In a very short time (the action time is less than theFundamental wavefree vibration The load acting on the member at a great speed (half of the period) is called impact load.The calculation of its stress and deformation is quite complicated.The calculation is generally based onLaw of conservation of mechanical energyconduct.
When a moving object with a certain speed impacts a stationary component, the speed of the impact object changes greatly in a very short time, that is, the impact object gets a large negative acceleration.This shows that the impactor is subjected to a great force opposite to its direction of motion.At the same time, the impactor also exerts a great force on the impacted component, which is called "impact force" or "impact load" in engineering.[1]
Figure 1
computing method
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Taking a simply supported beam as an example, the calculation method of impact load is explained.The simply supported beam shown in Figure 1 has a weight of W at the height h above it
After the free fall of the object, it impacts the midpoint of the beam.
During the impact process, the distribution of stress and deformation on the component is complex, so it is difficult to accurately calculate the impact load and the stress and deformation caused by the impact load in the impacted component.The simplified calculation method is mostly adopted in the project, which is based on the following assumptions:
(1) Assuming that the deformation of the impactor is negligible, it is considered that the impactor is a rigid body.From the beginning of the impact to the maximum displacement caused by the impact, the impact object and the impacted component move together without springback.
(2) Ignoring the mass of the impacted component, it is considered that the stress and deformation caused by the impact load are all over the impacted component at the moment of impact;It is assumed that the impacted component is still in the elastic range.
Corresponding energy conversion: the mechanical energy (potential energy) of the heavy object is converted into the elastic performance of the rod
Mechanical energy: E=P (h+△ d)
Strain energy:
Impact load displacement equation:
Impact displacement:
Maximum impact load:
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impact test
It is a method used to determine the impact resistance of materials under impact load.Impact strength is a measure to evaluate the impact resistance of materials or to judge the degree of brittleness and toughness of materials.resistImpact performanceIt is difficult to accurately characterize one of the mechanical parameters. Even with standard samples and standard experimental methods, only experimental results with relatively comparative significance can be obtained under specific conditions.The impact test methods of polymer materials include pendulum type, drop hammer type and tensile impact type.[2]
Impact fracture mode
Main pointsplasticityandbrittlenessTwo categories.The plastic fracture under impact has obvious yielding phenomenon, which is fracture in necking.Brittle fracture is a kind of multi split fracture, with small cusps and smooth surface areas.The first batch of active cracks in the impact instantly transformed into unstable growth cracks, triggering fracture.On the fracture surface of amorphous polymer, except for the smooth mirror surface, the rest are relatively rough fingerprints.The fracture surface of crystalline polymer is mostly in the state of pockmarks.
Falling hammer impact meterIt can change the weight and height of the drop hammer and crack or break the plate or sheet sample. It is also applicable toFoam plasticAnalysis of impact resistance.It is an impact test of plate bending.The instrumented drop hammer impact instrument can measure the impact energy and the impact stress-strain curve of the sample material.
