Explosive shock test of naval ship
Explosive shock test of naval ship

A mechanical or physical shock is a sudden acceleration caused, for example, by impact, drop, kick, earthquake, or explosion. Shock is a transient physical excitation.

Shock describes matter subject to extreme rates of force with respect to time. Shock is a vector that has units of an acceleration (rate of change of velocity). The unit g (or g) represents multiples of the acceleration of gravity and is conventionally used.

A shock pulse can be characterised by its peak acceleration, the duration, and the shape of the shock pulse (half sine, triangular, trapezoidal, etc.). The Shock response spectrum is a method for further evaluating a mechanical shock.[1]

Shock measurement

Shock measurement is of interest in several fields such as

Shocks are usually measured by accelerometers but other transducers and high speed imaging are also used.[12] A wide variety of laboratory instrumentation is available; stand-alone shock data loggers are also used.

Field shocks are highly variable and often have very uneven shapes. Even laboratory controlled shocks often have uneven shapes and include short duration spikes; Noise can be reduced by appropriate digital or analog filtering.[13][14]

Governing test methods and specifications provide detail about the conduct of shock tests. Proper placement of measuring instruments is critical. Fragile items and packaged goods respond with variation to uniform laboratory shocks;[15] Replicate testing is often called for. For example, MIL-STD-810G Method 516.6 indicates: at least three times in both directions along each of three orthogonal axes".

Shock testing

Military shipping container being drop tested
Military shipping container being drop tested

Shock testing typically falls into two categories, classical shock testing and pyroshock or ballistic shock testing. Classical shock testing consists of the following shock impulses: half sine, haversine, sawtooth wave, and trapezoid. Pyroshock and ballistic shock tests are specialized and are not considered classical shocks. Classical shocks can be performed on Electro Dynamic (ED) Shakers, Free Fall Drop Tower or Pneumatic Shock Machines. A classical shock impulse is created when the shock machine table changes direction abruptly. This abrupt change in direction causes a rapid velocity change which creates the shock impulse. Testing the effects of shock are sometimes conducted on end-use applications: for example, automobile crash tests.

Use of proper test methods and Verification and validation protocols are important for all phases of testing and evaluation.

Effects of shock

Mechanical shock has the potential for damaging an item (e.g., an entire light bulb) or an element of the item (e.g. a filament in an Incandescent light bulb):

Considerations

When laboratory testing, field experience, or engineering judgement indicates that an item could be damaged by mechanical shock, several courses of action might be considered:[17]

See also

Notes

  1. ^ JE, Alexander (2009). "The Shock Response Spectrum – A Primer" (PDF). Proceedings of the IMAC-XXVII, February 9–12, 2009 Orlando, Florida USA. Society for Experimental Mechanics. Archived from the original (PDF) on 2016-03-04. Retrieved 9 Feb 2015.
  2. ^ Dickensen, J A (1985). "The measurement of shock waves following heel strike while running". Journal of Biomechanics. 18 (6): 415–422. doi:10.1016/0021-9290(85)90276-3. PMID 4030798.
  3. ^ ASTM D3332-99(2010) Standard Test Methods for Mechanical-Shock Fragility of Products, Using Shock Machines
  4. ^ ASTM F1543-96(2007) Standard Specification for Shock Attenuation Properties of Fencing Surfaces
  5. ^ Walen, A E (1995). "Characterizing Shock Absorbers for Ground Vehicle Simulation". JTE. ASTM International. 23 (4). ISSN 0090-3973.
  6. ^ ASTM D1596-14 Standard Test Method for Dynamic Shock Cushioning Characteristics of Packaging Material
  7. ^ ASTM F429-10 Standard Test Method for Shock-Attenuation Characteristics of Protective Headgear for Football
  8. ^ ASTM STP209 Design and Tests of Building Structures: Symposiums on Seismic and Shock Loading Glued Laminated and Other Constructions.
  9. ^ Gibson, PW (1983). "Amplification of shock Waves by Textile Materials" (PDF). J Textile Institute. 86 (1): 167–177. Archived from the original (PDF) on 27 December 2016. Retrieved 14 February 2015.
  10. ^ Shock Design Criteria for Surface Ships (PDF), vol. NAVSEA-908-LP-000-3010, US Navy, 1995, archived from the original (PDF) on 2015-02-14, retrieved 14 February 2015
  11. ^ "MIL-S-901D (NAVY), MILITARY SPECIFICATION: SHOCK TESTS. H.I. (HIGH-IMPACT) SHIPBOARD MACHINERY, EQUIPMENT, AND SYSTEMS, REQUIREMENTS FOR"
  12. ^ Settles, Gary S. (2006), High-speed Imaging of Shock Wave, Explosions and Gunshots, vol. 94, American Scientist, pp. 22–31
  13. ^ ASTM D6537-00(2014) Standard Practice for Instrumented Package Shock Testing For Determination of Package Performance
  14. ^ Kipp, W I (February 2002), INSTRUMENTATION for PACKAGE PERFORMANCE TESTING (PDF), Dimensions.02, International Safe Transit Association, archived from the original (PDF) on 2015-02-07, retrieved 5 Feb 2015
  15. ^ ASTM Research Report D10-1004, ASTM International
  16. ^ Saitoh, S (1999). "Water hammer breakage of a glass container". International Glass Journal. Faenza Editrice. ISSN 1123-5063.
  17. ^ Burgess, G (March 2000). "Extensnion and Evaluation of fatigue Model for Product Shock Fragility Used in Package Design". J. Testing and Evaluation. 28 (2).
  18. ^ "Package Cushioning Design" (PDF). MIL-HDBK 304C. DoD. 1997. ((cite journal)): Cite journal requires |journal= (help)

Further reading