Pull-out Process Research Articles

Study on Crack Resistance Mechanism of Helical Carbon Nanotubes in Nanocomposites

Helical carbon nanotubes (HCNTs) with different geometrical properties were constructed and incorporated into nanocomposites for the investigation of the anti-crack mechanism. The interfacial mechanical properties of the nanocomposites reinforced with straight carbon nanotubes and various types of HCNTs were investigated through the pullout of HCNTs in the crack propagation using molecular dynamics (MD). The results show that the pullout force of HCNTs is much higher than that of CNTs because the physical interlock between HCNTs and matrices is much stronger than the van der Waals (vdW) interactions between CNTs and matrices. Remarkably, HCNTs with a large pitch length can not only effectively prevent the initiation of breakages but also hinder the growth of cracks, while HCNTs with a small diameter and tube radius cannot even effectively prevent the initiation of cracks, which is similar to straight CNTs. Moreover, the shear resistance of HCNTs increases with the increase in the helix angle, which remains at a high level when the helix angle reaches the critical value. However, HCNTs with a small helix angle and large diameter can carry out more polymer chains, while snake-like HCNTs and HCNTs with a small diameter and helix angle can hardly carry out any polymer chain during the pullout process and show similar interfacial properties to the straight CNTs.

Prediction of bond strength between fibers and the matrix in UHPC utilizing machine learning and experimental data

The bond strength between fibers and the matrix plays a crucial role in the tensile strength and post-cracking performance of ultra-high-performance concrete (UHPC). However, existing studies have not attempted to utilize machine learning models for the prediction of bond strength. In this study, machine learning techniques were employed to predict the bond strength of fibers in UHPC. A dataset comprising 658 experimental records was compiled and advanced unsupervised Isolation Forest techniques were utilized to identify and remove outliers, thereby ensuring the accuracy and reliability of the data. Six machine learning models, including ANN, GBDT and XGBoost are utilized in this study, with a focus on evaluating their performance in predicting the maximum pull-out force of fibers. The results indicate that the XGBoost model exhibits exceptional predictive performance, achieving R² value of 0.98, which demonstrates the model's capability to accurately predict the fiber pull-out process. Furthermore, feature importance analysis, visualized through advanced techniques, reveals the significant influence of fiber tensile strength on the pull-out force. A new equation is proposed to predict the maximum pull-out force of fibers and correction factors for different fiber shapes are introduced, significantly enhancing the precision of the calculations. The newly proposed predictive equation has R² value of 0.72, which increases to 0.74, 0.77, and 0.86 after the introduction of shape correction factors, significantly enhancing the accuracy of the computed results. This study not only provides an efficient and reliable method for predicting fiber bond strength in UHPC but also offers an innovative tool for calculating fiber pull-out force.

Reduced Graphene Oxide Reinforces Boron Carbide with High-Pressure and High-Temperature Sintering.

Introducing a second phase has been an effective way to solve the brittleness of boron carbide (B4C) for its application. Though reduced graphene oxide (rGO) is an ideal candidate for reinforcing the B4C duo's two-dimensional structure and excellent mechanical properties, the toughness is less than 6 MPa·m1/2, or the hardness is lower than 30 GPa in B4C-graphene composites. A barrier to enhancing toughness is the weak interface strength between rGO and B4C, which limits the bridging and pull-out toughening effects of rGO. In this work, internal stress was introduced using a high-pressure and high-temperature (HPHT) method with B4C-rGO composites. The optimal hardness and toughness values for the B4C-2 vol% rGO composite reached 30.1 GPa and 8.6 MPa·m1/2, respectively. The improvement in toughness was 4 times higher than that of pure B4C. The internal stress in the composite increased gradually from 2.3 GPa to 3.3 GPa with an increase in rGO content from 1 vol% to 3 vol%. Crack deflection, bridging, and rGO pull-out are responsible for the improvement in toughness. Moreover, the high internal stress contributed to the formation of good interface strength by embedding rGO into the B4C matrix particles, which further enhanced the dissipation of the crack energy during the pull-out process and led to high toughness. This work provides new insights into synthesizing high-toughness B4C matrix composites.

