Search Results (341)

To investigate the effect of print orientation (0°, 45°, and 90°) and artificial aging on flexural strength and fatigue resistance of 3D-printed denture bases compared to CAD/CAM milled controls, we fabricated 320 maxillary complete dentures, divided into 8 groups based on the fabrication method: horizontal, oblique, and vertical printing, alongside milled controls. Half of the specimens in each group were pre-conditioned via thermocycling and 240,000 cycles of chewing simulation. All specimens underwent static flexural strength testing and cyclic fatigue testing, followed by SEM fractography. The CAD/CAM milled bases demonstrated the highest mechanical durability, with non-aged specimens peaking at 149.43 ± 5.35 MPa. The horizontally 3D-printed non-aged specimens yielded the highest flexural strength (101.14 ± 4.80 MPa), while vertically printed aged specimens recorded the lowest (70.35 ± 8.18 MPa). Artificial aging degraded flexural strength uniformly across all orientations. Conversely, cyclic loading disproportionately devastated the older people’s vertical group, resulting in a 70% fracture rate. Fractography corroborated these findings, revealing severe interlaminar delamination in vertical builds, contrasting with cohesive, trans-layer fractures in horizontal prints. In conclusion, Horizontal orientation provided improved structural durability; however, CAD/CAM milled dentures remain superior and are recommended for long-term clinical applications. Full article

Carbon fiber textile-reinforced engineered cementitious composite (CTR-ECC) is widely utilized in structural strengthening applications due to its advantages of low weight and high strength. A comprehensive understanding of its mechanical behavior under off-axis tension is crucial for addressing the prevalent off-axis stress states in engineering practice. This paper presents an experimental investigation on the off-axis tensile properties of CTR-ECC. Specimens were fabricated with four off-axis angles: 0°, 15°, 30°, and 45°. The study revealed three main findings: (1) Under axial (0°) loading, failure is characterized by yarn fracture and interface slip, whereas off-axis tension induces a stable progressive delamination failure in textile-reinforced ECC systems. (2) While CTR-ECC exhibits higher tensile strength than plain ECC at all angles, its strength decreases significantly as the off-axis angle increases (e.g., a 27.1% reduction at 15°). Off-axis layouts, however, substantially improve energy absorption, with strain energy density increasing by up to 368.4% at 30°. (3) A phenomenological constitutive model was developed, which can adequately capture the stress–strain response of CTR-ECC under various off-axis angles, with coefficients of determination (R²) exceeding 0.9 in all cases. These results provide important insights into the failure mechanisms and performance design of CTR-ECC under off-axis tension conditions. Full article

Carbon fiber-reinforced polymer (CFRP) composites are widely employed in the aerospace industry due to their excellent properties such as high specific strength and corrosion resistance. However, the delamination and tearing of composites are prone to occur in the machining of CFRP, which significantly affect its performance. The existing laser-assisted cutting model generally simplifies the machining process into high-temperature conventional cutting, and only reflects the thermal effect by modifying the material parameters. The core selective ablation characteristics of laser–CFRP interaction are completely ignored, and the unique mechanical behavior of bare fiber under a large cutting angle is not modeled, and the quantitative correlation between cutting force evolution and machining damage is lacking. In this study, an innovative method of partially exposing fibers is proposed to simulate laser-assisted machining. A micromechanical model is developed to analyze the removal mechanisms of different phases during CFRP processing, and a cutting force prediction model from the micro to macro scale is also established. At the micro-scale, a micromechanical model for fiber cutting in orthogonal machining of CFRP is constructed based on the elastic foundation beam theory. The results show that the proposed cutting force prediction model has high reliability, and the relative error between the predicted value and the experimental measured value is only 7.81%~8.99%. All experiments were repeated three times. Statistical analysis showed that the repeatability of the results was excellent. Compared with conventional cutting, laser-assisted cutting fundamentally changed the failure mode of the fiber from matrix-constrained crushing fracture to controllable free-end large-deflection bending fracture. This transformation leads to a smoother and more regular fiber fracture surface, which effectively inhibits fiber breakage, matrix tearing, and fiber–matrix interface debonding. Quantitative analysis confirms that under laser-assisted processing conditions, the matrix tearing length is positively linearly correlated with the cutting depth, cutting speed, and bare fiber length. Full article

