Brittle Specimens Research Articles

Bond-Based Peridynamic Model for Tensile Deformation and Fracture of Polycarbonate and Polypropylene

Fracture mechanics has been a crucial aspect in the field of engineering science as technologies are rapidly growing nowadays. Various numerical methods have been developed to analyze fracture behaviour in different types of materials used in industries. Meanwhile, the application of polymers garners attention worldwide due to outstanding characteristics such as good strength, lightweight, and high temperature resistance, exemplified by polymers like polycarbonate (PC) and polypropylene (PP). Hence, failure aspects of such materials must be taken into consideration when conditions arise that may lead to failure, such as high-load impact, fatigue, and extreme temperatures. In this study, a bond-based Peridynamic model (PD) for the tensile behaviour, including fracture, of polymers has been developed. The PD model is constructed using the Centos software and encompasses both brittle and ductile fracture behaviours. Numerical results, including crack propagation, damage zone, and force-extension curves of notched specimens, are validated by comparison with experimental results of PC and PP. Through the validation process, PC specimens exhibit a difference percentage range for maximum load and rupture extension of 2.9% to 18.8% and 2.4% to 4.6%, respectively. PP specimens show a difference percentage range for maximum load and rupture extension of 31.2% to 43.5% and 0.9% to 30%, respectively. Consequently, the validation results indicate that the PD model for brittle specimens aligns more closely with experimental data compared to the PD model for ductile specimens.

Operando Quantification and Visualisation of Dendrite-Induced Stresses in Ceramic Electrolytes Via Photoelasticity

The measurement of stress fields around lithium metal dendrites in solid electrolytes in operating conditions is critical for the design of next-generation, dendrite-resistant solid electrolytes. Prior work, both experimental and theoretical, indicates that the developed stresses in the in the electrolyte can be significantly high. However, direct stress measurements are still missing as they entail inherent experimental difficulties associated with probing stress fields at small scale brittle specimens operando.By employing the principle of photoelasticity combined with electrochemical cycling in a plan-view cell the aforementioned challenges are bypassed, allowing not only to track the stress field as the dendrite events progress, but also to obtain whole-field stress information on a propagating dendrite in an LLZTO electrolyte. This new experimental methodology allows for direct stress measurements around the dendrite tip. The experimental data demonstrate that the dendritic events at such current densities can be understood by the classic Griffith - Irwin fracture theory.

Evaluation of the relationships in the tensile plastic deformations and the nano-micro structures of PA6 and PAMXD10 with different thermal histories

Polyamide (PA) is a semi-crystalline polymer in which the main chain is configured by repeating units of amide bonds (NHCO). The strength of PA is achieved by the intermolecular hydrogen bonds between the hydrogen and oxygen atoms of the amide bonds. The thermal history conditions change the crystallization rate and crystalline structure of the PA, which affects its macroscopic mechanical properties. In this study, the relationship between the nano-micro structure of PAs and their macroscopic plastic deformation behavior was investigated. PA6 and PAMXD10 specimens were prepared with different microstructures obtained from two different crystallization processes, called the “isothermal” and “annealing” conditions, respectively. The microstructures were investigated using differential scanning calorimetry, small angle X-ray scattering (SAXS), large angle X-ray scattering and polarized optical microscopy, and the mechanical behaviors were evaluated using uniaxial tensile tests. The maximum stresses of the stress strain curves for PA6 and PAMXD10 increased with temperature under isothermal and annealing conditions. Although the ductility of PA6 and PAMXD10 obtained under isothermal conditions at a higher temperature decreased, both PA specimens obtained under the annealing condition at a higher temperature elongated without fracture across a large strain range. The SAXS measurements revealed that the nano-micro structures of the brittle specimens in PA6 and PAMXD10 consisted of thick lamellae and thick amorphous phases. In contrast, large and small long-period structures co-existed in the specimen obtained under annealing conditions. This indicates that the presence of a composite structure, in which large and small long-period structures are mixed, inhibits the propagation of fractures in the later stages of deformation.

VO-Notches Subjected to Tension-Torsion Loading: Experimental and Theoretical Fracture Study on Polymeric Samples.

