Limited Ductility Research Articles

Experimental and numerical analysis on the limited ductility of H-shaped concrete cable-stayed bridge tower

When a cable-stayed bridge is subjected to strong ground motions during an earthquake, the vibration of the tower column is much stronger and more severe in the transverse direction than the longitudinal direction, due to the lateral restriction between the tower and the main girder. To address this issue, the current seismic design of cable-stayed bridge recommends that a larger reinforcement ratio in tower column be used to achieve the target seismic performance (i.e., remaining elastic). This entails an increased number of the piles and the reinforcement volume as well and thus such a design is often not cost-effective. Alternatively, the tower column can be designed to have allowable ductile behaviour for saving costs. Therefore, this study aimed to verify the ductility margin and the allowable ductile performance of the H-shaped concrete tower, by studying its lateral elastic-plastic behaviour and damage mechanism through incremental dynamic analysis, and its limited ductility against the lateral inertial forces based on quasi-static test.

High strength-ductility synergy of MgAlB4 whisker reinforced aluminum matrix composites achieved by in situ synthesis

For ceramic reinforced metal matrix composites, it is a critical issue to improve the limited ductility to achieve a combination of high strength and high toughness. Herein, in-situ synthesized MgAlB4 whisker reinforced Al (MgAlB4w/Al) composites with rational distribution of the reinforcement were successfully prepared by an efficient process combining vacuum pressureless sintering and hot-extrusion. The results demonstrate that the MgAlB4 whiskers noticeably coarsen with increasing sintering temperature, while the sintering duration has little effect on the size of the whiskers. The as-extruded composite with 15 wt% MgAlB4w has excellent comprehensive properties with the ultimate tensile strength of 380 MPa and the fracture elongation of ~9.6%. Fracture analysis reveals that the whiskers bear most loads and fracture firstly to form crack sources with the load increasing during drawing. In addition, the alignment distribution of MgAlB4w along extrusion direction is the main reason for the improved ductility of the MgAlB4w/Al composite.

A mechanistic interpretation of the strength-ductility trade-off and synergy in lamellar microstructures

Among various mechanisms responsible for the strength-ductility trade-off in metallic materials, the leading strategy is to delay the onset of necking by improving the work hardening rate via a number of metallurgical approaches such as heterogeneous or gradient microstructures. Recent research activities on high-entropy alloys also witness a wide range of alloy design capabilities that permit these microstructural designs such as the dual-phase lamellar microstructures. This work addresses the contrasting strength-ductility behavior of equiaxed and lamellar microstructures when geometric features are the only tuning parameter. It is found that failures in lamellae are preceded by necking in the hard phase, the growth of which is significantly confined and suppressed by the surrounding soft phase. Detailed finite element simulations reveal various degrees of strength-ductility trade-off and synergy, which mainly depend on the microscopic processes that govern the delayed neck growth and ductile fracture. The upper limit of tensile ductility is theoretically believed to be determined by short-wavelength necking in the hard phase.

Three-dimensional nonlinear response history analyses for earthquake damage assessment: A reinforced concrete wall building case study

Nonlinear dynamic analysis techniques have made significant progress in the last 20 years, providing powerful tools for assessing structural damage and potential building collapse mechanisms. The fact that several reinforced concrete shear wall residential buildings underwent severe structural damage in walls at the lower building levels during the 2010 Maule earthquake (Chile) presents a scientific opportunity to assess the predictive quality of these techniques. The objective of this research is to compare building responses using two completely different three-dimensional nonlinear dynamic models and study in detail the observed damage pattern and wall collapse of one reinforced concrete shear wall building in Santiago, Chile. The first model is a mixed fiber-shell model developed in MATLAB, and the second is a shell finite element model developed in the software DIANA. Results of both models are consistent with the hypothesis that high axial loads trigger a limited ductility failure in critical walls at roof-to-base drift ratios less than 0.34% with little capacity of hysteretic energy dissipation, which contradicts the ductile design philosophy of current code provisions.

