Showing papers on "Strain hardening exponent published in 2022"

High-strength high-ductility Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates

Abstract: In this study, Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates were successfully developed with high strength and high ductility. A multi-scale investigation was conducted to gain an in-depth understanding of the microstructure and ductility enhancement mechanism of geopolymer aggregate ECC (GPA-ECC). The use of geopolymer fine aggregates enabled the high-strength ECC to achieve higher tensile ductility and finer crack width compared to existing ones with similar compressive strength in the literature. It was found that the GPA reacted with the cementitious matrix, and the width of the GPA/matrix interfacial transition zone (ITZ) was larger than that of the silica sand/matrix ITZ. Moreover, the GPA achieved a strong bond with the cementitious matrix and could behave as “additional flaws” in high-strength matrix, resulting in saturated multiple cracking and excellent tensile ductility of ECC. This study provides a new avenue for developing high-performance fiber-reinforced cementitious composites based on artificial geopolymer aggregates.

High-strength high-ductility Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates

Abstract: In this study, Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) incorporating geopolymer fine aggregates were successfully developed with high strength and high ductility. A multi-scale investigation was conducted to gain an in-depth understanding of the microstructure and ductility enhancement mechanism of geopolymer aggregate ECC (GPA-ECC). The use of geopolymer fine aggregates enabled the high-strength ECC to achieve higher tensile ductility and finer crack width compared to existing ones with similar compressive strength in the literature. It was found that the GPA reacted with the cementitious matrix, and the width of the GPA/matrix interfacial transition zone (ITZ) was larger than that of the silica sand/matrix ITZ. Moreover, the GPA achieved a strong bond with the cementitious matrix and could behave as “additional flaws” in high-strength matrix, resulting in saturated multiple cracking and excellent tensile ductility of ECC. This study provides a new avenue for developing high-performance fiber-reinforced cementitious composites based on artificial geopolymer aggregates. • Geopolymer fine aggregates were successfully applied to develop high-strength high-ductility ECC with fine crack width. • Geopolymer fine aggregates reacted with cementitious paste, resulting in a strong interfacial bond. • Geopolymer fine aggregates acted as “additional flaws” in high-strength ECC matrix, leading to saturated multiple cracking. • Compared with existing ambient-cured high-strength ECC, geopolymer aggregate ECC exhibited superior tensile ductility.

Ultra-high-strength engineered/strain-hardening cementitious composites (ECC/SHCC): Material design and effect of fiber hybridization

Abstract: Abstract It is well known that an increase in the compressive strength of cementitious composites is usually accompanied by a loss of tensile ductility. Designing and developing ultra-high-strength cementitious composites (e.g., ≥200 MPa) with high tensile strain capacity (e.g., ≥3%) and excellent crack resistance (e.g., crack width ≤100 μm) remain challenging. In this study, a series of ultra-high-strength Engineered Cementitious Composites (UHS-ECC) with a compressive strength over 210 MPa, a tensile strain capacity of 3–6% (i.e., 300–600 times that of ordinary concrete), and a fine crack width of 67–81 μm (at the ultimate tensile strain) were achieved. Hybrid design of fiber reinforcement and matrix for UHS-ECC was adopted by combining the ECC and ultra-high-performance concrete (UHPC) design concepts, and the effect of fiber hybridization and aspect ratio on the mechanical behavior of UHS-ECC was comprehensively investigated. The overall performance of UHS-ECC was assessed and compared with the existing high-strength ECC and strain-hardening UHPC, and it was found that the currently designed UHS-ECC recorded the best overall performance among the existing materials. Finally, the multiple cracking behavior of UHS-ECC was analyzed and modeled based on a probabilistic approach to evaluate its critical tensile strain for durability control in practical applications. The results of this study have pushed the performance envelope of both ECC and UHPC materials and provided a basis for developing cementitious composites with simultaneously ultra-high compressive strength, ultra-high tensile ductility, and excellent crack resistance. • Hybrid design of fiber reinforcement and matrix for UHS-ECC was introduced by combining the UHPC and ECC design concepts. • UHS-ECC with a compressive strength over 210 MPa and a tensile ductility over 6% was developed. • UHS-ECC with 2% 18-mm PE fiber and 1% 13-mm steel fiber recorded the best overall performance. • Cracking behavior was modeled by a probabilistic approach to estimate the critical tensile strain of UHS-ECC.

