Failure Mode Characteristics Research Articles

Failure characteristics of surrounding rocks along the radial direction of underground excavations: An experimental study

For the in situ surrounding rocks of underground excavations, the radial stress corresponding to the minimum principal stress σ3 varies with increasing distance away from the excavated boundary. To characterize the rock failure along the radial direction of an excavation, true triaxial compression tests with different σ3 values (0.5, 5, 10, 15, 20, 25, and 30 MPa) were performed. The characteristic stresses (including the crack initiation stress σi, crack damage stress σd, and peak stress σs) and failure mode of the tested specimen under different σ3 conditions were systematically investigated. The experimental results indicate that with increasing σ3, the characteristic stresses increase monotonically and continuously. The increasing rate of the characteristic stresses with σ3 ranging from 0 to 10 MPa is higher than that with σ3 ranging from 10 to 30 MPa, indicating that low σ3 impose a greater effect on the characteristic stresses than high σ3. In addition, the failure mode of the tested specimen changes from tensile-shear failure mode I (with vertical tensile cracks and oblique tensile-shear cracks developing along the σ2 direction) to tensile-shear failure mode II (with oblique tensile-shear cracks developing only along the σ2 direction) to tensile-shear failure mode III (with oblique tensile-shear cracks along both σ2 and σ3 directions) as σ3 increases. Then, the failure characteristic of the surrounding rocks of the underground excavation was characterized based on the obtained experimental results. When maximum radial stress σr encountered near the excavated boundary is much smaller than the axis stress σa, the surrounding rocks will exhibit a “Ternary failure characteristic”, i.e., with an increase in the distance from the excavated boundary along the radial direction, the failure mode of the surrounding rocks after excavation changes from tensile/splitting failure mode to tensile-shear failure mode I and then to tensile-shear failure mode II. When the maximum σr encountered near the excavated boundary approximates the original σa, the surrounding rock masses will exhibit a “Quaternary failure characteristic”. Specifically, the corresponding failure mode along the radial stress direction away from the excavated boundary changes from tensile failure mode to tensile-shear failure mode I then to tensile-shear failure mode II and finally to tensile-shear failure mode III.

Evaluation on the ultimate bearing capacity for laminated bamboo lumber columns under eccentric compression

Engineered bamboo is gaining widespread interests from researchers to exploit its excellent functional characteristics in practical engineering applications. Laminated bamboo lumber (LBL) is one of many engineered products that are currently available with the potential to be used in the construction sector. The size and shape of LBL can be easily controlled to suit design requirements making it competitive with other commonly used building materials. This paper presents a recent investigation on the behaviour of LBL columns under eccentric compression loading, and proposes analytical formulations for predicting the ultimate resistance of columns with special emphasis given on observed failure mode types. Closer inspection revealed three characteristic failure modes, denoted as Mode I, II and III hereafter, for LBL columns under eccentric compression. In Mode I, failure was initiated from glue or bracket but all bamboo fibers remained elastic; in Mode II the outer most compression fiber reached elastic-plastic stage and eventually triggered failure; and in Mode III outer most part of compression fiber achieved plastic capacity prior to failure. Detailed analytical models have been developed and presented herein for all observed failure modes’ and the performance of the proposed design equations are compared using test evidences reported by the authors. The proposed equations produced very good agreement with the experimentally obtained results, and may be adopted in future design codes for engineered bamboo.

Seismic performance of lead-filled steel tube damper: Laboratory test, parameter identification and application

In this paper, a lead-filled steel tube damper (LFSTD) is presented consisting in a steel tube, a lead cylinder, and two steel plate caps. Seven LFSTDs were designed into three groups based on the thickness-diameter ratio, the weakening ratio, and the height-diameter ratio. They were subjected to cycling loading in a laboratory to determine device parameters such as hysteretic behavior, strength, ductility, energy dissipation and damping, and characteristic failure modes, Then, the Bouc-Wen model was used to analytically depict the hysteretic behavior of LFSTDs and the model parameters were identified using an improved squirrel search algorithm. The LFSTDs are later applied into a six-storey reinforced concrete (RC) frame model, and the seismic performance of the dampers with identified Bouc-Wen model parameters is numerically assessed when the RC frame model is under strong and destructive real earthquakes. Both laboratorial and numerical results indicate that the LFSTDs are effective in absorbing seismic energy and in reducing lateral displacements of the protected structure and they have potential of practical applications.