In this research, the fracture behavior of brittle specimens weakened by V-shaped notches with end holes (VO-notches) is studied. First, an experimental investigation is conducted to evaluate the effect of VO-notches on fracture behavior. To this end, VO-notched samples of PMMA are made and exposed to pure opening mode loading, pure tearing mode loading, and some combinations of these two loading types. As part of this study, samples with end-hole radii of 1, 2, and 4 mm are prepared to determine the effect of the notch end-hole size on the fracture resistance. Second, two well-known stress-based criteria, namely the maximum tangential stress (MTS) criterion and the mean stress (MS) criterion, are developed for VO-shaped notches subjected to mixed-mode I/III loading, also determining the associated fracture limit curves. A comparison between the theoretical and the experimental critical conditions indicates that the resulting VO-MTS and VO-MS criteria predict the fracture resistance of VO-notched samples with about 92% and 90% accuracy, respectively, confirming their capacity to estimate fracture conditions.

Effects of rock ductility on the fracability of ductile reservoirs: An experimental evaluation

Deep hydrocarbon resources below 4000 m are an important replacement for shallow-depth resources; however, as depth increases, the difficulty and cost of development increase sharply. One of the possible reasons is that deep reservoir rocks exhibit strong ductility, which causes difficulties to stimulate a well-developed fracture network in rocks. To date, the mechanism by which rock ductility affects reservoir fracturing remains unclear, and there is a lack of quantitative evaluation models for reservoir fracability. To overcome the problems, in this study, artificial rock specimens with different ductility were fabricated using similar materials based on the geological data and petrophysical properties of the target deep reservoir; the specimens were classified using the proposed ductility index BIf. The hydrofracturing and CT scanning experiments of the artificial specimens with different BIf values under various horizontal stress differences Kh were conducted. The spatial fractal index Df was proposed to characterize the three-dimensional morphology and development of the fracturing network. The fracability evaluation function of ductile rock correlating the indices BIf and Kh was established. It was shown that ductile rocks tended to form non-planar hydraulic fractures with obvious tortuosity and the degree of fracture network development was lower than that of brittle rocks. The maximum breakdown pressure was approximately twice that of the brittle specimen, while the corresponding injection energy was approximately 5–10 times and the energy dissipation of fracture propagation was approximately 4–6 times those of the brittle rock. When Kh ≥ 0.3 the maximum horizontal principal stress played a major role in controlling the hydraulic fracture propagation in ductile rocks. If Kh < 0.3, the fracture propagation in rocks was mainly governed by material ductility rather than the maximum horizontal principal stress. This study provides a new way to explore and quantify the effect of rock ductility on the fracability of deep reservoirs.

Rotational flexural strength of cylindrical brittle specimens

AbstractConventional static flexural strength testing of brittle cylindrical rods only subjects a small fraction of the entire specimen's area or volume to the maximum tensile stress. Thus, a nonconservative measured strength likely results since most flaws on the surface or in the bulk are not subjected to a sufficiently high tensile stress that can cause fracture. To mitigate this, a rotational flexural tester and corresponding test method were developed whereby rotation and monotonically increasing three‐point flexure were superimposed to investigate fracture response of solid glass cylinders. This combination of rotation and flexure subjects more area and volume of a cylindrical test specimen to tensile stress than a standard static (nonrotating) flexural test. As anticipated, failure stresses were lower for the rotational flexural test. Expressions for effective area and volume are provided for rotating solid rods and tubes subjected to three‐point, four‐point, uniform, and uniformly distributed load bending configurations.

Predicting the flexural strength of chemically strengthened aluminosilicate glass plates with fractography

This work introduces a novel, fractographic method to extend the current, state-of-the-art, fractographic techniques to chemically strengthened (CS) glasses with large, surface flaws. The proposed method uses proven relationships based on dimensional analysis to correlate the flexural fracture strength to newly-introduced, fractographic length-scales. The technique requires knowledge of the residual stress profile and access to standard visualization tools such as optical microscopes. The proposed methodology was validated on three sets of aluminosilicate CS glass plates with depth of the compressive layer (DOL)ranging between 28~54μm and surface stress 980–1140 MPa, and one set of untreated plates. For the CS and untreated glass plates considered, the proposed fractographic technique estimated the fracture strength within (2 ± 11)% of experimental values, which is comparable to the accuracy of fractographic techniques currently used for annealed, brittle specimens.

