Acceleration Of Crack Growth Rate Research Articles

Phase field modeling of underloads induced fatigue crack acceleration

This study proposes a novel methodology to model the fatigue crack growth acceleration under underloads using a phase field fracture framework. In this model, fatigue crack growth is characterized by the degradation of fracture toughness, with an emphasis on employing a representative loading strategy instead of explicit cyclic loading, thus accelerating simulations of high-cycle fatigue. The model integrates a zone-based crack acceleration approach that responds to single-cycle underload. Notably, the model adeptly captures the dynamics described by the Paris-Erdogan law. Post-underload crack growth acceleration is simulated by identifying an acceleration zone near the crack tip, inspired by existing models based on the plastic zone. This zone is defined by a strain energy density threshold, and the underload ratio governs the rate of fatigue damage accumulation attenuation within this area. The implementation leverages the UMAT user subroutine in Abaqus, utilizing coupled temperature-displacement elements where temperature analogously represents the phase field parameter. Experimental validation of the model confirms its ability to accurately reflect the loss of fatigue life and the acceleration of crack growth rates in compact tension and middle tension specimens. Additionally, the model shows promise for extension to periodic underloads, highlighting its potential for simulating real-world fatigue scenarios effectively.

Corrosion fatigue performance of modified 17-4PH in simulated geothermal environment

Corrosion fatigue behavior of a new candidate material for geothermal turbine blade, modified 17-4PH, was investigated in detail. The environmental acceleration of crack growth rate was discussed in terms of ∆K. Fracture surface and side surface were observed by SEM. The crack path on side surface was also analyzed by EBSD. Then, the corrosion mechanism was discussed. Striation width on the fracture surface was compared to the crack growth per cycle. Since pitting occurred during the corrosion fatigue test, anodic polarization test was exercised to measure the pitting potential, and the cause for the pitting was also discussed. The hardness was also characterized by micro-hardness test and auger electron mapping.

Effect of underloads in small fatigue crack growth

There is considerable cost incurred within the aircraft industry due to the inability to accurately predict fatigue crack growth rates for complex loading scenarios. This is particularly the case for small cracks. This paper aims to facilitate the improvement of fatigue crack growth by investigating the effects of underloads on small cracks grown in the common airframe structural aluminium alloy; AA7050-T7451. Six novel loading spectra were used to quantify how underload spacing, magnitude, and consecutive application impact crack growth. Quantitative fractography of post-test fractures enabled crack growth measurements to be made at crack lengths as small as 100 µm. It was found that underloads accelerate growth rates most significantly for small cracks, though this effect appears to progressively diminish as the crack grows. The results also suggest that underloads may have both short and long-term effects on crack growth. This research paves the way for accounting for the acceleration of crack growth rates caused for small cracks in predictions, which if not considered, could lead to non-conservative fatigue life predictions. Ultimately, this should allow for more accurate growth rate prediction of high strength aluminium structures subject to underloads, as well as provide quantitative basis for future mechanistic studies.

Multiscale study for the temperature effect on the mechanical properties and fatigue crack growth rate of polyamide 66

Sequential multiscale analysis was performed to investigate the effect of temperature on the mechanical properties and fatigue behaviors of a polymeric material. With increasing temperature, a nonlinear degradation of the elastic and plastic properties were clearly observed, which arose from the phase transition of polymer molecules. The influence of the temperature on the acceleration of crack growth rate was also evaluated via the plastically dissipated energy-based finite element analysis. Continuous curves of the Paris-regime crack growth rate were constructed by representing the Paris coefficients as a function of temperature. The predicted crack growth rates in the present study match well with the published experimental data on polyamide 66. The proposed framework provides an accurate estimation of the mechanical properties and fatigue crack growth rate of polymers, with a reduced reliance on experiments.

