Crack Initiation Strain Research Articles

Damage accumulation in carbon steel under low-cycle loading at the crack initiation stage and strain ageing temperature

Based on the strain criterion for fatigue fracture and experimental studies, the damage accumulation kinetics from recoverable plastic strain (plastic strain in the tensile half-cycle), from the unilateral accumulation and elastic strain with the increasing number of loading cycles was studied. The influence of stress ageing on the redistribution of the levels of these components of damage, depending on the number of cycles before destruction, was studied. Depending on the load level, the limit states (formation of a surface macro-crack or buckling failure of strain stability due to the formation of an internal crack in the quasi-static fracture range) were established. Strain ageing reduces the range of quasi-static fracture. It is shown that the critical values of strains and stresses correspond to the limit states. The parameters determining the capacity of structural materials under static and cyclic loading under conditions of stress ageing are confirmed.

Microscopic damage behavior in CFRP cross-ply laminates at cryogenic temperature

For the practical use of cryogenic propellant tanks made of CFRP laminates, experimental elucidation of the laminates’ microstructural damage propagation behavior at cryogenic temperatures is important. This paper presents a newly developed tensile test rig that enables in-situ observation of microscopic damage using an optical microscope at cryogenic temperatures using a Gifford–McMahon refrigerator. In-situ observations of microscopic damage under uniaxial tensile loading were done at room temperature (290 K, 17 °C) and at 30 K (−243 °C) on cross-ply thin-ply CFRP specimens of three kinds with different 90° layer thicknesses. The results demonstrated that the crack propagation behavior is independent of temperature, that the matrix crack density is higher, and that the onset of matrix crack initiation strain is lower at 30 K than at 290 K. Furthermore, the thermal strain within 90° layer at 30 K and at 290 K was estimated using finite element analyses (FEA). The FEA results suggest that the decrease in onset strain of matrix crack initiation at 30 K is mainly attributed to the increase in the thermal strains within the 90° layer.

Strength and Crack Propagation Analysis of Layered Backfill Based on Energy Theory

The strength of backfill is greatly influenced by its inclination angle and interlayer concentration. In order to study the influence of inclination angle and interlayer mass concentration on the strength of backfill, a group of layered cemented backfill with cement-sand ratio of 1:4, interlayer mass concentration of 66%, 67% and 68% and inclination angles of 0°, 10°, 20° and 30° were prepared by using tailings as aggregate. The uniaxial compression test was carried out to analyse the effect of interlayer mass concentration and inclination angle on layered cemented backfill. The crack propagation and energy change law of the specimen during compression were analysed by J-integral and energy conservation law. The relationship between the crack initiation and propagation and strain energy of two representative three-layer backfill specimens was analysed by numerical modelling. The results show that the increase in the layer number and the inclination angle of the backfill can weaken the strength of the backfill. In a certain range of inclination angles, the weakening coefficient of the backfill caused by the inclination angle is very consistent with the cosine value of the corresponding angle. Due to the release of crack energy and the existence of interface J integral, the uniaxial compressive strength of different mass concentration backfill is different at various positions. When the displacement reaches a certain value, the crack and strain energy no longer increase.

Study on the mechanism of energy evolution and bearing degradation in pre-cracked sandstone under non uniform cyclic loading

The engineering rock mass is affected by the original cracks, which increases the uncertainty of instability and failure mechanisms. Under the action of excavation unloading or other disturbance loads, its bearing properties are more complex and the deformation characteristics is more special, which has always been a difficult issue in the analysis of rock mechanics. In order to explore the propagation and failure mechanism of spatial micro cracks in fractured rock masses, acoustic emission (AE) experiment of sandstone was carried out under pre-static loading and low-frequency stepwise cyclic loading–unloading. The research results indicate that the peak strain and crack initiation stress of rocks with different pre-cracked positions have the highest differences of 12.62% and 18.65%, respectively. The difference between peak stress and crack initiation strain is within 4%. The pre-cracked position mainly affects the peak strain and crack initiation stress of pre-cracked rocks, and its effect on peak stress and crack initiation strain is limited. The crack initiation angle of prefabricated cracks is mainly distributed within the range of 45° − 90°, and it decreases with the increase of quantities of prefabricated cracks. The crack initiation angle of rocks with prefabricated three cracks is 30% lower than that of single crack. The research results have deepened the analysis of energy evolution and deformation damage of fractured rock masses.

