Apparent Ductility Research Articles

The defect sensitivity of brittle truss-based metamaterials

Architected strut-based metamaterials fabricated via three-dimensional printing exhibit a wide range of geometric and material heterogeneity, including variations in strut size, surface roughness, embedded micro-cracks and disconnected struts. The locations and severity of these defects are highly variable; combined with the complexity of the structure itself, it is exceedingly difficult to identify critical defects that limit performance and inform qualification protocols. To address this challenge, we consider the impact of distributions of defects for various strut-based lattice topologies. The role of defects is analyzed using Weibull distributions of strut failure strains. We relate the statistical distributions of strut properties to macroscopic stress-strain performance, using high through-put finite element predictions in thousands of virtual tests. Increasing the prevalence of defects decreases macroscopic strength; however, this has the complementary effect of introducing apparent ductility, i.e. load capacity even after the onset of strut failures. Hence, there is a trade-off between achieving high strength and gradual loss of stiffness that is desirable for thermal loading or lattice cores. Predictions of average strength as a function of Weibull modulus provide knock-down factors relative to the defect-free strength. In turn, these clearly identify quantitative processing targets to mitigate the impact of defects.

Toward accurate prediction of partial-penetration laser weld performance informed by three-dimensional characterization – Part II: μCT based finite element simulations

The mechanical behavior of partial-penetration laser welds exhibits significant variability in engineering quantities such as strength and apparent ductility. Understanding the root cause of this variability is important when using such welds in engineering designs. In Part II of this work, we develop finite element simulations with geometry derived from micro-computed tomography (μCT) scans of partial-penetration 304L stainless steel laser welds that were analyzed in Part I. We use these models to study the effects of the welds’ small-scale geometry, including porosity and weld depth variability, on the structural performance metrics of weld ductility and strength under quasi-static tensile loading. We show that this small-scale geometry is the primary cause of the observed variability for these mechanical response quantities. Additionally, we explore the sensitivity of model results to the conversion of the μCT data to discretized model geometry using different segmentation algorithms, and to the effect of small-scale geometry simplifications for pore shape and weld root texture. The modeling approach outlined and results of this work may be applicable to other material systems with small-scale geometric features and defects, such as additively manufactured materials.

Exploring the potential of the critical shear crack theory for reinforced and post-tensioned glass beams: Initial analysis and experiments

In the evolution of structural glass beam elements, the requirements for post-fracture load bearing capacity and safe failure behaviour have led to the development of reinforced and post-tensioned beams. Maximum bending capacity in the post-fracture state is normally associated with extensive yielding of the reinforcement, providing a safe failure mechanism through apparent ductility of the composite beam section. This can be achieved as long as the propagation of primary flexural cracks does not compromise the transfer of shear from the load points to the supports. Although shear failure is typically not critical for the ultimate limit state design of ’normal’ unreinforced glass beams, it may govern the load-bearing and deformation capacity in the post-fracture state for reinforced and post-tensioned glass beams. This paper presents exploratory experiments and initial analysis of the shear failure phenomenon in the post-fracture state of reinforced and post-tensioned glass beams. Potential shear transfer mechanisms are identified based on the critical shear crack theory developed for reinforced concrete members and applied in the analysis of shear failures observed in four-point bending tests of post-tensioned glass beams. The behaviour of fractured laminated glass under mixed-mode (tension+shear) loading is explored on a limited set of small-scale double-notched glass specimens, demonstrating the feasibility of the applied test methodology. Preliminary findings of the present study may serve as a basis for further investigations of shear resistance of glass beams. Typical shear failure kinematics and suitable constitutive laws of the applied materials need further investigation in order to provide design recommendations for the prediction of shear resistance of reinforced and post-tensioned glass beams.

