Fracture Surface Morphology Research Articles

Fracture‐mechanical properties of tailored epoxy nanocomposites at elevated temperatures

AbstractAn investigation was conducted to assess the influence of hybridized block‐copolymer (BCP) and core–shell rubber (CSR) particles on the tensile properties, fracture mechanical properties, and toughening mechanisms of a high strength epoxy/anhydride system at elevated temperatures (80°C). Utilizing standard tensile and compact tension (CT) specimens, substantial increases in fracture energy were observed for specimens enhanced with nanoparticles—831% for the 4 wt.% BCP‐toughened system and 600% for the 12 wt.% CSR‐toughened system. Field emission gun–scanning electron microscope imaging indicated that the inclusion of BCP and CSR significantly altered fracture surface morphologies, consistent with the improved mechanical properties. Glass transition temperature (Tg) determined through dynamic mechanical analysis—were not significantly influenced by additive inclusion, presenting a ~1°C decrease for BCP and ~3°C increase for CSR toughened systems relative to the unaltered matrix. Toughening mechanisms induced by the BCP and CSR particles were identified as (a) localized enhanced plastic shear‐band yielding around the particles and (b) cavitation of the particles followed by enhanced plastic void growth in the epoxy matrix. Outcomes from this study highlight the role of BCP and CSR particles in the toughening of epoxy/anhydride systems and provide a comprehensive assessment of their influence on mechanical characteristics at high temperatures.

Causes and analysis of failure of bearing cage at non-driven end of wind turbine generator

The non-driving end bearing cage of a wind turbine generator experienced a fracture and subsequent failure. In order to understand the reasons behind this failure, various analyses were conducted including examination of the metallographic structure, mechanical properties, fracture surface morphology, bearing vibration, and operating temperature of the cage. Through this analysis, the fracture process of the cage was deduced. The findings indicate that the fracture of the bearing cage was directly caused by the abnormal installation of the spacer. The eccentricity of the axial component resulted in ductile fracture, while the eccentricity of the radial component led to brittle fractures.

Fractographic investigation of carbon/epoxy PRSEUS composites exposed to flame after compressive failure

After a structural-related composite aircraft crash, a fractographic forensic analysis of the damaged surfaces is typically performed to assess the root causes of mechanical failures. Such accident reconstruction efforts, however, can be impeded if the aircraft catches on fire on the ground (i.e., a post-crash fire occurs), where flames or heat exposure can obscure or destroy the fracture surface morphologies of the fibers (i.e., the primary load carrying constituent). In this study, carbon/epoxy Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) skin-stringer assemblies were subjected to uniaxial compression and subsequently exposed to direct flame using a Bunsen burner. Specimens were oriented parallel, orthogonal, and at 45° to the flame axis for durations of 60 s. Additional vertical burn tests were performed for durations up to 300 s. Fractographic inspection of the failure surfaces before and after flame exposure was performed using a combination of destructive sectioning and scanning electron microscopy. The warp-knitted skin (fascia) surrounding the pultruded rod effectively served as a thermal protection layer, which shielded the rod’s broken filaments from significant thermal degradation and facilitated the identification of microbuckling and other mechanical failure mechanisms. This suggests that the presence of fascia, bulkheads, ribs, skins, and other intermediate layers in aircraft structures may significantly shield underlying principal structural element failure surfaces from fire exposure, facilitating post-crash forensic assessments of composite aircraft. Additionally, the through-thickness VectranTM stitching remained intact even after extended flame exposure, suggesting that such stitching can enhance the fire resistance of composite structures.