This work investigates the Mode I fracture behavior of adhesive joints manufactured from unidirectional carbon fiber-reinforced epoxy composites (CFRP) under static and fatigue loading. Specimens were exposed to two degradation environments: hygrothermal conditions (60 °C, 70% RH) and saline conditions (35 ± 2 °C, 89% RH), for 1 and 12 weeks, and compared with non-exposed material. Double Cantilever Beam (DCB) tests were conducted to evaluate the influence of aging on fracture toughness. Thermal (Differential Scanning Calorimetry, DSC) and spectroscopic (Fourier Transform Infrared Spectroscopy, FTIR) analyses were performed to identify degradation mechanisms. DSC results showed no significant variation in glass transition temperature (T_g) under saline exposure, whereas hygrothermal aging increased T_g, indicating post-curing effects. FTIR analysis revealed moisture uptake and oxidation under saline conditions, while hygrothermal exposure mainly led to structural rearrangement. Critical energy release rate (G_IC) values were used to define fatigue test conditions, enabling the construction of fatigue initiation (ΔG–N) and crack propagation (G–da/dN) curves. A Weibull-based model was applied to describe fatigue initiation behavior. Results show that saline exposure promotes progressive degradation, whereas hygrothermal conditions may enhance performance due to post-curing effects. Full article

The intralaminar and interlaminar mode I initiation fracture toughness of unidirectional IM7/8552 carbon/epoxy composites were re-evaluated using only the published experimental data. Classical statistics, two-parameter Weibull analysis (location fixed at zero), non-parametric kernel density estimation (KDE), bootstrap resampling (10,000 replications), and bootstrap-based uncertainty quantification were applied to the fatigue-precracked (FPC) initiation values (n = 12) and the corresponding R-curves. The pooled FPC mean initiation toughness was 0.1982 kJ/m² (COV = 8.50%). Weibull fitting yielded a shape parameter β = 12.33 and scale η = 0.2058 kJ/m², providing a B-basis value of 0.1715 kJ/m² (90% reliability) and an A-basis value of 0.1417 kJ/m² (99% reliability). The Kolmogorov–Smirnov test confirmed statistical equivalence between intralaminar and interlaminar groups (p > 0.05), validating the use of a single initiation toughness for both crack planes when sharp fatigue-precracked starter cracks are employed. Intralaminar R-curves exhibited significantly steeper propagation, rising to approximately 0.385 kJ/m² at Δa = 30 mm due to extensive fiber bridging, whereas interlaminar R-curves reached a near-plateau after 12–15 mm. Bootstrap 95% confidence bands quantified the higher uncertainty associated with the intralaminar R-curve. Teflon-insert data produced artificially high initiation values and unstable growth, confirming that only fatigue-precracked results are suitable for design allowables. This study demonstrates that a single, statistically robust initiation toughness (B-basis = 0.1715 kJ/m²) can be used interchangeably for intra- and interlaminar cracking in progressive-damage models and preliminary design analysis of IM7/8552 structures. The open-source statistical workflow (KDE + bootstrap) developed here is transferable to other small-sample composite datasets, though the numerical B-basis value (0.1715 kJ/m²) is specific to IM7/8552 and should not be generalized without validation. Full article

Acoustic emission (AE) was employed to characterize the mechanical behavior of repaired multi-layered woven lattice sandwich composite (MWLSC) in this paper. A patch repair strategy was adopted, in which damaged cores were reconstructed with polyurethane foam and fractured face sheets were restored using fiber fabric. Mechanical recovery was evaluated through mechanical testing, and AE monitoring was used to analyze damage evolution before and after repair. The repaired double-layered and triple-layered warp specimens recovered 123% and 104% of their original peak load, respectively, while the triple-layered weft specimen recovered 83%. Compared with pristine specimens, repaired MWLSC exhibited reduced cumulative AE counts and lower proportions of high-energy events. Continuous wavelet transform analysis revealed that the high-frequency components associated with interfacial delamination were significantly diminished after repair. These results indicate that repair modifies the dominant failure mechanism, shifting from delamination-dominated fracture toward core-related damage. The study demonstrates the effectiveness of AE techniques in capturing changes in damage evolution and mechanical response in repaired MWLSC. Full article

This study presents a novel methodology integrating high-resolution X-ray computed tomography, digital volume correlation and statistical visualisation mapping, to perform microscale observations and correlate delamination fracture mechanisms in heterogeneous materials. To demonstrate the utility of this integrated approach, it is applied to study the damage behaviour of aerospace carbon/epoxy composite laminates with an open hole, subjected to quasi-static tension and fatigue at a load ratio of 1:10. The study also explores the applicability of a Paris law type relationship to determine effective macroscopic fatigue delamination resistance in the load-bearing plies. The X-ray imaging for both load cases revealed extensive formation of delaminated fracture surfaces surrounding both glass fibre interlacing weaves and entrained voids within them, acting as preferential sites for localised strain hot spots. It is demonstrated that local energy dissipation is governed by the recurring weave pattern and topological order, which can help explain the typical damage state in quasi-static behaviour, establishing a direct link between microstructural features and macrostructural material response. Full article