On the dual-stage partial recrystallization and the corresponding mechanical response of the Cantor alloy

In this work, we systematically characterized the microstructural evolution of the Cantor alloy, FeCoNiCrMn, during recrystallization at a typical mid-temperature, 700 ℃, and carefully bridged up with the mechanical responses. Apart from the overall trade-off phenomenon, we evidenced the presence of a two-stage strength-ductility variation, where before 5 min the yield strength drops fast without apparent ductility compensation while in the following 5–20 min the ductility increases remarkably. The first stage covers the transition from cold-rolled non-recrystallized microstructure to a eutectic-like stacking of recrystallized regions and new non-recrystallized regions with a different texture. The second stage then includes the vanishing of the new non-recrystallized regions and the completing of recrystallization. This dual-stage transition is to our knowledge never reported before for HEAs, and we suppose the occurrence is associated with the initial inhomogeneity caused by cold-rolling, and thus the different pathways for such inhomogeneous microstructure to evolve, energetically, during heating.

Flexural behavior of carbon-textile-reinforced concrete I-section beams

In the present work, the flexural behavior of carbon textile reinforced concrete (TRC) I-beams is investigated. Four-point bending tests were performed in I-beams, considering SBR-laminated textile and the following conditions: (a) plain cementitious matrix; (b) plain matrix and sand-coated textile; and (c) strain hardening cement-based composite (SHCC) matrix. The main goal of the research was to correlate the improvements on interface and matrix properties with the crack pattern, failure mode and ductility. For a comprehensive discussion, full mechanical characterization was performed and a design model is proposed for a better evaluation of the experimental results. Cracking behavior was greatly improved for conditions (b) and (c) with respect to (a), due to enhanced bonding between reinforcement and matrix in the former and due to fiber bridging mechanism in the latter. Beams with plain matrix exhibited brittle failure, whereas the beam having SHCC matrix showed premature local failure near load application point, resulting in an apparent ductility due to concrete softening mechanism. The reduced cracking load and the premature flange detachment along web-flange junction for condition (a) are also discussed. The proposed model is also validated through comparison with experimental results available in the literature.

Dynamic response characteristics of dry and water-saturated schist under impact loading

Many geological engineering hazards are closely related to the dynamic mechanical properties of rock materials. However, most existing studies on the dynamic mechanical properties of rock materials were conducted on the hard rocks such as sandstone, granite, limestone, and marble, whereas soft rocks, such as schist, are less studied. Therefore, in this study, a series of triaxial impact tests were conducted on dry and saturated schist by employing a modified triaxial split Hopkinson pressure bar system to reveal the coupling effects of water, strain rate, and triaxial confining pressure on the mechanical properties of schist. The results show that schist is a type of watersensitive rock and the stress-strain curve of saturated schist has apparent ductility. The effects of strain rate on dynamic strain, deformation modulus and peak stress were analyzed. The results also show that the dynamic peak stress is affected by the combined softening effect and viscous effect of water under impact loading. Finally, it was found that the failure mode of schist belongs to typical axial tensile failure under uniaxial impact tests, and shear failure is the main failure mode under triaxial impact tests. With the increase in confining pressure, the failure modes of schist change from tensile failure to shear failure. This research can provide useful parameters for geological engineering hazard prevention in mountain areas.

A Comparative Study on Performance of Synthetic and Natural Fibers on Compressive and Flexural Strength of Concrete

Ordinary concrete - a stone like structure which is formed by the chemical reaction of the cement, aggregate and water and is a brittle material which is strong in compression but very weak in tension, which causes cracks under small loads. These cracks gradually propagate to the compression end of the member and finally, the member breaks. These increase in size and magnitude with time and finally fails. One of the successful reinforcing methods is providing steel reinforcement but even then, cracks in reinforced concrete members extend freely. Thus, need for multidirectional and closely spaced steel reinforcement arises. Fiber reinforcement gives the solution for this problem. So, to increase the tensile strength of ordinary concrete a technique of introduction of fibers in concrete is being used. These fibers act as crack arrestors and prevent the propagation of the cracks, improves the post cracking response of the concrete, i.e., to improve its energy absorption capacity and apparent ductility, and crack control. The Present study focuses upon, Synthetic (Polypropylene) Fiber Reinforcement (SFRC) of 1% and 3% and Natural (Jute) Fiber Reinforcement (NFRC) of 1% and 3% by weight and are compared with respect to their compressive strength and flexural strength. The present study concludes considering the practical issue of workability of fibers, that in between synthetic and natural fibers selected, 1% Polypropylene fibers can be added as a reinforcement to ordinary concrete to enhance both compressive strength by nearly 2 times at 28 days curing duration and flexural strength by 35%% at 28 days curing duration. History and Development

