Keywords

1 Introduction

In pursuit of energy-saving and environmentally friendly development strategies, the adoption of lightweight materials has become crucial for automotive manufacturers to achieve these goals. Carbon fiber reinforced polymers (CFRP) exhibit superior mechanical and physical properties compared to steel, such as high specific strength, high specific stiffness, fatigue resistance, and low density, indicating their vast potential as lightweight materials. However, they also present certain drawbacks, including poor toughness and high costs [1]. The localized application of CFRP to enhance metal components, forming steel-CFRP hybrid structures, successfully retains the original component's ductility while simultaneously improving rigidity and ultimate load capacity. Steel-CFRP hybrid structures have been widely utilized in the automotive and aerospace industries [2,3,4], advancing remarkable performance benefits and prospects in applications such as aircraft wings and fuselage skin, automotive drive shafts, and vehicle bodies. To circumvent additional mechanical damage, thermal damage, and weight associated with conventional joining techniques like riveting and welding, steel and CFRP are bonded using adhesives, resulting in a more even stress distribution and flexible design. The mechanical performance of steel-CFRP interfaces has been identified as a critical factor in determining the overall mechanical properties of steel-CFRP composite components [5]. Understanding the underlying bonding mechanisms at the steel-CFRP interface is instrumental for facilitating the successful application of steel-CFRP composites in vehicle body structures.

To elucidate the impact of various factors on the mechanical performance of adhesive interfaces, it is imperative to accurately characterize the influence of stress and fracture energy during the interfacial crack propagation process. The cohesive zone model (CZM) is widely adopted by researchers due to its strong physical significance, numerical stability, and simulation compatibility [6,7,8,9]. Wang et al. [7] investigated the influence of adhesive properties and thickness on the shear performance of steel-CFRP interfaces, revealing that ductile adhesive interfaces are well-suited for trapezoidal CZM characterizations with thickness impacting shear strength, whereas brittle adhesive interfaces are more amenable to bilinear CZM representations, but adhesive layer thickness has less pronounced effects on shear strength. Pang et al. [8] found that the shear strength of steel-CFRP interfaces bonded with Sikadur-30 (CN) adhesive increases with an increasing loading rate. Zhu [9] demonstrated that extreme temperatures can alter the mechanical properties of adhesive layers, subsequently reducing the mechanical performance of steel-CFRP interfaces and changing the interfacial failure modes.

To further enhance the mechanical performance of bonded interfaces, surface modification of the adherend is one of the most common approaches, including techniques such as grinding [10, 11], sandblasting [11, 12], plasma treatment [13,14,15], mechanical scoring [16], and laser processing. Russian et al. [11] demonstrated that increasing the surface roughness of steel through sandblasting and grinding treatments led to enhanced interfacial strength, with the strength post-sandblasting being 1.62 times. Manuel et al. [12] revealed that shot peening, compared to sandblasting, can further improve the shear strength. Studies by Williams et al. [14] and Lin et al. [15] showed that plasma-treated surfaces not only exhibit altered surface morphology but also host reactive chemical functional groups, resulting in an improved interfacial bonding performance. Guo et al. [16] investigated the impact of cutting mechanical grooves on the steel surface on the shear behavior of steel-CFRP interfaces, finding that resin flowed into the grooves and induced mechanical interlocking during shearing, consequently enhancing the shear strength of the interface.

Laser processing, characterized by its excellent controllability and capability to generate specific surface morphologies, has been widely investigated for creating dimples, linear grooves, and crossed grooves on metal surfaces. In studies focusing on dimpled microstructures, Zhang Yulong [17], Zou et al. [18], and Li et al. [19] conducted laser treatments with varying power and spacing on metal surfaces, revealing that surface roughness, surface energy after treatment were closely related to the bonding performance. Feng et al. [20] examined the influence of three structural morphologies on steel adhesive bonding performance, and found that although dimple morphology did not significantly enhance interfacial adhesive strength, both groove and grid morphologies notably increased the strength, with failure modes transitioning to cohesive failure.

Laser processing is a rapid and controllable surface treatment method. Current research indicates that laser processing can significantly enhance the interfacial strength of steel-CFRP interface. However, existing mechanical characterizations are limited to shear strength measurements, which cannot adequately describe the failure process. This paper investigates the influence of groove microstructures, created through laser treatment, on the interfacial shear strength, stress distribution, and failure modes of steel-CFRP double-lap shear specimens. Furthermore, we elucidate the mechanisms underlying the improved shear strength at the steel-CFRP interface using a cohesive zone model.

