Crack Propagation Experiments Research Articles

Calibrated the direct current potential drop method for fatigue crack propagation testing of nickel-based superalloy with film cooling hole

Turbine blades in aviation engines commonly feature a nickel-based superalloy structure integrated with film cooling holes to enhance inlet gas temperature. However, the presence of these film cooling holes often results in frequent occurrences of fracture failures nearby. The direct current potential drop (DCPD) method, renowned for its exceptional crack sensitivity, is frequently utilized for crack length monitoring. This study establishes a FEM model of the film cooling hole plate specimen to determine the optimal probe point location. Subsequently, a mapping relationship is derived to calibrate Johnson’s formula, accounting for the unequal crack lengths at the edges of film cooling holes. It is confirmed that the crack length measured by the DCPD method represents the cumulative crack lengths on both sides of the hole. Fatigue crack propagation experiments are then conducted, with crack length monitored using a microscope. The results affirm the successful application of the calibrated formula to the film cooling hole plate specimen, exhibiting an average error within 0.1 mm.

Guided-wave Dynamic Probability Model-based Crack Quantification Method for Metal Structures

In the process of applying guided-wave-based aircraft online structural health monitoring, the characteristics of guided wave signals used to represent structural status exhibit random uncertainty due to the interference of loads on guided wave propagation. The trend of signal feature changes caused by damage is often obscured, making the quantification of cracks under loads one of the challenges in online structural health monitoring for aircraft. To address this challenge, this paper proposes a crack quantification method using an adaptive Gaussian mixture model (GMM) and optimized probability migration distance measurement. This method effectively mitigates the uncertainty caused by loads by constructing an adaptive Gaussian mixture model to track the probability distribution changes of guided wave signal features under different structural health conditions. Subsequently, an optimized probability migration distance measurement method is employed to characterize the structural state, further enhancing the correlation between features and the extent of structural damage. Finally, an optimized probability migration distance relationship with crack length is established to estimate the crack length of new structural specimens. The effectiveness of this method is validated using load-induced crack propagation experiments on metal structures.

Effect of grain structure on fatigue crack propagation behavior of 2024 aluminum alloy under different stress ratios

The fatigue load that a material experiences and its microstructure are important factors influencing fatigue crack propagation behavior. This study employed laser scanning microscopy and electron backscatter diffraction (EBSD) technology, along with fatigue crack propagation experiments, to investigate the fatigue crack propagation behavior of 2024 aluminum alloy under varying stress ratios (R). The results showed that the stress amplitude (σp) was the main factor controlling the fatigue crack propagation life (N). Additionally, detailed characterization of the fatigue crack propagation path was conducted using crystal models and EBSD. A fatigue crack propagation model for 2024 aluminum alloy under different R-values was established based on the grain twist angle and Schmid factor. Finally, the impact of R on the crack tip shielding (Ks) was systematically analyzed, elucidating the intrinsic mapping relationship between R, microstructure, and crack propagation characteristics.

Investigation on the interaction evolution and growth behavior of multiple cracks under fatigue loading

In this study, the cracks interaction is investigated on plates with different crack geometry parameters through finite element analysis and experiments. The presence of multiple cracks may lead to an enhancement or shielding effect on stress intensity factors (SIFs) depending on crack relative positions and sizes, thereby potentially accelerating or decelerating the crack propagation rate. When the crack tips of double parallel non-collinear cracks begin to overlap, the interaction between cracks transitions from an enhancement effect to a shielding effect, and the KII values reach their peak at this point, which are explained by the stress field distributions surrounding the cracks. For non-collinear double cracks with an inclined angle α, the shielding effect of inclined cracks becomes increasingly pronounced in comparison to horizontal cracks as the angle α increases. To verify the numerical results, fatigue crack propagation experiments were carried out on Al–Li alloy samples with double non-collinear cracks with different crack vertical distance. The experimental results show that growth direction of two non-collinear cracks underwent a transition when they overlapped, leading to load mode I changing into mixed mode of I + II. Based on both numerical simulations and experimental results, the material parameters C and m in Paris equation are modified, and then a new method for predicting the multiple cracks growth life is presented. By comparing with the experimental values, it was found that this method exhibits high accuracy and reliability.