Carbon fiber-reinforced polymer (CFRP) laminates are susceptible to barely visible impact damage (BVID) under low-energy bird-strike-like conditions. However, in previous studies, most damage evaluations for BVID were limited to a single scale. In this work, a multiscale characterization and evaluation method integrating the analytic hierarchy process (AHP) and the CRITIC weighting method was proposed to investigate the damage evolution of CFRP laminates under low-energy impacts (approximately 12–33 J). Delamination area (S_Da), indentation depth (P_D), surface crack aspect ratio (R_A), energy dissipation, and compression-after-impact (CAI) strength were analyzed based on phased-array ultrasonic C-scanning, 3D optical profilometry, and scanning electron microscopy. The results showed that P_D, S_Da, and energy dissipation increased from 108.73 μm to 213.93 μm, from 228.6 mm² to 695.8 mm², and from 5.96 J to 21.40 J, respectively, with increasing impact energy. Meanwhile, CAI strength decreased from 202.2 MPa to 118.9 MPa, with a maximum degradation rate of 41.16%. A critical transition was observed in the medium-to-high energy range, where delamination growth gradually plateaued, while intralaminar cracking and fiber fracture became increasingly dominant. The proposed framework enables quantitative grading of BVID severity and provides a practical basis for assessing residual damage in impacted CFRP laminates. Full article

Aim: Fixed dental prostheses (FDPs) are increasingly fabricated from high-strength ceramic materials; however, their fracture behavior under flexurally dominated loading remains incompletely understood. This in vitro study aimed to compare the mechanical performance and fracture mechanisms of four FDP material systems under standardized bending conditions. Materials and Methods: Three-unit CAD/CAM-fabricated FDPs were produced from metal-ceramic (P1), zirconia-ceramic (P2), monolithic zirconia (P3), and monolithic lithium disilicate (P4) materials (n = 9 per group). Specimens were subjected to three-point bending until failure. Crack initiation load, maximum load, displacement, and stiffness were recorded, and fracture behavior was analyzed using stereomicroscopy, micro-computed tomography (μCT), and scanning electron microscopy (SEM). Results: Metal-ceramic FDPs (P1) exhibited the highest crack initiation load (0.89 kN) and maximum load (1.91 kN), with failure predominantly occurring through ceramic veneer delamination without complete framework fracture. Monolithic zirconia FDPs (P3) demonstrated the most brittle failure behavior, characterized by abrupt fracture and unstable crack propagation immediately after crack initiation. Zirconia-ceramic (P2) and lithium disilicate (P4) FDPs showed intermediate mechanical performance, with lithium disilicate exhibiting greater resistance to catastrophic failure (F_max = 0.94 kN) compared with zirconia–ceramic FDPs. Conclusions: These findings refine current assumptions regarding the mechanical reliability of monolithic zirconia FDPs under flexural loading and highlight the importance of fracture behavior, rather than peak strength alone, in material selection. Lithium disilicate and metal-ceramic systems exhibited more favorable damage-tolerant responses under static flexural loading. These findings should be interpreted within the limitations of this in vitro model and should not be directly extrapolated to long-term clinical performance. Full article

Carbon fiber-reinforced polymers (CFRP) are widely employed in advanced manufacturing sectors such as aerospace, wind energy, and new energy vehicles owing to their high specific strength and stiffness. The growing demand for lightweight, high-performance, and multifunctional materials has accelerated the development of structurally and functionally integrated CFRP. Introducing functional interlayers between composite laminates is an effective strategy to impart additional functionalities; however, such interlayers are often multi-component and structurally complex. A critical challenge remains to integrate functionality without compromising, and preferably enhancing, the load-bearing capability of CFRP, particularly their resistance to interlaminar delamination. In this study, electrically heated CFRP incorporating a sandwich-structured interlayer composed of glass fiber mesh fabric/CNT veils doped with carbon nanotubes/glass fiber mesh fabric (GF/CNTs-CNTv/GF) was investigated. The effects of interlayer architecture and CNT loading on the Mode II interlaminar fracture toughness were systematically examined. Delamination failure modes and interlaminar toughening mechanisms were analyzed using scanning electron microscopy and ultra-depth-of-field three-dimensional microscopy. The results demonstrate that an optimal CNT pre-impregnation concentration of 1.0 wt% yielded a maximum G_IIC of 1644.8 J/m², corresponding to a 103.06% increase relative to the reference laminate. The enhanced performance is attributed to simultaneous optimization of interfacial “nano-engineering” effects, including matrix toughening and a pronounced “nano-anchoring” mechanism induced by CNT. These effects promote a transition in failure mode from weak interfacial debonding to a mesh-block composite delamination pattern, thereby activating multiple energy-dissipation mechanisms such as crack deflection, fiber pull-out, rupture, and bridging. This work highlights the effectiveness of CNT-modified sandwich interlayers in improving delamination resistance and provides both theoretical insight and experimental validation for the design of multifunctional CFRP with superior interlaminar fracture toughness. Full article