2 Double Lap Shear Experiment of Steel-CFRP Based on DIC

2.1 Material Properties

The steel plates utilized in this research were 5.0 mm thick Q235, with a plate width of 5.0 mm and an elastic modulus of 205GPa. The cured CFRP plate used in this study, with a width of 25 mm and a thickness of 0.36 mm, was supplied by Composite Materials Easy Purchase (Beijing) Technology Co., Ltd. According to ASTM D3039, the tensile strength and elastic modulus were determined to be 2934 MPa and 147 GPa.

2.2 Laser Treatment of Steel Surface

To investigate the impact of laser treatment on bond behavior of the steel-CFRP interface, an IR laser (Wuhan Raycus Fiber Laser Technologies Co., Ltd.) with a wavelength of 1064 nm and spot diameter of 50 μm was used on steel plate. Linear groove microstructures were prepared on the steel surface through a filling method, as illustrated in Fig. 1a. The steel surface can be divided into two distinct areas: the laser-treated area and the untreated area. The laser-treated area has a width of W, while the untreated area has a width of S. The process parameters were controlled by adjusting the galvanometer, which was based on the computer numerical control (CNC) system. In the laser processing area, the laser scanning strategy was configured with a power of 24 W, a scanning frequency of 40 kHz, a scanning speed of 500 mm/s, and a scanning interval of 0.05 mm within the laser filling area, resulting in the achieved effect shown in Fig. 1b. The direction of laser scanning was perpendicular to the length direction of the sample. In order to quantify the enhancement of interfacial performance on the surface of steel after laser treatment, a control group was established in this study, which group solely employed the use of alcohol spray to remove surface impurities from the steel.

Fig. 1
Two diagrams a and b. A, a patterned steel surface featuring laser process area and untreated area in alternate sequence. B, the size and overlap of laser spots in a scanning process. An arrow at the bottom indicates laser scanning direction.

a Schematic diagram of linear groove microstructure of the steel surface; b schematic illustration showing the laser spot size and overlap

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2.3 The Preparation of Steel-CFRP Double Lap Shear Specimens

In this study, steel-CFRP double lap shear specimens were used to investigate the interfacial shear performance, as illustrated in Fig. 2a. The specimens were loaded at a rate of 1 mm/min using an MTS universal testing machine, and surface axial strain measurements were conducted using a three-dimensional digital image correlation (3D-DIC) system [21]. The testing area is located on the right side of the specimen, with a CFRP width of 25 mm and a length of 200 mm. The left end of the scatter region was taken as the origin and positive to the right. The first strain extraction region is positioned at 10 mm along the central axis, and the remaining strain extraction region are spaced at 20 mm intervals along the central axis. The steel-CFRP double-shear specimens were fabricated using the vacuum bagging process, as shown in Fig. 2b. The first step in the manufacturing process is to remove impurities from the surface of the steel plate using alcohol, which is then polished with sandpaper and treated with a laser. Subsequently, the prepared steel plate is coated with CFRP prepreg. The steel-CFRP double lap shear specimen is then placed into a vacuum bag and subjected to vacuum extraction. Finally, the vacuum bag is positioned within a heating furnace at a temperature of 120 °C for a curing duration of 2 h. Due to the resin in the prepreg, the CFRP is bonded to the steel plate during the curing process.

Fig. 2
An illustration depicts two processes, a and b. A depicts an assembly of a steel-C F R P double lap shear specimen, and b depicts the four-step application of vacuum bag pressure to the assembled specimen labeled surface treatment, lay the prepreg, vacuum sealing, and heating and curing.

a Construction of the steel-CFRP double lap shear specimen; b vacuum bag pressure process for the steel-CFRP double lap shear specimens

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2.4 Observation of Steel Surface Morphology

Surface topography observation of the laser-treated steel was conducted under a digital microscope (VHX-1000) to obtain dimensional parameters of linear groove microstructures and to observe the characteristics of the interface after failure.

