Brittle Failure Mechanism Research Articles

A dual 3D DIC-system application for DSL strain and displacement measurements

This paper describes a dual 3D Digital Image Correlation (DIC) system application for DLS strain and displacement measurements, where two 3D DIC-systems are used in parallel. The bonded specimens were tested to failure under monotonic loading in a uni-axial tensile testing machine at ambient temperature. Both surface in- plane strain and full-field displacement values were recorded using two DIC systems: high speed (HS) and high resolution (HR). The HS system was used in a parallel setup with the HR system in order to detect the initial failure location and crack propagation rate during the brittle failure mechanism, where an interface crack is propagating between the straps and the inner adherent. Using two inherently different DIC systems involve a number of problems. This involves synchronization of the HS and HR systems, a common illumination level and speckle pattern. This paper therefore describes guidelines for a mutual system setup, applied in an experimental study of steel/epoxy DLS joints under pure tension.

HVOF-sprayed WC–CoCr coatings on Al alloy: Effect of the coating thickness on the tribological properties

The microstructure, the micromechanical properties, the wear behaviour and the impact resistance of WC–CoCr cermet coatings, deposited onto an aluminium alloy substrate by High Velocity Oxygen-Fuel (HVOF) flame-spraying, were examined as a function of the coating thickness, which was varied between 50 μm and 150 μm by performing different numbers of scans of the HVOF torch in front of the substrate. The coatings became denser and significantly harder as the number of torch scans increased: the analysis of single WC–CoCr splats by combined SEM and Focused Ion Beam (FIB) techniques enabled the interpretation of the mechanisms underlying this phenomenon. In accordance to such densification, the sliding wear resistance increased with the number of torch scans, as abrasive grooving and brittle failure mechanisms were progressively suppressed. The resistance to cyclic impact was also enhanced. In comparison to anodised films, the WC–CoCr coatings appeared much more resistant against wear and cyclic impact; specifically, three torch scans seem enough to produce a coating having suitable characteristics.

Создание энергосберегающей установки для утилизации энергии доменного газа

The developed techniques of settlement research of a thermostressed state and life estimation taking into account survivability of hightemperature elements of power machinery are considered. The basis of techniques is made by solutions of non-stationary problems of heat conductivity, thermomechanics and mechanics of brittle failure. Examples of the solved problems are presented

Задачи термопрочности элементов турбоустановок

The developed techniques of settlement research of a thermostressed state and life estimation taking into account survivability of hightemperature elements of power machinery are considered. The basis of techniques is made by solutions of non-stationary problems of heat conductivity, thermomechanics and mechanics of brittle failure. Examples of the solved problems are presented

Brittle Rock Failure in the Steg Lateral Adit of the Lötschberg Base Tunnel

During the crossing of brittle rock formations at the Lotschberg base tunnel, failure phenomena have been observed both at the tunnel face and at the walls. A detailed analysis has been undertaken to explain these behaviours, based on the recent developments of Canadian research on brittle failure mechanisms. At the tunnel walls, a very good agreement is found between the calculated and observed damage and between two prediction methods, i.e. a semi-empirical failure criterion and elastic calculations with the “brittle Hoek-Brown parameters.” Near the face, due to the 3D nature of the stress conditions, some limitations of these approaches have been highlighted, and the growth of wall failure has been analysed. This research allowed a better understanding of the brittle rock mass behaviour at the Lotschberg base tunnel and showed that brittle failure processes dominate the behaviour of deep, highly stressed excavations in massive to moderately jointed rock. It also illustrates where improvements to the adopted approaches are required.

The quasi-static deformation and final fracture behavior of aluminum alloy 2219

In this paper is presented the results of an experimental study aimed at understanding the tensile deformation and fracture behavior of aluminum alloy 2219. Uniaxial tensile tests results reveal the alloy to have acceptable strength and ductility (elongation-to-failure) for both the anodized and non-anodized conditions. The ductility, quantified in terms of reduction in area, of the anodized sheet is marginally superior to the ductility of the non-anodized sheet. No drastic change in tensile fracture mode was evident as a function of anodized condition of the sheet. On a microscopic scale, tensile fracture surfaces of the alloy revealed features reminiscent of locally ductile and brittle failure mechanisms. The fracture behavior of the alloy is discussed in light of intrinsic microstructural features; deformation characteristics of the alloy, local stress state and grain boundary failure.

