Simplified Fracture Mechanics Analysis at the Zinc–Adhesive Interface in Galvanized Steel–CFRP Single-Lap Joints

The increasing reliance of the aerospace industry on lightweight yet robust materials, notably carbon fiber‐reinforced polymers (CFRPs), has emphasized the need for efficient joining methods for composite structures. This study investigates the influence of various bonding techniques on the strength of two distinct joint configurations: CFRP/CFRP and CFRP/301 steel interfaces. The CFRP/CFRP joints were configured as single lap joints, where two CFRP laminates and/or prepregs were overlapped and bonded using co‐curing, secondary bonding, or co‐bonding. The configuration ensured adhesion between the cured or uncured CFRP layers, depending on the bonding technique applied. For CFRP/301 steel, CFRP laminates and/or prepregs were similarly bonded to 301 steel substrates in an overlapping region. Both configurations were subjected to single lap shear tests following ASTM D3165 standards to assess the mechanical performance of each bonding method. The post‐test interface damage and fracture mechanisms were analyzed using optical microscopy, providing insights into the failure modes for each joint type. This study offers comprehensive experimental data on various joining techniques that can be useful during the initial design phase of aerospace systems, ultimately resulting in cost savings. The findings of this study, when analyzed from an industrial standpoint, indicate that co‐curing offers a promising solution due to its affordability and simplicity, making it especially advantageous for CFRP/301 steel overlap joining when combined with adhesive films.Highlights Co‐curing, co‐bonding, and secondary bonding techniques were applied to CFRP/steel joints in real aircraft components. The CFRP/CFRP joints obtained via co‐curing exhibited improved thermal stability and strength. Peel ply surface preparation enhanced adhesion the strength of the CFRP/CFRP bonded samples. Cohesion failure was observed in adhesive joints, with no failure at the bond lines.

Macroscopic structures consisting of two or more materials are called composites. The decreasing reserves of the world’s oil reserve and the environmental pollution of existing energy and production resources made the use of recycling methods inevitable. There are mechanical, thermal, and chemical recycling methods for the recycling of thermosets among composite materials. The recycling of thermoset composite materials economically saves resources and energy in the production of reinforcement and matrix materials. Due to the superior properties such as hardness, strength, lightness, corrosion resistance, design width, and the flexibility of epoxy/vinylester/polyester fibre formation composite materials combined with thermoset resin at the macro level, environmentally friendly sustainable development is happening with the increasing use of composite materials in many fields such as the maritime sector, space technology, wind energy, the manufacturing of medical devices, robot technology, the chemical industry, electrical electronic technology, the construction and building sector, the automotive sector, the defence industry, the aviation sector, the food and agriculture sector, and sports equipment manufacturing. Bonded joint studies in composite materials have generally been investigated at the level of a single composite material and single joint. The uncertainty of the long-term effects of different composite materials and environmental factors in single-lap bonded joints is an important obstacle in applications. The aim of this study is to investigate the effects of single-lap bonded GFRP (glass fibre-reinforced polymer) and CFRP (carbon fibre-reinforced polymer) specimens on the material at the end of seawater exposure. In this study, 0/90 orientation twill weave seven-ply GFRP and eight-ply CFRP composite materials were used in dry conditions (without seawater soaking) and the hand lay-up method. Seawater was taken from the Aegean Sea, İzmir province (Selçuk/Pamucak), in September at 23.5 °C. This seawater was kept in different containers in seawater for 1 month (30 days), 2 months (60 days), and 3 months (90 days) separately for GFRP and CFRP composite samples. They were cut according to ASTM D5868-01 for single-lap joint connections. Moisture retention percentages and axial impact tests were performed. Three-point bending tests were then performed according to ASTM D790. Damage to the material was examined with a ZEISS GEMINESEM 560 scanning electron microscope (SEM). The SEM was used to observe the interface properties and microstructure of the fracture surfaces of the composite samples by scanning images with a focused electron beam. Damage analysis imaging was performed on CFRP and GFRP specimens after sputtering with a gold compound. Moisture retention rates (%), axial impact tests, and three-point bending test specimens were kept in seawater with a seawater salinity of 3.3–3.7% and a seawater temperature of 23.5 °C for 1, 2, and 3 months. Moisture retention rates (%) are 0.66%, 3.43%, and 4.16% for GFRP single-lap bonded joints in a dry environment and joints kept for 1, 2, and 3 months, respectively. In CFRP single-lap bonded joints, it is 0.57%, 0.86%, and 0.87%, respectively. As a result of axial impact tests, under a 30 J impact energy level, the fracture toughness of GFRP single-lap bonded joints kept in a dry environment and seawater for 1, 2, and 3 months are 4.6%, 9.1%, 14.7%, and 11.23%, respectively. At the 30 J impact energy level, the fracture toughness values of CFRP single-lap bonded joints in a dry environment and in seawater for 1, 2, and 3 months were 4.2%, 5.3%, 6.4%, and 6.1%, respectively. As a result of three-point bending tests, GFRP single-lap joints showed a 5.94%, 8.90%, and 12.98% decrease in Young’s modulus compared to dry joints kept in seawater for 1, 2, and 3 months, respectively. CFRP single-lap joints showed that Young’s modulus decreased by 1.28%, 3.39%, and 3.74% compared to dry joints kept in seawater for 1, 2, and 3 months, respectively. Comparing the GFRP and CFRP specimens formed by a single-lap bonded connection, the moisture retention percentages of GFRP specimens and the amount of energy absorbed in axial impact tests increased with the soaking time in seawater, while Young’s modulus was less in three-point bending tests, indicating that CFRP specimens have better mechanical properties.

