An Investigation of The Behavior of Strain-Hardening Cement-Based Composites (SHCC) Obtained in a Split Hopkinson Tension Bar and the Influence of Structural Inertia

The performance of a normal-strength SHCC under impact loading was studied using the results obtained from a split Hopkinson tension bar (SHTB). The focus of the investigation is to explain the mechanisms behind the peculiar rate-dependent behavior of SHCC under tensile loading. With the help of frames obtained by high-speed cameras and the subsequent Digital Image Correlation (DIC) analysis, the stress-strain relation of the SHCC obtained in SHTB was analyzed. The investigation of the composite’s behavior was supported by constituent-level experiments on the non-reinforced matrix of the SHCC and on the fiber-matrix bond. In the case of the constituent matrix, the well-known apparent increase in the tensile strength of the cement-based matrix and its influence on the behavior of SHCC was studied. For this purpose, experiments on the SHCC specimens with different geometries were performed in the SHTB. The results obtained from these experiments and those obtained by DIC show that commonly used analytical models, in which the specimen is assumed elastic, cannot capture the effects of structural inertia on the results. Thus, an alternative novel method based on the results of DIC has been used to explain and quantify the contribution of structural inertia. The rate-dependent behavior of the fiber-matrix bond was studied by performing high-speed single fiber pullout tests in a miniaturized split Hopkinson tension bar. This novel experimental technique enabled explanation of the rate-dependent bridging action of the fibers in SHCC. Based on the results, the enhanced behavior of SHCC under impact loading is explained.

Stress Transfer Mechanism of Flange in Split Hopkinson Tension Bar

To reveal the stress transfer mechanism of the flange in a split Hopkinson tension bar, explicit finite element analyses of the impact of the hollow striker on the flange were performed across a range of flange lengths. The tensile stress profiles monitored at the strain gauge position of the incident bar are interpreted on a qualitative basis using three types of stress waves: bar (B) waves, flange (F) waves, and a series of reverberation (Rn) waves. When the flange length (Lf) is long (i.e., Lf > Ls, where Ls is the striker length), the B wave and first reverberation wave (R1) are fully separated in the time axis. When the flange length is intermediate (~Db < Lf < Ls, where Db is the bar diameter), the B and F waves are partially superposed; the F wave is delayed, then followed by a series of Rn waves after the superposition period. When the flange length is short (Lf < ~Db), the B and F waves are practically fully superposed and form a pseudo-one-step pulse, indicating the necessity of a short flange length to achieve a neat tensile pulse. The magnitudes and periods of the monitored pulses are consistent with the analysis results using the one-dimensional impact theory, including a recently formulated equation for impact-induced stress when the areas of the striker and bar are different, equations for the reflection/transmission ratios of a stress wave, and an equation for pulse duration time. This observation verifies the flange length-dependent stress transfer mechanism on a quantitative basis.

Mechanical behavior of polyimide filament tows under high strain rate tension

The tensile properties of polyimide (PI) filament tows were measured under quasi-static state and at high strain rates with a universal tensile testing machine and a split Hopkinson tension bar, respectively. Experimental results showed that mechanical behaviors of the tows were rather sensitive to strain rate, with failure stress and modulus increasing distinctly but the elongation at break declining as the strain rate increased. Besides, the PI filament tows exhibited a higher growth rate of fracture stress than para-aramid fiber and aramid III fiber did, and scanning electronic microscopy observation on the fracture surface indicated a ductile fracture mode. With the increase of strain rate, the axial splitting of fiber intensified. Further, strength distributions of the PI filament tows were evaluated by a single Weibull distribution function, and the curve predicted was in good accordance with the experimental data obtained.

Design guidelines for the striker and transfer flange of a split Hopkinson tension bar and the origin of spurious waves

Characteristics of the stress pulse generated by impact of a hollow striker on the flange of a split Hopkinson tension bar are investigated via an explicit finite element analysis. Design guidelines are extracted for the hollow striker and flange from the viewpoint of eliminating spurious waves located between the incident and reflected pulses. According to design guidelines, it is desirable to have a striker cross-sectional area the same as that of the flange. It is also desirable to make the cross-sectional area of the striker (flange) the same as that of the bar. As for the flange length, it is recommended to be comparable to the diameter of the bar. The magnitude and duration of the primary stress pulse are consistent with the results of a one-dimensional analysis even when spurious waves are present; meanwhile, overly long spurious waves should be avoided to eliminate their superposition with the reflected pulse. Spurious waves appear when general impedance of the striker is higher than the bar. The origin of spurious waves is a series of step-wise residual pulses generated by multiple cycles of striker impact that make the striker keep compressing the flange after the first cycle of impact. Step-wise residual pulses appear in two forms (continuous waves and discrete waves) in spurious waves due to the secondary impacts during the entrance process of step-wise residual pulses to the flange. The consequences of spurious waves in the use of split Hopkinson tension bars are discussed.