Common Terms Used in Protective Package Design and Testing

  1. Displacement: A vector quantity describing the change of position of a body and usually measured from a position of rest.
  2. Velocity: The rate at which displacement changes; it is a vector quantity with both magnitude and direction. It is the differential of displacement with respect to time. It is also the integral of acceleration.
  3. Acceleration: the rate at which velocity changes. It is the differential of velocity with respect to time. The acceleration of gravity (g) is a constant (386 in/sec) or (9.8 m/s). Impacts are normally measured in multiples of Earth's gravitational constant (G's).
  4. Velocity Change: the difference between impact and rebound velocity (Vi – Vr). This quantity can be thought of as the change in momentum associated with a given impact event. The difference in system velocity magnitude and direction from the start to the end of a shock pulse.

    Velocity Change is also directly related to an impact drop height by the following equation:
    ΔV = (1 + e)√(2gh)
    where: e = the coefficient of restitution of the impact services (Vr / Vi)(0 ≤ e ≤ 1)
         g = Earth's gravitational constant (386 in/s or 9.8 m/s)
         h = drop height

    Velocity Change is also equal to the area under the acceleration versus time pulse from a monitored impact event, that is, the magnitude may be determined from the integral of the acceleration versus time signature.
  5. Vibration: The oscillation of an element of a mechanical system about a fixed reference point.
  6. Mechanical Shock: A sudden, severe, non-periodic excitation of an object or system.
  7. Vibration, Periodic: A vibration consisting of a wave form that is repeated at equal time intervals.
  8. Vibration, Random: An oscillation having an instantaneous frequency and amplitude that can be specified only on a probability basis.
  9. Simple Harmonic Motion: Periodic vibration that is a sinusoidal function of time.
  10. Cycle: A complete sequence of values of a periodic quantity occurring over a definite time period.
  11. Period: Smallest interval of time in which a reoccurring event repeats itself.
  12. Frequency: The reciprocal of the period necessary for one complete oscillation. This is often described in cycles per second or "Hertz" abbreviated Hz.

    Diagram
  13. Frequency, Forcing: the frequency of excitation.
  14. Frequency, Natural: the frequency of free oscillation of a spring/mass system.
  15. Frequency, Resonant: the frequency at which a spring/mass system displays its maximum response.
  16. Harmonic: a sinusoidal quantity having a frequency that is an integer multiple of a fundamental or natural frequency.
  17. Resonance: resonance of a system in forced vibration exist when any change, however small, in the frequency of excitation causes a decrease in the response of the system. Resonance represents a maximum of response of a spring/mass system to forced vibration input.
  18. Damping: the dissipation of oscillatory or vibratory energy with motion or with time. Critical Damping is the minimum viscous damping that will allow a displaced system to return to its initial position without oscillation.
  19. Single Degree of Freedom System: a system consisting of a rigid mass attached to a referenced foundation by a mass-less spring that is constrained along a straight line.
  20. Shock Spectrum: a plot of the maximum response experienced by a single degree of freedom system as a function of its own natural frequency in response to an applied shock input. The response may be expressed in terms of acceleration, velocity, or displacement.
  21. Shock Pulse: a substantial disturbance characterized by a rise in BK and acceleration from a constant value in a short period of time. Shock pulses are normally displayed graphically as curves of acceleration as a function of time.