Experimental study on the influence of horizontal stress difference coefficient on coal rock fracture

Hydraulic fracturing is a pressure-relief and permeability-enhancing technique widely used in the mining of low-permeability coal seams. The horizontal stress difference coefficient is an important factor that affects the fracture propagation during the fracturing. To study the mechanism of the hydraulic fracturing process, in this paper, the initiation and propagation of cracks under horizontal stress difference coefficient are studied by using the developed true triaxial hydraulic fracturing system. The fracture surface morphology was obtained by 3D scanner. The experimental results are verified by the mathematical model. The results show that the fracture pressure and the peak values of AE count rate and b value increase and the cumulative AE count decreases with the increase of the horizontal stress difference coefficient. After the first pressure drop, the “pressure built-up” in the specimen increases and the peak value of AE count rate decreases with the increased horizontal stress difference coefficient. The 3D scanning results show that when the horizontal stress difference coefficient is larger, the hydraulic fracture propagation is straighter and the fracture surface area is smaller. The experimental results are in good agreement with the theoretical values. The experimental results provide a basis for engineering practice.

Effect of AlCrCuFeNi high entropy alloy reinforcements with and without B4C on powder characteristic, mechanical and wear properties of AA5083 metal-metal composites

This study presents a novel approach to enhancing the properties of the AA5083 alloy by incorporating a high entropy alloy (HEA), specifically AlCrCuFeNi, through mechanical alloying. The Al/HEA composites were thoroughly characterized in terms of powder morphology, microstructure, fracture and wear surface morphology, phase composition, and oxidation behavior using XRD, SEM-EDS, and TGA. The impact of HEA, both with and without B4C reinforcement, on the hardness, density, tensile strength, and wear properties of the AA5083 matrix composites was comprehensively evaluated. The findings revealed that the AA5083 alloy exhibited a baseline hardness of 74.7 HB and a tensile strength of 174.1 MPa. However, with the addition of HEA + B4C, these properties were markedly enhanced, with the hardness increasing to 150.1 HB (a 100 % increase) and tensile strength to 247.1 MPa (a 50 % increase). Furthermore, the wear resistance of the composite experienced a substantial improvement, with a nearly six-fold increase due to the HEA + B4C reinforcement.

Development and investigation of a polysaccharide ternary nanocomposite based on basil seed gum/graphene oxide/anthocyanin for intelligent food packaging

A new pH-sensitive intelligent packaging system was developed composed of extracted and purified basil seed gum (BG) containing aqueous malva sylvestris extract (MS) and varying amounts of synthesized graphene oxide (GO). In the following, the characteristics of prepared films including spectroscopic, physio-mechanical, thermogravimetry, fracture-surface morphology, anthocyanin release, and pH and TVB-N sensitivity, were investigated. Our results revealed that the addition of 0.5 wt % MS into the BG matrix induced pH sensitivity to the film and resulted in a visible color change from pH 2.0 to 14.0; however, it reduced the thermal and physio-mechanical properties. In this regard, the effective presence of the optimum concentration of GO (0.25 wt%) in enhancing the mechanical and thermal properties of the BG-MS films was shown. Moreover, inspecting the release kinetics demonstrated a controllable release for BG-MS-GO film compared to the BG-MS film in 48 h. Furthermore, the total volatile basic nitrogen (TVB-N) content and pH value were shown to be highly correlated with the color changes of the freshness indicator film during the storage of salmon fillets at 25 °C for 36 h. Therefore, it was shown that BG-MS-GO film can be used as a highly effective freshness/spoilage indicator of proteinic products.

Influence of fish-scale powder addition and stacking order on mechanical and thermal properties of hybrid kenaf/glass polyester composites