Influence of crack aperture and orientation on the uniaxial compressive behavior of rock

The pre-existing cracks with various geometric and physical properties significantly affect the mechanical behaviors of rock mass. In this study, uniaxial compressive tests are performed on numerical models of specimens containing a single pre-existing crack with different apertures and orientations based on PFC 2D to shed light on the impact of different crack apertures and orientations. The results reveal that the uniaxial peak strength and elastic modulus tend to decrease with increasing apertures, but they tend to increase with increasing inclination angles. Crack apertures can affect the stress intensity factor in the vicinity of the pre-existing crack tips, which further influences the crack initiation and crack type. Furthermore, shear cracks are more prone to occur in a pre-cracked specimen with a larger crack aperture and larger crack orientation. The crack initiation stress and strain initially show a slight decrease until the inclination angle reaches 45°, and then significantly increase with the further increase of angle from 45° to 90°. The microcracks occur earlier and easier when the inclination angle is 45°. As the inclination angle of the pre-existing crack increases, shear-sliding along the crack becomes more prominent, resulting in the formation of coplanar secondary cracks. The macro failure patterns switch from tensile failure to shear failure with increasing crack inclination angle, which is in good agreement with trend lines of displacement in the displacement field.

Response of interlayer-bonded bilayer graphene to shear deformation

We report results on the mechanical and structural response to shear deformation of nanodiamond superstructures in interlayer-bonded twisted bilayer graphene (IB-TBG) and interlayer-bonded graphene bilayers with randomly distributed individual interlayer C–C bonds (RD-IBGs) based on molecular-dynamics simulations. We find that IB-TBG nanodiamond superstructures subjected to shear deformation undergo a brittle-to-ductile transition (BDT) with increasing interlayer bond density (nanodiamond fraction). However, RD-IBG bilayer sheets upon shear deformation consistently undergo brittle failure without exhibiting a BDT. We identify, explain, and characterize in atomic-level detail the different failure mechanisms of the above bilayer structures. We also report the dependence of the mechanical properties, such as shear strength, crack initiation strain, toughness, and shear modulus, of these graphene bilayer sheets on their interlayer bond density and find that these properties differ significantly between IB-TBG nanodiamond superstructures and RD-IBG sheets. Our findings show that the mechanical properties of interlayer-bonded bilayer graphene sheets, including their ductility and the type of failure they undergo under shear deformation, can be systematically tailored by controlling interlayer bond density and distribution. These findings contribute significantly to our understanding of these 2D graphene-based materials as mechanical metamaterials.

Mechanical properties and thermal stability of ZrCuAlx thin film metallic glasses: Experiments and first-principle calculations

In this work, we provide a holistic picture about the relationship between atomic structure, mechanical properties, and thermal stability of ZrCuAlx thin film metallic glasses (TFMGs) varying the Al content from 0 to 12 at.%, carrying out a broad characterization involving experiments and ab initio molecular dynamic simulations (AIMD). We show that the addition of Al resulted in a change of average interatomic distances by ∼10 pm with the formation of shorter bonds (Al-Zr and Al-Cu), influencing the mechanical response (shear/elastic moduli and hardness) which increases by ∼15% for 12 at.% Al. Moreover, tensile tests on polymer substrate revealed a maximum value for the crack initiation strain of 2.1% for ZrCuAl9, while the strain-to-failure rapidly decreases at higher Al contents. The observed reduction in damage tolerance is correlated to a transition in atomic configuration. Specifically, a maximum in density of full and defective icosahedral cluster population is observed at 9 at.% Al, inducing a more shear-resistant behavior to the material. Thermal stability is investigated by high-energy and conventional x-ray diffraction and electrical resistivity measurements as a function of the temperature. Glass transition (Tg) and crystallization (Tx) temperature increase by Al addition reaching 450 and 500 °C, respectively for ZrCuAl12. The increase in thermal stability is related to the reduction in atomic mobility due to the formation of shorter chemical bonds, inhibiting atomic reconfiguration during crystallization. In conclusion, we provide guidelines to the design of compositional-tailored ZrCuAlx TFMGs with tuned mechanical properties and thermal stability with potential impact on industrial applications.

