Compressive Behavior Research Articles

GFRP-Reinforced Concrete Columns: State-of-the-Art, Behavior, and Research Needs

This comprehensive review paper delves into the utilization of Glass Fiber-Reinforced Polymer (GFRP) composites within the realm of concrete column reinforcement, spotlighting the surge in structural engineering applications that leverage GFRP instead of traditional steel to circumvent the latter’s corrosion issues. Despite a significant corpus of research on GFRP-reinforced structural members, questions about their compression behavior persist, making it a focal area of this review. This study evaluates the properties of GFRP bars and their impact on the structural behavior of concrete columns, addressing variables such as concrete type and strength, cross-sectional geometry, slenderness ratio, and reinforcement specifics under varied loading protocols. With a dataset spanning over 250 publications from 1988 to 2024, our findings reveal a marked increase in research interest, particularly in regions like China, Canada, and the United States, highlighting GFRP’s potential as a cost-effective and durable alternative to steel. However, gaps in current knowledge, especially concerning Ultra-High-Performance Concrete (UHPC) reinforced with GFRP, underscore the necessity for targeted research. Additionally, the contribution of GFRP rebars to compressive column capacity ranges from 5% to 40%, but current design codes and standards underestimate this, necessitating new models and design provisions that accurately reflect GFRP’s compressive behavior. Moreover, this review identifies other critical areas for future exploration, including the influence of cross-sectional geometry on structural behavior, the application of GFRP in seismic resistance, and the evaluation of the size effect on column strength. Furthermore, the paper calls for advanced studies on the long-term durability of GFRP-reinforced structures under various environmental conditions, environmental and economic impacts of GFRP usage, and the potential of Artificial Intelligence (AI) and Machine Learning (ML) in predicting the performance of GFRP-reinforced columns. Addressing these research gaps is crucial for developing more resilient and sustainable concrete structures, particularly in seismic zones and harsh environmental conditions, and fostering advancements in structural engineering through the adoption of innovative, efficient construction practices.

Analyses of bending performance of innovative paper-based sandwich beams with cup-box core

Paper-based materials are being alternative candidates to build load-bearing components for the high demanding building sector to be committed to the green transition, but more knowledge of structural mechanics of such components is needed. In this work, three categories of innovative load-bearing sandwich beams with cup-box core fully made of different paper materials were produced, tested and analyzed in four-point bending. The first failure mode was observed at the top facesheets due to the low compressive strength of paper materials; Beams with thin facesheets had premature buckling failure, whereas those with thicker facesheets exhibited ductility reaching higher deflection before the compressive failure. The developed finite element model, calibrated with the experiments for the equivalent bending and shear rigidities, provided figures of the modulus of the facesheet as well as the properties of the core. Furthermore, the compressive plasticity behavior of the facesheets was assessed by fitting the model with load-deflection curves from the tests. Using the model for the structural optimization of the thickness and height of the core, the work suggests optimal values 3 times higher than the original ones.

Composite Sand–Clay Infrastructural Soil Fills: Characteristic Consolidation and Hydraulic Properties

In the design construction of infrastructural projects comprised of geotechnical applications, including composite soil fill layers, compacted sand-clay soil fills are widely preferred as barrier layers, particularly in solid waste landfills, to minimize leakage, to prevent leachate from entering into groundwater. When bentonite clay with high water absorption capacity and low hydraulic conductivity is mixed with sand possessing relatively enhanced frictional properties, greater shear strength capacity, an effective fill material exhibiting low sensitivity to frost, and low volume change in case of wetting, drying can be obtained. On the other hand, when montmorillonite clay is loaded, due to highly critical volumetric contraction or dilation characteristics (high compressibility nature of clay), the soil fill composed of sand-clay will significantly consolidate. This situation may cause differential settlement problems of infrastructural fills employed in geotechnical applications. In this regard, the load conditions (mechanical effects) and the environmental conditions (physicochemical effects) in the field control compressibility characteristics and consolidation properties of sand-bentonite clay mixtures. This will ultimately impact the desired stability conditions of sand-clay soil layers built for constructed infrastructural fill, resulting in a deviation from anticipated performance conditions. To this end, in this study, the specimens of sand-bentonite clay mixtures prepared with different contents of sand-bentonite clay were subjected to one-dimensional consolidation tests to investigate the effect of bentonite content used in the mixture on consolidation behavior, hydraulic properties, and effect of sand amount on rate of consolidation and on resulting compressive strength behavior.