The use of SIMS in quality control and failure analysis of electrodeposited items inspected for hydrogen effects

In high-strength steels it is often difficult to distinguish between hydrogen embrittlement and various other brittle failure mechanisms. The objective of this work was to develop a sensitive analytical procedure based on secondary ion mass spectrometry (SIMS) that would allow in-service identification of local hydrogen accumulation, either during quality control or during failure analysis of electroplated items. Dynamic SIMS was found useful in identifying when baking of Cd-plated AISI 4340 steel was not carried out, thus potentially leading to hydrogen embrittlement. In all non-baked samples, an increase in the hydrogen signal was found at the Cd/steel interface. In baked samples, either a peak was not observed at the interface, or it was insignificant based on determination of the ratios between the hydrogen signals in the coating, interface and substrate. This reproducible effect was monitored even after 16 months storage in a desiccator. These observations make the procedure practical in suggesting more accurate, reliable and cost effective recommendations for prevention of failures. The main effect of baking was found to be effusion of hydrogen from the interface and the substrate steel into the atmosphere. A mechanism for delayed failure is suggested.

Fiber Reinforced Laminates: Progressive Damage Modeling Based on Failure Mechanisms

In the present article, computational modeling of progressive damage in continuous fiber reinforced laminates is considered. After a general review of modeling approaches and experimentally observed behavior of laminates, the focus is laid on predicting non-linear laminate behavior by models based on continuum damage mechanics. The wide variety of continuum damage models is demonstrated by example of three different damage models from the literature which are described in more detail. Finally, a ply level damage model developed by the authors is presented. The model is based on brittle failure mechanisms postulated by Puck and is able to capture several characteristics of the damage behavior of laminates. Furthermore, this brittle damage model is extended to include plastic shear deformations. It is shown, that the extended model capturing brittle damage and plastic strains leads to significant improvements in the prediction of the non-linear laminate behavior.

Thermal-mechanical coupled effect on fracture mechanism and plastic characteristics of sandstone

Scanning electronic microscopy (SEM) was employed to investigate fractographs of sandstone in mine roof strata under thermal-mechanical coupled effect. Based on the evolution of sandstone surface morphology in the failure process and fractography, the fracture mechanism was studied and classified under meso and micro scales, respectively. The differences between fractographs under different temperatures were examined in detail. Under high temperature, fatigue fracture and plastic deformation occurred in the fracture surface. Therefore, the temperature was manifested by these phenomena to influence strongly on micro failure mechanism of sandstone. In addition, the failure mechanism would transit from brittle failure mechanism at low temperature to coupled brittle-ductile failure mechanism at high temperature. The variation of sandstone strength under different temperature can be attributed to the occurrence of plastic deformation, fatigue fracture, and microcracking. The fatigue striations in the fracture surfaces under high temperature may be interpreted as micro fold. And the coupled effect of temperature and tensile stress may be another formation mechanism of micro fold in geology.

Analysis of FRP–concrete debonding via boundary integral equations

The problem of debonding of FRP plates glued over a concrete element is studied making use of boundary integral equations. Mode II cohesive crack model is adopted for the interface, whereas linear elasticity is used for the two materials outside the process zone. Symmetric Galerkin boundary element method is used, adopting the arc-length technique to follow the equilibrium path beyond its critical point. It is shown that, due to the presence of a softening branch in shear stress-slip law, the behavior of a specimen undergoing debonding may be strongly non-linear, and is associated with a very brittle failure mechanism. For bond lengths longer than minimum anchorage length, a snap-back branch typically occurs after the attainment of the maximum force. Two different test setups have been numerically simulated and results in good agreement with experimental tests are found.

