Increase In Crack Growth Rate Research Articles

Experimental study and theoretical analysis of ECC type I fatigue fracture after subjected to sub-high temperature

Three-point bending (TPB) fatigue fracture tests were conducted on Engineered Cementitious Composites (ECC) prefabricated notched beam specimens at three target temperatures (T = 20℃, 100℃, 200℃) and three stress levels (S = 0.7,0.8,0.9). The fatigue life, maximum crack opening displacement (CMODmax), and stiffness attenuation were characterized. The evolution of cracks is recorded in real time. The fatigue crack growth curve (af-N) is obtained by fitting a logistic function, and the crack growth rate is determined based on its first derivative. It is found that compared to 20℃, there is a slight increase in the fatigue life of ECC, and the critical crack growth rate decreases after 100℃. After 200℃, the fatigue life of ECC decreases, the brittleness increases, and the crack growth rate increases. This is mainly related to the degradation of PVA fibers at 200℃, with most fibers being pulled off during fatigue loading, which leads to a decrease in crack propagation resistance.

Effects of Ion Irradiation on Stage II Fatigue Crack Growth and Plasticity

AbstractFatigue crack growth characteristics of ion irradiated compact tension (CT) specimens were evaluated in the Paris law region. Fatigue crack growth was monitored under tension‐tension loading and the thermoelastic response was measured. Two stages of crack propagation were identified. In the first 300,000 cycles, crack growth rates of irradiated and unirradiated specimens were comparable and plastic zone area was found to be independent of crack length. Beyond 300,000 cycles, irradiated specimens showed a greater crack growth rate; additionally, the plastic zone area increased with crack length. The increase in crack growth rate was attributed to irradiation hardening. The plastic zone area was found to be dependent on the crack path, especially in the initial stages of crack propagation. Local peaks in the value of the area of the plastic zone were found to be associated with greater crack tortuosity and secondary cracks. As a result, decreases in plastic zone area were associated with greater crack growth rates.

Correlating stress ratio effects on the fatigue crack growth rate behavior of a nickel-based superalloy GTM718

Damage tolerance evaluation under service loads requires a unified constant amplitude fatigue crack growth law for the material which takes care of stress ratio effects. Several types of crack driving force parameters have been proposed in the past to correlate stress ratio effects on fatigue crack growth rates. In the present investigation, a unified crack growth law is derived for a nickel-based superalloy by considering various crack driving force parameters to eliminate stress ratio effects. Initially, the constant amplitude fatigue crack growth rate (FCGR) behavior of GTM718 alloy was experimentally determined at various stress ratios, R ranging from R = 0.1 to 0.7. As expected, increasing the stress ratio, increased crack growth rates, and decreased threshold stress intensity factor range, ΔKth. The conventional crack growth rate, da/dN vs stress intensity factor range, ΔK data was then modified and re-plotted as a function of different crack driving force parameters viz., (a) Elber’s parameter, ΔKeff, (b) Walkers’ parameter, Kw (c) Kujawski’s parameter, K*, (d) Forman’s parameter, Kf, and (e) Two-parameter crack driving force, ΔK*. It was observed that Kf and ΔK* crack driving force parameters were correlating stress ratio effects better than other models in all three regimes of crack growth rates. Further, a unified crack growth law was derived based on the Kf and ΔK* crack driving force parameters which is useful in predicting FCG behavior under service loads.

The Effect of Crack-Initiating Feature on the Environment-Assisted Cracking Behavior of Sensitized AA5456-H116 in Marine Environments

The influence of crack-initiating feature on the environment-assisted cracking (EAC) behavior of sensitized AA5456-H116 exposed to marine environments is assessed via fracture mechanics-based testing. Specimens that contained either a traditional fatigue precrack or purposefully introduced intergranular corrosion fissures were immersed in 0.6 M NaCl and polarized to select electrochemical potentials while held at a constant force. The measured crack length vs. time relationships from these experiments reveal that the two specimen geometries yield similar crack growth rates at −900 mVSCE and after the onset of accelerated crack propagation at −800 mVSCE. However, precorroded specimens exhibit significantly shorter times to failure than the precracked specimens at −800 mVSCE due to increased crack growth rates at the start of the experiment. The mechanical, environmental, and material factors that could contribute to the initially increased EAC susceptibility of the precorroded specimens are identified using a generalized model for EAC. Analysis of these possible causal factors suggests that the increased susceptibility is due to a residual, initially more deleterious crack chemistry at the occluded corrosion fissure tip from the aggressive galvanostatic polarizations used to accelerate fissure growth. The implications of these results on the efficacy of traditional fracture mechanics-based methods for quantifying EAC susceptibility are discussed.

