Carbon fiber reinforced composites (CFRP) are widely used in the aerospace and automotive industries due to their light weight and high strength. Traditional metal connection methods such as welding have more restrictions on the connection between metal and CFRP, so the current CFRP and steel connections are mainly glued and mechanically connected. This article mainly summarizes the mechanism of bonding and the failure modes and the advantages of glue-riveted joints, and summarizes the main process parameters of glue: the thickness of the glue layer, the bonding length and the effect of surface treatment on the strength of the joint. Finally, it still exists in this field. The main issues and future research work put forward their views.
Carbon fiber reinforced plastic (CFRP) is a composite material made of stacked carbon fiber tow as a reinforcement and resin (plastic) as a matrix. It has ultra-high specific strength and stiffness [1-3], It has been more mature in the aerospace field; in recent years, with the cost reduction and technology development, CFRP has been widely used in the international automotive field [4], such as BMW I3 and Lamborghini's Murciélago. Full CFRP single shell body. In addition, a variety of CFRP structural components are also widely used in car companies such as BMW, Volkswagen, Audi, Nissan GTR, Aston Martin and other car companies [5]. However, in the domestic automotive field, the application level of CFRP still lags behind the international level.
Compared with traditional steel, aluminum, and other metal materials, CFRP has obvious performance advantages, such as low density, high strength, good toughness, high damping, good vibration resistance, excellent heat resistance, and excellent fatigue resistance. Anti-corrosion and anti-radiation performance (multiple molding processes), good designability, good impact resistance and energy absorption capacity [2], etc. At the same time, CFRP can meet the automotive industry's requirements for light weight, safety and comfort [5] Therefore, the connection between steel or aluminum alloy and CFRP is a key issue that needs to be solved urgently. For the traditional connection methods such as welding, it is not suitable for the connection of non-metal and metal heterogeneous materials. Therefore, the advent of glue bonding and glue-rivet hybrid connection has greatly promoted the use and development of CFRP. Gluing and glue-rivet connection technology are playing an increasingly important role in aviation and automotive manufacturing, and steel as a common material and CFRP connection is of great significance for automotive lightweighting. This article mainly summarizes the current development status of glue bonding and glue-riveting, discusses the effects of process parameters such as the thickness of the adhesive layer, the bonding length, and the surface treatment on the strength of steel and CFRP glued joints. Problems and prospects for future development.
1 Status of development of gluing
Adhesive connection is a method of joining two parts by using an adhesive. The method of mechanical bonding and chemical bonding between the interface of the adhesive and the connected part is used to achieve the connection of the connected part [6].
Since gluing is an interdisciplinary discipline involving multiple disciplines such as polymer, physics, and chemistry, the bonding mechanism is more complicated. At present, the main adhesive mechanisms are as follows [7-11]: (1) adsorption theory, which believes that the interface bonding strength mainly comes from intermolecular forces; (2) electrostatic theory, which considers that the surface of the substrate and the surface of the adhesive can be regarded as one The capacitor forms an electric double layer, thus generating electrostatic attraction; (3) diffusion theory, which believes that the bonding surface forms a transition region through molecular diffusion to achieve the connection between the connected piece and the adhesive; (4) mechanical bonding theory, which considers the source of the strength of the joint Due to friction, increasing the surface roughness of the connected component can increase the strength of the joint; (5) The chemical bond theory believes that a chemical reaction between the adhesive and the surface of the connected component generates a new chemical bond, and finally achieves the interface bonding between the adhesive and the connected component (6) Weak interface layer theory, which believes that the bonding strength of the bonding interface will be affected by the weak boundary layer, thereby reducing the joint strength. However, the above theories can only explain part of the phenomenon, and the complexity of bonding should be further studied.
There are four main types of failure of the cemented joints of composite plate members, including three types of pure fracture and mixed fracture. The adhesive layer failure (cohesive failure) perfectly displays the performance of the adhesive and is an ideal form of adhesive joint failure. However, in actual application, pure adhesive layer failure rarely occurs, but mostly mixed with adhesive layer failure and interface failure. Mixed failure is represented by the ratio of three pure failure forms. The higher the ratio of adhesive layer damage or substrate failure, the better the quality and performance of the adhesive joint.
There are many kinds of connection methods, such as single lap joint, double lap joint, miter joint, trapezoidal joint, etc. Generally, the single joint is widely studied, which provides a mechanical basis for complex connections [11].
