Fatigue Crack Growth Experiments Research Articles

Uncertainty Quantification in Small-Timescale Model-Based Fatigue Crack Growth Analysis Using a Stochastic Collocation Method

Due to the uncertainties originating from the underlying physical model, material properties and the measurement data in fatigue crack growth (FCG) processing, the prediction of fatigue crack growth lifetime is still challenging. The objective of this paper was to investigate a methodology for uncertainty quantification in FCG analysis and probabilistic remaining useful life prediction. A small-timescale growth model for the fracture mechanics-based analysis and predicting crack-growth lifetime is studied. A stochastic collocation method is used to alleviate the computational difficulties in the uncertainty quantification in the small-timescale model-based FCG analysis, which is derived from tensor products based on the solution of deterministic FCG problems on sparse grids of collocation point sets in random space. The proposed method is applied to the prediction of fatigue crack growth lifetime of Al7075-T6 alloy plates and verified by fatigue crack-growth experiments. The results show that the proposed method has the advantage of computational efficiency in uncertainty quantification of remaining life prediction of FCG.

Stress intensity factors for mixed-mode crack growth in imitation models under biaxial loading

In this study a procedure to stress intensity factors calculation for imitation models made of titanium alloy is described. In service fatigue cracks are detected in a disk and blade “dovetail type” attachment. On the base of this attachment dimensions and with taking into account the biaxial loading conditions of rotating compressor disk the two geometries of imitation model of gas turbine engine compressor disk are developed. To accurate verification of biaxial loading conditions, the first imitation model of constant thickness is used. In order to fully reproduce the geometry of the compressor disk and conditions of mixed mode crack growth, the second imitation model with reduced cross section is proposed. The fatigue crack growth experiments of imitation models were carried out at room temperature on a biaxial testing machine. Two different stress ratio values are applied several times to each imitation model in order to fix the experimental crack front positions. The elastic and plastic stress intensity factors used for the representation of the experimental results are computed by using full-size 3D Finite Element analysis of the imitation models with surface quarter elliptical and through-thickness cracks.

Analysis of Surface Crack using Various Crack Growth Models

This research focusing on fatigue crack growth experiment and assessments of fatigue crack growth model. Fatigue has been considered as the most important phenomena in the engineering problems. It has been found to occur in every engineering field. Many methods were introduced to overcome fatigue problem. One of it is fatigue crack growth prediction models. Fatigue crack growth model prediction can determine the residual life of the mechanical component. Fatigue crack growth is an important parameter in engineering. It may begin from undamaged area and propagate afterwards. In this case, a mechanical component condition before an unstable crack still can be used until fatigue. With the study of fatigue crack growth, it can predict the crack life of the component and can reduce cost to repair the component. In this thesis, primary objective is to develop, test and provide computational fracture mechanics model that emphasized an uncertainties quantification for surface crack. Fracture and failure study that has been promoted by S-version finite element method is focus on the analysis of the surface-crack problem. In this project, a few limitations of the study have been implemented as to present a specific scope of the investigation. Both fatigue crack growth model is considered; Paris law model and modified Forman model. The specimen in this analysis is aluminium al 2024 T3. The effect of load on the fatigue crack growth rate is investigated on aluminium 2024 T3. It is concluded that modified Forman has a low exponent compared to the Paris law. Further the crack growth prediction based on modified Forman is lower than Paris law. Paris law only can predict the crack in the region II while modified Forman can predict the crack in region II and III. After finish setup the project based on the instruction, the project is proceeded and the data is collected. The result is analysed based on the objective by using data collected from the project. After that, conclusion is made based on the result of the project.

FATIGUE CRACK GROWTH AND FATIGUE FRACTURE MORPHOLOGY OF RECYCLED RUBBER POWDER–FILLED NR/BR BLEND COMPOUND

ABSTRACTThe effect of two different types and particle sizes (micronized cryo-ground 74 μm or ambient-ground 400 μm) of recycled rubber powder (RRP) was studied during fatigue crack growth (FCG) in a natural rubber/butadiene rubber (NR/BR) compound using a fracture mechanics approach. Absolute and relative hysteresis losses using single-edge notch tensile specimens were determined with a displacement-controlled strain compensating for the permanent set of the samples throughout the FCG experiments. Differences in relative hysteresis loss showed that additional energy dissipation, due to multiple new crack surfaces at the crack tip, contributes to the FCG of the RRP compounds. At higher tearing energy, beside other factors affecting the FCG performance of the RRP compounds, both higher absolute and relative hysteresis loss are slightly detrimental to the crack growth rates. At lower tearing energy, the larger RRP-filled compound showed slower, but not significant, different crack growth rates than the NR/BR control compound. Fracture morphologies for NR/BR and RRP-filled compound were associated with different fracture surface topographies at various tearing energies, which revealed the dependency of the crack growth microstructure on the tearing energies.

