Thermomechanical Fatigue Crack Research Articles

Estimation of thermo-mechanical fatigue crack growth using an accumulative approach based on isothermal test data

A procedure to estimate thermo-mechanical fatigue crack growth (TMFCG) rates in a coarse grain nickel-based cast alloy is presented. The implemented model relies on the linear accumulation of temperature dependent and independent crack growth contributions. Isothermal fatigue and creep crack growth experiments were performed to generate the model data base. To quantify oxidation damage, the formation of subsurface γ′-depletion was measured. For validation, TMFCG experiments on corner-crack specimen were conducted. The model gives reasonable estimations for the crack growth of an initial defect. Furthermore it allows the assessment of the causes of the crack growth and associate them to crack propagation characteristics observed from the crack path.

Cracking due to combined TMF and HCF loading in cast iron

Critical locations in diesel engine cylinder heads may be susceptible to damage as a consequence of thermomechanical fatigue (TMF) and/or high vibratory firing pressure (high cycle fatigue, HCF) loading. Evidence is presented for a vermicular cast iron to show that superimposed HCF loading below a threshold strain amplitude of ∼0.03% does not reduce the crack initiation endurances due to a service-type TMF cycle with Tmax of 450°C and a dwell/hold time at peak mechanical strain in compression of 180s. For superimposed strain amplitudes above the threshold, there can be a significant reduction in the TMF crack initiation endurances. The threshold superimposed HCF strain amplitude is found to be temperature sensitive, reducing with increasing Tmax (e.g. to 480°C). The magnitudes of the thresholds are judged to be determined by the strain rate sensitivity of oxide scale cracking at these temperatures. Importantly, for TMF cycles with Tmax 450°C and low mechanical strain ranges (i.e. low levels of constraint), a limiting value for total mechanical strain range is identified to be responsible for significantly longer endurances (>104cycles). The limit is found to be similar for both TMF and TMF(HCF) testing conditions (i.e. insensitive to the superimposed HCF loading) and coincides with the isothermal fatigue-limit for HCF loading of the iron at the TMF cycle TTRD equivalent temperature with the same mean stress level encountered during the low constraint TMF tests.With the improved capabilities of modern digital controllers in mechanical testing units, the possibilities for complex cycle control such as that required for combined HCF and TMF testing is readily attainable. New-efficient algorithms for TMF and TMF(HCF) testing and data acquisition are introduced in the paper.

Effects of Crack Closure and Cyclic Deformation on Thermomechanical Fatigue Crack Growth of a Near α Titanium Alloy

In this study, closure corrected in-phase (IP) and out-of-phase (OP) thermomechanical fatigue crack growth rates at two temperature intervals viz. 573 K to 723 K (300 °C to 450 °C) and 723 K to 873 K (450 °C to 600 °C) of Timetal 834 near α titanium alloy are presented. It is found that closure mechanisms significantly influence the stage I crack growth behavior. Surface roughness-induced crack closure (RICC) predominantly modifies the crack growth rate of near-threshold region at 573 K to 723 K (300 °C to 450 °C) test conditions. However, oxide-induced crack closure further strengthens RICC at 723 K to 873 K (450 °C to 600 °C) TMF loading. In stage II crack growth behavior, the alloy shows higher crack growth rates at 723 K to 873 K (450 °C to 600 °C) OP-TMF loading which is attributed to the combined effect of cyclic hardening occurring at the crack tip and weakening of interlamellar regions due to oxidation.

Thermomechanical Fatigue Crack Growth Modeling in a Ni-Based Superalloy Subjected to Sustained Load

Thermomechanical fatigue (TMF) crack growth modeling has been conducted on Inconel 718 with dwell time at maximum load. A history dependent damage model taking dwell damage into account, developed under isothermal conditions, has been extended for TMF conditions. Parameter determination for the model is carried out on isothermal load controlled tests at 550–650 °C for surface cracks, which later have been used to extrapolate parameters used for TMF crack growth. Further, validation of the developed model is conducted on a notched specimen subjected to strain control at 50–550 °C. Satisfying results are gained within reasonable scatter level compared for test and simulated number of cycles to failure.