A novel personalized homogenous finite element model to predict the pull-out strength of cancellous bone screws

BackgroundOrthopedic surgeries often involve the insertion of bone screws for various fixation systems. The risk of postoperative screw loosening is usually assessed through experimental or finite element (FE) evaluations of the screw pull-out strength. FE simulations are based on either personalized complex but accurate heterogeneous modeling or non-personalized simple but relatively less accurate homogeneous modeling. This study aimed to develop and validate a novel personalized computed tomography (CT)-based homogeneous FE simulation approach to predict the pull-out force of cancellous bone screws.MethodsTwenty FE simulations of L1-L5 vertebral screw pull-out tests were conducted, i.e., 10 heterogeneous and 10 homogenous models. Screws were inserted into the lower-middle region of vertebrae. In our novel homogeneous model, the region around approximately twice the diameter of the screw was used as a bone material reference volume. Subsequently, the overall material property of this region was homogeneously attributed to the entire vertebra, and pull-out simulations were conducted.ResultsThe mean error of the predicted pull-out forces by our novel homogenous simulations was ~ 7.9% with respect to our heterogeneous model. When solely the cancellous bone was involved during the pull-out process (i.e., for L1, L2, and L3 vertebral bodies whose cortical bone in the inferior region is thin), the novel homogenous model yielded small mean error of < 6.0%. This error, however, increased to ~ 11% when the screw got involved to the cortical bone (for L4 and L5 vertebrae whose cortical bone in the inferior region is thick).ConclusionThe proposed personalized CT-based homogenous model was highly accurate in estimating the pull-out force especially when only the cancellous bone was involved with the screw.

Mechanical and electromagnetic interference shielding properties of in-situ grown Si3N4nw synergistic defective-graphene reinforced alumina ceramics

Ceramic matrix composites have versatile application potential but are astricted by brittleness and single function. It can be ameliorated assisted by reinforcements, but the uneven distribution of reinforcements seriously limits the reinforcing efficiency. In this work, the layered porous skeleton of alumina (Al2O3) and silicon dioxide (SiO2) was prepared, then defective-graphene (DG) and silicon nitride nanowires (Si3N4nw) were successively grown in-situ in the skeleton (Al2O3/SiO2-G-Si3N4nw) to concurrently strength and toughen, as well as endow Al2O3 ceramic with electromagnetic interference (EMI) shielding performance. Subsequently, Al2O3/SiO2-G-Si3N4nw preform was sintered to construct a uniform Si3N4nw synergistic DG enhancement network. The optimum flexural strength and fracture toughness of the sintered ceramic reached 388.52 MPa and 11.29 MPa m1/2, respectively. This was mainly since DG can fine the ceramic grains, induce crack deflection and furcation, while the uniformly distributed Si3N4nw consumed additional energy during the pull-out process. In addition, the EMI shielding effectiveness of the sintered ceramics in X-band was up to 31.77 dB, which is mainly attributed to the conductive loss, dipole polarization loss and interfacial polarization loss of DG. Remarkably, this work provides an idea for efficient strengthening, toughening and integration of structure and function.

Mechanical properties of steel fibre–UHPC interface: Experimental study and numerical simulation

The aim of this study was to investigate the interfacial mechanical attributes between steel fibre (SF) and an ultra-high-performance concrete (UHPC) matrix. Using both experimental testing and numerical simulation, a comprehensive analysis on the pull-out process of SF from the UHPC matrix was conducted. The effects of fibre embedment depth, fibre diameter and fibre embedment angle on the mechanical properties of the SF–UHPC interface were examined. In the fibre pull-out tests, the maximum pull-out force of the group of specimens with different embedment depths showed a trend of rapid rise and then slow development, and the pull-out work increased with an increase in embedment depth. The maximum pull-out force and pull-out work of the group with variable fibre diameters increased with an increase in fibre diameter. The maximum pull-out force and pull-out work of the group with different embedment angles increased first and then decreased with an increase in embedment angle. The maximum pull-out force was 72.5 N when the embedment angle was 45° and the maximum pull-out work was 214.4 N.mm when the embedment angle was 30°. The results of finite-element simulations were found to be in good agreement with the experimental results.

Experimental and finite element analysis on pullout behavior of steel fibers embedded in cement grouting material

With increasing demands for enhanced safety and durability in offshore structures, steel fibers are attracting great attention as fiber admixtures to limit the cracking within cement grouting material. To investigate the pullout behavior and bond properties of steel fibers crossing the cracks in cement grouting material, the present study carried out a single-sided pullout test and three-dimensional numerical models based on the finite element method (FEM). The cohesive zone model and the concrete damage plasticity model were adopted for the analysis of the fiber-matrix interaction and compressive damage in the cement matrix during the pullout process. The outcomes indicated that the proposed numerical models achieved good agreement with the pullout test results. Moreover, the parametric analyses using the FEM showed that the anchoring effect of the hooked end can greatly improve the pullout resistance of the steel fiber but is not fully utilized due to the low compressive strength of the ordinary Portland cement grout. Furthermore, with an increase in the embedded length, the energy consumptions of the straight and hooked-end steel fibers during the pullout process were improved linearly. Besides, the distinct plastic deformation of steel fibers and compressive damage of the cement matrix could be observed during the pullout process of inclined fibers, and the optimal bond properties for both straight and hooked-end steel fibers were achieved with an inclined angle of 30°.

