Fatigue Crack Propagation Rate Research Articles

The Positive Role of Nanometric Molybdenum-Vanadium Carbides in Mitigating Hydrogen Embrittlement in Structural Steels.

The influence of hydrogen on the fracture toughness and fatigue crack propagation rate of two structural steel grades, with and without vanadium, was evaluated by means of tests performed on thermally precharged samples in a hydrogen reactor at 195 bar and 450 °C for 21 h. The degradation of the mechanical properties was directly correlated with the interaction between hydrogen atoms and the steel microstructure. A LECO DH603 hydrogen analyzer was used to study the activation energies of the different microstructural trapping sites, and also to study the hydrogen eggresion kinetics at room temperature. The electrochemical hydrogen permeation technique was employed to estimate the apparent hydrogen diffusion coefficient. Under the mentioned hydrogen precharging conditions, a very high hydrogen concentration was introduced within the V-added steel (4.3 ppm). The V-added grade had stronger trapping sites and much lower apparent diffusion coefficient. Hydrogen embrittlement susceptibility increased significantly due to the presence of internal hydrogen in the V-free steel in comparison with tests carried out in the uncharged condition. However, the V-added steel grade (+0.31%V) was less sensitive to hydrogen embrittlement. This fact was ascribed to the positive effect of the precipitated nanometric (Mo,V)C to alleviate hydrogen embrittlement. Mixed nanometric (Mo,V)C might be considered to be nondiffusible hydrogen-trapping sites, in view of their strong hydrogen-trapping capability (~35 kJ/mol). Hence, mechanical behavior of the V-added grade in the presence of internal hydrogen was notably improved.

A study of automatic measurement method of RPG (Re-tensile Plastic zone’s Generated) Load

ABSTRACT Identification of the relationship between fatigue crack propagation rate (da/dN) and stress intensity factor range (∆K) is inevitable to apply the fracture mechanics approach to assess the growth of fatigue crack growth. The relationship between da/dN and ∆K is widely applied to evaluate fatigue crack propagation behaviour. To evaluate the fatigue crack growth history under variable loading history, it is necessary to replace ∆K to the effective stress intensity factor range, which can quantitatively consider fatigue crack opening and closing behaviour. ∆K eff proposed by Elber is well known as the effective stress intensity factor range, but even if ∆K eff is applied to evaluate the fatigue crack propagation behaviour, a threshold value ((∆K eff)th) was occurred. On the other hand, it is known that fatigue cracks propagate even at ∆K eff below (∆K eff)th under variable loading history. This implies that ∆K eff is an insufficient parameter to describe fatigue crack propagation behaviour. ∆K RPG, which has a close relationship with the cyclic plastic behaviour in the vicinity of the crack tip proposed by Toyosada and Niwa, can overcome the shortcomings of ∆K eff even under complicated variable loading history including multiple frequency components. To apply the fatigue crack propagation law with ∆K RPG as a parameter, it is necessary to experimentally measure the RPG load and identify the propagation law constants (∆K RPG = C(∆K RPG) m ) C and m. A conventional method for identifying RPG loads requires the superposition of the hysteresis loop near the crack tip and its reversal loop as measured by the unloading elastic compliance method. However, advanced skills in this method, such as understanding the characteristics of measurement errors associated with loops, is required. In this study, we have proposed an automatic method for measuring the RPG load that is equivalent to manual measurement results by an expert and we have validated the automatic measurement method by comparing the results of automatic measurement using our proposed method with the past conventional measurement results for several materials and loading conditions.

Embedding artificial neural networks into twin cohesive zone models for composites fatigue delamination prediction under various stress ratios and mode mixities

This paper presents for the first time a novel numerical technique for modelling fatigue delamination growth in fibre reinforced composites, which is based on coupling two twin cohesive zone models with a single-hidden-layer artificial neural network. The simulation approach proposed here can describe composites fatigue delamination under negative & positive stress ratios and the full range of mode mixities. In the modelling strategy, each segment of a composites interface is described by two twin cohesive elements, which jointly provide local fracture mechanics parameters into a feedforward single-hidden-layer neural network, without the need to know the global load R ratio. In turn, the neural network algorithm feeds the fatigue crack propagation rate da/dN back into the twin cohesive elements, which follow a static and fatigue cohesive law in a synchronous fashion. The novel modelling methodology has been implemented in an explicit finite element scheme. The modelling strategy is first verified and validated by several benchmark cases, involving mode I Double Cantilever Beam tests, mode II End Loaded Split tests with and without reversal, as well as Mixed-Mode Bending tests. A relevant application of the modelling technique is demonstrated considering a tapered laminate, which experiences non-proportional loading due to the presence of combined static tension and cyclic bending.

