Brittle Failure Mechanism Research Articles

Investigation of Charpy impact behavior of porous twisted wire material

Abstract

The performance of paving block structures with mortar filled joints under temperature loading, accessed by means of numerical simulations

Paving block structures with mortar filled vertical joints can exhibit a significantly higher load capacity as comparable structures with sand filled joints. For this reason, this pavement construction method is increasingly being used or would be used, respectively, if its performance could be estimated reliably. Specifically, the exact cause of cracks due to cooling in winter has not been fully understood yet and appropriate prediction tools do not exist. This motivated the development of a numerical simulation tool for such paving block structures under temperature loading, which is able to take the brittle failure mechanisms in-between paving blocks and paving blocks and the underlying mortar bed into account. By means of the proposed simulation tool in this paper, basic structural failure mechanisms of such paving block structures, due to different temperature events, could be identified and relationships between crack widths and different bonding strengths as well as installation temperatures were obtained. Finally, estimates for necessary bonding strengths between paving blocks and mortar bed to prevent large (visible) cracks due to temperature loads are given.

Axial compressive behavior of seawater coral aggregate concrete-filled FRP tubes

The use of seawater coral aggregate concrete (SCAC) instead of ordinary aggregate concrete (OAC) in concrete-filled FRP tube (CFFT) members is highly attractive due to the beneficial effect of fiber-reinforced polymer (FRP), which has good resistance to salt and can effectively counteract the greatest shortcomings of the chloride-containing ions in SCAC. In the development of marine construction, SCAC-filled FRP tubes are a potential attractive alternative for pile and column applications in corrosive marine environments. The axial compressive behavior of SCAC-filled FRP tube (SCFFT) was first experimentally investigated and compared with OAC-filled FRP tube (OCFFT) in this study. The chosen coral aggregates (CAs) and SCAC were systematically prepared and tested, including classification and materials testing on CAs, uniaxial load testing on SCAC and microstructure analysis of SCAC. The porous nature and low strength of CAs led to a different and brittle failure mechanism of SCAC under uniaxial loading, and result in SCAC experiencing a compacting behavior before the development of its rapid expansion and the activation of confinement in SCFFT. As a consequence, the axial loading of SCFFT displayed a slight drop in the transition zone for the range of axial strains between approximately 0.002 and 0.004. The non-homogeneity and brittleness of SCAC led to a non-uniform hoop strain distribution in SCFFT under compression and thus discounted the effect of confinement. The ultimate load of SCFFT was approximately 60% of that of its OCFFT counterpart. The key elements and basic framework used to develop a detailed analysis-oriented model for predicting FRP-confined SCAC under axial compression was illustrated. Furthermore, the model developed originally for FRP-confined lightweight concrete with ceramsite aggregates was extended to cover FRP-confined SCAC, and the extended model was confirmed to be capable of producing reliable predictions.

Mechanical, structural and dissolution properties of heat treated thin-film phosphate based glasses

Here we show the deposition of 2.7μm thick phosphate based glass films produced by magnetron sputtering, followed by post heat treatments at 500°C. Variations in degradation properties pre and post heat treatment were attributed to the formation of Hematite crystals within a glass matrix, iron oxidation and the depletion of hydrophilic P-O-P bonds within the surface layer. As deposited and heat treated coatings showed interfacial tensile adhesion in excess of 73.6MPa; which surpassed ISO and FDA requirements for HA coatings. Scratch testing of coatings on polished substrates revealed brittle failure mechanisms, amplified due to heat treatment and interfacial failure occurring from 2.3 to 5.0N. Coatings that were deposited onto sandblasted substrates to mimic commercial implant surfaces, did not suffer from tensile cracking or trackside delamination showing substantial interfacial improvements to between 8.6 and 11.3N. An exponential dissolution rate was observed from 0 to 2h for as deposited coatings, which was eliminated via heat treatment. From 2 to 24h ion release rates ordered P>Na>Mg>Ca>Fe whilst all coatings exhibited linear degradation rates, which reduced by factors of 2.4–3.0 following heat treatments.

