced) peak-and-valley spectra allows for exploration of accelerated fatigue testing. Experimental results of fatigue crack propagation are compared to results of calculations using a Python based numerical framework.

Fatigue Failure Characteristics of Single-Crystal Fe-3%Al Alloy

To obtain a better understanding of the mechanical properties and failure characteristics of the single-crystal Fe-3%Al alloy (SC), tensile and fatigue properties of the SC and polycrystalline Fe-3%Al alloy (PC) have been investigated. The tensile strength and fracture strain for the SC samples were different depending on the loading direction, i.e., 〈111〉, 〈110〉 and 〈100〉, which could be explained by the Schmid factor. Fatigue strength for the SC samples was basically correlated with their tensile strength. Different fatigue properties were obtained for the PC sample, e.g., the high fatigue strength was obtained in the early fatigue stage, and significant reduction in the fatigue strength was appeared in the later fatigue stage, e.g., low endurance limit at 107 cycles. The reason for this was caused by the material brittleness due to the accumulated dislocation at the grain boundaries. Because the high ductile SC samples made severe plastic deformation ahead of the crack tip, the crack growth rate did not enhance even if the high stress intensity factor range was applied in Region II (da/dN versus ΔK). The crack growth rate dropped just before the final fracture. Effective stress intensity factor range was estimated using the crack closure model, but the crack growth rate is found to be affected strongly by the slip system of SC sample.