Crack Extension Research Articles

Ballistic and delamination mechanism of CFRP /aluminum laminates subjected to high velocity impact

This work focus on the experimental study of ballistic and delamination mechanism on carbon/epoxy laminates with [-45/90/45/0] stacking sequence. A series of impact tests were carried out on the monolithic CFRP and Al/CFRP/Al laminates targets. The two targets were compared in terms of ballistic limit, energy absorption, penetration process, damage mechanism and interfacial delamination. Results showed that the Al/CFRP/Al laminates with equal thickness had better ballistic performance than that of the monolithic CFRP laminates. The delamination of adjacent layers was mainly controlled by bending moment, uneven stress distribution of interface and mismatch of wave impedance between layers with different lay-up directions. The shear wave propagated between fiber layers in different layering directions, resulting in interlayer crack extension.

Experimental Study on Flexural Performance of Recycled Steel Fiber Concrete Beams

We sorted the waste from mechanical processing to form recycled steel fibers. In order to study the flexural mechanical properties of reinforced concrete beams after the addition of recycled steel fibers, four recycled steel fiber concrete beams (RSFCBs) and one normal concrete beam (NCB) were designed and poured using the volume fraction of steel fibers (0%, 0.5%, 1%, 1.5%, 2%) as the variables. Normal section bending tests were conducted on them under a concentrated load. We obtained experimental data such as the cracking load, ultimate load, mid-span deflection, and steel and concrete strain of the beam by gradually loading the test beam, and we observed and recorded the development of cracks. The results indicate that the NCB exhibits crushing failure, while the RSFCBs exhibit equilibrium failure. The addition of recycled steel fibers effectively controls the extension of cracks, resulting in a better bending toughness of the beam. The bending performance of RSFCBs with different volume additions shows a trend of first increasing and then decreasing with the increase in steel fiber content. The peak value was reached when the steel fiber content was 1.5%, which increased the bending bearing capacity by 54.72% compared with the NCB. With the increase in steel fiber content, the required load value for tensile steel bars to yield also increases, reaching a peak at a content of 1.5%, which increases the bending bearing capacity by 44.64% compared with the NCB. The addition of recycled steel fibers enables the beam to improve its bearing capacity while limiting the development of longitudinal reinforcement strain, allowing the longitudinal reinforcement to yield under higher loads and improving the overall bending performance of the beam.

Tough and Moldable Sustainable Cellulose-Based Structural Materials via Multiscale Interface Engineering.

All-natural materials derived from cellulose nanofibers (CNFs) are expected to be used to replace engineering plastics and have attracted much attention. However, the lack of crack extension resistance and 3D formability of nanofiber-based structural materials hinders their practical applications. Here, a multiscale interface engineering strategy is reported to construct high-performance cellulose-based materials. The sisal microfibers are surface treated to expose abundant active CNFs with positive charges, thereby enhancing their interfacial combination with the negatively charged CNFs. The robust multiscale dual network enables easy molding of multiscale cellulose-based structural materials into complex 3D special-shaped structures, resulting in nearly twofold and fivefold improvements in toughness and impact resistance compared with those of CNFs-based materials. Moreover, this multiscale interface engineering strategy endows cellulose-based structural materials with better comprehensive performance than petrochemical-based plastics and broadens cellulose's potential for lightweight applications as structural materials with lower environmental effects.

Creep crack growth characterization of SS316LN

ABSTRACT In creep conditions, conventionally, C* and C(t) line integrals are used for characterising the crack tip. However, the true crack driving force or its rate cannot be described by any of the conventional parameters. The theoretical validity of conventional crack tip-characterising parameters like J or Ct is also restricted, making it hard to anticipate the driving forces underlying cracks. To investigate the creep crack growth behaviour of this material an experimental analysis of SS316LN at 650 °C is performed. The crack extension is modelled through the node-release technique, and a configurational force-based rate of change of J integral is calculated by post-processing of the finite element results. The results show that the Ct and dJepc/dt calculated from FE analyses are similar and the trend is also similar to the experimentally measured C*-values.