Electron back scattered diffraction characterization of thermomechanical fatigue crack propagation of a near α titanium alloy Timetal 834

Fatigue crack growth (FCG) mechanisms have been studied in light of the interaction of a propagating crack with local crystallographic orientations of primary alpha (αP) and secondary alpha (αS) colonies in a near α Timetal 834 Ti-alloy under thermomechanical fatigue (TMF) loading using electron backscattered diffraction (EBSD). FCG testing at in-phase (IP) and out-of-phase (OP) TMF loading have been carried out at two temperature intervals 300°C↔450°C and 450°C↔600°C. EBSD analysis and microhardness measurements have confirmed that larger cyclic plastic zone size at the crack tip leads to lower crack propagation under IP-TMFCG loading as compared to OP-TMFCG loading. EBSD analysis has also confirmed that crack dissipates more energy to propagate when it passes from a soft grain oriented with its c-axis normal to the loading direction and encounters a hard grain with its c-axis parallel with the loading direction.

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.

Three-dimensional EBSD characterization of thermo-mechanical fatigue crack morphology in compacted graphite iron

In cylinder heads made of compacted graphitic iron (CGI), heating and cooling cycles can lead to localized cracking due to thermo-mechanical fatigue (TMF). To meticulously characterize the complex crack path morphology of CGI under TMF condition, in relation to microstructural features and to find out how and by which mechanisms the cracks predominantly develop, three-dimensional electron back scattering diffraction (EBSD) was employed. Based on the precise quantitative microstructural analysis, it is found that graphite particles not only play a crucial role in the crack initiation, but also are of primary significance for crack propagation, i.e. crack growth is enhanced by the presence of graphite particles. Furthermore, the density of graphite particles on the fracture plane is more than double as high as in any other arbitrary plane of the structure. The obtained results did not indicate a particular crystallographic preference of fracture plane, i.e. the crystal plane parallel to the fracture plane was nearly of random orientation.

Thermomechanical fatigue crack growth in a cast polycrystalline superalloy

Thermomechanical fatigue (TMF) crack growth testing has been performed on the polycrystalline superalloy IN792. All tests were conducted in mechanical strain control in the temperature range between 100 and 750 °C. The influence of in-phase (IP) and out-of-phase (OP) TMF cycles was investigated as well as the influence of applying extended dwell times (up to 6 hours) at the maximum temperature. The crack growth rates were also evaluated based on linear elastic fracture mechanics and described as a function of the stress intensity factor KI. Without dwell time at the maximum temperature, the crack growth rates are generally higher for the OP-TMF cycle compared to the IP-TMF cycle, when equivalent nominal strain ranges are compared. However, due to the fact that the tests were conducted in mechanical strain control, the stress response is very different for the IP and OP cycles. Also the crack closure level differs significantly between the cycle types. By taking the stress response into account and comparing the crack growth rates for equivalent effective stress intensity factor rages ΔKeff defined as Kmax − Kclosure, very similar crack growth rates were actually noticed independent of whether an IP or OP cycle were used. While the introduction of a 6 hour dwell time significantly increased the crack growth rates for the IP-TMF cycle, a decrease in crack growth rates versus ΔKeff were actually seen for the OP-TMF cycle. The fracture behaviour during the different test conditions has been investigated using scanning electron microscopy.

Evaluation of thermo-mechanical fatigue crack propagation behavior of Ni-based single crystal superalloy CMSX-4

Evaluation of thermo-mechanical fatigue crack propagation behavior of Ni-based single crystal superalloy CMSX-4

Thermomechanical fatigue crack growth from laser drilled holes in single crystal nickel based superalloy

The test results undergoing thermomechanical fatigue of single crystal PWA1484 crack growth showed that the life of TMF specimens with notches (in this case laser drilled holes) is 4 times shorter than the number of cycles to failure observed on smooth gage section specimens (without holes) under the same loading conditions. Such a significant change in number of cycles to failure must be accounted in any damage tolerant turbine airfoil design system. The detailed fractographic analysis demonstrated that all cracks start crystallographically along the {111} octahedral crystallographic planes and later change to mixed mode fracture. Most of the crack propagation takes place at the low temperature portion of the cycle in the out-of-phase test; however there is noticeable damage accumulation during the high temperature compressive load portion of the cycle. Crack propagation under TMF loading conditions is considerably faster than corresponding isothermal LCF crack growth tested at the temperature and similar loading conditions of the tensile part of the TMF cycle. As results show, the applicability of the LEFM methods for single crystal TMF crack growth prediction is limited and at least should consist of mixed mode crack analysis.A new method for detecting cracks during a TMF test using induction thermography was employed. This method, coined the Active Inferred Crack Detection System (AICD), demonstrated high effectiveness in following crack progression under cyclic loading making it well suited to perform TMF crack growth testing. Using this experimental technique we also investigated the effect of secondary crystallographic orientation on crack propagation.

