Mechanical shock testing is the application of high levels of acceleration, for a very short period of time, to a component, product, system or structure in order to observe its response or degradation. Mechanical shock testing simulates the effects on a test sample of an acceleration pulse that may be caused by an impact, drop, explosion, or other high amplitude and short duration acceleration or deceleration.
Although there is overlap between the two definitions, vibration testing is distinguished from mechanical shock testing by being applied for a longer period of time, ranging from a few minutes to many weeks. Mechanical shock tests have typical durations of less than one second per shock, although multiple shocks over longer time periods are commonly performed.
Mechanical shock testing is also distinguished from thermal shock testing which is performed by very rapidly changing the temperature of the test sample.
Typical Mechanical Shock Testing Equipment
High level mechanical shock testing generally involves the use of a mechanical shock testing machine that accelerates the test sample until it receives the shock by impacting a barrier. However, unlike driving a car into a wall, the test sample does not generally directly impact the barrier. Instead, the test sample is attached to a metal plate (or “platen”) that typically has a much larger mass than the test sample. The test sample is attached to the side of the platen away from the impact surface. A compressible material is placed between the impacting surfaces. The mechanical characteristics of this material, such as thickness and hardness, control the pulse shape during the shock event.
The amplitude of the shock is controlled by the test sample velocity at the time of impact. The test sample and platen can sometimes simply be dropped from the required height at which gravity results in the desired impact velocity. Mechanical shock testers often use compressed air to provide greater acceleration than gravity alone, thereby reducing the required height of the machine or to get much higher velocities than can be achieved by gravity alone. Hydraulics are also typically used to lift the test sample and platten to the starting height.
The mechanical shock tester computer controls may also directly measure the shock pulse received from an accelerometer mounted to the platen (also, of course, on the side facing away from the impact). The computer can then adjust the drop height and air pressure to more precisely match the desired shock for future impacts, and to continually make modification as the impacted material changes its properties with successive impacts.
This type of mechanical shock tester generally operates in only one direction, downwards (with a resulting upward acceleration as a result of the impact deceleration). Since the effect on a test sample may be different if the shock is applied in the 180 degree opposite direction, the mechanical shock test may need to be repeated with the test sample “upside-down”. Special fixturing is required to mount the test sample in all 6 orientations (3 axes times 2 directions). A typical test specification might call for 3 shocks in each of 6 directions, for a total of 18 shocks.
Mechanical Shock Testing Using an Electrodynamic Vibration Testing System
For lower shock amplitudes and durations, the mechanical shock input to the test sample is typically applied using an electrodynamic vibration testing system (or “shaker”). The use of this type of general purpose test equipment allows for the application of almost any desired shock pulse shape and frequency content to the test sample. It also allows the test sample to remain in its normal vertical orientation (using a slip-table for horizontal shocks). However, the displacement limits of the shaker do limit the maximum mechanical shock amplitude and duration that can be achieved.
In general, no physical impact occurs when using an electrodynamic shaker. The shaker simply has the power to provide the necessary deceleration. However, prior to the rapid deceleration shock event, the shaker must also accelerate the test sample to the required velocity so that the end velocity after the shock event is zero. (This can also be done in the reverse sequence.) As a result of needing to both accelerate and decelerate the test sample, substantial displacement is required, especially if the pulse duration is relatively long. As a result, even if the shaker has sufficient force to perform the desired mechanical shock, it may not be able to do so because the shaker’s maximum displacement is not sufficient.
Mechanical Shock Testing Examples
Other than drop testing of packaged products, the most most common mechanical shock test for typical electronic products is a classical half-sine (more precisely a haversine). Approximately triangular shaped (“sawtooth”) pulses are the next most common. Shock pulses with very high amplitudes are sometimes specified in the frequency domain as an SRS (Shock Response Spectrum) rather than the classical time domain pulse shape. The most common mechanical shock test for electronic products is a half-sine of 20 or 30 g peak amplitude and a duration of 11 msec.
Quest Engineering Solutions provides a broad range of vibration testing and shock testing services. We have a variety of vibration and shock test systems of different sizes and broad test capabilities. Vibration and shock testing are important to achieving a high product quality. Our experience can help you choose the tests which will optimize your product’s design and reliability.
Quest’s vibration and shock test equipment is highly adaptable and can be customized to meet your special vibration/shock test and analysis requirements. Our test equipment includes:
- Electronically controlled mechanical shock tester
- Electrodynamic shakers that can be programmed for shock testing
- Drop shock machines
- Transportation simulator for repetitive bounce testing
- Slip Tables
- Head Expanders
- Load bearing platform
- Custom fixturing
Common Shock Tests:
- Repetitive Shock Test
- Half Sine Shock Pulse
- Saw Tooth Shock Pulse
Mechanical Shock Standards:
- ASTM D4169
- EN/IEC 61373
- EN/IEC 68-2
- ISO 16750-3
- MIL-STD 810G
- and more!