Dynamic Crack Growth Research Articles

Experimental studies of crack growth kinetics and bearing capacity of steel pipes of underground water distribution systems

It was established that the value of the critical stress SK for all experimental steels increases with the increase in the service life, and the impact viscosity decreases, which indicates the structural embrittlement of the pipe steels associated with their sudden flooding. It is shown that the new 20 FA steel has the highest visco-plastic properties and resistance to brittle fracture, which is economically modified with a carbide-forming element (vanadium) and has a fine-grained structure and a low conten to fharm fulimpurities (sulfur, phosphorus). The microstrain of the α-Fe crystallattice, as well as the quantitative decay of cementite andt here distribution of carbon between ferrite and pearlite, were estimated by X-ray diffraction methods. The new steel grade 10FA is recommended for use in the construction of oil and gas pipelines and, for example, bridge structures, which are constantly under cyclic loads with simultaneous contact with a corrosive and aggressive environment. For the first time, the influence of thes ervice life of oil pipelines on the hydrogen content and microcracks in pipe steels of oil pipelines was determined.

Dynamic crack growth propagation of Al 2024 thin plates reinforced with carbon nanotube subjected to non-proportional cycling loading

In this study, the dynamics crack growth in a thin plate made of aluminum alloy 2024T3, which was coated with two types of coating, electrocoating (Ni-CNTs) and electroless coating (Ni-CNTs), were investigated under a non-proportional cyclic loading. Pure multi-walled carbon nanotubes (MWCNTs) were added to the coating solution at a concentration of 2 g/l to enhance the mechanical properties and study the effect of adding carbon nanotubes to the coating solution.Fracture mechanics analysis was performed using two methods. First, a novel apparatus was designed for experimental analysis of the application of a non-proportional cyclic loading, consisting of a repeated, cyclic inplane shear stress with constant tension stress. Secondly, the numerical analysis was carried out using ABAQUS 2019 programming to obtain numerical results and compare them with the experimental results to validate the study. Paris law parameters, including Paris law constants c and m, K (change in stress intensity factor), and da/dN (crack growth per cycle), which were included in the ABAQUS software to simulate the model and compare the results with the experimental data.The specimens were analyzed using XRD, FESEM, and EDXRF after the coating process to determine the quality, distribution, and thickness of the coating layer on the specimen's surface. The hardness tests showed a 57% increase in the hardness of the specimens coated with electrocoating and a 72% increase in the hardness of specimens coated with electroless coating. The Young's modulus also showed an increase of 37.5% for specimens coated with electrocoating and 50% for specimens coated with electroless coating, compared to specimens without coating.

Dynamically propagating cracks in anisotropic plates subjected to hyperbolic thermal shock

ABSTRACT In this paper, dynamically propagating cracks in anisotropic plates exposed to a generalised thermal shock are investigated. Here, the governing equations are considered according to the Lord-Shulman (LS) model as a fully coupled generalised thermoelasticity theory. The crack is modelled in the context of the eXtended Finite Element Method (XFEM). To evaluate Stress Intensity Factors (SIFs), a technique based on the J integral and crack tip opening and sliding displacements, previously applied for stationary cracks, is developed for a dynamically propagating crack in orthotropic solids. Besides, a variety of functions for enriching the displacement field are derived relying on the behaviour near the tip of a dynamically propagating crack in anisotropic materials. The maximum hoop stress criterion is also modified by using the asymptotic stress field for a dynamically propagating crack. In addition, the effect of the angular variation of fracture toughness in orthotropic materials is accounted in computing the crack propagation speed. In some examples, dynamic crack growth in complex cracked orthotropic structures subjected to LS thermal shock is studied in detail. The impacts of fibre orientations and LS relaxation time are also investigated in these examples. In conclusion, applying non-Fourier thermal shock for modelling dynamically propagating cracks in orthotropic structures is recommended to obtain more realistic results. In addition, by increasing the LS relaxation time, the crack grows less oscillatory and is more aligned along the material fibres.