GPU-based digital image correlation system for uniaxial and biaxial crack growth investigations

Experiments to describe the crack growth behavior are expensive and complex, while the evaluation depends mainly on the determined crack lengths over the testing period. Digital image correlation (DIC) is well established for crack-length and displacement measurements, but it normally requires sample preparation with speckle paint and interferes with mechanical extensometers. Novel GPU-based DIC system capable of measuring total strain at rates up to 850 Hz and strain-fields overcome these limitations. We present first results of fatigue crack growth experiments with uniaxially und biaxially loaded specimens. The force controlled tests under room-temperature conditions were accompanied by conventional measuring systems comprising a side extensometer, an ACPD-measurement system and the GPU-based DIC system. The results indicate that DIC-measured crack depth correlates well with ACPD crack depth values. Furthermore, the crack-flank displacements derived from DIC-evaluation are in good agreement with 2D-FEM simulations. The GPU-based DIC-System appears as a promising measurement technique for crack growth investigations.

Fatigue properties of aluminium triangular lattice plates

We propose a method to predict the fatigue life of triangular lattices by using fatigue data from tests on single lattice struts. The method was validated using fatigue tests on 6 identical triangular lattice plates divided into three groups with different initial crack lengths. The predicted fatigue lives of lattices agree very well with the experimental results. A three-stage mechanism of lattice fatigue was observed in the tests, and the crack propagated in the direction 30° to the horizontal axis in the upper half of lattice plate in all three groups. The fatigue crack propagation rate was also studied and calculated by using the effective mechanical parameters from the homogenized structure. Experimental fatigue crack growth rate data were compared with predictions, showing that the proposed method can also predict the crack growth rate in triangular lattices very well.

Numerical Modelling of Plasticity Induced Crack Closure with Rough Fracture Surfaces

A two-dimensional elastic-plastic finite element model was built to simulate the closure of a long fatigue crack with arbitrarily shaped crack faces. The model growth is simulated by the successive mesh splitting along the crack path defined by element edges. To obtain a realistic morphology of the fracture surface, fatigue crack growth experiments with CT specimen made from AISI 304 stainless steel were performed and fracture surface topology was determined using a single camera and a depth-from-focus method. Simulated closing loads and closure lengths for the cracks with rough and smooth faces and for plane-stress and plane-strain conditions are compared. A mismatch of rough crack faces, resulting in an additional contact, is visualized.

Mixed mode fatigue crack growth studies in AISI 316 stainless steel

Damage tolerance assessment requires the accurate prediction of fatigue crack growth lives. Numerical simulation techniques using mixed mode Paris' law and various equivalent stress intensity factor models (ΔKeq) are widely employed for fatigue life prediction. In the present work, mixed mode (I/II) fatigue crack growth experiments are performed using the compact tension shear specimens made of AISI 316 austenitic stainless steel for various loading angles. Finite element fatigue crack growth simulations are carried out, and the effect of ΔKeq model in fatigue life prediction is studied. To achieve this, a three parameter double exponential type best fit is proposed for fitting the experimental mixed mode crack length vs fatigue life. The performance and capability of various selected models are assessed by comparing the predicted life with the experimental results. Fractographic studies at different stages of crack propagation for different loading angles are also presented to aid the above assessments. Based on the overall consistent performance, Irwin's and Tanaka models are predicting life close to the experimental data and Richard's and Yan's models provide conservative solutions.

Digital twin approach for damage-tolerant mission planning under uncertainty

The digital twin paradigm that integrates the information obtained from sensor data, physics models, as well as operational and inspection/maintenance/repair history of a system (or a component) of interest, can potentially be used to optimize operational parameters of the system in order to achieve a desired performance or reliability goal. In this article, we develop a methodology for intelligent mission planning using the digital twin approach, with the objective of performing the required work while meeting the damage tolerance requirement. The proposed approach has three components: damage diagnosis, damage prognosis, and mission optimization. All three components are affected by uncertainty regarding system properties, operational parameters, loading and environment, as well as uncertainties in sensor data and prediction models. Therefore the proposed methodology includes the quantification of the uncertainty in diagnosis, prognosis, and optimization, considering both aleatory and epistemic uncertainty sources. We discuss an illustrative fatigue crack growth experiment to demonstrate the methodology for a simple mechanical component, and build a digital twin for the component. Using a laboratory experiment that utilizes the digital twin, we show how the trio of probabilistic diagnosis, prognosis, and mission planning can be used in conjunction with the digital twin of the component of interest to optimize the crack growth over single or multiple missions of fatigue loading, thus optimizing the interval between successive inspection, maintenance, and repair actions.

