Elsevier

Composite Structures

Volume 241, 1 June 2020, 112108
Composite Structures

Thermo-stamping co-curing process for CFRP/steel hybrid sheets and its interface strength improvement

https://doi.org/10.1016/j.compstruct.2020.112108Get rights and content

Abstract

The co-curing interface between CFRP and metal has a significant influence on the comprehensive mechanical properties of the composite structure. In this paper, a series of single lap specimens were fabricated by a thermo-stamping process with fast curing prepreg and dual phase steel DP980. The thermo-stamping co-curing experiments were conducted under different forming conditions based on a grit-blasted metal surface to study the effect of process parameters on the interface strength. In addition, two types of interface strength improvement methods were tested: cutting mechanical grooves on the metal surface and smearing different carbon nanotube or graphene coatings on the metal and prepreg surfaces. The interface strength improvement mechanism was further explained by microscopic characterization. Finally, a finite element model combining cohesive element damage and three-dimensional Hashin failure was established to predict the failure mode of the co-curing joint. The results show that the initial pressure, heating curve and laying up methods have a certain degree of influence on the interface strength, and the best interface enhancement effect could be obtained by coating a carbon nanotube/epoxy resin and a curing agent on the metal and prepreg surfaces, respectively.

Introduction

CFRP/metal hybrid structure has attracted increasing attention in recent years [1]. Some typical fiber–metal laminates (FML) applied in the aerospace industry include TiGr, ARALL and GLARE [2], which are essential components of fuselage skin that use titanium or aluminum as the strengthening metal. In the automotive industry, high-strength steel (HSS) is mainly used in hybrid structures to improve energy absorption properties, especially in the crucial safety parts of the car body such as center pillar [3]. The joint between CFRP and metal plays an important role in the manufacture of CFRP/metal hybrid structures. As a special form of adhesive bonding, the co-curing process has attracted scholarly interest [4], [5], especially for the thermo-stamping co-curing process, where the metal is bonded with CFRP by the matrix resin under the thermal and pressure load of a stamping die. The advantages of the thermo-stamping co-curing process are as follows: i) since the prepreg is used as the raw material, the process of resin mixing, injection and infiltration is omitted; ii) it can achieve one-step integrated forming of the CFRP/metal hybrid structure with higher efficiency; iii) the process is easy to realize large-scale continuous production, which offers broad application prospects in the automotive industry.

Surface preparation of the adherend has a crucial effect on the bonding behavior. Various surface treatment methods have been investigated to increase the bonding strength, such as grit blasting, anodizing [6], laser treatment [7], plasma treatment [8], etc. However, interface strength enhancement based on a physical mechanism is relatively limited. In recent years, nanoparticles such as carbon nanotubes (CNTs) and graphene have also been used to increase the bonding strength between metal and CFRP [9], [10], [11], most of these methods focus on bonding cured CFRP with CNT-reinforced resin adhesives or are based on liquid resin co-curing forming processes such as vacuum-assisted resin transfer molding (VARTM). There are few relevant studies on the interfacial modification method of nanoparticles in the prepreg-based co-curing process, especially in the thermo-stamping process.

The latest research results show that for high-strength steel sheets such as DP980, the out-plane shear strength is approximately 15% lower than in-plane shear strength [12], [13], [14], but it can also reach hundreds of MPa, which is far higher than the general bonding strength of heterogeneous materials such as CFRP and steel. Improving the structural design is an effective way to enhance the integrity of heterogeneous materials [15], but achieving excellent curing quality and increasing the fracture strength of the co-curing interface remains a significant challenge.

In this paper, we conducted a number of thermo-stamping co-curing experiments of DP980/CFRP hybrid structures. A series of single lap specimens were designed, and the interfacial shear strength was tested with a tensile machine and DIC (digital image correlation) system, after which the influence of different co-curing parameters on the interfacial shear strength was analyzed. In addition, we tested two types of surface treatment methods, including mechanical methods such as cutting grooves and chemical methods such as smearing CNT and graphene coatings, and investigated the internal mechanism by microscopic characterization experiments. Finally, we established a finite element model combining cohesive elements and a three-dimensional Hashin damage constitutive model to provide an accurate method for predicting the interface failure mode and calibrated the relationship between shear strength and the maximum tensile force.

