Probable Failure Mechanism Research Articles

Weft direction pseudo-thermomechanical fatigue behaviors of T300/ Polyimide-380 2.5D woven composites

This work elaborates the weft direction in-phase thermomechanical fatigue (IP TMF) behaviors of T300/PI-380 2.5D-WCs for the temperature variation between 100 and 300 °C for the first time. Results show that the IP TMF responses of 2.5D-WCs exhibit a low cycle fatigue characterization, which approximately linear decreases with the increase of cycle number. The hysteresis loops shifted to the right confirm an elongation of specimen during the IP TMF process. The residual stiffness and residual strength behaviors are also investigated in the case of TMF, and the corresponding numeric models are proposed based on the damage mechanics. Moreover, fracture morphologies of 2.5D-WCs under the IP TMF loading have been explored by means of SEM and X-rays observations. Finally, a probable failure mechanism for the weft direction IP TMF behaviors of 2.5D-WCs is proposed. This work provides a new benchmark in studying high-temperature fatigue properties of resin matrix 2.5D woven composites.

3D Discrete Fracture Network Modelling from UAV Imagery Coupled with Tracer Tests to Assess Fracture Conductivity in an Unstable Rock Slope: Implications for Rockfall Phenomena

The stability of a rock slope is strongly influenced by the pattern of groundwater flow through the fracture system, which may lead to an increase in the water pressure in partly open joints and the consequent decrease in the rock wall strength. The comprehension of the fracture pattern is a challenging but vital aspect in engineering geology since the fractures’ spatial distribution, connectivity, and aperture guide both the water movement and flow quantity within the rock volume. In the literature, the most accepted methods to hydraulically characterise fractured rocks in situ are the single borehole packer test, the high-resolution flow meters for fractures, and the artificial tracer tests performed in boreholes. However, due to the high cost a borehole requires and the general absence of wells along coastal cliffs, these methods may not be appropriate in rockfall-prone areas. In this study, an unsaturated rocky cliff, strongly affected by rockfalls, was investigated by combining kinematic analysis, Discrete Fracture Network (DFN) modelling, and artificial tracer tests. The DFN model and potential rock block failure mechanisms were derived from high-resolution 3D virtual outcrop models via the Structure from Motion (SfM) photogrammetry technique. An artificial tracer was injected using a double ring infiltrometer atop the recharge zone of the slope to determine the infiltration rate and validate the DFN results. The DFN and tracer test methods are frequently used at different spatial scales and for different disciplines. However, the integration of digital photogrammetry, DFN, and tracer tests may represent a new step in rockfall and landslide studies. This approach made possible the identification of groundwater flow patterns within the fracture system and revealed about a 10-day tracer transit time from the injection area and the monitored slope, with similar conductivity values gathered from both the DFN and tracer test. Planar and wedge failures with volumes ranging from 0.1 and 1 m3 are the most probable failure mechanisms in the areas. The results were consistent with the delay between the intense rainfall and the slope failures previously documented in the study area and with their mechanisms.

Overtopping-Induced breaching process of concrete-faced rockfill dam: A case study of Upper Taum Sauk dam

The back analysis of the dam breaching process based on reported failure case data is of great significance for understanding dam breaching mechanisms and the ability to predict breach flow hydrograph during emergencies. According to the essential information and post-failure investigation report of the Upper Taum Sauk dam, the first pumped storage power station in the United States, the most probable failure mechanism for dam breach is studied. Furthermore, a detailed physically-based numerical model is developed to simulate the overtopping-induced breach process of the concrete-faced rockfill dam (CFRD). Highlights of the model include the establishment of a simulation method of the dam-break flow based on the actual three-dimensional terrain considering the water-retaining characteristics of concrete face slabs, and the times and lengths of failure for each concrete face slab under water load and self-weight, as well as the adoption of different transportation modes for the widely graded rockfill materials. The breach side slope sliding mechanism is considered during the dam breach process. It is worth noting that the hydraulic features of dam-break flow and breach morphology can be well described with a combination of renormalization group (RNG) k-ε turbulence model and volume of fluid (VOF) method. The calculated results of the Upper Taum Sauk dam breaching parameters agree with the measured values, which testify to the rationality of the proposed model. The obtained time-varying dynamic dam breach process of CFRD can be divided into three stages. Moreover, the sensitivity analysis indicated that, the presence and absence of concrete face slabs affect the breach process of the Upper Taum Sauk dam significantly. The performance comparison with the existing dam breach models also verifies the advancement of the detailed physically-based model. Hence, the addressed numerical method can be used as an effective tool for simulating the CFRD breach process.