Dual-scale reinforcement of co-cure single lap joints through graphene nanoparticles and CFRP Z-pin

Externally bonded carbon fibre reinforced polymer (CFRP) is widely employed for strengthening steel structures. The CFRP-steel bond behaviour is typically conducted using the single-lap joint (SLJ) and double-lap joint (DLJ) shear tests. This study utilizes advanced distributed optical fibre sensors (DOFS) to measure strains in CFRP-steel SLJs and DLJs, with a specific focus on examining the bending effect. An experimental investigation was conducted to verify the appropriate adhesive and fibre coating materials, achieving synergistic deformation between optical fibre sensors and adherends. The strain results reveal that PI-coated fibres bonded with epoxy resin enhance deformation compatibility. The CFRP strain distributions within the joints are accurately measured by DOFS, enabling the calculation of bending strain to assess joint bending effects. The observed strain aligns well with the finite element (FE) predictions. A more significant bending effect is observed in SLJs compared to DLJs, predominantly concentrated at the loaded end. It can be inferred that the SLJs are subjected to mixed-mode I/II loading in shear tests, and the DLJs appears to be mainly influenced by mode II loading. Additionally, the verified FE model demonstrates that the shear strain across the thickness of the adhesive layer exhibits significant variation at the bond ends.

This paper described a comprehensive study of thermo-mechanical reliability of high input/outputs (I/Os) flip-chip on laminated substrate by using finite element analysis(FEA), response surface methodology(RSM) and interfacial fracture mechanics. Accelerated thermal cycling(ATC) tests for six specimens with different structures and materials were conducted firstly to determine the fatigue life and failure mechanism of the solder joints. Two-dimensional and Three-dimensional FEA corresponding to ATC tests were then conducted individually to analyze the mechanical behavior of the package under ATC conditions. Global-local FEA approach was used because of the large number of solder bumps and the complicated structure of the package. The simulation results of FEA accorded with the tests results well and a fatigue life model in related with accumulated strain energy density was built to estimate the solder joint fatigue life of the package. The combined effect of structural parameters including pad diameter, pad thickness, solder mask open, solder mask thickness and standoff height on the solder joint fatigue life were analyzed by the integration of two-dimensional FEA and central composite design (CCD) based RSM, and a response surface model was established to make the optimization. Due to imperfect manufacturing process, very small interfacial cracks commonly exist between the interface of solder joint and copper pad after reflow, the interfacial fracture behavior of the crack for different crack lengths under ATC conditions were investigated with two-dimensional FEA. Interfacial fracture mechanics and crack surface displacement extrapolation method were used to analyze the variations of strain energy release rate and phase angle at the crack tip