Features of Shear Crack Growth Dynamics in Heterogeneous Brittle Materials

Features of Shear Crack Growth Dynamics in Heterogeneous Brittle Materials

Finite element modeling of dynamic fracture in concrete through the initial fracture toughness-based criterion and field variable transference technique

Objective of the present study is to model the mode I dynamic crack propagation of concrete three-point bending (TPB) beams by using an initial fracture toughness-based criterion, and to evaluate the contribution of the rate and the inertia effects to the dynamic responses of concrete beams during crack growth. Firstly, the theory and implementation of the initial fracture toughness-based criterion are described in detail. Specifically, a field variable transference (FVT) technique is introduced to realistically reflect the coupling effect of the moving cracks and stress waves. Then, analysis procedures for static and dynamic crack growth are established. The comparison between the simulated and experimental impact and reaction forces indicates that, the initial fracture toughness-based criterion can accurately predict the mode I dynamic crack extension in concrete TPB beams. Moreover, the effects of the inertia and rate effects on dynamic responses (i.e., load, deformation, crack propagation velocity, and energy evolution and partition) are evaluated by performing numerical experiments on the mode I crack growth of TPB beams under a wide range of loading rates from 0.0002 mm/s to 2000 mm/s. It is concluded that, for the adopted concrete TPB beams, the inertia effect can be neglected and only the rate effect needs to be taken into account if the loading rate does not exceed 2 mm/s. Otherwise, if the loading rate exceeds 500 mm/s, only the inertia effect needs to be considered. In addition, the effect of damping on dynamic responses is investigated, and it is found that the damping effect can be ignored at high loading rates.

Experimental and numerical analysis of dynamic splitting mechanical properties and crack propagation for frozen sandstone

To reveal the dynamic tensile properties and crack growth law of frozen sandstone under impact loading, dynamic splitting tests (−30–20 °C) with different strain rates were carried out using a split Hopkinson pressure bar (SHPB), and the crack growth process of the specimens was captured using a high-speed camera. The study focused on the analysis of dynamic tensile mechanical properties, energy dissipation, crack growth, and failure modes of frozen sandstone. Numerical simulation analyses were also performed using LS-DYNA. The results indicated that: (1) The dynamic tensile strength has an obvious strain rate enhancement effect, and the strain rate sensitivity coefficient increased with a decrease in temperature. The Logistic function can characterize the relationship between the dynamic strain rate enhancement factor and the strain rate of frozen sandstone. (2) In the dynamic Brazilian disk (BD) test, the energy absorption rate showed a transition between the strain rates of 40 s−1 and 50 s−1, and there were two main failure modes of the specimen. The Y-direction secondary crack in mode II was caused by tensile stress in the Z-direction. (3) In the dynamic BD test, the central main crack (CMC) did not occur at the central position first, and its Z-direction propagation rate was linear with the strain rate. In the dynamic flattened Brazilian disk (FBD) test, the condition of CMC initiation was that the platform loading angle (2α) was greater than 10°. The dynamic tensile strength increased with an increase in 2α, and the correction coefficient was introduced in the calculation. This study can serve as a theoretical reference and basis for the study of the dynamic tensile properties of rock-like materials.

Inversion-based correction of Double-Torsion (DT) subcritical crack growth tests for crack profile geometry

Because of its simplicity and the ability to produce a stable, slow-propagating crack, the Double-Torsion (DT) method has been used widely for investigating the critical and subcritical propagation of a slow-propagating tensile (mode-I) crack. However, to determine the complex relationship between the crack velocity vc vs. the strain energy release rate G (or the stress intensity factor K) from laboratory measurements, several corrections must be made to account for the impact of sample and crack geometry. Particularly, DT test typically produces a crack with a curved edge profile instead of a straight line, causing the local vc and G vary along the crack front. The experimentally measured vc and G data merely reflect collective, averaged behavior of the crack. This makes inversion for the intrinsic, “true” crack growth kinetics necessary, based upon the knowledge of the crack geometry. Simple and effective correction methods have been proposed and validated for the slow, chemical-reaction-controlled part (Region I) of the vc−G curve. However, reliable methods for the highly nonlinear, transport-dominated part (Region II) and its sudden transition to the dynamic propagation part (Region III) are still lacking. In this paper, we propose a method for determining the intrinsic vc−G relationship across all three Regions based upon DT test data, using a simple model function and its numerical inversion. The performance of this approach is examined and demonstrated using both synthetic and laboratory data for subcritical crack growth in soda lime glass.