Frictional pull-out work capacity of macro-SSF in concrete composites and evaluation of matrix deterioration due to kinematic fiber-end-slip after full-debonding point

ABSTRACT This paper presents a parametric study on the effect of frictional pull-out work capacity on the performance of fiber/matrix regime during the kinematic pull-out process. This study involves an experimental program of pull-out test of straight steel fiber embedded in concrete composites, numerical study based on finite element modeling, and theoretical approaches based on mathematical model. Various parameters were considered such as fiber geometry, Poisson’s ratio, elasticity modulus, and aspect ratio. The experimental results were calibrated with the results of numerical study, and the theoretical approaches were controlled using deterioration coefficient based on a given curve of uniaxial stress–displacement relationship plotted experimentally. The results showed that the capacity of pull-out work decreases 62.6% when the fiber diameter decreases 6 times. Decreasing the fiber diameter also improves the resistance of the matrix against deterioration. While decreasing the fiber embedded length by 1.8 times, the frictional pull-out work decreases by 56.3% and boosts relatively the resistance of matrix against crumbling, where the frictional shear bond strength improved by 91%. Likewise, decreasing the fiber aspect ratio by 1.8 times decreases the done frictional pull-out work by 56.3% and boosts the fiber duct resistance against damage by improving the frictional shear bond strength.

DEM evaluation of the mobilization of mechanisms governing the geogrid-aggregate interaction

Pullout tests of geogrids embedded in a single clean aggregate type were conducted and subsequently simulated to investigate the geogrid-aggregate interaction mechanisms. The Discrete Element Method (DEM) model, which was carefully calibrated and validated against the experimental results, was adopted for the simulations. The three-dimensional deformation behavior of geogrids and the shear behavior of aggregates with complex particle shapes were successfully predicted. Analysis of the particle displacement distribution and contact force distribution allowed determination of the particle-scale interaction mechanisms of the geogrid-aggregate system. In particular, the mobilization of pullout resistance components was tracked based on the contact identification method and the influence of geogrid stiffness on the mobilization of pullout resistance components could be evaluated. The results indicate that the activation of the different geogrid resistance components that develop during pullout are not synchronized. During the pullout process, DEM predictions indicate that the frictional resistance of the geogrid is activated first, peaking rapidly and being followed by the development of passive resistance of the transverse ribs. Also, an increased geogrid stiffness was found to enhance the passive resistance of the transverse ribs but to influence only negligibly the frictional resistance.

Multi-scale simulation of stress transfer across ‘polymer bridge’ in graphene oxide/halloysite organic-inorganic hybrid aerogel

In our previous work, organic-inorganic hybrid aerogels were successfully constructed utilizing graphene oxide (GO) sheets and halloysite nanotubes (HNTs), and interesting stress transfer behavior were found. Through preliminary experimental studies, we found that the movement of polymer molecules significantly affected the stress transfer behavior of aerogels. The mechanism might involve macroscopic and microscopic scales, which were difficult to be adequately studied by conventional experimental means. Therefore, this paper investigated the stress transfer behavior of hybrid aerogels utilizing finite element analysis (FEA) combined with molecular dynamics (MD) multi-scale simulation methods. The influence of the connection angles of GO and HNTs on the stress transfer ability was studied utilizing the FEA combined with theoretical formulas. The GO polymer pull-out systems were established by MD. The pull-out processes of polymethylmethacrylate (PMMA) and polysiloxane (PSO) were compared. The effect of oxygen-containing functional group coverage of GO on interface strength was studied. The results indicated that connection angles of 60° and 30° contributed more to the stress transfer between GO and HNTs under tensile and compression stress, respectively. The interfacial enhancement effect of the GO/polymer systems reached saturation when the coverage of oxygen-containing functional groups on the GO surface was 20 %. Si–O–Si agglomeration networks were formed by silane-modified GO and PSO, which possessed stronger interfacial strength.

