The impact experienced by the products during use and transportation is mainly due to the impact caused by the emergency braking and impact of vehicles, the air drop and crash of aircraft (emergency landing), the launch of artillery, the explosion of chemical energy and nuclear energy, the ignition separation and re-entry of missiles and high-performance weapons. Impact is to apply a high-level input pulse force to the product in a relatively short time.
Impact is a very complex physical process. Like random vibration, it has a continuous frequency spectrum, but it is also a transient process and does not have the condition of steady-state random. After the product is impacted, the movement state of its mechanical system will change suddenly and produce transient impact response. The response of products to mechanical impact environment has the following characteristics: high frequency oscillation, short duration, obvious initial rise time and high-order positive and negative peaks.
The peak response of mechanical shock can generally be enveloped by an exponential function that decreases with time. For products with complex multi-modal characteristics, the impact response includes the following two frequency response components: the forced frequency response component of the external excitation environment applied to the product and the natural frequency response component of the product during or after the excitation application.
From the physical concept, the impact response generated by the product after impact (i.e. transient excitation) represents the actual impact strength of the product. If the instantaneous response amplitude of the product exceeds the structural strength of the product itself, the product will be damaged. It can be seen that the damage caused by the impact of the product is different from the damage caused by the cumulative damage effect, but belongs to the peak damage of the ultimate stress relative to the structural strength of the product.
This peak damage will cause structural deformation, looseness of installation, crack or even fracture, loose electrical connection, poor connection, breaking and making the product unstable. This peak damage can also change the relative position of each unit in the product, resulting in performance degradation or out of tolerance, and even breaking the components or parts, making them unable to work.
To sum up:
(1) Product failure caused by increase or decrease of friction force between parts or mutual interference.
(2) Product insulation strength changes, insulation resistance decreases, and magnetic and electrostatic field strength changes.
(3) Product circuit board failure, damage and electrical connector failure (sometimes, the product is impacted. The surplus on the circuit board may migrate and cause a short circuit).
(4) When the structural or non structural parts of the product are overstressed , the product will produce permanent mechanical deformation.
(5) When the ultimate strength is exceeded, the mechanical parts of the product are damaged.
(6) Accelerated fatigue of materials
It can be seen from the above description that the impact will have a harmful impact on the structure and functional integrity of the whole product. The degree of this harmful effect generally changes with the increase or decrease of the magnitude and duration of the impact. When the impact duration is consistent with the reciprocal of the natural frequency of the product or the main frequency component of the input impact environment waveform is consistent with the natural frequency of the product, the adverse impact on the product structure and functional integrity will be further increased.
Therefore, to ensure that the products have good impact strength and work reliably and stably under the impact environment or after the impact, impact test is an important method and means to solve this problem.
This method is used to evaluate the structural and functional characteristics of the product under mechanical impact during its service life. Such machinery is generally limited to a frequency range of not more than 10000 Hz and a duration of not more than 1.0 sec. In most cases, the main response frequency of the product does not exceed 2000 Hz, and the response duration is less than 0.1 sec
(1) Peak acceleration
The magnitude of the peak acceleration can directly reflect the magnitude of the impact force applied to the product. Because the structure of products is mostly linear system, even if it is a nonlinear system, it can be regarded as a linear system when the strain is small. Therefore, the response acceleration generated after the product is impacted is proportional to the excitation acceleration. It can be seen that in general, the greater the peak acceleration, the greater the destructive effect on the product.
(2) Pulse duration
The duration of the impact pulse refers to the time interval during which the acceleration is maintained at the specified peak acceleration ratio. The impact of impact pulse duration on the product is very complex, and its impact on the impact effect is related to the natural cycle of the tested system.
(3) Impact times
Since the impact mainly considers the impact on the ultimate strength of the product, rather than the cumulative damage, it is not necessary to conduct repeated tests on the product. However, in order to avoid contingency, a certain number of impact times are also required. Generally, it is required to continuously impact 3 times in each direction. In addition, since the maximum response caused by impact may occur in the same direction as the excitation pulse or in the opposite direction to the excitation pulse, it is generally specified that the impact test shall be conducted in each direction of the three mutually perpendicular axes of the specimen, that is, in six directions, so the number of impact tests is specified as 3 x 6 = 18.
3.Requirements for Test Equipment
There are many equipment that can generate impact, of which the electric shock test system is the most important one. It can not only generate the impact response spectrum and the impact time history of the heavy site, but also generate the nominal impact pulse waveform. However, in addition to the electric vibration table, there are other equipment that can generate the nominal impact pulse waveform: free drop type, compressed air type, gas-liquid pressure type and momentum conversion type. No matter what test equipment is used to generate impact, the requirements for them are the same. The requirements here are the same as the requirements of verification (calibration) on the test bench, that is, it does not refer to the requirements when the impact test equipment is unloaded, but refers to the requirements that the impact test equipment should meet at the test point after the samples (including clamps) and necessary loads are installed.