Crack Propagation Speed Research Articles

Quantity effect of radial cracks on the cracking propagation behavior and the crack morphology.

In this letter, the quantity effect of radial cracks on the cracking propagation behavior as well as the circular crack generation on the impacted glass plate within the sandwiched glass sheets are experimentally investigated via high-speed photography system. Results show that the radial crack velocity on the backing glass layer decreases with the crack number under the same impact conditions during large quantities of repeated experiments. Thus, the “energy conversion factor” is suggested to elucidate the physical relation between the cracking number and the crack propagation speed. Besides, the number of radial crack also takes the determinative effect in the crack morphology of the impacted glass plate. This study may shed lights on understanding the cracking and propagation mechanism in laminated glass structures and provide useful tool to explore the impact information on the cracking debris.

Molecular dynamics simulation of crack propagation behaviors at the Ni/Ni3Al grain boundary

The primary purpose of this paper is to study intergranular and transgranular crack propagation behavior at the grain boundary (GB) of Ni/Ni3Al by Molecular Dynamics (MD) simulation. Cracks are loaded in tension mode I. The stress–strain curves, the changes of dislocation network, the crack propagation speeds and the changes of interfacial energy are compared with those of two models. The results show that the yield stress of the intergranular crack model is greater than that of the transgranular crack model. It is found that the intergranular crack propagation does not extend along the GB, but the crack extends into the γ′ phase. In addition, the transgranular crack extends at an angle of 45° and the dislocation moves on the slip system {11} [01]. The transgranular crack model breaks completely under a small strain. Moreover, the tranagranular crack propagation results from deformation damage of the model, and its propagation mechanism is shear rupture along the slip plane. The intergranular crack initiates propagation at the most dense position of the slip bands.

Experimental procedure to characterize the mode I dynamic fracture toughness of advanced epoxy resins

This research aimed at developing an experimental procedure to characterize the dynamic fracture toughness of a fast running crack in advanced epoxies enriched with thermoplastic particles and demonstrating quasi-brittle fracture. A strain gauge method was defined to determine the dynamic stress intensity factor KID for the mode I crack opening with various crack speeds. This strain gauge measurement was associated with high speed cinematography on a three point bending test dedicated to the fracture of notched specimens made of the Hexply®M21 epoxy resin. Results demonstrate that the crack propagation speed has a large influence on the dynamic fracture toughness of advanced epoxy resins.

Velocity-Toughening and Crack Speed Oscillations in the Dynamic Fracture of PMMA Plates

To study the propagating behavior of a dynamic crack in brittle materials, an experimental technique was developed that measures the propagation speed of a fast crack in a preloaded brittle strip. The specimen is a rectangular PMMA plate clamped by two heavy steel fixtures that are attached to a MTS test machine. After the specimen was tensile loaded to a prescribed load level, a sharp crack was initiated at the middle point on one side of the specimen. Due to the brittleness of the material, the crack propagates quickly across the specimen. The propagation velocity of the crack was measured by using the equally spaced conductive lines that were painted on the specimen surface before the tests. High speed video camera was also used for monitoring the propagation behavior of the cracks. The experimental results show that for each preloaded plate, the crack arrives at a steady velocity v0 after a short acceleration stage. In the steady stage crack propagation is in self-similar state. The steady velocity of the crack was found to be an increasing function of the energy Gc stored in the preloaded strip, which means that the material has a "velocity-toughening" property. When the crack speed exceeds a threshold, the crack exhibits apparent speed oscillations. This speed oscillation corresponds to the occurrence of the periodic grooves on the fractured surface. Further increase of the preload results in the bending, micro branching, and full bifurcations of the crack paths.

