Brittle Failure Mechanism Research Articles

RC dapped-end beams with various reinforcement layouts: An experimental investigation

Reinforced Concrete (RC) discontinuity regions (D-regions), such as dapped ends, can represent vulnerable zones in existing RC structures since they can exhibit a brittle failure mechanism. In common design practice, the ultimate strength of D-regions is assessed through Strut and Tie (S&T) models. Current design codes recommend two resistant mechanisms for dapped ends but do not provide information about the identification of steel bars involved in the resistant mechanism when the reinforcement is formed by distributed bars. An experimental campaign on dapped-end beams with various reinforcement layouts, including configurations without diagonal bars or with one or multiple layers of diagonal bars, has been carried out at the Structures and Material Testing Laboratory of the Department of Civil and Environmental Engineering in Florence. Results allow for identifying the influence of different reinforcement layouts on the crack pattern at the Service Limit State (SLS) and on the post-peak response. Both the strain on bars and the crack opening displacements are investigated until failure. The work highlights that layouts with distributed diagonal bars reduce the crack widths at the SLS compared to other configurations, increase the ductility, and allow for the timely detection of damaged dapped ends during in situ inspections.

Experimental behaviour of ductile diagonal connections for rack supported warehouses

Steel racking systems are widely adopted for storage purposes: they are thin-walled structures composed of consecutive trusses, connected with beams on which the palletized goods are stored. Their geometry and structural configuration strongly depend on market and operator necessities, and, in modern applications, racks can also function as the supporting structure of the warehouse itself in the form of Rack Supported or High-Bay Warehouses. With the increase of the overall geometric dimensions and the global weight of the stored material, the seismic action becomes more relevant for the design. Along these lines, the development and experimental testing of a dedicated seismic design approach for ductile steel racks is here presented, with particular attention to Rack Supported Warehouses. This approach exploits the ductility of trusses introduced via the plastic ovalization mechanism of the diagonal-to-upright connections while a tailored capacity design is used to assure the elastic behaviour of the rest of the structure and to keep the brittle failure mechanisms at bay.

Experimental investigation of tunnel damage and spalling in brittle rock using a true-triaxial cell

Deep underground excavations in brittle rocks are subject to several ground stability hazards such as spalling and rockburst. These hazards are typically associated with brittle failure mechanisms for hard and massive rock mass. In this study, an experimental investigation has been carried out to evaluate the mechanisms underlying these induced hazards in deep underground excavations. The main objective of this study is to investigate the behavior of a freshly excavated circular unsupported tunnel in a brittle synthetic rock with a focus on induced damage and spalling response. The experiment used a miniature tunnel boring machine (TBM) to excavate a tunnel in a cubical specimen placed in a true-triaxial cell. The material selected for this experiment was a reproducible synthetic rock analogous to sandstone with brittle characteristics. In the experiment, the specimen was loaded with increasing confining stress under incremental isotropic conditions in the true-triaxial cell until the tunnel failed. Macro-photography was utilized to verify the excavation damage zones and failure mechanisms around the tunnel boundary. The main observed failure mechanism of the model tunnel was spalling, which occurred due to brittle failure and a sudden stress release at the tunnel excavation boundary. In addition to spalling, three zones of damage were identified by macro-photography in the tunnel due to damage from construction, fracturing, and plastic rock deformation. The outcomes point to a unique and in-depth comprehension of how damage and spalling failure during underground excavation develop and its impact on tunnel stability.

Cyclic Behavior of Concrete-Filled Tube Columns with Bidirectional Moment Connections Considering the Local Slenderness Effect

In this research, the cyclic behavior of concrete-filled thin tube (CFTT) columns with bidirectional moment connections was numerically studied within the context of thin-walled structures. Novel considerations in the design of CFTT columns with slenderness sections are proposed through a parametric study. A total of 70 high-fidelity finite element (FE) models are developed using ANSYS software v2022 calibrated from experimental research using similar 3D joint configurations. Furthermore, a comparison of different width-to-thickness ratios in columns was considered. The results showed that the models with a high slenderness ratio reached a stable cyclic behavior until 0.03 rad of drift, and a flexural strength of 0.8 Mp was reached for 4% of the drift ratio according to the Seismic Provisions. However, this effect slightly decreased the strength and the dissipated energy of the moment connection in comparison to columns with a high ductility ratio. Moreover, an evaluation of concrete damages shows concrete cracked for cyclic loads higher than 3% of drift. Finally, the joint configurations studied can achieve a good performance, avoiding brittle failure mechanisms and ensuring the plastic hinges in the beams.