The increasing concern for the environment has driven a surge in popularity of utilizing modern technology to develop new products from renewable resources. To address this, polymer composites are incorporating various bio-waste materials as filler components. This study focuses on fabricating four-layered hybrid polyester laminates using kenaf (K), glass (G), and reinforced with fish scale (FS) powder at concentrations ranging from 5 wt.% to 15 wt.%. Mechanical properties are assessed following ASTM standards to evaluate the optimal FS filler percentage. The results reflect that the KGKG laminate with 15 wt. % FS exhibits utmost tensile strength and hardness, while the same laminate with 10 wt. % FS shows optimal flexural and impact properties. Further, numerical analysis is performed using ANSYS 19.0 software to validate the experimental findings. It is marked that, the results of numerical analysis are align intimately with the experimental data, with a deviation of 5 to 10%. Additionally, the moisture absorption behavior is investigated, and the results revealing that FS filler reduced moisture uptake and enhanced dimensional stability. Further, the study is extended to investigate the thermal attributes of laminates through Thermogravimetric Analysis (TGA) and Dynamic Mechanical Analysis (DMA). The fractured surface morphology are studied through Scanning Electron Microscopic (SEM) and the findings reveal significant occurrences of matrix cracking, fiber withdrawal, and the separation of fibers from the matrix. Overall, this study suggests the incorporation of FS bio-fillers into hybridized kenaf/glass laminates in order to improve performance and create environmentally friendly options that suitable for automotive applications.

Effect of wood varnish coating on the water absorption and mechanical properties of jute fiber reinforced epoxy composites

Natural fiber-reinforced composites are becoming popular day by day because of their low cost and renewable nature. However, a major concern of these composites is water absorption. To minimize the water absorption in natural fiber-reinforced composites, wood varnish coating was used in this paper to investigate the effect of this coating on water absorption and mechanical properties of jute fiber-reinforced composites. Four types of composites were fabricated using the hand lay-up technique, among them on Type 1 no coating was used, Type 2 was surface coated, Type 3 was fiber coated and Type 4 was both fiber and composite surface coated. Water absorption, tensile, bending, short beam shear, and impact tests were performed on the composite specimens according to their respective ASTM standards to see the effect of coating. Both fiber and fracture surface morphology were observed using Scanning Electron Microscope. It was found that the water absorption was significantly reduced for Type 2 (50.31 %) and Type 4 (56.01 %) specimens. However, the tensile strength and bending strength were reduced by 74.17 %, and 72.71 % for Type 3 and 77.07 % and 84.24 % for Type 4 composites while for Type 2, they were slightly increased (7.26 % and 1.13 %). The ILSS of Type 3 and Type 4 were reduced by 69.34 % and 58.07 % respectively while it was reduced by only 4.49 % for Type 2 composite. On the other hand, the impact energy absorption was increased by 85.32 % for Type 4 composite. The findings of this study suggest that Type 2 composite (only composite surface coating) has the overall best performance and wood varnish coating has the potential to reduce the water absorption on natural fiber-reinforced composites.

Visualization experiment on dynamic migration and sealing mechanism of irregular materials in rough fractures

Lost circulation is one of the main problems causing low drilling efficiency and high drilling costs in the oil and gas industry. Due to an insufficient understanding of the migration and sealing mechanism of lost circulation materials (LCMs) in rough fractures, the success of fracture sealing is highly unpredictable. This study conducted visualization experiments using four types of LCMs to observe the entire dynamic sealing process of rough fractures. Adaptive fracture sealing experiments were designed to improve the sealing performance of LCMs at low concentrations. The results indicated that the key to sealing rough fractures is forming an effective sealing zone inside the fracture space rather than in the inlet buffer zone. The dynamic sealing process has experienced four typical stages: sporadic bridging, bridging structure expansion, sealing front formation, and sealing layer thickening. The first three typical stages correspond to flow from plate flow to trench flow and seepage. The formation of an effective sealing zone within fractures depends on forming a practical sealing front and subsequent particle accumulation of LCMs that thicken the sealing layer. Compared with regular particles, the sealing front of irregular particles is an area with a specific aperture range. The increase in irregular particle concentration makes the sealing front migrate to the fracture entrance, which is more likely to lead to inlet blockage. The distribution of a sealing zone depends upon the surface morphology of rough fracture. Increasing the particle size distribution improves the favorable bridging area and enhances the sealing performance. The adaptive sealing strategy uses low-concentration granular materials to form multi-point bridging within rough fractures and low-concentration flaky materials to adaptively and quickly extend and connect dispersed bridging structures. Mica flakes are more conducive to extending along the contour of fracture aperture than calcium carbonate particles. Adding mica flakes promotes the expansion of the bridging structures, forming a sealing front and a robust sealing layer in advance. This study delves into irregular materials' migration and sealing mechanisms in rough fractures, enriching theoretical understanding and providing guidance for on-site sealing operations.