Experimentally-informed lattice model to simulate the fiber pull-out behavior at the microscale

Engineered cementitious composite (ECC) is widely employed in engineering due to its high toughness and ductility. The Interfacial Transition Zone (ITZ) between the fibers and the matrix plays a vital role in influencing the strength and durability of ECC. This study introduces a numerical method to simulate fiber pull-out behaviors, specifically the fiber debonding and slipping. A birth-death method is proposed to account for the mechanical transition from fiber debonding stage to slipping stage. The contributions of various phases in the ITZ are explicitly considered. Furthermore, nanoindentation tests and Backscattered Electron (BSE) imaging were conducted to determine the microstructures of ITZ and local mechanical properties of each phase within the ITZ. A series of fiber pullout experiments with polyvinyl alcohol (PVA) fibers were conducted to calibrate and validate the model. Subsequently, the validated model was employed to explore the influence of w/c ratios, fiber orientations and bonding properties on the interfacial behavior. The microstructure-informed model proposed herein effectively predicts fiber pull-out behavior, facilitating a thorough exploration of fracture mechanisms throughout the pull-out process, and serves as the basis for multiscale modeling of ECC.

Mechanical behavior and cement sheath damage characteristics of casing pullout based on peridynamics

In petroleum exploration and production, the recovery and replacement of casing are crucial. However, difficulties often arise during casing pullout operations due to the strong bonding strength between the casing and the cement sheath. Currently, there are few researches on casing pullout process, which leads to the fact that casing pullout operations often relies on experience and lacks theoretical support. To comprehend the mechanical behavior, interface shear failure mechanisms of casing pullout process, and to propose rational solutions for practical engineering applications, this study establishes a Peridynamics (PD) numerical model for casing pullout to conduct research and analysis on its process. Based on the characteristics of the cement sheath, a damage model for the cement sheath is proposed in this paper. Additionally, a kernel function is introduced to reflect the variation of long-range forces with the distance between material points, aiming to characterize the changes in the stiffness of bond constant within the horizon of material points. The mechanical responses of the cement sheath and casing at different phases of the casing pullout process are analyzed and characterized. The research results indicate that an increase in the casing-cement sheath thickness ratio promotes the damage at the casing-cement sheath interface, reducing the time required for casing pullout but inhibiting the expansion of cement sheath cracks. As the thickness ratio increases from 0.1 to 0.18, the degree of cement sheath damage decreases from 6.904% to 6.453%. The increase in Young's modulus of the cement sheath is negatively correlated with the time required for casing pullout, positively correlated with the interface failure rate, and it does not exhibit a linear relationship with the degree of cement sheath damage. Within the range of Young's modulus from 4 to 20 GPa, the maximum difference in cement sheath damage degree is 3.715%. In engineering practice, the Young's modulus of well cement and the thickness ratio of casing-cement sheath can be appropriately increased to enhance pullout efficiency.

Bamboo as a natural optimized fiber reinforced composite: interfacial mechanical properties and failure mechanisms

Bamboo is a typical natural fiber-reinforced composite with an optimized distribution of vascular bundles as reinforcement and parenchyma tissues as bio-matrix. The interfacial bonding performance between vascular bundles and parenchyma tissue is critical for the mechanical properties and failure mechanisms of bamboo. This study employed pull-out tests to determine the interfacial shear strength (IFSS) between bamboo vascular bundles and parenchyma tissue and evaluate the critical embedded lengths () of vascular bundles. The effects of embedded vascular bundle lengths on interfacial strength and failure behaviors were also investigated. The results revealed a value of 2.51 mm, lower than the majority of plant fiber-reinforced composite materials, with an IFSS of around 20 MPa, surpassing most artificial bamboo fiber composites. The pull-out process of the vascular bundles involved elasticity, debonding, and sliding friction stage, where debonding energy absorption (DEA) outweighed frictional energy absorption (FEA) and increasing with embedded length. The primary failure features include interfacial debonding, parenchyma tissue ripping, delamination of fiber thin layers, and fiber breakage. Moreover, the parenchyma cells between the two fiber sheaths and at the top of the bamboo block readily detached. The interfacial failure mechanisms of bamboo included debonding, reinforcement, and matrix failure, with the proportions varying as the embedded length increased. Quantitative analysis of the interfaces structure and mechanical properties between vascular bundles and parenchyma tissues could provide a reference for the biomimicry of bamboo structures and the manufacturing of natural fiber-reinforced composite materials.