Surface wear resistance and microstructure characteristic in longitudinal–torsional ultrasonic vibration side milling of GH4169 superalloy

The intrinsic link between the surface wear resistance and microstructural evolution of a material was revealed. The surface wear resistance of the GH4169 superalloy was systematically investigated in relation to the surface machining topography and mechanical properties at different cutting widths in conventional side milling (CSM) and longitudinal–torsional ultrasonic vibration side milling (LTUVSM). The wear resistance was evaluated by analyzing the wear topography, friction coefficient, duration of the initial wear stage (IWS), and wear volume of dry sliding wear. The specimens in LTUVSM exhibited higher wear resistance than those in CSM owing to the inhibition of friction and growth of inter-surface friction nodes by the uniform ultrasonic vibration texture (UVT) and the inhibition of subsurface crack nucleation and extension by the superior mechanical properties. The mechanism of the ultrasonic vibration intervention in grain refinement was revealed. The involvement of ultrasonic vibration enhanced the dislocation accumulation, the grain boundary fractures, and increased the recrystallization fraction, which resulted in a thorough grain refinement with a 7.3 % decrease in average grain diameter (AGD) relative to that obtained with CSM. The increased grain refinement in LTUVSM improved the mechanical properties of the surface-layer material, thereby improving the wear resistance of the specimen.

Fabrication of TA2–304 SS laminated metal composite using directed energy deposition-arc: Microstructure, mechanical property, and corrosion resistance

Conventional fabricating methods of TA2–304 SS laminated metal composite (LMC) (e.g., hot rolling, hot stamping, and explosive welding) have difficulty in removing brittle TiFe intermetallic compounds (IMCs) and are not suitable for the preparation of the small size structure components or transition joints. Additive manufacturing of LMCs has the potential to achieve the desired microstructure, properties, and structure. In this study, TA2–304 SS LMC, which has been extensively employed in pipe flanges of nuclear plants and bows of sonar dome, was well fabricated by utilizing directed energy deposition-arc (DED-arc) and copper‑nickel alloy transition interlayers, and the formation of brittle TiFe IMCs was suppressed. The results of this study indicated that the standard deviation of the height of the surface morphology reconstructed by adopting Python VTK technique was evaluated as 0.67. Furthermore, copper‑nickel transition interlayers possessed the capability of isolating Ti from Fe atoms through the formation of laminar-distributed microstructure. Specifically, the Interlayer1 was comprised of (Fe, Cr) grains with different shapes and small-size (Cu) equiaxial grains, Interlayer2 covered large-size (Cu, Ni) dendritic grains and intergranular (Cu), and the TA2 layer comprised α-Ti and Ti2Cu. The result also showed that thermal cycle-induced recovery and recrystallization and diffusion-induced elemental interaction accounted for the different microstructures. Moreover, the interface between the TA2 and Interlayer2 layers contained TiCu4 + (Cu), Ti3Cu4 + TiCu4, Ti2Cu + TiCu, and TiCu + Ti2Cu eutectoid microstructures, which resulted in the highest hardness of 450Hv as well as crack initiation and extension. Furthermore, due to the uniform-distribution of microstructure in the absence of brittle IMCs, the average shear strength was 234.0 MPa, which reached the strength of the conventional method and conformed to the standard requirements. Multiple groups of galvanic reactions between different transition materials can result in reduced corrosion resistance, and the oxidation-reduction reactions can lead to the formation of massive oxides.

Critical slowing down precursor information for the acoustic emission response characteristics of defective tuffs

Fracture characteristics of rock and rational structural design are necessary to ensure the stability of underground rock engineering. Early warning of rock damage is a hot and challenging issue for research in geotechnical engineering. In this paper, uniaxial compression tests and acoustic emission (AE) tests were conducted on different defective specimens. The AE characteristics during rock damage were statistically analyzed based on the critical slowing theory. The results showed that the stress fluctuation in the stress–strain curve appeared earlier with the increase of initial damage. The failure pattern shifted overall from tensile to shear cracks. With the load increased, the AE amplitude changed from low amplitude and low density to high amplitude and high-density characteristics. The AE RA/AF was more sensitive to tiny cracks and had better immunity to attenuated AE signals. Both cumulative AE energy and RA/AF showed a precursor characteristic of a sudden increase in slope before rock damage. The AE b-value experienced a period of rising, fluctuation, and decline, with a sudden drop before the large-scale crack extension, showing a decreasing trend overall. The magnitude of fluctuations and density of distribution of the critical slowing down parameters (variance and autocorrelation coefficients) reflected the severity of specimen failure. The early warning characteristic of the variance was more significant, while the autocorrelation coefficient had an apparent disorder, which interfered with the judgment of the system phase transition. The average warning time of different AE parameters' “critical precursor point” was greater than 42 s, and the stress level was lower than 91.50%. The warning time sequence of different parameters in descending order was as follows: variance of AE parameters > autocorrelation coefficient of AE parameters > traditional AE parameters. The overall warning effect of the critical slowing down phenomena was better than that of classic AE parameters.

