Influence of shear stress state on dynamic fracture of a glass ceramic Macor

The effect of shear stress on the dynamic failure of ceramic materials is not sufficiently investigated in the published literature. With the use of a bespoke split Hopkinson pressure bar, this paper presents an effort to investigate the dynamic shear compressive response of Macor, a model ceramic material with zero porosity and light weight characteristics. A cone specimen and cylindrical specimens with varying inclined angles are used to introduce the shear stress to the Macor ceramic. The dynamic failure initiation and crack propagation are monitored by the high speed photography and Digital Image Correlation techniques. It is found that the equivalent stress of Macor at the initiation of failure decreases nonlinearly with the increase of shear stress. The high speed images show that the crack originates from the minimum cross-section of the cone specimen and the obtuse angle corner of the inclined cylindrical specimens. The cracks propagate parallel to the inclined plane instead of the axial loading direction. The fractographic analysis shows the compacted zone in the shear fracture surfaces of the cone specimen and the inclined cylindrical specimens. This indicates a significant role of shear loading in the dynamic failure process of Macor.     

Crack growth angle prediction of an internal crack under mixed mode load for unfilled elastomer using the strain energy density factor

This article investigates the prediction of the crack growth angle of an existing internal crack under mixed mode loading at the crack tip for an unfilled ethylene propylene diene terpolymer rubber (EPDM). For the realization of mixed mode loading, the cracks of the uniaxial loaded specimens were oriented with different angles to the loading direction. The energy density factor was used as a potential criterion for determining the crack growth angle. The determination of the strain energy density factor was carried out simulatively in Abaqus. The second-order Ogden model was used to describe the rubber-like material behavior. The relative local minimum of the strain energy density factor provides the possible growth angle. The experimental investigations show that the initial cracks grow orthogonally to the loading direction for the different crack orientation angles. For the crack orientation angle parallel to the load direction, the crack growth was observed because the strong stretching of the specimen caused strong necking in the crack region. The crack growth for the remaining crack orientation angles were induced due to shear loading at the crack tip. The predictive angle of different crack orientation angles shows very good accordance to the measured crack growth angles.     

Effects of residual stress and orientation on the fatigue of injection molded polysulfone

The tensile fatigue behavior of unnotched injection molded polysulfone specimens has been investigated. The effects of orientation and residual stress were studied by comparing asmolded specimens with annealed or annealed and quenched specimens with a known residual stress pattern. The treatments are shown to have differing effects at high stresses, where failure is by shear yielding and necking, and at intermediate stresses, where failure is by fatigue crack propagation. The geometries of fatigue cracks are described for each case. An attempt is made to separate the effects of crack and craze initiation from crack propagation, and cyclic loading from cumulative time under load.     

Experimental and numerical investigations of crack face adhesive bonding effect on the mixed-mode fracture strength of PMMA

Experimental and numerical investigations have been conducted to evaluate the effect of adhesive bonding of crack surfaces on the mixed-mode (I and II) fracture strength and effective stress intensity geometry/loading factor of a plate with an edge crack. The experimental tests were carried out on five batches of simple edge crack and specimens in which adhesive bonding was used on crack faces at different distances from the crack tip. The cracked specimens made from poly methyl-methacrylate rectangular plates. The specimens’ fracture strength was obtained by employing a tensile testing machine at different loading angles using a modified Arcan fixture. In the numerical part, finite element simulations were used to model the test specimens and thereby establishing their stress intensity geometry/loading factors. The results show that the adhesive bonding of the crack surfaces has a significant effect on reducing the equivalent mixed-mode stress intensity factor for all loading angles. The bonded specimens show considerable fracture force enhancement compared to the simple edge crack specimens.     

Fracture and Subcritical Crack Growth in Soda-Lime Glass under Combined Modes I and II Loading

Fracture and subcritical crack growth characteristics under combined modes I and II loading were studied using the compact tension shear specimens of soda–lime glass. The maximum normal stress criterion gives a good agreement with the experimental mode I–mode II fracture toughness envelope for initially straight cracks and kinked cracks. Subcritical crack growth characteristics were determined under sustained modes I and II loading in water. The values of K _I and K _II were calculated approximately by replacing the subcritical kinked crack with an assumed straight crack ā, and the K _Imax value based on the maximum normal stress criterion was used to describe this subcritical kinked crack growth. The experimental results show that subcritical crack growth under pure mode I, pure mode II, and various combined modes I and II loading can be well described by the K _Imax value based on the approximate maximum normal stress criterion.     