Prediction of forming limits for porous materials using void-size dependent model and bifurcation approach

The scientific literature has shown the strong effect of void size on material response. Several yield functions have been developed to incorporate the void size effects in ductile porous materials. Based on the interface stresses of the membrane around a spherical void, a Gurson-type yield function, which includes void size effects, is coupled with the bifurcation theory for the prediction of plastic strain localization. The constitutive equations as well as the bifurcation-based localization criterion are implemented into the finite element code ABAQUS/Standard within the framework of large plastic deformations. The resulting numerical tool is applied to the prediction of forming limit diagrams (FLDs) for an aluminum material. The effect of void size on the prediction of FLDs is investigated. It is shown that smaller void sizes lead to an increase in the ductility limits of the material. This effect on the FLDs becomes more significant for high initial porosity, due to the increase of void-matrix interface strength within the material.

Effect of Alkali Resistant Glass Fiber on Performance of Cement Concrete - A Review of Experimental Investigations

Cement concrete, a universally accepted construction material is enough strong in compression but weak in tension with limited ductility and therefore its resistance to cracking is low attributed to the inherent presence of micro-internal cracks. In order to improve its resistance to cracking, and post peak response, alkali-resistant glass fiber (ARGF) as an additive is one of the options. This is added in certain percentage during mixing and the concrete is generally called alkali-resistant glass fiber reinforced concrete (ARGFRC). This paper presents review of the experimental research works carried out on use of type with regards to aspect ratio and quantity of ARGF in cement concrete. Type and quantity both influence the properties of fresh and hardened ARGFRC. The dosage effect with quantity of the fiber on fresh concrete such as workability, and mechanical performance, seismic vulnerability evaluation, microstructural analysis, durability aspects on hardened concrete have been reviewed. It is found that alkali-resistant glass fiber together with silica fume can be utilized to enhance the toughness and load carrying capacity and improve the stiffness of the composite concrete.

Fracture envelopes on the 3D-DIC and hybrid inverse methods considering loading history

This paper deals with the comparative investigation of fracture envelopes constructed by the 3D-DIC and hybrid inverse analyses. For the evaluation of fracture limits of the DP980 1.2t steel over a wide range of loading conditions, tensile tests were conducted using three different specimens that are expected to induce certain stress states of plane strain tension, in-plane shear, and uniaxial tension at the potential location for the fracture initiation. The modified Mohr–Coulomb ductile fracture criterion was selected in this study to investigate the influence of the fracture envelope identification based on each way of loading history evaluation not only on the level of fracture limits but also on the fracture prediction especially for the simulation of square cup deep drawing. For an in-depth understanding of non-linear loading history on the variation of ductility limit, apparent fracture with respect to strain was numerically computed according to the change of loading path. Finally, a comparison of results from the square cup deep drawing test and the FEA prediction was performed in terms of punch force and stroke to confirm the model performance at various ways of the loading history evaluation.

Mechanical properties of proton bombarded SS316L thin foils using the small punch technique

The mechanical properties of thin foils (∼25 μm), to be used as a target’s window in a high intensity accelerator, require non-standard characterization techniques. In the current research, the innovative small punch test technique (SPT) had been used to map and determine the mechanical properties of SS 316L foil irradiated by high intensity of proton beams. The SPT results and the energy to fracture are presented and the fracture modes were determined by scanning electron microscopy (SEM) observations and electron backscatter diffraction (EBSD) analysis. The irradiated samples were exposed to 3.6 MeV proton bombardment at 250 μA and 290 μA, for 42 and 5 h, respectively. The major damage, as was reflected by significant ductility and energy to fracture losses, was associated with the irradiated zones which experienced the highest temperature and protons flux. Based on the observed evidence of deformation twins and dense dislocation bands, as well as the EBSD analysis, it was revealed that the limited deformation in the irradiated samples is related mainly to radiation damage and probably a minor effect of hydrogen embrittlement phenomenon. The limited ductility was explained by the accumulation of radiation damage that most probably hinder dislocations mobility through crystallographic glide. Nevertheless, these alternative deformation mechanisms involve nucleation and propagation of dislocation slip bands (DSBs) and intra-grain fragmentation. The DSBs intersections were proposed as the source for stress localization, which initiates crack formation which was followed by crack propagation through slip bands. This cracking mechanism was exhibited by a unique “saw tooth” fracture mode.