The glue-riveted hybrid connection uses a combination of mechanical and adhesive connections. Generally, there are two methods, one is to glue first, and then riveting after the glue layer is cured, and the other is when the glue layer is not cured. That is, riveting, and bonding after the adhesive is cured can improve the quality of the joint.
The mixed application of the two connections can make the glue and rivets at the joint share the load and improve joint performance, especially under overload or high fatigue stress conditions [12-15]. If properly designed, compared to the simple adhesive connection, the glue-riveted hybrid connection can overcome the shortcomings of weak anti-peeling and splitting stresses, effectively delay or prevent the damage expansion of the adhesive layer, and improve the impact resistance (anti-peeling) Anti-fatigue, anti-creep and other properties. Moreover, compared to pure mechanical connections, glue-rivet hybrid connections can reduce the use of mechanical fasteners, thereby reducing quality and improving structural efficiency. Unfortunately, there is still a problem of hole stress concentration in glue-rivet hybrid connections.
Bodjona et al. [16] calculated load sharing by modeling a single-bolt single-lap glue-rivet composite connection, and found through experiments that the bolt can share nearly 40% of the total external load. Kweon et al. [17] compared the joint performance of two different adhesives of CFRP and aluminum alloy and three different connection methods (glue, riveting, glue-rivet connection) through experimental tests. It was found that when using paste adhesive, the glue-rivet connection The average strength is higher than glued and riveted, and reached 84% of the combined strength of the other two connection methods. In addition, Kelly et al. [18] tested CFRP glue-rivet hybrid joints and found that the fatigue life of glue-rivet joints has been extended by an order of magnitude compared to glued joints and riveted joints. At the same time, it is found through the back strain technology that crack initiation of the glue-rivet connection joint is slowed down, and the presence of the bolt also hinders the crack propagation, thereby extending the fatigue life. The above research shows that the adhesive-rivet connection can improve the fatigue resistance of the joint, but the effect of the properties of the adhesive on the joint performance and the influence of environmental factors such as humidity and temperature on the joint performance need to be further studied.
2 the influence of process parameters on the strength of glued joints
2.1 Effect of the thickness of the adhesive layer on the strength of the adhesive joint
A large number of literatures have studied the effect of the thickness of the adhesive layer on the mechanical strength of the glued joint [19-28]. Generally, the performance of the thinner adhesive layer is higher than that of the thicker adhesive layer, because the joint end will have stress concentration, The stress concentration is smaller than when the adhesive layer is thicker.
[19] conducted a fracture toughness test on a double cantilever beam (DCB) joint and found that as the thickness of the composite laminate increased, the interface strength and fracture toughness increased. However, when steel is used as the base material, the joint performance is just the opposite. As the bonding thickness increases, the interface strength and fracture toughness decrease. Naito et al. [20] conducted tensile and shear tests on single lap joints and butt joints (polyurethane glue as adhesive and aluminum plate as substrate) and found that as the bonding thickness increases, the tensile strength of the butt joint decreases, but the adhesive layer The thickness has no effect on the shear strength of the single lap joint. And a large number of studies have found that the most suitable thickness of the adhesive layer is 0.1 ~ 0.5mm [21-22, 27-28].
Lee et al. [23] explored the mechanism of adhesive layer thickness to joint strength through microscopic characterization, and learned about five different adhesive layer thicknesses (0.1mm0.10.3mm ､ 0.7mm ､ 1.5mm ､ 2.1mm) versus compact tension (CT) The effect of the fracture toughness of the sample is found that when the thickness of the adhesive layer is less than 0.3mm, the fracture toughness increases with the increase of the thickness of the adhesive layer. When the thickness of the adhesive layer increases to 0.7mm, the fracture toughness decreases significantly. Increased, fracture toughness is basically stable on a platform. Through microscopic characterization, it is found that when the thickness of the adhesive layer is small, the distance between the interface damage zone and the main crack is close. Since energy can be dissipated at the interface, the stress concentration at the crack tip will be reduced, so the fracture toughness is higher than the bulk fracture toughness. When the thickness is large, the damage area near the interface is relatively small and the distance from the middle main crack is far, which has little effect.