A printable shear stiffening composite with enhanced fracture toughness and impact resistance for intelligent wearable applications

The continuous advancement in shear stiffening materials has sparked innovation in the field of flexible wearable devices. However, conventional preparation techniques for these materials have drawbacks such as limited toughness and lack of structural design. Herein, we propose a convenient fabrication method for shear stiffening elastomer (SSE) composite using a direct-ink-writing (DIW) assisted manufacturing technique. The influence of SSE structural design strategy on the mechanical behavior of composite is comprehensively investigated using crack propagation and dynamic impact experiments. Benefiting from the mechanically complementary effect of two-phase materials, coupled with robust interfacial adhesion, the composite with a densely arranged rectangular SSE structure exhibits high fracture toughness (2776.10 J⋅m−2) and impact force dissipation (61.78 %). Further, by integrating the printable composite with deep learning-based wireless sensing modules, a wearable elbow pad with attractive man–machine interaction is developed, which verifies the broad application prospect of this composite in intelligent protection.

A FEM/DEM adaptive remeshing strategy for brittle elastic failure initiation and propagation

AbstractThis article presents an adaptive remeshing strategy between the finite element method (FEM) and the discrete element method (DEM). To achieve this strategy, an edge‐to‐edge coupling method based on Lagrange multipliers has been set‐up to ensure the continuity of velocities at the interface. To switch from a computation initially purely FEM to a FEM‐DEM one, a field transfer method was required. In particular, a displacement field transfer method has been set‐up. The switching from a FEM subdomain to a DEM one is activated by a transition criterion. Each time a FEM subdomain is substituted by a DEM one, the DEM subdomain microscopic properties are set‐up with respect to the subdomain geometry and desired particle refinement. This is performed thanks to the linking to the so‐called “Cooker,” a tool distributed along with the GranOO Workbench. Two subdomain remeshing cases were dealt with: that of an initially FEM subdomain that is converted to DEM, and that of DEM subdomains which coalesce. A numerical test case shows that the dynamic remeshing method behaves as expected: FEM subdomains are substituted by DEM ones when the transition criterion is met, and DEM subdomains coalesce when required. The final numerical test case shows a good agreement with a crack propagation experiment of the literature, while a speedup of about 20 was observed when compared to pure DEM computation.

Fatigue failure analysis of U75V rail material under Ⅰ+Ⅱ mixed-mode loading: Characterization using peridynamics and experimental verification

Fatigue failure behavior of U75V rail material under Ⅰ+Ⅱ mixed-mode loading is analyzed and characterized in this work. A fatigue crack propagation experiment was carried out on U75V rail material under mixed-mode loading for R = 0.1. Then, based on the Ordinary State-Based Peridynamic (OSB PD) theory, bond strain rate and bond energy release rate methods were used to characterize the fatigue failure behavior of the rail material. Finally, the simulation results were compared with experimental results in terms of crack propagation length, path and angle to evaluate the characterization effects of both methods. The conclusions were as follows: under single mode I loading, both methods could accurately characterize the fatigue failure behavior of the rail material. However, compared with the bond strain rate method, the bond energy release rate method could provide a more accurate characterization under I + II mixed-mode loading.

Modeling brittle crack propagation for varying critical load levels: a dynamic phase-field approach

AbstractBrittle materials are known for their violent and unpredictable cracking behavior. A behavior which is dictated by a combination of microscopical material defects and the competition between the potential energy of the system and the surface energy of the material. In this study, we present the implementation of a dynamic fracture phase-field model with a new crack driving force into a commercial finite element (FE) solver and examine its behavior using three different tension-compression splits. After validating the implementation, we use the model to investigate its predictive capacity on quasi-statically loaded L-shaped soda-lime glass specimens with varying critical load levels. The dynamic fracture phase-field model predicted similar crack propagation to what was found in the literature for quasi-static and dynamic validation cases. By varying the critical load level for the L-shaped soda-lime glass specimens using the new crack driving force, the model predicted a positive correlation between the initial crack propagation speed and the critical load level, similar to what was seen in the experiments. However, the predicted crack propagation speed decreased quicker than the experimental crack propagation speed. The tension-compression splits had an impact on the predicted crack propagation paths. Overall, the proposed crack driving force used in the dynamic fracture phase-field model seems to capture the relation between critical load and initial crack propagation speed and thus enables crack predictions for specimens of varying strength.