Qualification of the in-situ bending technique towards the evaluation of the hydrogen induced fracture mechanism of martensitic Fe–C steels

This paper proposes a new in-situ hydrogen (H) charging bending technique to investigate the susceptibility to hydrogen embrittlement (HE) of high strength steels with limited ductility. The methodology is tested with generic martensitic Fe–C steels with a carbon (C) content of 0.2 wt%, 0.4 wt% and 1.1 wt%, respectively. The in-situ bending technique is developed to evaluate the hydrogen susceptibility of these brittle materials and is compared to uncharged samples as a reference. Moreover, as a crucial step in the validation of the technique, a comparison with conventional in-situ tensile testing for the most ductile material (i.e. Fe-0.2C) is performed. The bending results show that charging with H causes a significant ductility loss, which is characterized by a transition from a microvoid (Fe-0.2C), intergranular (Fe-1.1C) or mixed (Fe-0.4C) fracture surface for the uncharged samples to a hydrogen induced cleavage fracture appearance with additional cracking. The transition to the cleavage fracture type is found to be caused by the Hydrogen Enhanced Plasticity Mediated Decohesion mechanism, indicating that hydrogen is preferentially trapped at packet or block boundaries in high carbon steels without alloying additions. The fracture surface of the Fe-0.2C alloy after in-situ tensile testing was very similar to the fracture surface obtained after in-situ bending testing, which indicates that the fracture mode during bending is mainly dominated by the tensile field. This supports the applicability of the in-situ bending technique for intrinsically brittle materials.

Structural Steel Tensile Fracture-Inception Prevention Limit State and Ductility Criteria

Abstract This paper proposes using the tensile-fracture-inception-prevention (TFIP) limit state and ductility parameter/criterion for a safer fracture-resistant design of the tension members of steel structures. Finite element (FE) analysis revealed that the tensile band localization strain (TBLS) is significantly lower than the fracture strain currently employed as the structural steel elongation-based ductility parameter/criterion. The TBLS is thus, a safer fracture-resistant parameter/criterion than the fracture strain. FE analysis reveals that the TBLS at the center of tension specimens (mid-thickness and mid-width) is lower, and is thus a safer TFIP elongation-based ductility criteria/parameter than the TBLS at the surface of tension specimens.

Opportunities and Issues in the Application of Titanium Alloys for Aerospace Components

The metal titanium (Ti) and its alloys have many attributes which are attractive as structural materials, but they also have one major disadvantage, high initial cost. Nevertheless, Ti and Ti alloys are used extensively in airframes, gas turbine engines (GTE), and rocket engines (RE). The high cost is a deterrent, particularly in airframe applications, in that the other alloys it competes with are, for the most part, significantly lower cost. This is less of a concern for GTE and RE where the cost of titanium is closer to and sometimes even lower than some of the materials it competes with for these applications. In spacecraft the weight savings are so important that cost is a lesser concern. Ti and its alloys consist of five families of alloys; α-Ti, near α-alloys, α + β alloys, β-alloys, and Ti-based intermetallic compounds. The intermetallic compounds of primary interest today are those based on the compound TiAl which, at this time, are only used for engine applications because of their higher temperature capability. These TiAl-based compounds are used in a relatively low, but growing, amounts. The first production application was for low pressure turbine blades in the GE engine (GEnx) used on the Boeing 787, followed by the GE LEAP engine used on A-320neo and B-737MAX. These air foils are investment cast and machined. The next application is for the GE90X which will power the Boeing B-777X. These air foils will be made by additive manufacturing (AM). Unalloyed titanium and titanium alloys are typically melted by vacuum arc melting and re-melted either once (2X VAR) or twice (3X VAR); however a new and very different melting method (cold hearth melting) has recently become favored, mainly for high performance applications such as rotors in aircraft engines. This process resulted in higher quality ingots with a significant reduction in melt-related defects. Once melted and cast into ingots, the alloys can be processed using all the standard thermomechanical working and casting processes used for making components of other types of structural alloys. Because of their limited ductility, the TiAl-based intermetallic compounds are quite difficult to process using ordinary wrought methods. Consequently, the low-pressure turbine blades currently in service are investment cast and machined to net shape. The AM air foils will require minimal machining, which is an advantage. This paper describes some relatively recent developments as well as some issues and opportunities associated with the production and use of Ti and its alloys in aerospace components. Included are new Ti alloys, new applications of Ti alloys, and the current status of several manufacturing processes including a discussion of the promise and current reality of additive manufacturing as a potentially revolutionary method of producing Ti alloy components.