Banea et al. [24] tested the effect of the thickness of the polyurethane adhesive (SikaForce17888 L10) on the interface strength and tensile and shear strength. Tool steel (DIN 40CrMnMo7) was used to make DCB specimens, and high-strength steel (DIN C65 heat treated) was used as a specimen for testing the tensile shear strength of the lap joint. The DCB sample and the overlapped sample are all surface treated: sandblasted and acetone cleaned, and then glued. The relationship between the fracture toughness (GIC) of the DCB sample and the thickness of the adhesive layer is shown in Figure 3. When the thickness of the adhesive layer is less than 1 mm, the GIC increases approximately linearly with the increase of the thickness of the adhesive layer. However, when the thickness of the adhesive layer increases from 1 mm to 2 mm, the GIC Increase by about 20%. This is because as the thickness of the adhesive layer increases, the number of defects (such as micropores) in the adhesive layer increases, and the constraint of the substrate on the adhesive layer also decreases. Therefore, as the thickness of the adhesive layer increases, GIC will slowly become a platform.
The tensile shear test of the lap joint of the CT sample found that as the thickness of the adhesive layer increased, the maximum shear stress that the lap joint could withstand gradually decreased. Banea et al. [24] found that when the thickness of the adhesive layer is increased from 0.2mm to 0.5mm, the tensile shear strength is reduced by about 4%, when it is increased to 1mm, the tensile shear strength is reduced by about 13%, and when it is increased to 2mm Reduced by approximately 31%. Grant et al. [25] also used steel as a substrate and epoxy resin as an adhesive for single-lap joint pull-shear tests. It was found that with the increase of the thickness of the adhesive layer, the tensile strength of the joint decreased rapidly, and the thickness of the adhesive layer increased to 2mm. As a result, the tensile and shear strength drop reached 66%, indicating that the epoxy resin adhesive is more sensitive to the increase in thickness and tensile and shear strength. In the above, only one kind of adhesive is used to study the effect of the thickness of the adhesive layer on the strength of the joint. However, due to the different properties of different adhesives, such as viscosity, the optimal thickness of the adhesive layer will also be different. Therefore, the next stage of research should consider the relationship between the inherent properties of the adhesive and the thickness of the adhesive layer, and further study the mechanism of the thickness of the adhesive layer affecting the joint strength.
2.2 Effect of bonding length on the strength of glued joints
Grant et al. [25] studied the influence of three bonding lengths (15mm ､ 30mm ､ 45mm) on the strength of joints, of which the length of the steel plate was 102.5mm. The study found that when the bonding length was 30 mm, the joint strength was the best (as shown in Figure 4). Yang Yan [29] set up five parameters (10mm ､ 20mm ､ 30mm ､ 40mm ､ 50mm) to conduct a finite element simulation of the strength of a single lap joint with a steel plate length of 100mm. It was found that when the bond length is less than 30mm, the failure load varies with The increase in bonding length is basically linear, and when the bonding length exceeds 30mm, the failure load increases slowly and tends to a platform. Considering economic benefits, the most suitable bonding length is 30mm.
2.3 Effect of surface treatment on the strength of glued joints
Surface treatment is usually the first and most important step in the gluing process. The surface treatment of the bonding surface in the gluing process directly affects the quality of the joints of the bonded parts and the joints of the glue-rivet connection [30- 31]. The purpose of bonding surface treatment includes removing surface contamination, increasing surface energy to improve wettability and chemical bonding force, and improving surface roughness to improve the contact area of mechanical occlusion and adhesive, thereby improving joint quality. The commonly used surface treatment methods can be divided into mechanical treatment, high energy treatment and chemical treatment. The specific surface treatment methods and classifications are shown in Table 1.
The surface of steel is usually characterized by surface energy, surface chemical composition, and surface roughness and morphology [32], all of which have an important effect on joint strength. The common surface treatments for steel are mechanical treatment, laser treatment, and chemical treatment.
Fernando et al. [33] found that the steel surface was polished by sandpaper after five mechanical treatments (acetone cleaning, 100Cw sandpaper sanding, 0.5mm sandblasting, 0.25mm sandblasting, and 0.125mm sandblasting) The roughness is the highest, followed by 0.5mm sand blasting, and as the diameter of the sand used for blasting becomes smaller, the surface roughness of the steel decreases. After acetone cleaning, the steel surface has the lowest roughness. The surface energy of the steel surface after sandblasting is the highest, followed by sandpaper sanding. The tensile stress test of the butt joint found that the tensile strength of the steel surface after sandblasting at 0.125mm was higher than that of the steel surface after sandblasting at 0.5mm, and the strength of the steel surface after sandblasting at 0.25mm was the smallest. The tensile strength is higher than that after sandpaper grinding and acetone cleaning. However, it was found in the single lap tensile-shear test that the maximum shear force on the steel surface after sandblasting increased as the diameter of the sandblast decreased. Similarly, the maximum shear force on the steel surface after sandblasting is greater than the maximum shear force on the steel surface after sandpaper sanding, and the maximum shear force on the steel surface after acetone cleaning is the smallest. The results show that the surface energy has a decisive effect on the strength of the glued joint.