Failure Modes and Effect Analysis of Gear Pair

In the present study, the failure mode and effect analysis (FMEA) process has been used to determine the probable failure mechanisms of gear pairs and their implications on the performance, availability, and cost. The risk priority number (RPN) is calculated to rank the failure modes and identify high-risk failures. The methods are proposed to minimize high-risk failure modes.

Thermomechanical fatigue behaviors and failure mechanism of 2.5D shallow curve-link-shaped woven composites

Thermomechanical fatigue (TMF) performance is one of the most challenges for aero-engine materials. This work experimentally elaborated the in-of-phase (IP) TMF behaviors and failure mechanism of high-temperature resistant 2.5D woven composites (2.5DWCs) for the first time. In this study, both fatigue load and thermal load varying from 160 °C to 260 °C are synchronously imported in the form of trapezoidal wave. An experimental platform consisting of quasi-static mechanical tester, strain collection device, heating/cooling equipment and temperature control system was firstly established. The relationship between stress level and TMF life was obtained. The fracture morphologies were investigated using scanning electron microscope (SEM) and X-ray computed tomography (Micro-CT). Results show that the TMF behaviors of 2.5DWC exhibit a low cycle fatigue characterization with a fatigue limit of 0.75 stress level. The hysteresis loops gradually shift to the right, confirming an elongation of specimen during the TMF. It indicates a reduction of inclination angle of warp yarns. Particularly, the failure behavior is fiber-dominated brittle fracture mode, without clear necking phenomenon. The fiber pull-out length and the extensive debonding are more severe than those failed in the static loading. Finally, a probable failure mechanism for the TMF behaviors of 2.5DWC is proposed.

A levee breach induced by internal erosion in Western Australia

This case study presents a levee breach induced by piping erosion under cyclonic conditions in 2019. The levee is a 2.5 m-high, 500 m-long, mass earth-fill embankment; with no cutoff trench, core or ancillary structures. Located near Port Hedland on the north coast of Western Australia, its purpose is diversionary: to redirect cyclonic surface water away from the nearby Great Northern Highway. The levee was founded directly on alluvium in 1987; it is formed of locally sourced clayey sandy gravel. In 2003, the levee was partially excavated to enable the placement of a buried pipeline through the levee. Following a cyclonic event in 2019, a 27 m length of the levee breached, resulting in significant scour of the foundation and downstream soil. A site visit and investigation were conducted shortly thereafter, where in situ testing and laboratory soil tests on the levee and foundation materials were conducted. Analysis of the site observations and laboratory testing data led to the probable failure mechanism being theorized as having been initiated within the foundation by piping erosion within sand-rich beds of alluvium. The large quantity of water ponding upstream of the levee then caused a progressive washout and breach of the levee. Thematic collection: This article is part of the Role of water in destabilizing slopes collection available at: https://www.lyellcollection.org/cc/Role-of-water-in-destabilizing-slopes

Bearing capacity-relative density behavior of circular footings resting on geocell-reinforced sand

Geocell reinforcement (GR) improves load-deformation behavior of shallow foundations. A series of instrumented large-scale plate load tests was performed to ascertain such improvements and to correlate that to the relative density (Dr ) of geocell-reinforced sand (GRS). Variable effects on stiffness, bearing capacity (BC), load dispersion and strain-stress behaviors are discussed. The extensive tests made it possible to arrive at a simplified analytical procedure for predicting the BC of circular footings on GRS, taking into account probable shear failure mechanisms and monitored GR deformation. These results allowed the validation and generalization of the procedure reduced from an equivalent-layer BC model considering ‘confinement’ and ‘stress dispersion’ effects on the failure mechanism. Load dispersion angle and strains increase with increasing the sand Dr resulted in significant stress reduction below the GR. This was found most beneficial for the higher densities ranges. The proposed analytical approach can confidently be used for practical design purposes.

