Increased Crack Growth Research Articles

The effect of bond-line thickness on fatigue crack growth rate in adhesively bonded joints

The effect of adhesive thickness on fatigue crack growth in an epoxy film adhesive (FM94) was investigated, using a combination of experiments and numerical modelling. For the range of thicknesses investigated an increased thickness led to an increased crack growth rate. It was found that the energy required per unit of crack growth did not depend on the adhesive thickness. In contrast, the energy available for crack growth does depend on the adhesive thickness.The numerical analysis confirms that the energy required per unit crack growth is not sensitive to the adhesive thickness, but that the plastic energy dissipation increases with the thickness. The experimental results imply that this increase of plasticity has an anti-shielding effect, as the crack growth rate is increased.

Strain-induced crystallisation in natural rubber: a thermodynamically consistent model of the material behaviour using a multiphase approach

Strain-induced crystallisation (SIC) is the phenomenon of elastomers to experience a pronounced nonlinearity under large deformations of several hundred percentages of strain, which is advantageous for industrial applications due to the resulting positive properties such as increasing crack growth resistance and fatigue behaviour. The overall objective of the current study is the constitutive modelling of the material behaviour of natural rubber when stretched uniaxially. Initially, unfilled natural rubber is considered. Cyclic traction experiments in which crystallinity, elongation and stresses are simultaneously measured are found in the literature. The same vulcanisate is used to conduct additional experiments in the present laboratory. The experimental investigations focus on uniaxial tension tests, conducted with a variety of parameters such as time and stretch rate. The current work presents an approach to model the SIC phenomenon. The model considers thermoelasticity and crystallisation. The approach of the hybrid free energy is used to derive constitutive equations and to meet thermomechanical consistency. However, the effect of temperature is not the focus of the current work. Furthermore, the contribution of a mixing energy is represented in the free energy. The derivation of this mixing entropy is explained in detail by making use of different assumptions and approaches, e.g. the consideration of ideal mixtures. Here, a one-dimensional constitutive model for strain-crystallising rubber is developed, which can be extended with a micro-sphere approach (concept of representative directions) to a full thermodynamically consistent anisotropic model.

The influence of plasticity-induced crack closure on creep-fatigue crack growth in two heat-resistant steels

Finite element simulations are performed to predict plasticity-induced crack closure and creep-fatigue crack growth rates in two heat-resistant steels, the austenitic 20Cr-25Ni (Alloy 709) and martensitic 9Cr-1Mo (modified 1Cr-9Mo) steels. In a creep-fatigue cycle, total crack growth rate is usually computed by the addition of the fatigue crack growth rate during cyclic loading and creep crack growth rate during hold time. Two-dimensional finite element analyses of compact tension specimens are performed to simulate crack growth under cyclic and time-dependent loading conditions, with the consideration of elastic-plastic and creep deformations of the material at the crack tip. The simulations quantify the effect of hold time on crack opening load induced by the combined action of plasticity-induced crack closure and creep-induced stress relaxation at the crack tip. It is shown that increasing hold time during a creep-fatigue cycle results in a decrease of crack-tip opening load, thus increasing crack growth rate during the next cyclic loading. Experimental results of creep-fatigue crack growth rates in Alloy 709 are presented. The finite element simulations predicted crack growth rates under creep-fatigue loading in agreement with experimental values for both Alloy 709 and modified 9Cr-1Mo steels.

Fracture behavior of carbon fiber reinforced polymer composites: An optical study of loading rate effects

Crack initiation and growth in single-edge notched unidirectional T800s/3900-2 CFRP laminates are studied under stress wave and quasi-static loading conditions. An optical technique called reflection-mode Digital Gradient Sensing is also extended to study fracture mechanics of CFRP by using it in conjunction with ultrahigh-speed photography to perform full-field measurement of crack-tip deformations in the pre- and post-crack initiation regimes. DGS is capable of measuring two orthogonal surface slopes in the crack-tip vicinity as angular deflection of light rays. A method for extracting crack-tip parameters – the instantaneous crack speed and stress intensity factor (SIF) histories – associated with the stationary and propagating cracks using measured surface slopes is presented. The effect of fiber orientation in the range 0°–60° relative to the initial notch and two loading rates are investigated. Nominally mode-I fracture occurs when the fiber orientation is 0° whereas mixed-mode fractures ensue in others. Besides crack initiation occurrence at higher loads as fiber orientation increases, the SIF histories imply strong fiber bridging at low fiber orientations under quasi-static conditions. Furthermore, this CFRP shows significant loading rate dependence during crack growth. Unlike stress wave loading conditions, an increasing crack growth resistance immediately after crack initiation is seen under quasi-static conditions.