Exploring the acoustic activity in brittle materials in terms of the position of the acoustic sources and the power of the acoustic signals—Part I: Founding the approach

The acoustic activity in brittle specimens submitted to mechanical loading is explored in terms of two alternative approaches. The first one is based on the Euclidean three-dimensional distance between the sources of successive acoustic events while the second is based on the power of the acoustic signals recorded. In the direction of quantifying the acoustic activity according to these alternative approaches two functions are introduced, the time evolution of which is considered in juxtaposition to that of the externally applied mechanical load and the respective one of a previously introduced function, namely, the F-function. The latter is determined by taking advantage of the interevent time intervals between successive acoustic signals. Data gathered during an experimental protocol, with notched marble plate-shaped specimens subjected to uniaxial tension, are used to outline the procedure for calculating the newly introduced functions. Analysis of the time evolution of these functions exhibits series of common qualitative characteristics with the respective one of the F-function. Moreover, it is indicated that proper interpretation of the respective data in terms of the “time-to-failure” parameter provides interesting signs that could be considered as pre-failure indices.

Uji Kekuatan Bending Pipa Komposit Jute-Epoxy pada Perlakuan Rendaman Air Panas

The main purpose of this study was to determine the comparison of the bending strength of composite pipes when treated with hot water. In this study, composite pipes were made using woven sacks and tied with an epoxy matrix which was molded using the VARTM (Vacuum Assisted Resin Transfer Molding) method. Jute fiber laminated composite on epoxy binder is made by laminated 3 layers with fiber direction 450 and has a thickness of 2.55 mm, the jute-epoxy composite pipe will be treated with heat treatment. Then tested on bending loading. The results of this study were the bending stress and strain on the control specimen compared to the heat treated specimen, an increase in the strain of the heat treatment specimen was 0.007 [MPa], this was due to the heat treatment temperature on the specimen which caused a change in the properties of the specimen itself, which was initially a brittle specimen changes to plastic. Meanwhile, the value of bending stress and modulus of elasticity on control specimens and heat treatment specimens. The obtained stress on the control composite pipe is 67.946 [MPa] with a modulus of elasticity of 10.398 [GPa]. While the heat-treated composite pipe has a bending stress of 28.613 [MPa] with an elastic modulus of 2.225 [GPa].

Global and local fracture behavior in a brittle solid with a set of pre-existing small-scale cracks

For understanding the mechanics of complicated seismic activities, not only global observations of large-scale material behavior but also more local tracing of smaller-scale fractures is needed. In this study, therefore, by using the technique of dynamic photoelasticity, the evolution of fractures and waves is experimentally traced in a two-dimensional linear elastic brittle specimen that has a set of pre-existing small-scale parallel fracture regions. The small-scale fracture regions or cracks, modeling a large-scale geological normal fault plane, are prepared by a digital laser cutter. Our experimental observations with a high-speed video camera, together with finite difference numerical simulations, indicate that the global and local fracture behavior significantly depends on the initial inclination angle and distribution pattern of the set of parallel cracks. In the case of pre-existing cracks that are more vertically inclined with respect to the quasi-static external load applied by a tensile testing machine, the main fracture develops dynamically and then the secondary fractures appear at spatiotemporally remote distances from the main fracture. The secondary fractures extend also dynamically, into the direction opposite to the main fracture. On the other hand, in the case of smaller inclination angles, the main fracture tends to evolve quasi-statically. Nevertheless, in both cases, it is found that fracture can be arrested and jump without difficulties in a brittle solid having a number of small-scale cracks, which implies that such small-scale cracks may play an important part in the generation of a cluster of seismic events.

Cracking Patterns of Brittle Hemispherical Domes: an Experimental Study

Crack formation in hemispherical domes is a distinguished problem in structural mechanics. The safety of cracked domes has a long track record; the evolution of the cracking pattern received less attention. Here, we report displacement-controlled loading tests of brittle hemispherical dome specimens, including the evolution of the meridional cracking pattern. The 27 investigated specimens, 20 cm in diameter, were prepared in 3D printed molds, and their material is one of the three mixtures of gypsum and cement. We find that neither the (limited) tensile strength nor the exact value of the thickness significantly affects the statistical description of the cracking pattern, i.e., the cracking phenomenon is robust. The maximal number of the meridional cracks never exceeds seven before the fragments’ disintegration (collapse). We find that the size distribution of the fragments exhibits a lognormal distribution. The evolution is reflected in the load-displacement diagrams recorded in the test, too, as significant drops in the force are accompanied by an emergence of one or more new cracks, reflecting the brittle nature of the phenomenon. A simple, stochastic fragmentation model, in which a segment is fragmented at either in the middle or at the fourth point, fairly recovers the observed size distribution.

Finite Element Analysis on Initial Crack Site of Porous Structure Fabricated by Electron Beam Additive Manufacturing.

