3.1. Mechanical properties of epoxy matrix modified by particles
The mode Ⅰ fracture toughness and tensile properties of epoxy resin were tested, respectively. As shown in
Figure 1a, with the increase of CTBN addition, the epoxy resin matrix's
KIC value showed an increasing trend. The
KIC value of pure epoxy resin was 0.38 MPa.m
1/2, and after adding 5wt%, 8wt%, and 10wt% CTBN, the
KIC value was 0.65 MPa.m
1/2, 0.95 MPa.m
1/2, and 0.90 MPa.m
1/2, respectively, which increased by 71%, 150% and 137% compared with pure epoxy resin. It can be seen that the addition of the CTBN toughening agent can improve the toughness of epoxy resin. When the addition amount is 8wt%, the fracture toughness of the epoxy matrix is the best, and the continued addition will lead to the decline of toughness. Since the toughening mechanism of epoxy by submicron rubber particles is mainly to produce holes and matrix plastic shear, which weakens the stress field at the crack tip, when the number of particles inside the material reaches the peak, the addition of CTBN cannot produce more holes or matrix plastic yield and also reduces the efficiency of internal stress transfer. Hence, the filling amount continues to increase. The fracture toughness of the material decreased instead.
As shown in
Figure 1b, the tensile strength of epoxy resin decreased with the increase of CTBN addition. The tensile strength of pure epoxy resin is 82.3 MPa. When 5wt%, 8 wt% and 10 wt% CTBN were added, the tensile strength was 77.2 MPa, 72.6 MPa, and 68.3 MPa, respectively, which decreased by 6.2%, 11.8%, and 17.0% compared with pure epoxy resin, respectively. This decrease is because the strength of the material mainly depends on its cohesion energy density, and the addition of flexible CTBN particles reduces the cohesion energy density of the epoxy matrix, so the strength decreases. In summary, with the 8 wt% CTBN introductions, the fracture toughness of epoxy resin composite is the best, reaching 0.95 MPa.m
1/2, while the tensile property decreases by 11.8%. The modulus reduction upon adding the CTBN was expected since the particles contained soft polymer polybutadiene, and its modulus was considerably lower than that of the epoxy resins[
11]. For the neat epoxy, the Young's modulus was 1.35 GPa. At 8 wt% CTBN, the epoxy matrix Young's modulus showed a 19.3% decrease relative to the neat epoxy.
The authors tried to reduce the influence of rubber particles on the strength of the epoxy matrix. SiO
2 nanoparticles were added to the epoxy resin toughened by 8wt% CTBN for collaborative toughening. As shown in
Figure 2a, in the epoxy resin matrix containing 8wt% CTBN, the fracture toughness
KIC value of the material first increased and then decreased with the increase of nano-SiO
2 addition. When nano SiO
2 is not added, the
KIC value of 8wt% CTBN epoxy resin is 0.95 MPa.m
1/2. When 0.25wt%, 0.5wt%, 0.75wt%, and 1wt% nano-SiO
2 were added, the
KIC values of the modified epoxy matrix were 0.99 MPa.m
1/2, 1.20 MPa.m
1/2, 1.26 MPa.m
1/2 and 1.22 MPa.m
1/2, respectively. Compared with the matrix modified by 8 wt% CTBN, the increases were 4.2%, 26.3%, 32.6%, and 28.4%, respectively. Adding nano-SiO
2 further improves the fracture toughness of the epoxy resin matrix. When 0.75 wt% nano-SiO
2 is added, the toughness of the epoxy resin matrix is the best, and the fracture toughness is 131.6% higher than that of the unmodified epoxy matrix.
Figure 2b shows the effect of nano-SiO
2 on the tensile strength of 8wt% CTBN modified matrix. As shown from the figure, the addition of nano-SiO
2 makes the tensile strength of the epoxy matrix modified by 8 wt% CTBN first increase and then decrease. The tensile strength of the epoxy matrix modified by 8 wt% CTBN is 72.6 MPa, and the addition of 0.25 wt%, 0.5 wt%, 0.75 wt% and 1wt% nano-SiO
2. The tensile strength of the epoxy matrix modified by two-component particles is 73.3 MPa, 80.4 MPa, 76.6 MPa, and 73.8 MPa, respectively. The nano-SiO
2 concentration was found to be optimal at 0.5 wt%, resulting in a 10.7% increase in tensile strength relative to the one-component modified matrix. Compared with the unmodified epoxy matrix, the fracture toughness of the matrix is only 2.3% lower, and the fracture toughness is 215.8% higher than that of the unmodified epoxy matrix. Meanwhile, it can be seen that the introduction of nano-SiO
2 dramatically improved the matrix in stiffness. For the 8 wt% CTBN modified epoxy, the Young's modulus is 1.09 GPa. At 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt% nano-SiO
2, the Young's modulus of matrix are 1.18 GPa, 1.25 GPa, 1.33 GPa, and 1.35 GPa, corresponding to the increases of 8.2%, 14.7%, 22%, and 23.9%, respectively. The Young's modulus of epoxy matrix slightly reduces as the concentration of nano-SiO
2 reaches 0.5 wt% (1.25 GPa) comparing with neat epoxy (1.35 GPa).