Dwell-fatigue crack growth behaviour of Alloy 709

Dwell-fatigue crack growth behaviour of an advanced austenitic stainless steel Alloy 709 has been investigated and compared with that of a conventional Type 316H stainless steel. The test procedure employed alternation of 1 h dwell-fatigue loading and 0.25 Hz fast cycling so that crack growth rates (da/dN) obtained from dwell-fatigue loading can be compared to those purely result from fatigue mechanism on the same test-piece. Tests were conducted at 550, 650 and 750 °C in air using 0.5 T compact tension test-pieces under a fixed maximum load of 8 kN and a stress ratio, R, of 0.1. For the investigated temperature and ΔK ranges, crack growth mechanisms of fatigue alone, creep alone and mixed fatigue-creep have all been observed. Detailed fractographic and metallographic observations were conducted to interpret failure mechanisms and regimes of different failure modes. Compared to crack growth rates obtained under 0.25 Hz fatigue loading, dwell-fatigue produces: no obvious increases in crack growth rates at a test temperature of 550 °C; a moderate increase (∼ 2–5 times) at a test temperature of 650 °C; and, over a tenfold increase at a test temperature of 750 °C. Compared to 316H, Alloy 709 has much improved resistance against creep-fatigue crack growth, here confirmed at the single test temperature of 650 °C.

Development of a methodology for mechanical testing of steel samples for predicting the durability of vehicle wheel rims

The reliability and durability of parts of aggregates and mechanisms of motor vehicles depend on the efficiency of the basic and responsible load-bearing structures. The responsible elements of the motor vehicle are the wheel rims, whose reliability will increase the energy efficiency of the motor vehicle as a whole. One of the directions for increasing the reliability of wheel rims is the use of optimized low-pearlite steels with the increased impact strength of the 10HFTBch type. When determining the mechanical characteristics of steels with increased impact strength, it is worth considering a mixed form of failure; therefore, the scheme of four-point asymmetric bending is optimal for laboratory research. Conditions of mixed failure for an oblique crack lead to a 25–45% decrease in the value of ΔКІm* relatively to the stress intensity factor ΔКІ* obtained for the pure I mode. The same can be said about the II mode, when ΔКІІ* determined during the growth of an oblique crack is 10% smaller than the value of ΔКІІ*, which is calculated for the pure mode of failure, This indicates an increase in the fatigue crack growth rate for mixed failure compared to pure modes.

A full-stage fatigue crack growth model for metallic materials

In this study, based on the understanding of blunting and re-sharpening to micro-void coalescence mixed mechanism, a new full-stage fatigue crack growth model is proposed by exploiting the energy distribution concept. As the crack growth rate increases, the transformation efficiency of three types of mechanisms exhibited different trends. The parameter m in the Paris’ law, derived from the new model, is in accordance with the experimental results. The new model is accurately verified by considering some engineering materials, including steels, titanium and aluminum alloys, which would be further applied to other innovative materials such as composites and high-entropy alloys.

Experimental studies of fracture and fatigue crack growth in dissimilar metal welded joints of nickel-based superalloys: Cracks oriented along the weld

Nickel-based superalloys, Inconel 718 and Nimonic 80A, were welded without any interlayer for Blisk manufacturing in aerospace industries. Fracture toughness and fatigue crack growth rate of the dissimilar metal welded joints were experimentally studied to confirm their structural integrity. Cracks were oriented in and along the weld. Base metals, Inconel 718, and Nimonic 80A differ in yield strength but have the same modulus of elasticity. Electron beam welding was used to butt weld Inconel 718 to Nimonic 80A. J resistance curve ( J vs. crack extension) was experimentally found to determine the elastic–plastic fracture toughness. The welded specimens showed lesser fracture toughness and higher crack growth rates than base metals. An increase in load ratio resulted in an increase in fatigue crack growth rate in the high Δ K region and a decrease in Δ Kth. Fractography studies confirmed linear elastic and plane strain fracture all along the fatigue crack growth rate path except for the final fracture with plane stress.