With the opening of complex service routes, the importance of the service performance of brake pads under long slope braking conditions is increasing. It is necessary to analyze the slope braking adaptability of current brake pad products. This work takes the copper-based powder metallurgy brake pads of a certain in-service high-speed train as the research object and conducts friction and wear behavior tests of the brake pads based on a full-scale brake test bench. Through microscopic observation and damage analysis, the differences in friction and wear behavior of the brake pads under stop braking and slope braking conditions are compared, revealing the wear mechanism and damage evolution characteristics of the brake pads. The results show that under the impact of high speed, high braking force, and severe thermal load in the stop braking conditions, the uneven wear of brake pads is high, and the eccentric wear of friction blocks is affected by both the friction radius and friction direction. The friction surface has a large number and size of damages, and the stability of the friction interface is poor. The brake pad exhibits a composite wear mechanism dominated by abrasive wear and brittle fracture induced exfoliation. In the slope braking condition, under the action of low speed, low braking force, and long-term stable thermal load, the uneven wear of the brake pads is relatively low, the surface damage size is small, and the friction block only has eccentric wear along the friction direction. The brake pad mainly initiates cracks along the interface of the components, which propagate parallel to the friction surface, exhibiting a progressive delamination and flaking exfoliation mechanism with a low wear rate. Although the friction interface of the brake pad is relatively stable under slope braking conditions, the cumulative delamination wear of the brake pads under long-term braking action needs further attention. Full article

Repeated impact loading can induce progressive fatigue delamination in composite laminates, in which both damage accumulation and strain-rate sensitivity of the interlaminar interface play important roles. In this work, an adopted rate-dependent fatigue cohesive formulation is extended to a three-dimensional framework for simulating interlaminar delamination in composite laminates subjected to repeated impact. The constitutive formulation incorporates separation-rate-dependent critical tractions and fracture toughness together with cumulative fatigue damage, enabling a unified description of dynamic rate effects and progressive interface degradation. A time-incremental algorithm is developed and implemented in ABAQUS 2020/Explicit through a user-defined cohesive element subroutine (VUEL). The cohesive formulation is further coupled with the Hashin intralaminar failure criterion to represent the interaction between interlaminar delamination and intralaminar damage. Numerical simulations are conducted for composite laminates with three structural configurations—conventional, drop-off, and wrapped drop-off—to systematically examine the influence of rate dependence on fatigue delamination under repeated impact. The results show that the developed framework captures the progressive evolution of delamination and impact response under repeated impact and indicate that the sensitivity to rate-dependent interlayer properties depends on both laminate configuration and impact velocity. The present study provides a feasible computational framework for the comparative simulation and assessment of fatigue delamination under repeated impact and offers numerical insight into the role of structural configuration and interfacial rate dependence in composite laminates. Full article

MoS₂ acts as a high-performance lubricant, enhancing friction material stability, reducing wear and noise under extreme conditions, and preserving friction pair performance. However, its tendency to decompose and poor matrix wettability make surface modification essential for effective use in Cu-based composites. In this study, comprehensive investigations combining macro-scale and micro-scale friction experiments were conducted to examine the interfacial friction behavior of MoS₂ with different coatings and its tribological effects on copper-based composites under varying braking energy densities. The results indicate that the nickel coating suppressed MoS₂ decomposition, forming a high-strength diffusion interface with the matrix. This enhances the frictional stability and suppresses interfacial defect formation during micro-friction tests. However, the copper coating formed a poor-strength diffusion-reacting interface with matrix, leading to unstable friction at the interface and interface failure. Coating-dependent interfacial properties and micro-friction behaviors lead to varying tribological performance in Cu-based composites with MoS₂ during macro-friction tests. Nickel-plated MoS₂ (MoS₂@Ni) exhibits superior lubrication and frictional stability. The friction coefficients of Cu-based composites with MoS₂@Ni under low, medium and high working conditions are 0.36, 0.3 and 0.24, respectively, which are 6%, 12% and 13% lower than those of copper-plated MoS₂ (MoS₂@Cu). Meanwhile, its friction stability is 0.8, 0.6 and 0.58, respectively. With rising braking energy density, wear in Cu-based composites transitions from ploughing to oxidation and then to delamination. Defective MoS₂@Cu/matrix interfaces intensify delamination wear caused by the unstable fracture of subsurface plastic deformation layer cracks at higher energy density. Full article