Environmentally assisted crack growth in adhesively bonded composite joints

Adhesively bonding composite components is a reliable alternative to conventional joining processes that minimizes part weight and reduces fabrication costs. Regarding performance and reliability, of particular interest is developing adherend surface treatments that enhance adhesion of the joint interfaces in aggressive chemical environments. Using fracture mechanics-based adhesion metrologies, critical and subcritical crack growth were evaluated for several peel-ply-treated, adhesively bonded composite joints. Fracture toughness, Gc, and corresponding failure modes were evaluated for specimens constructed using two different bonding processes (co-bonding and secondary bonding) and four different peel ply treatments. Environmentally assisted crack growth was evaluated as a function of time in several environments: humid, high temperature humid, and hydraulic fluid immersion. It is shown that humid environments accelerate crack growth rates, da/dt, relative to the strain energy release rate, G. This effect was amplified at elevated temperatures and further amplified in the presence of hydraulic fluid.

Thermo-Mechanical Fatigue Crack Growth of RR1000

Non-isothermal conditions during flight cycles have long led to the requirement for thermo-mechanical fatigue (TMF) evaluation of aerospace materials. However, the increased temperatures within the gas turbine engine have meant that the requirements for TMF testing now extend to disc alloys along with blade materials. As such, fatigue crack growth rates are required to be evaluated under non-isothermal conditions along with the development of a detailed understanding of related failure mechanisms. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180° OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.

Fatigue crack growth behavior of aerospace and ship-grade aluminum repaired with composite patches in a corrosive environment

Investigation of the performance for a composite repair patch to prolong the service life of pre-cracked 7050, 7075, 5083 and 6061 aluminum plates under fatigue conditions in a corrosive environment was conducted. Both insulated graphite–epoxy and boron–epoxy composite patches were evaluated for the effects of corrosion fatigue. The repair patches consisted of unidirectional plies (laminae) oriented in the loading direction. The improvement of service life and the effect that a corrosive environment had on crack propagation rates for the repaired aluminum plates were examined. The bond durability between the aluminum plates and the boron patch were also assessed. As expected, the introduction of salt water during testing greatly increased crack growth rates and can be quantified for comparison. The graphite patch consistently showed positive results compared to the boron patch for 6061 aluminum samples. The same system on 5083 aluminum had mixed results. Examination of potential 5083 sensitization due to elevated cure cycles is discussed. The boron–epoxy repairs showed a positive life improvement in both lab air and salt water exposed environments for 7050 and 7085 aluminums while the graphite–epoxy repair accelerated crack growth rates in the salt water environment.

OS0610-476 等2軸繰返し荷重下におけるステンレス鋼のき裂進展加速要因の検討

Studies of crack growth under equi-biaxial fatigue which is caused by thermal stress have been conducted and show that crack growth rate under equi-biaxial stress is faster than under uniaxial one. This study focuses on the difference of crack growth rate between equi-biaxial and uniaxial and compared them quantitatively based on fatigue life and crack growth rate. Then the mechanism of occurrence of different crack growth rate between them is examined. The acceleration of crack growth rate under equi-biaxial stress is estimated as the main reason to decrease fatigue life.

Localised Corrosion Damage Based Fatigue Crack Extension Model of Aluminium Alloy with Prior Corrosion Damage

A modified model was presented for describing corrosion fatigue crack growth rate of aluminum alloy accurately. Considering the local damage around the crack tip is more reasonable for accelerating crack growth rate under corrosive environment, a damaged influence factors which characterize the damage degree of local damage near the crack tip has been introduced to revise the corrosion fatigue crack growth rate, and the experimental results are in good agreement with predictions of amendment fatigue crack growth rate model. The proposed model based on damaged influence factors is reasonable, and proposed a theoretical reference for aluminum alloy damage tolerance analysis.

The role of local strains from prior cold work on stress corrosion cracking of α-brass in Mattsson's solution

The dynamic strain rate ahead of a crack tip formed during stress corrosion cracking (SCC) under a static load is assumed to arise from the crack propagation. The strain surrounding the crack tip would be redistributed as the crack grows, thereby having the effect of dynamic strain. Recently, several studies have shown cold work to cause accelerated crack growth rates during SCC, and the slip-dissolution mechanism has been widely applied to account for this via a supposedly increased crack-tip strain rate in cold worked material. While these interpretations consider cold work as a homogeneous effect, dislocations are generated inhomogeneously within the microstructure during cold work. The presence of grain boundaries results in dislocation pile-ups that cause local strain concentrations. The local strains generated from cold working α-brass by tensile elongation were characterized using electron backscatter diffraction (EBSD). The role of these local strains in SCC was studied by measuring the strain distributions from the same regions of the sample before cold work, after cold work, and after SCC. Though, the cracks did not always initiate or propagate along boundaries with pre-existing local strains from the applied cold work, the local strains surrounding the cracked boundaries had contributions from both the crack propagation and the prior cold work.