Residual life assessment of deep rolled railway axles considering the effect of process parameters

Deep rolling is an efficient technique to increase the fatigue life of railway axles. The application introduces compressive residual stresses into the area near the surface, which are the most effective positive influence on fatigue behaviour and remaining service life in presence of a crack. This paper investigates the remaining service life of deep rolled railway axles. Previous research studies are used as basis for this work whereas, firstly, an elaborated crack growth model for common railway axle steels was developed. This model is based on experimental crack propagation results and can consider crack closure as well as load sequence effects and local residual stress conditions. Secondly, a numerical deep rolling simulation model validated with experimental residual stress measurements to determine the introduced residual stress state and to investigate the influence of various process parameters. In this study, the comprehensive outcome of both studies is connected in order to evaluate and compare the residual life of railway axles with the presence of different deep rolling-induced residual stress distributions in-depth. The crack growth study involves combinations of the deep rolling force as one of the most influencing process parameters, load scenarios as well as initial crack sizes and finally discusses the comprehensive results.

Re-Distribution of Welding Residual Stress in Fatigue Crack Propagation Considering Elastic–Plastic Behavior

The welding residual stress re-distribution behavior during fatigue crack propagation in butt-welded high-strength steel plates for ship construction is investigated based on experimental test results and numerical analyses. The specimens’ initial welding residual stresses are obtained from X-ray for middle tensile (MT) specimens cut from butt-welded high-strength steel plates. Then, fatigue crack propagation experiments on MT specimens are conducted, and a strain gauge is used to measure the residual stress re-distribution field around cracks. A practical fatigue crack propagation simulation procedure is developed with a dynamic update of in-situ welding residual stress, where the residual stress intensity factor Kres of the MT specimen is deduced. The stress ratio effect on Kres during fatigue crack propagation is analyzed and a good agreement between experimental and numerical results is achieved.

Crack propagation and mechanical properties simulation of G/(HfNbTaTiZr)C–Al2O3 gradient composites

The rapid development of computer simulation technology has provided theoretical and technical support for the performance prediction of ceramic materials. A two-dimensional simulation model was established to investigate the effect of the graphene (G) and gradient structure on the mechanical properties of (HfNbTaTiZr)C. Voronoi mosaics were applied in Abaqus to characterize the particle distribution of high entropy ceramics (HECs). The G/(HfNbTaTiZr)C–Al2O3 gradient ceramics were characterized through the formation of cohesive elements within grain boundaries and grains of each element using Python language. The fracture toughness, flexural strength and hardness of G/(HfNbTaTiZr)C–Al2O3 gradient ceramics were investigated through simulation model. The toughening mechanisms of G/(HfNbTaTiZr)C–Al2O3 gradient composites were investigated by establishing crack growth model. The simulation consequences are consistent with the experimental results in the literature, which verifies the accuracy of the simulation consequences. The toughening mechanism is validated through crack propagation experiments and its correctness is testified. Therefore, the simulation consequences are conducive to improving the design efficiency of HECs.

Crack propagation and induced vibration characteristics of cracked cantilever plates under resonance state: Experiment and simulation

Cracks in aero-engine blades have caused great economic losses and safety hazards in the past decades. High cycle fatigue, as one of the main causes of blade crack propagation, has been of increasing interest in recent years. In this paper, the cantilever plate specimens are selected as the research object to analyze the real crack propagation path and the change of resonance frequencies under the resonance state. First, a joint simulation analysis method based on FRANC3D and ANSYS software is proposed for the analysis of the crack propagation path. The proposed method is verified through crack propagation experiments. Second, a dynamic model for cantilever plates with a real crack propagation path is proposed based on the shell element theory. Finally, the associations between crack propagation lengths and resonance frequencies are studied. Some interesting phenomena are found: the resonance frequency decreases as the crack propagation length increases and the resonance frequencies show a slow decrease, then a constant decrease, and finally a rapid decrease; the nearer the initial crack location is to the root of the cantilever plate, the greater the curvature of the crack propagation path. This study can provide a better understanding of blade fatigue and the reference for analysis of blade crack propagation paths.

Fatigue Crack Propagation Behavior and Life Prediction of Welded Joints of SMA490BW Steel for Bogies

In this paper, the precrack propagation behavior of an SMA490BW welded joint in the core zone and heat-affected zone was analyzed through experiments based on the FRANC 3D and ABAQUS cosimulation method, and life prediction of these cracks was also investigated. The results show that the experimental crack propagation path and crack propagation rate in the weld core zone and heat-affected zone agree well with the simulation results, with a small relative error. The life span of the prefabricated crack in the heat-affected zone was higher than that in the core zone. Furthermore, regarding the life prediction results for the two samples, the relative error obtained using FRANC 3D was about 5%, which was verified using the method of joint simulation of crack propagation behavior in the zone simulation and their life prediction with accurate feasibility.