Hydrogen Embrittlement Detection Technology Using Nondestructive Testing for Realizing a Hydrogen Society.

Crack detection in high-pressure hydrogen gas components, such as pipes, is crucial for ensuring the safety and reliability of hydrogen infrastructure. This study conducts the nondestructive testing of crack propagation in steel piping under cyclic compressive loads in the presence of hydrogen in the material. The specimens were hydrogen-precharged through immersion in a 20 mass% ammonium thiocyanate solution at 40 °C for 72 h. The crack growth rate in hydrogen-precharged specimens was approximately 10 times faster than that in uncharged specimens, with cracks propagating from the inner to outer surfaces of the pipe. The fracture surface morphology differed significantly, with flat surfaces in hydrogen-precharged materials and convex or concave surfaces in uncharged materials. Eddy current and hammering tests revealed differences in the presence of large cracks between the two materials. By contrast, hammering tests revealed differences in the presence of a half size crack between the two materials. These findings highlight the effect of hydrogen precharging on crack propagation in steel piping and underscore the importance of early detection methods.

Biomass-derived epoxy resin and its application for high-performance natural fiber composites

The development of bio-based materials with superior comprehensive properties from biomass resources remains a significant challenge, providing an alternative to conventional petroleum-based materials. This study explores the lignin-derived vanillyl alcohol epoxy (VAE) resin-containing mono and di (m&d) epoxy structure as an adhesive to develop environment-friendly natural fiber-reinforced composites to reduce the carbon footprint. The synthesized m&dVAE containing mono and di-epoxidized (m&d) aromatic rings, when cured with 4, 4´-diaminodiphenyl methane (DDM) hardener, exhibits higher record tensile strength ∼124.0 ± 8.43 MPa and tensile modulus ∼2.88 ± 0.35 GPa compared to a commercial petroleum-based epoxy, diglycidyl ether bisphenol A (DGEBA) thermoset. Additionally, it demonstrated higher adhesion shear strength (∼19.16 ± 0.58 MPa) with cellulose nanofibers film than DGEBA. Moreover, fabricated green composite possesses excellent flexural strength of ∼203.72 ± 2.08 MPa and stiffness of ∼11.58 ± 0.38 GPa than the petroleum-based thermoset composite. The composite’s fracture surface morphology shows that the resin and fibers have good interfacial adhesion. Notably, the sustainable composite showed good hydrophobicity and an excellent heat-resistant index of 144.4°C. The m&dVAE resin can be an alternative to petroleum-based resins as an adhesive, and its environment-friendly sustainable composite could be a promising candidate to replace synthetic materials for high-performance structural applications.

The influence of various welding wires on microstructure, and mechanical characteristics of AA7075 Al-alloy welded by TIG process

Owing to their exceptional mechanical properties, the various welding wires used to combine aluminum can meet the needs of many engineering applications that call for components with both good mechanical and lightweight capabilities. This study aims to produce high-quality welds made of AA7075 aluminum alloy using the GTAW technique and various welding wires, such as ER5356, ER4043, and ER4047. The microstructure, macrohardness, and other mechanical characteristics, such as tensile strength and impact toughness, were analyzed experimentally. To check the fracture surface of the AA7075 welded joints, the specimens were examined using optical and scanning electron microscopy (SEM). A close examination of the samples that were welded with ER5356 welding wire revealed a fine grain in the weld zone (WZ). In addition, the WZ of the ER4043 and ER4047 welded samples had a coarse grain structure. Because the hardness values of the welded samples were lower in the WZ than in the base metal (BM) and heat-affected zone (HAZ), the joints filled with ER5356 welding wire provided the highest hardness values compared to other filler metals. Additionally, the ER4047 filler metal yielded the lowest hardness in the weld zone. The welding wire of ER5356 produced the greatest results for ultimate tensile stress, yield stress, welding efficiency, and strain-hardening capacity (Hc), whereas the filler metal of ER4043 produced the highest percentage of elongation. In addition, the ER4047 fracture surface morphology revealed coarser and deeper dimples than the ER5356 fine dimples in the welded joints. Finally, the highest impact toughness was obtained at joints filled with the ER4047 filler metal, whereas the lowest impact toughness was obtained at the BM.