Crack Growth of a Polycrystalline Nickel Alloy under TMF Loading

Thermo-mechanical fatigue (TMF) is an important factor for consideration when designing aero engine components due to recent gas turbine development, thus understanding failure mechanisms through crack growth testing is imperative. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180°OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.

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.

S–N fatigue properties of a stable high-aluminum austenitic stainless steel for hydrogen applications

The fatigue properties of a novel high aluminum austenitic stainless steel with a high resistance against hydrogen embrittlement were investigated. S–N tests in 40 MPa H2 at −50 °C resulted in a reduction in fatigue life by a factor of about 2 compared to air. Striation analysis revealed no acceleration of crack growth rate but accelerated crack initiation or accelerated short crack growth in H2. No apparent difference in fatigue fracture characteristics and striation morphology between the air and H2 tested specimens could be identified.

Estimation of Corrosion Fatigue-Crack Growth through Frequency Shedding Method

Abstract Corrosion fatigue-crack growth characteristics are important for the design of marine and off-shore structures. Design of critical components requires data on fatigue-crack growth rate at very low frequencies of the order of 10−2 to 10−3 Hz. Experiments at low frequencies pose practical difficulties of enormous test duration. To address this, it is proposed to estimate corrosion crack growth characteristics using a frequency shedding method where the frequency is shed with crack advance using an exponential law. Fatigue-crack growth rate tests have been conducted on Ni–Mn–Cr steel at a constant ΔK range of 18 MPa√m (lower Paris regime) under lab air conditions as well as 3.5 % NaCl solution. Crack growth rate data plotted as a function of test frequency presents a straight-line trend in log–log scale for a frequency range of 1–0.1 Hz; however, there is a change in trend when the frequencies are dropped further, which could be due to domination of corrosion mechanism. To understand the role of crack closure, crack closure estimates were obtained at periodic intervals of crack length and the effective stress intensity graphs suggest acceleration in crack growth rate due to corrosion as the frequency is reduced.

Fatigue crack growth due to overloads in plain concrete using scaling laws

Scaling laws are represented in power law form and can be utilized to extract the characteristic properties of a new phenomenon with the help of self-similar solutions. In this work, an attempt has been made to propose a scaling law analytically, for plain concrete when subjected to variable amplitude loading. Due to the application of overload on concrete structures, acceleration in the crack growth process takes place. A closed form expression has been developed to capture the acceleration in crack growth rate in conjunction with the principles of dimensional analysis and self-similarity. The proposed model accounts for parameters such as, the tensile strength, fracture toughness, overload effect and the structural size. Knowing the governed and the governing parameters of the physical problem and by using the concepts of self-similarity, a relationship is obtained between the different parameters involved. The predicted results are compared with experimental crack growth data for variable amplitude loading and are found to capture the overload effect with sufficient accuracy. Through a sensitivity analysis, fracture toughness is found to be the most dominant parameter in accelerating the crack length due to application of overload.

Fatigue crack analysis of EBR-II Ni-bonded duplex tubing

Small, notched three-point bend specimens machined from duplex tubes, which were extracted from an EBR-II superheater, were fatigued through the nickel interlayer to determine propensity for crack arrest within this interlayer. Several of these specimens were fatigued in the near threshold, and steady state regimes of Paris Law behavior. Additionally, two specimens were fatigued to the edge of the nickel interlayer and then monotonically loaded. Micro-hardness profiles of the nickel interlayer were also measured. Fatigue behavior was found to be similar to previous studies in that arrest was only noted in the near threshold Paris regime (attributed to the presence of voids) and in the steady state regime exhibited an acceleration of crack growth rate through the nickel interlayer followed by a slight retardation. Monotonic loading resulted in crack branching or delamination along the interlayer. Although archival material was not available for this study, the hardness of the nickel interlayer was determined to have been lowered slightly during service by comparison to the expected hardness of a similar nickel braze prepared as specified for fabrication of these tubes.