Dynamic crack propagation under thermal impact

This paper addresses the dynamic crack growth in an isotropic media subjected to a non-classical thermal shock by implementing the Green-Naghdi (GN) generalized thermoelasticity theory in the eXtended Finite Element Method (XFEM). To satisfy the crack tip singularity for modeling dynamic crack growth via XFEM, proper enrichment functions for field variables are needed, which are extracted from the asymptotic fields for a dynamically growing crack in a thermoelastic media. These near tip fields of a dynamically moving crack are also used in the formulations of the maximum circumferential stress criterion and the equivalent stress intensity factor. Finally, numerical examples of the dynamic crack propagation under thermal shock are given out, and the effect of the dissipation coefficient in the GN model and the thermal shock amplitude on the crack growth path is studied in detail. In addition, the crack propagation under GN thermal shock is compared with that under classic thermal shock. The main difference between them is discussed in a numerical example.

Assessment of Commercially Available Polyethylene Recyclates for Blow Molding Applications by a Novel Environmental Stress Cracking Method.

The transition to a circular economy has a major impact on waste management and the reuse of materials. New mandatory recycling targets for plastics will lead to a high availability of recyclates. For these recyclates, useful applications need to be found. One potential application for recyclates is blow molding bottles as packaging for non-food contents. This study investigates commercially available post-consumer high-density polyethylene recyclates together with virgin blow molding grades in terms of their short-term mechanical properties and environmental stress cracking resistance. While the short-term mechanical properties showed only slightly lower performance than the tested virgin grades, the overall environmental stress cracking failure times of the recyclates were much lower compared to virgin materials, even though the crack-growth kinetics could be similar. Although neither the tensile nor the notched impact strength results of the two polyethylene recyclates revealed large differences, the stress intensity-factor-dependent crack-growth rates of both materials were significantly different.

Fatigue crack growth kinetics in titanium alloys at different interaction of cyclic loading amplitudes

Fatigue crack growth kinetics of various titanium alloys BT6 (α+β structure) and 3M (α-structure) under constant amplitude loading with different types of cyclic disturbances in the form of over- and underloading at a loading frequency of 25 Hz was investigated.The tests were conducted on a servohydraulic testing machine under controlled load with simultaneous automatic recording of loading parameters and readings from a crack opening gauge. Tests were carried out on compact specimens with an edge crack which were subjected to off-centre loading. The crack opening data was converted into crack length using a yield strength measurement method. Experimental results, namely fatigue crack growth curves and fatigue fracture kinetic diagrams, were analyzed. It is shown that crack growth kinetics depends mainly on the type of alternating loading amplitude sequences. Reliability estimation of crack growth on the basis of crack opening gauge and study of fractographic features of materials fracture has been carried out.

Damage Monitoring of Ceramic Matrix Composites Under Tension Loading Via NDE-based DIC Approach

Environmental barrier coatings (EBCs) are used as a coating material on fiber-reinforced ceramic matrix composites (CMC) for various aerospace and turbine engines applications. In order to validate physics-based analytical models for predicting the spallation life of EBCs, the fracture strength of the EBC and the kinetics of crack growth in EBC layers need to be experimentally determined under engine operating conditions. In this study, a coating layer of barium strontium aluminum silicate (BSAS)–based melt-infiltrated silicon carbide fiber-reinforced silicon carbide matrix composite (MI SiC/SiC) is applied on a CMC specimen and tensile tested at room temperature. Multiple tests are performed on a single specimen with increasing predetermined stress levels until final failure. Damage progression due to the load applied is monitored using a digital image correlation (DIC) system. After unloading from the predetermined stress levels, the specimen is evaluated by optical microscopy and computed tomography (CT). The inspection forms the imaging which implied that primary and secondary cracks developed during tensile loading until failure. DIC showed formation of a primary crack at ~50% of the ultimate tensile strength, and this crack grew with increasing stress and eventually led to final failure of the specimen.