Assessment of reduced beam section (RBS) moment connections subjected to cyclic loading

The disastrous results of the “1994 Northridge” and the “1995 Kobe” earthquakes have shown that there was a poor level of seismic response in moment resisting frame buildings. These buildings were found to have developed limited ductility level, leading to brittle collapse. Extensive scientific research took place studying new design methods for steel connections, providing the acceptable post-elastic behavior for the steel joints and subsequently the desirable seismic response for the post-Northridge/Kobe steel structures.Reduced Beam Section (RBS) moment resisting connections are among the most economical and practical rigid steel connections that were developed, aiming to satisfy the above mentioned target. Although the RBS connection effectiveness was widely investigated using US design and construction practices, research data from European research exists only in the last decade. Recommendations of RBS applications in steel frames were prescribed in EC8, Part3, however the reliability of these recommendations is under consideration.This paper presents the experimental results of two specimens, where the RBS technique was applied. The extended endplate moment beam to column connection (widely applicable in European practices) is used under cyclic loading, and Finite Element Analysis was conducted in order to provide a safe assessment for the RBS application in European steel profiles.

Damage micromechanisms in high Mn and Zn content 7XXX aluminum alloys

The nucleation, growth, and coalescence of microvoids are examined in high Zn and Mn content 7XXX aluminum alloys using high-resolution synchrotron X-ray microtomography. The results have clearly shown that the addition of Mn content (0.6% mass) increases the ultimate tensile strength with limited ductility. The loading step in which each microvoid was nucleated together with its nucleation site is determined by tracking the microvoids in reverse chronological order from the final loading step towards the initial stress-free loading step. It was observed that microvoids were initiated due to particle cracking, and void nucleation occurs continuously with the applied strain. Furthermore, it was also observed that the particle underwent multiple fractures. It was concluded the ductile fracture was dominated by the nucleation and growth of microvoids due to particle fracture.

Ductility limit diagrams for superplasticity and forging of high temperature polycrystalline materials

A mechanistic understanding of the ductility limit diagrams is of critical importance, but it still remains elusive for a multitude of high temperature materials processing techniques, such as superplastic forming and hot forging. The relevant failure modes for the former are necking at high strain rates and intergranular cavitation at low strain rates, while those for the latter include the competition between longitudinal fracture and shear band. The comparison between the Arrhenius processes for grain boundary diffusion and grain interior creep defines a length scale that dictates whether the grain boundary cavity growth is diffusive or creep-constrained. A quantitative assessment of these damage evolution processes leads to the delineation of the dominant parametric spaces for individual failure modes, and thus superplasticity and forging limit diagrams are derived and compared to available experiments in literature.

A molecular dynamics study of Fe50-XMXAl50 ternary alloy (M=Ag, Pt, Pd)

ABSTRACTIron aluminide intermetallic alloys are of great importance in many industries due to their excellent oxidation resistance, low cost, low density, resistance to corrosion and good ductility at room temperature. However, these alloys suffer limited room temperature ductility above 873 K. In this paper, a molecular dynamics-based LAMMPS-EAM was used to model Fe50-XMXAl doped systems with either Ag, Pt or Pd. The lattice side preferences of the dopant were deduced from their energy landscape, and Fe sub-lattices showed promising properties. It was found that the addition of Ag, Pt and Pd enhances the stability of Fe50-XMXAl composition. More importantly, Ag and Pd doped systems gave comparable transition temperatures to experimental findings of 1273 K and 1073 K, respectively. Their thermodynamic and the mechanical stability trends showed promising properties for industrial applications, displaying stability at a high temperature below 1300 K.

Seismic assessment of existing gravity load-designed RC framed building: a case study from Warangal, India

Most of the existing reinforced concrete (RC) buildings in India are either designed before the existence of seismic design codes or without proper provisions to resist lateral loads. These structures generally possess low strength and limited ductility, thereby always prone to global failure and are a primary reason for causalities during an earthquake. Hence, seismic evaluation of these structures and strengthening, if necessary, is essential to prevent a disaster during its serviceable lifetime. In this investigation, a four-storied (G+3) RC building structure in Warangal city, Telangana State, India, has been selected as a case study. Furthermore, the structure is to be extended vertically by two stories above the existing configuration. The structural and material inputs to the analytical model are updated from visual inspection and nondestructive tests, followed by an analytical study. From the analytical study, the seismic capacity of the existing gravity load-designed structure itself is found to be inadequate for seismic requirements. Since the structure is to be extended vertically, the existing building needs to be strengthened appropriately to withstand additional loads and remain functional throughout its lifetime. The outcome of the seismic analysis is checked for conformity using demand and capacity requirement of the structural components of the six-storied (G+5) structure. This behavior mimics the current scenario of existing RC buildings in this locality, which needs to be appropriately strengthened. It has been observed from the analysis that the RC building was vulnerable to the seismic forces and needed retrofitting to remain functional during its service life.

