Search Results (186)

Post-installed rebars are extensively used in the strengthening and rehabilitation of concrete structures, where compressive stresses in the anchorage zone provide transverse pressure and significantly affect bond behavior. However, it remains unclear how different transverse pressure conditions, particularly one-way and two-way transverse pressure, influence the bonding behavior of post-installed rebars and how their effects differ. To address this gap, this study investigates the effects of one-way and two-way transverse pressure on the bond mechanism and failure mode of post-installed rebars. To achieve this, 22 pull-out tests were carried out under two transverse pressure configurations, namely one-way and two-way transverse pressure, with pressure levels ranging from 0 to 12 MPa. The results show that, without confinement, concrete splitting was the dominant failure mode, whereas under transverse pressure, failure shifted to adhesive failure or adhesive–rebar interface failure. Transverse pressure significantly improved bond strength, with maximum increases of 49.9% under one-way transverse pressure and 82.9% under two-way transverse pressure. Both the transverse pressure configuration and pressure level had a significant influence on failure evolution and bond performance. In general, increasing the pressure level enhanced the interfacial bonding capacity; however, one-way transverse pressure tended to induce stress concentration in the adhesive layer, thereby promoting adhesive-related failure. These findings clarify the role of transverse pressure conditions in the anchorage behavior of post-installed rebars and provide a basis for the design and analysis of post-installed rebar anchorage systems. Full article

Resin and interlayer properties play significant roles in the resistance to impact of fibre-reinforced polymer composites (FRPCs). To investigate the contribution of each factor within the coupled variables to the impact resistance ability of FRPCs, in this work, waterborne polyurethane (WPU) with different tensile elastic modulus, tear strength and bonding strength was obtained. To systematically evaluate the impact resistance and failure mechanisms of the composite materials under varying external loads, impact resistance tests, numerical simulations, and relative weight analysis were conducted. The relative weight analysis results quantified the individual contributions of these three factors to the overall energy absorption capacity across diverse loading conditions. The results indicated that with the increasing rate of the external loading, the resin modulus consistently contributed more significantly to energy absorption than tear strength of resin and interlayer strength, reaching up to 44.3%. In ballistic penetration tests, with the increase in resin modulus, the ballistic performance of PE/WPU laminates demonstrated an S-shaped downward trend. Composites prepared with more rigid matrix could lead to unsatisfactory interlayer damage. A more robust structure could result in fibre pull-out and breakage to a greater extent at the point of forced impact while less in the secondary affected area, presenting comparatively lower impact resistant performance. Full article

This study investigates the early-age bond behavior between steel reinforcement and lightweight alkali-activated concrete (LWA-AAC) using pull-out tests and modeling. Deformed and plain steel bars with different diameters were embedded in two LWA-AAC matrices to examine the effects of curing age, matrix strength, confinement, and bar surface geometry. The bond of plain bars is governed primarily by adhesion and friction and shows weak dependence on matrix strength or confinement. In contrast, the bond strength of deformed bars increases with curing age and matrix strength, while reduced confinement promotes a transition from ductile pull-out to brittle splitting failure. This confinement-sensitive transition highlights the dominant role of matrix tensile capacity in controlling bond stability in LWA-AAC. Compared with lightweight ordinary Portland cement (OPC) concrete, LWA-AAC exhibits more brittle bond behavior, characterized by smaller peak slip, steeper post-peak softening, and lower residual bond stress. Existing OPC-based bond models show limited applicability to LWA-AAC due to differences in failure mechanisms and confinement sensitivity. New empirical models incorporating matrix tensile strength and geometric confinement are proposed to predict bond parameters and bond–slip responses, providing a mechanism-informed basis for the design of reinforced LWA-AAC structures. Full article

This study numerically examined the anchorage mechanism of rebar hooks under varying straight development lengths, including high stress levels. A three-dimensional rigid body spring model (3D RBSM) was used for the investigation and successfully reproduced the experimental pullout test stress–slip relationships and inner–outer strain distributions for the rebar hook with and without a straight development length. A validated numerical model was used to assess local concrete stresses and internal crack propagation, enabling a clear interpretation of how straight development length influences the anchorage mechanism. The results revealed that increasing straight development length increases stiffness, reduces rebar strains and concrete stresses in the hook region, promotes crack formation around the rebar surface, and forms maximum tensile stresses closer to the top surface, ultimately resulting in earlier splitting failure at high rebar stress levels. A comparison of cases with and without hooks shows that combining the hook with straight development length improves stress distribution, delays crack propagation, and increases anchorage by reducing tensile stress concentrations near the top surface and side faces. These findings provide valuable insights into the role of straight development length in the anchorage performance of 180-degree rebar hooks. Full article