2.5 Cohesion Model Calculation

The interfacial shear stress and relative slip for the steel-CFRP double lap shear joint are determined by the integral and difference of strain values from the strain extraction regions, whose Computational methods have been used in various literature [7, 10]. Assuming that the CFRP section's stiffness is significantly lower than that of the steel section, and that the strain varies linearly between two adjacent strain extraction region. The interfacial shear stress and relative slip of steel-CFRP double lap shear joint at the middle point between two adjacent strain extraction region can be expressed as

$$\tau_{i + 1/2} = E_{f} t_{f} \frac{{\varepsilon_{i + 1} - \varepsilon_{i} }}{{x_{i + 1} - x_{i} }}$$
(1)
$$\delta_{i + 1/2} = \frac{{\delta_{i + 1} { + }\delta_{i} }}{2}{ = }\frac{{\varepsilon_{i} + \varepsilon_{i + 1} }}{4}(x_{i + 1} - x_{i} ) + \sum\limits_{i + 1}^{{\text{n}}} {\frac{{\varepsilon_{i} + \varepsilon_{i + 1} }}{2}} (x_{i + 1} - x_{i} )$$
(2)

where \(\tau_{i - 1/2}\) and \(\delta_{i + 1/2}\) are the interfacial shear stress and relative slip at the middle point between the \(i_{{{\text{th}}}}\) strain extraction region and the \((i + 1)_{{{\text{th}}}}\) strain extraction region; \(E_{f}\) and \(t_{f}\) are the elastic modulus and the thickness of CFRP; \(\varepsilon_{i}\) and \(\varepsilon_{i + 1}\) are the \(i_{{{\text{th}}}}\) and \((i + 1)_{{{\text{th}}}}\) strain extraction region, respectively; \(x_{i}\) and \(x_{i + 1}\) are the distances of the \(i_{{{\text{th}}}}\) and \((i + 1)_{{{\text{th}}}}\) strain extraction region from free end, respectively; and n is the total number of strain extraction regions.

3 Experimental Results and Analysis

3.1 Surface Morphology of Steel Before and After Laser Treatment

Figure 3 presents the surface morphology of the steel prior to and following laser treatment. As show as Fig. 3a, the steel surface exhibits primarily smooth characteristics, albeit with size variations attributed to manufacturing process complications. After laser treatment, the current microstructure of the steel surface is generated with a depth of D 25.2 μm, a width of W 110.6 μm, and a spacing of S 489.4 μm. Meanwhile, the metal undergoes a phase of melting due to the high pulse, followed by propulsion and reshaping caused by the impact of the laser pulses. Consequently, an accumulation with an average height of 6.6 μm forms on one side of the groove, while a stack of 18.6 μm is generated on the other side. Simultaneously, the metal spatter generated by the laser pulses envelops the entirety of the untreated area, as illustrated in Fig. 3b, c. The preexisting pits in the untreated region have been filled by the spatter, resulting in a color transformation from the metal's original silver hue to a yellow oxide.

Fig. 3
A close-up, a microscopic view, and a 3-D graph depict three stages of a steel surface namely its original state, post-laser treatment, and after laser treatment, demonstrating the transformation of the steel surface due to laser application.

a The original surface of steel; b laser treated steel surface; c 3D morphology of steel surface after laser

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3.2 Load–displacement Curve and Failure Mode

In Fig. 4, the load–displacement curves of the original and laser-treated steel-CFRP double-lap shear specimens are presented, whose behavior can be divided into three stages: an initial stage with linear load increase, an elastic–plastic stage with nonlinear load increase indicating the beginning of interface failure, and a platform stage suggesting equilibrium sustained by interlocking, friction, or interfacial bonding. During the initial stage, there was no significant difference in the initial stiffness of both specimens. However, after laser treatment, the load and displacement at the onset of the steel-CFRP interface failure increased by 133.1 and 229.4%, respectively. In the plateau stage, the load and displacement of the laser-treated specimens increased by 81.6 and 81.7% respectively. This analysis indicates that laser treatment does not alter the stiffness of the steel-CFRP interface, but it can enhance the maximum stress and relative slip at the interface. As shown in Fig. 4b, c, the failure mode of the original specimens is interface failure, while the laser-treated specimens exhibit debonding crack parallel to the longitudinal direction of the CFRP throughout the entire region, indicating CFRP delamination failure. The incorporation of Fig. 4b demonstrates that the presence of linear grooves and metal splatters causes a shift in the failure mode from interface failure to cohesive failure, owing to mechanical contraction and frictional influences.