Influence of surface treatment with infrared nanosecond laser on adhesion performance of adhesion-bonded carbon fiber/epoxy composite

The surface treatment of carbon fiber/epoxy composites with a pulsed infrared (1064 nm wavelength) nanosecond (7 ns pulse duration) laser has been investigated. It is found that single 1064 nm laser pulse can remove the polymer matrix from composite surface to expose the carbon fibers by expansion of the pyrolysis gas resulting from laser-induced polymer degradation. The influences of laser power density and scan strategy on matrix removal process have been studied. The bonding strength of laser processed, adhesion-bonded CFRP (carbon fiber reinforced plastic) increase with lower matrix residues on the fibers and a linear relation is found. The adhesion-bonding strength of CFRP surfaces, tested by lap-joint shear test, exceeds the strength measured for untreated and abrasion treated (SiC paper) samples. The main origin of fiber damage of the laser-treated surface is mechanical fracture rather than thermal damage as found by laser scanning confocal microscope and scanning electron microscope. The characteristic morphologies and failure modes of shear-tested surfaces are analyzed. The obvious improvement of adhesive strength is achieved by the laser-treated surfaces attributed to matrix removal and carbon fibers protruding. A 1 D model is established to describe temperature and internal gas pressure of the irradiated surface. The new laser treatment method needs less laser energy and low pulse numbers and prove the way to an efficient, low-cost surface treatment for preparation of high quality adhesion bonded CFRP components.

Analysis of time, frequency and time-frequency domain features from acoustic emissions during Laser Powder-Bed fusion process

Sensor integration for in situ monitoring during additive manufacturing promises to enhance control over the process and assures quality in the fabricated workpieces. Acoustic emissions from the process zone of the laser powder-bed fusion process carry information about the events and failure modes of the printed workpiece. Analysis of acoustic signals emitted during different laser regimes, such as conduction, keyhole, etc. in time, frequency and time-frequency domains could provide quantitative information about the underlying physical mechanisms. This article reports a statistical analysis of the features in acoustic signals to perceive the characteristics of failure modes occurring during layering of stainless steel 316L. The visualization of the feature space distribution that corresponds to different failure modes shows the potentials of applying machine learning for in situ classification. The paper also proposes strategies in terms of data acquisition and preprocessing for building a comprehensive monitoring system.

Bending of Composite Cylindrical Shells with Circular Cutouts: Buckling and Failure Analysis

This study investigates the effects of cutout size and position on the progressive failure characteristics of constant-stiffness and steered-fiber variable-stiffness carbon-fiber-reinforced-polymer cylindrical shells under pure bending. The geometries of the shells with the proposed cutouts are described, and the analysis methods that are used for the evaluation of the buckling and failure characteristics of the shells are detailed. Measured geometric imperfections are introduced during nonlinear progressive analyses. Results of a parametric study varying the location and size of the cutouts and investigating the maximum load levels and predominant failure modes for specific constant-stiffness and variable-stiffness layups are presented. Three specific failure modes are identified and related to the cutout size and location. Candidate configurations for future testing were selected based on the failure mode characteristics to validate the shell analysis conclusions.

Seismic behavior of recycled plastic lumber walls: An experimental and analytical research

Recycled Plastic Lumber (RPL) is a wood-like material made from recycled plastics that aims to diminish the environmental pollution resulting from plastic wastes. This material is used in different kinds of nonstructural and structural applications. Recently, RPL has been proposed as a suitable material to develop structural walls that comprise the seismic resistant system of housings, in order to lessen the housing deficiency. This article presents the results drawn from an experimental campaign carried out over three full-scale RPL walls, which were tested under cycling loading conditions to determine structural parameters such as strength, hysteretic behavior, ductility, energy dissipation, equivalent damping and characteristic failure modes of the RPL walls, which are necessary to design and to assess seismically the housings. Finally, a multilinear hysteretic model capable of simulating the nonlinear dynamic behavior exhibited by RPL walls was implemented, in order to simulate and to assess the seismic behavior of them under strong and destructive real earthquakes.

