Ductile Failure Mechanisms Research Articles

Nonlinear dynamic study on existing masonry-infilled concrete frames retrofitted with timber panels

This study investigates the dynamic performance of a timber-based seismic retrofit for existing reinforced concrete (RC) buildings, named RC-TP. The core of the proposed retrofit intervention is the replacement of the existing masonry infills with structural timber panels that are connected to the concrete elements using metal dowel-type fasteners and a timber subframe. The unreinforced frames were designed according to codes in force in the 60 s and 70 s, considering exclusively gravity loads to simulate a condition common to a large part of the built heritage across Europe. The seismic performance of single-storey single-bay frames before and after the application of the RC-TP retrofit was assessed via nonlinear simulations, for which the incremental dynamic analysis method was adopted. The results of the extensive dynamic study show the effectiveness of the RC-TP retrofit in increasing the seismic acceleration resisted by the frames and their base shear capacity and in promoting a more ductile failure mechanism. Some general considerations helpful in guiding the implementation of the retrofit system were also drawn.

The effect of Al2O3 content on the microstructure and properties of Al/Al2O3 functionally graded material

This study aims to survey effect of Al2O3 content on the microstructure and also the physical and mechanical properties of Al/Al2O3 which functionally graded material (FGM) that are manufactured by the powder metallurgy technique. For this purpose, five-layer Al/Al2O3 green part samples with Al:Al2O3 different weight ratios were fabricated by cold pressed under 620 MPa. After compaction, the samples were sintered for 1 h in a nitrogen atmosphere at a temperature range between 600 °C and 640 °C. An optical microscope and a field emission scanning electron microscope (FESEM) were used to evaluate the Microstructural characterization. The Archimedes density, Vickers hardness and flexural strength of sintered samples were measured. The highest relative density of 95 % was achieved at a sintering temperature of 640 °C on lower Al:Al2O3 weight ration variation on Al/Al2O3 FGM sample. The hardness values gradually changed in cross section of different FGM samples from 31 HV to 65 HV which depend on the sequence of Al2O3 content in different layers. However, the highest flexural bending strength of 70 MPa was obtained at the sample on Al/Al2O3 FGM included the compositions of 35, 25, 15, 5, 0 wt% Al2O3. By decreasing Al2O3 content along Al/Al2O3 FGM, the changing semi-brittle or brittle to ductile failure mechanism was observed. In some fabricated FGM samples observed a few cracks in the interfacial region between layers with high Al2O3 content.

Retrofitting of Damaged Crumb Rubber Concrete Frame Using Fiber Reinforced Polymers

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

Root cause analysis of flywheel gear failure in a marine diesel engine

Engine gear failures are of significant concern across a wide range of industries. It is important to investigate each gear failure in light of its unique loading and operating conditions for root cause analysis (RCA) and further corrective actions to avoid such failures in the future. The aim of this study was to investigate the premature failure of a flywheel gear of a marine diesel engine and establish the root cause and damage mechanisms using experimental and numerical analysis techniques. A bottom-up creative approach was adopted to assess the loading conditions on the failed gear by first estimating the bending deformation of the failed securing bolt, assuming overload torque on the flywheel caused an equivalent bending moment permanently bending the bolt. Von Mises equivalent stress acting on the gear was numerically estimated as σVM = 737 MPa, which was found significantly more than the specified yield strength of gear governing material AISI 1055 (σY = 550 MPa). SEM fractography revealed a quasi-cleavage failure mechanism in the ‘case’ region near the gear root and a ductile failure mechanism inside the ‘core’ region of the flywheel gear. A crack was found to initiate from the gear root consisting of high-stress concentration under overload stresses. RCA established that the flywheel gears had failed by overload stresses, caused due to overload torque generated from sudden inertial thrust in the internal combustion (IC) engine due to excess fuel throttle opening.