Fig. 4
A line graph, a close-up, and a microscopic view depict a load-displacement curve for a steel-C F R P specimen, and the failure modes of original and laser-treated specimens, demonstrating the impact of laser treatment on the specimen’s structural integrity.

a Load–displacement curve of steel-CFRP double lap shear specimen; typical failure modes of steel-CFRP double lap shear specimens: b original specimens; c laser-treated specimens

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3.3 The Axial Strain Distribution of CFRP

Figure 5a, b show the axial strain distribution of CFRP for the original and laser-treated specimens during the entire test. Throughout the strain evolution process, similarities can be observed between the two: during the initial loading stages, as the applied load increases, the strain initially increases continuously on the side farthest from the free end. Once it reaches its maximum value and remains unchanged, it indicates debonding of CFRP and steel at this specific interface. At the same time, the region where the strain reaches its Upper limit continues to expand towards the free end until the entire CFRP surface, that is, the steel and CFRP are completely unbonded. Figure 5c displays the strain history of the strain extraction region located 140 mm from the free end for both the original and laser-treated samples. The strain at the interface debonding time of the laser-treated sample exhibits a significant increase from 4058 με to 7829 με, representing a 92.9% increase, while the bearing time also shows a 60.2% increase. Furthermore, the results demonstrate that the stress transferred between the CFRP and steel interfaces is significantly higher after laser treatment, leading to an increase in relative slip.

Fig. 5
Three graphs, a, b, and c. A and b are heatmaps depicting the axial strain distribution in an original and a laser-treated specimen. C is line graph that plots C F R P versus time for the strain history at a specific point, for the effects of laser treatment on strain distribution.

CFRP axial strain distribution of a original specimen; b laser treated specimen; c Strain history at x = 140 mm

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3.4 The Steel-CFRP Bond-Slip Relationship

The figure presented in Fig. 6 illustrates the bond-slip relationship at the steel-CFRP interface for untreated and laser-treated specimens, as obtained using Eqs. (1) and (2). Both the original and laser-treated steel-CFRP interfaces exhibit similar distribution trends in terms of bond-slip relationship. These trends can be divided into two stages: (1) the elastic stage, where the interface shear stress gradually increases with increasing slip until reaching the maximum interface shear stress, and (2) the plastic softening stage, where the interface shear stress gradually decreases and approaches zero with increasing slip. Based on the features of the interfacial bond-slip curves in Fig. 6, the bond-slip response for the steel-CFRP interface can be simplified as follow:

$$\tau (s) = \left\{ {\begin{array}{*{20}c} {\frac{{\tau_{\max } }}{{s_{{1}} }}s} & {if} & {0 \le s \le s_{{1}} } \\ {\frac{{\tau_{\max } }}{{s_{\max } { - }s_{{1}} }}(s_{\max } - s)} & {if} & {s_{{1}} < s \le s_{\max } } \\ 0 & {if} & {s > s_{\max } } \\ \end{array} } \right.$$
(3)
Fig. 6
A line graph plots interfacial stress versus slip for the interfacial bond-slip curves of original and laser-treated specimens. The laser-treated specimen depicts a steeper ascent and descent covering a larger area compared to the original specimen.

The interfacial bond-slip curves of original and laser treated specimens

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where \(\tau_{\max }\) is interfacial maximum shear stress, \(S_{{1}}\) is the relative slip corresponding to \(\tau_{\max }\), \(S_{\max }\) is maximum relative slip.

The specific values for \(\tau_{\max }\), \(S_{{1}}\), \(S_{\max }\) and fracture energy of the interface \(G\) (the area enclosed by the two linear segments) are provided in Table 1. The research demonstrates that the enhancement of shear performance in the steel-CFRP interface due to laser treatment is attributed to several factors, including a 83.8% increase in maximum shear strength, a 111.6% increase in relative sliding at the moment of maximum stress, a 116.8% increase in relative sliding, and a 298.4% increase in fracture energy.

Table 1 Key parameters of the bilinear bond-slip relationship
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4 Conclusion

This study employs the 3D-DIC-assisted strain measurement method to investigate the influence of laser treatment on the failure mode, ultimate load, and shear performance of the steel-CFRP interface. Additionally, a cohesive force model is established. The following conclusions can be drawn:

  1. 1.

    Laser treatment generates a steel surface microstructure with specific dimensions and metal accumulation, while metal spatter covers the untreated area

  2. 2.

    The failure mode of untreated specimens is interface failure, while laser-treated specimens exhibit CFRP delamination failure. After laser treatment, the maximum load of the steel-CFRP double lap shear specimens increases by 81.6%.

  3. 3.

    The strain at the interface debonding time of the laser-treated sample exhibits a significant increase from 4058 με to 7829 με, representing a 92.9% increase.

  4. 4.

    Laser treatment significantly enhances the steel-CFRP interface by increasing maximum shear strength (83.8%), relative sliding at maximum stress (111.6%), overall relative sliding (116.8%), and fracture energy (298.4%), resulting in improved performance of double-lap shear specimens.