Ti6Al4V specimens with porous structures can be fabricated by additive manufacturing to obtain the desired Young’s modulus. Their mechanical strength and deformation behavior can be evaluated using finite element analysis (FEA), with various models and simulation methodologies described in the existing literature. Most studies focused on the evaluation accuracy of the mechanical strength and deformation behavior using complex models. This study presents a simple elastic model for brittle specimens followed by an electron beam additive manufacturing (EBAM) process to predict the initial crack site and threshold of applied stress related to the failure of cubic unit lattice structures. Six cubic lattice specimens with different porosities were fabricated by EBAM, and compression tests were performed and compared to the FEA results. In this study, two different types of deformation behavior were observed in the specimens with low and high porosities. The adopted elastic model and the threshold of applied stress calculated via FEA showed good capabilities for predicting the initial crack sites of these specimens. The methodology presented in this study should provide a simple yet accurate method to predict the fracture initiation of porous structure parts.

An Experimental Study on the Creep Characteristics of Sandstone in the Interval of Different Critical Stresses

Creep tests on brittle sandstone specimens were performed to investigate the time-dependent characteristics in the interval of different critical stresses. The results showed that failure will not occur when the loaded stress σ1 is less than the critical stress of dilation σcd, while all specimens were destroyed when σ1 is larger than σcd. In addition, the value of σcd was very close to the long-term strength obtained by the method of the isochronous stress-strain curve. Therefore, σcd can be regarded as the long-term strength of the sandstone specimens. When σ1 is larger than σcd, the time required for the failure of specimen tf decreases with the increase of σ1; the creep rate dε/dt increases with time t, and the specimen will be destroyed when it reaches a maximum value (dε/dt)max. Both relationships tf and σ1 and (dε/dt)max and σ1 can be described by the exponential function. Then, a nonlinear damage creep model considering the deformation damage and strength damage in the interval of different critical stresses was established, which can describe the whole creep process and predict the failure time of sandstone specimens.

Research on triaxial compression failure characteristics and meso-simulation of brittle gypsum material

The laboratory tests and Particle Flow Code numerical simulation of triaxial compression are conducted to analyze fractal dimensions of fracture surface, strength properties, energy dissipation, damage laws and crack evolution. Under low confining pressure, the fracture surface of the brittle specimen is rough and the angle relative to the maximum principal stress is small. The specimen eventually exhibits local tensile-shear failure mode. With the increase of confining pressure, the strength of the specimen increases and yield platform for the stress–strain curve is more obvious. The main fracture surface becomes increasingly flatter and the fracture angle increases with the increase of confining pressure. The specimen exhibits a shear failure mode macroscopically. With the increase of confining pressure, the trend of transgranular shear failure becomes obvious and the micro-fracture surface has a smaller fractal dimension. In the triaxial compression test, energy storage, dissipation and release occur. When the storage limit is reached, the internal elastic strain energy releases sharply and the damage variable increases rapidly. As the confining pressure increases, the number of tensile and shear cracks is accumulated in an ‘S’ shape. Meanwhile, the proportion of shear cracks increases gradually.

Mechanical and Acoustic Responses of Brittle Geomaterials with a Hole under a Compressive Disturbance

The objective of this article was to investigate the acoustic emission (AE) precursor information and crack propagation mechanism of brittle granite specimens containing a hole under uniaxial compression using AE and digital image correlation (DIC) techniques. The results show that the existence of the opening in the specimen leads to the degradation of the mechanical properties, and the stress‐strain curve exhibits several stress drops. Each deformation stage of rock had different AE characteristics, so AE signals can be used to characterize the microscopic damage evolution of rock and predict the macroscopic failure process of the surrounding rock of the opening. The deformation field evolution process of the specimen containing an opening gradually develops from a uniform distribution to prominent local deformation, and the deformation of the local area near the opening is greater than that of other areas. The initiation of cracks begins around the opening, and the propagation of tensile cracks around the opening is primarily influenced by the nucleation and propagation in the strain localization area. The crack propagation evolution derived from DIC images can be combined with AE monitoring to accurately reveal the deformation and failure mechanism of the intact specimen containing an opening.