Mechanical behavior of freestanding 8YSZ thin films under tensile and bending loads

Thermal barrier coatings (TBCs) with yttria stabilized zirconia (YSZ) as topcoat, are used to provide thermal insulation for the hot sections of gas turbine engines and other high temperature components. The aim of the present study is to evaluate the in situ tensile properties of plasma sprayed freestanding ZrO2 8 wt% Y2O3 top coat of ~250 μm thickness. Air plasma sprayed films were prepared, using pre-fabricated templates, in the form of tensile specimens. Digital image correlation with optical imaging was used to obtain full-field deformation, and a strain resolution less than 0.01% was achieved. Further, the probability of failure in these coatings was investigated using Weibull analysis. The tensile stress vs. Strain curves of the coatings exhibited a non-linear behavior, with an elastic modulus of 12 ± 4 GPa, and failure strength of ~16 ± 4 MPa with a Weibull modulus (m) of 5. The bending strength and flexural modulus of these coatings were higher, and found to be ~69 ± 12 MPa with ‘m’ of 10 and 37 ± 8 GPa, respectively. Successive unloading-reloading experiments indicated the absence of non-linearity in tensile behavior with increase in number of cycles demonstrating the inhibition of frictional sliding of microcracks. Furthermore, fractography indicated intersplat shear and decohesion as probable failure mechanisms.

Geological and geophysical investigations to analyse a lateral spreading phenomenon: the case study of Torrioni di Rialba, northern Italy

We combined geological, geomechanical, geophysical and remote sensing investigations, including persistent scatterer interferometry and bathymetry, to study a slope where four conglomerate towers laterally spread over a shale layer. Electrical resistivity tomography surveys confirm a shale layer that underlies the rock towers with an attitude parallel to the slope. Field mapping reveals that the stability of the rock towers is threatened by weakly cemented conglomerate layers, large eroded zones and karstic weathering due to water circulation. We deem that the most probable failure mechanism would be the toppling of the southernmost tower, promoted primarily by the weak conglomerate layer in its lower section. The plastic shale layer underneath the rock cliff is very likely to promote lateral spreading that may have triggered the toppling of an additional former rock pillar, whose rock blocks were found in the lake during a bathymetric survey. Close- and long-range remote sensing of displacements provide no results that could be interpreted with confidence. Seismic noise recording sessions with 3C low-frequency velocimeters suggest that the dynamic conditions of the towers do not show appreciable variations across the recording sessions, especially considering the two rock towers featuring the most interesting spectral characteristics. However, both the resonance frequencies and the preferential oscillation directions estimated from the seismic noise datasets are consistent with the analytical relationships and will support the design of an effective monitoring strategy.

Characterization of the geological and geotechnical conditions at the village of Monsanto

The stability of slopes and hillsides involving rock or soil masses depends crucially on the occurring geological and geotechnical conditions. The village of Monsanto, municipality of Idanha-a-Nova, is located on a granitic inselberg. The tourist interest of the village lies largely in the fusion of the granitic rock mass with the man-made constructions and in the aesthetics of several granitic boulders scattered throughout the village. In the present work the geological and geotechnical characterization of the rock mass has been carried out, namely by the field survey of the entire intervention area and by a set of laboratory tests on samples collected in the field (ultrasound propagation velocity and the uniaxial compression test). Based on this information, the shear strength of the discontinuities was estimated and a kinematic analysis was performed using the Dips 7.0 commercial software, with the objective of defining the predominant sets of joints and identifying the most probable failure mechanisms in each hillside.