Out-of-plane non-intrusive seismic retrofitting of in-plane damaged masonry infill through 3D printed recycled plastic devices

The current techniques used for strengthening existing buildings made out of masonry infilled frames are characterized by high impact on the daily internal activities, as people living or working in the building need to move out while the retrofitting activities take place.The seismic vulnerability of infilled concrete frames is related to their high stiffness that, being due to the presence of masonry infills, induces a very brittle failure mechanism. The available retrofitting techniques do not allow to decrease the frames stiffness without making demolitions, especially for what concerns the double layer infills, a typology that characterizes a large part of the Italian buildings built from 1970s. For such infills only traditional techniques are available, mostly lime/cement-based mortars strengthened with various fibers that increase the frame stiffness. This study proposes an innovative technique for the out-of-plane strengthening of masonry double layer infills. The system is based on a 3D printed system made of recycled plastic, placed between the two masonry layers composing the infill and acts: i) triggering preferential in-plane sliding surfaces for reducing stiffness and therefore damaging, ii) warranting out-of-plane stability avoiding the snap-through mechanism, iii) ensuring low environmental impact by using recycled materials and, iv) no disrupting the daily internal activities. The full scale out of plane test is carried out on an infill specimen previously damaged in the in plane direction, by using a hydraulic jack placed at mid-height of the infill. The experimental results are compared with analytical models available in literature combined with a model specifically developed for the resistant mechanism provided by the plastic devices. The obtained results suggest that plastic joints are effective in preventing instability and can be used as effective strengthening system for concrete frames with masonry infills.

Retrofitting of Damaged Crumb Rubber Concrete Frame Using Fiber Reinforced Polymers

Abstract: The research work is focused on the retrofitting of damaged crumb rubber concrete frame. The retrofitting of these frames was carried out using Carbon Fiber Reinforced Polymers (CFRP) wrapping technique accompanied with beam weakening technique, carried out at certain locations to achieve the desired failure mechanism. Two previously tested 1/3rd reduced scale models Crumb rubber concrete (CRC) and Normal concrete (NC) were CFRP retrofitted along with beam weakening and then tested dynamically. From the as-built testing strong-beam weak-column effect was observed, for that purpose beams weakening through cores removal away from the face of the column equal to two times depth of the beam, so as to shift the damage from the brittle failure mechanism of column and joint damage to more ductile damage in the beam. In addition to beam weakening the damaged beam column joints were CFRP retrofitted. A comparative study between the retrofitted and as-built models was done. The results obtained from testing has recovered the ductility up to 82% and response modification factor up to 90% for normal concrete while in case of crumb rubber concrete frame the ductility is recovered up to 76% and its response modification factor up to 81%. As intended, a much-distributed damage behavior/cracking pattern as compared to the reference models, especially in the case of normal concrete model. The intended purpose of the retrofitting in recovering seismic properties and a comparatively distributed damage/cracking pattern was achieved. The purpose of beamweakening through core removal for shifting the damage from columns and joints towards the beams for achieving a more ductile failure mechanism was achieved to some extent.

Evaluating the impact of calcite and heterogeneity on the mechanical behavior of coal: A numerical study with grain-based finite-discrete element method

Understanding the mechanical properties and fracture behaviors of coal is essential for the safety of underground mining engineering. This research aims to evaluate the impact of calcite and heterogeneity on the mechanical properties of coal. Neper, a software package for polycrystal generation and meshing, is used to create numerical models with a non-uniform grain size distribution. A random parameter assignment method is proposed to simulate the mechanical properties and failure behavior of coal using the combined finite-discrete element method (FDEM). A comprehensive parametric investigation of the properties of triangular and joint elements is conducted, and calibration procedures are suggested for numerical modeling. The model was calibrated to experimental results and then used to understand the impact of calcite grains on the mechanical behavior of coal. After calibration, a triaxial compression was performed on coal sampleswith and withoutcalcite grains at differentconfining pressures. It is found that the presence of calcite in a coal sample increases Young's modulus and decreases the peak strength of coal. This research uses Voronoi tessellation technology to present a Weibull statistical grain-based model (GBM) to investigate the microstructural heterogeneity induced by grain morphologies. The deformation response, strength characteristics and cracking behavior are investigated by varying the heterogeneity of numerical models under compressive tests with different confining pressures. The simulation results of triaxial compression indicate that the compressive strength increases linearly as the heterogeneity m value increases. The consistency of the numerical results with the experimental observationssuggests that the GBM approachcan be used to better understand the mechanics of brittle rock failure at the micro-structure scale.

