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Plastic fatigue performance

by:CSSSLD     2021-01-23
Discriminant fiber reinforced with notch specimen or plastic with inorganic filler added impact resistance performance, the results show the larger deviation, such as glass fiber reinforced nylon in lzod notched impact tests showed higher than that of pure nylon impact strength value, but in fact it is not. With lzod impact toughness test method for evaluation of injection molding processing materials is not real. A similar situation, that is, by the limitations of experimental method itself, affect the accuracy of judging the material toughness, in other kinds of impact test method also has the varying degree existence method.

general material toughness data provided by the manual can't be used in products design directly. Products design and material selection are also need to products of the same or similar model is similar to the actual conditions of use of testing, test conditions can be finalized. Although commonly used experimental method exported data can't accurate characterization of the actual toughness, and the results of different test methods export and cannot compare, but to know something about the possible destruction of materials under impact load behavior, for material selection is still have important reference value.

products stress by periodic repeated actions, including tensile, bending and compression or distortion of the different types of stress, and alternate deformation phenomenon, become tired. Caused fatigue is the direct result of material mechanics performance attenuation, and even lead to material in bending. Due to the internal or surface of a material usually some micro defects, these defects can lead to local stress concentration, under the effect of cyclic stress of continuous, the defect can produce micro-crack first, and gradually expand, the final break.

due to the characteristic of the viscoelastic characteristics of plastic materials under the action of external force and deformation stress changes may couldn't keep up with the speed of the speed, so that no time cycle there will always be part of the mechanical energy as heat consume, this kind of phenomenon be lagging heating, due to the poor thermal conductivity of plastic, not the transfer of heat in time will be accumulating, makes the local temperature rising and mechanical performance is reduced, eventually lead to thermal damage.

there are a variety of factors affecting plastic fatigue behavior, including the characteristics of material itself, such as strength, thermal conductivity, notch sensitivity, products of geometric shape, size, thickness and surface quality and stress of the role in product type, size of stress, effect of frequency and the external environment, size, thickness and surface quality. Stress of the role in product type, size of stress, effect of frequency and the external environment, such as temperature, humidity, environment the participation of grafting, etc.

different materials all have lay particular stress on the cause of fatigue damage. Some hard brittle plastic, such as rigid PVC, PPO, PSF and some thermosetting plastic, and its fatigue damage is mainly caused by cracking, and some toughness plastic, such as fluorine plastic, PP, PE, PA can be caused by thermal damage, there are some plastic varieties, such as PMMAD, POM, PET, PC, two factors are present. However, for most of the plastic, heat is an important factor of fatigue damage.

in engineering design, the choice of materials concerned about most is the injection molding processing, the service life of the material under the action of alternating stress, or to determine the service life under design stress. Generally by material fatigue life curve for rough estimates of the parameters. The so-called S - N curve describes the failure stress N and destruction of the loop S relation curve, he is through the test many samples, and each sample under different stress levels of cyclic stress fatigue test, will damage under various stress cycles.

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