On the mechanical behavior of polymeric lattice structures fabricated by stereolithography 3D printing

AbstractThe utilization of 3D printing technology has transformed the possibilities for design adaptability and manufacturability. This study delves into the mechanical response and energy absorptivity of resin‐based lattice structures when subjected to compression, specifically examining structures fabricated from Tough 2000 (ductile) and Rigid 10K (brittle) resin materials using a stereolithography 3D printer. The analysis encompasses various types of lattice designs (such as cubic‐primitive, circular, triangular, and hexagonal), gradient structures, and combined shape configurations with varying strut dimensions. The primary objective is to provide significant findings regarding the compressive performance of these resin lattice structures produced through 3D printing. Analysis results show that graded and combined lattice designs have better compressive behavior compared to regular shapes with the same strut thickness. In addition, and for strut thickness of 0.5 mm, combined lattice designs show better energy absorption capabilities compared to regular shapes.

Experimental and Numerical Insights into the Multi-Impact Response of Cork Agglomerates

Due to their extraordinary qualities, including fire resistance, excellent crashworthiness, low thermal conductivity, permeability, non-toxicity, and reduced density, cellular materials have found extensive use in various engineering applications. This study uses a finite element analysis (FEA) to model the dynamic compressive behaviour of agglomerated cork to ascertain how its material density and stress relaxation behaviour are related. Adding the Mullins effect into the constitutive modelling of impact tests, its rebound phase and subsequent second impact were further examined and simulated. Quasi-static and dynamic compression tests were used to evaluate the mechanical properties of three distinct agglomerated cork composite samples to feed the numerical model. According to the results, agglomerated cork has a significant capacity for elastic rebound, especially under dynamic strain rates, with minimal permanent deformation. For instance, the minimum value of its bounce-back energy is 11.8% of the initial kinetic energy, and its maximum permanent plastic deformation is less than 10%. The material’s model simulation adequately depicts the agglomerated cork’s response to initial and follow-up impacts by accurately reproducing the material’s dynamic compressive behaviour. In terms of innovation, this work stands out since it tackles the rebounding phenomena, which was not previously investigated in this group’s prior publication, either numerically or experimentally. Thus, this group has expanded the research on cork materials’ attributes.

Microstructural evolution and dynamic mechanical behavior of PM 2195 Al-Li alloy with resistance to shear instability in high strain rate deformation

The risk of adiabatic shear instability is common in traditional alloys covering high-strength steels, copper alloys and aluminum alloys during high strain rate deformation. However, the research on shear instability of 2195 Al-Li alloy prepared by powder metallurgy (PM) at high strain rates is still blank. The purpose is to explore its microstructure evolution and dynamic compression behavior. In this work, the 2195 Al-Li alloy prepared by PM amazingly demonstrates excellent resistance to shear instability at high strain rates during Split-Hopkinson pressure bar (SHPB) experiment. The microstructural evolution of grains and T1 (Al2CuLi) precipitate phase, and dynamic compression behavior of PM 2195 Al-Li alloy at various strain rates from 3000 s−1 to 9000 s−1 are investigated in detail employing correlative electron-backscatter diffraction (EBSD) and transmission electron microscopy (TEM). During high strain rate deformation, the alloy undergoes grain fragmentation and rotation, increasing dislocation density around the grains. When the strain rate is from 3000 s−1 to 7500 s−1, the Kernel Average Misorientation (KAM) value increases, indicating intensified localized strain. At a strain rate of 9000 s−1, a small number of grains are surrounded by nanocrystals (around 200 nm) that undergo stress release, while the average size of micrometer-sized grains is 0.81 μm. As the strain rate increases, the T1 phases in the alloy extends, vibrates, and fractures before dissolving in the matrix, while high-rate deformation promotes dislocation entanglement and defects, leading to the decomposition or ordering of precipitated phases through rapid plastic deformation. By virtue of energy dissipation strategy via the formation of nanocrystallines and dislocation behavior induced by the dissolution of T1 phases, the "positive-positive" effect is observed. The PM 2195 Al-Li alloy undergoes no shear failure even at the strain rate of 9000 s−1, showing great prospects in future applications in dynamic-loading conditions.