Brittle Failure Modes and Mechanisms in Foliated Rock Under Uniaxial Compression: Laboratory Testing and Particle Flow Modeling

Brittle Failure Modes and Mechanisms in Foliated Rock Under Uniaxial Compression: Laboratory Testing and Particle Flow Modeling

Non-contact embedded sensing by Magnetostrictive Carbon Fiber Reinforced Polymer (MagCFRP): A smart material for early inter-lamina localized damage detection

Although failure mechanics and plasticity of composite materials is a relatively new and volatile field, it has been long realized in the composite materials community that a composite’s true integrity lies in the constituents’ interfacial health. Composite materials allow scientists and engineers to design structural architectures with directional stress, strain, and thermal fields in mind while simultaneously reducing the system’s overall weight. While there are advantages to using composite materials like carbon fiber reinforced polymers (CFRPs), designing and implementing long-term sustainable aerospace structures out of CFRPs is bottlenecked by the brittle catastrophic failure mechanism high strength carbon composites exhibit. As the demand for these materials in critical loading regimes increases, it is paramount that scientists and engineers understand how CFRPs will behave in real-time and in predictive models for load profiles. This research’s motivation comes from the US Army’s future vertical lift vehicle initiative to transition from interval-based maintenance to condition-based maintenance (CDB). This paper explores a real-time, non-contact, and non-destructive evaluation (NDE) method for composite materials by performing localized magnetic flux scans (32 mm2 field of view) of CFRP embedded with Terfenol-D ([Formula: see text] microns in diameter), a magnetostrictive material. For Magnetostrictive Carbon Fiber Reinforced Polymer (MagCFRP) elastic regime testing, there was an observed localized magnetic flux gradient of more than 5 mT (4%) with a reversible flux of 100%. For MagCFRP elastic-plastic regime testing, a localized magnetic flux gradient of more than 3 mT (2%) with a reversible flux of only 25% was observed. Terfenol-D embedded CRFPs have shown promising results for detecting instantaneous stress and strain levels and detecting deviations in inter-lamina residual stress after critical loading. Acoustic emission (AE), Digital Image Correlation (DIC), and X-ray computed tomography (CT) scanning were used to validate the observed results.

The hydrogen embrittlement mechanisms and hydrogen induced cracking process of selective laser-melted GH4169

This study investigates the hydrogen embrittlement sensitivity and brittle failure mechanism of selective laser-melted GH4169 alloy under hydrogen precharged conditions. Fractographic analysis reveals that a high concentration of hydrogen facilitates the formation of small quasi-cleavage steps near the fracture edge. Additionally, hydrogen promotes the migration of {111} planar dislocations, resulting in shear failure of the δ phases and the nucleation of microvoids. These microvoids then propagate, leading to the development of hydrogen-assisted transgranular cracks along the dislocation slip bands. The deformation behaviors of the δ and γ" phases were characterized using high-resolution transmission electron microscopy.

Behaviour of small-scale glulam timber connections with slotted-in steel plates and dowel-type fasteners reinforced with self-tapping screws

Braced frames are a common lateral force-resisting system in mass timber structures. In timber braced frames, timber-steel dowel connections are relied upon to provide ductility and energy dissipation capacity under earthquake loads. However, the ductility and energy dissipation capacity of these connections can be limited by the onset of a brittle failure mechanism (e.g., row-shear or group tear-out) prior to significant dowel yielding. This paper presents an experimental campaign investigating the structural performance of timber-steel dowelled connections reinforced with self-tapping screws. Twelve connections were tested; with 8 tested under monotonic loading and 4 tested under a cyclic loading regime. The tested connections included two different fastener diameters of 11 mm and 16 mm, with one or two internal steel plates, both with and without reinforcing screws designed to prevent brittle shear failure. The results demonstrate that the reinforcing screws are most effective for improving the ductility and energy dissipation for connections with similar design values of dowel yield and row shear capacities. In addition, the increases in ductility and energy dissipation with reinforcement were greatest for connections with high slenderness (defined as the ratio of dowel diameter to timber thickness) dowels. There was little improvement in the ductility of connections whose design capacity was dominated by row shear, although there was improvement in the energy dissipation of the connections. Overall, results of this study demonstrate the potential for using self-tapping screws to reinforce steel dowel connections and improve the seismic performance of mass timber braced frames.