This study presents an experimental investigation into the effects of subsurface kerosene contamination on the behavior of model piles in clay. Three instrument piles were placed in a compacted clay bed and tested in three saturations in a two-week period: natural moisture content, water-saturated, and kerosene-flooded. The tests performed on the piles were axial compression and pullout tests, supported by unconfined compression and consolidation tests for the clay. The saturating fluid controls the pile capacity, with the water-saturated condition producing the lowest ultimate capacity and the natural condition the highest. Moreover, it was shown that kerosene contamination significantly modified the mechanical characteristics of the clay, thus leading to an order of magnitude increase in the pre-consolidation pressure and a decrease in the compression index by 50% compared to the water-saturated cases. Under pullout loading, the unit skin friction varied linearly with depth, and failure consistently occurred at the pile–soil interface. This demonstrates the applicability of geotechnical design that considers hydrocarbon contamination to significantly decrease pile capacity and substantially change the consolidation characteristics of clayey soils. Full article

Rod-end spherical bearings are widely used in heavy machinery, wind power, and transportation. Their bolted connections directly determine structural safety but are prone to pull-out failure under maximum articulation angle and heavy load. This study employs finite element (FE) simulation to elucidate the failure mechanism and, combined with Timoshenko beam theory, systematically analyses the effects of end cap parameters (size, height, modulus) on bolt head lateral force and bending moment. Results show that two-piece end caps induce abnormal contact and severe stress concentration under combined lateral and axial loads. A spigot design with optimised bolt number and contact geometry is proposed, reducing the additional bending moment from $1.882\times {10}^4$ N·mm to $2.193\times {10}^3$ N·mm and lateral load from $8.236\times {10}^5$ N to $7.092\times {10}^4$ N (over 88% reduction), bringing stress within a safe range. Although the numerical analysis was not directly verified experimentally, the experimental confirmation of the design’s functionality supports the optimisation. This study clarifies the pull-out mechanism and provides insight for anti-pull-out connections under high lateral forces. Full article

This study investigates the fatigue failure of fiber-reinforced rubber used in automotive shock-absorbing elements subjected to cyclic loads. A quantitative simulation model integrated with material analysis to predict the service life and performance decay of these viscoelastic dampers was introduced. Failure is governed by a degradation factor that models accumulating fatigue damage and results in a predictable, cyclic loss of maximum force capacity; specifically, the model accurately predicts a 36.3% reduction in peak force (from 111.44 N to 70.97 N) over the first 10 fatigue cycles. Crucially, the model incorporates the non-linear stiffness behavior caused by a fiber pull-out mechanism, which transitions load resistance from high elastic integrity to lower frictional forces post-critical displacement. These findings establish a direct, quantitative link between microstructural failure (verified via SEM) and observed performance decay, offering key insights for maintenance planning and material selection. Full article

This study investigates the influence of fiber bridging on the interlaminar strength of carbon fiber-reinforced polymer (CFRP) made from recycled carbon staple fiber yarn (rCF), compared to CFRP made from new fibers (vCF). Double-cantilever beam (DCB) tests measure the resistance of both materials against crack formation and the corresponding energy release rate (ERR). Several microscopic tools (SEM, CT) were then used to analyze the fracture surfaces and characterize the underlying failure mechanisms of the fiber bridges. The resulting ERR of rCFRP is four times (2140 J/m² compared to 587 J/m²) higher than that of vCFRP. SEM images of the fracture surface reveal that the fracture mechanism is fiber debonding followed by fiber pull-out with constant friction. This finding is confirmed by calculating the fiber bridging stress using the mathematical formulation of this effect resulting in a fiber bridge tension of approximately 70 N/mm². The main reason for the increased ERR of rCFRP compared to vCFRP is the extensive occurrence of fiber bridges in rCFRP due to the inhomogeneity of the rCF roving. This results in a pronounced nesting effect between adjacent rCF layers. The influence of the nesting effect on the ERR was investigated by testing samples with an increased layer orientation difference of 3° and 5°. This results in an ERR decrease of 26% in rCF and 30% in vCF. The nesting effect can be eliminated in vCFRP, but in rCFRP higher layer orientation, nesting is still visible. This finding suggests that the coarse, inhomogeneous structure of the rCFRP roving causes nesting regardless of the layer orientation and leads to a pronounced tendency to form fiber bridges. Full article