Isotropic compression behavior of salinized unsaturated agricultural soil: An experimental and constitutive investigation

Salinity-induced soil degradation is a significant challenge in coastal reclamation areas, impacting agricultural productivity and infrastructure development. Variations in soil compression and deformation caused by changes in salinity warrant further investigation, particularly for agricultural applications. This study explores the relationship between soil pore water salinity and compressibility by conducting isotropic compression tests on salinized unsaturated agricultural soils treated with distilled water, 0.5 mol/L and 1 mol/L sodium chloride (NaCl) and calcium chloride (CaCl2) solutions. The results demonstrate that the type and concentration of salts significantly affect the compression behavior of these soils. The constitutive parameters were calibrated based on the experimental data. To account for osmotic suction, Barcelona Basic Model (BBM) was adjusted. The findings indicate that as pore water salinity increases, soil compressibility decreases, reflected by a compression index (Cc), and higher pre-consolidation stress (py). This modification aimed to quantify the impact of pore water salinity on soil compression. To validate the constitutive model, a numerical analysis of an isotropic compression test was carried out. This study contributes to our understanding of the isotropic compression behavior of coastal saline soil, proposes a constitutive framework for predicting soil responses under different salt conditions, and provides theoretical support for engineering construction in coastal reclamation areas.

Axial compressive behavior of multi-celled corrugated-plate CFST walls: Tests and numerical simulations

In recent years, steel-concrete composite walls have been widely used in practical engineering, primarily attributed to their exceptional properties. This paper proposed a novel type of composite wall called the multi-celled corrugated-plate concrete-filled steel tubular (MC-CFST) wall, which consisted of corrugated cells and interval flat plates connected alternately. Axial compression tests involving 14 MC-CFST wall specimens were performed to investigate their compressive resistant behavior. According to the test results, the steel and concrete showed good composite action. It was found that all specimens exhibited ductile failure with residual resistance-to-ultimate resistance ratios varying within the range of 0.503 to 0.672. Local buckling occurred in the interval flat plates, corrugated steel plates, and boundary flat plates. In addition, a finite element (FE) model was established and validated. The results showed that the vertical normal stress of concrete in the trough section was obviously enhanced, and the load-bearing capacity of the MC-CFST wall was primarily carried by the concrete. Moreover, a parametric analysis was conducted to investigate the effects of geometric dimensions and material strengths. The numerical results were then compared with the calculation results of existing codes. It was found that the design formulas in existing codes could not accurately calculate the load-bearing capacity of the MC-CFST wall. Finally, a design formula with good prediction accuracy was suggested, which could be used to predict the ultimate resistance of the MC-CFST wall.

Experimental and numerical analysis of L-shaped concrete-filled steel tube stub columns

This paper reports experimental and numerical studies on the cross-sectional compression resistance and behaviour of a novel L-shaped concrete-filled steel tube system. The novel composite system is proposed to eliminate concave corners and enhance the confinement of the outer steel tube to the concrete infill. A testing programme was firstly conducted, including nine tensile steel coupon tests, six compressive concrete cube tests and ten stub column tests. Upon testing, all the ten L-shaped concrete-filled steel tube stub column specimens were found to fail by local buckling. Finite-element models were then developed in the finite-element analysis software ABAQUS and validated against test results. The validated models were then employed to perform parametric studies to obtain an additional set of 48 numerical data. Test and numerical results were used to perform a design analysis, in which the suitability of the design rules in the European code EN 1994–1-1, the American specification ANSI/AISC 360 and the Chinese code GB 50936 for the developed L-shaped concrete-filled steel tube stub column system was assessed. The analysis results reveal that both EN 1994–1-1 and GB 50936 yield unsafe compression resistance predictions for the studied L-shaped concrete-filled steel tube stub columns, owing to the improper consideration of the contributions of the outer steel tube and the concrete infill to the overall capacity, while ANSI/AISC 360 offers safe design with high accuracy. The research findings are expected to contribute to the safe and wide use of the proposed novel L-shaped concrete-filled steel tube system in structural engineering and also to the further development of the current international design standards.