Numerical study on the impact-induced failure mechanism of cylindrical glass projectile using the FEM-SPH conversion method

Brittle materials have been used for projectiles in recent years. To investigate the failure mechanism of glass cylinder during the impact, a light gas gun was adopted to conduct dynamic experiments on aluminum targets with polyurea coating. The residual glass projectiles collected perform a discrepancy after impacting different sides of the targets. By comparing different numerical method to simulate the experiments, the FEM-SPH adaptively conversion method was validated with significant advantage and reliability, and it was then used to reveal the brittle material failure mechanism. In addition, the effect of different velocities and impact surfaces on glass failure was numerically and theoretically analyzed characterized by the residual mass of elements and corresponding failure mass of particles converted from failure elements. An initial-contact-stress-based equivalent model was proposed to provide analytic and satisfied assessment of the projectile failure extents when impacting different targets. Our findings provide a reliable reference for further revealing the fracture mechanism of brittle materials during impact.

Anisotropic mechanical behavior of ultra-deep shale under high in-situ stress, a case study in the Luzhou block of the southern Sichuan Basin, China

The safe and efficient development of ultra-deep shale gas deeper than 4500 m is closely related to the comprehensive evaluation of the mechanical behavior, brittleness and anisotropy characteristics of reservoir rocks under in-situ stress conditions. However, the above-mentioned information about ultra-deep shale is rarely reported. This study aims to fill this gap by systematically investigating the mechanical and anisotropic characteristics of cores, with 0° and 90° bedding orientations, from different layers in the vertical depth of 4900–4930 m, under confining pressure of 110 MPa. Furthermore, a new brittleness index with three parts related to brittle failure, i.e., the energy storage performance in the pre-peak stage, the performance of elastic energy used to maintain the post-peak failure, and the severity of the energy released during the post-peak stage, all of which are physically meaningful, was proposed. Subsequently, the index was validated and applied to the brittleness evaluation of ultra-deep reservoir rocks. The brittleness was evaluated in the following descending order: black shale of Longmaxi Formation ＞ gray-black shale of Wufeng Formation ＞ limestone of Baota Formation. Overall, the Wufeng- Longmaxi Formation shale still exhibits brittle failure under such high confining pressure, while the Baota Formation limestone exhibits quasi-ductile failure, which is consistent with the phenomenological behavior of cores. Further, various experimental techniques have been employed to study the brittle and quasi-ductile failure mechanisms of reservoir rocks. The anisotropy evaluation results show that the anisotropy degree of the mechanical and brittleness parameters corresponding to the deep formation is significantly lower than that of the shallow formation. The anisotropy degree of strength and deformation parameters (Young's modulus, Poisson's ratio) is closely related to the content of quartz and clay minerals, respectively. The above results are expected to provide data support and theoretical guidance for the development of deep and ultra-deep shale gas in the southern Sichuan region of China.

Shear strength formula for interior beam-column joints with plain bars in existing buildings

Beam-column joint behavior in Reinforced Concrete (RC) structures is a key issue for earthquake engineering, because, under seismic loads, these elements are subjected to high stresses, which could lead to unexpected collapses, due to the development of brittle failure mechanisms. In Italy, many existing RC buildings constructed before the mid-1970's present structural deficiencies, as they were designed in the absence of the prescriptions of modern seismic codes. In particular, the use of plain reinforcing bars, inappropriate anchorage solutions and the lack of joint horizontal hoops are rather widespread.This paper presents an in-depth overview on seismic behavior of interior beam-column joints reinforced with plain bars, under cyclic loads, based on experimental results available in the literature. The inadequacies of certain reinforcement arrangements and details of tested joints are highlighted. A critical discussion of the damages and failure mechanisms developed in the joints, in relation to the reinforcement details and to the effects of the main influencing parameters, is provided. These parameters are the mechanical properties of materials, the geometry of the joint and the converging elements, and the column axial load. A formula to predict shear strength of joints with plain bars is proposed.Due to the lack of similar papers in the literature, the present analysis constitutes the basis for a comprehensive understanding of behavior of interior joints with plain bars under seismic actions. Moreover, due to the absence in the main current Codes of design formulas for the shear strength assessment of existing beam-column joints with plain bars, a design formula is also proposed. This formula is a valuable tool to design the retrofit intervention of existing buildings reinforced with plain bars, constituting a not negligible part of the built heritage.