Effects of Pore–Crack Relative Location on Crack Propagation in Porous Granite Based on the Phase-Field Regularized Cohesion Model

This study employs the phase-field regularized cohesion model (PF-CZM) to simulate crack propagation and damage behavior in porous granite. The impact of the pore radius (r), initial crack–pore distance (D), and pore–crack angle (θ) on crack propagation is investigated. The simulation findings reveal that, with a fixed deflection angle and initial crack–pore distance, larger pores are more likely to induce crack extension under identical loading conditions. Moreover, with r and θ remaining constant, the crack extension can be divided into two stages: from its initiation to the lower edge of the pore and then from the lower edge to the upper boundary of the model. Multiple combinations of different D/r ratios and pore radii are derived by varying the values of D and r. These results demonstrate that with a constant r, cracks tend to deflect towards the pore closer to the initial crack. Conversely, when D remains constant, cracks will preferentially deflect toward pores with a larger r. In summary, the numerical simulation of rock pores and initial cracks, based on the PF-CZM, exhibits remarkable predictive capabilities and holds significant potential in advancing rock fracture analyses.

Effects of delay time on crack coalescence between two boreholes

Delayed detonation widely occurs in rock blasting, which has a significant influence on the propagation and interaction of explosion-induced stress waves. This affects the rock fracture interconnection between adjacent boreholes in controlled blasting. In this study, the effects of delay time on crack coalescence between two blastholes in contour blasting are experimentally, theoretically and numerically investigated. The rock cracking in two-hole blasting on 200 × 200 × 50 mm granite plate is first experimentally examined, in which the delay time is determined by high-speed photography and the rock fracturing is identified by digital image correlation. Then, two-hole blasting with delayed initiation is analytically modelled, and the fracture propagation behaviours in blast tests are explained based on the evolution of explosion-generated stress waves and peak stress distribution between two holes. Subsequently, 3D finite element models are built in LS-DYNA to simulate the evolution of explosion-produced pressure, rock fracturing and energy dissipation in two-hole controlled blasting with delayed initiation. The results indicate that in two-hole contour blasting, the stress superposition zone, which produces local stress enhancement in the vicinity of the stress wave collision point, moves closer to the later detonated hole. The superposed stress facilitates the preferential extension of blast-created cracks along the connection line of boreholes, and the crack from the later detonated hole extends farther than that from the first detonation hole. The optimal crack coalescence in contour blasting archives when the delay time allows stress waves to approximately propagate through the hole spacing.

Acoustic Emission-Based Modeling of Fiber Tailings Cementation and Filling Body Dynamics and Damage Ontology

Optimizing the mechanical characteristics of cemented tailings backfill (CTB) and quickly identifying its damage state under external loading, this study compares and prepares CTB specimens without fiber, doped with polypropylene fiber (PF), doped with glass fiber (BL), and doped with polypropylene and glass blended fiber (PB). Uniaxial compression and acoustic emission (AE) monitoring experiments are also conducted. Based on the cumulative energy of AE, the damage ontology model of CTB was developed. As shown by the study’s findings, adding various fibers can greatly enhance the filler body’s uniaxial compressive strength (UCS). BL has the greatest effect, followed by PB, while PFs have the least effect. Furthermore, the fibers primarily prevent the growth of crack extension by extending or breaking themselves, The results of the tests on acoustic emission revealed that the fiberless filler’s signals were more active prior to the peak point and less intense in the later stages of the damage, whereas the fiber-doped filler’s signals began to increase following the peak point and remained high. Thus, the damage model curves of various fiber-filled bodies are constructed based on the cumulative energy of acoustic emission, and the experimental data verification shows that the two have good consistency, suggesting that the established theoretical model can serve as a basis of reference for assessing the filled bodies’ damage state.