Experimental test setup design for ultimate limit state evaluation of ductile wrapped composite KK‐joints

AbstractComplex circular hollow section double KK‐joints are widely used in, e.g. offshore wind turbine jackets. As an alternative to conventional welded joints of the CHS multi‐membered structures, wrapped composite joints have shown superior structural performance in previous pilot tests. Experimentally characterizing the joint behaviour under uniplanar loading can be complicated, because of the ductile failure mechanisms of wrapped composite joints and second‐order bending moments. This paper presents the design of a test frame in combination with a conventional tensile 2.5 MN test rig. Ductile joint failure, joint imperfections and second‐order effects are considered in the design. Structural finite element software SOFiSTiK is used to analyse the effects of wrapped joint behaviour and imperfections on the test system and interaction of joint, frame and loading rig behaviours. Non‐linear joint behaviour and misalignments of the braces increased the second‐order effects significantly. Mitigation strategies have been explored to reduce additional bending moments in the KK‐joint. A complete test setup is proposed to perform static experiments on double KK‐joints in uniplanar axial loading conditions.

A Horizontal Stiffener Detailing for Shear Links at the Link-to-Column Connection in Eccentrically Braced Frames

This study investigates the performance and viability of a horizontal stiffener detailing (HSD) to (1) be used at the link-to-column connection in a D-braced eccentrically braced frame layout where conventional link-to-column connections show poor performance, (2) minimize the overstrength factor of shear links caused by the hardening of link flanges, and (3) mitigate an undesirable failure mechanism previously observed in shear links fabricated using ASTM A992 steel. HSD and conventional stiffener detailing (CSD) were tested using W460×60 sections under two loading protocols, including one simulating near-collapse excitation. While the flanges of CSD shear links are greatly stiffened by vertical stiffeners, webs of HSD shear links exhibit inelastic buckling at large rotation angles. This allows the flanges to freely deform, thereby reducing the likelihood of brittle fracture at the flange connections, as well as undesirable link overstrength induced by the flange contribution. This study shows that the horizontal stiffener configuration developed a ductile failure mechanism resulting from gradually increasing local buckling in the web. The strength degradation of links with horizontal stiffener detailing was much more gradual due to the ever-increasing web buckling, unlike the sudden brittle fracture at the flange-to-column face of links with CSD. This research shows that the HSD is a viable and economic alternative for shear links, which has the potential to decrease welding in the shear link while exhibiting adequate seismic performance. HSD also maintains a low overstrength factor and utilizes a simplistic design approach. Finally, the bolted extended end-plate connection and its details at the link ends used in the test experiment provided a viable solution for replaceable links.

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

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

Shear behavior of externally prestressed ultra-high-performance concrete (UHPC) T-beams without stirrups

Externally prestressed ultra-high-performance concrete (UHPC) beams without stirrups (EPUBs-WS) that simplify the reinforcement design and construction procedure are becoming a competitive option in bridge engineering. To investigate the shear behavior of EPUBs-WS, five specimens were designed and tested considering several critical parameters, such as the shear span-to-depth ratio (λ), shear stirrups, and longitudinal reinforcement ratio. The EPUBs-WS exhibits diagonal tension failure as the fibers pull out and the specimen shears off into two parts along the critical diagonal crack. However, the beam with λ = 3.67 exhibited flexural failure due to the large λ. The stiffness, shear-cracking strength, and shear strength of EPUBs-WS decrease as λ increases. Stirrups enhance the shear resistance of EPUBs but result in a more brittle shear failure mode. A higher reinforcement ratio augments the dowel action in EPUBs-WS, which contributes to a ductile shear failure mechanism. The web-shear cracking force accounts for 46% to 87% of the shear capacity, implying that the shear-cracking strength is a significant factor in the design of EPUBs-WS. An equation for predicting the shear-cracking strength of prestressed UHPC beams is proposed and validated using a database of experimental results reported in the literature. A parametric analysis is performed using 40 available specimens to investigate the effects of compressive strength, λ, reinforcement ratio, fiber reinforcing index, and prestressing level on the shear strength of prestressed UHPC beams without stirrups. By considering the key parameters, equations for estimating the shear strength are proposed and exhibit good accuracy.