Friction Stir Processing Influence on Microstructure, Mechanical, and Corrosion Behavior of Steels: A Review.

Friction stir processing (FSP) technology has received reasonable attention in the past two decades to process a wide range of materials such as aluminum, magnesium, titanium, steel, and superalloys. Due to its thermomechanical processing nature, FSP is used to alter grain structure and enhance mechanical and corrosion behavior in a wide range of steels. The refinement in grains and phase transformations achieved in steel after FSP affects hardness, tensile properties, fracture toughness, fatigue crack propagation rate, wear resistance, and corrosion resistance. A number of review papers are available on friction stir welding (FSW) or FSP of nonferrous alloys. In this article, a comprehensive literature review on the FSP/FSW of different types of steels is summarized. Specifically, the influence of friction stir processing parameters such as advancing speed, rotational speed, tool material, etc., on steels’ performance is discussed along with assessment methodologies and recommendations.

Investigation of crack growth in CFRP-concrete interface under fatigue loading

In this paper, the fatigue crack growth behavior along the interface between carbon fiber reinforced polymer (CFRP) sheets and concrete induced by intermediate flexural crack in CFRP-sheet strengthened concrete beams was studied, both experimentally and theoretically. The fatigue crack growth behavior of the intermediate crack-induced debonding was investigated by modified beam tests. The major variables considered in the test were loading amplitude, concrete strength, and the width ratio of CFRP-sheet to concrete. Test results indicated that three stages were observed in the evolution of CFRP-concrete interfacial crack propagation under cyclic loading, including debonding initiation, steady propagation, and instability propagation. Furthermore, the fatigue crack propagation rate grew with the increase of the load level, concrete strength, or the width ratio of CFRP-sheet to concrete, and was reduced as load cycles increased in the steady propagation stage due to the interfacial friction existed in the debonding zone. Based on the test results and fracture mechanic method, a crack growth prediction model, which considered the effects of concrete strength and width ratio of CFRP-sheet to concrete, was proposed for predicting the whole crack growth process and fatigue life of intermediate crack-induced debonding of CFRP-concrete interface under cyclic loading. The predicting results using this proposed model agreed well with test results.

Compliance-based testing method for fatigue crack propagation rates of mixed-mode I–II cracks

Compliance-based testing method for fatigue crack propagation rates of mixed-mode I–II cracks

Fatigue crack propagation within Al-Cu-Mg single crystals based on crystal plasticity and XFEM combined with cohesive zone model

A method combining crystal plasticity (CP), the eXtended Finite Element Method (XFEM), and cohesive zone model (CZM) with traction separation law is developed for an Al-Cu-Mg alloy to predict the effect of grain orientation on fatigue crack propagation (FCP) during stage ΙΙ within a single crystal. The simulation results show that of all the orientations, Goss grain possesses the largest fatigue crack deflection, then followed by Cube orientation. Comparatively, Brass, Copper, S and Random grains have relatively small crack deflection angles. Besides, it is found that Goss grain with the largest fatigue crack deflection possesses the lowest FCP rate as compared with other orientations. The results of simulations are consistent with previous experimental observations. This indicates that this coupled CP XFEM CZM simulation method is capable of well predicting the effect of grain orientation on FCP during fatigue stage ΙΙ of Al-Cu-Mg single crystal.

Influence of macrozones on the fatigue cracking behavior and fracture mechanisms of rolled Ti–6Al–4V alloy

In this work, rotating bending fatigue, scanning electron microscope in-situ fatigue, pre-fatigue (and post-fatigue) electron backscatter diffraction tests were carried out for rolled Ti–6Al–4V alloys. The macrozone sizes and orientations of the materials were classified and the effect of macrozones on fatigue cracking behavior and fracture mechanisms was studied. The results indicated that intra-granular fracture was dominated in the process of crack propagation. Basal and prismatic slip systems were favored for the macrozones whose crack paths were straight and zigzag, respectively. Multiple active slip systems can also induce the fatigue crack deflections inside grains. Moreover, fatigue crack propagation rate, threshold stress intensity factor range (ΔKth) and fracture toughness (KIC) were measured for different kinds of macrozones. For the macrozones favorably orientated for prismatic slip, their crack propagation resistance and ΔKth were excellent. However, the KIC values for different macrozones were similar. Finally, the effect of macrozone orientations on fracture mechanisms (cleavage and plastic fracture) was discussed through a combination of Schmid factor, active slip systems and the Δθ angle (between ɑ-Ti phase (0001) plane normals (c-axes) and cyclic load directions). The Δθ is an adequate parameter to predict the fatigue fracture modes.