Externally bonded carbon fiber-reinforced polymer (CFRP) composites have been widely adopted for strengthening and repairing aging concrete structures. However, premature fracture at the CFRP-concrete interface remains a significant concern, necessitating a better understanding of the interfacial bond behavior and fracture mechanisms. Traditional experimental studies and numerical simulations have limitations in accurately capturing the complex nonlinear factors influencing this fracture behavior. This paper explores the novel application of machine learning (ML), specifically artificial neural networks (ANNs), for predicting the fracture strength of CFRP-reinforced concrete (CFRP-RC) members. ANNs enable mapping the intricate relationships between input parameters (e.g., CFRP/concrete properties, geometric configurations) and fracture responses through iterative training on experimental data. Key aspects include comprehensive data collection, preprocessing techniques, feature selection methods like mean impact value (MIV) analysis, and neural network architecture design. The paper highlights successful ANN applications in predicting CFRP-RC bond strength, shear capacity, and overall fracture behavior. A systematic approach is proposed for developing robust ANN models tailored to CFRP-RC fracture prediction, encompassing data curation, input variable screening, network training/validation, and model interpretability analyses. By leveraging ML’s ability to handle multifaceted nonlinearities, this data-driven framework offers a powerful alternative to traditional methods, potentially enhancing the design, analysis, and performance evaluation of CFRP-strengthened concrete structures.

Influence of temperature on concrete beams strengthened in flexure with CFRP

Enhancing shear strength of single lap composite adhesive joint with graphene nano-particles and Z-pins reinforcement through co-curing technique

Fracture mechanics analyses of ceramic/veneer interface under mixed-mode loading

Strengthening structures using CFRP (Carbon Fiber Reinforced Polymer) strips is a widely used method because of superior mechanical properties of CFRP. Along with the overall impact of the FRP strengthening, FRP application type is one such important parameter that directly affects the strengthened beam behavior. FRP can be applied in a number of different ways. FRP sheets, either full wraps or strips are the mostly preferred methods for strengthening in recent years. These methods have more or less advantages according to each other. In other words, CFRP application influences the strengthened beam behavior since application type has a direct impact on the failure mechanism. In this study, it was aimed to identify the effect of CFRP application type on behavior of reinforced concrete (RC) beams. For this purpose, CFRP-strengthened RC beams, which were strengthened by different application types such as side-bonded and U-jacketed CFRP sheets, were tested under cyclic loading and monitored by Acoustic Emission (AE) technique. AE data obtained from the tests were processed by parameter and SiGMA analyses. Crack locations, types and orientations were determined and were compared with mechanical results. Thus, failure mechanisms of the specimens were evaluated and effects and discrimination of CFRP application type on the fracture mechanism were revealed.

