a) Transient Event

What is shock?

Non-periodic excitation of a system characterized by sudden and severe relative displacements in the system. Early shock tests called for classical pulses such as half-sine, and these tests could be accomplished via drop methods. Presently, many shock tests require complex, oscillatory pulses that require other methods such as pyrotechnic, impact or shaker induced environments.

How is it defined?

A shock is defined by a transmission of kinetic energy into a system in a short event interval relative to the system’s natural period. A steady-state response is not achieved.

How is it measured?

Shock is commonly measured via time histories or shock response spectrum (SRS).

b) Time Histories

• Measure amplitude and time duration of shock pulse event
• Commonly used for measurement of classical shock pulse (eg. half-sine), also used for transient events as well
• Typically, amplitude measured in g’s and time in seconds, can also measure velocity, displacement, force, pressure, stress or torque

• Plot of maximum response of a SDOF system, as a function of its natural frequencies, in response to an applied shock input
• May be expressed in terms of acceleration, velocity or displacement, commonly acceleration
• Does not uniquely define the shock input, since 2 different shock pulses can have same maximum peak response
• Maximum of peaks is only a single point in the time of the response

d) Shock test specifications

The goal in defining a shock test is to specify a test rationally, with a known level of conservatism that represent a single or composite of measured in service environments

The engineer must determine if the shock test can be modeled as a classical shock (reviewed in the next section) or a SRS type shock.

If an SRS is chosen the type of definition must be selected. There are many ways the SRS can be defined. A few are absolute response, relative response, acceleration, velocity, displacement, primary, residual and maximax. The 3 most common are:

1) Maximax, absolute acceleration – aerospace industry

2) Maximax, absolute pseudovelocity – navy

3) Maximax, absolute velocity plotted on tripartite paper – earthquake industry

Maximax if the envelope of all conditions through the environment

Tolerances: Typically given in ± dB value from nominal test level for g range.

Number of shocks: Typically for qualification testing of a product there are 3 shocks per direction per axis for a total of 18 shocks. For acceptance testing it may be reduced to 1 shock per direction per axis for a total of only 6 shocks.

MIL-STD 810 is a good reference for shock testing and definition for both classical and SRS shocks

c) Pyroshock

• High G, short duration shock pulse
• Oscillates somewhat symmetrically around a zero baseline value
• Typically decays in very short time <20ms
• Measured using SR

d) Seismic Shock: Low g, high displacement, long time duration

Sources of Shocks

Pyrotechnic Excitation
• Point Sources
• Line Sources
Mechanical Excitation
• Collision impact
• Handling / Drop
• Evasive Maneuvers in Aircraft or Missiles (Gust Loading)
• Ballistic impact
• Aircraft landing
• Braking
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• Missile / Rocket Launching
• Gunfire
• High-speed fluid entry
• Transportation, uneven surfaces, rough terrain
Natural Phenomenon
• Earthquake
• Wind gust
• Air Blast
• Ocean Waves
• Ice Impact

  Shock Testing Methods

• Shock testing is commonly performed via the following methods. Each method imparts the kinetic energy to the system in a different manner

- Drop

- Hammer / Impact

- Shaker (Electrodynamic / Hydraulic)

- Pyroshock

- Hopkinson Bar

- MIPS

• In drop testing DV ¹0 and Dd ¹0, while others maintain zero change in velocity and displacement

a) Drop Shock

• Test item at velocity (Moving towards impact surface)
• Usually free-fall or assisted (bungee-cord) fall
• Massive target surface
• Shock programmer (target surface) determines pulse shape, peak g’s and duration
• Drop height and/or assist determine impact velocity
• Little energy outside pulse bandwidth
• General purpose machines
• Large DV and Dd
• Simple (Classical) pulses shapes

b) Hammer / Impact Testing

• Navy prescribed testing to simulate severe but non-lethal, non-contact underwater explosions
• Categorized by lightweight, medium-weight and heavy-weight machines depending on size of test article
• Lightweight machine is pendulum hammer that strikes an anvil plate to which specimen is attached
• Medium-weight machine is pendulum hammer that strikes a group of steel channels that are attached to an anvil plate. The anvil plate is allowed to move up to 3” before being stopped by a retaining ring.
• Heavy-weight machines are floating barges where the specimen is located and explosive charges are set off underwater at prescribed distances away from the barge.

	c) Shaker Testing Limited to shaker velocity, force and displacement capabilities For shock pulses less than 3000Hz typically Can be controlled within test tolerances better than other methods due to control system and shaker setup of allowing pre test levels

d) Pyroshock

• Caused by explosive device or propellant activated device releasing stored strain energy
• Input is highly localized
• High g (100~200,000g), high freq (100Hz~1Mhz), very lightly damped
• Low vel and disp
• Short time duration < 20 msec
• Damping of SRS model is held at 5% or Q=10
• Measured using SRS, not time history of shock
• Plotted on log-log paper and typically is terms of accelerations vs. frequency
• Imparts energy in all 6 directions simultaneously for reduced number of overall shock events.
• Very little rigid body motion of a structure in response to pyroshock
• Characteristics of pyroshock vary greatly with distance from shock event (Near-field, mid-field and far-field pyroshock)
• Near-field
• within 6 inches of source
• materials stress waves dominate response
• peak accelerations greater than 5000 g’s and contains frequency content up to and above 1 MHz
• very difficult to measure due to instrumentation limitations
• Mid-field
• 6–24 inches from source
• peak accelerations greater than 1000 g’s and contains frequency content up to and above 10 KHz
• Far-field
• greater than 24 inches from source
• low g levels and frequencies usually less than 10KHz

Rarely causes structural failure, more frequently failure in electronic components (eg. relay chatter, short circuits, circuit board failures)

e) Hopkinson Bar

• Provides extremely high g levels
• A controlled projectile impacts the end of a metallic bar, and a stress wave of know magnitude propagates through the bar.
• UUT is at end of bar and experiences high g rapid rise time stress when wave arrives
• Also used in calibration of accelerometers
• Exotic materials such as Beryllium and Titanium sometimes used for Hopkinson Bars

Example Failures

a) Broken electronics

b) Contamination

c) Component level failures