Effect of heat treatment on the shear fracture and acoustic emission properties of granite with thermal storage potential: A laboratory-scale test

The exploitation of geothermal resources enhances the energy structure but poses challenges related to thermal–mechanical issues. This study utilizes short core in compression (SCC) specimens of granite to investigate thermo-mechanical properties. We examined the effects of heat treatment temperature on physical properties, shear fracture toughness, as well as acoustic emission (AE) characteristics. Fracture surface morphology and microstructure of the heat-treated SCC specimens were also analyzed using 3D laser scanning and scanning electron microscopy. Results indicate that both mass loss and P-wave velocity decrease with increasing heating-treated temperature, and similar trends are observed in mechanical properties. Specifically, fracture energy and shear fracture toughness decline by 51.48 % and 61.27 %, respectively, as temperature rises from 25 °C to 750 °C. The b-value and proportion of shear cracks initially increase before falling, with an inflection point at 300 °C, linked to thermal-induced microstructural deterioration. Fractal dimension (D) and joint roughness coefficient (JRC) show significant increases with higher heat treatment temperatures. Microstructural degradation, including dehydration and thermal-induced microcracking, drives the reduction in shear properties of heat-treated granites. Overall, our findings offer valuable insights into determining various methods of heat storage reconstruction with great application potential.

Study on the mixed fracture characteristics of concrete-rock Brazilian disks with different fracture angles

The fracture characteristics of cracked straight-through Brazilian discs for concrete-rock composites (CSTBD-CRC) with different fracture inclination angles (Mode I-II) were investigated to evaluate the stability of concrete reinforcement engineering. Acoustic emission (AE) and digital image correlation (DIC) techniques were employed to study crack propagation and fracture modes in CSTBD-CRC Brazil splitting tests, as well as the entropy evolution of AE and DIC. Additionally, 3D scanning technology and fractal theory were utilized to investigate the effect of the fracture mode on the fracture surface morphology. The results indicate that both the fracture energy and peak load of the CSTBD-CRC increase with increasing fracture inclination angle. Moreover, the section fractal dimensions of 0°, 10°, 15° and 25° samples are 2.01879, 2.01528, 2.01186, and 2.01072. Shear fracture mode increases as the dip angle increases and the fracture surface morphology decreases in fractal dimension. Notably, sharp increases in entropy values for both AE and DIC occurred at peak load, suggesting their use as failure criteria for CSTBD-CRC specimens. Furthermore, cracks were found to initiate at the tips of preexisting cracks within the concrete. The displacement in the x direction undergoes the most significant changes on both sides of the crack during crack growth, while the strain in the x direction experiences its greatest variations at the crack location.

Synthesis and characterization of additively manufactured microcapsule‐reinforced polylactic acid composites for autonomous self‐healing