A dynamic description of material brittle failure using a hybrid phase-field model enhanced by adaptive isogeometric analysis

The objective of this work is to study dynamic crack propagation in brittle materials under time-dependent loading conditions by using the recently developed adaptive isogeometric phase-field approach. The current approach owns several ingredients including the advantages of the phase-field method (PFM), which can be used to model complex crack morphologies without any fracture criterion, and the isogeometric analysis, which possesses some excellent features such as exact geometry and high-order continuity. In addition, the model is further enhanced by employing the locally refined non-uniform rational B-spline (LR NURBS) basis, which is utilized for spatial discretization and phase-field discretization, while the generalized-α or HHT-α method is employed for the temporal discretization. For the coupled elasto-phase field system, a staggered approach is adopted to compute the unknown field variables. Moreover, a hybrid formulation of the PFM is employed to recover the linearity of momentum equation. The structured mesh adaptive refinement is conducted based on the prescribed threshold on the phase-field variable, thus it can effectively alleviate the burden of computational overhead, which is the main shortcoming of the PFM. Three problems involving dynamic crack growth is solved to show the robustness and the applicability of the proposed approach. It is shown that the proposed approach can obtain accurate results at a fraction of degrees-of-freedom without compromising accuracy in comparison with uniformly refined mesh.

A 2D image-based multiphysics model for lifetime evaluation and failure scenario analysis of self-healing ceramic-matrix mini-composites under a tensile load

We propose a multi-physics numerical model for a self-healing ceramic matrix mini-composite under tensile load. Crack averaged PDEs are proposed for the transport of oxygen and of all the chemical species involved in the healing process and studied in the dimensionless form to perform the most appropriate discretization choices concerning time integration, and boundary conditions. Concerning the fibres’ degradation, a slow crack growth model explicitly dependent on the environmental parameters is calibrated using a particular exact solution and integrated numerically in the general case. The tow failure results from the statistical distribution of the fibres’ initial strength, the slow crack growth kinetics, and the load transfer following fibres breakage. The lifetime prediction capabilities of the model, as well as the effect of temperature, spatial variation of the statistical distribution of fibres strength, and applied load, are investigated highlighting the influence of the diffusion/reaction processes (healing) on the fibre breakage scenarios.

Incorporation of high‐altitude environmental effects in the linear elastic fracture mechanics‐based modeling of aluminum alloy 7075

AbstractAirframe structural components experience coupled load–environment spectra that are currently not captured in traditional damage‐tolerant management approaches. The effect of incorporating fatigue crack growth kinetics relevant to high‐altitude flight environments into LEFM‐based predictions of fatigue progression for aluminum alloy 7075‐T651 was quantitatively assessed for a variety of load–environment spectra using AFGROW. The results of this study demonstrated that in all conditions, the incorporation of high‐altitude‐relevant growth kinetics enhanced the fatigue life predictions. However, the magnitude of the impact on the overall life is dependent upon the subtleties of the coupled load–environment spectrum, despite the fact that environmental effects are most potent in the near‐threshold regime of the da/dN versus ΔK relationship. The potential impact to the structural integrity management community is discussed, and remaining challenges and opportunities are identified.

Fatigue crack growth in bearing steel under cyclic mode II + static biaxial compression

Mode II fatigue crack growth under reversed shear and static biaxial compression was investigated in two bearing steels. Many aborted branches, quasi-orthogonal to the main crack, were observed along the crack face. The compressive stress parallel to the main crack hindered the growth of these branches and favored coplanar mode II crack growth. The crack face sliding displacement profiles measured by DIC were used to derive ΔKII,eff, at the main crack tip, using elastic–plastic FE simulations with crack face friction, by an inverse method. Friction-corrected crack growth kinetics were obtained for mode II crack growth in both steels.

Crack Growth Kinetics in Structural Materials and Their Welded Joints

Crack Growth Kinetics in Structural Materials and Their Welded Joints

Development and implementation of a simultaneous fatigue crack growth test setup for polymeric hybrid laminates

Delamination is a common and highly relevant in-service failure mode of laminated structures. Cyclic mechanical loads superimposed with environmental factors, such as hot-humid air, are most critical. To evaluate the cyclic debonding resistance of polymeric hybrid laminates, fracture mechanics approaches based on double cantilever beam (DCB) specimens are well established. However, conventional fatigue test setups allow for measurement of just one specimen which is rather time consuming. The main objective of this paper was to develop and implement a novel fatigue test setup for simultaneous characterization of multiple DCB specimens. To validate the simultaneous fatigue testing system, steel/epoxy laminates of different number of steel plies and epoxy adhesive layers with or without filler were characterized using the novel fatigue test setup and a conventional electrodynamic test machine. Fatigue crack growth data was fitted by a form of the Hartman-Schijve crack growth equation. To derive a statistically significant threshold value a fitting procedure was employed. A good agreement of the crack growth kinetics data at ambient conditions was obtained confirming the high quality and reliability of the novel simultaneous fatigue test setup. Moreover, reliable data were gathered at the beginning of the displacement–controlled experiments in the instable crack propagation regime, as the displacement amplitude was reached from the first cycle onward. The novel test methodology is of special relevance for development and screening of adhesive formulations.