Propagation of a semi-infinite conducting crack in piezoelectric materials: Mode-I problem

In this paper, the mode-I transient response of a semi-infinite conducting crack propagating in a piezoelectric material with hexagonal symmetry under normal impact loading is investigated. The integral transform methods together with the Wiener–Hopf technique are used to solve the mixed boundary value problem under consideration. The solutions of the coupled fields are derived for two cases, i.e., generalized Rayleigh wave exists or not. The dynamic stress intensity factor and dynamic electric displacement intensity factor as well as their universal functions are obtained in a closed form. The numerical results for two universal functions are provided to illustrate the characteristics of dynamic crack propagation. It is found that the universal functions for the dynamic stress and electric displacement intensity factors vanish when the crack propagation speed reaches the generalized Rayleigh speed which is the propagation speed of surface wave in a piezoelectric half-space with metallized surface. It is noted that the electromechanical coupling coefficient has an important influence on the dynamic fracture characteristics.

High-speed camera study of Stage III crack propagation in chemically strengthened glass

Thermally or chemically strengthened glass is more resistant to damage and breakage compared to non-strengthened glass. Both strengthening mechanisms are based on incorporation of a compressive stress profile in the surface of the glass, which must be balanced by an equivalent amount of integrated tensile stress in the interior of the glass. This tensile stress is believed to affect the kinetics of Stage III crack propagation upon fracture of the sample. In this study, we use a high-speed camera to perform direct measurement of the kinetics of Stage III fracture in a strengthened glass sample. Data including crack propagation speed, crack bifurcation distance, and bifurcation angles are collected at a rate of 500,000 frames per second and then characterized. The authors believe that these data will provide a foundation for understanding the physics of Stage III fracture in strengthened glass samples.

Investigation of surface integrity of aluminum lithium alloy in high speed machining

Aluminum-lithium alloy takes the advantage of low density, high specific strength and specific stiffness that make it the candidate for the structural material in the new generation of aerospace vehicles. Aircraft skin is under the cabin pressure changes caused by alternating loads while the aircraft taking off and landing, so the fatigue strength of structural components is very important. Structural fatigue crack initiates from the surface of microscopic defects, and surface and subsurface mechanical state affects the crack initiation and propagation speed. The surface integrity includes surface contours, micro-hardness, microstructure, residual stress, etc. These factors affect the fatigue performance of the structure. Meanwhile, the surface integrity associates with the material character, machining parameters, and heat treatment process. This paper studies the surface integrity of aluminum- lithium alloy in mechanical milling process. Traditional skin structures are obtained using chemical machining methods, by which the plant occupy large area with serious pollution and the waste disposal costs higher. At present, studies on superplastic forming process of aluminum-lithium alloys are carried out extensively and those on surface integrity of aluminum-lithium alloy in cutting process are very lacking. In this paper we investigate the contributions of machining parameters and the previous rolling process on the surface integrity of aluminum-lithium alloy with liquid nitrogen and dry cutting conditions. Also we give the comparison of the surface topography as well as the residual stress in liquid nitrogen and dry conditions. The experiment results verify the enhancement of surface integrity of aluminum-lithium alloy in liquid nitrogen condition, which supplies the experimental basis for the improvement of the fatigue properties of aluminum-lithium alloy by using the mechanical milling process.

The Fracture During Drop-weight Tear Test of High Performance Pipeline Steel and its Abnormal Fracture Appearance

The dynamic cracking arrest capability is very important for pipeline in failure control. DWTT (Drop-Weight Tear Test) is widely applied as a standard test for evaluating crack arrest characteristics of pipelines. But for high performance pipeline steel, the abnormal fracture appearance is frequently observed on fracture surface of DWTT specimens and it make DWTT technique difficult to evaluate the dynamic toughness by the current test standard. In this study, various types of DWTT specimens of a commercial API-X70 pipeline steel were conducted at different temperature. The fracture appearances of specimens are observed. The intrinsic causes of abnormal fracture appearance are analyzed basing on the three-dimensional fracture theory. It is found that the dynamic propagation speed of crack in high performance pipeline steel is largely scattered. The propagation speed of fracture can change the three-dimensional stress constraint and lead to difference in the fracture appearance of specimen. Compress deformation, local thickness increase, toughness decrease and stress increase can make the crack dynamic propagation manner change from ductile to brittle and produce abnormal fracture appearance. The approaches to prevent or reduce the occurrence of abnormal fracture appearance for high performance pipeline steel are discussed in this paper.