Carbon fiber pull-out analysis for composite materials by considering interface properties

ABSTRACT An improved shear-lag model is adopted in this paper to study the fiber pull-out by considering interface properties in a representative volume element (RVE). The surface roughness, represented by grooves distributed around the fiber, is introduced to analyze the influence of roughness on fiber pull-out comparing it with a round fiber. The influence of groove shapes is also discussed. The interfacial stiffness is introduced in the improved shear-lag model by the cohesive zone modeling to simulate a practical interface connection. To represent the residual stress in composites, a pressure load is added on the surface of the RVE to analyze its influence on the fiber pull-out. The results show that the proposed theory can accurately predict the stress distribution in the RVE. Furthermore, the maximum reaction force increases with an improvement in roughness and interfacial stiffness, while the pressure can improve the reaction force in the third stage of fiber pull-out process. The proposed theory can predict the stress field and fiber pull-out process accurately. It can improve the efficiency and effectivity of micro-experiment with the reference of theoretical results. The results obtained using the proposed theory in this research can also be useful for the design and analysis of composite materials.

Experimental study and multi-objective optimization of the shear mechanical properties of recycled aggregate concrete with hybrid fibers and nano SiO2

Structural imperfections (microcracks, micropores, and weak interfacial transition zones) impact the shear mechanical properties of recycled aggregate concrete (RAC). Fibers and nanoparticles can mitigate these issues, enhancing the overall quality of recycled aggregates (RAs). Various hybrid systems, including basalt fiber (BF), polypropylene fiber (PPF), and nano-SiO2 (NS), were added to the RAC for double shear testing. Regression models were developed using the response surface method to optimize the hybrid systems. The study showed that the NS-BF-PPF hybrid system achieved the optimal shear strength, shear strain, shear toughness index, and shear fracture energy (6.99 MPa, 9.58 × 10−4, 0.716, and 34.603 J, respectively). These values were superior to those of the BF-PPF hybrid system and single system. The three-dimensional reticulated supporting structure devised by the BF-PPF effectively remedied the structural defects in the RA at various levels. Furthermore, the fibers integrated into the hybrid system act as bridges, absorbing shear energy during the pullout process. The incorporation of nanoscale NS stimulated the hydration response of the cement, resulting in a more compact pore structure within the RAC. Nonlinear multivariate regression models were fabricated utilizing the response surface method to maximize strength. The optimal contents of BF and PFF in the BF-PPF hybrid system were 0.344 % and 0.3 %, respectively. The optimal contents of NS, BF, and PPF in the NS-BF-PPF hybrid system were determined to be 3.5 %, 0.5 %, and 0.1 %, respectively. The combination of fibers and nanoparticles elicits a synergistic effect, significantly augmenting the shear performance of RAC.

Study of the tensile mechanical properties and failure mechanisms of CFRP screwed joints

Removable joint technology is commonly used in composite laminates for various load-bearing structures. However, existing research primarily focuses on bolted joints, there is relatively limited research on screwed joints in composite materials. This study investigates the influence of connected layer thickness and hole diameter on the tensile behavior of threaded joints in carbon fiber-reinforced polymer (CFRP) laminates. After fabricating different CFRP screwed joint specimens, tensile tests were conducted. The digital image correlation (DIC) technique captured the deformation process. The experiment results indicate a significant increase in load-bearing capacity with the increase in diameter. For instance, joints with an 8 mm diameter exhibited a load-bearing capacity of 10.82 kN. The increase in the connected layer thickness correspondingly enhanced the load-bearing capacity of the joint. The joint with a thickness of 7 mm had the highest load-bearing capacity of 8.83 kN. Besides, with the increase in the thickness of the connected layer, the failure mode transitioned from shear failure in the connected layer to screw pull-out. The tilt angle of the screw during the pull-out process also decreases with the increase in the connected layer thickness. Strain and out-of-plane displacement measurements under ultimate load conditions verify these observations.

3D meso-scale modeling of bond behavior between helical strand and concrete

Excellent interfacial bonding is the foundation for the strand and concrete working together. Helical structure of strand causes its rotation during the pull-out process, which makes it difficult to quantify the bond-slip at the interface. A three-dimensional (3D) meso-scale numerical model is first established to investigate the bond behavior at strand-concrete interface in this study. A refined modeling method is proposed to build a helical strand with the longitudinal helix shape and transverse flower-like shape. The heterogeneous concrete composed of three-phase composite material is built based on the random aggregate model. The interaction between strand and concrete is implemented by the surface-to-surface contact strategy. The rationality of the proposed model is verified by the experimental results. The effects of the helical structure of strand, the lay length of strand, aggregate content and eccentricity of strand on interfacial bond behavior are discussed. The results indicate that considering the helical structure of strand and heterogeneity of concrete can more accurately describe the stress distribution at the strand -concrete interface. The proportion of twisting slip is lower than that of longitudinal slip in the whole pull-out process, and its proportion increases first and then decreases. The interfacial bond strength increases first and then decreases with the increase of lay length of strand and aggregate content, and it reaches the best when the lay length is 14 time of diameter with the aggregate content of 45 %.