Integration of reverse engineering and non-linear numerical analysis for the seismic assessment of historical adobe buildings

Abstract This paper aims at presenting a methodology that integrates reverse engineering tools with a combination of advanced and simplified analytical methods to perform predictive analysis of the structural behavior of historical adobe buildings under seismic loads. This methodology proposes the joint use of terrestrial laser scanner and photogrammetry to obtain accurate geometrical models. The methodology also proposes the in-situ measurement of structural system properties through experimental modal tests. All the collected information is used to create representative Finite Element models which are then considered for performing predictive non-linear analyses and identifying the most probable collapse mechanisms. Limit analysis method is finally used to complement the analysis and to assess the structural performance of the adobe building under study considering different seismic scenarios. The methodology proposed in this paper is validated in the church of San Juan Bautista de Huaro, located in Cusco, Peru. This church was built in the 16th century and is known for its impressive mural paintings covering the entire indoor surface of the church. The results of the seismic assessment methodology proposed in this paper allowed to estimate the global behavior and possible damage patterns in the church during seismic events, showing that the most probable failure mechanisms would be the global rocking of the facade and the partial collapse of tympanum of the facade. The application of this methodology also allowed defining the performance levels of this church when facing different seismic scenarios. The results showed that the church should stay in a safe state until occasional earthquakes with a return period of 72 years. However, the results also indicate that rare earthquakes (return period of 475 years) will produce an unsafe structural condition with partial collapses of structural elements.

Damage survey, simplified assessment, and advanced seismic analyses of two masonry churches after the 2012 Emilia earthquake

ABSTRACTRecent Italian earthquakes (L’Aquila 2009, Emilia 2012, Central Italy 2016–2017) have shown the high vulnerability of existing and historical masonry constructions to seismic actions. This article investigates the damage distribution and the seismic response of two masonry churches located in the province of Ferrara (Northern Italy) and severely damaged by the 2012 Emilia earthquake. A proper knowledge of the geometry and structural details of the two churches was achieved through accurate on-site surveys and documentary research conducted after the earthquake. Moreover, field surveys provided detailed information about the crack patterns, highlighting the presence of significant damage in several parts of the churches. The seismic vulnerability assessment of the two churches was conducted on the basis of both the simplified approach given by Italian Guidelines on the Built Heritage and sophisticated nonlinear dynamic analyses performed on detailed finite element models assuming an elasto-plastic damage constitutive law for masonry. Limit analysis with pre-assigned failure mechanisms was carried out to preliminarily identify the most probable active failure mechanisms. Nonlinear dynamic analyses with different PGA levels allowed for a deep insight into the damage distribution in the churches: moreover, the energy density dissipated by tensile damage and the maximum normalized displacements were computed for the different macro-elements. A comparison between the numerical results and the damage observed during detailed field surveys was also provided. The results obtained in this study may represent a useful step to improve the knowledge of the seismic behavior of similar masonry churches located in the same region.

Design, Fabrication and Testing of Carbon Fiber Reinforced Epoxy Drive Shaft for All Terrain Vehicle using Filament Winding

Filament winding is a composite material fabrication technique that is used to manufacture concentric hollow components. In this study Carbon/Epoxy composite drive shafts were fabricated using filament winding process with a fiber orientation of [852/±452/252]s. Carbon in the form of multifilament fibers of Tairyfil TC-33 having 3000 filaments/strand was used as reinforcement with low viscosity epoxy resin as the matrix material. The driveshaft is designed to be used in SAE Baja All Terrain Vehicle (ATV) that makes use of a fully floating axle in its rear wheel drive system. The torsional strength of the shaft was tested and compared to that of an OEM steel shaft that was previously used in the ATV. Results show that the composite shaft had 8.5% higher torsional strength in comparison to the OEM steel shaft and was also lighter by 60%. Scanning electron microscopy (SEM) micrographs were studied to investigate the probable failure mechanism. Delamination, matrix agglomeration, fiber pull-out and matrix cracking were the prominent failure mechanisms identified.