Three-layer phase-field model of finite strain shell for simulating quasi-static and dynamic fracture of elasto-plastic materials

Shell structures are widely used in the aerospace, automobile, pipeline, and construction industries. Under quasi-static and dynamic loading, the failure modes of the thin and thick shells are quite different. To explain the correct fracture behavior of the moderated thick shell under bending loads, this paper proposed a new three-layer phase-field model for elasto-plastic materials based on the crack regularized phase-field model. Three independent phase-fields corresponding to the shell’s upper, middle, and lower surfaces are used. This provides a realistic behavior in bending-dominated problems, which is illustrated in the classical beam and plate problems. Five typical numerical examples are given, including the bending of bi-clamped beam, the Muscat-Fenech and Atkins plate problem, the dynamic crack growth problem in a stiffened cylinder under internal pressure, the radial cracks problem in perforated sheets, and the circular steel plate subjected to an underwater blast load. All the problems have been successfully solved, and the proposed phase-field model is proved to be effective.

Temperature dependent fatigue crack growth in electrical steel laminates with μm-thick epoxy layers

The effect of adhesive layer thickness on monotonic roll peel strength and fatigue crack-growth kinetics was investigated for electrical steel laminates with bond line thickness values ranging from 7 to 48 μm. Roll peel strength was increasing with epoxy layer thickness up to 20 μm. In contrast, roll peel strength was leveling at higher thickness values. The fatigue fracture mechanics approach in mode I delivered an equivalent ranking such as roll peel testing. However, energy-based fracture mechanics analysis was much more sensitive than monotonic roll peel testing, also at thicker bond lines. The different failure modes were correlated with the plastic deformation zone size developing at the propagating crack. In case of a constrained plastic zone, cohesive failure mainly within the epoxy layer was ascertained. In contrast, interfacial failure and interestingly also a better fatigue crack growth resistance was deduced for a weakly or non-constrained plastic zone. Regarding the temperature dependency, an enhanced fatigue crack resistance was obtained at 60 °C. At this temperature about 50 °C below the maximum of the loss factor of the epoxy adhesive, the effect of epoxy layer thickness was much smaller than at 23 °C. At testing temperatures slightly below or above the glass transition temperature, the fatigue crack propagation rate was a factor of 10 to 100 higher.

Fatigue fracture of steel with ferrite-martensite composite structure

The article presents results of the fatigue tests of natural steel composite material for cyclic bending in a zero loading cycle. Natural ferritemartensite composite (NFMC) has a structure of alternating layers of viscous ferrite and strong martensite, which determines special mechanism of crack deceleration under loading. Zero loading cycle presumes presence of tensile forces directed only in one side, which makes it possible to avoid work hardening of crack edges during its growth. A diagram of fatigue fracture was constructed using data obtained on kinetics of fatigue crack propagation and its growth rate, depending on the number of vibration cycles. Comparison of test results for the samples made of steel of the same chemical composition was carried out. In one case, the secondary sorbite structure ran through the traditional heat treatment. In the other, quenching of the initial row ferrite-pearlite structure in intercritical temperature range, led to obtaining ferrite-martensite composite layered structure. These materials had the same hardness, but the difference in structure organization caused the NFMC structure steel advantage in terms of resistance to fracture under cyclic loading. When crack approaches the martensite-ferrite interface, delamination occurs in ferrite due to tensile stresses parallel to the crack plane. Growth of a crack stops before additional energy is supplied for a new crack generation under conditions close to the uniaxial stress state. Method for determining characteristics of kinetics of crack growth under fatigue loading is presented and recommended for testing steels and alloys under conditions of cyclic load changes.