AbstractMaterial extrusion‐based additive manufacturing (AM) process builds the objects/structures through a precise feedstock deposition in a layer‐by‐layer manner. Polylactic acid (PLA) is a popular biodegradable feedstock in AM, while octyl methoxycinnamate (OMC) is known for its eco‐friendliness and ultraviolet (UV) protection properties. The present study focuses on the novel infusion methodology of OMC‐based microcapsules into PLA to develop self‐healing composite filaments. Post‐composition iterations, the optimum compositions for the filler and plasticizer were determined, and the filaments were extruded. Microcapsule‐infused PLA and the neat PLA samples were printed as per the American Society for Testing and Materials (ASTM) standard. The uniaxial tensile test results showed that the failure strain endured by the microcapsule‐infused samples was about 10 times more than the neat PLA counterparts. It is attributed to the effective load distribution and the complex polymerization reaction (due to the interaction of OMC with the matrix). Fracture surface morphology of the samples via optical microscopy (OM) and field emission scanning electron microscope (FESEM) affirmed the strong PLA‐OMC interface. A depreciation in the Brinell Hardness for the microcapsule‐based samples was due to the localized indenter force, causing greater damage in a narrow area than microcapsule ruptures' healing ability.Highlights The optimized composition of PLA: plasticizer:microcapsule is 1:0.04:0.05. Microcapsule‐infused PLA has improved Young's modulus and failure strain. Interaction with microcapsules improves elastic behavior and self‐healing. FESEM reveals close bonding of microcapsule with the PLA matrix.

Unravelling the relation between free volume gradient and shear band deflection induced extra plasticity in metallic glasses

Previous experiments have revealed that the controllable introduction of structural gradients in metallic glasses (MGs) can endow the materials with extra plasticity due to the gradient-induced deflection of shear bands. However, the relation between the spatial structural gradient and the initiation of shear band deflection remains unclear. The current study has been focused on investigating the relationship between the improved mechanical properties of MGs and structural gradients specified by the distribution of the intrinsic free volume. Molecular dynamics (MD) simulations are firstly performed on homogeneous MG models containing various initial free volume values, showing that the shear band angle increases with decreasing free volume under uniaxial compression, whereas higher shear band angle is observed under uniaxial tension with increasing free volume. Based on the asymmetric behaviors of MGs under compression and tension, a theoretical model is developed to quantitatively characterize the influence of free volume on the mechanical response of MGs, which incorporates a failure criterion based on free volume generation during external loadings. The model can be further utilized to interpret and predict the fracture strain, shear band angle, maximum stress, and fracture surface morphology of gradient structured MGs in both simulations and experiments. The relationship between free volume gradient and shear band deflection induced extra plasticity established in this study provides valuable guidance for the structural design of MGs with enhanced mechanical properties.

Stress Corrosion Cracking of 304 Austenitic Stainless Steel in H2SO4/NaCl Media at Room Temperature

The effects of Cl− ion concentration, H2SO4 concentration, the applied potential, and the strain rate on the stress corrosion cracking (SCC) of 304 stainless steel were investigated. The postfracture scanning electron microscope micrographs of the fractured surfaces revealed that solutions of high H2SO4 concentration combined with low Cl− ion concentration resulted in corrosion (active dissolution), while solutions of very low H2SO4 concentration combined with a relatively moderate Cl− ion concentration resulted in ductile failure. Solutions with moderate concentrations of H2SO4 and Cl− ions resulted in transgranular SCC. The susceptibility to SCC and the fracture surface morphology were found to depend on the [H2SO4]/[Cl−] ratio, the strain rate, and the applied potential. The average crack-growth rate (ACGR) was found to depend on the strain rate and the applied potential. For specimens undergoing SCC at the open-circuit potential, the ACGR increased with increasing the strain rate. For specimens undergoing SCC under applied potential in the active range, the ACGR increased with increasing the applied potential. The specimen eventually failed in a ductile manner when tested under an applied anodic potential in the passive range. The increase in the ACGR with increasing strain rate and the applied anodic potential is attributed to the enhanced dissolution at the crack tip. Moreover, secondary cracks formed when relatively high strain rates were used. The secondary cracks are believed to propagate mainly along the slip planes. The fracture surface morphology shows the {110} planes are the preferred fracture planes, with the fracture surface consisting of parallel facets separated by steps. Finally, the results indicate that hydrogen may play an indirect role in SCC.