Anisotropy in microstructural features and tensile performance of laser powder bed fusion NiTi alloys

From the viewpoint of the poor formability and mechanical anisotropy of laser powder bed fusion (LPBF)-processed NiTi alloys, three different energy densities, i.e., minimum value, median value, and maximum value, were adopted in this work. The anisotropic behavior in the microstructural distribution and tensile properties were explored to evaluate the performance stability of NiTi components within a wide processing window. The results showed that an excellent deposition quality was acquired when the energy density was within the range of 45–75 J/mm3. The phase constitution and microstructural features were similar even under different processing parameters. In a single melting pool, the columnar grains within the body zone exhibited {110}<001> and {100}<011> textures. However, no preferred orientation was observed for the fine dot-like sub-structures within the overlapping zone. The anisotropy in tensile strength and plasticity was mainly attributed to the columnar grains with preferred crystallographic orientation in the body zone. Only a dimple-like morphology was observed on the fracture surface, indicating a ductile tensile failure mechanism. The findings demonstrated that the stable microstructural features and tensile performance of LPBF-processed NiTi components were ensured within a wide processing window (45–75 J/mm3), despite the anisotropic behavior along the transverse and longitudinal tensile directions.

Influence of material anisotropy on void coalescence by necking for face-centered cubic single crystals

This paper presents the coalescence of microvoids embedded in an anisotropic copper single crystal using a micromechanics approach. Crystal plasticity framework was used to account for the anisotropy arising from the orientation and slip. A full 3D representative volume element (RVE) with void was considered to circumvent spurious loading. The constant load path parameters were enforced on the RVE using multi-point constraints. Various loading conditions that lead to necking were studied. The key finding from the present study is the interplay of the load path parameters (triaxiality and Lode parameter), material anisotropy, and initial void volume fraction on the void coalescence. It is noticed that at high triaxiality, the ductile failure mechanism is dominated by the necking mechanism, and at medium to low triaxiality, the ductile failure mode is a combination of shearing and necking mechanisms. It was observed that non-homogenous crystallographic slip manifests the material anisotropic effects in void cell RVE across various crystallographic orientations. The crystal orientation [110] exhibited higher shearing than the orientation [100] and [111]. Further, material anisotropy significantly affected void morphology but not the void coalescence strains at high triaxial values.

Debonding model for nonlinear Fe-SMA strips bonded with nonlinear adhesives

The application of adhesively-bonded joints for strengthening of structures using iron-based shape memory alloys (Fe-SMAs) has recently emerged in construction. Fe-SMAs and the majority of structural adhesives exhibit a pronounced nonlinear material behavior, which may result in a favorable ductile failure mechanism. The development, however, of a mechanical model to predict the structural behavior of the joint is non-trivial due to the presence of nonlinearity in the adherent and adhesive. This study aims to propose a semi-analytical and semi-numerical model for describing the mechanical behavior of Fe-SMA-to-steel adhesively bonded joints. The developed model serves three main functions: (i) estimating the bond capacity for a given interfacial fracture energy, and vice versa; (ii) processing the bond–slip (τ−s) behavior directly from the load–displacement (F−Δ) curve, and vice versa; and (iii) delivering a numerical method to simulate the full-range mechanical behavior of the bonded joints, namely the behavior at different loading stages. The model is validated using the experimental testing of 26 Fe-SMA-to-steel lap-shear joints, as well as 24 further bonded joints subject to shear with different adherents (e.g., stainless steel strips and Nickel–Titanium SMA wires) and base materials (e.g., concrete and composite polymer). An experimental data processing protocol, on the basis of the experimentally measured force–displacement (F−Δ) behavior and the distributed displacement along the bond line (s−x) via the Digital Image Correlation (DIC) technique, is further proposed to assess the full-range behavior of bonded joints.

Analysis of ductile damage evolution and failure mechanisms due to reverse loading conditions for the aluminum alloy EN‐AW 6082‐T6

AbstractThis paper deals with experiments and numerical simulations for reverse uniaxial tension‐compression and shear tests to investigate the ductile damage and fracture behavior of the aluminum alloy EN‐AW 6082‐T6. For this purpose, experiments with different loading sequences and number of loading cycles with uniaxial tensile and new shear specimens have been performed. Furthermore, the digital image correlation (DIC) technique captures the deformations and strain fields during the experimental processes. A modified anisotropic stress‐state‐dependent elastic‐plastic‐damage continuum model is proposed and is validated by experimental strain fields measured by DIC. Moreover, numerically predicted distributions of the principal damage strains are compared with fracture images taken by scanning electron microscopy (SEM).