Crack propagation in pre-strained single layer graphene sheets

We use molecular dynamics simulations to delineate crack propagation speed as a function of the crack length and the axial prestrain in a single layer graphene sheet. A covalent bond between two carbon atoms is assumed to break when the bond length has been stretched by 100%. For a pristine single layer graphene sheet the maximum axial force is attained at a nominal axial strain of 15.5%. A pristine graphene sheet is first deformed in tension in the armchair direction to the desired value of the axial strain, and then one or two cracks are simultaneously inserted in it at central locations by breaking the bonds. Five such problems have been studied with four different values of the axial prestrain up to 15.3%. For each problem, crack-tip speeds are found to reach steady state values as the crack elongates. The steady state crack speed increases with an increase in the axial nominal prestrain. The crack propagation is found to be stable in the sense that the value of the J-integral increases with an increase in the crack length. For the same normalized crack length the value of the J-integral increases with an increase in the nominal axial prestrain.

Shear and flexural characterization of grid-reinforced asphalt pavements and relation with field distress evolution

The use of geogrids at the interface of asphalt layers is currently adopted to improve pavement performance in terms of rutting, fatigue and reflective cracking. Several test methods have been proposed in order to simulate the complex mechanical behavior of reinforced pavements and assist practitioners in the selection of the appropriate reinforcement product. A particular subject of debate is the evaluation of geogrid effects in terms of both flexural strength and interlayer bonding. In this context, an interlaboratory experiment has been organized as part of the RILEM TC 237-SIB/TG4 with a twofold objective: to compare the predictive effectiveness of different experimental approaches and to analyze the behavior of different geogrid types. For this purpose two experimental reinforced test sections have been realized, the first one to prepare samples for the interlaboratory experiment, the second one to analyze the geogrid field performance under heavy traffic conditions. This paper describes the test results obtained by one participating laboratory on double-layered asphalt samples extracted from the first experimental section and compares them with the periodic visual observation of the reflective cracking evolution occurred in the second test section. The laboratory tests were performed following a specific testing protocol that combines interlayer shear tests, repeated loading tests in a four-point bending configuration and quasi-static three-point bending tests, in order to investigate the overall performance of double-layered asphalt systems. Results have shown that geogrid reinforcement does not noticeably influence the flexural stiffness and strength in the pre-cracking phase, whereas the crack propagation speed can be significantly reduced and the failure behavior may change from quasibrittle to ductile, depending on the interlayer shear resistance. Laboratory results were confirmed by periodic visual observation of field performance in terms of reflective cracking evolution.

Effect of Contact between the Crack Faces on Crack Propagation

In mixed-mode crack propagation the crack faces frequently touch each other. The ensuing friction is expected to decrease the crack propagation speed. This effect is usually not taken into account, however, a realistic prediction of this effect may increase the calculated life and consequently increase the length of the inspection intervals. In this paper, penalty contact conditions are introduced in between the crack faces of the automatically generated mesh in a cyclic crack propagation. Special attention is given to the contact formulation and the area in which contact is defined. It is shown that the resulting crack propagation rate is significantly reduced by the introduction of friction provided that positive Mode-I is not significantly involved.

