Dynamic Failure of Borosilicate Glass Under Compression/Shear Loading Experiments

Dynamic compression/shear experiments on a borosilicate glass at an average strain rate of 250 s⁻¹ are conducted using a modified version of a split Hopkinson pressure bar. Instead of applying confining pressure, cuboid specimens with the material axis inclining to the loading direction at different angles (0°, 3°, 5°, and 7°) are used to generate higher shear stresses. A high-speed digital camera, synchronized with the loading stress pulse, is used to record the dynamic crack initiation and propagation. Experimental results show that the equivalent stress at failure decreases with increasing shear portion in the stress. Digital images show that the cracks initiate randomly in the right specimen, whereas cracks initiate from the stress-concentrated corners in the inclined specimens. Subsequent crack propagation, despite specimen inclination angles, is along the specimen axis rather than the compressive loading direction.     

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.     

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.     

Dynamic compressive strength of alumina ceramics

Dynamic (220–510 s^?1) and quasi-static (0.001 s^?1) compression experiments are conducted on alumina ceramics implemented with two types of tungsten carbide inserts, cylindrical and step-shaped. Split Hopkinson pressure bar (SHPB) tests with in-situ, high-speed optical imaging are adopted to capture the damage and failure of ceramic samples under dynamic compression. The compressive strength of alumina ceramic samples with step-shaped inserts is 15%–30% higher than that with cylindrical inserts commonly used in previous studies, under both dynamic and quasi-static loading. Damage occurs first at the two ends of ceramic samples with the cylindrical inserts, followed by edge fracture and splitting cracks penetrating the sample. However, damage is initiated in the sample region away from the sample ends for the step-shaped inserts, and oblique and secondary transverse cracks dominate the failure process. The different damage modes in the case of step-shaped inserts result in the delayed damage initiation and sample failure, and consequently high compressive strengths. Finite element modelling (FEM) of the SHPB tests provides strength and damage evolution features consistent with the experiment using the Johnson–Holmquist (JH-2) model. FEM reveals equivalent, tensile and shear stress concentrations at the two ends of samples with cylindrical inserts. The stress concentrations are responsible for the damage initiation and growth at the sample ends and the following splitting cracks, consistent with the high-speed images. In contrast, homogeneous stress distributions are achieved in the sample with the step-shaped inserts, ensuring simultaneous damage development across the sample. Overall, the step-shaped inserts in conjunction with cylindrical samples can yield reliable strength measurements for ceramics and ceramic-like materials.     

Static and Dynamic Compressive Behavior of Aluminum Nitride under Moderate Confinement

An experimental technique for imposing lateral confinement on specimens subjected to dynamic or quasi-static uniaxial compression has been developed. Lateral confinement is provided by a shrink-fit metal sleeve installed on the lateral surface of a cylindrical ceramic specimen. Experiments using this technique were performed on sintered aluminum nitride (A1N). The results show that the failure mode changes from fragmentation by axial splitting under conditions of uniaxial stress to localized faulting under moderate lateral confinement. The compressive failure strength of the A1N increases with the increase of confinement pressure under both static and dynamic loading conditions. The effect of strain rate on the failure strength appears to be independent of the confinement pressure.     

A Test Method to Measure Shear Strength of Ceramic Joints at High Temperatures

The asymmetrical four-point bend (AFPB) method, which provides a pure shear stress state in the middle of the test specimen, was utilized to measure the shear strength of the joints between Nicalon-fiber-reinforced SiC composites. The small test specimens were prepared as butt-joints with uniform depth (no notch). It was shown experimentally that the position of loading points is critical to induce shear fracture at the joint without the failure of base material. The appropriate positions for the present test specimen were determined and, as a result, an articulated ceramic fixture was designed. This test method is economical and involves small test specimens with simple geometry, and can be used at high temperatures.     