Theoretical model for spontaneous combustion gangue coarse aggregate concrete filled steel tube stub columns

Spontaneous combustion gangue coarse aggregate concrete filled steel tube (SCGCA-CFST) provides a possibility for the structural application of spontaneous combustion gangue coarse aggregate (SCGCA) concrete. To establish an accurate stress-strain model for SCGCA-CFST, 12 SCGCA-CFST specimens and 18 SCGCA concrete specimens were tested under monotonic axial compression with steel tube thickness and SCGCA replacement ratio (0%, 50%, 100%) as variables. The experimental results demonstrated that as the SCGCA replacement ratio increased, the elastic modulus of SCGCA concrete decreased by 6.65%–11.65%, the cylinder compressive strength decreased by 11%–21.95%, and the axial strain at peak axial stress increased by 7.1%–14.27%. Furthermore, the crack extension forms and dilation properties of the core concrete in SCGCA-CFST differed significantly from those of normal coarse aggregate concrete filled steel tube (NCA-CFST). The incorporation of SCGCA reduces the effectiveness of steel tube confinement, which causes the existing stress-strain models of NCA-CFST overestimate the enhancement effect of steel tube confinement on the strength and deformability of core SCGCA concrete. Based on this, the analysis-oriented stress-strain model for NCA-CFST was modified to be applicable for SCGCA-CFST by incorporating the influence of aggregate characteristics, and the accuracy of that was verified.

Experimental study on effects of cyclic loading paths on cracking behavior and fracture characteristics of granite

To investigate the impact of cyclic loading paths on the Mode I fracture characteristics of granite, a series of fracture tests were conducted on semi-circular bend (SCB) specimens using acoustic emission (AE), digital image correlation (DIC) and 3D scanning. The tests included monotonic loading, variable amplitude (VA) cyclic loading, and tiered constant amplitude (TCA) cyclic loading. The results demonstrated that VA cyclic loading leads to an average enhancement of 5.6% in fracture toughness, whereas TCA cyclic loading results in an average reduction of 3.7% in fracture toughness. During cyclic loading and unloading, the TCA loading path exhibits a higher rate of cumulative plastic deformation growth and greater magnitude cumulative plastic deformation compared to the VA loading path. Additionally, detailed observations from 3D scanning revealed that under cyclic loading, the fracture trajectories and surface morphology exhibit more tortuosity and roughness compared to monotonic loading, particularly in the case of TCA cyclic loading. Furthermore, according to the AE and DIC analysis, the proportion of microcrack initiation and stable propagation stage, cumulative AE counts and fracture process zone (FPZ) size generated under VA cyclic loading are greater than those generated under TCA cyclic loading, displaying a consistent variation pattern with fracture toughness. The changes of fracture properties can be attributed to a competition between the compaction-induced hardening effect and the cracking-induced damage effect. A higher cyclic upper limit and incomplete unloading are more likely to accelerate the growth of the irreversible crack damage and rapid crack extension, resulting in weakened rock fracture properties. These findings provide important references for rock instability control in underground rock engineering.

Resistance and strain during tearing for tubular joints under reversed axial actions

This article aims to predict the load versus crack growth (P−Δa) and strain versus crack extension (ε−Δa) relations in the ductile tearing process of welded joints made from circular hollow sections (CHS). The experimental program includes fracture tests on monotonically and cyclically loaded CHS joints, with a fatigue crack near the weld toe, under axial loadings applied in the brace. Coupled with the tearing assessment diagram, this study integrates fracture resistance curves measured directly from joints and that from a fracture specimen under monotonic and reversed loadings in the experimental procedure. The tearing assessment estimates the evolution of the load resistance and the nominal strain versus the crack extension, both showing satisfactory agreement with the experimental results. This study translates the ductile tearing process into the load resistance and strain levels, which provides an interface between the fracture-mechanics based assessments and the engineering analysis and monitoring.