The rising environmental concerns over cement-based construction materials have led to the development of sustainable alternatives. Among these, geopolymers represent a promising class of low-carbon binders offering environmental benefits and competitive mechanical properties; however, their intrinsic brittleness limits their tensile and post-cracking performance. This study investigates the adoption of flax fibers as natural reinforcement to enhance ductility and post-peak behavior of metakaolin-based geopolymers. The performance of metakaolin-based geopolymers with flax fibers (MKFLAX) was experimentally evaluated in terms of strength, stiffness, toughness, and failure behavior. The addition of flax fibers enhanced ductility, toughness, and post-peak load-carrying capacity while slightly improving stiffness due to the bridging of cracks and the fiber pull-out mechanism. In comparison with the available literature on sisal, flax, and jute fibers, flax fibers showed improved performance due to the better dispersion within the matrix and higher tensile modulus. These findings highlight that flax fiber-reinforced metakaolin geopolymers show enhanced post-cracking behavior at the laboratory scale and could be of interest for sustainable cementitious materials, subject to further validation at the structural scale. Furthermore, a nonlinear finite element model was adopted based on damage mechanics to simulate the damage localization, stress–strain response and post-peak behavior of geopolymer composites. The numerical results showed a reasonable agreement with the experimental trends, particularly in the elastic and early softening phases. The findings are limited to the studied material system, fiber content, and small-scale samples and should be viewed as trend-level observations rather than generalized performance claims. Full article

Polyester yarn bundle geogrids are widely used materials in flexible retaining structures due to their high toughness and high-strength mechanical properties. To investigate the mechanical characteristics and the interfacial mechanical properties of these geogrids, a series of pull-out tests were conducted under different pull-out rates and filling water contents. Based on the test results, a DEM-FDM coupled numerical model for pull-out behavior was established to analyze the pull-out deformation behavior of the geogrids. Combined with the theoretical analysis of the load-bearing characteristics of the geogrids, the reinforcement mechanism of polyester yarn bundle geogrids was revealed. The results show that there exists a critical pull-out rate of 1 mm/min that maximizes the pull-out resistance; the interface friction angle decreases with an increase in pull-out rate, while the interface cohesion shows an opposite trend. The filling water content presents a more significant weakening effect on the soil–geogrid interface strength under low stress, resulting in a strain-softening type of pull-out curve. Unlike fine-ribbed plastic geogrids, the sliding frictional resistance of polyester yarn bundle geogrids accounts for 80% of the total pull-out resistance during the pull-out process. The mechanical interlocking force, which arises from the bulges on the mid-section of transverse ribs and the downward bending of longitudinal rib edges, is subject to dynamic changes in the course of the pull-out process. The geogrid exhibits overall shear failure under low normal stress (${\sigma }_n<$ 200 kPa) and penetration shear failure under high normal stress (${\sigma }_n\ge$ 200 kPa). In practical engineering installation, polyester yarn bundle geogrids should be placed as parallel as possible to maximize the frictional resistance with filled soil and should take care of the geogrid joints for enhanced durability of the geogrids. Full article

This study investigates the mechanical performance and surface morphology of polyamide-based materials commonly used in plastic injection molding. Two resins, PA6 and PA66, were analyzed in both neat and 20 wt% glass fiber-reinforced (GF20) forms. The influence of reinforcement and material type on tensile strength and ductility was examined through integrated experimental and numerical approaches, complemented by microstructural and elemental analyses. PA6 and PA66 specimens were produced in accordance with ISO 527, and tensile tests revealed a significant increase in elastic modulus and tensile strength with glass fiber reinforcement, accompanied by a reduction in elongation at break. Flammability was evaluated via Glow Wire and Tracking tests. SEM–EDS analyses provided insights into fracture morphology and elemental distribution, showing that fiber–matrix interfacial debonding and fiber pull-out dominated failure in reinforced specimens, whereas neat polymers exhibited homogeneous surfaces. Finite element simulations performed in ANSYS Explicit Dynamics supported the experimental findings by identifying stress concentration zones and failure initiation regions. Although numerical simulations successfully captured stress distribution trends, quantitative differences were attributed to idealized modeling assumptions and processing-induced microstructural effects. Overall, this work provides a comprehensive assessment of the reinforcement effects in PA6 and PA66 systems, offering valuable guidance for material selection and design optimization in polymer-based engineering components. Full article

In the present study, the effects of electrode geometry and welding current on the tensile-shear strength, failure energy, fracture type, and joint microstructure were investigated during the RSW of ultra-high-strength MS1500 steel to high-strength low-alloy SPFC590 steel, both 1.2 mm in thickness. Three electrode geometries—designated as G0-6 mm, G0-8 mm, and A0—recommended for 1.2 mm sheets according to ISO 5821 were examined. It was found that in the G0-6 mm electrode geometry, which has the smallest contact area, excessive expulsion occurred at lower current levels compared to the other geometries. Consequently, this configuration resulted in lower maximum tensile-shear strength and failure energy values. The highest mechanical performance was achieved with the G0-8 mm electrode geometry, where the tensile-shear strength and failure energy were measured as 19.42 kN and 43.81 J, respectively. For the A0 electrode, although the maximum tensile-shear strength (19.68 kN) was comparable to that of the G0-8 mm geometry, the failure energy was approximately 7% lower (40.94 J). For all electrode geometries corresponding to maximum mechanical strength, a pull-out failure mode was observed, where the nugget region of the SPFC590 steel detached from the base metal and remained adhered to the ultra-high-strength MS1500 sheet. Full article