Design of pure aluminum laminates with heterostructures for extraordinary strength-ductility synergy

Abstract: Heterogeneous metals and alloys are a new class of materials with superior mechanical properties. In this paper, we engineered sandwich-structured pure aluminum laminates composed of middle coarse-grained layer and outer fine-grained layer via extrusion, rolling and annealing. By controlling the post-annealing regimes, a larger degree of microstructure heterogeneities such as boundary spacing, misorientation and texture across the hetero-interface were obtained, which resulted in obvious mechanical differences. Tensile tests indicated that the 300 °C/30 min annealed laminates enabled a relatively high tensile ductility while simultaneously retaining a high strength, which was better than prediction by the rule-of-mixture. To explain the reasons behind it, the evolution of geometrically necessary dislocations and strain gradient at the hetero-interface zone were detected using in-situ tension and microscopic digital image correlation technique. It was found that with the increasing applied strain, a significant strain gradient was developed near the interface, which was accommodated by geometrically necessary dislocations, thereby contributing to higher hetero-deformation induced (HDI) strengthening and hardening.

A new strategy for fabrication of unique heterostructured titanium laminates and visually tracking their synchronous evolution of strain partitions versus microstructure

Abstract: Heterostructured (HS) material with extraordinary mechanical properties has been regarded as one of the most promising structural materials. Here, we reported a new strategy for preparing heterostructured pure titanium laminates that possess a good combination of strength and ductility by combining gradient structure (GS) and heterogeneous lamella structure (HLS). The deformation characteristic versus microstructure evolution of GS/HLS titanium laminates, namely the strain partitions between different-sized grains (480–25 μm) was visualized using a scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) mode combined with the digital image correlation (SEM-DIC) with an ultrahigh spatial resolution for the first time. As a result, the hetero-deformation of unique GS/HLS structure by the characteristic of strain partitions could be accurately captured. While the hetero-deformation could result in the hetero-deformation induced (HDI) stress strengthening and HDI hardening, which were regarded as the key reason that the resulting GS/HLS Ti laminates showed a superior combination of strength and ductility. This could promote a more in-depth understanding of the strengthening-toughening mechanism of heterostructured material.

Configuration of new fiber-like structure driven high matching of strength-ductility in TiB reinforced titanium matrix composites

Abstract: To fulfill the combination of high strength and ductility, configuration of novel fiber-like structural TiB reinforced titanium matrix composites was designed and successfully fabricated through series of powder metallurgy and hot working process. The influence of the fiber-like structure and microstructures on the mechanical response is investigated to clarify their strengthening and plasticizing mechanisms. Smaller equiaxed α-Ti grains generated in both the composites region and Ti region of the TiB/Ti composites. The new configuration improved the tensile strength from 573.5 MPa to 659.1 MPa while the ductility (24.4%) was comparable with that of pure Ti (23.5%). Besides, prismatic slips with Schmid factors above 0.4 had higher distribution in both the composites region and Ti region of the TiB/Ti composites than pure Ti along the loading direction, suggesting the higher deformation ability of the TiB/Ti composites. Moreover, the loading-unloading-reloading (LUR) true stress-strain curves showed much higher back stress in fiber-like structural TiB/Ti composites compared with unreinforced Ti during deformation. The existence of the back stress increased the strain hardening rate when the true strain was above 0.05 and strengthened the Ti region, which was caused by deformation incompatible between the composites region and Ti region during tensile loading in the TiB/Ti composites. The higher mechanical response of the TiB/Ti composites is attributed to the introduction of TiB reinforcements with the new fiber-like structure, and the coupling effect of consequent grain refinement, load transferring effect, dislocation pinning effect of the fine TiB particles, back stress strengthening and the higher Schmid factors of prismatic slips.