Impact of foreign object damage on the leading edge of TC11 titanium alloy aeroengine blade like specimen

ABSTRACT Foreign object damage (FOD) generally occurs when hard body particles are ingested into an aero-engine fan and compressor blade when the engine is taxing, flying, or moving on the runway with definite speed. Ingested foreign object debris induces severe structural damage on the leading edge of the aero-engine blade due to the time-dependent axial fatigue loading under service conditions. Induced residual stresses due to foreign object damage play a major role in mitigating the rate of fatigue crack initiation and propagation, resulting in failure of components. In this present investigation, an explicit module of commercial software package LS-DYNA was used to simulate the impact of the GCr15 stainless steel ball (52,100 steel equivalent) against leading-edge specimen (TC11 titanium alloy) by varying speeds like the in-service operating speeds. A comparative study was carried out among obtained deformed geometry (Crater) and its dimensions from the numerical simulation with experimentation results. Variation of residual stresses in three mutually perpendicular directions along with depth has been determined to predict the possible location of crack initiation at different projectile impact velocities. A stress concentration factor has also been calculated at the vicinity of the impact, which is very much an essential factor for the fatigue strength evaluation of TC11 titanium alloy.

ON THE LAW OF DISTRIBUTION OF THE RATE OF FATIGUE FRACTURE

The article discusses the correction of the law of development of fatigue cracks based on the application of the Fokker–Planck–Kolmogorov equation. The coefficient of variation of the fatigue crack propagation rate was chosen as the correction criterion. Its decrease brings the descriptions of the fatigue fracture process closer to engineering. The latter is especially important for specialists dealing with the assessment of the fatigue strength of metal-consuming structures, where mass formation of fatigue cracks is inevitable.

Fatigue crack propagation behaviors in Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with STA and BASCA heat treatments

Fatigue crack propagation behaviors of a novel high strength Ti-5Al-3Mo-3V-2Zr-2Cr-1Nb-1Fe alloy with equiaxed and lamellar microstructures were systematically investigated. Fatigue crack propagation tests at both increasing and decreasing stress intensity factor range values were carried out with a frequency of 15 HZ and stress radio R of 0.1 by using compact tension samples. The sample with lamellar microstructure displayed the faster fatigue crack propagation rates in near-threshold regime compared to the sample with equiaxed microstructure. The fatigue threshold (ΔKth) was 2.7 MPa·m1/2 for lamellar microstructure, and 3.2 MPa·m1/2 for equiaxed microstructure. The steady state crack growth rate in lamellar microstructure was slightly slower than that in equiaxed microstructure in the Paris regime, while much faster in the rapid growth regime. The interactions between crack path and microstructures as well as the microscopic fracture surface morphology in both microstructures were detected and analyzed by scanning electron microscope (SEM), revealing the different crack propagation behaviors in three regimes. The results showed that the crack propagation behaviors were a result of two contributing factors: interface obstacles and crack front profile. The increased fatigue crack propagation resistance of equiaxed microstructure in near-threshold regime can be attributed to the interface obstacles which dominated over the rougher crack front profile. While the higher fatigue crack growth resistance for lamellar microstructure in Paris regime was attributed to the more positive effect of rougher crack front profile. In the rapid growth regime, the crack in lamellar microstructure was found to be extended along the grain boundary α, which was regarded as a low energy crack path and led to the faster crack propagation rate.

