| Low relative velocity friction-induced-vibration(LRV-FIV)is induced by friction when the relative velocity between a friction pair is low.Stick-slip vibration is one typical kind of LRV-FIV.It is very common in engineering and daily life.Generally,LRV-FIV is harmful and unwanted,for example,it not only accelerates damage of machine parts and shortens their service life,causing accidents,but also produces noise pollution and affects people's physical and mental health.Therefore,researches have carried out a large amount of research to study the characteristics of LRV-FIV and its suppression methods.The main focus of LRV-FIV has been on its theoretical analysis based on multi-degree of freedom theoretical models to study their vibration behaviours.However,there are obvious limitations in purely theoretical analyses because they cannot very well capture the influence of friction characteristics at the interface on LRV-FIV.Therefore,conducting experimental studies of LRV-FIV are of great significance to investigate the vibration and friction properties and their interrelationship,to further understand the characteristics of LRV-FIV and propose methods to suppress LRV-FIV.At the same time,to make the best of theoretical and experimental studies,a combination of the two methodologies can not only enrich the research field,but also help to study LRV-FIV more comprehensively and provide more reliable theoretical guidance to suppress LRV-FIV.In this study,experimental and theoretical studies are conducted to investigate the generation mechanism of LRV-FIV,its behaviour and suppression methods,including construction of reliable theoretical models for LRV-FIV based on experimental data and judicious combination of experimental and theoretical studies.The main conclusions are as follows:1.A self-designed stick-slip vibration test rig of horizontal pin-on-disc configuration is manufactured and constructed.Stick-slip vibration tests,under different normal forces,different velocities,different friction pair materials,different numbers of friction pairs,can be conducted on this test rig,which lays the foundation for the experimental study of stick-slip vibration in this project.2.A self-constructed friction vibration experimental setup based on the CETR UMT-3test rig is used to conduct an experimental study to investigate the effect of Mn-Cu damping alloy(Mn-20Cu-5Ni-2Fe)working as the friction material(disc material)on stick-slip vibration of the friction system(the test rig).Wear of the friction interface is also studied.Moreover,four other discs respectively made from forged steel,cast iron,Al alloy(7075)and Mn-Cu alloy(Mn-20Cu-5Ni-2Fe without heat treatment)are tested for comparison.The results show that the forged steel and cast iron discs show visible stick-slip oscillation,the Mn-Cu alloy disc shows only a limited reduction of stick-slip vibration,the Al alloy disc can suppress stick-slip vibration at steady stage,and the Mn-Cu damping alloy disc exhibits pure sliding during the whole experimental stage.The wear test results show that forged steel and cast iron discs undergo slight wear and its wear volume is much lower than the other three materials.The Mn-Cu damping alloy and Al alloy suffer much more severe wear than the others — obvious deep ploughing and a large amount of wear debris can be observed and their wear volumes are also very high.It is confirmed that the wear debris on the surface of Mn-Cu alloy,Mn-Cu damping alloy and Al alloy discs acts as a third body “lubricant” and contributes to reduction of the stick-slip oscillation.Moreover,the high damping capacity of the Mn-Cu damping alloy may be another key important factor to suppress the stick-slip oscillation in the whole experimental stage.3.A self-designed horizontal stick-slip vibration test rig(with only one pin)is used to study the influence of rubber blocks with various surface features in a friction system on stickslip vibration.Rubber blocks whose contact surfaces are modified with grooves in three configurations: vertical(the groove is perpendicular to the relative velocity,referred to as VG),diagonal(the angle between the groove and the relative velocity is 45o,referred to as DG)and horizontal(the groove is parallel to the relative velocity,referred to as H-G).The dynamic behaviour of stick-slip oscillation is changed when adding different rubber blocks to the friction system.The Plain rubber block can reduce the stick-slip oscillation compared with the Original system(without any rubber block)to a certain degree.Adding a grooved rubber block to the friction systems produces a better ability to reduce the stick-slip oscillation.Among them,the H-G system shows some ability to reduce the stick-slip oscillation which is followed by the D-G system.The V-G system has the best performance to suppress the stickslip oscillation.The wear results shows that Area ?(the region that experiences severe wear)of the pad surface for the Original system and the Plain system is smaller compared with that of the other three systems and the former obviously has compacted wear debris layer in Area?.The pad surface of the V-G system shows the largest Area ? and the wear of this zone is complex-in addition to a friction layer,crack and detachment can be observed.The highest contact pressure of the pad surfaces is significantly reduced,the contact area is increased,and the likelihood of separation on the trailing side between the pad and the disc surfaces is greatly reduced by adding a rubber block to the friction system,which may be due to the cushioning effect of a rubber block as a softer material.The maximum contact pressure of the V-G system is the lowest and the contact pressure spreads over the largest area,and the separation between pad and disc is the smallest,among all the five systems under this investigation.This is the likely reason for visible reduction of stick-slip vibration of the V-G system.4.A combined experimental and theoretical study of friction-induced stick-slip vibration is conducted for an experimental setup with a sophisticated dual-pin-on-disc(two pins is used)configuration designed by the student,and a series of modal tests are conducted to investigate the dynamic properties of the test rig under different working conditions(various levels of the normal force and disc velocity).A 2-degree-of-freedom(Do F)model is built and various parameters,such as the stiffness,mass and moment of inertia,are identified based on experimental results of several kinds of tests and aided by a detailed solid model and a finite element model of the test rig.For validation,the theoretical results obtained by a numerical method are shown to agree quite well with the experimental results under the same working conditions.The errors between the theoretical and experimental results are very small,which indicates that the built 2-Do F model is reliable and can be used to simulate the experimental stick-slip vibration with high accuracy.It can be used to explore FIV beyond the parameters of the test rig.5.It is well known that a numerical study by using finite element software is very popular,but this method is computationally expensive for real complicated structures like the several test rigs used in this Ph D project.To overcome this numerical difficulty,the student takes part and has played a major and active role in the study of model reduction for friction systems,in collaboration with researchers in Dalian University of Technology and University of Liverpool.Firstly,the established model reduction strategy is applied to a 9-degree-of-freedom model(whole model 1)to get the reduced model 1,and a theoretical analysis is conducted for the whole model 1 and reduced model 1,and the results of the reduced model 1 agree with these of the whole model 1 quite well,which indicates that the model reduction strategy works well for theoretical study of friction systems.Secondly,an experimental study of LRV-FIV is conducted and the corresponding finite element model(FE model,whole model 2)is built.To get the corresponding reduced model 2,the model reduction strategy is applied.The results show that the theoretical results of reduced model 2,the finite element analysis results of whole model 2,and the experimental results agree with each other quite well,which further proves that the established model reduction strategy works well for LRV-FIV study. |
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