Fatigue Crack Growth and Closure of Stainless Casting SCS6 and Carbon Steel Casting SC49 for Hydraulic Turbine Runner

In hydraulic turbine runner, unexpected accidents due to fatigue often occur. In this study, in order to clarify fatigue crack growth and closure properties on SCS6 and SC49 used as hydraulic turbine runner over 27 years, fatigue crack growth test under the stress ratio R-constant were carried out using compact tension specimens of both materials. Fatigue crack growth rate and crack closure were measured by means of an unloading elastic compliance method. The results are summarized as follows:(1) Although the stress ratio dependency on the fatigue crack growth rate is found in both materials, the influence is recognized remarkably in SCS6 except for threshold value. (2) Although the threshold stress intensity factor ΔKth of SCS6 is almost same as that of SC49 when R<0.5, the value of stress ratio of SCS6, when the R-dependency of growth rate disappears, was found to be higher than that of SC49. (3) Fatigue crack growth rates of SCS6 and SC49 could be correlated with the effective stress intensity factor range ΔKeff, and da/dN-ΔKeff relationships of both materials agree well in the low and mid growth rate region except for threshold value. In high ΔKeff, however, the rapid acceleration of crack growth rate due to the change of growth mechanism to cleavage fracture was found only in SC49.

OS2201 水素は如何に材料の疲労強度に影響するか?(OS22-1 基調講演,OS-22 水素環境の材料強度特性への影響)

This paper overviews the recent progress of the experimental study on hydrogen embrittlement (HE) carried out at HYDROGENIUS and the brief perspective toward Hydrogen Economy. The effects of various ranges of hydrogen content and cyclic frequency on fatigue crack growth of various steels were investigated. Crack opening was smaller in presence of hydrogen. However, the fatigue crack growth rate was accelerated compared to the case without hydrogen. The effect of hydrogen on fatigue crack growth rate was strongly dependent on load frequency both in BCC and FCC metals. The acceleration of crack growth rate due to hydrogen in a high strength steel was caused by nucleation of secondary cracks along grain boundaries and carbide boundaries ahead of main crack. These secondary cracks are produced by hydrogen-enhanced deformation twins impinging on grain boundary.

Mechanisms and modelling of fatigue crack growth under combined low and high cycle fatigue loading

In this paper a mechanism-based model is presented, which is able to describe the evolution of microcracks under pure low cycle fatigue (LCF) and combined LCF and high cycle fatigue (HCF) loading conditions. In order to verify the model and to calibrate the model parameters, the crack length evolution of microcracks is followed at room temperature for pure LCF and combined LCF/HCF loading in a 10%-chromium steel. These studies reveal accelerated crack growth rates under LCF/HCF interaction as soon as a critical crack length is reached. The model is capable of accounting for this effect and needs only few parameters, including the threshold for fatigue crack growth, whose knowledge is crucial for the accuracy of the model.

Sliding Contact Fatigue Damage in Layered Ceramic Structures

Porcelain-veneered restorations often chip and fracture from repeated occlusal loading, making fatigue studies relevant. Most fatigue studies are limited to uni-axial loading without sliding motion. We hypothesized that bi-axial loading (contact-load-slide-liftoff, simulating a masticatory cycle), as compared with uni-axial loading, accelerates the fatigue of layered ceramics. Monolithic glass plates were epoxy-joined to polycarbonate substrates as a transparent model for an all-ceramic crown on dentin. Uni-and bi-axial cyclic contact was applied through a hard sphere in water, by means of a mouth-motion simulator apparatus. The uni-axial (contact-load-hold-liftoff) and traditional R-ratio fatigue (indenter never leaves the specimen surface) produced similar lifespans, while bi-axial fatigue was more severe. The accelerated crack growth rate in bi-axial fatigue is attributed to enhanced tensile stresses at the trailing edges of a moving indenter. Fracture mechanics descriptions for damage evolution in brittle materials loaded repeatedly with a sliding sphere are provided. Clinical relevance is addressed.