Research Of The Contact Stress At Interface Based On The Nonlocal Effect In Strip Rolling

In rolling process,accurate prediction of the rolling contact stress at the interface between strip and work roll is of great significance to producing high quality strip.Traditionally,the contact stress model is derived from the classical continuum mechanics.It can solve many practical engineering problems,but sometimes the theoretical result does not accord with actual situation.In the traditional theoretical model,the contact stress at rolling contact surface is often thought to be local,namely,stress of one point is only related to the stress of this point,but do not associated with the stress state of the other points in a neighborhood.In fact,the stess at a given point on contact interface is not only associated with the stress state of the point,but also associated with the stress state of the other points in a neighborhood of the same point,that is,the nonlocal effect is existence.In order to reflect the nonlocal effect on rolling interface,it is necessary to use nonlocal theory to further analyze the rolling contact stress at the interface.Therefore,the nonlocal theory into the rolling forming problem is introduced in this thesis,then the model contact stress models based on Oden nonlocal model and nonlocal stress gradient model are established,respectively.Meanwhile,the differences between two kinds of nonlocal model are studied.In order to determine the nonlocal characteristic parameter in nonlocal model,an indirect sensor based on the(improve)inverse method for measurment of the contact stress at the contact interface is designed and developed,and the scope of the nonlocal characteristic parameter is discussed in this thesis.The main works of this thesis are as follows:1.Based on the Oden nonlocal model,the contact stress models are established of no lubrication rolling and lubrication rolling,respectively.The difference between nonlocal model and local model in no lubrication rolling condition is studied.The influences of nonlocal effect on contact stress at entrance zone,plastic deformation zone and exit zone are researched in lubrication rolling,respectively.Then the influences of nonlocal characteristic parameter,friction coefficient,tension and reduction on nonlocal effect are discussed.2.The nonlocal stress gradient model is derived from the Eringen gradient theory model,then the contact stress models of no lubrication rolling and lubrication rolling are established by the nonlocal stress gradient model,respectively.The influences of nonlocal effect on normal pressure predicted by the Coulomb friction model and the absolutely constant friction model are studied,respectively.The differences of contact stress predicted by local(Coulomb)model and nonlocal model in lubration rolling were studied.The significant of nonlocal effect at entrance area,plastic deformation area and exit zone in lubrication condition are studied.The relationships between difference of the nonlocal model and local model and nonlocal characteristic parameter and reduction are discussed.In addition,the differences of the Oden nonlocal model and the local stress gradient model for prediction of rolling contact stress are studied.3.On the basis of traditional contact stress inverse method,an envelope fitting method is proposed to improve the inverse method for prediction of contact stress.Then a indirect contact stress sensor is designed based on the(improved)inverse method in this thesis.The superiority of improved inverse method is validated by simulation analysis and experiment test.Meanwhile,The influences of the order of truncation points and the reduction(corresponding to the sampling points with constant sampling frequency)on the accuracy of reconstructed contact stress are discussed.4.The nonlocal characteristic parameters are determined from the contact stress measured in experiment.Then the difference of nonlocal characteristic parameter in Oden model and nonlocal stress gradient model for given experimental data is studied.Then,the principle of choosing the nonlocal characteristic parameter for practical application in no lubrication rolling is discussed.And the applicable scope of nonlocal model for modified the theory model is also discussed.In this thesis,the contact stress at the rolling interface is studied by nonlocal theory.It will increase the complexity of problem,but the result may be more real and reliable.Additionally,the indirect contact stress sensor designed in this thesis overcomes the deficiencies of traditional contact stress sensor,and it can accurately measure the contact stress at the rolling interface,which is beneficial to analysis and suppression of vibration and providing the potential application value for production of high performance strip.