Development of Accelerated Durability Tests for Rear Suspension Components under Failure-Correlated Load

Abstract Validation tests are essential for assuring the durability of automotive structures and components. The sporadic nature of mechanical failure requires that durability testing be accelerated to reduce the test time while faithfully replicating the characteristic fatigue damage and failure modes obtained during service. This paper develops accelerated durability tests for the torsion beam axle of a rear suspension under failure-correlated load, where a particular loading input is highly correlated with a characteristic fatigue failure mode and therefore dominates the fatigue damage obtained at a failure location. This study involves several steps. First, the load histories, i.e., the wheel loads and strains at critical regions, are measured on a proving ground. Second, failure locations are identified by critical plane and biaxial ratio analyses of the strain histories and local strain states on the axle. Then, the failure-correlated load is identified by determining the correlation between the absolute maximum principal strain at the failure locations and the corresponding full-wheel loads recorded in the measured load histories. Finally, two accelerated load spectra based on the identified failure-correlated load are developed in either the time domain or the amplitude domain, and the results of the accelerated durability tests are compared with those obtained for durability tests conducted under full-wheel loading. The results show that the failure locations and fatigue lifetimes obtained using the accelerated load spectra coincide with those obtained under full-wheel load conditions, and acceleration factors between 1.75 and 2.85 were observed using the two developed spectra. The proposed methodology is expected to contribute to the development of accelerated durability tests of automotive structures and components subjected to complex multiexternal loads.

Failure particularities of adhesively bonded joints between pultruded GFRP composite profiles

The failure particularities of the adhesively bonded joints between fibre reinforced polymer (FRP) composite elements represent a set of critical parameters in assessing the performance of these systems. If the failure process is not properly evaluated and controlled, it can lead to significant decrease of the overall efficiency of the joint. Usually, if the bond length is greater than the effective one, the failure process for these types of joints can be either progressive, if the initial failure is initiated in the adhesive layer, or brittle, if the failure occurs in the FRP elements. The ultimate failure occurs if any stress or strain component reaches a limit state, referring to the components (adherents and adhesive) or to the interfaces between them.This paper presents some particularities related to the failure process of the adhesively bonded joints between pultruded glass fibre reinforced polymer (GFRP) composite members. The results were obtained through a complex experimental program conducted at the Faculty of Civil Engineering and Building Services from Iasi, where the structural behaviour of adhesively bonded single lap joins (SLJ) and thick adherents’ joints (TAJ) between pultruded GFRP composite flat profiles was studied. For the experimental program, 30 specimens were prepared by varying the connection type (SLJ, TAJ), the bond length (70, 100, 150 mm), the adhesive type and its thickness (1, 2, 3 mm). All specimens were loaded in axial tension, up to failure and the characteristic failure modes were investigated by in-depth microscopic analysis of the failure surfaces.

Influence of construction method on the load bearing capacity of skew masonry arches

This paper investigates the influence of construction method (e.g. false skew, helicoidal and logarithmic method) on the mechanical behaviour and load carrying capacity of single span skew masonry arches. Simulations were performed with the three dimensional computational software, based on the Discrete Element Method of analysis. Each stone/brick of the masonry skew arch was represented by a distinct block. Mortar joints were modelled as zero thickness interfaces which can open and close depending on the magnitude and direction of the stresses applied to them. The variables investigated were the construction method, the angle of skew, the size of masonry blocks and the critical location of the live load along the span of the arch. At each skew arch, a full width vertical line load was applied incrementally until collapse. From the results analysis, it was found that for a skew masonry arch constructed using the false skew method, as the angle of skew increase, sliding between voussoirs in the arch increases and failure load decreases. However, for skew masonry arches constructed using the helicoidal and logarithmic method, as the angle of skew increases, the failure load increases. Three different characteristic failure modes were identified depending on the contact friction angle and the method of construction. These observations provide new insight into the behaviour and lead to suggestions for understanding the load carrying capacity and failure mode of skew arches.