ModLayer: A MATLAB GUI Drawing Segmentation Tool for Visualizing and Classifying 3D Data

Characterizing a material’s microstructure, especially as it relates to the manufacturing processes used to fabricate it, is of great interest to engineers and researchers. In recent years, state-of-the-art imaging techniques have been able to yield a plethora of high resolution 3D data that can be used to study materials at various length scales. This 3D data is usually organized as stacked serial sections of 2D images and almost always requires some combination of enhancement and segmentation (the process of separating an image into subsets), in order to extract meaningful information. To aid in this process, ModLayer was created as a MATLAB® executable. ModLayer is an interactive graphical user interface that seeks to remove the burden of import/export redundancies when interacting with 3D data in MATLAB during visualization, modification, or segmentation through manual drawing across image stacks. The utility of ModLayer is demonstrated here through three case studies; (1) classifying regions of damage with in-situ time lapse X-ray micro-computed tomography ($$\mu $$-CT) of a glass fiber reinforced polypropylene (GFRP), (2) correcting multi-class segmentation errors in segmented X-ray $$\mu $$-CT images of a GRFP composite, and (3) capturing features of interest within in-situ 3D X-ray $$\mu $$-CT images during fatigue crack growth experiments of aluminum 7050. Overall, this tool is especially useful to engineers and researchers interested in correcting—within MATLAB—automated segmentation of noisy 3D images which can yield erroneous microstructural features in segmentation procedures.

Biaxial fatigue crack propagation behavior of ultrahigh molecular weight polyethylene reinforced by carbon nanofibers and hydroxyapatite.

Ultrahigh molecular weight polyethylene (UHMWPE) artificial joint has remained the preferred polymer component in total joint replacement surgery. However, more and more concerns have been raised about the failure of UHMWPE components due to the initiation and propagation of cracks at the notches with fixed functions. For this reason, biaxial fatigue crack growth (FCG) experiments of UHMWPE reinforced by carbon nanofibers (CNF) and hydroxyapatite (HA) were carried out using elastic-plastic fracture mechanics theory. The FCG resistance of UHMWPE, UHMWPE/CNF, and UHMWPE/HA was compared, and the effects of stress ratio (R) value and phase difference on FCG rate were investigated. At the same time, the influence of loading path was considered, and the corresponding crack path was analyzed. Results suggest that UHMWPE/CNF has better FCG resistance and the FCG rate increases with the increase of R value and the existence of 180° phase difference. In addition, crack bifurcation behavior is not observed under nonproportional loading conditions. The findings in this study will provide experimental validation and data support for better clinical application of UHMWPE-modified materials.

Experimental and numerical investigation of mixed mode fatigue crack growth models in aluminum 6061-T6

Damage tolerance philosophy and safe life design principles are widely used for critical structural components, and in such cases, fatigue life prediction using the numerical techniques is essential. In the present work, mixed mode fatigue crack growth experiments are performed using the compact tension shear specimens made of Al 6061-T6 alloy for mode mixity angles of 30°, 45°, and 60°. These experimental studies are supported by numerical and fractographic studies on the selected specimens. A three-parameter double exponential model for fitting the fatigue crack growth curves (crack length vs. the number of fatigue life cycles) is also proposed for both the mode I and mixed mode (I/II) loading conditions. Numerical prediction of mixed mode (I/II) fatigue life is carried out using the Paris’ law in combination with various ΔKeq models. The results of the present investigation show that highly satisfactory best fits to the scattered experimental data can be obtained with the help of the proposed double exponential model. The predicted life is compared with the experimental fatigue life using various measures of error. Based on the overall performance of the models during the entire crack growth regime, the results of the present investigation clearly show that Irwin’s model and one of the Tanaka’s model are consistently found to predict the mixed mode fatigue life close to the experimental data. Whereas, Richard’s and Yan’s models, again based on the overall performance, are found to be conservative models consistently for the prediction of the mixed mode fatigue life. These conclusions are verified with the published results.