Methodology for the Design of Accelerated Stress Tests for Non-Precious Metal Catalysts in Alkaline Fuel Cell Cathodes

As the interest in alkaline fuel cells (AFCs) rises, the development of new materials to increase their activity and lower their cost, the question of their durability has become critical. The durability testing protocols suggested by agencies such as the US DOE is limited to the study of platinum on carbon in acid. These protocols have become the generic testing protocols to all fuel cell catalysts but the they possess one significant flaw: they do not necessarily take into account the vulnerabilities of other types of catalysts. When studying the durability of fuel cells, one can conduct a constant lifetime test; however, this method is time-consuming and expensive. Instead of long-term measurements, the fuel cells industry along with government agencies such as the US-DOE and researchers develop protocols and perform accelerated stress tests (ASTs). ASTs are methods for determining the durability of the whole cell or a particular component in the fuel cell system in short periods of time by exposing the cell to extreme, yet realistic, operating conditions that may cause some of its components to degrade rapidly (e.g. support, catalyst and membrane). Using such methods, the weaknesses of the fuel cell can be determined and then improved in order to increase its lifetime. The performance, degradation rate and component damage level under specific working conditions are examined during and after the ASTs. The aim of the AST is to predict the fuel cell's lifetime and explain the probable failure and degradation mechanisms. So far, most of the AST protocols were designed for commercial polymer electrolyte membrane fuel cells (PEMFCs) which consist on Pt-based catalysts, carbon supports and Nafion membranes. In the case of other catalysts, supports and membranes, specifically in AFCs, these ASTs become irrelevant due to the different chemical and electrochemical properties of the different components. For example, in some of the US-DOE ASTs, the electroactive surface area (ECSA), an indication of the catalysts activity, of Pt-based catalysts is measured from the hydrogen adsorption/desorption peaks: this cannot be done with non-precious group metal catalysts since most of them do not adsorb hydrogen. Hence, ASTs need to be considered as case sensitive, or at least divided into categories of materials, since not all fuel cells are composed of the same materials and/or operate under the same conditions. Therefore, other methods and measurable parameters need to be found in order to assess the catalyst degradation in such cases. Here, we developed the methodology which will allow groups who are studying non-conventional materials for fuel cells to study the stability and durability of their catalysts and supports, develop case-sensitive ASTs for their fuel cells in general and for AFCs in particular. In this work a wide array of methods are used for testing non-precious group metal catalysts and support degradation alkaline fuel cell cathodes. In this case study, we used a cathode composed of a pyrolyzed non-precious metal group catalyst (NPMGC) on activated carbon. The vulnerabilities of the cathode components were studied in order to develop the methodology and design an accelerated stress test for NPMGC-based cathode in alkaline environment. Cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS) were used to characterize the electrochemical behavior of the cathode and to follow the changes that occur as a result of exposing the cathodes to extreme operating conditions. Rotating ring disk electrode (RRDE) was used to study the cathodes kinetics; Raman spectroscopy and X-ray fluorescence (XRF) were used to study the structural changes in the electrode surface as well as depletion of the catalysts’ active sites from the electrode. The changes in the composition of the electrode and catalyst were detected using X-ray diffraction (XRD). For the first time, we show that NPMC degrade rapidly at low operating potentials whereas the support degrades at high operating potentials. We developed a tailor-made AST for the studied electrodes based on real operating statistics and operating conditions.

Molecular level detection and localization of mechanical damage in collagen enabled by collagen hybridizing peptides

Mechanical injury to connective tissue causes changes in collagen structure and material behaviour, but the role and mechanisms of molecular damage have not been established. In the case of mechanical subfailure damage, no apparent macroscale damage can be detected, yet this damage initiates and potentiates in pathological processes. Here, we utilize collagen hybridizing peptide (CHP), which binds unfolded collagen by triple helix formation, to detect molecular level subfailure damage to collagen in mechanically stretched rat tail tendon fascicle. Our results directly reveal that collagen triple helix unfolding occurs during tensile loading of collagenous tissues and thus is an important damage mechanism. Steered molecular dynamics simulations suggest that a likely mechanism for triple helix unfolding is intermolecular shearing of collagen α-chains. Our results elucidate a probable molecular failure mechanism associated with subfailure injuries, and demonstrate the potential of CHP targeting for diagnosis, treatment and monitoring of tissue disease and injury.