Influence of Tertiary Gamma Prime (\u03b3\u2032) Size Evolution on Dwell Fatigue Crack Growth Behavior in CG RR1000

This work investigates coarsening behavior of strengthening precipitates (γ′) in coarse-grained RR1000 after isothermal exposure for various times (50 to 500 hours) at 700 °C and 750 °C. The impact of these isothermal treatments is then studied by carrying out dwell (1 hour) fatigue crack growth tests at 700 °C in air. Such dwell periods can increase crack growth rates by nearly two orders of magnitude compared with baseline (0.25 Hz trapezoidal waveform) fatigue crack growth tests. Predominantly, scanning electron microscopy was used for γ′ analysis. Transmission electron microscopy was also utilized when necessary. Overaging as a thermodynamically driven and diffusion-controlled process strongly affects tertiary γ′ precipitates for the temperatures and treatment times investigated here. No influence of overaging on baseline fatigue behavior is observed. However, after overaging at 750 °C for 500 hours (which increases the mean tertiary γ′ size by a factor of two from 17 to 37 nm), dwell crack growth rates at 700 °C are reduced by one order of magnitude. Increased dwell fatigue crack growth resistance is also measured after overaging for 100 hours at 700 °C. The potential influence of γ′ distributions on dwell fatigue crack growth resistance is discussed in terms of stress relaxation behavior during dwell periods.

Experimental and Numerical Investigations on Crack Propagation in Titanium Alloys

The titanium alloys are the objects of wide experimental analysis in the terms of crack growth characteristics and mechanism of fracture due to their applications. The Ti-6.4%Al-2.6%Mo-1.7%Cr-0.5%Fe-0.5%Si (wt %) alloy was used in the investigation. The tensile tests were conducted on plate specimens. The test variables considered are width of tensile specimen, crack size and tensile loading. The mechanism of fracture that appeared in the specimens was established from the TEM micrographs of the replicas taken from different zones of the fracture surfaces. The mechanism of fracture was also studied using finite element analysis. The results obtained from the FEA were verified with experimental results. It can be concluded that there is a general trend of increasing stress intensity factor with increasing applied tensile load, crack size and width of the flat specimen. The strain energy release increases with increasing applied tensile load. Crack extension can occur when crack-driving force is equal to the energy required for crack growth. For a particular stress the energy release rate is proportional to the crack size. As the tensile load and the size of the initial crack increase, there is an increased crack growth in the Ti alloy. In the crack initiation zone evidence of quasi-cleavage fracture with limited plastic striations symptoms was found in the specimen tested under  =1000MPa. Micro fracture analysis of the specimens (tested under =1100MPa) has revealed quasi-cleavage fracture with small symptoms of plastic shearing in the early part of cracking.

Fatigue behaviour of AA6061-T6 alloys in the corrosive environment

The combined effects of corrosion and fatigue are known to be hazardous threats to structural integrity of aluminium alloys that are being extensively used in marine applications. This work investigated the fatigue crack initiation and growth behaviour of AA6061-T6 alloys in 3.5 wt% NaCl simulated seawater using scanning electron microscope and electron backscatter diffraction characterisation techniques. It was found that the fatigue resistance of AA6061-T6 is drastically downgraded when subjected to the corrosive environment of 3.5 wt% NaCl solution. High stress concentration at both sides of a pit mouth in conjunction with attacked grain boundaries facilitates fatigue crack nucleation, while the presence of hydrogen formed by corrosion reactions causes crack tip embrittlement and thus increases crack growth rate. Fractographic analysis reveals that there is a change in fatigue crack growth mechanism of AA6061-T6 alloys tested in the NaCl solution. At short crack length, the crack develops transgranularly along crystallographic planes due to hydrogen-enhanced decohesion process. Further crack growth is dominated by adsorption induced dislocation emission process, resulting in the mixed mode of intergranular and transgranular crack growth.