Effect of tungsten-particle size on the room temperature compressive deformation behaviour of Zr55Cu30Al10Ni5 amorphous alloy

W-particle-reinforced Zr-based amorphous composites (Wp/Zr55Cu30Al10Ni5) with a W-particle volume fraction of 50 % and W-particle sizes of 30, 75 and 250 μm were prepared through spark plasma sintering. The effects of different W-particle sizes on the quasi-static and dynamic compressive behaviours of the composites were investigated using a scanning electron microscopy/energy-dispersive X-ray spectroscopy instrument, a universal testing machine and the split-Hopkinson pressure bar. The results showed that the addition of W particles considerably enhanced the strength and ductility of the amorphous composites. Under quasi-static compression, the compressive strength of the W-particle-reinforced Zr-based amorphous composites increased as the W-particle size decreased; the fracture strain also increased accordingly. At a W-particle size of 30 μm, the composite achieved a maximum compressive strength of 1905 MPa and a maximum strain of 18.4 %. Under dynamic-compression conditions, the W-particle size was inversely proportional to the dynamic-compression strength of the material, with 3074 MPa obtained as the maximum strength. An analysis of the fracture morphology of the amorphous composites reveals that the fracture mode transitions from shear fracture to a mixed mode dominated by the W particles, a process involving interface debonding, cleavage fracture and ductile fracture.

Fracture modeling in composite structures containing orthotropic materials by incorporating strain orthogonal decompositions into phase-field method of interfacial damage

PurposeThe phase-field method of interfacial damage is used to simulate the damage in composite structures containing the brittle orthotropic materials and their interface.Design/methodology/approachIn the brittle fracture modeling, the strain tensor is decomposed into positive and negative parts characterizing tension and compression behaviors. By requiring an elastic energy preserving transformation involving the elastic stiffness tensor, these two strain parts must satisfy the orthogonality condition in the sense that the elastic stiffness tensor responds as a metric. However, most of the recent phase-field methods for brittle fracture do not verify this orthogonality condition. Additionally, to describe the damage in structures with anisotropic phases, recent studies have used multiple phase-field variables, with each preferential orientation represented by a phase-field variable to describe the bulk damage of component materials. This approach increases the complexity of simulation procedure. These disadvantages motivate the present study aimed at enhancing the simulation method.FindingsThe present study improves the phase-field method of interfacial damage by (1) incorporating the strain orthogonality condition into the phase-field method; (2) using only one phase-field variable instead of multiple phase-field variables to simulate damage in component orthotropic phases; and (3) investigating the interaction between interfacial damage and bulk damage as well as the effect of orientation tensor of preferential orientation in each orthotropic phase and the interfacial parameters on crack branching in composite structures.Originality/valueThrough several simulation examples, the present simulation method is proven to be accurate, effective, and helps the simulation process simpler than previous relevant methods.

Study of the one-dimensional consolidation and creep of clays with different thicknesses using different hypotheses and three elastic visco-plastic models

The one-dimensional consolidation analysis of clays considering creep compression is a classical issue in soil mechanics and geotechnical design. The major debate lies in how to predict the consolidation settlement for a thick layer in the field using parameters obtained from a thin specimen from the laboratory. Different hypotheses have been advocated, based on which various methods and constitutive models have been developed. However, there are still some questions unaddressed and concepts inconsistently used, which may mislead engineers in the selection of methods/models and may result in settlements underestimated on a risk design side. In this paper, a state-of-the-art review and a thorough comparison study are performed on the existing methods and models for the consolidation analysis of clays exhibiting creep, from theoretical derivations to numerical simulations in comparison with soil test data. An in-depth discussion is carried out on several key issues related to the thickness effects on the time-dependent compression behaviour of clays. The arguments of Hypothesis A and Hypothesis B are revisited based on the current development of constitutive theories. Three existing elastic visco-plastic (EVP) models that consider the creep compression implicitly during the whole consolidation process can perform well in predicting the settlement of clay layers with different thicknesses, and are in line with Hypothesis B. It is concluded that using existing EVP models based on porous-media continuum mechanics is a rigorous scientific method (also called “rigorous” Hypothesis B method), which is superior to the old Hypothesis A method which has logic errors and may result in unsafe underestimation of settlements.