Load and temperature interaction modeling of fatigue crack growth in a Ni-base superalloy

An approach was developed to predict the thermo-mechanical fatigue crack growth rates under typical gas turbine engine spectrum loading conditions. The material studied in the development of this model was a polycrystalline superalloy, Inconel 100. Load interaction effects were determined to have a major effect on the crack growth life. A yield zone load interaction life prediction model was modified to include temperature dependent properties. Multiple overload effects were included in the model to incorporate enhanced retardation compared to single overload retardation behavior. Temperature interaction effects were included and proved to be very important because of the wide temperature ranges to which turbine engine components are subjected. The effects of oxidation and temperature changes were accounted for in the model by accelerating crack growth in regions that had been previously affected by elevated temperatures. Experimental data of isolated, first order effects were used to calibrate and verify the model. Temperature dependent mechanical properties were determined and were essential in the model’s development. Parametric studies were performed using this model to assess the sensitivity of specific crack growth variables on life predictions.

Fatigue Crack Initiation and Propagation in Piles of Integral Abutment Bridges

Abstract: A new continuum damage modeling approach of “successive initiation” is used to determine the location of a thermomechanical fatigue crack initiation and the propagation path and rate in piles of integral abutment bridges. A global‐local modeling approach is introduced to determine the critical location in the pile where a crack is initiated using a 3‐dimensional nonlinear finite element model and to implement “successive initiation.” A simulated case study is used to showcase the multistep procedure. The results indicate that for a pile subjected to the maximum stress, the first fatigue‐induced crack initiates in the tip of the flange at the element immediately below the abutment. Several other cracks at different locations form in the flange of the pile while the initial crack continues to propagate in the flange to the web. The crack propagation rate increases as more cracks initiate in the flange. The propagation rate decreases when the crack reaches the web. Based on the case study presented, a crack could initiate in the pile in as little as 6 years, but it may take about 20 years for it to reach the web; however, final failure of the pile may not take place for several decades. The method can also be used as a guide in bridge foundation inspection and in the determination of the remaining life of an existing bridge.

Hold-time effect on the thermo-mechanical fatigue crack growth behaviour of Inconel 718

In-phase TMF crack growth testing with different lengths of the hold time at the maximum temperature of 550 °C has been conducted on Inconel 718 specimens. Focus has been on establishing a method f ...

Advanced thermomechanical fatigue crack growth testing for component life validation

Abstract The increasing performance requirements of gas turbines are driving higher operating temperatures in critical components, leading to greater creep and fatigue interactions. There is thus a requirement for thermomechanical fatigue (TMF) test data, including TMF crack growth rates. The test equipment required to meet this challenge is briefly discussed, along with the development of the test methodology. Thus, the accurate measurement and maintenance of thermal performance during TMF are considered. The determination of fatigue crack growth rates by conventional electrical potential difference measurements has been employed, utilising convenient dwells within the TMF cycles, where both load and temperature are held constant for a brief period. These experimental methods have been demonstrated with trials on the advanced nickel base superalloy RR1000 used for turbine discs. This work has also highlighted significant microstructural effects on the crack growth rates, with the coarser grain siz...

Thermo-mechanical fatigue crack propagation experiments in Inconel 718

An experimental test facility was developed to perform thermo-mechanical fatigue crack growth experiments. The thermal cycles were generated using hot and cold air flows distributed by a nozzle onto the test specimen. With the equipment it was possible to obtain TMF cycles with cycle times of 120 s with less than 10 °C temperature variation over the measurement length of the sample, cycling between 200 °C and 550 °C. The equipment allows TMF crack propagation tests up to, at least, 600 °C. The crack length and the crack closure level were determined using the potential drop technique. Thermo-mechanical fatigue crack propagation experiments were performed in-phase and out-of-phase with various R-values on samples of Inconel 718. Cylindrical specimens with a small starter crack were cycled in nominal total strain control. The crack propagation rate was determined and the correlation to the effective J-integral range, Δ J eff , corrected for crack closure presented. The fracture surfaces showed a dominance of trans-granular crack propagation with striations, indicating a low degree of time dependency in the procedure.

