Simulation Analysis Of Vibratory Source Properties Of Reflected Wave Test For Low Strain

The impact force character of reflected wave test for low strain, such as pulse width, the incident wave amplitude,have important influence to the reliablity of the test results. Using the finite element program ANSYS/LS-DYNA, this thesis simulated the influence factors of the force pulse width and its effect to the test results.Firstly,the basic theory of reflected wave test for low strain and the modeling process by applied ANSYS/LS-DYNA are simple described, followed by the definition of a bell-shaped force pulse width. The pulse width of four materials, such as steel, aluminum, nylon and polyethylene plastic are calculated by the numerical simulation. The most important investigation is the Influence of pulse width of contact area, impact velocity, weight of hammer and shim thickness, reached the following conclusions:(1) the greater the contact area, the smaller the pulse width of impact force, the incident wave amplitude is also smaller;(2) impact velocity had no effect on the pulse width, but the incident wave amplitude increases with the impact velocity increased by a linear law;(3) Pulse width increases with the impact vibration hammer weight increases by a power function relationship(4) Thicker shim, the greater the pulse width,and impact force pulse width with shim thickness increased by a linear law.Lastly apply a impact hammer with different pulse width to verify the effect of the test. It found that narrow pulse wave which produced by the steel hammer is propitious to identify defects in the vicinity of pile top, whereas steel hammer impact large diameter pile may cause wave speed decreased and wave graph oscillated.At the same time,wide pulse wave which produced by the polyethylene plastic hammer can overcome these problem.So the appropriate pulse width of the incident wave which are adjusted by hammer material,contact area,impact velocity, weight of hammer and shim thickness should be determined according to the geometry size of pile and testing purposes to meet the test requirements.The pulse width and impact velocity as small as possible on the premise that can obtain reflection from the pile toe.