Analysis of a pull-out test with real specimen geometry. Part II: the effect of meniscus

This paper continues our study on the platelet model of the pull-out specimen, in which the matrix droplet shape is approximated by a set of thin parallel disks with the diameters varying along the embedded fiber. Using this model, the fiber tensile stress and the interfacial shear stress profiles were calculated for real-shaped matrix droplets, including menisci (wetting cones) on the fibers, taking into account residual thermal stresses and interfacial friction. Then, these profiles were used to numerically simulate the processes of crack initiation and propagation in the pull-out test and to obtain theoretical force-displacement curves for specimens with different embedded lengths and wetting cone angles. Our simulations showed that the interfacial crack in real-shaped droplets initiated at very small (practically zero) force applied to the fiber, in contrast to the popular ‘equivalent cylinder’ approximation. As a result, the equivalent cylinder approach underestimated the interfacial shear strength (IFSS) value determined from the pull-out test and at the same time overestimated the interfacial frictional stress; the smaller was the wetting cone angle, the greater the difference. We also investigated the effects of the embedded fiber length and interfacial frictional stress in debonded areas on the calculated IFSS. The simulated force–displacement curves for the real-shaped droplets showed better agreement with experimental curves than those plotted using the equivalent cylinder approach.     

Dynamic failure and inelastic deformation behavior of SiC ceramic under uniaxial compression

The investigation of the dynamic compressive properties and failure process of ceramics can broaden their applications under extreme conditions. In this study, the online dynamic deformation and failure processof silicon carbide (SiC) ceramic were directly observed in the split-Hopkinson pressure test by high speed photography. The failure strength of the silicon carbide has been quantified as a function of strain rate. There appears to be a critical value (a transition strain rate), above which the strain rate dependency becomes significant. The failure in the dynamic test is an outcome of the concurrent incline major crack and longitudinal micro-cracks. Scanning electronic microscopy (SEM) observation of the dynamic fragments shows that the transgranular fracture dominates the failure process. High-resolution transmission electron microscopy (HRTEM) observation of the post-collected fragments indicates that the local shear stress causes the nucleation of dislocations responsible for the macroscopic inelastic deformation behavior.     

Combined Mode I-Mode III Fracture of Fatigue-Precracked Alumina

The mixed-mode fracture behavior of (cyclic) fatigue pre-cracked ceramic specimens was studied in combined tension-torsion loading. Circumferentially notched cylindrical rods of polycrystalline alumina were precracked in uniaxiai cyclic compression to introduce a concentric mode I fatigue crack. Subsequently, the rods were quasi-statically fractured in pure tension, pure torsion, and various combinations of tensile and torsional stresses to obtain the mode I-mode III fracture envelope. The introduction of torsional loads promotes severe abrasion between the crack faces. The critical stress intensity factor for fracture initiation increases by a factor of °2.3 as the loading mode is changed from pure tension to pure torsion. Fracture surface tortuosity and abrasion "shield" the crack-tip from the far-field tensile and torsional loads to cause an apparent toughening effect. The mechanisms of mixed-mode fracture in alumina are examined and consequences of the breakdown of the similitude concept implicit in the nominal use of fracture mechanics are discussed.     

Mechanical behavior of ceramic-metal joint under quasi-static and dynamic four point bending: Microstructures,damage and mechanisms

This paper studies the mechanical behavior of Alumina ceramic-Kovar joint under quasi-static and dynamic four-point bending (FPB). The joint is fabricated by molybdenum-manganese (Mo-Mn) metallization method with extra additions. The bend strength of the joint is improved by glass phase migration. Electronic universal testing machine and modified split Hopkinson pressure bar (SHPB) are employed to realize the loading process. The microstructure of the joint is investigated by scanning electron microscope (SEM) and the chemical composition is determined by energy dispersive spectrometer (EDS). Digital image correlation (DIC) technique is used to determine the displacement contours and ultra-high speed camera is used to monitor the deformation and crack evolution around the joint. It is found that the specimen will slip due to the different Young's modulus of base material. The dynamic bend strength of the joint is lower than the quasi-static bend strength. The failure mechanism of the ceramic-metal joint is mainly intergranular failure for the dynamic bend but mixed transgranular/intergranular failure for the quasi-static case. The crack starts from the inherent voids inside the ceramic and then expands along the metallization band between solder and ceramic.     

Fatigue crack initiation and damage characterization in Brazilian test specimens for adhesive joints

The present study is focused on the fatigue failure initiation at bimaterial corners by means of a configuration based on the Brazilian disc specimens. These specimens were previously used for the generalized fracture toughness determination and prediction of failure in adhesive joints, carried out under static compressive loading. Under static loading, local yielding effects might affect the asymptotic two-dimensional linear elastic stress representation under consideration. Fatigue loading avoids this fact due to the lower load levels used. The present tests were performed using load control; video microscopy and still cameras were used for monitoring initiation and crack growth. The fatigue tests were halted periodically and images of the corner were taken where fatigue damage was anticipated. Damage initiation and subsequent crack growth were observed in some specimens, especially in those which presented brittle failure under static and fatigue tests. These analyses allowed the characterization of damage initiation for a typical bimaterial corner that can be found in composite to aluminium adhesive lap joints.     