Water Saturation Effects on the Brazilian Tensile Strength of Gypsum and Assessment of Cracking Processes Using High-Speed Video

This study investigates the water weakening effect on the tensile strength, as well as the fracturing behavior, of an artificially molded Hydrocal B-11 gypsum rock. Brazilian disc tests, with the aid of a high-speed video system to monitor and record the cracking processes, are conducted on dry and wet specimens to determine their tensile strengths. The dry specimens are oven-dried, while the wet specimens are prepared by soaking in water for 1, 3, and 10 weeks to achieve different levels of water content. The test results show that the tensile strength drops to nearly half of its dry value after being soaked in water for only 1 week. The tensile strength reduces only slightly further after the specimens have been immersed in water for 3 and 10 weeks. An analysis of the recorded high-speed footage shows that the primary crack initiates at the center as observed from the surface for the majority of the tested specimens. Most importantly, the cracking processes of dry and wet specimens are distinctly different with regard to the speed of crack propagation and the number of cracks developed.

Crack propagation induced by thermal shocks in structured media

This paper describes the propagation of an edge crack in a structured thermoelastic solid. A rapid change of temperature, represented by a time-periodic series of high-gradient temperature pulses, is applied at the boundary of the structured solid. A lattice approximation is employed in the model analysis discussed here. In order to describe the crack advance through the lattice a failure criterion is imposed, whereby the links break as soon as they attain a critical elastic elongation. The elongations of the links are produced both by a variation in temperature and by elastic waves generated at the boundary due to thermal shocks, as well as waves created by the propagating crack through the breakage of the elastic ligaments. The analysis is compared to the quasi-static and dynamic models of thermal striping in thermally loaded solids containing edge cracks. The emphasis is on the effect of the structure on the crack trapping. The nonlinear simulations presented in this paper show that the average speed of crack propagation can be estimated from the analysis of the dispersion properties of waves initiated by the crack. Temperature and inertia contributions to crack propagation are also investigated. It is found that inertia amplifies the elongations of the links, and thus influences the crack advance through the structured solid.

Detection of Reinforced Concrete Crack Using Mechano-Luminescence Paint

As a nonuniform and unisotropic material with a relatively low tensile strength in spite of high compression strength, a concrete material is vulnerable to bending and tension. Due to the mechanical properties of the current reinforced concrete structures, it is hard for concrete materials to avoid the damages caused by cracks. Although cracks are the easiest things to detect and the most effectively repairable things due to their characteristics, it is very hard to measure them efficiently. In this research, the author measured cracks by visualizing them through mechano- luminescence(ML) paint. By applying ML paint on the surface of the specimen and using the 3-point bending test, the author conducted a quantitative evaluation on the mechanical properties of cracks such as the cracking aspect and length of reinforced concrete. Through the results of this research, the author confirmed the crack propagation speed by section and the mechanical correlation such as between loads and cracks and between deflection and cracks, which means this research was quite successful in analyzing the characteristics of cracks.

Reduced-thickness CVN testing to represent slant failure of pipelines

To avoid longitudinal ductile crack propagation along a gas pipeline, the Batelle Two Curve method is used during pipeline design. This method states that a running crack will be arrested if the gas decompression velocity exceeds the crack propagation speed at the internal gas pressure. The crack propagation curve is scaled by impact energy values obtained through Charpy V-Notch (CVN) testing. However, for high-strength steel grades this scaling leads to unconservative predictions, because the experiment does not sufficiently represent the pipeline failure mode. The CVN specimen exhibits mainly mode I failure, without significant shear lips, while real failure is a combined mode often described as slant failure. In the present study, instrumented CVN tests are carried out on samples with different thickness reduction levels. To get a better insight in the crack initiation and propagation behaviour, the CVN test is simulated by finite element analysis. The dissipated energy and resulting fracture surfaces can be successfully represented. It is observed that slant failure is promoted by reducing the specimen thickness. In addition, the specific absorbed energy is decreased. However, most of the difference of absorbed energy is in crack initiation. This means that the fraction of the total energy dissipated in crack propagation is increased for reduced thickness specimens, making it a possible tool to assess the resistance of a material to crack propagation, provided that brittle fracture is avoided.