An analytical model for necking strains in stretched plates under dynamic biaxial loading

A model for the prediction of localised necking in plates subjected to dynamic biaxial loading is presented. The proposed model extends the classic two-zone localisation analysis introduced by Marciniak and Kuczyński (1967) to include the contribution of inertia. Several examples of computations are presented to illustrate the influence of inertia, material rate-dependence and yield surface shape on plate necking under dynamic biaxial loading. It is found that inertia may significantly delay the occurrence of localised necking and increase the apparent ductility of the material. The results of the proposed model are compared to those of three-dimensional finite element simulations and a very good agreement is observed.

Thermal effects on the enhanced ductility in non-monotonic uniaxial tension of DP780 steel sheet

To understand the material behavior during non-monotonic loading, uniaxial tension tests were conducted in three modes, namely, the monotonic loading, loading with periodic relaxation and periodic loading-unloadingreloading, at different strain rates (0.001/s to 0.01/s). In this study, the temperature gradient developing during each test and its contribution to increasing the apparent ductility of DP780 steel sheets were considered. In order to assess the influence of temperature, isothermal uniaxial tension tests were also performed at three temperatures (298 K, 313 K and 328 K (25 °C, 40 °C and 55 °C)). A digital image correlation system coupled with an infrared thermography was used in the experiments. The results show that the non-monotonic loading modes increased the apparent ductility of the specimens. It was observed that compared with the monotonic loading, the temperature gradient became more uniform when a non-monotonic loading was applied.

Fundamental properties and mechanical characteristics of high performance cement composite with steel fibres under high temperature

High-performance fibre-reinforced cementitious composite (HPFRCC) is characterized by multiple functioning composite materials comprising fibre and cement matrix. HPFRCC is able to withstand tensile stress forming distributed micro-cracks since embedded fibres in concrete are to improve energy-absorption capacity and apparent ductility. This characteristic can be applied to protect structures under an extreme state of loading, such as blast or impact. In order to determine a mix proportion, this study considered various variables in the mix. Following experimental results, this study suggests an optimum mix proportion of HPFRCC and investigated fundamental properties of the considered mixes. Flexural performance is one of the representing characteristics of the high-fracture energy-absorbing materials. This study investigated the flexural performance of HPFRCC, which used high-strength steel fibres with volume fraction 8%. Under ASTM 1609, load–deflection curves were obtained. The major test variables include silica fume replacement ratio 0 and 15% and exposure temperature, ambient and 400 °C. The flexural strength was similar to the compressive strength and the absorbed fracture energy was relatively greater than other typical HPFRCC. This may be a composite reaction of the strong bonding between fibre and matrix and high-compressive strength of the matrix itself.

Additive Manufacturing of SiCN Ceramic Matrix for SiC Fiber Composites by Flash Pyrolysis of Nanoscale Polymer Films

The potential of polymer‐derived ceramic matrices for silicon‐carbide fiber composites is demonstrated by additive layering of thin films on SiC fiber bundles. The thin liquid films, which naturally wet the fiber surfaces, are cross‐linked and pyrolyzed in‐situ into the silicon carbonitride ceramic in just a few seconds to yield defect‐free layers that are 10 to 100 nm thick. The infiltration is completed by repeating the cycles. A nearly fully dense and defect free SiCN matrix could be obtained. Room‐temperature tensile tests show a tensile strength of ~1200 MPa. Good matrix‐fiber interface behavior is seen with pull‐out character which is most likely responsible for the apparent ductility. The SiC fibers were uncoated.

