The Forgotten Damage Boundary Method

The Forgotten Damage Boundary Method

The D-Bound-curve measures the fragility level of a product. A programmable shock-testing machine is used to determine the D-Bound-curve. Two types of shock pulses are used for this determination: a half sine and a square wave shock pulse. The duration and acceleration of the square wave is adjustable.

The measured critical acceleration (Ac) and critical velocity change (ΔVc) determine the D-Bound curve. The product is tested per axis (+/-). Depending on the planes of symmetry, a 2x3 test series is required to determine the critical velocity change, and 2x3 test series is required for the critical acceleration. The maximum number of test series is 12, requiring 4-5 shocks per direction to determine the first point of damage. Above the critical velocity change and acceleration levels, the possibility of damaging a product is almost 100%.

Based on our knowledge and experience, only one prototype product is needed to carry out a full D-Bound test. However, the test must be stopped just before the anticipated damage occurs, and in doing so, the results of the D-Bound curve might be slightly conservative. Nevertheless, the number of prototype products (most of the time expensive) is kept to a minimum this way. The margin between actual testing and the D-Bound-curve outcome can become significant when the single product method is applied on products or components that are brittle.

The critical velocity change is determined with a half sine shock pulse. This shock pulse has a fixed duration (1-1.5 ms) and a wide range of acceleration levels (1000-6000m/s²). The drop height of the shock table is increased gradually until the first point of (unacceptable) damage to the product is attained.

The critical acceleration is determined by means of a square wave shock pulse (20-1000m/s² - 5-60ms). A square wave is defined by 19 frequencies that excite all frequencies present in a product. A half sine shock pulse will usually excite only one frequency. At a constant drop height, the acceleration level increases gradually as duration decreases. The decreasing duration ensures that every possible frequency of the product will be excited eventually.

Some of the applications are:

• Reducing or determining the minimum required amount of cushioning material in packaging.
• Verifying shock resistance of equipment and products for the defence and civil industries (transport, earthquake, reliability).
• Optimising products or component joints, e.g. (adhesives, bolts, pop rivets).
• Comparing similar products or verifying product modifications.
• Guarding against excessive safety measures implemented in the design (overkill).
• Reduction of the number of tests, e.g. skip long duration bump testing (unless intended for low-cycle fatigue).

The S2T philosophy is that given identical product costs, a higher shock resistance will result in less weight, better quality, and lower production and service costs.

A good shock design is realised when material properties concerning strength and stiffness are optimised and superfluous material is kept to a minimum. Superfluous material causes higher loads on joints because each kilo/gram of mass has to be multiplied by 50 - 500 m/s², or more.

Higher loads require stronger, and/or more bolts and glue; bending moments increase and create the stresses on the same material thickness or dimensions.

The damage usually affects the joints; the incorrect loading of (brittle) materials, stress concentrations, possibly in combination with bending moments. Yet, most damage is unnecessary and can be easily avoided, even without complex analyses.

Dedicating more time and effort into the design and engineering phase will prevent 80 to 90% of this type of damage from occurring. Full integration of transport or shock requirements in the design phase gives a near 100% reliability factor towards damage prevention.

Do we have your attention? Sebert trillingstechniek would be pleased to send you information about the possibilities we have to offer. You are also welcome to visit us for a demonstration or support.