Effects of thermal aging level on the morphological structures, rheological properties and fatigue characteristics of high-viscosity asphalts

High-viscosity asphalt (HVA) provides excellent cracking and rutting resistance for ecological permeable pavement. However, HVA suffers from severe aging due to higher construction temperatures and rich porous structures, which limited its applications for urban sustainability. Currently there is still a lack of knowledge about the thermal aging effects and mechanisms of HVA. In this study, the effects of thermal aging on the morphological structures, rheological and fatigue properties of two HVAs (i.e. HVA-1 and HVA-2) at five aging levels via Simple Aging Test (SAT) are systematically investigated. HVA-1, characterized by loosen and coarse reticulated structures, undergoes significant structural changes during aging, whereas HVA-2, characterized by dense and fine reticulated structures, experiences fewer alterations. The changes in the modulus master curves of HVAs become more gradual after aging, indicating the reduced temperature sensitivity. Multiple stress creep recovery results reflect that two types of HVAs exhibit totally different dominance of oxidation or polymer degradation depending on aging level. Liner amplitude sweep (LAS) results based on damage mechanics analysis reveal that aging leads to an increase in the accumulated strain energy observed in stress-strain curves for both HVAs. As aging progresses, the proportion of elastic energy decreases due to polymer degradation, resulting in a reduction in fatigue life. Especially, the fatigue life of aged HVAs significantly decreases after SAT40. LAS results based on fracture mechanics analysis show that the crack length increases with aging due to polymer degradation and asphalt phase hardening. Aging effect on crack length was limited at initial stage but becomes more significant as crack extends. As aging deepens, the crack initial stage of HVAs tends to lengthen while the crack extension stage tends to shorten. Moreover, the crack growth process of HVAs follows a two-stage behavior of crack initiation and propagation under certain aging levels from the relationship between energy release rate and crack propagation rate. Overall, the thermal aging kinetics of various HVAs differ significantly in the structures, rheological and fatigue properties as aging deepens. HVA-2 with loosen and coarse structures exhibits greater aging resistance and lower temperature sensitivity than HVA-1 with dense and fine structures.

Hidden ring crack in a rotating hollow cylinder under torsion

We consider the impact of a ring crack within a rotating hollow cylinder of fixed height under axisymmetric (torsion) loading. The form of the displacement is obtained from the equation of motion using the Fourier sin transform. The displacement jump over the crack is obtained from the boundary condition on the tangential stress, formulated as a singular integral equation which is solved by the method of orthogonal polynomials. The stress intensity factors on the opposing crack surfaces are calculated. The dependence of the crack extension on the problem geometry is investigated, including the impact of the crack’s location, cylinder’s height, torsion loading and rotation frequency. Possible extensions of the model to cover fatigue cracking are considered. Whether the presented model can be used to develop a practical test for detecting cracks is investigated.

A new method for predicting multi-crack fracture mechanism in the finite plate of red sandstone

A novel theoretical approach was put forth to forecast multi-crack propagation trajectories in a finite plate. First, a weighted residual approach and complex function are used to create the formula for computing the maximum (stress intensity factor) SIF of multi-crack in the finite plate. And then, three cracks were then chosen as a model for computation to determine how the plate size affects the SIF. The formula of multi-branch crack in finite plate is derived and the crack initiation and propagation criterion of multi-crack in finite plate was established. Finally, the propagation paths of three cracked red sandstone in a finite plate were predicted and verified by the uniaxial compression test. It demonstrates that the SIF of multiple cracks in a finite plate is consistently larger than that of an infinite plate. Under the uniaxial compression test, the first crack initiation point is often A2 or B2, and each specimen’s first crack initiation time increases and subsequently reduces when the relative horizontal spacing (Ds’) of cracks varies. The crack initiation stress increases and the unstable stress first increases and then decreases with Ds’. The total crack extension length initially goes down and then goes up. All of the fracture initiation and propagation mechanisms are Mode I. The effectiveness of the multi-crack propagation theory in finite plates is established by the good agreement between predicted and test findings.