Studying the Effect of PWHT on Microstructural Evolution and Mechanical Properties of Welded A517 Quenched and Tempered Steel

Sometimes post-weld heat treatment (PWHT) is required for stress relieving both the weld metal and heat-affected zone in A517 Quenched and Tempered (QT) steels. This process unavoidably results in further tempering which adversely affects the mechanical properties of the welded joints. Investigations on A517 QT steel are presented in as-welded condition and after PWHT at 560 and 630 °C. Microstructural variation upon applying PWHT and its influence on impact absorbed energy, fatigue behavior, tensile and hardness properties of the material in as-welded condition and after PWHT are studied. The decrease in hardness is very remarkable after PWHT at 630 °C. Increasing heat treatment temperature from 560 to 630 °C results in reduction of YS and UTS. Void coalescence and coarsening of carbides after PWHT makes the fracture easier and results in reduction of absorbed impact energy. Elimination of acicular ferrite after PWHT at 630 °C increases crack growth rate, and fatigue endurance limit is reduced.

Relation between crack growth behaviour and crack front morphology under hold-time conditions in DA Inconel 718

The crack growth behaviour of Direct Aged Inconel 718 was studied at 550°C. Experiments were carried out under pure fatigue cycles, hold-time cycles of different durations and a mix of both. Hold-time cycles were systematically associated with complex crack front morphologies. A new numerical approach was developed to assess the effect of crack front morphology on the direct current potential drop technique, mechanical fields at the crack tip and ultimately, measured crack growth rates. Using this approach, a clear relation was established between crack front morphology and its evolution, and the crack growth behaviour under hold-time conditions. Complex crack front morphologies are demonstrated to be responsible for increased crack growth rates. From this, a crack growth mechanism under hold-time conditions is proposed. Finally, the numerical framework here presented is to be considered as a new, easily reproducible, way to properly analyse experimental data when dealing with complex loading cycles and complex crack front morphologies.

Crack closure effect and crack growth behavior in GH2036 superalloy plates under combined high and low cycle fatigue

The present work aims to investigate the crack closure effect and crack growth behavior of GH2036 superalloy subjected to a combined high and low cycle fatigue (CCF) loading. GH2036 superalloy plates were tested under stress ratios of 0.1, 0.4, and 0.7 in order to examine the crack closure effect. Together with Digital Image Correlation measurement, it is found that crack closure is present at low stress ratio (R<0.7) and subsequently a crack closure model was developed. Moreover, experiments by varying high cycle fatigue (HCF) amplitude and cycle ratio were performed under CCF loading at 550°C, showing increased crack growth rate under large HCF amplitude and cycle ratio while decreased crack growth rate under small HCF amplitude. Furthermore, a new CCF crack growth model incorporating the crack closure effect, time-independent crack increment, and transient vibrational analysis was proposed and validated by comparing with experimental results to be able to accurately predict fatigue crack growth life under different HCF amplitudes and cycle ratios. Finally, fractography analysis was also performed to discuss the mechanism underlying the crack growth behavior. Our study provides comprehensive and significant experimental and modeling insights into the crack growth behavior of superalloys under CCF loading.

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.

Influence of superimposed vibrational load on dwell time crack growth in a Ni-based superalloy

Sustained loads have for some Ni-based superalloys been shown to give rise to increased crack growth rate at elevated temperature. Such loads generate a history dependent fatigue problem due to weakening and cracking of grain boundaries during dwell times, later broken apart during subsequent load cycles. So far most studies have focused on the interaction of load cycles, overloads, and temperature. However, vibrations of different kinds are to some extent always present in engine components, and an investigation of how such loads affect the dwell time cracking, and how to incorporate them in a modelling context, is therefore of importance. In this paper a study of the most frequently used gas turbine material, Inconel 718, has been carried out. Mechanical crack propagation testing has been conducted at 550°C for surface cracks with and without the interaction of superimposed vibrational loads. Subsequent investigation of the fracture behaviour was performed by scanning electron microscopy and the modelling work has been conducted by incorporating the vibration load description within a history dependent crack growth law. The obtained results show reasonable accuracy with respect to the mechanical test results.