Behavior of CFRP confined rectangular high-strength concrete-filled steel tubular columns under eccentric compression

This paper presents an experimental study on the eccentric compression behavior of rectangular high-strength concrete-filled steel tubular columns confined with carbon fiber reinforced polymer (CFRP- CFST). Seventeen specimens were tested under eccentric compression, examining key parameters involving eccentricity, number of transverse CFRP layers, slenderness ratio, corner radius, CFRP partial wrapping scheme, and CFRP orientation. The effects of these parameters on failure mode, load-displacement relationship, strain development, and ultimate mechanical properties were analyzed and discussed. The ultimate bearing capacity (Nu) of CFRP-CFST columns improved with an increase in transverse CFRP layers, corner radius, and CFRP wrapping proportion, while it decreased with increasing eccentricity and slenderness ratio. Partial wrapping schemes reduced the effectiveness of CFRP confinement, with the outward local buckling of the steel tube primarily occurring in areas without CFRP confinement. Longitudinal CFRP provided less improvement to Nu compared to transverse CFRP under small eccentricity conditions. A theoretical model based on the fiber element method was developed to predict the ultimate bearing capacity and implemented through a custom program to facilitate calculations. The model was validated using a test database comprising results from this study and previous research. Additionally, axial force-bending moment (N-M) interaction curves were rapidly generated, providing a theoretical basis for the design and practical application of CFRP-CFST columns in practical engineering.

Flexural wave compression behaviors of programmable graded piezoelectric meta-beams

Active electromechanical metamaterials have drawn significant attention due to their remarkable tunability and adaptability in vibration and wave control. Exploiting the electromechanical coupling effect via controlled biased fields enables precise manipulation of the structural vibrations and wave transmissions. In this research, we have developed a programmable graded piezoelectric meta-beam that successfully imitates the wave compression behavior of an acoustic black hole without modifying the structural geometries. The effective parameters of the meta-beam are reshaped into a gradient distribution by tuning the electrical impedances of the digital shunting circuits. With such graded effective parameters, it becomes feasible to tailor the local wavenumber distribution of the meta-beam and hence achieve the desired wave compression. Numerical results validated our proposed paradigm of the programmable black hole in both straight and curved beams. In contrast to the straight beam, curvature of the curved beam reduces the tunable range of the local wavenumber. A comprehensive parametric study was conducted to investigate the influences of the gradient profile, electric damping, and curvature on the wave control function. For a given desired frequency, programmable control of wave compression location and amplified output of electrical signals are achieved.

Compressive Strength Characteristics of Compressed Stabilized Earth Block (CSEB) Units and Walls

This paper is mainly focused on compressive strength behavior of Compressed Stabilized Earth Block (CSEB) units and CSEB walls. Compressed Stabilized Earth Block (CSEB) is a rectangular block used in wall construction. The ingredients of CSEB are soil, cement, fine aggregate, crusher dust, and a small amount of water. These blocks have less energy consumption and carbon emission, and they provide improved thermal insulation. In addition, they use local resources and disseminate appealing aesthetics with elegant profile and uniform size. Due to these advantages CSEB can be used as a green construction material. This research aims to study the strength characteristics of CSEB wall in compression and evaluate the suitability of CSEB walls as load bearing walls in structures. This research studies physical and mechanical characteristics of CSEB units made from red residual soil of Lele (Lalitpur) with 8% cement for stabilization. This paper discusses the compressive strength behavior of walls constructed of size 0.660m x 1.100m x 0.220m using CSEB units in cement sand mortar and stabilized mud mortar separately which were tested after 28 days. The experimental values after laboratory testing of CSEB masonry wall with height to thickness ratio 5:1 for cement sand mortar (1:6) and stabilized mud mortar (stabilized with 8% cement and 16% extra sand) separately are compared with relevant values from different codes. Results obtained from compressive strength tests of masonry walls constructed in the laboratory and those values from different codes concerning the strength of masonry unit and mortar are compared and found to be in agreement. The comparison of laboratory results with codal provisions of design of masonry walls illustrate that CSEB masonry walls can be designed in the similar way as brick masonry walls.