Flexural creep tests and long-term mechanical behavior of fiber-reinforced polymeric composite tubes

An experimental investigation of the long-term creep behavior of fiber reinforced polymeric composite tubes subjected to flexural loading was performed. The tubes were first tested under a three-point bending scheme at room temperature to determine the ultimate flexural strength (UFS) and to identify the characteristic failure mode and possible failure locations. Creep tests were then carried out at stress levels of 45%, 55%, 65%, and 75% of the UFS at constant temperatures ranging from −60 °C to 100 °C for 500 h, and strain measurements over time were recorded. The tubes were further tested at varying temperatures by applying 22.5 thermal cycles between −60 °C and 100 °C and 9.5 thermal cycles between −160 °C and 80 °C. Similar flexural loads were applied simultaneously on the tubes. Long-term creep deformation of the tube was evaluated using the time-temperature-stress superposition principle, the derived creep main curves, and the Findley model. Finally, the creep deformation and mechanical strength of the tube at expected service conditions over its entire lifetime were predicted.

Damage assessment of channel section columns subjected to uniform compression

The paper presents the failure analysis of compressed channel section profile made of GFRP material. Three symmetrical arrangements of layers were considered: [90/0/90/0]s, [90/-45/45/0]s, [45/-45/90/0]s. In order to characterize the failure process, visual inspection, ultrasonic phased array, non-destructive testing and computer microtomography were used. Damage analysis based on studies of external and internal damage of investigated structure allowed for identification of characteristic failure modes and mechanisms. It was shown that the dominant form of destruction in the quasi-orthotropic system are matrix cracks coupled with the presence of extensive delamination while in quasi-isotropic systems more concentrated damage was noticed, in which the cracking/rapture of fibers localized in the column’s corners determined the initiation of the failure mechanism and the carrying capacity loss. In addition, it was noted that the fiber directions of the outer layers determined the directions of matrix cracks and delamination propagation.

Modelling Coastal Cliff Recession Based on the GIM–DDD Method

The unpredictable and instantaneous collapse behaviour of coastal rocky cliffs may cause damage that extends significantly beyond the area of failure. Gravitational movements that occur during coastal cliff recession involve two major stages: the small deformation stage and the large displacement stage. In this paper, a method of simulating the entire progressive failure process of coastal rocky cliffs is developed based on the gravity increase method (GIM), the rock failure process analysis method and the discontinuous deformation analysis method, and it is referred to as the GIM–DDD method. The small deformation stage, which includes crack initiation, propagation and coalescence processes, and the large displacement stage, which includes block translation and rotation processes during the rocky cliff collapse, are modelled using the GIM–DDD method. In addition, acoustic emissions, stress field variations, crack propagation and failure mode characteristics are further analysed to provide insights that can be used to predict, prevent and minimize potential economic losses and casualties. The calculation and analytical results are consistent with previous studies, which indicate that the developed method provides an effective and reliable approach for performing rocky cliff stability evaluations and coastal cliff recession analyses and has considerable potential for improving the safety and protection of seaside cliff areas.

Using finite element analysis to influence the infill design of fused deposition modelled parts

Additive manufacturing (AM) has and continues to experience considerable market and technological growth with many forecasting a tripling in market value over the next decade. One of the primary drivers for this growth is the increased freedom afforded to the design of both the external form and internal structure of fabricated parts. This freedom presents greater opportunities in optimising a parts mechanical properties, (such as strength, stiffness and mass), which in turn leads to enhanced performance whilst potentially reducing material use and hence, environmental impact. Realising this potential will further increase the viability of AM for a greater range of engineering contexts. Correspondingly, the contribution of this paper lies in the creation and validation of a method for the topological optimisation of the infill structure of fused deposition modelled (FDM) components. The proposed method uses results attained from finite element analysis (FEA) to influence the design of the internal structure (i.e. infill) by locally varying the composition of the infill based upon the associated stress values. This paper presents and discusses the proposed method, and demonstrates the generalisability of the method through its ability to handle complex geometries and loading conditions, and manufacturing process constraints. In addition, the paper validates the method through testing of FDM beams comprised of FEA influenced and standard honeycomb infill designs undergoing four different loading scenarios. The validation reveals that a three and a half times increase in strength can be achieved where the stress profiles are well defined within the structure. In addition, the FEA-influenced beams exhibited more consistent failure mode profiles, which maybe desirable for designing parts with specific failure mode characteristics.