A pull-out equivalent telescopic layered failure model of carbon fiber bundles in seawater sea-sand cementitious matrix

Based on the excellent mechanical properties and piezoresistive effect of carbon fiber, carbon fiber reinforced cementitious matrix (CFRCM) has been used in the strengthening and self-monitoring research of reinforced concrete (RC) structures. In this study, the assumptions of the telescopic layered model theory of CFRCM were refined. The factors influencing the piezoresistive effect of carbon fibers were subsequently analyzed. Simultaneously, the resistance calculation theory for the pull-out process of CFRCM bundles was established by combining the parallel circuit theory. Based on the above theories, a pull-out equivalent telescopic layered failure model of carbon fiber bundles in the cementitious matrix was developed, considering the trilinear cohesive material law (CML) and the piezoresistive effect. Pull-out tests on carbon fiber bundles in both freshwater standard sand and seawater sea-sand matrices were conducted. This model was successfully used to simulate the piezoresistive effect during the pull-out process of CFRCM bundles with two matrices and different embedded lengths. The number of fiber filaments in each layer, the rupture damage of the fiber filaments during the loading process, and the degree of matrix infiltration into the carbon fibers were calculated. This work lays the foundation for advancing the development of structural strengthening and structural health monitoring.

A Study of Preload Detection Technology in Suspension Bridge Cable Clamp Bolts Based on the Pull-Out Method

The pull-out method is a simple and effective method for detecting the preload of suspension bridge cable clamp bolts. However, research on the pull-out method is currently limited. The force principles governing the bolt during the pulling process are unclear, and the relationship between tension force and the desired preload remains uncertain. This paper aims to explore the force principles of bolts during the pull-out method detection process through a combined approach of theoretical analysis, full-scale test, and finite element simulation. The results indicate that the bolt preload increases during the pulling process. The preload detected by the pull-out method is not the initial preload of the bolt, but rather it exceeds the initial preload. The force relationships among various components are determined as follows: the preload subtracts the change value of the force exerted by the nut at the tension end, which equals the change value of tension force. Additionally, an analysis of the impact of the length of the bolt clamping section and the bolt area on the preload was conducted. Under the same bolt area, a shorter clamping section length corresponds to a greater increase in preload. With the same clamping section length, the increment of preload increases with the bolt area. These findings can serve as references for detecting and specifying the preload of the bolts.

Failure mechanism of steel fiber pullout in UHPC affected by alternating cryogenic and elevated variation

This paper investigates the pullout response of straight and hooked-end steel fiber embedded in Ultra-high Performance Concrete (UHPC) subjected to alternating cryogenic (−170 °C) and elevated (200 °C) temperature variation. The temporal evolution of Acoustic Emission (AE) parameters and failure behavior during the pullout process were monitored in situ using the AE test. Additionally, changes in the UHPC microstructure were also evaluated. Results showed that in addition to the UHPC matrices exposed to elevated temperature, the compressive and flexural strength of UHPC matrices subjected to single or multiple cycles were generally higher than those at ambient temperature. Whereas the bond strength/pullout energy of straight fiber increased with a single cycling exposure of UHPC specimens to either cryogenic or elevated temperature, the improvement in the pullout performance of the hooked-end fiber was obtained only after the post-cryogenic thawing of specimens. Moreover, the exposure to an increased number of temperature-variation cycles caused the pullout strength and energy of both fiber groups to gradually decrease. Nonetheless, the pullout performance of the hooked-end fiber was significantly and consistently superior to that of the straight fiber, regardless of the exposure temperature or cycles. The AE test confirmed that the pullout process of both fibers was mainly characterized by tensile failure, with a marginal occurrence of shear failure for the hooked-end fiber. When the UHPC was exposed to various temperatures, the fiber pullout behavior was predominantly influenced by the physical structure of the fibers, thermal deformation proclivity of the matrix, and the competition between mechanisms such as pore moisture phase change, secondary hydration, and the composition of hydrates.