Theoretic Study On Thermal Shock And It's Application In Engine

Piston's thermal load is mainly characteristiced by shock under different working conditions, which is the main factor that leads to piston's thermal damage. In order to find mechanism of piston's damage under thermal shock, theoretic background of elastic components and FEA simulation of piston's thermal behavior are both carried .At first step, theoretical study on thermal shock is carried, four items related to thermal shock are explored, which are expressed as effect of relaxation time, dynamics, coupling and dynamical coupling. Then piston's thermal behavior under different shock mode is simulated during engine's cold start working condition. Suitable boundary setting for piston is put forward. Digital image processing is adopted to find new application in extracting crack at piston's ceiling at thermal shock test bench.GL model is the most complicated thermo-elastic model with two relaxation times. Small relaxation time require short time step that is very difficult to execute when the shock time is longer. The small time solution of GL model in an infinite pole is gained by Laplace transform and Rona's Series. The influence of relaxations time on thermal behavior of semi infinite body is discussed. Considering relaxation time, the response time for semi infinite body to attain thermal rebalance is longer, also the working domain of thermal shock is enlarged. Effect of relaxation time is decreased as the shock processing proceeds. Simulation results express that the relaxation time can be omitted when natural gas is used as the heat source, so the classic coupled model can simulate heated elastic component's thermal behavior under thermal shock with shock time longer than relaxation time.Infinite plane is selected as the physical model to study the influence of coupling factor and inertia factor in classic model. Constitutive equations under different boundary constraint are given both in time domain and non-dimension domain. Also simulation under different initial temperature and shock form is carried and the basic factor that affect the low cycle shock is pointed out. Effect of coupling on plane's temperature and thermal stress is apparent only at the start of thermal shock and to a finite distance from the plane being heated. On the contrary , effect of dynamics on plane's thermal stress is very apparent during the whole shock...