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

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

Seismic reliability analysis of a r.c. arch bridge

Seismic reliability analysis (SRA) is a powerful tool able to combine seismic hazard and structural fragility for computing the probability of failure of a specific damage state of interest. In this work, a SRA has been applied to a specific case study of an open spandrel RC arch bridge located in North-eastern Italy, adopting as suitable engineering demand parameters for both the ductile and fragile failure mechanism of the columns between the RC arches and the RC beams grillage. Collapse seismic fragilities are computed for each structural element and, finally, the bridge seismic reliability is computed in a system reliability-based format.

Brittle failure of laterally loaded self-tapping screw connections for cross-laminated timber structures

The performance of structural timber connections is of utmost importance since they control the global response of the building. A ductile failure mechanism on the global scale is desirable, especially in the design of structures in seismic areas, where dissipative components in which ductile failure modes need to be ensured are considered. Therefore, the knowledge of possible brittle failure modes of connections is crucial. The paper investigates the brittle failures of laterally loaded dowel-type connections in cross-laminated timber subjected to tensile load in a lap joint configuration through experimental investigations and analytical estimations. A set of 13 different test series has been performed with fully threaded self-tapping screws of 8 mm diameter and different lengths (40 to 100 mm) in cross-laminated timber composed of 3 or 5 layers (layer thickness range from 20 to 40 mm), giving rise to the activation of different brittle failure modes at different depths. Plug shear was among the most typically observed failure modes. A previously proposed model for the brittle capacity was applied to the tested connections at the characteristic level. As shown by the performed statistical analysis, the existing model is not reliable and mainly unconservative. A very low performance is observed (CCC = 0.299), but with a good correlation (c = 0.750) for the tests in the parallel direction. Further research work is required to improve the current model predictions and to gain a better understanding of the underlying resisting mechanisms.

A rate-dependent phase-field model for dynamic shear band formation in strength-like and toughness-like modes

The phase-field model has proven a promising tool to simulate crack propagation, and special attention has recently been paid to the prediction of crack nucleation. Dynamic shear banding (or the so-called adiabatic shear banding) is a significant ductile failure mechanism in metals and alloys under impact loading, and its nucleation has been considered either strength-like or toughness-like in the literature. In this work, a rate-dependent phase-field model incorporating stored-energy-based shear banding criteria is proposed to simulate dynamic shear band formation within a thermodynamically consistent framework, emphasizing the capability to capture shear band nucleation in all possible modes, be it strength-like, large-scale yielding, or small-scale yielding. The model is first applied on dynamic shear banding under coupled damage and thermal softening, where the phase-field length scale is demonstrated to play an important role in energy dissipation when either the rate- or temperature-dependence during shear band formation is non-negligible, which is significantly different from its interpretation in quasi-static conditions. Then the capability of the proposed model in capturing shear band nucleation is thoroughly investigated by simulating both designed numerical tests and reported physical experiments. Strain-based and stress-based criteria for shear band nucleation can be well retrieved with the energy-based phase-field model. Validation against shear banding experiments of all possible modes over a wide range of specimen geometries, material properties, and loading rates is performed to demonstrate the performance of the model and shed light on a unified understanding of shear band formation in various scenarios.

Evaluation of the mechanical performance and sustainability of rubberized concrete interlocking masonry prism

Waste materials may be used as raw materials for interlocking masonry products in order to contribute to sustainable development and environmental protection. Rubberized concrete Interlocking Brick (RCIB) was developed by volumetric replacement of 56% of the ordinary Portland cement with fly ash and 20% of the sand with crumb rubber (CR) to reduce the production cost of conventional concrete bricks (CCB) and restrict the depletion of natural resources and contributing to solving the environmental problems associated with the accumulation of scrap tires in landfills. The mechanical and sustainability evaluation of masonry prism made of the developed brick is the aim of this research. Consequently, compressive strength, failure mechanism, stress–strain behaviour, and energy absorption of grouted and ungrouted prisms made of RCIB were measured experimentally under axial compression load. The thermal resistance, fuel consumption, CO2 emission, and cost analysis of RCIBs were estimated. The findings reveal that grout had a significant impact on the compressive strength of rubberized concrete interlocking masonry prisms where the compressive strength of grouted and ungrouted prisms was 10.99 MPa and 5.83 MPa, respectively. Web splitting and vertical cracks were the common failure modes observed in both prisms. Moreover, the rubberized concrete interlocking masonry prisms revealed greater energy absorption as well as a gradual and ductile failure mechanism. The RCIB exhibited higher thermal resistance than CCB (increased from 0.106 to 0.171 m2 K/W) which could contribute to a 62% reduction in annual fuel consumption and CO2 emission. Further, more than 25% of the material cost could be saved.