Mechanical properties and dynamic compression behaviour of basalt/polypropylene fibre-reinforced high-strength concrete

Concrete is a typical brittle material. Although it has excellent compressive performance, it can be damaged by dynamic loads such as explosions and impacts due to its poor tensile strength and cracking susceptibility. In an attempt to improve the impact resistance of concrete, a ∅100 × 50 split Hopkinson pressure bar was used to carry out impact tests on high-strength concrete (HSC) specimens with different volume contents (0.1%, 0.3% and 0.5%) of basalt fibre (BF) or polypropylene fibre (PPF) under three different impact pressures. The compressive strength, splitting tensile strength, flexural strength and dynamic compression performance of BF/PPF-reinforced HSC (C60) were studied. With an increase in impact pressure, increasing the volume content of fibre significantly improved the impact resistance of the concrete, due to the crack resistance and bridging effects of the fibre. The BF and PPF were found to be sensitive to the strain rate. Within a certain range of strain rate, the relationship between peak stress and strain rate was positively correlated. Dynamic stress–strain curves of concretes with optimal dosages of BF and PPF were obtained through dynamic compression tests.

Experimental study on constitutive relation of coal gangue coarse aggregate concrete under uniaxial compression

As an efficient method for utilizing coal gangue (CG), concrete incorporating coal gangue as coarse aggregate has significantly reduced the reliance on natural aggregates, offering substantial environmental and economic benefits. In this study, coal gangue concrete was prepared with coal gangue replacement rates of 0, 20, 40, 60, 80, and 100%, and mechanical tests under unconfined compression were conducted to evaluate the stress-strain behavior and failure mechanism of coal gangue coarse aggregate concrete (CGC). Utilizing scanning electron microscope (SEM) microscopic characterization, the microscopic failure mechanism of CGC was further elucidated. With increased coal gangue replacement, the CGC's uniaxial compression failure mode shifts from shear to longitudinal splitting failure. The slope, peak stress and elastic modulus of the stress–strain curve's rising section are negatively correlated with the coal gangue content, while the falling section's slope, peak strain and ultimate strain are positively correlated. Next, building upon the classical constitutive model, we adjust the constitutive parameters utilizing the uniaxial compressive strength and coal gangue content. Finally, we introduce a predictive model for the CGC's constitutive compressive behavior across various content levels. There is a notably high agreement between the model and experimental data.

Triaxial compressive behaviour of ultra-high performance geopolymer concrete (UHPGC) and its applications in contact explosion and projectile impact analysis

Ultra-high performance geopolymer concrete (UHPGC) is increasingly recognised as a promising construction material in protective engineering owing to its exceptional material strength and eco-friendly characteristics. However, there is currently limited research data on the behaviour of UHPGC under complex stress states, and the applicability of some common concrete dynamic constitutive models to UHPGC requires calibration and evaluation. This paper presents an experimental study on the triaxial compression (TXC) behaviour of UHPGC under varying confining pressures with a range of 0–40 MPa. The stress-strain curves under various confinement levels and failure patterns of UHPGC are discussed. The results indicate that the peak axial stress and toughness of UHPGC increase with higher confining pressure levels, but it still exhibits noticeable softening behaviour. Under uniaxial compression, UHPGC typically fails with inclined shear and axial tensile cracks, while under TXC, particularly at confining pressures above 20 MPa, the failure mode transitions to shear failure with prominent oblique shear cracks. Additionally, existing failure criteria are calibrated to match the strength envelop of UHPGC, with the Power-law failure criterion providing the closest fitting results. Subsequently, utilising both current and pre-existing TXC data, three commonly used concrete constitutive models, including HJC, K&C and CSC, are calibrated with respect to the dominant strength parameters for UHPGC in the finite element (FE) hydrocode ANSYS/LS-DYNA. Further, numerical simulations are conducted to evaluate the effectiveness of such three calibrated concrete models in replicating the response of UHPGC panels exposed to the contact explosion or projectile impact. The simulation results demonstrate that the K&C and CSC models effectively replicate the localised damage of UHPGC panels under contact explosions, while the HJC model more accurately simulates the depth of penetration (DOP) for thick UHPGC panels under projectile impact.

Compressive Properties and Fracture Behaviours of Ti/Al Interpenetrating Phase Composites with Additive-Manufactured Triply Periodic Minimal Surface Porous Structures

Compressive Properties and Fracture Behaviours of Ti/Al Interpenetrating Phase Composites with Additive-Manufactured Triply Periodic Minimal Surface Porous Structures

Effect of the Multidirectional Free Hot Forging on the Hot Compression Behavior and Microstructure of the Zr–Nb Alloy

Effect of the Multidirectional Free Hot Forging on the Hot Compression Behavior and Microstructure of the Zr–Nb Alloy