Source parameters of microearthquakes at Mount St Helens (USA)

SUMMARY We estimate the source parameters for a selection of microearthquakes that occurred at Mount St Helens in the period 1995–1998. Excluding the activity of 2004 September, this time period includes the most intense episode of earthquake activity since the last dome-building eruption in 1986 October. 200 seismograms were processed to obtain seismic moments, source radii, stress drops and average fault slip. The source parameters were determined from the spectral analysis of P waves, after correction for attenuation and site effects. In particular, P-wave quality (Qp) and site (S) factors have been previously calculated in the frequency ranges 2–7 Hz and 18–30 Hz. Because it was impossible to perform corrections for Qp and S over the whole spectrum we applied a new approach, based on the notion of ‘holed spectrum’, to estimate spectral parameters. The term ‘holed spectrum’ indicates a spectrum lacking corrected spectral amplitude values at certain frequencies. We carried out a statistical study to verify that dealing with the ‘holed spectrum’ does not lead to significant differences in the estimates of spectral parameters. We also investigated the dependence of spectral parameters (low-frequency level, corner frequency and high-frequency decay) on the bandwidth of spectral hole, and defined the threshold values for three different spectral models. Displacement ‘holed spectra’, corrected by attenuation and site response, are then used to determine spectral parameters in order to calculate seismic source parameters. Seismic moments range from 1017 to 1019 dyne-cm, source dimensions from 100 to 350 m, and average fault slip from 0.003 to 0.1 cm. Self-similarity seems to break down in that stress drops are very low (0.1–1 bars). We postulate that seismicity is associated with a brittle shear failure mechanism occurring in a highly heterogeneous material under a relatively low stress regime.

Effects of oxide and hydrogen on the circumferential mechanical properties of Zircaloy-4 cladding

Mechanical properties of Zircaloy-4 cladding were evaluated by the ring tension test to simulate the cladding behavior under the reactivity-initiated accident (RIA) in a high burnup environment. Zircaloy-4 cladding was controlled to have a hydrogen content up to 1000 ppm and a oxide thickness up to 100 μm, followed by a circumferential ring tension test with a strain rate in the range of 0.01 and 1/s. The results showed that both the ductility and toughness decreased as the hydrogen content and the oxide thickness increased. Hydride inside the cladding played an important role in the brittle failure mechanism by a decohesion between the hydride and the metal matrix as well as a preferential fracture of the brittle hydride. Surface oxide also affected the mechanical property such as a reduction of the load bearing area, stress relieving effect, and so on. Correlation between the in-reactor RIA parameters such as the fuel enthalpy and out-reactor mechanical toughness was postulated and it was concluded that the change of the mechanical toughness caused by the oxide was more significant than that by the hydrogen.

Micromechanical Modeling of the Static Loading of an Al 359-SiC MMC

The objective of this work was to use micromechanical finite element models to simulate the static mechanical behavior of a metal matrix composite: a cast Al 359 alloy reinforced with 20% SiC particles, at two different temperatures: room temperature and 150°C. In the simulations, periodic unit cell models incorporating the explicit representation of the matrix, reinforcing particles and precipitated primary silicon crystals in both 2D and 3D were used. Micromechanical models with both idealized and realistic reinforcing particle geometries and distributions were generated. The realistic particle geometries and distributions were inferred from experimental SEM micrographs. The pattern and intensity of the plastic deformation within the matrix was studied and the macroscale behavior of the composite was inferred from average stress and strain values. In order to include the effects of residual stresses due to the processing of the material, a quenching simulation was performed, prior to mechanical loading, and its effects on the macroscopic and microscopic behavior of the MMC was assessed. The effects of introducing the damage mechanisms of ductile void growth and brittle failure of the reinforcing particles was also investigated. The results of the simulations were compared with experimental results for the MMC in terms of macroscopic tensile stress–strain curves and conclusions were drawn.