Marco et al. [34] used a low-energy laser ablation method to surface-treat 304 stainless steel, and found through various characterization methods that the content of CO polar groups and surface wettability on the surface of the aluminum alloy after treatment had been improved, and after laser treatment The surface roughness of the rear steel has also been improved, and finally the performance strength of the adhesive connection has been improved. Kamil et al. [35] used different pressures (50MPa ､ 100MPa ､ 150MPa) for water treatment on stainless steel, and found that compared with 320 mesh sandpaper sanding treatment, the surface roughness of stainless steel after water impact treatment was significantly improved, and the average surface roughness (Ra ) To 9.3 μm. When the water impact is 50 MPa, the maximum shear stress of the stainless steel after surface treatment is the same as that of 320 mesh sandpaper sanding. When the pressure is increased to 100 MPa, the shear strength is increased by 28%. When the pressure is increased to 150 MPa, the shear strength is decreased. The shear strength is lower than the sandpaper sanding process, which may be due to the large number of micron-scale grooves, and the adhesive is not easy to enter, which reduces the wettability and causes the shear strength of stainless steel to decrease.
The surface treatment of CFRP generally adopts mechanical treatment [36] ､ plasma treatment [37-38] ､ laser treatment [39-40] ､ ion-assisted reaction [41-43] and other methods. Encians et al. [44] found that mechanical treatments, such as sandpaper sanding and sandblasting, can reduce the fluorine on the CFRP surface, increase surface roughness, and improve surface activity and wetting.
In 2002, Rhee et al. [43] used Ar + ion-assisted reaction for surface treatment of CFRP, and found that the contact angle of CFRP decreased from 80 ° to 8 °, and the surface energy increased from 31 × 10-7J / cm2 to 72.4 × 10-7J / cm2, T-peel strength and shear strength have been greatly improved. In 2011, Zaldivar et al. [39] used O2 ､ CO ､ CO2 as an active gas to perform plasma treatment on the CFRP surface and compared it with mechanical treatment. They found that plasma treatment can incorporate oxygen-containing functional groups on the surface of CFRP and improve the hydrophilicity of the material. Compared with mechanical treatment, the joint's lap shear strength is increased by 75%. In 2012, Fischer et al. [35] used UV lasers and CO2 lasers to perform laser surface treatment on CFRP surfaces, and found that the laser surface treatment selectively removed CFRP matrix resin without damaging carbon fibers, and improved the bonding strength. At the same time, this treatment method avoids thermal damage, making its main failure mode cohesive failure of the adhesive layer, and giving full play to the strength of the adhesive layer.
The above research shows that proper surface treatment can greatly improve the mechanical properties of joints, but the mechanism of surface treatment is different, some are to change the surface energy and some are to change the roughness, so the influence on joint strength is different. The next research can focus on these two The respective proportions of the effects, so as to better understand the effect of surface treatment on joint strength.
3 conclusion
The development of the aerospace and automotive industries has made CFRP more and more widely used. At present, the connection methods of CFRP and metal steel are mainly glued and glue-rivet connected. Since different process parameters such as the thickness of the adhesive layer, the bonding length, and the way of surface pretreatment have a significant impact on the quality of the joint, it is necessary to study their mechanism and influence mode. In order to further improve the stability of joint performance, further research can be carried out in the following directions:
(1) The mechanism of action of the adhesive and the surface of the substrate. At present, there are many gluing mechanisms and can only explain some phenomena. A deeper study can help achieve better joint quality and stability.
(2) the influence of environmental factors. Because the glued parts are put into use, the subsequent influence of environmental factors such as temperature and humidity on the quality of the joint must be considered.
(3) The influence of the thickness of the adhesive layer on the quality of the joint is studied from the nature of the adhesive itself. Due to the different inherent properties of the adhesive, such as viscosity and curing time, the optimum thickness of the adhesive layer may be different. Therefore, considering the inherent properties of the adhesive, considering the optimal thickness of the adhesive with different viscosity has guiding significance for the bonding process.
(4) Mechanism of surface treatment on joints. At present, it is mainly from the aspect of surface characterization to explore how the surface treatment affects the quality of the joint, such as increasing the roughness to increase the mechanical interlocking ability or the surface energy, and increasing the content of polar groups to make the surface of the connected component wettable and molecular diffusion ability Lift, thereby increasing joint strength. However, the proportion of the two methods is unclear, and their respective effects or interactions need further research.