New model of crack propagation of aluminium wire bonds in IGBT power modules under low temperature variations

In this paper, a new model for wirebond degradation during power cycling in IGBT power module is proposed. This model is based on experimental crack propagation analyses and on a plastic strain empirical law. For the experiments, two power cycling tests in switching mode under high voltage were carried out respectively with junction temperature swings of ∆ Tj=30°C and 40°C at a minimal temperature of Tj,min=55°C. The DUTs and the test conditions were chosen so that only degradations of chip top-side interconnections could occur. The on-state voltage (VCE) was measured online during the tests as an indicator of wire and metallization degradations, moreover, the samples were removed at different aging stages. The removed samples were cross-sectioned in order to observe the crack propagation evolution with cycling. Concerning the plastic strain law, we used results from literature giving the evolution of the plastic strain variation (∆εpl) with bond-wire contact crack growth. As results, this new lifetime prediction model based on a modified Paris's law for aluminium wire bonds fatigue of IGBTs shows a good fitting of the test results and is it can be used to predict the lifetime under other load conditions. Finally, the lifetime of tested IGBT modules are estimated and used to verify the effectiveness of the proposed model.

Development of Fatigue Life Model for Rubber Materials Based on Fracture Mechanics.

In this paper, the research on the fatigue damage mechanism of tire rubber materials is the core, from designing fatigue experimental methods and building a visual fatigue analysis and testing platform with variable temperature to fatigue experimental research and theoretical modeling. Finally, the fatigue life of tire rubber materials is accurately predicted by using numerical simulation technology, forming a relatively complete set of rubber fatigue evaluation means. The main research is as follows: (1) Mullins effect experiment and tensile speed experiment are carried out to explore the standard of the static tensile test, and the tensile speed of 50 mm/min is determined as the speed standard of plane tensile, and the appearance of 1 mm visible crack is regarded as the standard of fatigue failure. (2) The crack propagation experiments were carried out on rubber specimens, and the crack propagation equations under different conditions were constructed, and the relationship between temperature and tearing energy was found out from the perspective of functional relations and images, and the analytical relationship between fatigue life and temperature and tearing energy was established. Thomas model and thermo-mechanical coupling model were used to predict the life of plane tensile specimens at 50 °C, and the predicted results were 8.315 × 105 and 6.588 × 105, respectively, and the experimental results were 6.42 × 105, with errors of 29.5% and 2.6%, thus verifying the accuracy of thermo-mechanical coupling model.

Computation of energy evolution during the dynamic fracture of elastomers using the finite viscoelastic model: Its implementation in Abaqus

Cracks propagating in elastomers consume the strain energy available in the body through viscoelastic effects and the creation of new fracture surfaces. Determining the energy consumed by the dissipative and the fracture processes explicitly helps understand the fracture mechanisms better and if they are rate-dependent. In literature, crack propagation experiments are usually performed on thin sheets of elastomers. The displacement fields in the body as the crack propagates through it during the experiments can be accessed using the Digital Image Correlation (DIC) technique. These displacement fields can then be used as boundary conditions in finite element analysis to compute the energy evolution in the body. Using an appropriate material model to describe the bulk material, the strain energy and the viscoelastic dissipation can be computed, thereby allowing to study the energy evolution in the body as the crack passes through.In the current study, the behavior of the elastomer material through which the crack passes has been described by the finite viscoelastic (FV) model. In such models, the viscoelastic effects are described by using some internal variables, whose evolution is prescribed through certain constitutive equations. This article first discusses the implementation of the FV model for plane stress conditions and applies it to study the dynamic fracture of elastomer membranes. The crack propagation through an elastomer is simulated by imposing the displacement fields extracted from the experiments along the crack faces onto the finite element model (thereby implicitly imposing the crack speed). The energy evolution in the body is studied to compute the energy release rate and the dissipation in the body as a consequence of viscoelastic effects. A significant portion of the energy has been observed to be consumed as the viscoelastic dissipation in the bulk material as the crack propagated.