Interlaminar shear behaviors of 2D needled C/SiC composites under compressive and tensile loading

The interlaminar shear strength of 2D needled C/SiC composites was measured using the double-notch shear test method. Interlaminar shear tests were performed under compressive and tensile loading. Shear stress–strain response and shear strain field evolution were studied using the digital image correlation (DIC) technique. The results show that the interlaminar shear strength of the specimen using the compressive loading method is 15% higher than that of the tensile loading method. Severe shear strain concentration was observed near the upper notch of the tensile loading specimen. Acoustic emission (AE) was utilized to monitor the damage during the tests. Typical damage mechanisms were categorized according to AE signal characteristics. The statistical results show that more matrix cracks were produced in the tensile loading specimen and no separate fiber/matrix debonding signal was detected in both specimens.     

Compressive test and numerical simulation of center‐notched composite laminates with different crack configurations

Experimental and computational studies of the composite laminates with thin center notches under axial compressive loading are carried out. A series of compressive testing of the composites with different crack lengths and angles between the loading vector and 0° fiber direction were conducted. The damage mechanisms as well as load–displacement curves are obtained from the test to analyze the effects of crack dimensions on stress distribution and ultimate load. It was shown that the compressive strength of composites drastically reduces when the crack angle goes from 0° to 90°. By studying the fracture surfaces of the tested specimens, all initial cracks within the laminates are found to extend without a straight crack path until fibers fracture simultaneously. Cases that involve crack propagation are modeled for different crack dimensions with a 3D progressive damage finite element analysis using the Abaqus. Numerical simulations qualitatively reproduce the general observations made in the laboratory experiments. POLYM. COMPOS., 38:2631–2641, 2017. © 2015 Society of Plastics Engineers     

双轴压缩下混凝土预制双裂纹扩展与应力应变关系研究

对混凝土预制双裂隙板试件进行了双轴压缩试验,研究了裂隙倾角以及岩桥倾角对双裂隙的扩展演化影响.通过在裂隙尖端贴放应变片,分析了裂隙扩展与应力应变关系,探讨了裂隙尖端应变集中对裂隙扩展演化的作用规律.试验结果表明,裂隙倾角以及岩桥倾角对裂隙的扩展、贯通有较大影响.实验结果共观测到7种裂纹贯通模式(T1和T2;S1和S2;TS1、TS2和TS3)及两种贯通失败模式(剪切失败和拉伸-剪切失败),且随着岩桥角的增加,裂纹贯通模式由剪切裂纹贯通到翼型-剪切复合式贯通,然后再到翼形裂纹贯通逐渐转化.应力应变曲线与裂隙扩展贯通密切相关,拉应变集中是翼形裂纹产生的原因,而压应变集中则是引起剪切裂纹产生的原因.     

Mechanical Properties of Two Plain-Woven Chemical Vapor Infiltrated Silicon Carbide-Matrix Composites

The elastic and inelastic properties of a chemical vapor infiltrated (CVI) SiC matrix reinforced with either plain-woven carbon fibers (C/SiC) or SiC fibers (SiC/SiC) have been investigated. It has been investigated whether the mechanics of a plain weave can be described using the theory of a cross-ply laminate, because it enables a simple mechanics approach to the nonlinear mechanical behavior. The influences of interphase, fiber anisotropy, and porosity are included. The approach results in a reduction of the composite system to a fiber/matrix system with an interface. The tensile behavior is described by five damage stages. C/SiC can be modeled using one damage stage and a constant damage parameter. The tensile behavior of SiC/SiC undergoes four damage stages. Stiffness reduction due to transverse cracks in the transverse bundles is very different from cross-ply behavior. Compressive failure is initiated by interlaminar cracks between the fiber bundles. The crack path is dictated by the bundle waviness. For SiC/SiC, the compressive behavior is mostly linear to failure. C/SiC exhibits initial nonlinear behavior because of residual crack openings. Above the point where the cracks close, the compressive behavior is linear. Global compressive failure is characterized by a major crack oriented at a certain angle to the axial loading. In shear, the matrix cracks orientate in the principal tensile stress direction (i.e., 45° to the fiber direction) with very high crack densities before failure, but only SiC/SiC shows significant degradation in shear modulus. Hysteresis is observed during unloading/reloading sequences and increasing permanent strain.     