This study evaluated the ballistic performance and failure mechanisms of epoxy-based hybrid laminates reinforced with abaca/UHMWPE and pineapple leaf fiber (PALF)/UHMWPE fabrics fabricated by using vacuum-assisted hand lay-up. Ballistic tests utilized 9 mm full metal jacket (FMJ) rounds (~426 m/s impact velocity) under NIJ Standard Level IIIA conditions (44 mm maximum allowable BFS). This experimental test was complemented by finite element analysis (FEA) incorporating an energy-based bilinear fracture criterion to simulate matrix cracking and fiber pull-out. The results showed that abaca/UHMWPE composites exhibited lower backface signature (BFS) and depth of penetration (DOP) values (~23 mm vs. ~42 mm BFS; ~7 mm vs. ~9 mm DOP) than PALF/UHMWPE counterparts, reflecting superior interfacial adhesion and more ductile failure modes. Accelerated weathering produced matrix microcracking and delamination in both systems, reducing overall ballistic resistance. Scanning electron microscopy confirmed improved fiber–matrix bonding in abaca composites and interfacial voids in PALF laminates. The FEA results reproduced major failure modes, such as delamination, fiber–matrix debonding, and petaling, and identified stress concentration zones that agreed with experimental observations, though the extent of delamination was slightly underpredicted. Overall, the study demonstrated that abaca/UHMWPE hybridcomposites offer enhanced ballistic performance and durability compared with PALF/UHMWPE laminates, supporting their potential as sustainable alternatives for lightweight protective applications. Full article

The article presents a comparative analysis of mechanical properties of M8 threaded joints produced using three different methods, in rectangular nylon (PA 12) specimens manufactured in SLS technology. Threaded holes in specimens were made by direct thread printing (specimens marked PT), thread reinforcement with Helicoil inserts (HT), and the use of heat-set inserts (IT). The specimens were subjected to a tensile testing at a constant displacement rate of 2 mm/min. The maximum force and the displacement at failure were recorded. The results indicated that the lowest load-bearing capacity F_MF was observed in the printed thread specimens, with an average value of 3.41 kN. The use of heat-set inserts increased F_MF to 3.83 kN, representing a 12% improvement. The highest load-bearing capacity was achieved in specimens reinforced with Helicoil inserts, which enhanced joint strength by 40% compared to printed thread specimens, reaching an average F_MF of 4.78 kN. In all cases, failure occurred due to the thread or insert pull-out from the specimen material. Studies have shown that the use of metal inserts significantly enhances the strength of threaded joints in SLS-printed PA12 components. Helicoil inserts provide the highest F_MF load capacity, while heat-set inserts offer better technological advantages. Although printed threads are easier to manufacture, their applicability is limited to larger thread sizes and lower mechanical loads. Full article

The embedded through-section (ETS) technique is a promising method for fiber-reinforced polymer (FRP)-strengthening reinforced concrete (RC) structures, offering higher bond resistance and reduced surface preparation compared to externally bonded or near-surface mounted FRP systems. A common failure in ETS applications is debonding at the FRP bar-to-concrete interface. However, current design standards often assume uniform bond stress and lack predictive models that account for debonding propagation and its effect on load capacity. Furthermore, a detailed analysis of interfacial stress development, including debonding initiation and progression along varying bond lengths, remains limited. To address these gaps, this study introduces an analytical model that describes the complete debonding process in ETS FRP bar-to-concrete joints, incorporating both long and short bond lengths and frictional effects. Based on a trilinear cohesive material law (CML), closed-form expressions are deduced for the load–slip response, maximum load, interfacial shear stress and strain distribution along the FRP bar. The proposed model is validated experimentally through pull-out tests on glass FRP (GFRP) bars adhesively bonded to concrete with different strength grades. The results show that the analytical predictions agree well with both the self-conducted experimental data for short joints and existing test results for long joints given in the literature. Therefore, the developed design-oriented solution enables accurate evaluation of the actual contribution of ETS FRP reinforcement to RC members by explicitly modeling debonding behavior. This provides a rigorous and mechanics-based tool for performance-based design of ETS FRP-to-concrete joints, addressing a critical gap in the future refinement of current design standards. Full article