Considering the matrix's fracture toughness and tensile properties, the synergistic modification of 0.5wt% nano-SiO2 and 8 wt% CTBN rubber particles has a balanced strengthening and toughening effect on the epoxy matrix.
Figure 3 shows the SEM diagram of the SENB specimen ductile section of pure epoxy, single-component CTBN modification, and two-component co-modification of CTBN and nano-SiO
2. As can be seen from
Figure 3a, the surface of the Mode Ⅰ fracture toughness section of pure epoxy resin is smooth and flat, and there are relatively regular parallel banded lines, indicating that cracks expand faster inside the material without crack deflection or bifurcation, which is a typical brittle fracture feature. As shown in
Figure 3b, the 8wt% CTBN-toughened epoxy matrix mode Ⅰ fracture toughness section has multiple curved and bifurcated river-like lines with a certain depth. The curved and bifurcated river-like lines indicate that cracks have experienced more crack deflection or bifurcation during the propagation process and have a longer propagation path, thereby improving the toughness of the resin. With the introduction of nanoparticles (
Figure 3c), there were more bifurcated and curved lines on the surface of the section. There were wrinkle-like protrusions, indicating that the introduction of nanoparticles extended the propagation path of cracks in the epoxy resin before failure and caused a certain degree of plastic shear deformation of the epoxy, which was consistent with the further increase of fracture toughness of the matrix material.
3.2. Interlayer properties of GF/EP laminated composites toughened by SiO2 and CTBN
(1) Interlayer fracture toughness
Figure 4a shows the typical load-displacement curve obtained in the Mode Ⅰ interlayer stripping experiment of GF/EP laminates with or without CTBN or SiO
2 added. It can be seen that the load-displacement curve (black line) of the DCB specimen of unmodified GF/EP laminate composite material is relatively smooth, indicating smooth crack propagation. When 8wt% CTBN is added, the load-displacement curve (red line) of the DCB specimen is zigzags, meaning that the cavitation of rubber particles and the plastic deformation of the matrix material are obstructed in the crack propagation, and the displacement load and total displacement of the initial crack increase significantly. On this basis, when 0.5wt% SiO
2 is added, the total displacement of the load exceeds 85mm, and the area under the load-displacement curve is much larger than that of the GF/EP composite DCB specimen without the modifier.
Through the load-displacement curve and the crack length read synchronously online, the relationship between the energy release rate of interlayer peel toughness and the crack length change can be obtained, and then the GIC platform value of the mode Ⅰ fracture toughness of the laminated composite with or without CTBN and nano SiO
2 added can be obtained (as shown in
Figure 4b).
The energy release rate of unmodified GF/EP laminates has a GIC platform value of 0.76 kJ/m2, and when 8wt% CTBN is added, the GIC platform value is 1.18 kJ/m2, which is 55.3% higher than that of unmodified GF/EP laminates. When 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1 wt% nano-SiO2 were added to the 8 wt% CTBN toughened EP system, the GIC platform value of the laminates increased to 1.28 kJ/m2, 1.42 kJ/m2, 1.37 kJ/m2 and 1.43 kJ/m2, respectively. When 0.5 wt% nano-SiO2 was added, the GIC platform value of the laminated composite was 86.8% higher than that of the unmodified GF/EP laminated composite.