Fractographic and Fracture Mechanical Investigations of Refractories under Different Loading Conditions

Crack initiation and propagation have been investigated under tensile and shear loading in ceramically and carbon bonded refractories.A wedge splitting test procedure and a modified shear test have been applied.Test results have been used for material characterization especially with respect to brittleness.Furthermore a microscopic fractographic test procedure was developed and applied on fractured test specimens.In order to explain brittleness dependence on structure properties correlation of fractographic and fracture mechanical results has been evaluated.Frequently brittleness reduction is achieved by a lower amount of transgranular crack propagation associated with a strength decrease while maintaining specific fracture energy unchanged.Deviations from pure linear fracture mechanics increase with decreasing brittleness and contribute to specific fracture energy.Shear specimens may show two generations of cracks,a first one initiated by tensile loads （stable propagation） and a second one by shear loads （unstable propagation）.     

Discrete element simulation of SiC ceramic containing a single pre-existing flaw under uniaxial compression

A model of a SiC ceramic containing a single pre-existing flaw was established based on the discrete element method. The effects of the flaw inclination angles, which ranged from 0° to 75°, on the mechanical properties of the specimen under uniaxial compression were studied. The evolution of the force-chain field, displacement field and stress field around the pre-existing flaw in the process from the load to failure was also analysed. The results showed that the flaw inclination angle affected the mechanical properties of the specimen as well as the initiation and propagation of the first crack. Based on the investigation of the force chain field, it was found that the distribution curve of the normal force carried by the parallel bond in the specimen with the corresponding angles under compression is similar to the “peanut” rose diagram, while the shear force distribution curve is similar to the "butterfly wings" rose diagram. In addition, in the analysis of the displacement field and the stress field, the displacement field around the flaw can be divided into four types in the process from specimen loading to its failure. Meanwhile, it was found that initiation of the first crack was affected by tensile stress. With the propagation of the first crack, the tensile stress concentration region at the flaw tip moved and dissipated correspondingly.     

Oblique Impact of Spherical Particles onto Silicon Nitride

Oblique impact damage to gas-pressure-sintered silicon nitride is investigated by examining changes in the stress field beneath the impact site of the silicon nitride. Varying the impact angle changes the morphologies of the initiated cracks from Hertzian cone crack to surface ring crack as the impact angle decreases. Moreover, impact at greater angles (90° and 60°) creates Hertizian cone cracks that drastically decrease the strength of the target materials; impact at smaller angles (45° and 30°), on the other hand, creates surface ring cracks that cause only moderate strength degradation.     

Fatigue crack propagation behavior of polyether ether ketone under biaxial loading

Polyether ether ketone (PEEK) has become a promising material in total joint replacement. However, it still faces the risk of fatigue fracture during service. In this paper, the effects of biaxial stress ratio λ, cyclic stress ratio R, and load phase difference θ on fatigue crack propagation (FCG) behavior of PEEK are investigated. In the case of vertical cracks, results show that the FCG rate of PEEK increases with the R value, while decreases with the increase of λ value. Furthermore, the effective stress intensity factor range ΔK_eff can uniformly describe the biaxial FCG behavior at different cyclic stress ratios. In the case of 45° slant cracks, compared with mode-I intensity factor range ΔK_I, the energy release rate range ΔG is more accurate for describing the FCG behavior under various load phase differences. In addition, the investigation on the 45° crack propagation path shows that a bifurcated Y-shaped crack appears under 180° load phase difference, while no bifurcated crack appears under 90° load phase difference and uniaxial loading. Three different methods are used to predict the crack propagation path. The comparison results show that the maximum circumferential stress (MTS) criterion can well predict the crack propagation path under out-of-phase biaxial loading and uniaxial loading.     

The mechanical response characteristics of sapphire under dynamic and quasi-static indentation loading

Sapphire is widely used as optical materials and substrate materials due to its excellent physical and chemical properties. The mechanism of crack propagation and fracture damage evolution has important significance for improving the manufacturing quality and application performance of sapphire parts. In this study, dynamic and quasi-static indentation tests have been performed on the c-plane and a-plane of sapphires by Hopkinson pressure bar tester and continuous indentation tester, respectively. The crack propagation path in sapphire has been captured by High-speed camera and the crack velocity has been calculated. The crack propagation and fracture damage evolution has been analyzed based on the fracture morphology of specimen. It was found that the bearing capacity of sapphire is related to the loading velocity, while the crack propagation is affected by the crystal orientation. Under the indentation loading, the cracks in sapphire first propagate steadily, and then the cracks begin to propagate uncontrollably after reaching the critical conditions, where the crack propagation velocity obviously increases, typically from 204?m/s to 1006?m/s (dynamic indentation) or from 0.0032?m/s to 820?m/s (quasi-static indentation). And the crack propagation velocity depends on the loading speed at stable stage. The r-planes of sapphire are weaker than other crystal planes and are prone to crack propagation.     