Strain-rate-dependent compressive and compression-shear response of an alumina ceramic

This paper assessed the microstructure and properties of CeramTec ALOTEC 98 SB alumina ceramic through microscopic characterization and mechanical experiments. The rate-dependent strength and failure response of an alumina ceramic were studied under both uniaxial compression and compression-shear loading. Under quasi-static uniaxial compression at rates of 10^?5 to 10³ s^?1, the strength had an average of 3393 ± 306 MPa, and at dynamic strain rates of 10² to 10³ s^?1, the strength ranged from 3763 to 4645 MPa. The CeramTec ALOTEC 98 SB alumina ceramic was found to have greater mechanical properties than other commercial alumina ceramics from the literature (i.e., AD-995). To monitor the strain field and the failure process of the alumina ceramic during testing, an ultra-high-speed camera coupled with digital image correlation (DIC) was used to visualize crack initiation and propagation processes, and obtain quantitative stress-strain information. A new data processing method was then proposed in this study to calculate the shear components for the compression-shear tests. Validation of the proposed method was confirmed by the shear strain obtained from the DIC analysis with the ultra-high-speed camera. Using the results obtained by the proposed model and the DIC analysis, new observations and understandings of failure mechanisms are obtained. (1) In compression-shear tests, the shear failure happens before complete failure, and shear behavior plays an important role during the failure process. (2) The equivalent peak stress (strength) of compression-shear test is smaller than the uniaxial compression one. (3) The directional cracks have weak influence on the compressive stiffness, but have a strong influence on the shear response.     

Critical Tractions for Initiating Adhesion Failure at Interfaces in Encapsulated Components

Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.     

Determination of dynamic delamination toughness of a graphite‐fiber/epoxy composite using Hopkinson pressure bar

Modified dynamic three‐point‐bending and compact shearing test configurations based on Hopkinson pressure bar (HPB) and crack detection gage (CDG) (Vishay Intertechnology, Inc) were used for the determination of the dynamic mode I and mode II delamination‐initiation toughness of a unidirectional graphite‐fiber/epoxy composite made of P7051S‐20Q‐1000 prepregs (Toray Composites America). The transient loading history was recorded precisely by the HPB installed with a high‐resolution digital oscilloscope, and the crack initiation and delay time were captured using the CDG. By means of dynamic finite‐element analysis (FEA) of the impact processes with the loading history and crack initiation time as input, the critical dynamic stress intensity factors (DSIFs) (K_IDC/K_IIDC) were extracted from numerical results of the crack opening displacements (CODs). Results show that under the present transient loadings, the K_IDC value is about 80–90% of the static one, while the K_IIDC value is nearly unchanged. Dynamic failure mechanisms of the composite specimens were evaluated by fractography using a scanning electron microscope (SEM). POLYM. COMPOS., 26:165–180, 2005. © 2005 Society of Plastics Engineers     

Critical Tractions for Initiating Adhesion Failure at Interfaces in Encapsulated Components

Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.     

Effect of Loading Rate on the Monotonic Tensile Behavior of a Continuous-Fiber-Reinforced Glass-Ceramic Matrix Composite

The stress-strain behavior of a continuous-fiber-reinforced ceramic matrix composite has been measured over a wide range of loading rates (0.01 to 500 MPa/s). It was found that the loading rate has a strong effect on almost every feature of the stress-strain curve: The proportionality stress, the composite strength and failure strain increase with increasing loading rate. The microstructural damage varies also with the loading rate; with increasing loading rate, the average matrix crack spacing increases and the average fiber pullout length decreases. Using simple models, it is suggested that these phenomena are caused partly by time-dependent matrix cracking (due to stress corrosion) and partly by an increasing interfacial shear stress with loading rate.     