Characterization and mesosimulation of the triaxial compression failure of rock-like materials

The laboratory tests and the numerical simulation of the particle flow code of triaxial compression are conducted to analyze the fractal dimensions of fracture surface, strength properties, energy dissipation, damage patterns, and crack evolution. The following results are obtained. (1) Under low confining pressure, the fracture surface of a brittle specimen is rough, and the angle relative to the maximum principal stress is small. The specimen eventually exhibits a local tensile–shear failure mode. As the confining pressure increases, the strength of the specimen increases, and the yield platform for the stress–strain curve is more obvious. The main fracture surface flattens, and the fracture angle increases as the confining pressure increases. The specimen macroscopically exhibits a shear failure mode. (2) As the confining pressure increases, the trend of transgranular shear failure becomes obvious, and the microfracture surface has a smaller fractal dimension. (3) In the triaxial compression test, energy is stored, dissipated, and released. When the storage limit is reached, the internal elastic strain energy becomes released sharply, and the damage variable increases rapidly. (4) As the confining pressure increases, the number of tensile and shear cracks accumulates in an “S” shape, and the proportion of shear cracks increases gradually.

Failure characteristics of brittle rock-like specimen with fractured internal structure under uniaxial compression

The methods commonly used to prepare fractured structures can be used to form only 2D cracks or holes in rock specimen. In engineering, however, the geological structures considered usually have prominent 3D characteristics. To prepare specimens with 3D internal structures, a method to form real cavity in a hard-brittle rock-like model is proposed in this paper. The uniaxial compression test was then conducted on it using three cylinder samples (solid, internal fracture, and internal cavity), and main conclusions were obtained: (1) By using the volume loss method and super absorbent polymer as material in a small-scale laboratory test, rock-like samples with internal open-type flaws and cavities were prepared such that they were not in contact with the outer surfaces of the samples. (2) The results of the compression test show that the internal fractures reduced the uniaxial compressive strength as their volume increases. However, the size of the internal fracture had little influence on the elastic modulus and peak-time axial strain of the samples over a certain range. (3) Unlike the final failure surface obtained in 2D studies, the splitting surfaces shown in this test always extended diagonally with respect to the internal structures and were vertical to the preset fractures. (4) Because the horizontal section of the internal cavity/flaw was square in shape (the diagonals were perpendicular to each other), the direction of extension of the tensile failure surface was not along the horizontal diagonals of the internal structures but perpendicular to the corresponding diagonals.

63‐3: Translating 2 Point Bend with Step Stress Methodology

Translating 2‐point bend with step stress was developed to increase thin brittle specimen area tested under peak stress. The benefits include improved weakest link defect detection and readily extractable survivor data. Highly censored Weibull distributions can be obtained for life modeling applications. A description of the method, it's validation, and an example safe lifetime stress determination follow.

Effect of Thickness Layer of Kenaf Fibre Reinfoeced Fibre Glass, Against Impact of Hybrid Composite Sandwich with Core Sengon

The purpose of this research is to know the impact strength of composite structures of sengon laut sawdust. Experimental results show that the impact toughness of sandwich composite will increase as the thickness of composite sandwich skin increases. The impact failure is due mostly to the shear failure of the core. Brittle specimen failure occurs on the sandwich composites structured with skin and core manufactured with the same thickness of 5 mm. This brittle sandwich composite has a flat cross section on both sides of the fault. The shear failure of the cores occurs in samples with 2 mm thick, 3 mm, 4 mm thick, and 10 mm thick core. In some samples, the shear failure of the cores is accompanied by cracks on the core so that the sandwich composite is broken in several parts. The highest value of absorption energy and the highest impact strength is found on thick composite sandwich variation of 10 mm thick with 4 mm thickness of 2,7860 J and 0,01032 J/mm2.

Noncontact evaluation of full elastic constants of perovskite MAPbBr3 via Photoacoustic eigen-spectrum analysis in one test

Elasticity is one basic property of hybrid organic-inorganic perovskites. It highly relates to many fundamental processes in solid physics. The investigation of elasticity is of interest not only to explore the intrinsic properties of a material, but also to improve their potential application performance. In this study, we predict photoacoustic eigen-spectrum (PAES) of single crystal. Then by solving the inverse problem of the generation of PAES, we propose a noncontact method to determine a complete set of elastic constants of single crystal in one test. Experiments confirm the proposed method accurately determines all elastic constants of MAPbBr3. Since this method is totally noncontact and does not require multiple specimens cutting along different crystal axes, it could be more competent for rare, tiny and brittle specimen, or when the specimen is immersed in turbid or opaque medium. Benefitting from these superiorities, the proposed method might be found prominent values in materials science and applications.