Mechanical Behaviour of 3D Multi-layer Braided Composites: Experimental, Numerical and Theoretical Study

Mechanical properties and failure mechanisms of a newly designed 3D multi-layer braided composites are evaluated by experimental, numerical and theoretical studies. The microstructure of the composites is introduced. The unit cell technique is employed to address the periodic arrangement of the structure. The volume averaging method is used in theoretical solutions while FEM with reasonable periodic boundary conditions and meshing technique in numerical simulations. Experimental studies are also conducted to verify the feasibility of the proposed models. Predicted elastic properties agree well with the experimental data, indicating the feasibility of the proposed models. Numerical evaluation is more accurate than theoretical assessment. Deformations and stress distributions of the unit cell under tension shows displacement and traction continuity, guaranteeing the rationality of the applied periodic boundary conditions. Although compression and tension modulus are close, the compressive strength only reaches 70% of the tension strength. This indicates that the composites can be weakened in compressive loading. Additionally, by analysing the micrograph of fracture faces and strain-stress curves, a brittle failure mechanism is observed both in composites under tension and compression.

A microstructure-based multisurface failure criterion for the description of brittle and ductile failure mechanisms of clear-wood

The failure behavior of wood, a complex natural composite, highly depends on structural features on several lower length scales. Thus, a new approach for the prediction of failure mechanisms of wood is proposed, aiming at a better description of mechanical processes in wood by taking the influence of microstructural characteristics into account.Previously proposed failure criteria for wooden cells are now linked to unit cells at the annual ring scale. Extensive investigation of load combinations allow the identification and classification of the main failure mechanisms of clear-wood. Based on these findings, a multisurface failure criterion is formulated, indicating brittle as well as ductile failure mechanisms and giving interesting insight into the failure behavior. A new algorithm is presented, which enables the simultaneous description of plastic failure and cracking within numerical simulations. Finally, the failure criterion is validated for biaxial experiments and applied to a timber engineering example.

Pull-out behavior of different fibers in geopolymer mortars: effects of alkaline solution concentration and curing

Reinforcing geopolymer materials with fibers can enhance tensile and flexural strengths and fracture toughness. The bond between fiber and geopolymer matrix is a critical factor that needs to be investigated to optimize the performance of the fiber reinforced composite. In this study, single fiber pull-out tests are conducted on steel and polypropylene fibers embedded in geopolymer matrices; in addition, OPC mortars are tested as control condition. The following parameters are investigated: fiber type (i.e. steel and polypropylene) and shape, concentration of alkali solution in the geopolymer matrix, and curing conditions. Bond-slip performance, failure modes, and slip resisting mechanisms of different matrices and fibers are compared and discussed. The fiber deformation ratio, a novel parameter, is introduced to quantitatively investigate the effect of fiber shape on the mortar performance. In case of steel fibers, the geopolymer-fiber composite performs better for lower fiber deformation ratios, where the full fiber pull-out mechanism can be exploited. For higher deformation ratios, the strong bearing forces developed, combined with the high adhesion strength of the geopolymer-steel fiber interface, lead to more brittle failure mechanisms, such as fiber breakage or matrix failure, as observed in end-deformed and length-deformed steel fibers, respectively.

The influence of coupled horizontal–vertical ground excitations on the collapse margins of modern RC-MRFs

With the increasing interest in vertical ground motions, the focus of this study is to investigate the effect of concurrent horizontal–vertical excitations on the seismic response and collapse fragilities of RC buildings designed according to modern seismic codes and located near active faults. It must be stressed that only mid- to high-rise buildings are of significant concern in the context of this research. The considered structures are categorized as intermediate and special RC-MRFs and have been remodeled using distributed and lumped plasticity computational approaches in nonlinear simulation platforms, so that the utilized NL models can simulate all possible modes of deterioration. For better comparison, not only was the combined vertical and horizontal motion applied, but also a single horizontal component was considered for direct evaluation of the effect of the vertical ground motions (VGMs). At the member level, axial force variation and shear failure as the most critical brittle failure mechanisms were studied, while on the global level, adjusted collapse margin ratios (ACMRs) and mean annual frequency of collapse (λ Collapse) using a new vector-valued intensity measure were investigated. Findings from the study indicate that VGMs have significant effects on both local and global structural performance and cannot be neglected.

