the temporary bridge materials, these resin polymers tend to constantly absorb water, which could induce the colors change of the patching materials over time. Even worse, when they exposed to coffee, tea, or drugs like chlorhexidine [4], their color degrade more dramatically. In order to find out the most suitable materials for tooth repair, we also need to have a clear understanding of the dynamic mechanical properties for human teeth. However, because of the medical ethics concerns, human teeth are difficult to obtain for biological experiments. Until this year, through the help of human trials committee (IRB) at Kaohsiung municipal triumph hospital, the authors were finally certified to perform human teeth researches. All of the human wisdom teeth samples were supplied by Kaohsiung dentist inscription cooperation in an effort to study the mechanical properties of human molar teeth and filler materials under high strain rate conditions. 9.2 Method and Material 9.2.1 SHPB Dynamic response was obtained using a modified split Hopkinson pressure bar, which consist of the striker, incident, and transmission bar. The lengths for striker, incident, and transmission Al-7075 bars were 300, 1,000, and 900 mm, respectively. The semiconductor transducers strain gage used to measure the wave signal, and via amplifying, it was recorded by Tektronix DPO 4104 digital phosphor oscilloscope, as shown in Fig. 9.1. The SHPB is based on the one dimension wave theory, when elastic wave propagation through incident bar, partly of the elastic wave propagated to the specimen and remained to reflect to the incident bar. Nevertheless, when testing the soft material within the conventional SHPB, the dynamic equilibrium is not satisfied automatically [5–8]. To closely approximate the dynamic stress equilibrium, conventional SHPB was using the pulse shaper to attain the constant strain rate before damaging [5–10]. According to onedimensional wave theory, the stress–strain formula are as following: _ε tð Þ¼ C L εi tð Þ εr tð Þ εt tð Þ ½ ð9:1Þ ε ¼ 2C L ð t 0 εr tð Þdt ð9:2Þ σ ¼Eεr tð Þ ð9:3Þ where _ε represents strain rate, C represents wave speed, L represents length of specimens, εi represents strain of incident wave, εr represents reflected wave and εt represents transmitted wave. ε represents strain, σ represents stress, and E represents elastic modulus. Fig. 9.1 SHPB device 46 J. Ren et al.
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