Ductile ultrastrong China low activation martensitic steel with lamellar grain structure

The low high-temperature mechanical properties of reduced activated ferrite martensitic (RAFM) heat-resistant steels limit the maximum operating temperature of nuclear fusion reactors. However, it is difficult to improve the strength of the material without reducing its plasticity. In this study we prepared a China low activation martensitic (CLAM) steel with a layered structure through hot rolling followed by low-temperature tempering, which successfully improved its strength and ductility. In addition, we elucidate the ductility mechanisms in such lamellar structural materials. When the strength of the grain boundaries exceeds that of the grains, cracks will continuously extend along the layered grain boundaries, eventually resulting in a delamination fracture mechanism. When the grain strength exceeds that of the grain boundaries, cracks will continuously form through the relative sliding of the grain boundaries, and a large number of small cracks will initiate and propagate longitudinally along the layered boundaries. Both forms of crack propagation cause longitudinal expansion along the layered grain boundaries. The resulting longitudinal extension of cracks markedly diminishes local stress concentration to promote plastic deformation, thereby significantly increasing the ductility of the material. Employing the combination of this delamination ductility mechanism with interface strengthening and precipitation strengthening, the strength of the CLAM steel for nuclear applications is shown to be enhanced by ∼30 % without loss of elongation. The current delamination ductility strategy provides a unique pathway to develop materials with ultrahigh strength and superior ductility at economical material costs.

Effects of crystallographic orientation and lamellar configuration on fatigue crack propagation in single-colony structures of Ti–6Al–4V alloy: Alternating shear crack growth vs. damage accumulation crack propagation

In this study, the fatigue crack propagation mechanisms in lamellar colonies of Ti–6Al–4V alloy were examined using miniature compact-tension specimens with a single-colony structure at the crack tip. Fatigue tests were performed on colonies with different α-phase crystallographic orientations and lamellar configurations with respect to the notch plane and direction, followed by post-fatigue metallographic analysis. Crack propagation occurs by three main mechanisms: 1) alternating shear due to in-plane prismatic slip, 2) damage accumulation via dislocation–dislocation interaction due to out-of-plane slip, and 3) interlamellar decohesion due to damage accumulation via lamellar interphase boundary–dislocation interaction. The slope of da/dN vs. ΔK is higher for alternating shear crack growth than for damage accumulation crack propagation via dislocation–dislocation interaction. This is due to an incubation period before substantive crack extension in the latter case, which is dominated by degree of strain accommodation associated with the activated slip systems. When the lamellar interphase boundaries are nearly perpendicular to the loading axis, the crack growth rates drastically increase by interlamellar decohesion. This is attributed to the reduced incubation period due to slip incompatibility at the interphase boundary where numerous crack nuclei were formed during damage accumulation process.

Effect of Stress Ratio, Constraint, and Load History on the Intrinsic and Extrinsic Components of Threshold Stress Intensity

Abstract Threshold intensity range, ΔKth, was experimentally investigated as the sum of two components, both of which are sensitive to applied loading conditions. The intrinsic component, ΔKth,i, is characterized as a sole function of a near-tip stress parameter, σ*, in much the same way as fatigue limit depends on mean stress or stress ratio. The extrinsic component, ΔKcl, is the attenuated fraction of crack tip response because of crack wake contact, whose exact value is not readily measured at threshold. The ΔKth,i versus σ* relationship is established using a specially designed load shedding process under constant baseline maximum load, with σ* modulated through periodic overload-underload combinations whose periodicity is regulated to avoid undue crack extension at threshold. This relationship is then used to extract the ΔKcl component from ΔKth readouts obtained on the same batch of material using conventional load shedding procedure. The effect of constraint on ΔKth and its two components was studied using specimens with sharp side grooves to maximize constraint across the crack front. The study was performed on a medium strength low-alloy steel.

Peridynamic modeling of elastic-plastic ductile fracture

A peridynamics-based framework is established for the elastic-plastic ductile fracture analysis. In this frame, a new state-based peridynamic elastic-plastic model is presented using the novel bond extension state forms of yield function and plastic flow rule. The nonlinear energy release rate is computed by the extended peridynamic finite crack extension (PFCE) method, and an energy dissipation rate-based bond failure criterion is proposed for ductile crack growth modeling. Numerical methods for plastic states update and yield function solution are also given. Then, examples of plates with a center hole or a center crack, and compact tension (CT) tests are analyzed by the proposed models, and compared to those from theoretical and FEM solutions. The results demonstrate that the proposed peridynamic models can well capture the characteristics of elastic-plastic deformation and ductile fracture, including the plastic shape and size, energy distributions, nonlinear energy release rate, load-displacement and resistance curves, etc.