Civil infrastructure is witnessing an increasing number of degraded and deteriorated steel structures. With current retrofitting and rehabilitation techniques becoming outdated the implementation of carbon fibre reinforced polymer (CFRP) materials have excelled with their impressive strength to weight ratios and chemical resistance. Unfortunately the longevity of bonded CFRP systems is largely unproven under industrial conditions, in particular under combined environmental exposure and fatigue loading. Environmental exposure for such applications often involves elevated temperature and moisture. Moisture ingress can be detrimental to adhesive integrity and can provide the necessary environment for galvanic corrosion between steel and CFRP materials. Temperature changes, particularly elevated temperatures, can soften epoxy adhesives as they reach their glass transition temperature (Tg), in turn, significantly reducing their strength. Additionally, this rubberisation can further intensify the level of moisture absorption, compounding and exacerbating the degradation. Finally these deteriorated materials are likely to be more detrimentally affected by the application of strenuous loading which can significantly reduce their strength even further. Thus, the broad aim of this research is to better understand the durability and fatigue performance of CFRP/steel systems. The first stage of research into the bond performance of CFRP/steel was to quantify the amount of localised corrosion (pitting) created between CFRP and steel during submergence in simulated seawater solutions at elevated temperature. Pitting creates high stress concentrations and can become the site of premature cracking and fracture. Specimen submergence in seawater solutions showed that isolated pitting was insignificant, with general chloride corrosion being more substantial than localised galvanic damage. Consequently, the potential degradation of the adhered joint under such conditions appeared to be more influential on the durability and fatigue performance of CFRP/steel systems. The next stage focused on an investigation of the bond strength of CFRP patched steel double lap specimens after fatigue loading and environmental exposure. To investigate the most damaging and destructive scenarios to durability and strength of the joints, the service loading, fatigue loading, exposure temperature and submergence duration were altered. Normal modulus materials were unable to survive the application of high stress fatigue cycles after environmental exposure. On the other hand, high modulus sheeting specimens survived all loading and environmental conditioning, experiencing strength losses of only 10% on average. The final stage was to investigate methods to improve the bond integrity of double-lap joints to reduce the amount of degradation during submergence and loading. Firstly, a high Tg adhesive was implemented to maintain joint integrity and strength under elevated temperature. Furthermore, carbon nanotubes (CNT) were dispersed into common structural epoxies to increase the physical and mechanical properties of the adhesives, however their addition increased adhesive viscosity and decreased their workability. Also a chemical bond promoter, silane, was introduced to enhance the chemical bond of CFRP/steel joints to ease the strength losses resulting from submergence and fatigue. However, levels of strength reduction remained comparable between un-treated and silane pre-treated samples, implying that degradation is not necessarily due to an issue between the interface of steel and epoxy, but more likely within the bulk adhesive. Thus, silane pre-treatment is perhaps more effective when combined with multi-layered patches as they commonly witness more steel and adhesive interfacial failures. After bond performance was explored in detail, investigations progressed to examine the fatigue performance of CFRP repaired damaged steel after seawater submergence. Tri-layered CFRP patches were applied to pre-cracked steel plates prior to exposure. Configurations consisted of either single or double sided repair, with or without silane pre-treatment. Several double sided repaired specimens managed to survive in excess of 6 million cycles without any visible damage after exposure, while single sided specimens maintained at least an 80% increase in fatigue life over bare steel. Finally, a numerical model was developed to predict the fatigue life of pre-exposed CFRP repaired steel, which was validated via the experimental investigations. The influence of environmental conditioning was incorporated into linear elastic fracture mechanics theory to accurately predict the degradation and fatigue performance of exposed repairs. This research provides advanced understanding into the durability of adhered CFRP to steel. Investigations focused on the combination of fatigue loading and environmental exposure, which is likely for structural elements expected to utilise CFRP strengthening. Studies showed that CFRP systems were capable of surviving conditions that are more severe than those expected during their industrial life cycle. It can be concluded that CFRP can potentially provide revolutionary rehabilitation performance for steel structures, even under extreme environmental and loading scenarios.

This paper studied favorable low-temperature plasma (LTP) surface treatment modes for Carbon Fiber Reinforced Polymer (CFRP)/Al7075 single-lap joints using complex experimental methods and analyzed the failure modes of the joints. The surface physicochemical properties of CFRP after LTP surface treatment were characterized using scanning electron microscopy (SEM), contact angle tests, and X-ray photoelectron spectroscopy (XPS). The influence mechanism of LTP surface treatment on the bonding properties of CFRP/Al7075 single-lap Joint was studied. The results of the complex experiment and range analysis showed that the favorable LTP surface treatment parameters were a speed of 10 mm/s, a distance of 10 mm, and three repeat scans. At these parameters, the shear strength of the joints reached 30.76 MPa, a 102.8% improvement compared to the untreated group. The failure mode of the joints shifted from interface failure to substrate failure. After low-temperature plasma surface treatment with favorable parameters, the CFRP surface exhibited gully like textures, which enhanced the mechanical interlocking between the CFRP surface and the adhesive. Additionally, the surface free energy of CFRP significantly increased, reaching a maximum of 78.77 mJ/m2. XPS results demonstrated that the low-temperature plasma surface treatment led to a significant increase in the content of oxygen-containing functional groups, such as C-O, C=O, and O-C=O, on the CFRP surface.