Effects of dielectric layer on ductility for dielectric/Au/dielectric multilayers on polycarbonate substrate

The ductility of transparent conductive films on polycarbonate (PC) substrate is of great concern as it significantly affects the stability and longevity of aeronautic equipment. Three types of dielectric/Au/dielectric multilayers, including ITO/Au/ITO (IAI), IZO/Au/IZO (ZAZ) and AZO/Au/AZO (AAA) were fabricated to obtain highly ductile films on thick rigid PC substrate. The ductility of multilayers was comparatively investigated using in situ scanning electron microscopy test and in situ electrical resistance test under uniaxial tension. The effect of dielectric layer on ductility was elucidated according to the results of film stress and elastic modulus, and analyzed using the energy release rate approach based on the mechanics model. The results of in situ tests revealed that the crack initiation strain by morphology and the conductive failure strain of AAA were found to be 1.04 ± 0.04% and 1.47 ± 0.07%, which were superior to those of IAI and ZAZ. This result can be attributed to differences in layer stress state and layer-substrate mechanical contrast induced by different dielectric layers. Although AAA has the lower fracture toughness, the higher compressive residual stress and the smaller elastic mismatch give AAA the smallest crack driving force under the same conditions, resulting in excellent ductility.

The Influence of Thermal and Mechanical Stress on the Electrical Conductivity of ITO-Coated Polycarbonate Films.

The influence of thermomechanical stress on the conductivity of indium tin oxide (ITO)-coated polycarbonate (PC) films was investigated. PC is the industry's standard material for window panes. ITO coatings on polyethylene terephthalate (PET) films are the main commercially available option; as such, most investigations refer to this combination. The investigations in this study aim to investigate the critical crack initiation strain at different temperatures and crack initiation temperatures for two different coating thicknesses and for a commercially available PET/ITO film for validation purposes. Additionally, the cyclic load was investigated. The results show the comparatively sensitive behavior of the PC/ITO films, with a crack initiation strain at room temperature of 0.3-0.4% and critical temperatures of 58 °C and 83 °C, with high variation depending on the film's thickness. Under thermomechanical loading, the crack initiation strain decreases with increasing temperatures.

Experimental Study on Dynamic Tensile Properties of Macro-Polypropylene Fiber Reinforced Cementitious Composites

Using a high-speed photography system and a split Hopkinson pressure bar, macro-polypropylene fiber reinforced cementitious composites are tested to reveal the effects of the macro-polypropylene fiber volume fraction and loading rate on the dynamic tensile strength and failure mode. We also analyze the functional relationship between the dynamic tensile strength, loading rate, and fiber volume fraction, and study the splitting failure process using digital image correlation technology. The evolution law of the strain and displacement fields of the specimens is obtained, and the effect of the fiber volume fraction on the crack initiation strain value is quantitatively studied. The results show that the appropriate fiber content (1.5–2%) can significantly improve the dynamic tensile strength, while a higher fiber content (2.5%) leads to deterioration of the specimen. Adding macro-polypropylene fiber prevents the specimen from undergoing central tensile fracturing under dynamic loading, and distributes the impact load more evenly, thus improving the ability of the specimen to resist cracking.

FORMATION AND FAILURE BEHAVIOR OF OPEN-GRADED FRICTION COURSE AT LOW TEMPERATURES

To study the deformation and failure behavior of the open-graded friction course (OGFC) before cracking at low temperatures, a four-point bending test of the trabecula was conducted, and the deformation process was monitored by the digital speckle method (DSM). The distribution law of the strain field of the trabecular specimens was analyzed. The horizontal strain field gradually changed from a uniform distribution to a strain concentration area with loading. The crack initiation time, crack initiation strain, and deformation period were obtained from the strain curve. The combination of stress reconstruction and DSM makes the stress measurement of the OGFC more authentic.