Analysis of Test Specimens for Cohesive Near-Bond Failure of Fiber-Reinforced Polymer-Plated Concrete

The failure mechanisms of reinforced concrete (RC) members change due to the application of externally bonded fiber-reinforced polymer reinforcement. Although an extensive literature is available describing the failure mechanisms of poststrengthened flexural systems, brittle failure modes caused by bond failure, such as midspan debonding and end peeling, need to be further investigated in order to identify and quantify the fracture processes that result in bond failure. Simplified experimental tests have been designed to idealize the bond between the laminate and the RC member. However, it is unclear how the simplified test results can be related to the actual flexural debonding failures. This paper investigates and compares two bond failure tests: a simplified test (or simple shear test) and a recently proposed shear/normal test. After discussing the characteristics of both tests and how they relate to the midspan debonding and end peeling failures, the shear/normal test is studied in more detail using a nonlinear finite-element fracture mechanics program. The program accounts for cohesive localized and distributed concrete crack damage and is capable of describing the geometrical discontinuities that induce different brittle failure mechanisms. The numerical results compare well with available experimental data and help explain the crack formation and propagation pattern up to specimen failure. Parametric studies are presented to elucidate the influence of different material parameters on the failure mechanisms.

Using a deformable discrete-element technique to model the compaction behaviour of mixed ductile and brittle particulate systems.

This paper illustrates the application of a combined discrete- and finite-element simulation to the compaction of assemblies comprising both ductile and brittle particles. Through case studies, the results demonstrate the importance of using a fine mesh on the particle boundary, the effect of fragmentation and its impact on the form of the compression curve, and the effect of inclusion of ductile particles at ca. 25% by volume suppressing brittle failure mechanisms. Although, the calculations can be extended to three dimensions, the computational cost is a current limitation on such calculations. The novelty of this approach is in its ability to predict material yield surfaces for the compaction of a mixture of particles. The initial results are optimistic, but there is a need for model improvement, principally through the ability to capture the random packing of irregular particles since this will eliminate a key problem in defining an initial density for the simulation. The main advantage of this technology is in its ability to minimize the need for expensive triaxial testing of samples to develop the yield-surface history.

A discrete deformable element approach for the compaction of powder systems

The paper describes the application of a discrete deformable element modelling approach to simulate the compaction of a mixture of ductile and brittle powders together with an exploration of its suitability to establish the yield characteristics of powders. Two-dimensional rod models comprising ductile and brittle particles are assembled and subject to uniaxial compression in a rigid die. The utility of the method to represent the yielding of the powder is also explored by the application of a biaxial loading sequence. The model was found to be capable of modelling the behaviour of powder mixtures successfully. It captured ductile–brittle failure mechanisms in the case of brittle particles and demonstrated the effectiveness of a proportion of ductile particles in preventing the fragmentation of brittle particles in a mixture, illustrating agreement with experimental observation. The exploration of yield surface prediction was subject to a number of assumptions driven by the two-dimensional nature of the simulation, coupled with the high initial density level enforced by the particle packing. Despite these constraints, the results were encouraging when compared with experimental measurement of yield characteristics. Further work is required to establish more realistic initial density and to develop the capability to include a larger number of particles within the simulation.

Integrated analysis for die design including brittle damage evolution

Brittle failure mechanism has been well known as growth of penny-shaped micro-cracks, referred as brittle damage, which eventually cause macro-cracks and failure. A brittle damage model is proposed, which consists of a damage function and an evolution equation for brittle damage. The damage function determines whether a micro-crack begins to grow as a yield function in plasticity, while an evolution equation gives the growth rate of a micro-crack when it grows. An integrated analysis is then proposed to model the shrink-fit, elastic deformation, fatigue, wear as well as brittle damage evolution in a die. Applications of the proposed analysis to the extrusion die with multi-layers of stress-rings are given.