Stress Rate Dependence of Fracture Strength in Pre-Cracked Silicon Nitride Ceramics Doped with Ytterbium Oxide

High-temperature dynamic fatigue behavior has been investigated in 6 wt% ytterbium oxide and 2 wt% alumina-doped silicon nitride ceramics by nitrogen gas pressure sintering. The specimens were pre-cracked by Vickers indentation to prevent creep damage and to ensure dynamic fatigue dominating. The tests were performed in four-point flexure in air at temperatures of 1000°, 1200°, 1300°, and 1400°C and by varying the loading rate from 1, 0.5, 0.1–0.01 mm/min at each temperature. The analyses were conducted by plotting fatigue stress against loading rate at each testing temperature in double logarithm coordinates. The material was found to be the least susceptible (the highest slow crack exponent number N ) to slow crack growth at 1200°C, as reflected by the comparison of the plot slopes for the four testing temperatures. The explanation and analyses take into consideration the grain-boundary phase crystallization, crack healing, and oxidation during testing evidenced by X-ray diffraction and transmission electron microscopy. The fracture surfaces were characterized by three well-defined zones, namely zone I, II, and III, referring to the pre-cracked area, slow crack growth area, and fast fracture area, respectively.     

Deformation Mechanisms in Compression-Loaded, Stand-Alone Plasma-Sprayed Alumina Coatings

Cylindrical, stand-alone tubes of plasma-sprayed alumina were tested in compression in the axial direction at room temperature, using strain gauges to monitor axial and circumferential strains. The primary compression-loading profile used was cyclic loading, with monotonically increased peak stresses. Hysteresis was observed in the stress–strain response on unloading, beginning at a peak stress of 50 MPa. The modulus decreased as the maximum applied stress increased. The stress–strain response was only linear at low stresses; the degree of nonlinearity at high stresses scaled with the stress applied. One-hour dwells at constant stress at room temperature revealed a time-dependent strain response. Using transmission electron microscopy and acoustic emission to investigate deformation mechanisms, the stress–strain response was correlated with crack pop-in, growth, and arrest. It is proposed that the numerous defects in plasma-sprayed coatings, including porosity and microcracks, serve as sites for crack nucleation and/or propagation. As these small, nucleated cracks extend under the applied stress, they propagate nearly parallel to the loading direction along interlamellae boundaries. With increasing stress, these cracks ultimately link, resulting in catastrophic failure.     

Mechanical response and crack propagation of oil well cement under dynamic and static loads

The dynamic mechanical properties and fracture mechanism of three types of oil well cement with different formulations were investigated using a Φ50?mm split Hopkinson pressure bar (SHPB) and quasi-static mechanical tests were conducted with a hydraulic universal testing machine. The stress-strain diagram, time-stress diagram, total energy absorption diagram, and the dynamic growth factor (DIF) under different strain rates were obtained. The crack propagation process of the oil well cement under dynamic loading is evaluated using high-speed photography to determine the fracture mechanism. The test results show that the strength of the cement increases under a dynamic impact. The compressive strength of the pure cement increases from 37?MPa to 184.80?MPa under static loading. However, the peak stress of the cement stone strengthened with cellulose fiber is lower under a dynamic load than a static load. Under dynamic loading, the absorption energy is higher for the pure cement stone than for the cement stone reinforced with whiskers and cellulose. Furthermore, the crack initiation, crack propagation, and fracture characteristics of the oil well cement are different under dynamic and static loads. Under a static load, the rupture of the cement is the result of the propagation of the tensile cracks. Under dynamic loading, there are fewer micro cracks on the cement surface and a composite fracture results from tensile and shear cracks.     

Fracture of Brittle Materials under a Simulated Wear Stress System

Currently no data are available for fracture toughnesses associated with the compression/shear (mixed mode) stress distribution of the sliding-wear loading system. To provide such information, diametrically compressed disk specimens of electrical porcelain were tested and used to develop a fracture criterion for a body containing a crack which is subjected to mixed-mode loading. The role of applied shear loads in creating local tensile stresses near the crack tip is discussed. While these stresses are embodied in the maximum-hoop-stress theory of mixed-mode fracture, it was found that the toughness values had only a fair fit with theory. However, the direction of initial crack growth, which is out-of-plane, is consistent with theory. Since effects due to crack-face rubbing were limited by using a finite-thickness notch in the test specimen (instead of a crack), the data, although indicative, are not fully representative of debris formation during sliding wear. The paper concludes with the formulation of a simple model of the formation of a debris particle.     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.