Failure Transition and Validity of Brazilian Disc Test under Different Loading Configurations: A Numerical Study

The Brazilian disc test is a popular tensile strength test method for engineering materials. The fracture behavior of specimens in the Brazilian disc test is closely related to the validity of the test results. In this paper, the fracture process of granite discs under different loading configurations is simulated by using a coupled finite–discrete element method. The results show that the maximum tensile stress value is located within 18 mm (0.7 times the disc radius) of the vertical range of the disc center under different loading configurations. In small diameter rods loading, the invalid tensile strength is obtained because the crack initiation and plastic strain is at the end of the disc. The crack initiation points of flat platen loading and curved jaws loading are all within the center of the disc, and the valid tensile strength can be obtained. The tensile strength test results under different loading configurations show that the error of small diameter rods loading is 13%, while the errors of flat platen loading and curved jaws loading are both 1%. The curved jaws loading is the most suitable for measuring the tensile strength of brittle materials such as rock, followed by flat platen loading. The small diameter rods loading is not recommended for the Brazilian test.

Effect of transparent polymer encapsulation overlayers on bending fracture behavior of flexible organic lead halide perovskite thin films

Although halide perovskites have significant potential for flexible devices, the bending mechanical behavior of halide thin films has not been investigated in detail. Herein, we report the quantitative bending fracture behavior of flexible methylammonium lead triiodide (MAPbI 3 ) thin films encapsulated with an overlayer of either polystyrene (PS) or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). The significantly retarded cracking behavior of the halide films demonstrated that adjusting the polymer overlayers with different thicknesses resulted in substantial mechanical enhancements. For example, the minimum strain for crack initiation was ~1.96% for a highly concentrated PS overlayer with a thickness of ~50 nm, which corresponds to an increase of ~40% relative to the non-encapsulated sample. In addition to quantifying the fracture behavior with fracture energy and film strength, the chemical effects of encapsulation were also investigated in terms of the phase dissolution of the perovskite, which was correlated with changes in the Hall electrical properties. • Bending fracture behavior of polymer-encapsulated perovskite halide films is studied. • Less cracks were observed with the thicker polystyrene overlayer under bending. • A 50 nm-thick polystyrene layer resulted in a 96% improvement of fracture energy. • Phase dissociation with atmospheric exposure was suppressed by polymer-encapsulation. • Hall electrical resistivity was correlative to the level of phase dissociation.

Mechanical Properties of Tensile Cracking in Indium Tin Oxide Films on Polycarbonate Substrates

The electro-mechanical behaviors of transparent conductive oxide film on polymer substrate are of great concern because they would greatly affect the stability and lifespan of the corresponding devices. In this paper, indium tin oxide (ITO) films with different thicknesses were deposited on a polycarbonate (PC) sheet; meanwhile, in situ electrical resistance, in situ scanning electron microscopy and profilometry were employed to record the electrical resistance, morphologies and residual stress in order to investigate the fracture behavior and electrical-mechanical properties of ITO films under uniaxial tension loading. The electrical resistance changes, crack initiation, crack propagation and crack density evolution of ITO films were systematically characterized by in situ tests. Three fracture stages of ITO films were summarized: Ⅰ crack initiation, Ⅱ crack propagation, Ⅲ crack saturation and delamination. The crack initiation and electrical failure in a thinner ITO film occurred at relatively higher applied tensile strain; namely, the ductility of the film decreased as the film thickness increased. Residual compressive stress was recorded in the ITO films deposited on PC at room temperature and increased as the film thickness increased. Intrinsic crack initiation strain (CIS*) showed an opposite thickness dependence to residual strain (εr); the increase in residual compressive strain was counteracted by the decrease of intrinsic cohesion, leading to an overall decrease in effective crack initiation strain (CIS) when the film thickness increased. In addition, integrated with a formulated mechanics model and the analysis of the three fracture stages under tension, the fracture toughness and interfacial shear strength were quantitatively determined. As the film thickness increased (in the range of 50~500 nm), the fracture toughness decreased and the films were more prone to crack, whereas the interfacial shear strength increased and the films were less likely to delaminate.

