GFRP (Glass Fiber Reinforced Plastic) is a composite material reinforced with glass fibers, commonly known as “fiberglass”. It is a composite material using glass fibers and their products (yarn, tape, glass cloth, felt, etc.) as reinforcement and synthetic resin as the matrix. Depending on the resin, it is classified into epoxy fiberglass, phenolic fiberglass, polyester fiberglass, etc. GFRP has advantages such as light weight, high strength, corrosion resistance, good dielectric properties, and good formability, making it one of the preferred materials to replace metal materials. GFRP has a wide range of applications, including bridges, concrete reinforcement, pipe materials, decorative materials, and thermal and sound insulation. In these applications, GFRP may be subjected to impact loads. Under impact loads, the material often exhibits different physical properties than under quasi-static tests; that is, the material’s physical properties are strain rate-dependent, and the strength of the material increases with increasing strain rate.
Table 1 Comparison of Physical Properties of GFRP and Common Metallic Materials
Common methods for studying the impact dynamic properties of materials include drop hammer impact test, Taylor impact test, and Split Hopkinson pressure bar (SHPB) test. This paper will use SHPB technology to study the dynamic properties of GFRP, and use the two-wave method to process the waveform data to obtain stress-strain curves and strain rate-time curves for different strain rates.
02 Analysis and Discussion
2.1 Stress-Strain Curves The stress-strain relationship of GFRP materials with different sample sizes at room temperature (20℃) is shown in Figures 2 and 3. In Figure 3, the curve with a strain rate of 500/s shows that due to the low air pressure and low impact velocity of the bar, the sample did not undergo large deformation and was destroyed, resulting in only a partial compression curve. Both figures show a jittering phenomenon in the curves. This is because the theoretical assumption of the Hopkinson test—the one-dimensional stress wave assumption—cannot be fully satisfied during the test, resulting in wave dispersion and abnormal jittering in the final curve. However, the jittering amplitude is not very large, and it can be approximately assumed that the wave does not scatter within the bar. The stress-strain curves of GFRP materials of both sizes exhibit an elastic stage, a strengthening stage, and a final strain softening stage. Therefore, the dynamic compressive properties of GFRP can be characterized using parameters such as elastic modulus, yield strength, compressive strength, and strain corresponding to maximum strength.
Figure 2: Stress-strain curve of specimen φ5.6mm*5mm
Figure 3: Stress-strain curve of specimen φ5.6mm*10mm
Table 3 compares the compressive strength of specimens of the two sizes at different strain rates. It can be seen that the compressive strength of GFRP increases with increasing strain rate, indicating that GFRP is sensitive to strain rate at room temperature (20℃) and exhibits a strain strengthening effect at high strain rates.
Table 3: Comparison of compressive strength at different strain rates
03 Strain Rate Analysis
Another assumption of the Hopkinson test is a constant strain rate during the test. From the strain rate curves of the φ5.6mm5mm and φ5.6mm10mm sizes, it can be seen that the strain rate remains constant during the test. In Figure 4, the aspect ratio of the specimen size is less than 1, resulting in large strain rate fluctuations during the test, but on average remaining at a relatively constant level. In Figure 5, the aspect ratio of the specimen is greater than 1, and the strain rate generally decreases during the test. The constant strain rate assumption cannot be achieved because the impedance of GFRP material is low, and its elastic modulus differs greatly from that of the steel rod system. Therefore, for testing materials with low impedance, rod systems with even lower elastic modulus, such as aluminum or nylon rods, should be selected. Furthermore, when selecting specimen size, cylindrical specimens are preferred, with an aspect ratio between 0.5 and 1 being optimal.
Figure 4: Strain rate-time curve of specimen φ5.6mm*5mm
Figure 5: Strain rate-time curve of specimen φ5.6mm*10mm. Observing the strain rate curves of both sizes, the rising phase is very volatile, and the strain rate only falls back after an abnormal surge. A slower increase in strain rate makes it easier to achieve the constant strain rate assumption, as shown in the 900/s curve in Figure 4. When testing low-impedance materials, to achieve an approximately constant strain rate during the test, some literature suggests adding a buffer component between the impact rod and the incident rod; this component is called a “waveform shaper,” as shown in Figure 6. The choice of waveform shaper material and size varies depending on the test material, requiring experimental verification. Commonly used waveform shaper materials include copper.
Figure 6: Waveform Shaper
04 Conclusion
This paper utilizes the Hopkinson test technique to test the dynamic compressive properties of GFRP materials. The experiment shows that GFRP materials exhibit strain rate sensitivity and strain strengthening effects. In the high-impedance SHPB testing system, the impedance difference is too large when testing GFRP materials, making it impossible to achieve the constant strain rate assumption, resulting in unreliable test results. When testing low-impedance materials, steel is unsuitable as a rod material; materials with even lower impedance, such as aluminum or nylon, should be selected as waveguide rods. Simultaneously, a waveform shaper component can be added to the SHPB testing system to ensure the constant strain rate assumption.
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