Influence of processing temperature on formability of thin-rolled DP590 steel sheet

ABSTRACT DP590 steel is used as an alternative material in automobile applications due to its high strength and considerably good other mechanical properties. However, major concern of DP590 steel forming is limited ductility and malleability. Warm forming is used as an effective alternative process to produce complex shape components from such high strength material. The present work is mainly focused on the investigation of processing temperatures (room temperature [RT], 200°C, and 400°C) on stretch forming behavior of DP590 thin-rolled sheet. The formability of the material increased by approximately 26% as the forming temperature increased from RT to 400°C. Further, six ductile fracture criteria were calibrated in order to predict the fracture locus of material in effective plastic strain versus triaxiality space. Oh’s model is found to have best predictability of fracture locus with least average absolute error at RT and 200°C while Cockcroft and Latham model best predicted at 400°C. Furthermore, fracture behavior of DP590 steel has been analyzed in the biaxial, plane strain, and uniaxial regions. The specimens in the biaxial region fractured without any prior hint of necking while predominantly ductile type of fracture has been observed for specimen lying in the uniaxial region.

Enhanced strain delocalization through formation of dispersive micro shear bands in laminated Ni

Nanocrystalline metals usually exhibit limited ductility due to catastrophic shear fracture caused by the preferential formation of a few premature shear bands during tensile deformation. In this work, the tensile and fracture behavior of laminated Ni with varied degree of difference in grain size of hard and soft layers were investigated. Compared with monolithic nanocrystalline Ni, laminated Ni with a large difference in the grain size of hard and soft layers exhibited enhanced elongation to failure without sacrificing tensile strength, because the formation of dispersive micro shear bands could undertake large plastic strain and lead to strain delocalization. With decreasing the difference in the grain size of hard and soft layers and the corresponding resistance to propagation of micro shear bands, there is a transition of fracture mode from ductile necking fracture accompanied with dispersive micro shear bands to shear fracture dominated by a few macroscopic premature shear bands. A mechanical model based on the transition from grain interior dislocation emission to grain-boundary-mediated deformation within the soft layers was proposed and the critical grain size of the soft layers corresponding to the optimum synergy of high strength and ductile necking fracture was obtained.

Evaluation of Seismic Performance of Improved Rocking Concentrically Braced-frames with Zipper Columns

Concentrically braced frames (CBFs) as one of well-known stiff and common lateral force resisting systems often show limited ductility capacity under severe earthquakes. This study proposes rocking zipper braced frame (RZBF) to improve the drift capacity of CBFs which is based on combination of rocking behavior and zipper columns. In the RZBF system, rocking behavior permit the braced frame to uplift during the earthquake and then restoring force induced through post-tensioned bars self-center the frame to its initial state. Also, zipper columns can decrease the concentration of damage by distributing the unbalance force at the mid bay over the frame’s height. To assess the performance of RZBF, a comparison study is carried out considering CBF, rocking concentrically braced frame, zipper braced frame and RZBF. For this purpose, some frames structures are designed and nonlinear time history analysis conduct under a set of earthquake records. Seismic responses such as roof drift ratio, gap opening at the column-base interface, forces of top story braces and post-tensioned bars are taken into consideration. The results show that the proposed RZBF has better performance among the others and zipper columns can improve the behavior of rocking systems.

Ductility of an ultrastrong glass-crystal nano-dual-phase alloy in sub-micron

We report an excellent tensile property of the Mg-based nano-dual-phase glass-crystal alloy in sub-micro size. It shows room temperature large ductility (>50% true strain, resulted from the localized necking deformation) with ultrahigh tensile strength (2.4 GPa), compared with 1.4 GPa strength and limited ductility of its metallic glass counterpart. To unraveling the underlying reasons, we have made transmission electron microscope (TEM) in-situ tensile investigation and molecular dynamic (MD) simulation for the materials, showing that homogenous plastic flow of nano-sized metallic glass phase activated by strain non-localization mechanism is responsible for the large ductility.