Effect of tetragonal zirconia nanoparticle content on enamel strength

The fracture failure of enamel is caused by hole-type flaws and cracks generally present in enamel. In this study, the effect of tetragonal zirconia nanoparticles (nano-t-ZrO2) on enamel strength was investigated. The addition of nano-t-ZrO2 improved the surface morphology and reduced the porosity of the enamel as determined by morphological and porosity distribution analyses. The strain on the enamel surface was recorded using a strain gauge attached to the enamel surface. The crack initiation force and strain were measured using the strain gauge, and the crack propagation diagrams at various stages of the tensile process were recorded. The relationship between the crack initiation energy and nano-t-ZrO2 content was found to be non-linear. Nano-t-ZrO2 affected the strength of the enamel. Moreover, the tensile force required for the appearance of cracks first increased and then decreased with an increase in the nano-t-ZrO2 content. The addition of nano-t-ZrO2 to the enamel matrix suppressed the formation of stress concentration regions and decreased the brittleness of the enamel. At a nano-t-ZrO2 content of more than 5 wt%, the ZrO2 particles agglomerated.

The influence of material microstructural characteristics on the strength of porous or composite ceramic coatings

This paper is a follow-up study to our earlier research that identified several types of preferred stress concentration regions in a material with a porous coating and suggested certain microstructural factors to induce them (Zinoviev et al., 2020). This study takes a closer look at each of the factors indicated earlier, examining their effects on the deformation and fracture of a coating and a coated material as a whole. Porosity, including a pore distribution, size, number, and distance between them, elastic property difference in a composite coating, curvilinear coating-substrate interface, and thickness of the pore-free coating layer are among the causative factors analyzed in microstructure-based mechanical simulations. It is found that the pore distribution controls the crack-initiation strain and first crack location while having a low effect on the elastic modulus. A dimensionless parameter is proposed to characterize the effect of the pore size and minimum distance between them on the local stress–strain state of a coating. The research findings provide insights for the choice of filler materials to be induced in a polymer-derived ceramic coating, suggesting the material resulting in the highest strength properties of the coated material among the considered ones. Analysis of the non-porous layer effect suggests that cracks form either at the coating-substrate interface or pore surface, and defined the layer thickness when the location of crack initiation changes. The insights gained from this study may be of assistance for coating design and engineering.

Research on fracture behavior of lightly reinforced concrete beam under different loading rates

Under impact loads, the crack resistance of reinforcement in concrete is the focus of impact dynamics research. In this study, the three-point bending tests of lightly reinforce concrete beams with notch were conducted with a drop hammer testing machine. The high speed camera and Digital Image Correlation (DIC) technology were applied to catch the fracture process and analyse the displacement field. The shape and geometry of specimen followed the RILEM recommendation, i.e., 150 mm × 150 mm in cross section, 800 mm in length, notch-depth ratio was around 1/3 and span was kept constant 600 mm. And the impact force, mid-span deflection, steel strain beneath the loading point and concrete strain at the notch tip were measured over four loading rates. The experiment results showed that the crack initiation strain rate increased linearlywith the loading rate, and the growth trend weakened when loading rate was 1.771×103 mm/s. Based on the test results, the empirical formula of crack initiation strain rate with respect to loading rate wasgiven, which was meaningful to simulate the crack initiation in concrete under dynamic loading. In addition, with the increase of loading rate, the steel yielded gradually, which resulted in the response time difference between impact force and steel strain decreased. With the recovery of elastic deformation of steel bars, the crack started to close and the macro crack became visible clearly. Meanwhile, the crack mouth opening displacement (CMOD) declined to a constant value after reaching the maximum value. Then, the crack mouth opening displacement rate was obtained from the fitting curve, and a linear growth relationship between CMOD rate and loading rate was founded. The failure process of the lightly reinforced beams was analyzed based on the CMOD rate, which provided an idea for comparing the fracture process of the beam under dynamic and static loads.