(2) Interlaminar shear stresses are the major cause of delamination in fiber reinforced resin composites. GF/EP laminated composites' interlayer shear properties are shown in
Figure 5. CTBN rubber particles and nano-SiO
2 significantly improve the shear properties of GF/EP laminate composites. The shear strength of pure EP enhanced by GF is 28.4 MPa, and 36.1 MPa when 8wt% CTBN is added, which is 27.1% higher than that of pure EP. When 0.25 wt% nano-SiO
2 is added to 8wt% CTBN-toughened GF/EP laminate composite, the interlayer shear strength of the composite is significantly increased, reaching 45.9 MPa, which is 27.1% higher than that of the single-component CTBN-modified composite. When the filling amount of nano-SiO
2 increases to 0.5 wt%, the improvement of interlayer shear strength of the composite increases (the slope of the red dashed line increases in
Figure 5), reaching 59.4 MPa, which is 64.5% higher than that of the single-component modified composite and 109.2% higher than that of the unmodified composite. In the 8 wt% CTBN toughened GF/EP laminates, the filling amount of nano-SiO
2 continues to increase, and the improvement of interlayer shear strength of the composite slows down (the slope of the red dotted line in
Figure 5 decreases). When the additional amount of nano-SiO
2 is 0.75 wt%, the interlayer shear strength of the two-component modified composite is the best. The toughened GF/EP laminated composite is 69.1% higher than single-component CTBN and 114% higher than unmodified GF/EP laminated composite.
3.3. Fracture Behavior Analysis of GF/EP laminate composites toughened by nano SiO2 and CTBN
Figure 6 shows the SEM topography of the GF/EP laminates' DCB specimen with different components, and the crack propagation direction is from bottom to top.
Figure 6a~c shows the section morphology of the DCB specimen of unmodified GF/EP laminate. It can be seen from the figure that the glass fiber (black arrow) with delamination failure has a smooth surface and no matrix adhesion (as shown in
Figure 6c), and there is a resin matrix (red arrow) with parallel river-like lines between adjacent glass fibers (black arrow). These characteristics indicate that the interface bonding between the unmodified epoxy matrix and the glass fiber is weak, and the cracks spread rapidly along the interface between the matrix and the fiber before failure. The failure mode belonged mainly to the interface-debonding failure[
30], so its
GIC platform value is low, only 0.76 kJ/m
2.
Figure 6d~f shows the SEM cross-section of the DCB specimen of GF/EP laminates modified by 8wt% CTBN. It can be seen from the figure that the surface of the glass fiber is partially covered with a thin matrix (blue arrow in
Figure 6e, f). In contrast, some glass fibers have smooth surfaces without matrix adhesion, indicating that cracks are spreading in the matrix. It is a mixed failure mode in which matrix cohesion failure and interface adhesive failure coexist. At this time, the surface of the matrix (blue arrow) between the parallel glass fibers is rougher than the surface of the unmodified matrix, with more irregular mountain ridges. There are uniformly distributed voids (blue box) in the enlarged figure (
Figure 6e, f), which indicates that when the cracks expand in the matrix, the rubber particles will be hollow and cause the plastic deformation of the nearby matrix.
The coexisting failure mode of cohesion failure and interface adhesive failure and the increased matrix plastic deformation makes the crack propagation in GF/EP laminate composite more load consumption, the interlaminar fracture toughness is significantly improved, and the
GIC platform value increases to 1.18 kJ/m
2. It is worth noting that due to the negative effect of CTBN rubber particles on the modulus of the epoxy matrix, the stress transfer efficiency between the fiber and the matrix decreases, and part of the interface debonding also occurs under load (the exposed glass fiber and blue dotted elliptic crack in
Figure 6e).
After 0.5wt% nano-SiO
2 was added to the epoxy matrix modified by 8wt% CTBN, the glass fiber surface of the DCB specimen of GF/EP composite was covered with the matrix (black and purple arrows in
Figure 6g~i). As can be seen in the enlarged figure, the glass fiber surface was rough, and there were many spherical holes (purple boxes in
Figure 6i). The matrix surface between the fibers becomes rougher. The bond between the fiber and the resin is tight without cracks (purple dashed ellipse in
Figure 6i). This no-crack bond indicates that cracks mainly propagate in the matrix, and the failure mode of the DCB specimen of GF/EP laminate composite is mostly cohesion failure mode[
31]. The introduction of rigid nano-SiO
2 improves the modulus of the matrix, thereby improving the stress transfer efficiency between the matrix and the fiber and avoiding interface adhesive failure with low energy consumption. At this time, cracks propagate in the matrix between the layers. The interlaminar toughness of the composite is mainly determined by the fracture toughness of the matrix, and the fracture toughness of the matrix is higher[
32], so the
GIC platform value of the composite further increases to 1.42 kJ/m
2, which is 88.2% higher than that of the unmodified composite.