Study on the influence of wood ray morphological characteristics on the tensile strength of wood parallel to grain

The tensile strength of wood parallel to grain is one of the important indicators of wood physical and mechanical properties. Cross-sectional shape possesses a significant effect on the tensile strength parallel to grain, especially for broad-leaved trees with broad wood rays. However the influence of the structural feature of wood rays on the test results, which was commonly ignored. The article focuses on the beech tree with two types of wood rays: broad and narrow, poplar wood with extremely fine wood rays, and dilleniad with extremely rich wood rays. The relationship between the inclination angle and fracture mode of wood rays and the microstructure of wood rays is studied, in order to provide a more accurate scientific basis for determining the tensile strength of wood parallel to grain. The results showed that the inclination angle between the wood ray and the tensile direction has a significant impact on the fracture mode and tensile strength, and the tensile strength parallel to grain without inclination angle is greater than that with inclination angle. Different types of wood rays have a effect on the coefficient of variation of the tensile strength parallel to grain of the specimen. Under the same inclination angle, the Pearson correlation between the wood ray parenchyma ratio and the tensile strength parallel to grain is significantly negative at the 0.01 level (double tailed) or 0.05 level (double tailed). Besides, as the wood ray parenchyma ratio gradually increases, the overall tensile strength of wood parallel to grain shows a downward trend. The fracture surface morphology characteristics of specimens with different fracture modes are different.

Fatigue crack extension mechanism and mode-type analyses of martensitic steels for proposing fatigue strength evaluation method: Example of 18% Ni BCC martensitic steel

The fatigue properties of martensitic steels tend to differ significantly from those of low- and medium-strength steels. Even if the hardness is the same, the fatigue strength varies substantially depending on the microstructure. Thus, for using martensitic steel rationally and developing materials with high fatigue strength, it is desirable to establish a fatigue strength evaluation method that can consider the microstructural effects on fatigue properties. For this purpose, 18% Ni bcc martensitic steel was used as a model metal to conduct rotating-bending fatigue tests in this study. The fatigue crack behavior on the smooth specimen surface was examined, and the fracture surface morphology on both sides of the broken specimen and the microplastic strain distribution near the fatigue crack tip were analyzed. The results clarified the mechanism underlying fatigue crack extension (FCE). The analytical results revealed three types of FCE mechanisms dependent on the stress amplitude, stress state (i.e., plane stress or plane strain), and martensitic microstructure. Considering these mechanisms, a unique stress–life curve was predicted, and a particular fatigue crack shape and fractography were confirmed. The material indices were speculated to be the representative properties of martensitic steels.

Improving the Interfacial Adhesion of Long Carbon Fiber-Reinforced Polyamide 6 Composites by Electrochemical Oxidation and Polyethylenimine-Carboxymethyl Cellulose Grafting.

Carbon fiber (CF)-reinforced thermoplastic composites have notable ascents in various sectors and applications. For high-performance composites, strong interfacial adhesion between the polymer matrix and the CF is crucial. This is achieved by introducing functional groups on the CF surface. In this paper, a water-based surface treatment was applied to long carbon fibers to enhance the interfacial bonding with the polyamide 6 (PA6) matrix. For that, PEI and CMC were grafted onto the surface of carbon fibers after electrochemical oxidation. The PEI-CMC sizing reduced the carbon fiber/water contact angle to 26.42° from 111.69°. The clear improvement in wettability resulted in a 164.8% increase in the interfacial strength of 26.7 MPa after the application of PEI-CMC sizing on carbon fibers (CFs). The resultant tensile and flexural strength increased by 19.3 and 11.7% from 2009.6 and 378.3 MPa for desized CF/PA6 composites to 250.3 and 422.7 MPa for PEI-CMC-sized CF/PA6 composites, respectively. Moreover, the fractured surface morphologies were also investigated to confirm the enhancement of mechanical properties. The proposed one-step electrochemical oxidation and water-based sizing procedure is found to be promising for the production of high-performance long-fiber-reinforced thermoplastic composites.