Crack propagation behavior of ultra-high-performance concrete (UHPC) reinforced with hybrid steel fibers under flexural loading

This study investigates the crack propagation behavior of UHPC blended with straight steel fibers of different lengths (6, 13, and 20 mm), at varying replacement ratios (0.5%, 1.0%, and 1.5%) and a constant total fiber volume fraction (Vf = 2.0%), under flexural loading. The results indicate that blending with 0.5% medium length fibers and 1.5% long fibers yields the best flexural behavior compared to other investigated specimens. Using digital image correlation (DIC) technique, the evolution of surface deformation and strain field was analyzed. Based on the recorded images and corresponding analyses, the full debonding strain between the fibers and the matrix was determined to be about 0.32%. Moreover, the maximum strain for generating cracks was approximately 3.25%, irrespective of the fiber length. When longer fibers were replaced with shorter fibers at 0.5% or 1.0% replacement ratios, Crack opening displacement (COD) variations along the beam presented a linear shape. However, at higher replacement ratios (1.5% and 2.0%), the profile showed an abrupt change, and COD variations were non-constant. The samples comprising hybrid fibers presented a faster decline in the crack initiation strain rate at higher replacement ratios of the longer fibers. In the initial stage of cracking, the crack propagated rapidly; however, the crack propagation speed dropped dramatically during crack evolution before it stabilizes.

Mean and residual stress effects on fatigue behavior in a pre-strained corner of stainless steel sheet

There are many thin-walled corners in the components of automobiles, machines, and structures. It is essential to elucidate their fatigue properties and influence factors. Parameters for stress–strain behavior of ferritic stainless plane sheet and pre-strained sheet were obtained under tensile, bending, and cyclic bending loads using the Ramberg–Osgood model. The parameters were integrally used as the fundamental characteristics of bending fatigue behavior for obtaining the mean stress and crack initiation stress amplitude from the measured strains in the curved portion of corner specimens. Bending fatigue tests of pre-strained corner specimens were carried out, and the relationships between the crack initiation strain amplitudes and the mean strains were verified. The strain amplitude ratios of the curved inside to outside portion were analyzed. The residual stress and hardness were measured, and work hardening effect on the inside was higher than that on the outside for improving fatigue strength. The fatigue limit of the corner specimen with the radius ratio of 1.3 was 23% higher than the unworked plane sheet. The effects of mean stress and residual stress on the fatigue strength were analyzed using the fatigue limit–static stress relation equations, indicating that the Gerber parabola can be used for fatigue limit predictions in the stress ratio range of R < –0.5 and that the Goodman line can be used after the point of R = –0.5. The SWT function can be used to accurately predict the fatigue lives in various conditions of stress amplitudes combined with static stresses.

Computational parametric study for plastic strain localization and fracture in a polycrystalline material with a porous ceramic coating

Polysilazane-based ceramic coatings have ever-increasing applications in critical and high-power engineering. This paper analyzes the effects of the substrate grain size, the curvature of the coating-substrate interface and the pore-free coating layer on the plastic strain localization and fracture of the porous ceramic coating–polycrystalline substrate structures under compressive loading applied to the coated surface. Models for generating pore and polycrystalline structures of the coating and substrate are developed, with the experimentally observed microstructures of the coated steel being taken into account in calculations explicitly as the initial data of the boundary-value problem. Numerical results obtained by the finite-difference method show that the crack-initiation strain depends on the substrate grain size exponentially. A comparison between the models with serrated and plane coating-substrate interfaces suggests that in the former case, coating cracking initiates near the humps of the interface concavities and the top and bottom pore surfaces, while in the latter case, two sites of cracking nucleation can be distinguished, namely, close to the pores – in the fine-grained substrate, and near the coating-substrate interface – in the case of coarse grains. The results indicate that it is preferable to use the coated specimens characterized by a serrated coating-substrate interface.