Thermo-Mechanical Fatigue Crack Growth and Phase Angle Effects in Ti6246.

A bespoke TMF crack growth test set-up has been developed and validated for use throughout this study and the effects of phasing between mechanical loading and temperature have been investigated. The study shows that TMF cycles may show increased crack growth rate behaviour when compared to isothermal fatigue. The phase angle of the applied TMF cycle can also affect crack growth behaviour, with in-phase (IP) test conditions showing faster crack growth rates than out-of-phase (OP) test conditions. Propagating cracks interact with the microstructure of the material, in particular, the α/β interfaces within the prior beta grains and supporting fractography evidences subtle differences in fracture mechanisms as a result of phase angle.

IMPLEMENTATION OF EXTENDED FINITE ELEMENT METHOD IN CRACK PROPAGATION OF CONCRETE

The Extended Finite Element Method is a numerical solution based on the Finite Element Method (FEM) XFEM has really become a very important generalization of classical finite element techniques, by establishing a mesh independent generalization of classical finite elements to reduce the mesh-dependent shortcomings of the solution. The application of XFEM in crack simulation should improve the modeling of the crack tip environment and also apply to generalized advanced global failure criteria, which is specifically designed to deal with problems in the engineering field Such as the fracture behaviour model. The purpose of this paper is to identify the application of the Extended Finite Element Method to a technical problem, namely fracture behaviour model. The media used is pure boneless concrete modelled with COMSOL Multiphysics 5.6 software by combining stress ratio, lateral strain due to axial loading, concrete density, and crack growth rate. The crack growth process provides initial prolonged growth along with the increase in crack size. In the end, the growth is faster. The reason for this accelerated growth is the stress intensity factor at the crack tip. As the crack grows, the stress intensity factor increases, leading to faster growth. The crack grows until it reaches a critical value, and fracture occurs. The test results obtained the cause of failure: the critical stress intensity is exceeded. See a comparison of crack size and stress cycle as the crack size increases. This accelerated growth is because the growth rate depends on the stress intensity factor at the crack tip, and the stress intensity factor depends on the crack size. As the crack grows, the stress intensity factor increases, leading to faster growth. The crack grows to a critical size, and failure occurs. The results show a relatively strong relationship between increasing crack size and increasing crack growth rate.

A study of the effect of hydrogen on the fatigue behaviour of metals

Hydrogen Embrittlement is a phenomenon in which the presence of hydrogen can lead to various detrimental effects on the mechanical properties of the material. These effects include reduction in ductility, delayed fracture under constant loading, increase in fatigue crack initiation and growth rates, and subcritical cracking even below the threshold fracture toughness of the material. Fatigue is one of the most common modes of material nature during the operation of components during their service life. Hence it is very important to study the effect of hydrogen on the fatigue behaviour of steel and find methods for their prevention. This paper deals with the study of hydrogen on various types of fatigue in materials, various factors that affect the susceptibility to HE in fatigue, the effect on the microscopic morphology of the fatigue crack generated and the methods suggested for the prevention of HE in fatigue.

Influence of operating temperature on the measured and predicted lifetime of carbon black-filled hydrogenated nitrile butadiene rubber

This work analyses the influence of the operating temperature on the lifetime of carbon black filled elastomers. The crack growth properties and the lifetime of elastomers are temperature dependent. The increase of temperature at the same nominal stress leads to an increase of crack growth rate and consequently to a decrease of the lifetime. The lifetime prediction is performed using the Monte Carlo principle. Carbon black agglomerate are randomly distributed in the investigated test volume. By combining the crack growth properties, agglomerate distribution and the local deformation in the component, the lifetime prediction can be performed. The prediction curves show good agreement with the measured lifetime at different operating temperatures and different loading levels.