Preliminary Investigation on Steel Jacketing Retrofitting of Concrete Bridges Half-Joints

An innovative strengthening system for dapped-end beams is studied numerically and experimentally in this paper. The system is developed for the half-joint regions of bridge beams also commonly called “gerber saddles”, but it can be adapted to different scenarios. The strengthening system consists of two steel plates that are clamped on both sides of the webs of the beams by means of bolts. The purpose of the system is to transfer the highest possible amount of shear from the concrete webs to the steel plate elements reducing the resistance demand of the concrete half joint. Shear is transferred by friction from concrete to steel plates. The system is designed to be applied on existing bridges without heavy work interesting the carriageway, therefore reducing the interference with the traffic. Some interesting considerations emerge from the study, including the influence of the flange web connection on the structural behavior and the possible presence of brittle failure mechanisms that are difficult to model numerically using f.e.m. simulations.

An evaluation of mechanical properties and estimation of environmental reduction factors in welded API X70 steel pipeline in natural seawater

Due to the detrimental effect of damage induced by seawater in pipeline structures, there is a need to investigate the effects of natural seawater and air environments on mechanical properties of representative pipeline materials, to obtain useful data for estimation of their service lives. Hence, in this work, a X70 steel pipeline plate was welded using submerged arc welding technique; and subjected to air and natural seawater environments. Test specimens were soaked in seawater for twelve months at 28 °C. The parent plates, weld regions and the heat affected zones were investigated by evaluating their mechanical properties and fracture surfaces. The experimental findings revealed that the tensile strengths of parent and weld were 634.00 MPa and 674.00 MPa respectively, while the compressive stresses were 750.10 MPa and 750.40 MPa respectively with highest hardness value of 239HV. The findings also revealed that weld area and heat affected zone depend on material thickness, heat input and possible effect of residual stresses in the weldment. The fracture surfaces of test specimens showed combination of brittle and ductile failure mechanisms. Comparison of the test results revealed that seawater had significant effect on the mechanical properties and surface morphology of the API X70 steel pipeline with respect to immersion time.

A fault diagnosis method based on stiffness evaluation model for full ceramic ball bearings containing subsurface cracks

Most of the surface macroscopic faults of ceramic bearings are caused by the extended evolution of early subsurface cracks in mesoscopic and microscopic levels. Because of the greater contact stiffness and brittle failure mechanism of ceramic materials, the dynamic response becomes more sensitive, especially in the early failure stage with subsurface cracks. To evaluate the non-negligible effect for the cumulative expansion of subsurface cracks on the contact stiffness, this paper proposes a stiffness evaluation method based on the combination of theory and experiment. Combined with fracture mechanics, the quantified weakening effect of subsurface cracks on stiffness is substituted into the dynamics model to establish the stiffness evaluation model. Then the mapping relationship between the subsurface crack groups and the dynamic response is obtained. The results show that the different degrees of early subsurface crack failures in different directions have a significant effect on the magnitude of contact stiffness and the area of action. The model not only effectively evaluates the weakening effect on the contact stiffness of ceramic bearings when early subsurface failures occur, but also promotes the stiffness evaluation models of key components using ceramic materials.

A Brief Overview on Crack Patterns, Repair and Strengthening of Historical Masonry Structures.