Study on Fatigue Crack Propagation and Fracture Characterization of 7050-T7451 Friction Stir Welded Joints

The fatigue crack propagation (FCP) behavior of FSW welded 7050-T7451 joints was studied using compact tensile specimens. Based on Paris law and three-stage theory of FCP, the FCP rate of joint was obtained by data fitting and calculation. The fracture morphologies at different stages of FCP were characterized by metallography and scanning electron microscopy (SEM). Results showed that the FCP rate in each region for FSW joint was as follows: heat affected zone (HAZ) > weld nugget zone (WNZ) > base metal (BM). In the first stage of FCP, the fracture morphologies of BM and HAZ were mainly tearing ridges and river pattern. There were a small number of dimples on the fracture of BM, but no dimples on the fracture of HAZ. In the second stage of FCP, the fatigue striations of the BM were denser and finer than those in the HAZ, and have dimple fracture characteristics. In the third stage of FCP, dimples still existed in the fracture surface of the BM, and the fatigue striations disappeared. The HAZ was dominated by lamellar tearing and cleavage fracture without dimples. During the three stages of FCP, the fracture morphology of the WNZ in the first and second stages was fine dimple fracture with significant equiaxed deep dimple morphology. In the third stage, the fracture dimple in the WNZ changed from equiaxed deep dimples to tearing dimples.

Material pre-straining effects on fatigue behaviour of S355 structural steel

A commonly used material in offshore structures is S355 structural steel. For example, during the monopile fabrication process, the material is pre-strained to different levels at different depths through the thickness. Therefore, the influence of pre-straining on fatigue life and crack growth behaviour of the material needs to be examined and considered for design and life assessment procedures. In the present study, uniaxial fatigue and fatigue crack growth tests have been conducted on materials with different pre-strain levels and the results are compared with the un-strained material state. From the test data, it has been seen that the S-N fatigue life will reduce with increasing pre-straining level, while the fatigue crack propagation rate remains largely unchanged in pre-strained material. The results from this study are compared with the recommended S-N fatigue and fatigue crack growth trends available in standards and are discussed in terms of the applicability and level of conservatism in the recommended curves to account for the material pre-straining effects on the fatigue life assessment of offshore structures.

Fatigue crack propagation behavior of polyether ether ketone under biaxial loading

AbstractPolyether ether ketone (PEEK) has become a promising material in total joint replacement. However, it still faces the risk of fatigue fracture during service. In this paper, the effects of biaxial stress ratio λ, cyclic stress ratio R, and load phase difference θ on fatigue crack propagation (FCG) behavior of PEEK are investigated. In the case of vertical cracks, results show that the FCG rate of PEEK increases with the R value, while decreases with the increase of λ value. Furthermore, the effective stress intensity factor range ΔKeff can uniformly describe the biaxial FCG behavior at different cyclic stress ratios. In the case of 45° slant cracks, compared with mode‐I intensity factor range ΔKI, the energy release rate range ΔG is more accurate for describing the FCG behavior under various load phase differences. In addition, the investigation on the 45° crack propagation path shows that a bifurcated Y‐shaped crack appears under 180° load phase difference, while no bifurcated crack appears under 90° load phase difference and uniaxial loading. Three different methods are used to predict the crack propagation path. The comparison results show that the maximum circumferential stress (MTS) criterion can well predict the crack propagation path under out‐of‐phase biaxial loading and uniaxial loading.

Mechanical Behavior and Morphological Study of Polytetrafluoroethylene (PTFE) Composites under Static and Cyclic Loading Condition.

The key goal of this study was to characterize polytetrafluoroethylene (PTFE) based composites with the addition of bronze particles and mineral fibers/particles. The addition of individual fillers was as follows: bronze—30–60 wt.%, glass fibers—15–25 wt.%, coke flakes—25 wt.% and graphite particles—5 wt.%. Both static and dynamic tests were performed and the obtained results were compared with the microscopic structure of the obtained fractures. The research showed that the addition of 60 wt.% bronze and other mineral fillers improved the values obtained in the static compression test and in the case of composites with 25 wt.% glass fibers the increase was about 60%. Fatigue tests have been performed for the compression-compression load up to 100,000 cycles. All tested composites show a significant increase in the modulus as compared to the values obtained in the static compression test. The highest increase in the modulus in the dynamic test was obtained for composites with 25 wt.% of glass fibers (increase by 85%) and 25 wt.% of coke flakes (increase by 77%), while the lowest result was obtained for the lowest content of bronze particles (decrease by 8%). Dynamic tests have shown that composites with “semi-spherical” particles are characterized by the longest service life and a slower fatigue crack propagation rate than in the case of the long glass fibers. In addition, studies have shown that particles with smaller sizes and more spherical shape have a higher ability to dissipate mechanical energy, which allows their use in friction nodes. On the other hand, composites with glass fiber and graphite particles can be successfully used in applications requiring high stiffness with low amplitude vibrations.