A study of the morphological effect of an α-Al2O3 layer on the creep life for nickel-based superalloys using microstructure-based geometrical models

A study on the role of the irregular geometry of the α-Al2O3 film during the creep rupture test was conducted on samples of DD6 Ni-based single-crystal superalloy at 980 °C/250 MPa. This study involved experimental and finite element analyses of creep-oxidation specimens. The experimental results showed that the creep rupture life of the 0.67 mm-thin samples was shorter than that of the 0.81 and 1.12 mm specimens. Microscopic examination of the oxidation-affected zones showed that the morphology of the inner α-Al2O3 layer of a 0.67 mm-thin sample was discontinuous and incomplete, and the sample (1.12 mm thick) included a continuous and complete α-Al2O3 film. Hence, the influence of different morphologies of the α-Al2O3 layer on the stress distribution was studied numerically using the ABAQUS software. The finite element modeling results showed that the maximum stress was concentrated in the blocks of a discontinuous α-Al2O3 layer, which increased the diffusion of ions and metal cations. As a result, excessive growth of the oxidation-affected zone of the thin samples and the formation of AlN particles in high density occurred. It also contributed to an increase in the surface crack growth rate in the thin wall sample, which reaches the critical value of crack length in a shorter time than thick ones. Microstructure-based modeling has provided further insight into the role of the α-Al2O3 layer morphology in the deterioration of the mechanical properties of thin creep specimens.

Crack growth in rotor P2M steel at elevated temperature

The paper presents a comprehensive computational and experimental study of the crack growth rate during the creep-fatigue interaction in compact specimens of P2M steel at a temperature of 550 °C. The theoretical part of the study consisted in the formulation of the fracture resistance parameters through the classical and new constitutive equations of the cracked body state taking into account the accumulation of damage. Numerical calculations included the determination of the stress-strain state fields for the conditions of elasticity, plasticity, and creep, as well as the distributions of nonlinear stress intensity factors and the C*-integral along the crack length and along the crack front for each tested compact specimen. The interpretation of the experimental results for specimens of the same shape in plan but different thicknesses is given in terms of elastic and nonlinear stress intensity factors, taking into account the accumulation of creep damage. It was found that the crack growth rate under the fatigue and creep interaction increases monotonically as the crack size increases in comparison with harmonic fatigue by an order of magnitude or more on specimens of the same geometry. Taking into account the damage through the creep stress intensity factors causes differences in the cyclic fracture diagrams. The superposition of fatigue and creep contributions in terms of load holding time shows an order of magnitude increase in the total crack growth rate compared to the interpretation of the experimental data in terms of pure creep.

Effect of heat treatment on fatigue crack growth behavior of 316NG austenitic stainless steel in deaerated water at 325 °C

316NG (00Cr17Ni12Mo2N) austenitic stainless steel was heat-treated at 600, 725 and 1000 °C to investigate the effects of heat treatment temperature and holding time on the fatigue crack growth behavior of this steel in deaerated water at 325 °C. It is found that fatigue crack growth rates in air at room temperature are decreased after the heat treatments, due to a reduced dislocation density. Corrosion fatigue crack growth rates in water at 325 °C are also decreased after the heat treatments at 600 and 1000 °C for 1 h, while the heat treatment at 725 °C leads to a moderate increase in the fatigue crack growth rate. The latter phenomenon results from a higher amount of CrNbN (Z-phase) precipitates formed in the steel after the annealing at 725 °C.

Size effect in creep–fatigue crack growth interaction in P2M steel

AbstractAn investigation of the crack propagation under the creep–fatigue interaction in C(T) specimens of P2M steel was carried out. Numerical calculations included the determination of the stress–strain state fields for the viscoelastic material, as well as the distributions of stress triaxiality, nonlinear stress intensity factors, and the C‐integral along the crack front in specimen with different thickness. The interpretation of the experimental results under fatigue–creep interaction is given by the elastic and nonlinear stress intensity factors in terms of the traditional and new normalized cyclic fracture diagrams. Considering the size effects through the creep‐dominated stress intensity factors caused differences in the crack growth behavior. In specimens with the same geometry, the crack growth rate under the creep–fatigue interaction increased monotonically compared with harmonic fatigue, and the superposition of fatigue and creep contributions shows an order of magnitude increase in the total crack growth rate with respect to the pure creep.