In vitro shear bond strength test and failure mechanism of zinc phosphate dental cement

The cohesive shear bond strength (SBS) of hardened zinc phosphate cement (CeCe) was comparatively measured with those of the adhesive SBS of the human dentin–cement (DCe), artificial acrylic crown–cement (CrCe), dentin–cement–crown (DCeCr). Results from experiments found that the SBS of CeCe specimen is much higher than those of the adhesive SBS. The average maximum SBS of CeCe, DCe, DCeCr and CrCe specimens of approximately 6.91, 1.02, 0.67 and 0.25 MPa, are respectively obtained. Fractographs taken by scanning electron microscope and close-up camera images of the fracture surfaces were analyzed for their failure mechanisms. The crack initiation and propagation of DCe and CrCe bonding types occur along the bonding interface. In the DCeCr bonding type, the crack initiates at crown–cement interface, propagates downward and changes in to the dentin-cement interface until failure. The average fracture area ratio of the CrCe interface to DCe interface of about 85:15 are observed for the DCeCr de-bonded specimens. Results from force and stress analysis using the two point bonding model indicate that the shear stress is more evident for the bond fracture.     

Cyclic contact fatigue behavior of baria-silicate glass-ceramics as a function of crystal aspect ratio

Ceramic materials are potentially useful for dental applications because of their esthetic potential and biocompatibility. However, evidence of contact fatigue damage in ceramics raises considerable concern regarding its effect on the survival probability predicted for dental prostheses. To simulate intraoral conditions, Hertzian indentation loading with steel indenters was applied in this study to characterize the fatigue failure mechanisms of ceramic materials. Baria silicate glasses and glass-ceramics with different aspect ratios of crystals were selected because the glass and crystal phases have similar density, elastic modulus, and thermal expansion coefficients. Therefore, this system is a model ceramic for studying the effect of crystal geometry on contact cyclic fatigue failure. The subsequent flexural strength results show that the failure of materials with a low fracture toughness such as baria-silicate glass (0.7 MPa m^1/2) and glass-ceramic with an aspect ratio of 3.6/1 (1.3 MPa m^1/2) initiated from cone cracks developed during cyclic loading for 10³ to 10⁵ cycles. The mean strengths of baria-silicate glass and glass-ceramics with an aspect ratio of 3.6/1 decreased significantly as a result of the presence of a cone crack. Failures of baria-silicate glass-ceramics with an aspect ratio of 8.1/1 (K_c = 2.1 MPa m^1/2) were initiated from surface flaws caused by either grinding or cyclic loading. The gradual decrease of fracture stress was observed in specimens with an aspect ratio of 8.1/1 after loading in air for 10³ to 10⁵ cycles. A reduction of approximately 50 % in fracture stress levels was found for specimens with an aspect ratio of 8.1/1 after loading for 10⁵ cycles in deionized water. Thus, even though this glass-ceramic with an 8.1/1 crystal aspect ratio material is tougher than that with a 3.6/1 crystal aspect ratio, the fatigue damage induced by a large number of cycles is comparable. The mechanisms for cyclic fatigue crack propagation in baria-silicate glass-ceramics are similar to those observed under quasi-static loading conditions. An intergranular fracture path was observed in glass-ceramics with an aspect ratio of 3.6/1. For an aspect ratio of 8.1/1, a transgranular fracture mode was dominant.     

Direct numerical simulations on crack formation in ceramic materials under thermal shock by using a non-local fracture model

In this work, a non-local failure model was proposed and implemented into a finite element code. It was then used to simulate the crack evolution in ceramic materials subjected to thermal shock. By using this numerical model, the initiation and propagation of cracks in water quenched ceramic specimens were simulated. The numerical simulations reproduced faithfully the crack patterns in ceramic specimens underwent quenching tests. The periodical and hierarchical characteristics of the crack patterns were accurately predicted. The numerical simulations allow a direct observation on whole the process of crack initiation and growth, which is quite a difficult task in experimental studies. The failure mechanisms and the fracture procedure are discussed according to the numerical results obtained from the simulations. It is shown that the numerical model is simple, robust, accurate and efficient in simulating crack evolution in real structures under thermal shock.     

Static and Cyclic Fatigue of Alumina at High Temperatures: II, Failure Analysis

Failure mechanisms of an alumina, tested at 1200°C under static and various cyclic loading conditions, were examined. Slow crack growth of a single crack is the dominant mechanism for the failure in specimens under cyclic loading with a short duration of maximum stress at all applied stress levels, as well as at high applied loads for static loading and cyclic loading with a longer hold time at maximum stress. At low stress levels, failure of static loading and cyclic loading with a longer hold time at maximum stress might occur by formation and/or growth of multiple macrocracks. More importantly, for all the given loading conditions. The viscous glassy phase behind the crack tip could have a bridging effect on the crack surfaces. A simplified model for calculating effective stress intensity factor at the crack tip under static and various cyclic loading demonstrated a trend consistent with the stress–life data.