Effects of the Strengthening Mechanism on Fatigue Properties of Precipitation-Hardened Al Alloy

The effects of the strengthening mechanism on the fatigue properties of Al-Mg-Si alloy were investigated. Annealing was conducted to minimize the precipitation-hardening effects on Al-Mg-Si alloy, as it could be the most efficient method to stabilize the precipitates. Severe plastic deformation was adapted to obtain grain-refining and work-hardening effects after the annealing process. To evaluate the change in fatigue properties and to investigate the dominant strengthening mechanisms in the specimens, fatigue crack propagation and low-cycle fatigue tests were performed. The experimental results showed that the fatigue crack propagation rate of the annealed specimen was higher than those of other specimens that were grain-refined and precipitation-hardened. During the low-cycle fatigue test, the annealed specimen exhibited cyclic hardening and saturation behavior, whereas the grain-refined specimen exhibited saturation and softening behavior. The low-cycle fatigue life and characteristics of the hysteresis loops of the specimens were evaluated using various parameters. Precipitation-hardened specimens exhibited higher fatigue strength and ductility coefficients than those under other conditions.

Numerical Modeling of Plasticity-Induced Fatigue Crack Growth Retardation Due to Deflection in the Near-Tip Area

This article studies the retardation effect in plasticity-induced fatigue crack growth rate for a low-medium strength steel, due to the appearance of microdeflections in the crack path. To this end, the finite element method was used to model the crack with its kinked tip under several stress intensity factor (SIF) ranges. The results allowed a calculation (after a small number of cycles) of the fatigue crack propagation rate for the multiaxial and uniaxial fatigue configurations at the microscopic level. It was observed that the retardation effect rose with an increase in the initial kinked crack tip angle, an increase in the initial projected kinked crack tip length, and with a decrease in the SIF range.

Research on stress intensity factor and fatigue crack propagation rate of the general-purpose gondola car body

In order to study the crack propagation characteristics of key parts of the general-purpose gondola car body, according to the structural characteristics and crack statistics results of the car body, the floor along the cross bearer weld was taken as the research object. The stress intensity factors under different conditions of the floor were studied, based on linear elastic fracture mechanics and finite element analysis. In addition, the crack propagation rates of the floor were studied by the Paris formula. These research results will provide a reference not only for formulating maintenance procedures for the general-purpose gondola car body, but also for evaluating the safety and reliability of wagon bodies.

Coupling Effect of Grain Structures and Residual Secondary Phases on Fatigue Crack Propagation Behavior in an Al-Cu-Mg Alloy

The effect of solution treatments at high temperatures on the microstructure and fatigue performance of an Al-Cu-Mg alloy was investigated. Microstructural observation revealed that large secondary phases could be dissolved sufficiently by a strengthening solution at 507 °C for 20 min and shortening the solution time at the high temperature would bring about a significantly grain refinement. The fatigue test showed that the sample treated by the solution at the high temperature for a short time (507 °C/6 min) possessed the lowest fatigue crack propagation (FCP) rates. This was mainly attributed to the combined effect of the reduced large residual secondary phases and the conspicuous grain refinement. Reducing the large secondary phases remaining by the high temperature solution treatment was beneficial to limit the bridging of cracks, and refining the grains with Goss orientation could induce more crack deflection, consequently enhancing the fatigue resistance.

Crack Propagation and Predication on 2024 Aluminum Alloy under Combination Action of Friction and Fatigue

The crack propagation of 2024 aluminum alloy under the combination action of non-lubricated sliding friction and fatigue was investigated by numerical simulation analysis and experimental methods. The variation regulation of stress intensity factors (SIFs) KI and KII, and the fatigue crack propagation under different initial crack length, tension and contact pressure was obtained. Based on numerical analysis, experiment research and Paris formula, the predication model of fatigue crack propagation rate was established. The results show that the crack is closed owing to friction force when the upper specimen slides toward the crack, while is torn as sliding away. When the upper specimen moves in the near crack zone, there is relative motion between the crack faces caused by contact pressure. The crack appears slip-open effect. In according to numerical analysis and experimental results, the crack propagation is I-II composite crack forms. The comprehensive influence on I-II composite fatigue crack propagation from large to small is tension, contact pressure and initial crack length. The predication model is verified to reasonable and effective by the new test investigation.