| The study of rubber are mostly concentrated on the prediction of fatigue life or calibration of the performance,the study of rubber fatigue fracture is inadequate,the formation mechanism of various fatigue stages can not be explained from a microcosmic perspective,and failed to incorporate rubber fatigue properties,fracture morphology and composition analysis systematically.Using two kinds of natural rubber materials(M and G) as the research object,Tension and tension compression fatigue tests are carried out on the universal fatigue testing machine after static tension text,and the effects of material,strain and loading frequency on the fatigue life are studied.The micro topography of rubber fatigue fracture is observed by field emission scanning electron microscope(FESEM),and the characteristics of rubber fatigue fracture is studied.By in situ tensile fatigue crack propagation tests,fatigue crack propagation mechanism is analysed,and use IR,DSC,TGA,DMA,NMR and GC-MS analysis methods to determine the composition and source of product in the fatigue process.The fatigue test results show that the temperature of rubber rise in the fatigue process,the greater strain,the higher the loading frequency and higher specimen temperature make rubber fatigue life shorter,the influence of strain on the fatigue life is significant.The fatigue life of material M is longer than material G under the same loading condition,and it is more likely to generate fatigue products.FESEM observation shows that the fatigue fracture in mode of stretching is divided into the source area,extended area and transient breaking zone.The fatigue fracture in mode of tension compression is divided into the edge area and the middle area,and step characteristic is visible at the junction of two area.The fatigue crack is easily initiated from the edge of the sample and the large additive.The crack initiation is slow, which accounts for ninety percent fatigue life.The greater loading frequency make fatigue characteristics of the fracture more obvious,the greater the strain rate make the crack growth rate faster.The result of in situ tensile fatigue crack propagation test shows that the fatigue crack tip is consist of many diamond regions.Diamond regions contains additive and micro crack.Under cyclic stress,micro crack formed by the precipitation ofadditive,then formed small diamond with the propagation of micro crack.Ligament become thiner and thiner then rupture happens,two adjacent small diamond become a big diamond.The crack tip will continue to repeat the above process until the specimen broken completely.Besides,the crack propagation rates are different in different diamond regions,and the adjacent cracks can connect to form a larger crack.The results of meterial structure change shows that the crosslinking density of meterial M was decreased with the increasing of fatigue life, and the density crosslinking of meterial G was unchanged.The black powder is formed by extrusion,wear the rubber pieces piled up.The oil is small molecules which are formed by rubber chains rupture under cyclic stress,it's main components is alkane and olefin,also contains a small amount of ketones,ethers,lipids after thermal oxgygen aging,and a small amount of stearic acid,amines added in rubber. |
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