High-performance strain-hardening cementitious composites with tensile strain capacity exceeding 4%: A review

Abstract: A state-of-the-art review on the development of high-performance strain-hardening cementitious composites (SHCC) with over 55 MPa compressive strength, 4% tensile strain capacity, and 300 kJ/m3 strain energy was conducted. The different designs of high-performance SHCCs with respect to the type of ingredients (cementitious materials, aggregates, fillers, and nanomaterials), water/binder and sand/binder ratios, fiber type, and aspect ratio, along with diverse curing regimes to satisfy the performance criteria, were analyzed. Some fiber surface refinement processes, e.g., graphene oxide coating and oxidation, were explained, and their effects on the mechanical properties of high-performance SHCCs were discussed. The durability and impact/blast resistance were also evaluated and compared with those of conventional SHCCs, such as engineered cementitious composites (ECC) and ultra-high-performance fiber-reinforced concrete (UHPFRC). It was discovered that ductility-enhanced high-strength SHCC has a higher impact resistance than ECC and UHPFRC do. Because of its excellent mechanical properties, high-performance SHCC can be effectively used for various purposes, e.g., strengthening of existing structures, fireproofing of steel structures, elimination of stirrups, partial or total replacement of longitudinal steel rebars in reinforced concrete structures, and earthquake-resistant frame structures. Based on data analysis, a very-high-performance SHCC, which can absorb five times more energy than ECC, has also been recently developed. This SHCC has over 100 MPa compressive strength, 8% tensile strain capacity, and 800 kJ/m3 strain energy, respectively.

High-performance strain-hardening cementitious composites with tensile strain capacity exceeding 4%: A review

Abstract: A state-of-the-art review on the development of high-performance strain-hardening cementitious composites (SHCC) with over 55 MPa compressive strength , 4% tensile strain capacity , and 300 kJ/m 3 strain energy was conducted. The different designs of high-performance SHCCs with respect to the type of ingredients (cementitious materials, aggregates, fillers, and nanomaterials), water/binder and sand/binder ratios, fiber type , and aspect ratio, along with diverse curing regimes to satisfy the performance criteria, were analyzed. Some fiber surface refinement processes, e.g., graphene oxide coating and oxidation, were explained, and their effects on the mechanical properties of high-performance SHCCs were discussed. The durability and impact/blast resistance were also evaluated and compared with those of conventional SHCCs, such as engineered cementitious composites (ECC) and ultra-high-performance fiber-reinforced concrete (UHPFRC). It was discovered that ductility-enhanced high-strength SHCC has a higher impact resistance than ECC and UHPFRC do. Because of its excellent mechanical properties, high-performance SHCC can be effectively used for various purposes, e.g., strengthening of existing structures, fireproofing of steel structures, elimination of stirrups, partial or total replacement of longitudinal steel rebars in reinforced concrete structures , and earthquake-resistant frame structures. Based on data analysis, a very-high-performance SHCC, which can absorb five times more energy than ECC, has also been recently developed. This SHCC has over 100 MPa compressive strength, 8% tensile strain capacity, and 800 kJ/m 3 strain energy, respectively.

A ductile Nb40Ti25Al15V10Ta5Hf3W2 refractory high entropy alloy with high specific strength for high-temperature applications

Abstract: Refractory high entropy alloys (RHEAs) with good high-temperature softening resistance have been revealed as promising candidates for high-temperature structural materials. In this work, a low-density ductile RHEA, Nb40Ti25Al15V10Ta5Hf3W2, has been developed. The RHEA with a BCC matrix and B2 nanoprecipitates exhibits excellent specific yield strength. The compressive specific yield strength (σ0.2/ρ) at 1073 K is as high as 83.2 MPa g−1 cm3. The different deformation behaviors during compression at 1073 K and 1273 K are also identified. The dislocation-dominated deformation provides the initial strain hardening capability, and then microcracks and dynamic recovery accelerate the transition from strain hardening to softening at 1073 K. While the diffusion-controlled dislocation annihilation and continuous dynamic recrystallization (DRX) are the dominant reasons for persistent strain softening at 1273 K. Our work not only reports a promising RHEA with excellent high-temperature properties, but also promotes the development of RHEAs for high-temperature applications.