SEM analysis on rock failure mechanism by supercritical CO2 jet impingement

Supercritical carbon dioxide (CO2) jet-assisted radial drilling is regarded as a potential alternative drilling method because supercritical CO2 jet has higher efficiency in rock-erosion efficiency than water jet. Although researchers have investigated how the rock-erosion performance by supercritical CO2 jet impingement correlates with the influencing parameters, there is little on the rock failure mechanism and change of pore structure. Scanning electron microscopy (SEM) analysis has been an important means in mineral identification, material damage detection, and aiding in water jet research. Therefore, rock-erosion experiments and SEM observations and analyses were carried out on different rocks including sandstone, marble and so forth. Secondary electron (SE) images show obvious grain shapes on the complex impinged surface of sandstone and marble by supercritical CO2 jet, and cleavages on the mineral grains, and cracks and holes between the grains in different sizes. It suggests that cementations in weaker rocks are firstly removed and then mineral grains broken, generating the intergranular and intragranular cracks and micro-fractures and fissures. Supercritical CO2 jet cannot bring obvious changes to hard limestone and shale at the same condition because the rock substances are tightly packed and filled with large cementing forces and ultra-low permeability and porosity, but can produce a few micro-cracks vertical to bedding planes. The features show that supercritical CO2 jet broke rock substances mainly in the brittle tensile failure mechanism and made the rock much easier to further break, accompanied with shear failure mechanism in particular locations of the perforation hole. In contrast, water jet impingement forms flat cutting sections in marble with continuous parallel breaking traces, indicating the shear failure mechanism. Low viscosity and high diffusivity of supercritical CO2 are primarily attributed to the rock tensile failure mechanism, they facilitate supercritical CO2 penetrates into deep into rock pores and produces static pressure on the inner wall and at the tips of the fractures, besides exerting jet impinging pressure. Moreover, the energizing effect caused by the phase change from supercritical to gas leads to the energizing effect and intensify rock breaking into more shattered pieces and, concurrently, improve general rock connectivity and porosity in the surface layers. As a consequence, supercritical CO2 jet appears to be more efficient and suitable than water jet in slim-hole radial drilling and hydraulic fracturing, particularly in unconventional reservoirs with low permeability.

Experimental and analytical study of the dynamic debonding in FRP plated beams

This paper presents an experimental study of the dynamic debonding failure mechanism in FRP plated beams. The experimental study is accompanied by an analytical/numerical study. The focus in the experimental investigation is on the dynamic aspects of the brittle, rapid, and abrupt failure mechanism and on its characterization and quantification. The experimental technique is based on four point bending tests of FRP plated steel beam specimens. The main dynamic monitoring technique uses high-speed digital photography with rate of 88,050 frames per second. The analytical and numerical aspects of the investigation use an extended high order layered beam theory with a physical modeling resolution that considers nucleation and evolution of the debonding mechanism in each physical interface. This is achieved by using cohesive interfaces and a specially tailored finite element formulation that is based on the theory. Digital image processing of the experimental results reveals and quantifies the dynamics of the interfacial failure with propagation velocities in the order of 100–1000 m/s and duration of less than 0.2 ms. The results reveal the dynamic nature of the failure process and provide an experimental benchmark for its consideration. They also provide direct experimental data for supporting and validating the theory and for determining the fracture energy and length scale parameters of the cohesive interfaces. With that, the combined experimental and theoretical study further explores the dynamic features of the failure mechanism.

Failure analysis using acoustic and energy emission assessment of fibre reinforced polymer material performance under severe conditions

Laboratory compressive tests and acoustic emission analysis have been used to investigate the failure of pultruded fibre reinforced polymer materials after they have been subjected to temperature stress and compressive loading. The acoustic emission approach is then compared with the experimental values of energy released by each sample through the corresponded load-displacement curve. Samples subjected to severe thermal conditions showed more evidence of brittle failure mechanism. This analysis has been conducted in order to confirm the potential capacity of fibre reinforced polymer materials, known currently for their strength and lightweight but often ‘brittle’ physical characteristics, to perform under exaggerated conditions of temperature and compressive stress.

Experimental and numerical study on the effect of calcite on the mechanical behaviour of coal

The effects of the presence of calcite grains on the mechanical behaviour of a coal are investigated in this study. Coal specimens with and without calcite grains were obtained from the same coal seam and then subjected to confined compression with varying confining pressures. Mineral composition data of the coal containing calcite grains were obtained from petrographic analysis and subsequently used for constructing UDEC-GBM model. The model was calibrated to experimental results and then used to further the understanding of the influence of calcite grains on the mechanical behaviour of the coal. It is found that the presence of calcite increases the Young's modulus, reduces the peak strength and the brittleness of the coal. The numerical results suggest that the presence of calcite grains significantly increase the heterogeneities of the coal, leading to differential and extensional straining which in turn causes internal tensile stresses and consequently tensile cracking. The conformity of the numerical results with the experimental observations suggests the potential use of the UDEC-GBM approach model for better understanding the mechanics of brittle rock failure at micro-structure scale.