Effects of Mixed Mode Loading Conditions on Fatigue Crack Growth Rate

The fatigue crack growth rate is the most important factor in predicting the life of a product when applying the damage tolerance design concept. Studies related to pure mode I for structures under fatigue loading have been actively conducted, while not many studies are conducted on the mixed mode. In this study, therefore, mixed mode fatigue crack growth experiments were designed using the Compact-Tension-Shear (CTS) specimens and the loading devices, proposed by Richard. Furthermore, the finite element analysis was used in determining the stress intensity factors of CTS specimen. As the results, the fatigue crack growth rate using the equivalent stress intensity factors proposed by previous researchers was lower than that of pure mode I at the initial stage of crack growth when the load angle increases.

The sharp drop in fatigue crack growth life at a critical elevated temperature for a PM Ni-based superalloy FGH97

The fatigue crack growth (FCG) experiments for Ni-based powder metallurgy (PM) FGH97 superalloy were performed from RT to 800 °C in air. It has been found that the FCG life decreases in an accelerated manner with an increase in temperature in the service temperature range, and a sharp drop in FCG life was noticed at the inflection point temperature (Tc) of about 800 °C. The fractographic and microstructural analyses were carried out to investigate the primary reason for the phenomenon of observed sharp drop. The obtained results indicate that the fracture transforms from transgranular to intergranular mode during the crack propagation under elevated temperature, and the transition point of fracture mode (ΔKT) also declines rapidly with an increase in temperature. At 800 °C, ΔKT appears even from the crack starts to propagate, which is closely related to the significant decline of FCG life. Combined with the calculated apparent activation energies of FCG, it can be inferred that the sharp drop in FCG life is primarily attributed to the dynamic embrittlement induced weakening of grain boundary, which dominates in the intergranular FCG stage at 800 °C. Further, the evolution of the fracture mechanism governing the FCG process under the conditions of different temperatures and stress intensity factor range (ΔK) has been discussed. With an increase in temperature in the service temperature range, the dominant mechanism of the intergranular fractures transforms from stress assisted grain boundary oxidation to dynamic embrittlement, which results in the sharp decline of FCG life at Tc.

Determining finite-width-correction factors for fatigue crack growth prediction in GLARE using the equivalent compliance method

Finite-width-correction factors (FWCF) are required for GLARE to accurately predict fatigue crack growth using linear elastic fracture mechanics. As demonstrated in earlier work, these factors effectively correct for the change in specimen compliance, which in GLARE relates to intact fibre layers and the formation of delaminations between these layers and the cracked metal layers. Therefore, the development of delaminations in various GLARE grades were studied using digital image correlation during fatigue crack growth experiments under different maximum stresses and stress ratios. The elongation of GLARE in loading direction was recorded by crosshead displacement during fatigue testing, and was correlated to the observed development of the delamination area. The relationship between effective delamination area and effective GLARE specimen compliance is used as basis for determining explicit formulations for the FWCF for GLARE.

The perturbation series method based on the logarithm equation for fatigue crack growth prediction

This paper gives the perturbation series method for the fatigue crack growth prediction accounting for the error in initial crack length. The proposed method utilizes the logarithm form of the crack growth equation and the solution of it is expressed in the form of asymptotic series by introducing a small parameter. The initial crack length is considered as a sum of the nominal value and the perturbation which mainly originates from manufacturing and measurement errors. Based on the Taylor series expansion and perturbation series method, a series of perturbation equations to predict the fatigue crack growth history are obtained. Then these equations can be solved by the successive approximation approach and the a-N curve which describes the relationship between crack length and number of load cycles can be obtained. Two examples with respect to fatigue crack growth experiments are given to verify the accuracy and validation of the presented method. The results show that the perturbation series method is closer to the experimental data compared with the original solution of crack growth equation and it can provide a more accurate prediction for the fatigue crack growth process.

Evaluation of crack growth direction criteria on mixed-mode fatigue crack growth experiments

The predictive capabilities of different fatigue crack growth direction criteria in finite element simulations are investigated. Crack growth direction criteria based on stress intensity factors, energetic measures and kinematic (displacement) measures are evaluated for mixed-mode fatigue crack growth experiments in the literature. More specifically, the development of the experimentally found crack path is simulated numerically, and the directions from the different criteria are compared to the known crack path along the development of the fracture. The simulated experiments featured proportional and non-proportional loading. Of the evaluated criteria, those based on energetic and displacement measures are able to accurately capture the tensile-mode fatigue crack growth direction for all examined experiments. Furthermore, the displacement-based criterion is able to capture the direction of shear-mode crack growth as well as the transition from shear- to tensile-mode growth and the subsequent tensile-mode growth. Modeling the cyclic elastic–plastic material response does not improve the accuracy of the predicted directions for the considered cases.