Tensile deformation behavior of ferrite-bainite dual-phase pipeline steel

Abstract: In-situ tensile test accompanied by electron backscatter diffraction (EBSD) analyses were performed on polygonal ferrite (PF) and bainite dual-phase steel, selected regions of interest were analyzed following plastic deformation of the steel. Deformation-induced crystal orientation evolution, localized strain concentration, slip transfer, and geometrically necessary dislocation (GND) density were tracked. Results revealed that heterogeneity deformation facilitated formation subregions with crystal orientation deviation in grain and fragmented the grain by the new low angle grain boundaries (LAGBs) or medium angle grain boundaries (MAGBs). The PF grains with ND// preferred crystal orientation exhibited high orientation stability, and almost all load axes of the selected PF grains moved to the [101] pole, resulting in enhancing {111} orientation component at high strain levels. With the lattice rotation during deformation, the high angle grain boundaries (HAGBs) can change to MAGBs, which was beneficial to maintain coordination deformation among grains. Localized strain concentration can be decreased by the slip transfer across the PF grain boundaries or bainite/PF phase boundaries, which reduced the risk of micro-void formation. Additionally, the variation of α12 GND tensor average value (Ave. α12) revealed that the ferrite was continuous plastic deformation, while the bainite occurred stage hardening. The required strain for the coordination deformation was controlled by strain hardening behavior.

Strain Hardening of Polypropylene Microfiber Reinforced Composite Based on Alkali-Activated Slag Matrix

Abstract: A comparative study of the fracture features, strength and deformation properties of pseudo strain-hardening composites based on alkali-activated slag and Portland cement matrices with polypropylene microfiber was carried out. Correlations between their compositions and characteristics of stress–strain diagrams under tension in bending with an additional determination of acoustic emission parameters were determined. An average strength alkali-activated slag matrix with compressive strength of 40 MPa and a high-strength Portland cement matrix with compressive strength of 70 MPa were used. The matrix compositions were selected for high filling the composites with polypropylene microfiber in the amount of 5%-vol. and 3.5%-vol. ensuring the workability at the low water-to-binder ratios of 0.22 and 0.3 for Portland cement and alkali-activated slag matrices, respectively. Deformation diagrams were obtained for all studied compositions. Peaks in the number of acoustic signals in alkali-activated slag composites were observed only in the strain-softening zone. Graphs of dependence of the rate of acoustic events occurrence in samples from the start of the test experimentally prove that this method of non-destructive testing can be used to monitor structures based on strain-hardening composites.

Enhanced strength-ductility synergy via high dislocation density-induced strain hardening in nitrogen interstitial CrMnFeCoNi high-entropy alloy

Abstract: • Nitrogen addition induced a remarkable increase in strength but only a negligible decrease in uniform elongation of CrMnFeCoNi HEA. • First-principles calculations were carried out to analyze the effect of nitrogen on generalized stacking fault energy ( γ usf , γ isf , and γ utf ) and twinning. • The pinning effect of nitrogen on dislocations contributed to the increased dislocation density and thus the higher strain-hardening rate. The present work demonstrates that nitrogen doping inhibits the formation of deformation twins in a CrMnFeCoNi high entropy alloy, while significantly increasing the strength without sacrificing much ductility at 77 K. Microstructural characterization and first-principles calculations were employed to unveil the role of interstitial nitrogen atoms in obtaining such an excellent combination of strength and ductility at 77 K. It is found that nitrogen addition increases generalized stacking fault energy (GSFE) and reduces twinning. However, the pinning of dislocations by nitrogen atoms effectively suppresses dislocation cross-slip and dynamic recovery and in turn, promotes the accumulation of dislocations. The high dislocation density induces a high strain hardening capacity and improves uniform elongation, which compensates for the ductility loss accompanied by solid solution strengthening. The effect of nitrogen doping enriches the design concept of high- and medium-entropy alloys, providing an economical and effective strategy to develop ultra-high-performance alloys that are suitable for cryogenic applications.