08.30: Experimental investigation and specific behaviour of X‐HVB shear connectors in prefabricated composite decks

ABSTRACTConstruction industry is constantly facing new demands towards fast construction and smallest possibly quantity of work at the construction site. These increasing demands, in the field of composite construction of buildings and bridges, resulted in development of different types of prefabricated composite decks during decades and group arrangement of different types of shear connectors. Positioning of shear connectors into envisaged openings of prefabricated composite decks contributes to smaller quantity of concrete needed to infill openings of prefabricated composite decks. Composite action between steel profile and concrete decks can be achieved using X‐HVB shear connectors with powder‐actuated fasteners. Anchorage mechanisms developed between steel profile and fasteners during installation process of powder‐actuated fasteners are resulting in composite action between steel and concrete. Relatively simple and fast installation process, without requirements for additional equipment and electrical power on the site is characteristic for this type of shear connectors. Experimental investigation of X‐HVB shear connectors in prefabricated composite decks through standard push‐out tests, according to EN 1994‐1‐1 is presented in this paper. Shear connectors are positioned at the envisaged openings of prefabricated composite decks at the minimal longitudinal and transversal distances, recommended by the manufacturer. Two different orientations of shear connectors, forward and backwards to shear force direction are experimentally investigated and presented in this paper. Experimentally obtained characteristic shear resistance are compared with recommendations given by manufacturer. Characteristic slip at failure obtained from experimental investigation is compared with requirements given in EN 1994‐1‐1 in order to determine if this type of shear connector is considered as ductile. Eight specimens in two series are experimentally investigated through standard push‐out test. Different characteristic shear resistance and different failure modes are obtained for forward and backward orientation of shear connectors. Experimentally obtained results from standard push‐out tests for different orientations of shear connectors are compared with results obtained from extensive numerical analysis performed in Abaqus.

Characterization of Hybrid Joining Techniques for FRP/Steel-Structures under Combined Mechanical and Thermal Loading

In order to optimize the design of vibrating screening machines and realize significant weight reductions, the use of hybrid structures is gaining importance. In this context, the joining of FRP and steel and their interactions due to different material properties were investigated. Therefore, quasi-static tests with combined mechanical and thermal loads were carried out. To realize the simultaneous application of physical measurement techniques, e.g. optical and acoustic measurements, and thermal loads, short-wave infrared emitter technique was used instead of thermal chambers. Thus, the mechanical characteristics and acoustic emissions could be determined and assessed. The results show different structural mechanisms of hybrid joining at room and elevated temperatures. The characteristics of failure modes, shear stresses, strains and acoustic emissions could be correlated to determine the damage developments and mechanisms.

Seismic vulnerability enhancement of medieval and masonry bell towers externally prestressed with unbonded smart tendons

Medieval and masonry bell towers are highly vulnerable to suffer strong earthquake damage due to the mechanical and physical characteristics of masonry and other important factors. An approach for the seismic vulnerability reduction of masonry towers with external prestressing is proposed. The devices are vertically and externally located in order to be removable when needed. The characteristic flexural failure mode of medieval towers and the shear mechanism of bell towers are simulated. Both failure modes are in agreement with earthquake damage in similar towers. Medium prestressing level enhances force capacity of towers failing by bending without reducing ductility. High prestressing level slightly reduces the displacement capability of towers failing ductile. In case of belfry failure, both prestressing levels permit to increase displacement but lower force than towers failing by bending. The proposed medium prestressing level is the optimal for masonry towers and other slender structures failing by bending and shear.

