# Study of Mechanical Properties of Micron Polystyrene-Tughened Epoxy Resin
## 1 Introduction  
Fatigue cracking of steel structures is one of their common diseases [1], and the adhesion of carbon-fiber-reinforced polymer (CFRP) is an effective method to strengthen steel structures [2; 3; 4]. Epoxy resin is the most commonly used adhesive, and its performance is crucial to the reinforcement effect [5; 6; 7]. It has been shown that the damage mode of CFRP-reinforced steel structures under fatigue loading is often the adhesive-steel/CFRP interface damage [8; 9; 10], and the adhesive has a significant influence on the bonding performance of the bonding interface [11; 12; 13]. Epoxy resin is a thermosetting resin with good mechanical properties, bonding performance and thermal stability performance [14; 15], but its curing into a cross-linked mesh structure, brittleness, poor impact resistance and lack of toughness are shortcomings that affect the reinforcement effect to some extent [16; 17; 18]. Therefore, it is necessary to study the high-toughness adhesives applicable to CFRP-pasted steel structures.  
To clarify the toughening mechanism of micron (nano)-material-modified epoxy resins, improve their bonding properties, and prepare high-performance epoxy adhesives, numerous scholars have conducted a large number of related studies [19; 20; 21; 22]. [PERSON] et al. [23] investigated the effect of carbon nanotube (CNT)-modified epoxy adhesive on carbon-fiber-cloth-steel interface and found that the carbon-nanotube-epoxy adhesive can transfer moreload from the main structure to the carbon-fiber-fabric laminate, thus changing the bonding performance. [PERSON] et al. [24] used experimental methods based on in situ Fourier transform infrared spectroscopy, dynamic mechanics and SEM microstructure to determine the enhancement effect of alumina nanoparticles in bisphenol A (DGEBA)-type epoxy resin, diglycidyl ether, compared to pure epoxy resin, with relatively low concentrations of alumina-nanoparticle-modified adhesives showing significant improvements in mechanical and thermal properties. [PERSON] et al. [25] conducted tensile tests on modified epoxy resins using expanded polystyrene as filler and short-cut glass fibers as reinforcement, and the results showed that the modified composites had significantly lower density and higher specific modulus, but both had lower specific strength than the pure resin, and increasing the fiber/filler ratio would enhance the tensile strength and elastic modulus of the colloids. [PERSON] [26] et al., used end-epoxy-based polystyrene oligomers (PSG) to toughen the epoxy/anhydride resin system. The results showed that when the molecular mass of PSG was 3500-4900 and the weight fraction added was 2-7 wt%, the impact toughness was improved from 16.4 kJ/m\({}^{2}\) of unmodified epoxy resin to 31.1-39.8 kJ/m\({}^{2}\), which increased by 89.6-143%, and the strength and heat resistance of the epoxy resin were better maintained while toughening. However, epoxy resins have low strength in terms of crack extension and exhibit brittle fracture behavior because of their tight three-dimensional molecular network structure, which leads to limitations in the use of such polymers under loading conditions [27]. Moreover, most of the aforementioned studies used high-temperature curing processes, which are cumbersome and do not facilitate the practical engineering applications of CFRP-reinforced steel structures.  
Therefore, in this work, based on the epoxy resin adhesive ratios developed by the team in the early stage, the effects of mixing different weight fractions of micron PS-modified epoxy resin adhesives in tensile, flexural and impact properties were investigated by using an ambient-temperature curing process and ultrasonic cell crushing and dispersion techniques. Then, the tensile section morphology of the adhesive was observed by SEM electron scanning microscopy to further reveal the toughening mechanism of micron PS on epoxy resin.