Crash safety shock test comparison

[YossiD]

I am preparing a similarity analysis for a rugged computer for defense applications.

As usual, the customer is trying to minimize testing expenses and wants to accomplish as much environmental qualification as possible by means of similarity to other systems that have successfully completed the testing.

At the moment, I am trying to write a reasonable sounding justification for comparison between two systems in connection with Crash Safety Shock per MIL-STD-810F, Method 516.5, Procedure V. The test pulse is a 40g/11ms sawtooth applied in all six directions. The purpose of the test to to ensure that the tested system will not break free to cause damage or injury in the event of a vehicle crash. Functionality of the system during/after the test is not relevant.

The two systems I am comparing have similar mechanical designs, but the tested system is heavier (7.5 kg vs 2.5 kg) and has a significantly higher center of mass (~5cm vs ~1 cm above the mounting base). The heavier and higher system, which is the one that was tested, is mounted with 8x 10-32 screws, through mounting flanges in its base. The lighter system, which I am addressing in the similarity analysis, is similarly mounted using four of the same screws.

It seems to me that the stress on each mounting screw will be significantly less for the lower lighter system. First of all, there is less mass per mounting screw (2.5kg/4 screws vs. 7.5kg/8 screws). And the lower center of mass means that the load on the screws during the crash test pulses in the plane of the mounting surface will be almost entirely shear, while the screws of the higher system will experience a combined tensile-shear load. The perpendicular crash pulses will result in a purely tensile load for both systems, so the comparison is straightforward.

I do not have the means (or budget) to perform a dynamic analysis but I would think that the above combination of factors is a convincing argument that if the higher/heavier system passed the test, the lower/lighter one will also pass under the same conditions.

Does this sound reasonable.

Thanks.

 

[SWComposites]

Yes, but only if the screws are the critical failure mode. You have to also analyze your box. The flanges, etc may be critical.

Though the govt cert agency may want more than glorified arm waving.

 

[YossiD]

Thanks for the quick reply and for validating my approach. I agree that it is glorified arm waving, but I think it is a reasonable approach under the circumstances.

The mounting screws are the only reasonable point of failure, as there are no parts that can fly off.

I suspect that our customer will be performing some kind of testing at the next higher level, which is why they are willing to settle for a similarity analysis rather than to pay for the actual testing. Possibly a CYA exercise so they can check a box.

 

[Ng2020]

The designs might be similar but you need certainty there are no changes to gauges or material specifications to make an argument by loads comparison alone.