| Based on the theoretical study of rigid flat punch acting on an elastic half-space, aface-on-face torsional friction model was built. A program was compiled with Maple softwareto obtain normal stress distribution, the relationship between torsional torque and the radius ofadhesion and torsional angle. A face-on-face torsion friction tester was made for a real-timeobservation. The torsioal tribological behaviors of PMMA against45steel were conductedwith a high-speed microscopic video camera observing the adhesion and slip region. Therelationship of real-time wear morphology and cycles under different normal forces andtorsional angles was found. Previous experimental results of MC nylon and PTFE were usedto further validate the correctness of the torsional friction model. The main conclusions werelisted as follows:1. Normal stress in the center of torsional contact interface was smallest, and increasedrapidly. Stress concentration appeared at the adjacent edges of a flat punch. The syntheticalfactors of shear modulus torsional angle, friction coefficient and normal force nonlinearlychanged the size of the radius of the adhesive, which determined the contact status. With theadhesive radius increasing, friction torque at first increased, reached the maximum and thendecreased slightly.2. With increasing angular displacement amplitude, T-θ curves of PMMA changed fromstraight to ellipse and finally became a parallelogram. Under the same angular amplitude, theadhesion radius increased with the increasing normal force. In torsional angle of0.1°, innormal load83N,123N and163N, the theoretical adhesion radius of PMMA were5mm,which indicated that the interface was completely adhesive state. But experimental adhesionradius was respectively3.8mm,4mm and4.15mm. In the torsional angle of0.25°, under83N,123N and163N, the theoretical adhesion radius was1.48mm,3.88mm and5mm, whichindicated that the interface was partial slip or adhesive state. The measured adhesion radiuseswere1.8mm,3.15mm and3.4mm. In the torsional angle of0.5°, the theoretical adhesionradius under83N,123N and163N was0.074mm,0.5mm and1.2mm while the adhesionradius was measured for0,0, and0.5mm, which indicated the entire interface was already infull slip. Measured adhesion radiuses were close to the theoretical values. The torsionalfriction model was proved to be correct from the relationship between the adhesion radius andtorsional angle.3. Torsional wear morphology firstly occurred at the interfacial edge, but abrasionsmorphology was found in partial slip region, while crazing damage occurred in completely slip zone. With the increasing cycles, wear was more serious around stick-slip boundary andspread gradually to the contact center. Until the end of the cycles, the contact center was stillno damage. In partial slip region, unevenly distributed abrasions morphology appeared aroundthe stick/lip junction along the circumferential direction, and the wear scar continued growthin the circumferential direction, and gradually expanded in the radial direction, accompanyingby slight fine damage (i.e., crazing). In complete slip zone, corrugated features of crazes werevery obvious. Crazing radially generated, elongated, widened, blurred and disappeared duringthe process. At last, the traces of plastic flow were found. Under the same load, the wearvolume of PMMA presented a slightly increasing tendency with the increasing angularamplitude. Under the same angular amplitude, the wear volume of PMMA increased withincreasing normal load.4. Torsional torque of MC nylon composites and PTFE sharply reduced to the steadystate with increasing torsional angles. The total torque linearly increased as the frictioncoefficient and normal force increased. In partial slipping region, torque was higher than inthe complete slip state. Experimental and calculated torque curve was similar under differentangular displacements, revealing the torsional friction model could predict the approximatetorque of polymer in the elastic stage. |
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