Experimental tests on cross-laminated timber joints and walls

As a product, cross-laminated timber exhibits widely linear-elastic behaviour (except for compressive stress) and a tendency to have brittle failure mechanisms. Therefore, especially at dynamic loadings (e.g. earthquakes), the behaviour of cross-laminated timber structures is highly dependent on the connections used. Based on several single-joint and wall tests performed monotonically as well as cyclically, this paper provides improved insight into the behaviour of different joints that make up cross-laminated timber structures. In addition to information on the failure modes that occurred and specific values determined by way of experimental tests, interpretations of different contributions towards the overall deflection of the tested wall systems are presented. The influence of different connection types and their combinations within one wall system is another topic discussed. Finally, the sensitivity of resulting test properties concerning the post-processing models used is singled out as a central theme.

Threshold size for fluid inclusion decrepitation

AbstractFluid inclusions in igneous and metamorphic rocks are equilibrated at pressures reaching a few tens of kilobars at mantle depths. These microsystems experience decompression as a consequence of uplift processes, such as subaerial volcanic eruptions. On decompression, inclusion fluid overpressure is known to bring about the mechanical failure of the crystal matrix through either stretch or decrepitation, depending on ductile or brittle failure mechanism of the matrix, respectively. On the one hand, laboratory experiments performed on synthetic inclusions at one atmosphere show that the decrepitation temperature is strongly size dependent, with the smaller cavities observed to decrepitate at higher temperatures. On the other hand, natural fluid inclusions, which undergo migration through a pressure gradient, are often found intact below a critical size. Here by modeling fluid inclusions as spherical cavities in a continuous elastic medium and by adopting a nonlocal stress approach to fracturing, we demonstrate that the decrepitation phenomenon is predicted to be characterized by a threshold size of the cavity, below which decrepitation would not be allowed. In order to validate our model, two independent experimental data sets relating internal pressure to cavity size are utilized to calculate pertinent model parameters and to evaluate their consistency.

Thermal behaviour of unsaturated silt at high suction magnitudes

A triaxial cell was developed to investigate the shear strength of unsaturated silt under elevated temperatures and high suction magnitudes. The results from a series of drained triaxial compression tests on compacted silt specimens are presented in this paper. After anisotropic compression, some specimens were heated before suction was applied, while others were heated after the application of suction. The shear stress–strain curves of the soils under high suction magnitudes showed a brittle failure mechanism, with a clear increase in peak shear strength with net confining stress. Heating after suction application led to a greater peak shear strength than reference tests at ambient temperature, whereas heating before suction application led to a lower peak shear strength. Despite the observed path effects, a single peak failure envelope was defined when evaluating the data in terms of effective stress. The suction stress concept was used to define the effective stress, and the values of suction stress were found to be linked with a non-isothermal definition of the soil-water retention curve.

Strengthening of Aluminum Alloy 2219 by Thermo-mechanical Treatment

Strengthening of aluminum alloy 2219 by thermo-mechanical treatment has been compared with artificial aging. Three simple deformation modes including pre-stretching, compression, and rolling have been used in thermo-mechanical treatment. The tensile strength, elongation, fracture feature, and precipitated phase have been investigated. The results show that the strengthening effect of thermo-mechanical treatment is better than the one of artificial aging. Especially, the yield strength significantly increases with a small decrease of elongation. When the specimen is pre-stretched to 8.0%, the yield strength reaches 385.0 MPa and increases by 22.2% in comparison to the one obtained in aging condition. The maximum tensile strength of 472.4 MPa is achieved with 4.0% thickness reduction by compression. The fracture morphology reveals locally ductile and brittle failure mechanism, while the coarse second-phase particles distribute on the fracture surface. The intermediate phases θ″ or θ′ orthogonally precipitate in the matrix after thermo-mechanical treatment. As compared to artificial aging, the cold plastic deformation increases distribution homogeneity and the volume fraction of θ′′ or θ′ precipitates. These result in a better strengthening effect.

A new method for shale brittleness evaluation

High brittleness is the prerequisite for the creation of a hydraulic fracture network, which is crucial for shale reservoir development. The test evaluation methods are restricted in the field by lack of continuous brittleness profiles. Mineralogical evaluation cannot incorporate the external factors like stress state. The most widely used method is mechanical evaluation method based on statistical results of Young’s modulus and Poisson’s ratio. This method cannot explain the brittle tensile failure mechanism of shale and contradiction between high brittleness and barriers of high strength rock. With these methods, brittle evaluation and interval selection errors will happen in the application. Starting from the mechanism of rock failure and considering the effect of rock mechanical properties on brittleness, a new method for rock brittleness evaluation has been established. Firstly, the failure surface features were analyzed and the dominant mechanisms in complex failure were identified with triaxial test and scanning electron microscopy test. Secondly, a stress model for shale reservoirs with thin sand and shale interbedded was set up based on elasticity and the factors affecting the stress distribution and failure modes have been analyzed. Eventually, based on experiments and theoretical recognition, the brittleness index for shale was proposed by integrating Young’s modulus, Poisson’s ratio and fracture toughness. Moreover, a method for acquiring brittleness index from well logs was developed from which brittleness profiles for shale formations in Sichuan basin were obtained. Failure features analysis demonstrates that tensile failure dominates during rock brittle failure. It was observed that compression stress on rock is only relevant to mechanical parameters and independent of space location under both internal and external stresses. Finally, comparison with the results from Rickman’s method and microseismic monitoring demonstrates that the method developed in this work possesses more satisfactory results of brittleness evaluation and interval selection in field application.