Three-dimensional crack growth modelling of a Ni-based superalloy at elevated temperature and sustained loading

High temperature materials subjected to elevated temperature have been shown to be sensitive to dwell times, giving an increased crack growth rate. The interaction between these dwell times and rapid cyclic loads have been shown to constitute a complex problem. Many models have been developed for 1D conditions, but the application to general 3D conditions has seldom been seen, although this is the most common case in most structures. In this paper a model for taking care of the interaction between these load modes in general 3D crack growth has been developed. The model uses 1D results for extension to general 3D, thus providing for local crack front evolution with a minimum of numerical simulations. Consequently, a history dependent crack growth law in 3D is given in this paper, which is capable of tracking the damage from the sustained load, and the interaction with rapid cyclic loading. The model has been implemented for usage with finite element calculations and several different tests are simulated and compared with experimental results for the nickel based superalloy Inconel 718 at 550°C. The simulation results show crack shapes in agreement with experimental fracture surfaces and time to failure.

Fatigue Crack Growth Mechanisms for Nickel-based Superalloy Haynes 282 at 550-750 °C

The fatigue crack growth rates for nickel-based superalloy Haynes 282 were measured at 550, 650, and 750 C using compact tension specimens with a load ratio of 0.1 and cyclic loading frequencies of 25 and 0.25 Hz. The crack path was observed to be primarily transgranular for all temperatures, and the observed effect of increasing temperature was to increase the fatigue crack growth rates. The activation energy associated with the increasing crack growth rates over these three temperatures was calculated less than 60 kJ/mol, which is significantly lower than typical creep or oxidation mechanisms; therefore, creep and oxidation cannot explain the increase in fatigue crack growth rates. Transmission electron microscopy was done on selected samples removed from the cyclic plastic zone, and a trend of decreasing dislocation density was observed with increasing temperature. Accordingly, the trend of increasing crack growth rates with increasing temperature was attributed to softening associated with thermally assisted cross slip and dislocation annihilation.

Unique Appearance of Lamellar Cleavage Patterns on Fracture Surfaces of Ti-Based Amorphous Matrix Composite

In order to improve mechanical properties of Ti-based amorphous matrix composites basically composed of ductile β-Ti dendrites and brittle amorphous matrix by overcoming their inherent brittle nature, their fracture mechanisms should be verified in relation with microstructure, stress intensity factor level, and crack growth rate. In this study, thus, detailed fractographic observations including the unique appearance of lamellar cleavage patterns, which has not been reported in previous studies on conventional metals and alloys, were conducted. According to fractographic results, lamellar cleavage patterns were formed by repeated interruptions of crack propagation on {100} cleavage planes by difference between dendrite orientation and loading direction. Ductile-to-brittle transition phenomenon (ductile dimpled fracture → lamellar cleavage fracture → ordinary cleavage fracture in dendrite areas, and vein pattern → smooth pattern in amorphous matrix areas) occurred with increasing crack growth rate was also plausibly explained by the concept of time required for crack growth as well as dendrite orientation.

Influence of overloads on dwell time fatigue crack growth in Inconel 718

Inconel 718 is one of the most commonly used superalloys for high temperature applications in gasturbines and aeroengines and is for example used for components such as turbine discs. Turbine discs can be subjected to temperatures up to ~700°C towards the outer radius of the disc. During service, the discs might start to develop cracks due to fatigue and long dwell times. Additionally, temperature variations during use can lead to large thermal transients during start-up and shutdown which can lead to overload peaks in the normal dwell time cycle. In this study, tests at 550°C with an overload prior to the start of each dwell time, have been performed. The aim of the investigation was to get a better understanding of the effects of overloads on the microstructure and crack mechanisms. The microstructure was studied using electron channelling contrast imaging (ECCI). The image analysis toolbox in Matlab was used on cross sections of the cracks to quantify: crack length, branch length, and the number of branches in each crack. It was found that the amount of crack branching increases with an increasing overload and that the branch length decreases with an increasing overload. When the higher overloads were applied, the dwell time effect was almost cancelled out. There is a strong tendency for an increased roughness of the crack path with an increasing crack growth rate.

Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li

At high temperatures in air, introducing a dwell period at the peak stress of fatigue cycles promotes time dependent intergranular crack growth which can increase crack growth rates by upto a few orders of magnitude from the rates of transgranular fatigue crack growth in superalloys. It is expected that time dependent intergranular crack growth in nickel-based superalloys may not occur below a critical mechanical driving force, Δ K th−IG , analogous to a fatigue threshold (Δ K th ) and a critical temperature, T th . In this study, dwell fatigue crack growth tests have been carefully designed and conducted on Alloy 720Li to examine such thresholds. Unlike a fatigue threshold, the threshold stress intensity factor range for intergranular crack growth is observed to be highly sensitive to microstructure, dwell time and test procedure. The near threshold crack growth behaviour is made complex by the interactions between grain boundary oxidation embrittlement and crack tip stress relaxation. In general, lower Δ K th−IG values are associated with finer grain size and/or shorter dwell times. Often a load increasing procedure promotes stress relaxation and tends to lead to higher Δ K th−IG . When there is limited stress relaxation at the crack tip, similar Δ K th−IG values are measured with load increasing and load shedding procedures. They are generally higher than the fatigue threshold (Δ K th ) despite faster crack growth rates (d a /d N ) in the stable crack growth regime. Time dependent intergranular crack growth cannot be activated below a temperature of 500 ∘ C.

Corrosion fatigue crack growth behavior of 7075-T6 under biaxial tension–tension cyclic loading condition

Biaxial and uniaxial fatigue crack growth of aluminum alloy 7075-T6 was characterized under air and salt water (3.5%) environments. Biaxial fatigue increased crack growth rate in region I relative to uniaxial fatigue under both environments. Biaxial fatigue increased crack growth rate in region II relative to uniaxial fatigue under salt environment. Crack growth rates in region I were comparable in air and salt environments, while they were faster in salt water than air environment in region II. Biaxial fatigue in salt environment exhibited intergranular cracks along with main transgranular fracture. A mechanism for biaxial corrosion fatigue crack growth is proposed.

Modelling of high temperature fatigue crack growth in Inconel 718 under hold time conditions

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650°C. The main load cycle for such components is typically defined by the start-up and shut-down of the engine. It generally includes hold times at high temperatures, which have been found to have a potential for greatly increasing the fatigue crack growth rate with respect to the number of load cycles. However, these effects may be totally or partly cancelled by other load features, such as overloads or blocks of continuous cyclic loading, and the actual crack propagation rate will therefore depend on the totality of features encompassed by the load cycle. It has previously been shown that the increased crack growth rate found in hold time experiments can be associated with a damage evolution, where the latter is not only responsible for the rapid intergranular crack propagation during the actual hold times, but also for the increased crack growth during the load reversals. In this paper, modelling of the hold time fatigue crack growth behaviour of Inconel 718 has been carried out, using the concept of a damaged zone as the basis for the treatment. With this conceptually simple and partly novel approach, it is shown that good agreement with experimental results can be found.

The effect of welding process and shielding atmosphere on the AlMg4.5Mn weld metal properties

In this paper, the AlMg4.5Mn alloy has been welded using tungsten inert gas (TIG) and metal inert gas (MIG) processes with different gas shielding atmospheres. Tensile strength, hardness, impact and fracture toughness, fatigue crack growth parameters (ΔKth, da/dN), as well as microstructure were determined. By comparing results for different gas mixtures, the main conclusion for TIG welding was that increased helium content has an important effect on toughness and fatigue crack growth parameters, whereas its effect on other mechanical properties is not significant. On the other hand, in the case of MIG welding, adding helium does not affect mechanical properties, except the fatigue crack growth rate. It was also established that adding nitrogen (TIG welding) and oxygen (MIG welding) reduces toughness and increases crack growth rate, so their use in shielding mixtures is not recommended.