Section snippets

CFRP prepreg and metal sheet

A fast-curing UD prepreg with a thickness of 0.1 mm is used in this study. The prepreg is composed of T700 carbon fibers from Toray and FRD-YQ5-01S fast-curing epoxy resin with 40% resin weight. The physical properties of the epoxy resin provided by the supplier are listed in Table 1. Double-phase steel DP980 with a thickness of 1.6 mm is used for the thermo-stamping co-curing process. The stress–strain curves of DP980 along different angles relative to the rolling direction are shown in Fig. 1

Forming conditions in the co-curing process

A series of single lap specimens are fabricated under different forming conditions to study the influence of process parameters on the interface strength. The forming condition factors considered include the surface treatment method, initial pressure, heating curve and prepreg layup method. The specific forming conditions of the specimens are listed in Table 2.

When testing the influence of process parameters, a grit-blasted metal surface is used to obtain better interface strength and

Introduction of the mechanical grooves

The mechanical interlocking mechanism between CFRP and metal is an important source of joint shear strength. However, as the fluctuation amplitude of the grit-blasted surface morphology is too small, the influence of mechanical interlocking on the interface strength cannot be directly observed by DIC during the tensile process of a single lap specimen.

Therefore, a metal specimen with a grooved surface is designed to test the mechanical interlocking effect, as shown in Fig. 15. The grooves are

Introduction of CNT and graphene coatings

In this research, three types of reinforced coatings are smeared on metal and prepreg surfaces—a CNT/waterborne resin coating, a graphene/waterborne resin coating and a CNT/epoxy resin coating—to test the enhancement effect of coatings with different matrices and nanofillers on the interface strength. The CNT and graphene waterborne resin coatings are provided by Suzhou Tanfeng Graphene Technology Co. Ltd, of which the matrix is polyurethane (PU) and acrylic ester. The CNT/epoxy resin coating

Finite element model

A finite element model of the single lap specimen is established based on Abaqus 6.14/Standard. The mesh detail and boundary conditions are shown in Fig. 31. The dimension of the model is referred to the specimen size in Fig. 2(a). Since the friction coefficient of the Teflon spacer is rather small, it is omitted in the model. The materials of DP980 and CFRP laminates are both modeled with solid elements C3D8R. The mesh at the joint area is refined and the enhanced hourglass control method is

Evolution of the stress and damage variable of cohesive element layer

The comparison of the tensile curves between the simulation and experiment is shown in Fig. 34, where the experimental data of 1# specimens are taken as an example. The maximum load in the simulation is set to be equal to the average load in the experiment. In the first stage, both DP980 and CFRP are linearly elastic. When the tension force exceeds approximately 2500 N, the arm of CFRP begins to yield; as a result, the slope of the tensile curve decreases until the final failure of the joint,

Conclusion

The temperature and pressure are closely related during the co-curing process. At the beginning of the curing process, with increasing temperature, the viscosity of the matrix resin decreases, and the resin starts to flow, resulting in a downward trend of pressure. After the resin molecules are cured by the crosslinking reaction, the pressure increased with the temperature due to the thermal expansion of CFRP and metal.

A rough metal surface is more helpful for achieving higher co-curing bonding

CRediT authorship contribution statement

Cong Guo: Investigation, Validation, Writing - original draft. Ji He: Conceptualization, Methodology, Funding acquisition, Writing - review & editing. Youhuang Su: Visualization. Shuhui Li: Supervision.

Acknowledgements

Financial support from the Natural Science Foundation of Shanghai, China (Grant No.19ZR1425800), National Natural Science Foundation of China, China (Grant No.51975364), Fundamental Research Funds for the Central Universities, China and New Faculty Fund (SMC, China) of University are gratefully acknowledged.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Conflict of interest

The authors declare that they have no conflict of interest.

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