To address the engineering challenges of frequent flexural deformation and instability of composite roadway roofs and the difficulty in accurately controlling the support strength range during deep coal mining, this study takes the soft–hard interbedded composite roof of the working face in the West No. 1 Mining Area of Shuangyang Coal Mine in Shuangyashan as the engineering background. Typical fine sandstone (hard rock) and tuff (soft rock) from the on-site roof were selected to prepare layered composite specimens, and indoor four-point bending tests were conducted. Combined with theoretical calculations, strain monitoring, and acoustic emission (AE) real-time localization technology, the regulatory mechanisms of three key factors—lithological combination, loading rate, and span—on the flexural mechanical properties, deformation and failure modes, and fracture evolution laws of layered composite rock masses were systematically investigated. The research results show the following: (1) The flexural performance of layered composite rock masses is dominated by the interlayer interface effect. Their flexural strength is 46.7% and 41.1% lower than that of single hard rock and soft rock specimens, respectively, and the competitive mechanism between interface slip and delamination fracture is the core inducement of strength deterioration. (2) The strength and deformation characteristics of layered composite rock masses exhibit a significant loading rate effect. When the loading rate increases from 0.002 mm/s to 0.02 mm/s, the flexural strength decreases by 51.8% and the mid-span deformation deflection reduces by 50.1%. High loading rates will exacerbate the deformation mismatch between soft and hard rock layers, trigger premature failure of interface bonding, and inhibit the full development of structural plastic deformation. (3) Increasing the span significantly optimizes the flexural bearing performance of layered composite rock masses. When the span increases from 170 mm to 190 mm, the flexural strength increases by 65.7% and the mid-span deformation deflection synchronously increases by 65.7%. A large span can extend the flexural deformation path, promote the coordinated deformation of rock layers, and suppress local stress concentration. (4) The flexural failure of layered composite rock masses is dominated by Mode II shear cracks, while single-lithology specimens are mainly dominated by Mode I tensile cracks. Loading rate and span significantly change the crack propagation mode and energy release law. This study establishes a calculation method for the equivalent flexural stiffness of layered composite rock masses and reveals the mesoscopic mechanism of flexural failure of heterogeneous layered rock masses. The research results can provide a theoretical basis and experimental support for the optimization of support schemes and the prevention and control of roof collapse hazards for composite roofs of deep coal mine roadways. Full article

Fused deposition modeling/fused filament fabrication (FDM/FFF) enables architectural tailoring of mechanical response through layer configuration and multi-material manufacturing strategies. However, the combined effects of layer arrangement, infill ratio, and packing geometry in polymer–metal hybrid structures and interfacial load transfer mechanisms are still not sufficiently elucidated. In this study, the tensile behavior of single- and multi-material structures produced using PLA and 17-4 PH stainless steel filaments was systematically investigated. A total of 24 experimental parameter sets were created with four-layer configurations (PLA, 17-4 PH, PLA/17-4 PH/PLA, and 17-4 PH/PLA/17-4 PH), three infill ratios (20%, 60%, and 100%), and two packing patterns (linear and hexagonal); the samples were tested according to the ASTM D638 standard. Mechanical data were modeled using Response Surface Methodology (RSM) and ANOVA, and the developed regression models showed high accuracy (R² > 0.95). The findings showed that tensile and yield strength are primarily controlled by the layer arrangement, while infill ratio and infill pattern have a secondary effect. The highest strength was measured in 100% infill linear PLA samples (≈10.35 MPa), and the lowest value was measured in 17-4 PH “green part” samples without sintering (≈0.92 MPa). Hybrid structures exhibited intermediate performance in the range of 2.9–4.9 MPa. ANOVA results showed that the majority of the mechanical variance was explained by the layer arrangement (70–85% contribution), while infill ratio and infill pattern had a secondary effect. Fracture surface analyses showed that high performance was associated with homogeneous filament fusion and low porosity; Studies have confirmed that poor performance is associated with delamination and interfacial separation. Full article