Plastic design of eccentrically braced frames with shear panels

This paper introduces a new earthquake-resistant building design featuring eccentrically braced frames with steel wall shear panels. It also proposes closed-form expressions for analysis and extension of the existing failure mode control design method for the new structural system. Closed-form equations for internal forces were obtained, and probable failure mechanisms and corresponding lateral load multipliers for secondary effects were identified. Selection of member profiles was completed by the mechanism equilibrium curve concept. Pushover modelling was then performed with plastic hinge distribution corresponding to failure mechanisms. Only small differences were found between the analytical and modelled values of lateral force multipliers, meaning that the closed-form equations are a valid alternative approach for plastic analysis and design of eccentrically braced frames with shear panels.

A Review of Seismic Damage of Mountain Tunnels and Probable Failure Mechanisms

Severe cases of damages of mountain tunnels following 1995 Hyogoken-Nanbu (Japan), 1999 Chi-Chi (Taiwan), 2004 Mid-Niigata Prefecture (Japan) and 2008 Wenchuan (China) earthquakes have challenged the traditional belief of tunnel structures being seldom damaged in seismic events. These experiences are a reminder that seismic behaviour of mountain tunnels must be further studied in detail. Such investigations assume greater significance as more number of tunnels are being planned to be constructed to meet the infrastructural needs of mountainous regions all around the world. In this paper, seismic damages of mountain tunnels have been reviewed. Prominent failure patterns have been identified based on the case histories of damages. Damages in the form of cracking of tunnel lining, portal cracking, landslide induced failures, uplift of bottom pavement, failures of sidewalls, shearing failure of tunnel liner and spalling of concrete have been majorly observed. Based on the damage patterns and earthquake data, main factors leading to instabilities have been discussed. Probable failure mechanisms of mountain tunnels under seismic loading conditions have been explained. Seismic analyses of a circular lined tunnel in blocky rock mass have been carried out through discrete element based approach. The significant role of different seismic parameters like frequency, peak ground acceleration has been identified. Moreover, effect of tunnel depth on the seismic response of tunnels has been investigated. It is believed that the present study will help in advancing the present state of understanding with regard to the behavior of tunnels under seismic conditions.

Earthquake damage estimation of concrete gravity dams using linear analysis and empirical failure criteria

A new methodology is proposed to estimate earthquake damage of gravity dams. In this methodology the static pushover analysis is used to formulate a systematic and rational procedure to estimate the level of damage from the linear seismic analysis results. The tensile cracking of concrete is considered as the main potential damage. Examples of three existing concrete gravity dams are provided to illustrate the methodology and discuss the probable nonlinear response and failure mechanisms. The damage state of the dams under twelve proper earthquake ground motions scaled to increasing intensity levels is estimated using the new proposed criteria and verified using the actual nonlinear time-history analysis. Finally, a damage index is defined to quantitatively predict the earthquake damage of gravity dams.

Comparison the capacity of box steel columns, fabricated with groove and fillet welds

Currently, box columns are fabricated by welding plates using fillet or groove welds. According to probable seismic failure mechanism of these columns, their nonlinear behavior in terms of ductility and strength is necessary. In this research, a variety of columns with different welding patterns including groove and fillet welding with various overlap values and weld legs are numerically modeled and their nonlinear behavior is studied in finite element domain by using ABAQUS software. Suitable welding pattern is proposed by evaluation the energy absorption as well as stress distribution and concentration in box section. Results indicate that the section fabricated through groove welding has the best nonlinear performance and in the next priority, the sections with overlap size and weld leg equal to 20 and 80, or 50 and 50 percent of plate thickness, respectively, lead to suitable performance.

Structural performance of a parked wind turbine tower subjected to strong ground motions

The objective of this paper is to evaluate the structural performance of a typical wind turbine tower subjected to strong ground motions. A detailed finite element model of an 80m wind turbine tower was developed and subjected to strong ground motions. Two sets of input ground motions were used: one with pulse-type near-fault motions and the other one with far-fault motions. The structural performance of the wind turbine tower was investigated through seismic fragility analysis. The potential limit states were defined as global buckling of the tower, first occurrence of yielding, overturning of the foundation and permanent deformation of the tower. It was found that the wind turbine tower investigated in this study is most vulnerable to the overturning in the event of an earthquake. Yielding of the tower is the second most probable failure mechanism, which is followed by development of permanent deformation and global buckling of the tower. Similar trends in the failure mechanism were observed for both near-fault and far-fault ground motions.