Assessment of Strength Reduction Factor on Concrete Moment Frames According to the New Venezuelan Seismic Code

Nonlinear static analysis is a validated tool for the seismic evaluation of existing and new structures, specifically for reinforced concrete buildings. In order to assess the performance of reinforced concrete frames designed according to the new Venezuelan seismic code, configurations of low-, medium-, and high-rise concrete buildings are subjected to 20 different load patterns considering the nonlinear behavior according to FEMA P695. A total of 140 concrete frame models were analyzed using modal response spectrum analysis and nonlinear static pushover analysis. The parameters considered for analyzing the models were the response reduction factor (R), the overstrength factor (RΩ), and the ductility factor (Rµ). The results showed a performance controlled by ductile failure mechanisms in low-rise models unlike combined failure mechanisms with columns with plastic hinge in high-rise models. Reduction factor values between 4 and 14 were obtained. In addition, the pushover curves were affected by the load patterns; therefore, it was necessary to identify the representative patterns, refusing the rest of the patterns. A statistical adjustment was performed using a log-normal distribution. The strength reduction factor specified in the new Venezuelan code was higher than the values obtained for the 95% confidence levels according to the distribution assumed in the reinforced concrete frames models. Finally, the strength reduction factor more representative is R = 4.

Factors influencing the extent of hydrogen-enhanced brittle cracking in a Cu-strengthened HSLA steel during monotonic loading

Abstract The extent of brittle crack as a function of hydrogen charging conditions was studied for a HSLA steel using circumferentially notched cylindrical tensile samples. Two different notch depths were used. The effect of hydrogen could be well represented by an effective hydrogen potential which was defined using a representative hydrogen concentration and a diffusive time parameter, for relatively faster strain rates. The high triaxiality in deep-notched samples led to the initiation of ductile failure mechanisms overwhelming the brittle cracking process.

Mechanics of thin film coated Bulk Metallic Glass composites

Bulk Metallic Glasses (BMGs) exhibit superior mechanical properties such as high strength (≈2 GPa), high yield strain (≈2%) and high corrosion resistance. But, they lack ductility which prevents them from being used as structural materials. One of the methods explored by material scientists to improve their strain to failure is to coat BMGs with a thin layer of a ductile polycrystalline metal such as Copper. In this work, we numerically investigate the mechanical behavior of thin film coated BMGs through Finite Element (FE) Simulations. We simulate the effect of an extrinsic copper coating on the deformation and failure of monolithic BMG cylinders under uniaxial loading. We employ the Anand and Su (2005) constitutive model for BMGs implemented by us in ABAQUS FE software via user material subroutines: UMAT and VUMAT. An isotropic, elastoplastic material model is used for representing the Copper coating with properties of pure Copper. Three-dimensional, two-dimensional plane strain and axisymmetric simulations of uniaxial loading of monolithic BMG and Copper coated BMG composite at quasi-static strain-rate are performed using the Explicit/Dynamics formulation in ABAQUS. The simulations take into consideration a ductile damage and failure mechanism involving cracking inside the shear bands in the BMG matrix. We observe the formation of shear bands which transform into cracks in the BMG matrix. The simulations are able to correctly predict the deformation and fracture mechanism of the BMG matrix and copper coating under uniaxial loading. Results show that a thin copper coating of about 1–5% of the diametrical thickness of the cylinder increases the strain to failure of the coated BMG composite by about 2% which is in qualitative agreement with experimental observations reported in literature. The present results have implications for designing coated BMGs with improved malleability or strain to failure enabling their deployment in structural applications.