Role of prior austenite grain boundaries in short fatigue crack growth in hydrogen charged RPV steel

Short crack propagation under cyclic loading is compared experimentally in hydrogen charged and un-charged SA508 Gr. 3 Cl. I low alloy steel (LAS). This LAS is used in manufacturing of pressure vessels installed in nuclear power plants and is susceptible to hydrogen embrittlement (HE) during operation. Single edge notch tension (SENT) specimens with an initial notch of 85 μm–90 μm are used for this short fatigue crack propagation study. The short crack growth from the notch of SENT specimen subjected to cyclic loading is measured using moving digital microscope fitted on the servo-hydraulic fatigue testing machine. The short fatigue crack growth rate in hydrogen charged SA508 Gr. 3 Cl. I LAS is found to be one order higher as compared to the un-charged subject reactor pressure vessel (RPV) steel. Both trans-granular and inter-granular crack propagation is observed in un-charged and hydrogen charged specimens. In un-charged specimens, inter-granular crack propagation along the prior austenite grain boundaries (PAGBs) occurred only when the short fatigue crack encounters the martensite/austenite (M/A) island present along the PAGB. Whereas, in hydrogen charged specimens, inter-granular crack propagation occurred along the PAGBs of large size prior austenite grains and trans-granular crack propagation through the small size prior austenite grains. Strong resistance to short fatigue crack propagation is provided by PAGBs in un-charged specimens whereas, these PAGBs offered negligible resistance to short fatigue crack growth in hydrogen charged subject RPV steel. A new perspective of studying HE in materials by conducting short fatigue crack growth experiments is demonstrated.

Microstructure and mechanical anisotropy of CoCrW alloy processed by selective laser melting

We have investigated the correlation between the microstructure and mechanical anisotropy of biomedical CoCrW alloy processed by selective laser melting (SLM). A phase analysis revealed that the as-SLM CoCrW alloy consisted of mostly the γ-fcc phase and little ε-hcp phase. The as-SLM CoCrW microstructure featured the rotated cube texture, a strong <100> texture along the building direction (BD) and a weaker <110> texture along the front direction. To investigate the mechanical anisotropy, the external loading was applied either perpendicular or parallel to the BD (i.e., horizontal loading (⊥BD) or vertical loading (//BD), respectively). The rotated cube texture leads to a higher elastic modulus (~ 2.4 times), plastic flow stress, and microhardness in the horizontal loading. Macroscopic plastic anisotropy was elucidated by the Taylor factor analysis of polycrystalline aggregate. Strain-induced martensite transformation (SIMT) occurring in the horizontal loading (ef = 6.9%) is responsible for significantly reduced elongation to fracture compared to the vertical loading (ef = 32.8%). In the fatigue crack growth experiment, the threshold stress intensity range in the vertical loading is ~ 32% higher compared to the horizontal loading, leading to a retarded fatigue crack growth rate. A vertically elongated columnar granular structure due to the epitaxial grain growth along the BD results in anisotropic behavior of the fatigue crack growth.

Acquiring in situ Fatigue Crack Growth Curves by a Compliance Method for Micro Bending Beams to Reveal the Interaction of Fatigue Cracks with Grain Boundaries

To understand the interaction of dislocations with microstructural obstacles it is necessary to find test methods which are able to resolve the interaction of only a single or few defects with obstacles like interfaces. Therefore, the investigation of micro specimens has been established over the years as a suitable method to test the influence of microstructural features on the mechanical response. While quasi-static loading of micro specimens has been carried out extensively in the past decade and has given powerful insights on the mechanical behavior at small scales, cyclic loading and fatigue crack growth experiments still provide a challenge. In order to check the possibility to systematically initiate and monitor fatigue crack growth rates in micro specimens, we cyclically loaded micro bending beams made of a nickelbase superalloy. Furthermore, for grain boundaries of differing types we checked if the crack growth curves are suitable to measure crack - microstructure interactions. The fatigue cracks showed a deceleration when approaching the grain boundaries followed by an abrupt re-acceleration, which is in accordance to macroscopic experiments and connected to dislocation or slip transfer from the plastic zone of the crack through the grain boundary. Furthermore, we observed a dependency of the deceleration on the grain boundary type and the crystallographic orientation of the neighboring grains. A 3D-HR-orientation gradient map was gathered by HR-EBSD using the software CrossCourt in combination with a self-provided MATLAB tool to reveal detailed information about strain localization at the grain boundary in the process zone near the crack tip.