핫스탬핑강의 너트 프로젝션 용접시 너트 재질이 용접부 파단모드 변화에 미치는 영향

Abstract The use of materials for modern lightweight auto-bodies is becoming more complex than hitherto assemblies.The high strength materials nowadays frequently used for more specific fields such as the front and rear sub frames, seat belts and seats are mounted to the assembled body structure using bolt joints. It is desirable to use nuts attached to the assembled sheets by projection welding to decrease the number of loose parts which improves the quality. In this study, nut projection welding was carried out between a nut of both boron steel and carbon steel and ultra-high strength hot-stamped steel sheets. Then, the joints were characterized by optical and scanning electron microscope. The mechanical properties of the joints were evaluated by microhardness measurements and pullout tests. An indigenously designed sample fixture set-up was used for the pull-out tests to induce a tensile load in the weld. The fractography analysis revealed the dominant interfacial fracture between boron steel nut weld which is related to the shrinkage cavity and small size fusion zone. A non-interfacial fracture was observed in carbon steel nut weld, the lower hardnessof HAZ caused the initiation of failure and allowed the pull-out failure which have higher in tensile strengthsand superior weldability. Hence, the fracture load and failure mode characteristics can be considered as an indication of the weldability of materials in nut projection welding.Key Words : Nut projection welding, Hot stamped steel, Carbon steel nut, Boron steel nut, Failure mode, Shrinkage cavityISSN 2466-2232Online ISSN 2466-2100

MODELS FOR INTRALAMINAR DAMAGE AND FAILURE OF FIBER COMPOSITES - A REVIEW

In order to fully exploit the potential of structures made from fiber composites, designers need to know how damage occurs and develops and under what conditions the structure finally fails. Anisotropy and inhomogeneity cause a rather complex process of damage development which may be one reason for an exceptionally large number of existing models. This paper intends to provide an overview over those models and give some hints about current developments. As such it is an updated version of a recent publication [1]. The survey is limited to laminates from unidirectional layers out of straight continuous fiber polymer composites under quasi-static loading. Furthermore, focus is laid on intralaminar damage.Many failure models smear out the inhomogeneity between fibers and the matrix. Simply limiting each stress component separately can lead to surprisingly good results as documented in the first World-Wide Failure Exercise. Interpolation criteria consider mutual influence of normal and shear stresses, predominantly through a quadratic failure condition. Traditionally one distinguishes between interpolation criteria and physically based ones. As an important physical effect the difference between fiber failure and inter-fiber failure is considered. Furthermore, stress invariants are taken as a basis, increased shear strength under compression is accounted for, and characteristic failure modes are captured. Fibers and the matrix material are characterized by a large disparity in stiffness and strength. Micromechanical models consider this inhomogeneity but suffer from the difficulty to determine relevant material properties. Compressive strength in fiber direction has attracted special attention. However, the role of kink band formation, which is observed in the failure process, seems to be not yet fully understood.In summary it must be concluded that despite the tremendous effort which has been put into the model development the damage and failure simulation of fiber composites are not in a fully satisfying state. That is partly due to lack of accurate and reliable test results.

Strength criterion of plain recycled aggregate concrete under biaxial compression

This paper presents results of biaxial compressive tests and strength criterion on two replacement percentages of recycled coarse aggregate (RPRCA) by mass for plain structural recycled aggregate concrete (RAC) at all kinds of stress ratios. The failure mode characteristic of specimens and the direction of the cracks were observed and described. The two principally static strengths in the corresponding stress state were measured. The influence of the stress ratios on the biaxial strengths of RAC was also analyzed. The experimental results showed that the ratios of the biaxial compressive strength 3f to the corresponding uniaxial compressive strength for the two RAC are higher than that of the conventional concrete (CC), and dependent on the replacement percentages of recycled coarse aggregate, stress states and stress ratios; however, the differences of tensile-compressive ratios for the two RAC and CC are smaller. On this basis, a new failure criterion with the stress ratios is proposed for plain RAC under biaxial compressive stress states. It provides the experimental and theoretical foundations for strength analysis of RAC structures subject to complex loads.