Introduction to shock testing – Part b

Environmental effects of shock

The large impact force generated by the impact comes from the change in the momentum (mo) of the moving object and the object being hit (such as the hard ground) at the very short contact moment (A), that is:

F (Δt) = m ( v₁ – v₂ )

A_{max = F/M=} ( v₁ – v₂ ) / Δt

Where:

F–Shock force, kN;

Δt — Shock contact time, i.e. pulse width D, ms, s;

m –mass of the moving object, kg;

A_max — maximum acceleration of impact, m/s’;

v₁, v₂ –speed of the moving object before and after the impact, m/s.

The result of the impact is that a large impact force (or acceleration) is applied to the product in a very short time, so that the product’s response characteristics to the external impact environment show the following characteristics: high-frequency oscillation, short duration, obvious initial rise time and high-level positive and negative peak acceleration.

For products with complex multimodal characteristics, their impact response includes the following two frequency response components:

1) The forced dynamic characteristic response component given by the external excitation environment (impact force) applied to the product;

2) The response components generated by the product according to its inherent dynamic characteristics during or after the application of the stimulus. These responses will inevitably have an adverse effect on the structural integrity, functional indicators and performance parameters of the internal components of the product, and the degree of influence varies with the magnitude of the impact and the duration. In particular, when the duration of the impact is consistent with the inverse of the product’s frequency, or the main frequency component of the input impact environment is consistent with the product’s inherent frequency, the impact of the external impact on the product’s structural and functional integrity will be strengthened and aggravated. Specifically, it manifests in:

a) product failure caused by increased or decreased friction between parts, or mutual interference;

b) changes in product insulation strength, decreased impedance of electrical insulation, changes in magnetic field and electrostatic field strength;

c) product circuit board failure, damage and electrical connector failure;

d) the product may cause the excess material on the circuit board to shift and cause a short circuit under the impact of the product;

e) permanent mechanical deformation of the product due to overstress of the product structure or non-structural parts;

f) damage to the mechanical parts of the product due to exceeding the ultimate strength;

g) accelerated material fatigue (low-cycle fatigue);

h) potential piezoelectric effect of the product; i) product failure due to cracks in crystal, ceramic, epoxy resin or glass packaging, etc.