Integrated numerical–experimental analysis of interfacial fatigue fracture in SnAgCu solder joints

In ball grid array (BGA) packages, solder balls are exposed to cyclic thermo-mechanical strains arising from the thermal mismatch between package components. Thermo-mechanical fatigue crack propagation in solder balls is almost always observed at the chip side of the bump/pad junction. The objective of the experimental part of this study is to characterize the bump/pad interface under fatigue loading. Fatigue specimens are prepared by reflowing Sn3.8Ag0.5Cu lead-free solder alloy on Ni/Au substrates. Obtained results show that fatigue damage evolution strongly depends on the microstructure. Applied strain and solder volume both have an influence on the fatigue damage mechanism. In the numerical part of the study, fatigue experiments are modeled using the finite element technique. A cohesive zone approach is used to predict the fatigue damage evolution in soldered connections. Crack propagation is simulated by an irreversible linear traction–separation cohesive zone law accompanied by a non-linear damage parameter. Cohesive zone elements are placed where failure is experimentally observed. Damage evolution parameters for normal and tangential interaction are scrutinized through dedicated fatigue tests in pure tensile and shear directions. The proposed cohesive zone model is quantitatively capable of describing fatigue failure in soldered joints, which can be further extended to a numerical life-time prediction tool in microelectronic packages.

The manufacture and fatigue cracking resistance of grit free aluminide diffusion coatings

Abrasive blasting is an inexpensive and effective means of cleaning superalloy castings, and of roughening the surface of these castings prior to platinum electroplating or overlay coating deposition. However, during the blasting operations, some of the abrasive grit particles become embedded in the surface of the superalloy and this embedded grit can result in detrimental effects on the mechanical properties of gas turbine components. A research program was conducted to evaluate the use of molten caustic immersion to eliminate entrapped grit particles, and to determine the effects of entrapped grit removal on the Thermo-Mechanical Fatigue (TMF) crack resistance of aluminide diffusion coatings. Using aggressive TMF testing conditions, the average life of the samples increased by 47% using the molten caustic immersion to remove the embedded grit particles. The research program is described, the results are presented and discussed and all possible conclusions are summarized.

Three-dimensional thermo-mechanical fatigue crack growth using BEM

This paper describes the formulation and numerical implementation of an incremental procedure for fatigue crack growth simulation applied to three-dimensional thermoelasticity using the dual boundary element method. The efficiency of the formulation is demonstrated with a number of mixed-mode examples including embedded as well as edge cracks. The results are in accord with the generally accepted behaviour of cracks, and analogies were established with the elastic case under mechanical load.

Delamination Crack Growth of Unidirectional CFRP Laminates under Variable Temperature (2nd Report, 180.DEG.C. Cure Type Epoxy Resin Matrix).

The delamination crack growth behavior under iso-thermal and thermo-mechanical fatigue were investigated with unidirectional CF/epoxy laminates. Tests were conducted either in air or in water with double cantilever beam specimens. The crack growth tests were conducted under constant ΔK conditions. In air, the delamination crack growth rates for the iso-thermal fatigue tests were higher for higher temperature, and the crack growth rates for the thermo-mechanical fatigue tests were lower than those for the iso-thermal fatigue tests. The crack growth rates for the in-phase thermo-mechanical tests were lower than those for the out-of-phase thermo-mechanical tests. In water, the crack growth rates for the iso-thermal fatigue tests were higher for higher temperature under high stress ratio. The rates, however, were not monotonically changed with test temperature under low stress ratio. The rates for the themo-mechanical fatigue test were almost the same value as those for the iso-thermal fatigue test whose test temperature was equal to that at the maximum load for the themo-mechanical fatigue test. The most remarkable difference in the growth behavior between present material and the material employed for the 1st report appeared in the thermo mechanical fatigue crack growth tests in air

A methodology for thermomechanical fatigue crack growth testing in MMCs

This paper describes the experimental techniques used in an investigation of the crack growth characteristics of a four-ply, unidirectional, silicon carbide fiber reinforced, titanium matrix composite (SCS-6/Ti−6Al−2Sn−4Zr−2Mo) subjected to thermomechanical fatigue. A mechanical test system was assembled which is capable of conducting fully automated, computer-controlled thermomechanical fatigue crack growth tests. The system is able to simultaneously impose operator-defined arbitrary mechanical and thermal histories on the specimen. Crack lengths in single-edge tension [SE(T)] or middle tension [M(T)] specimens are measured by the direct-current electric potential method and optically using a unique telemicroscope system. A series of isothermal, in-phase and out-of-phase crack growth tests was conducted to obtain baseline data for material modeling purposes. The test temperatures ranged from 150°C to 538°C, and the highest thermal frequency was 0.0083 Hz.