The Effect of Loading Rate on the Environment-Assisted Cracking Behavior of AA7075-T651 in Aqueous NaCl Solution

The influence of loading rate on the environment-assisted cracking (EAC) behavior of AA7075-T651 immersed in 0.6 and 1.0 M NaCl solution was assessed at applied potentials ranging from −800 to −1200 mVSCE via a slow-rising stress intensity (K) testing methodology. Measured crack growth rates under rising K loading are compared to those obtained using a fixed K protocol, which revealed that rising K-based testing consistently yields increased crack growth rates relative to static K approaches across all tested conditions. However, relative to other alloy systems, EAC in AA7075-T651 is only modestly loading rate-dependent, as demonstrated by testing conducted at fixed dK/dt ranging from 0.25 to 2.0 MPa√m/h. The implications of the observed results are considered in the context of current EAC testing specifications, with specific focus on the conservatism and efficiency of rising K-based approaches.

Compactive Deformation of Sandstone Under Crustal Pressure and Temperature Conditions

AbstractThe transition from macroscopically brittle to macroscopically ductile deformation in porous sandstones is known to be pressure dependent, with compactive, ductile behavior occurring only once significant effective pressures have been reached. Within the crust, such effective pressures are associated with burial depths in the range 0.5–6 km, where the temperature is likely 35°C–200°C. To test the importance of such elevated temperature on the strength and deformability of sandstone, a series of constant strain rate, triaxial deformation experiments were performed on three different water saturated sandstones at either ambient temperature or 150°C. For each sandstone, an effective pressure range was used which spanned both the brittle and ductile deformation regimes, up to a maximum of 120 MPa. In the brittle regime, we observed a temperature‐dependent lowering of the yield stress of between 8% and 17%. Within the ductile regime, we observed an even greater reduction in the yield stress of between 9% and 37%. A further notable observation is that the transition from dilatant, brittle behavior to compactive, ductile behavior tends to occur at a lower effective pressure at elevated temperature. The weakening observed at elevated temperature can be explained by a reduction in fracture toughness, which is shown mathematically to cause greater weakening in the ductile regime than in the brittle regime. The apparent reduction in fracture toughness at elevated temperature is potentially driven by a combination of a reduction in surface energy and, to a minor extent, an increase in subcritical crack growth rate.

Corrosion-Fatigue Crack-Growth Resistance of Steel of the Frame of a Sprayer Boom

We construct diagrams of fatigue crack growth for St3ps steel placed in demineralized water and in saturated solutions of the Nurelle D insecticide and UN-32 complex liquid fertilizer and evaluate the corresponding values of threshold and critical stress intensity factors. It is established that, in a Nurelle D solutions, the fatigue crack-growth rate increases, whereas in UN-32 solutions, it decreases as compared with its value in air. For the demineralized water, there exists a characteristic value below which the crack-growth resistance of steel increases and above which it decreases. The obtained results may serve as a basis for the numerical analyses aimed at prediction of the serviceability and residual service life of the frames of sprayer booms.

On the Localization of Plastic Strain in Microtextured Regions of Ti-6Al-4V

The equiaxed microstructure of Ti-6Al-4V contains nominally 10µm to 15µm α-phase grains. Depending on processing, these fine grains may aggregate into large, millimeter-scale regions of similar crystallographic orientation. These so-called microtextured regions are detrimental to quasi-static and fatigue properties. Their presence may facilitate the formation of long-range strain localization—bands of plastic strain that traverse grain boundaries and terminate at the boundaries of the microtextured region which compromises strength and ductility—and can cause early fatigue crack nucleation. Furthermore, the low angle boundaries within microtextured regions offer little resistance to crack growth, and corresponding increases in crack growth rates have been observed in these regions. Despite significant research into the effects of microtextured regions on macroscopic properties, there is still a lack of rigorous definition of what constitutes a microtextured region. To this end, we present a computational study in which deformation simulations are performed to study the effect of the intensity and character of microtexture on the localization of plastic strain. Mean orientation and intensity of microtexture are parameterized in a suite of simulations. Simulations indicate that strain localization is facilitated both by the presence of some amount of orientation spread within a microtextured region, and further by the intensity of microtexture in neighboring regions. Simulations are further discussed with respect to high resolution experimental measurements of intragrain strain in a microtextured Ti-6Al-4V specimen, which exhibit similar mechanical trends.