Flexural Behavior of High-Volume Steel Fiber Cementitious Composite Externally Reinforced with Basalt FRP Sheet

High-performance fiber-reinforced cementitious composites (HPFRCCs) are characterized by unique tensile strain hardening and multiple microcracking behaviors. The HPFRCC, which demonstrates remarkable properties such as strength, ductility, toughness, durability, stiffness, and thermal resistance, is a class of fiber cement composite with fine aggregates. It can withstand tensile stresses by forming distributed microcracks owing to the embedded fibers in the concrete, which improve the energy absorption capacity and apparent ductility. This high energy absorbing capacity can be enhanced further by an external stiff fiber-reinforced polymer (FRP). Basalt fabric is externally bonded as a sheet on concrete materials to enhance the durability and resistance to fire and other environmental attacks. This study investigates the flexural performance of an HPFRCC that is externally reinforced with multiple layers of basalt FRP. The HPFRCC considered in the study contains steel fibers at a volume fraction of 8%.

Numerical study of the deformation and fracture behavior of porous Ti6Al4V alloy under static and dynamic loading

In a previous work, the dynamic compression behavior of Ti6Al4V with 0, 10 and 20% porosities, at the strain rate of 1 × 10−3, 1 × 103, 4 × 103 and 8 × 103/s, was experimentally characterized. The objective of this study is to use numerical simulations to gain insights on the observed material behavior and a general understanding of the dynamic responses of porous metals. The study indicates that the pore in general could serve as both a failure initiator and an inhibitor. The pore would lead to lower strength due to higher stress concentration as a result of the reduction of load bearing area and geometric discontinuity. On the other hand, the pore could also result in lower matrix stress at the onset of failure due to the reduced overall load bearing capacity. This lower stress and associated lower potential energy could lead to slower failure propagation and higher apparent ductility. The study also shows that the pore distribution and pore shape play significant role on the deformation and fracture behavior of porous material. For a fixed porosity, the more densely populated small pores could lead to faster failure propagation due to the enhancement of the failure propagation through void coalescence. However, the distribution does not seem to affect much the stress-stain response prior to the onset of failure. The effects of pore shape are similar to that porosity. Different pore shapes lead to different degrees of stress concentration and the apparent strengths. However, the higher stress concentration could also result in slower failure propagation due to the lower matrix stress and the associated potential energy at the onset of catastrophic failure. Besides pore density and morphology, matrix properties also played important roles on the response of porous metals. Although higher strength enhanced material's resistance to deformation, it could also lead to faster failure propagation once it was initiated due to the higher matrix stress. On the other hand, higher hardening rate could provide increasing resistance to failure propagation.

Towards electroformed nanostructured aluminum alloys with high strength and ductility

Abstract

Grain-level analysis of dynamic fragmentation of ceramics under multi-axial compression

Fragmentation of a small ceramic specimen subjected to a dynamic compressive load is analyzed numerically at the mesoscale level using a cohesive/volumetric finite element scheme. Special emphasis is placed on the effect of lateral confinement on the dynamic fragmentation process. The analysis is performed in plane strain condition and assumes intergranular crack motion. The scheme relies on Voronoi tessellation to generate the granular microstructure. A nonlinear kinematic description is used to account for the possible large deformations and/or rotations of the grains during the fracture event. A cohesive unilateral contact enforcement scheme is used to model the complex interaction between the fracture surfaces and the fragments formed. The brittle to ductile transition in the constitutive response along with the associated change in the mode of failure of this class of materials from axial splitting to shear localization are captured through detailed simulations. It is quantified together with the effect of the applied strain rate and granular microstructure on the dynamic fragmentation process. Results are in good agreement with the analytical wing crack model and experimental observations of dynamic fragmentation for this class of materials. The apparent ductility exhibited by confined ceramics is found to be associated with the suppression of lateral dilation rather than the onset of shear localization.

A proposal for an arch footbridge in Venice made of structural glass masonry

Glass presents high compression strength and excellent durability, but its intrinsic brittleness renders its use in structural applications quite problematic. In order to increase its apparent ductility, a system is conceived of, in which “bricks” made of float glass, either laminated or not, are connect by “mortar” joints of epoxy resins. Uniaxial compression experimental tests have been performed on cubes made with such a “glass masonry” and, to compare, on cubes formed by dry connected glass tiles. The compression strength of the composite package is about one half of that of the dry cubes, but the ductility is increased by one order of magnitude due to a well-marked strain-softening branch. In the post-critical response a crucial role is played by the lateral stress provided by the material filling the abreuvoirs. The considerable compression strength and the so-obtained ductility are combined in the design of a fixed-arch 50 m-span footbridge, crossing the Grand Canal in Venice. At least from a theoretical point of view, the possibility of constructing arch footbridges for short or medium span entirely made of glass has been confirmed.