Investigations of High Performance Fiberglass Impact Using a Combustionless Two-stage Light-gas Gun

Two types of high performance fiberglass panels were investigated at normal impact conditions of around 1km/s (2200 mph). Thin phenolic laminates of plain-weave glass cloth impregnated with synthetic thermosetting resins, one melamine and the other epoxy, were impacted by a 1.6mm diameter nylon sphere using a combustionless two-stage light-gas gun. The resulting impact and ejecta was captured using high-speed imaging, and the perforation characteristics and damage zone were examined post-mortem using optical microscopy. Both composites had a fiber volume fraction of an estimated 56% and identical fiber weave so that the role of the matrix on impact performance could be comparatively investigated. The epoxy fiberglass resulted in full perforation of the target, whereas the melamine fiberglass resulted in deformation, but not full perforation. The overall damage zone of melamine fiberglass was twice the size on the rear surface than the epoxy fiberglass. In addition, the melamine fiberglass rear damage zone was 5 times the impactor diameter and exhibited ductile debonding and delamination as the main failure mechanisms. In contrast, the epoxy matrix rear damage was 3 times the impactor diameter and exhibited brittle failure mechanisms of fiber breakage and pullout. These experimental results suggest that the matrix material is a driving factor in the impact damage and perforation characteristics, and is discussed in context of the compressive and tensile strength at break of the materials.

Morphologic Interpretation of Rock Failure Mechanisms Under Uniaxial Compression Based on 3D Multiscale High-resolution Numerical Modeling

Multiscale continuous lab oratory observation of the progressive failure process has become a powerful means to reveal the complex failure mechanism of rock. Correspondingly, the representative volume element (RVE)-based models, which are capable of micro/meso- to macro-scale simulations, have been proposed, for instance, the rock failure process analysis (RFPA) program. Limited by the computational bottleneck due to the RVE size, multiscale high-resolution modeling of rock failure process can hardly be implemented, especially for three-dimensional (3D) problems. In this paper, the self-developed parallel RFPA3D code is employed to investigate the failure mechanisms and various fracture morphology of laboratory-scale rectangular prism rock specimens under unconfined uniaxial compression. The specimens consist of either heterogeneous rock with low strength or relatively homogeneous rock with high strength. The numerical simulations, such as the macroscopic fracture pattern and stress–strain responses, can reproduce the well-known phenomena of physical experiments. In particular, the 3D multiscale continuum modeling is carried out to gain new insight into the morphologic interpretation of brittle failure mechanisms, which is calibrated and validated by comparing the actual laboratory experiments and field evidence. The advantages of 3D multiscale high-resolution modeling are demonstrated by comparing the failure modes against 2D numerical predictions by other models. The parallel RVE-based modeling tool in this paper can provide an alternative way to investigate the complicated failure mechanisms of rock.

Experimental and numerical studies of initial cracking in CFRP cross-ply laminates

This work is concerned with the conditions for formation of the first (initial) cracks in composite laminates with cutouts or ply drop-offs subjected to in-plane loading. We study here the crack formation on the free edge of CFRP cross-ply laminates experimentally and by numerical stress and failure analysis. The free-edge surface strains are measured by the digital image correlation (DIC) technique. The numerical analysis consists of a two-scale approach, where the macro-level analysis is performed with a three-dimensional finite-element method (3D FEM) and the micro-level analysis uses a periodic unit-cell (PUC) in the transverse plies. The constitutive assumption made for the macro-level analysis is an orthotropic linear thermo-elastic solid for the unidirectional plies with a thin isotropic viscoplastic layer between the longitudinal and transverse plies. In the PUC, the fibers are assumed linear elastic, while the matrix is modeled as an elastic–viscoplastic solid. Crack formation is assumed to occur in the matrix by the dilatation induced brittle failure mechanism for which the dilatation energy density criterion is used.