Given that a significant fraction of buildings and architectural heritage in Europe's historical centers are masonry structures, the selection of proper diagnosis, technological surveys, non-destructive testing, and interpretations of crack and decay patterns is paramount for a risk assessment of possible damage. Identifying the possible crack patterns, discontinuities, and associated brittle failure mechanisms within unreinforced masonry under seismic and gravity actions allows for reliable retrofitting interventions. Traditional and modern materials and strengthening techniques create a wide range of compatible, removable, and sustainable conservation strategies. Steel/timber tie-rods are mainly used to support the horizontal thrust of arches, vaults, and roofs and are particularly suitable for better connecting structural elements, e.g., masonry walls and floors. Composite reinforcing systems using carbon, glass fibers, and thin mortar layers can improve tensile resistance, ultimate strength, and displacement capacity to avoid brittle shear failures. This study overviews masonry structural diagnostics and compares traditional and advanced strengthening techniques of masonry walls, arches, vaults, and columns. Several research results in automatic surface crack detection for unreinforced masonry (URM) walls are presented considering crack detection based on machine learning and deep learning algorithms. In addition, the kinematic and static principles of Limit Analysis within the rigid no-tension model framework are presented. The manuscript sets a practical perspective, providing an inclusive list of papers describing the essential latest research in this field; thus, this paper is useful for researchers and practitioners in masonry structures.

Influence of Density Gradient on the Compression of Functionally Graded BCC Lattice Structure.

In this paper, five grading functional gradient lattice structures with a different density perpendicular to the loading direction were proposed, and the surface morphology, deformation behavior, and compression properties of the functional gradient lattice structures prepared by selective laser melting (SLM) with Ti-6Al-4V as the building material were investigated. The results show that the characteristics of the laser energy distribution of the SLM molding process make the spherical metal powder adhere to the surface of the lattice structure struts, resulting in the actual relative density of the lattice structure being higher than the designed theoretical relative density, but the maximum error does not exceed 3.33%. With the same relative density, all lattice structures with density gradients perpendicular to the loading direction have better mechanical properties than the uniform lattice structure, in particular, the elastic modulus of LF, the yield strength of LINEAR, and the first maximum compression strength of INDEX are 28.99%, 16.77%, and 14.46% higher than that of the UNIFORM. In addition, the energy absorption per unit volume of the INDEX and LINEAR is 38.38% and 48.29% higher, respectively, than that of the UNIFORM. Fracture morphology analysis shows that the fracture morphology of these lattice structures shows dimples and smooth planes, indicating that the lattice structure exhibits a mixed brittle and ductile failure mechanism under compressive loading. Finite element analysis results show that when the loading direction is perpendicular to the density gradient-forming direction, the higher density part of the lattice structure is the main bearing part, and the greater the density difference between the two ends of the lattice structure, the greater the elastic modulus.

An experimental investigation of brittle failure mechanisms in scratch tests of rock

This paper reports the results of scratch tests on three rocks (Bonne Terre Dolomite, Alabama Marble and Dunnville Sandstone) conducted at depths of cut ranging from 0.2 mm to 2.6 mm. The experiments were performed on slabs of thickness slightly smaller than the cutter width, with a camera recording the entire cutting process. The test results reveal the existence of three cutting regimes: ductile, fragmentation, and brittle with increasing depth of cut. The regimes differ by the failure mode, the dependence of the specific energy ϵ on the depth of cut d, and the fragment shapes. In the ductile regime, ϵ∼d0 and the fragments are constitutive grains of the rock or powder as a result of intense shearing. In the fragmentation regime, ϵ∼d−1/2 and the fragments is a mix of ductile-type particles and flake-like chips also the product of shear failure. Finally, the brittle regime is characterized by ϵ∼d−1 and is essentially associated with chip-like fragments that come in two varieties depending on whether they are created by shear or by tension. The d−1-dependence of the specific energy can only be observed if the rock is sufficiently homogeneous at the scale of the experiment. The distinction between different modes of failure is based on evidence from video recording and fractography with a Scanning Electron Microscope. It is shown that the dominance of tensile over shear fracture is intimately related to the existence of a crushed zone, which acts as a wedge to initiate a tensile macrocrack in a compressive stress field. Finally, an analysis of the energy partitioning in the brittle regime indicates that the energy dissipated in creating new surfaces with a tensile crack is actually negligible compared to the energy expended in the formation of the wedge. This result indicates that the mode I toughness cannot be determined from the specific energy in the brittle regime, on the basis of a model of a crack parallel to the free surface, in contradiction with published claims to that effect. Finally, this experimental investigation clarifies some confusion in the literature, rooted on assuming the existence of only two modes of failure, ductile and brittle.