On enhancing ductility of brittle-like materials by high confining hydro-pressure

An explanation is sought for the substantial increase in apparent ductility of brittle-like solids (increasing their tensile strain to failure by three orders of magnitude) when surrounded by sufficiently high confining hydrostatic pressure (practically activated by fluid pressure). The magnitude of this hydro-pressure which is required to initiate the ductility enhancement is hypothesized to be the one that causes a complete closure of pre-existing micro-flaws (cavities, voids, etc). With further more hydro-pressure it resists slip between the flanks up to the order of the material grains boundaries slip. In such event, the solids behave as ‘flawless’ materials whose tensile failure is determined by necking instability preceded by large deformation, akin to ductile-like materials. When the confining hydro-pressure is insufficient for complete closure of all voids, it is supposed that even a minute amount of micro-voids may grow rapidly, link with neighbors, and cause brittle-like failure with barely noticeable strain. These geometrical events are traced here in detail in time-like scale (namely, by incremental increase of the tensile load at a given confining pressure). It is done by solving the incremental form of the governing equations that define, in our view, the mechanics of the considered situation. The constitutive equation of the net material matrix follows a power-law hardening, but the overall evolutionary behavior of the cavitated material is continuously perturbed by the presence of the shrinking/expanding cavities. The forthcoming solution leads to results which are generally in satisfactory agreement with experiments found in the open literature.

A New Test Method for Fiber-Reinforced Concrete

Abstract It has long been recognized that the principal benefit gained by using fiber reinforcement in portland cement concrete is a conversion of the material from brittle to relatively ductile behavior. The apparent ductility, or toughness, is primarily related to the improved tensile strength of fiber-reinforced concrete (FRC) especially and even after significant matrix cracking has occurred. However, the methods used to measure tensile strength and toughness of FRC have been, at best, elaborate and controversial. A practical and much needed testing methodology for fiber-reinforced concrete has been developed and adopted as an ASTM standard. It was conceived from the need to have a test that is relatively simple and inexpensive to conduct and is yet capable of assessing the tensile strength of cracked FRC. The test method, ASTM C 1399, and an interlaboratory testing program conducted to help formulate a precision statement for the new methodology is discussed.

‘Ductile’ mixed biopolymer gel composites

The deformation and failure behaviour of novel ‘pseudo-ductile’ gelatin/maltodextrin mixed biopolymer gels have been assessed using dynamic mechanical tests, conducted on the stage of a confocal laser scanning microscope. These materials exhibit a phase-separated structure, with spherical maltodextrin-rich inclusions within a continuous gelatin-rich matrix. Using both conventional tension and notched compact tension tests, it was observed that the apparent ductility was due to inclusion/matrix interfacial debonding, resulting in reduced elastic modulus. Based upon a simple debonding model, an interfacial fracture energy of ∼0.25 J m −2 was determined.

A Constitutive Theory for Brittle Solids with Application to Concrete

A single-flow surface model unifying damage mechanics and plasticity is developed for quasi-brittle solids in general and concrete in particular. The necessity of using both plasticity and damage mechanics is discussed, and the preference for developing a single-flow surface model over the existing bi-surface models is outlined. The model is cast within the general setting of the continuum thermodynamics with internal variables. Invoking the internal dissipation inequality and guided by physical arguments, a flow surface is developed via introducing pressure functions. The flow surface displays both hardening and softening. The model is capable of predicting the essential features of material inelasticity including dilatation, anisotropic degradation, inelastic flow, and enhancement in strength and apparent ductility with lateral pressure, and is simple for computer implementation. These features are shown together with comparisons of theoretical and experimental data.