The emergence of high strength epoxies has enhanced the feasibility of increasing the shear strength of concrete beams by wrapping with carbon fiber reinforced polymer (CFRP) fabric. The objective of this investigation is to evaluate the increase in shear strength of concrete beams wrapped with different configurations of CFRP fabric. Shear tests are conducted up to failure on two reinforced concrete control beams and twelve reinforced concrete beams wrapped with four different configurations of CFRP fabric. An analytical procedure is presented to predict the shear strength of beams wrapped with CFRP fabric. Comparisons are made between the test results and the analytical calculations. The shear strength is increased up to 33% on concrete beams wrapped with CFRP fabric at an angle of +/- 45 deg to the longitudinal axis of the beam.

This study developed a kind of non-woven carbon tissue reinforced adhesive film (NWCTRAF) with high bonding strength and reliability by interleaving the NWCT into the adhesive film. The manufacturing process, bonding strength, Weibull distribution, metallography of cross-section, and fracture surface morphology of NWCTRAF specimens were investigated. The results showed that the average bonding strength and Weibull characteristic strength (WCS) of single lap joint (SLJ) were 27.4a and 28.2 MPa, which increased by 17.6% and 15.1%, respectively compared with the blank specimen, and the Weibull modulus risen from 9.51 to 18.72, which indicated that NWCTRAF specimens had higher bonding reliability. The mechanism of micro-crack formation was obtained by analyzing the cross-section of loading 95% WCS without visible damage. The metallographic section showed that the microcracks of the blank specimen originated from the interface between carbon fiber-reinforced polymer (CFRP) and adhesive layer, while the microcracks of the NWCTRAF specimen originated from the resin-rich regions between the carbon fiber fabric layers in CFRP. Typical failure modes were gained from optical microscope and SEM. Failure modes of blank specimen included more interface failure of CFRP-adhesive and Al-adhesive, while NWCTRAF specimen had more carbon fiber tearing and delamination of CFRP. The fracture morphology of adhesively cohesive failure included the pull-out of short carbon fibers and the appearance of rough and multi-stage resin in the NWCTRAF specimen, which improved the toughness of the adhesive layer. The bridging effect of short carbon fibers in the NWCTRAF disperse and transfer stress to CFRP and fail at resin-rich regions of CFRP, decrease the bonding interface's direct failure, and improve the bonding strength and reliability.

This paper presents an investigation on the thermal damage characteristics and mechanism of carbon fiber reinforced polymer (CFRP) during fiber laser joining between CFRP and mild steel with Cr electroplating. The results show that laser heat causes nonuniform thermal damage of CFRP at the bonding interface. In the center region of the fracture after shear testing of the joint, CFRP endures serious ablation characterized with the formation of large cavities and the joint failure happens at the bonding interface. In the peripheral region of the fracture, a large amount of micron porosities are observed and the joint failure happens at the porosity zone in CFRP. According to the thermochemical analysis of CFRP, the critical temperatures of resin matrix decomposition, carbon residue oxidation and carbon fiber oxidation are 350°C, 484°C and 650°C respectively. When the temperature is higher than 350℃, the resin matrix of CFRP decomposes, producing gaseous products such as CO2, NH3 and the water vapor, which form the porosities in CFRP. When the temperature is higher than 484℃, the residual carbon is oxidized and ablation happens. The thermal histories at different locations of bonding interface show that Gaussian distribution of the laser beam results in nonuniform temperature distribution in the bonding interface. The peak temperature in the center of bonding interface is 654℃, higher than the CFRP oxidation temperature. CFRP in this region endures severe thermo-oxidation ablation. In the peripheral region of the bonding interface, the peak temperature drops from 484°C to 350°C sharply with the increase of distance from the centerline and CFRP in this region endures decomposition. Through strict control of laser heat input, the thermal ablation can be completely eliminated and the amount of the porosities can be reduced. Thus the shear strength of the joint is apparently improved from 16.8MPa to 27.2MPa. Therefore, using the laser with uniform intensity distribution instead of fiber laser and controlling the temperature of interface lower than CFRP decomposition temperature is a promising approach to suppress the thermal damage in CFRP.