Seismic Behavior of Post-Tensioned Gravity Dams: Shake Table Experiments and Numerical Simulations

Displacement controlled shear tests on small concrete lift joint specimens with different surface roughnesses reinforced with an unbonded post-tension cable are conducted to study a related joint constitutive model, and to determine the failure mechanisms associated to it. To investigate the applicability of the joint model, shake table tests are conducted on a 3.4-m high plain concrete post-tensioned gravity dam that has a cold lift joint about midheight. Numerical simulations using the joint constitutive model are compared with the experimental results. From the displacement controlled shear tests, it is shown that the post-tension cable provides additional strength to the joint by applying an additional normal load on the joint, and by the shear resistance that can be mobilized (cable dowel action). The tests performed on rough surface joints have demonstrated that the dilatancy phenomenon due to asperities increases the post-tension force. Shake table tests on the 3.4-m high dam model have shown that post-tensioning largely reduces residual sliding displacements of the joint. This reduction is a function of the frequency content of the base excitation. However, a single post-tension cable placed near the upstream face increases the upstream rocking response of the upper block, which is also a function of the frequency content of the excitation. Strong seismic excitations can induce cable failure at a lower value than its uniaxial tensile strength when significant shear displacements take place. To describe this brittle failure mechanism, a cable failure criterion that considers the mobilized shear and tensile strengths was proposed and was found to be adequate. Moreover, the post-tensioned joint model proposed showed good agreement with the static and dynamic tests performed to evaluate the joint shear force.

RELEVANCE OF BEAM-COLUMN JOINT DAMAGE AND COLLAPSE IN RC FRAME ASSESSMENT

The role of joint damage and collapse in the seismic response assessment of existing reinforced concrete frame buildings is herein investigated. Based on recent results from experimental investigations on frame system and subassemblies designed for gravity-load-only, considerations on structural performance based on hybrid local and global failure mechanisms related to joint damage are provided, with particular attention to displacement demand, interstorey drift and damage distribution. Effects of bond deterioration and slip of reinforcing bars passing through an interior joint are discussed in terms of local hierarchy of strength and sequence of events. A simple analytical model for joint response is proposed and adopted for preliminary investigations on frame systems with substandard structural details. The occurrence of a “shear hinge” in the joint might protect to some extent soft-storey mechanisms, reducing the interstorey drift demand, with no significant effects on the global displacement demand. On the other hand, typical inadequacies of structural details (i.e. end-hook anchorage in beam bars combined with use of smooth bars) might cause severe strength degradation leading to particularly brittle failure mechanism.

Source Properties of Earthquakes near the Salton Sea Triggered by the 16 October 1999 M 7.1 Hector Mine, California, Earthquake

We analyze the source properties of a sequence of triggered earthquakes that occurred near the Salton Sea in southern California in the immediate aftermath of the M 7.1 Hector Mine earthquake of 16 October 1999. The sequence produced a number of early events that were not initially located by the regional network, including two moderate earthquakes: the first within 30 sec of the P-wave arrival and a second approximately 10 minutes after the mainshock. We use available amplitude and waveform data from these events to estimate magnitudes to be approximately 4.7 and 4.4, respectively, and to obtain crude estimates of their locations. The sequence of small events following the initial M 4.7 earthquake is clustered and suggestive of a local aftershock sequence. Using both broadband TriNet data and analog data from the Southern California Seismic Network (SCSN), we also investigate the spectral characteristics of the M 4.4 event and other triggered earthquakes using empirical Green's function (EGF) analysis. We find that the source spectra of the events are consistent with expectations for tectonic (brittle shear failure) earthquakes, and infer stress drop values of 0.1 to 6 MPa for six M 2.1 to M 4.4 events. The estimated stress drop values are within the range observed for tectonic earthquakes elsewhere. They are relatively low compared to typically observed stress drop values, which is consistent with expectations for faulting in an extensional, high heat flow regime. The results therefore suggest that, at least in this case, triggered earthquakes are associated with a brittle shear failure mechanism. This further suggests that triggered earthquakes may tend to occur in geothermal–volcanic regions because shear failure occurs at, and can be triggered by, relatively low stresses in extensional regimes.