This study investigated the recovery stress and bond resistance of cold drawn crimped SMA fiber using two different initial diameters of 1.0 and 0.7 mm. These characteristics are important to the active prestressing effect and crack-closing of the fiber. NiTi SMA fiber was used for the cold drawing, and then crimped shapes were manufactured with various wave heights. After that, tensile, recovery, and pullout tests were conducted. The cold drawn crimped fiber showed softening tensile behavior more clearly than the cold drawn straight fiber when not subjected to heating, whereas they had the same tensile behavior under heating. The recovery stress and the residual stress of the crimped fibers were less than those of the straight fiber with the same diameter. Moreover, crimped fibers with a large diameter and higher wave height would induce more recovery stress and residual stress. The maximum pullout resistance of the crimped fiber was a function of the wave depth, embedded length, yield strength, and flexural rigidity of the fiber.

The straining behavior of the shape memory alloy (SMA) fibers-reinforced mortar was investigated in this study by the monotonic compressive and cyclic compressive tests. Two types of SMA fibers with a crimped and dog-bone shape were used due to the high pullout resistance capacity, which guaranteed that the fibers and mortar matrix were composited well. The plain mortar was mixed with two different compositions to create the higher elastic modulus mortar matrix and the lower elastic modulus mortar matrix compared with the elastic modulus of SMA fibers. The results of the experimental test indicated that the non-heated SMA fibers could control the strains in both elastic and plastic phases; in which, the crimped fiber was more effective in precracking due to the higher composite capacity while the dog-bone-shaped fiber had a higher effect in post-cracking. After heating, the dog-bone-shaped fiber slipped more than that of the crimped fiber; thus, the heated crimped fiber was more effective than the heated dog-bone-shaped fiber in controlling strains after cracking. The effect of SMA fibers on the elastic modulus depended on both the elastic modulus of mortar matrix and the property of SMA fibers. In the plastic phase, the fibers were effective on reducing the speed of damage in monotonic case. An equation using reinforcing index was suggested for damage evolution in the cyclic case.

In this study, a compressive impact test was conducted using the split Hopkinson pressure bar (SHPB) method to investigate SMA fiber-reinforced mortar’s impact behavior. A 1.5% fiber volume of crimped fibers and dog-bone-shaped fibers was used, and half of the specimens were heated to induce recovery stress. The results showed that the appearance of SMA fibers, recovery stress, and composite capacity can increase strain rate. For mechanical properties, the SMA fibers reduced dynamic compressive strength and increased the peak strain. The specific energy absorption of the reinforced specimens slightly increased due to the addition of SMA fibers and the recovery stress; however, the effect was not significant. The composite behavior between SMA fibers and the mortar matrix, however, significantly influenced the dynamic compressive properties. The higher composite capacity of the SMA fibers produced lower dynamic compressive strength, higher peak strain, and higher specific energy absorption. The composite behavior of the dog-bone-shaped fiber was less than that of the crimped fiber and was reduced due to heating, while that of the crimped fiber was not. The mechanical properties of the impacted specimen followed a linear function of strain rate ranging from 10 to 17 s⁻¹; at the higher strain rates of about 49–67 s⁻¹, the linear functions disappeared. The elastic modulus of the specimen was independent of the strain rate, but it was dependent on the correlation between the elastic moduli of the SMA fibers and the mortar matrix.

The composite girder, which combines a steel beam with an ultra-high-performance concrete (UHPC) slab, has gained significant attention in recent years as a new type of bridge structure. However, accurately estimating the bending capacity of such girders remains a challenge, as practical methods are limited. In this article, the authors propose a theoretical approach based on the Euler-Bernoulli beam theory to determine the bending capacity of composite girders. This approach considers the assumptions of plane sections remaining plane and infinitesimal strains during bending. By applying this theoretical approach, the authors derive a formula that allows engineers to calculate the bending capacity of the composite girder. The formula takes into account the dimensions and properties of both the steel beam and the UHPC slab. The derived formula serves as a valuable tool for evaluating the structural behavior and performance of composite girders. To validate its accuracy, the authors compare the results obtained from their calculations with numerical simulations of composite girder failures caused by bending. The close agreement between the theoretical calculations and the numerical simulation results confirms the reliability and applicability of the proposed formula. This research significantly contributes to the field of composite girder design by providing a practical and reliable method for estimating the bending capacity of steel beam-UHPC slab composite girders. The proposed theoretical approach, validated through numerical simulations, offers valuable insights for the design and optimization of these composite girders in various engineering applications.

A finite-element (FE) pullout model was developed and validated by an earlier experimental pullout test of cold-drawn shape memory alloy crimped fibre. Then, how the dimensions of crimped fibre, such as diameter, wavelength, embedded wave and wave depth, influence its pullout behaviour was investigated. The FE results indicate that the fibre diameter must be large enough to produce a good flexural rigidity. The recommended diameter is ∼1.0 mm. However, it should be noted that the pullout resistance is not improved much for thicker fibre. A short wavelength is more effective than a long one due to a higher maximum pullout stress and more peak points. More embedded waves produce a higher pullout resistance; however, the increasing pullout resistance slows down when the maximum pullout stress exceeds the yield strength. A five-embedded wave is proposed to produce a reasonable pullout resistance while making efficient use of materials. Finally, deep-crimped fibre generates a higher pullout resistance compared with shallow-crimped fibre. On the basis of the effect of the dimensions on the pullout behaviour, an optimum dimension concept is proposed for crimped fibre.