Viscoelastic effects on electromechanical instabilities in dielectric elastomers

We present a computational study of the effects of viscoelasticity on the electromechanical behavior of dielectric elastomers. A dynamic, finite deformation finite element formulation for dielectric elastomers is developed that incorporates the effects of viscoelasticity using the nonlinear viscoelasticity theory previously proposed by Reese and Govindjee. The finite element model features a three-field Q1P0 formulation to alleviate volumetric locking effects caused by material incompressibility. We apply the formulation to first perform a fundamental examination of the effects of the viscoelastic deviatoric and volumetric response on dielectric elastomers undergoing homogeneous deformation. Specifically, we evaluate the effects of the shear and bulk relaxation times on the electromechanical instability, and demonstrate that while the bulk relaxation time has a negligible impact, the shear relaxation time substantially increases the critical electric field needed to induce electromechanical instability. We also demonstrate a significant increase in the critical voltage needed to induce electromechanical instability in the presence of a distribution of relaxation times, compared to a single relaxation time, where the former is more representative of viscoelastic behavior of polymers. We then study the effects of viscoelasticity on crack-like electromechanical instabilities that have recently been observed in constrained dielectric films with a small hole containing a conductive liquid. Viscoelasticity is shown again to not only significantly increase the critical electric field to induce the electromechanical instability, but also to substantially reduce the crack propagation speeds in the elastomer.

Measuring high speed crack propagation in concrete fracture test using mechanoluminescent material

Measuring crack length in concrete fracture test is not a trivial problem due to high speed crack propagation. In this study, mechanoluminascent (ML) material, which emits visible light under stress condition, was employed to visualize crack propagation during concrete fracture test. Three-point bending test was conducted with a notched concrete beam specimen. The cracking images due to ML phenomenon were recorded by using a high speed camera as a function of time and external loadings. The experimental results successfully demonstrated the capability of ML material as a promising visualization tool for concrete crack propagation. In addition, an interesting cracking behavior of concrete bending fracture was observed in which the crack propagated fast while the load decreased slowly at early fracture stage.

Moving cracks in viscoelastic materials: Temperature and energy-release-rate measurements

A novel set up is proposed to measure the energy release rate vs. crack propagation speed in viscoelastic materials. The new test is referred to as the square sample tear test. In comparison to other techniques as the pure shear specimen and the pure tensile specimen tests, the square sample geometry guarantees at the same time: (i) a constant flux of elastic energy at the crack tip, (ii) the full development of the annular dissipative region around the crack tip, where the material is in the transition region of the viscoelastic response spectrum. Both conditions allow to precisely control the crack propagation speed and to test the predictions of existing theories. By employing the new set-up we measure the energy release rate as a function of the crack propagation speed. We also measure with a high precision infrared camera the increase of temperature at the crack tip, showing that, notwithstanding the low speed regime considered during the experiments, the viscoelastic dissipation at the crack tip leads to a measurable temperature increase of about 1°C.

Effect of retraction speed on adhesion of elastomer fibrillar structures

We studied the dependence of adhesion on retraction speed in elastic fibrillar structures. The evolution of the real contact area and crack propagation speeds at the contact interface was characterized during retraction using a custom measurement system. The experimental results showed an increase in the average crack propagation speed and the rate of change of real contact area during pull-off, for increasing retraction speed. Our results indicate that during pull-off, the critical contact area does not have significant contribution to rate-dependent adhesion, whereas rate of change in the contact area and the crack propagation speed contribute significantly to rate-dependent adhesion.

The Meaning, Selection, and Use of the Peridynamic Horizon and its Relation to Crack Branching in Brittle Materials

This note discusses the peridynamic horizon (the nonlocal region around a material point), its role, and practical use in modelling. The objective is to eliminate some misunderstandings and misconceptions regarding the peridynamic horizon. An example of crack branching in a nominally brittle material (homalite) is addressed and we show that crack branching takes place without wave interaction. We explain under what conditions the crack propagation speed depends on the horizon size and the role of incident stress waves on this speed.