Temporal evolution of crack propagation characteristics in a weak snowpack layer: conditions of crack arrest and sustained propagation

Abstract. For a slab avalanche to release, we need sustained crack propagation in a weak snow layer beneath a cohesive snow slab – a process we call dynamic crack propagation. Field measurements on crack propagation are very scarce. We therefore performed a series of crack propagation experiments, up to 9 m long, over a period of 10 weeks and analysed these using digital image correlation techniques. We derived the elastic modulus of the slab (0.5 to 50 MPa), the elastic modulus of the weak layer (50 kPa to 1 MPa) and the specific fracture energy of the weak layer (0.1 to 1.5 J m−2) with a homogeneous and a layered-slab model. During crack propagation, we measured crack speed, touchdown distance, and the energy dissipation due to compaction and dynamic fracture (5 mJ m−2 to 0.43 J m−2). Crack speeds were highest for experiments resulting in full propagation, and crack arrest lengths were always shorter than touchdown lengths. Based on these findings, an index for self-sustained crack propagation is proposed. Our data set provides unique insight and valuable data to validate models.

A robust Moore–Penrose pseudoinverse-based static finite-element solver for simulating non-local fracture in solids

In this work, we develop a pseudoinverse-based static finite-element solver to model the elastic deformation and non-local brittle fracture of solids. The pseudoinverse of the finite-element stiffness matrix is calculated using a QR decomposition-based method which is faster than using the traditional approach based on the singular value decomposition method. This new finite-element framework has two advantages: (1) the finite-element equations can still be solved even when the finite-element stiffness matrix is singular, and (2) there is no need for introducing an artificial elastic rest energy or viscous regularization for the sole purpose of keeping the finite-element stiffness matrix non-singular in order to solve the finite-element equations. We also show that the proposed method is robust in solving chosen boundary value problems involving the elastic deformation and abrupt non-local mode I and mixed-mode brittle fracture of solids when compared to the traditional element kill method where a small but finite residual stiffness is maintained at a material in order to prevent the finite-element stiffness matrix from being singular. Hence, this proposed method allows the modeling of separation/fragmentation of solids when fracture occurs.Finally, we use the new computational framework to model the fracture of PMMA beam samples at room temperature. By calibrating the material parameters in the constitutive theory using analytical methods and fitting to a Mode I fracture experiment force–displacement response, we show that our newly-proposed computational method is able to predict the experimental fracture loci and crack propagation characteristics in PMMA beam samples undergoing mixed-mode fracture conditions to good accord.

Initiation and propagation behaviour of cohesive cracks at high temperatures: A dynamic experimental study

Temperature, as an important factor, affects the fracturing effect in deep reservoirs, so it is urgent to carry out high-temperature crack propagation experiments. Here, by combining a high-speed camera and DIC technology, Brazilian splitting tests of three types of rocks at different temperatures are carried out to study the crack initiation and propagation characteristics from a mesoscopic perspective. A new way to identify the cohesive crack tip is proposed based on the high-temperature cohesive zone model and opening displacement patterns. In addition, the dynamic propagation process and changes in the characteristics of cohesive cracks at different temperatures are analysed. The experimental results demonstrate that dynamic process of high-temperature cohesive crack initiation and propagation can be subdivided into three stages: stress concentration, cohesive microcrack initiation and propagation and cohesive macrocrack penetration. Interestingly, multiple interconnected rather than single cohesive microcracks develop in the centre region of sandstone and limestone specimen surfaces at 260 ℃. Notably, the duration of the stress concentration stage of sandstone and limestone shows an increasing trend with temperature. The average propagation speed and the opening displacement change rate of the cohesive crack in sandstone, limestone and shale specimens show a declining trend with increasing temperature. The results can provide some insights into the high-temperature crack propagation mechanism in deep reservoirs.

Revealing the fatigue crack propagation mechanism of a Ni-based superalloy electron beam welded joint through in-situ SEM observation

In this study, in-situ scanning electron microscope (SEM) fatigue crack propagation experiments for electron beam welded joints of GH4169 Ni-based superalloy are carried out. Based on material characterization and analysis, the corresponding fatigue crack propagation mechanism of the welded joint (WJ) is investigated. Compared to the base material (BM), the WJ has comparable static strength, but worse plasticity. Significantly, the WJ has longer fatigue crack growth life than that of the BM. The fatigue crack growth rate (FCGR) curves of the WJ and the BM intersected with each other. In particular, under low stress intensity factors (ΔK) level, the FCGR of the WJ is lower than that of the BM. Under high ΔK level, the FCGR of the WJ is higher than that of the BM. The steep FCGR curve of the WJ can be attributed to the unique weld texture and a shielding effect caused by the weld strength mismatch.