Final Design

Problem Definition:

ATA needs a machine that generates the following shock profiles of specific amplitude and pulse length, as seen in the following table:

Table 1: Acceptance Test Pulse Shapes

The requirements for the machine are:

• It can test samples up to 61 cm x 61 cm x 61 cm (2' x 2' x 2') samples, up to 34 kg (75 lbs).

• It can achieve the given pulse profiles in Table 1 with a measurable repeatability of 10% of the specified amplitude and 10% of the specified pulse width.

• Shock impulse shape must be approximately be a half-sine wave

  • This means that all major motion is in one direction (NO negative acceleration)

  • Any ringing that occurs after the approximate half-sine wave has been achieved can be ignored

Acceptance Testing:

The shock pulse will be measured at the attachment location for the test article.

The test article will be a 30.48 cm x 30.48 cm x 2.54 cm, 18.1438 kg steel plate.

The steel plate will be mounted to an excitation surface as shown (Dimensions in images are in inches):

Figure 2: Test Article (Side View)

Figure 3: Test Article (Top View)

Final Design:

Standardized tab preview (4 of 13 tabs shown)

Closeup of impactor/excitation surface interface

Impactor/excitation interface, shows where impactors (not shown) are attached.

Waxed wood/Acetal Delrin® slider

Underside of wood platform; shows Acetal Delrin® contactors and foam bonded to platform

Device Specifications

Excitation Surface:

10.2 cm x 10.2 cm 3/8"-16 UNC bolt pattern (up to 51 cm in size)

#10-32 threaded holes for ATA accelerometers

Sacrificial steel strike plate to prevent aluminum fatigue (bolt on)

Keenserts in all threaded holes

Optimized to reduce mass but retain stiffness and minimum mode frequencies

Waxed Wood Slider:

Delrin Contact Strips on waxed wood surface make for a low friction interaction

The excitation plate and foam stack sit on a waxed wood slider so that the excitation plate can achieve displacements required by certain pulse profiles without the foam experiencing too much deformation.

The foam has a low natural frequency and is intended to act as a free boundary so that the pulse into the excitation plate is uncorrupted by outside fixtures for longer than the duration of the impact pulse.

Impactor Masses:

Impactor masses; steel plates can be interchanged

CAD of Impactor Actual Impactor

Frame:

Hot rolled steel square tube frame

35 tubes

85 tabs

12 types of tabs

3.5 m x 1.5 x 2.5 m footprint\

Impact Tips:

Delrin Tip Teflon Tip Rubber Tip

Test Results

Theoretical Predictions

Modeling the mass-spring-mass system as a 2 degree of freedom system, the equations for maximum pulse amplitude and pulse length were derived Chapter 3 of the full report. Using these equations, the mass of the impact, stiffness of the impact tip, and height of the drop can be set for each pulse profile to be tested. However, as damping and other losses were not considered in the analysis, their effects on prototype performance cannot be reliably predicted. Therefore, the results from the analysis can be used to achieve pulses close to the desired pulse and empirical methods can be used to hone in on the desired results.

Scale Model Tests

Additionally, scale model tests were performed and qualitative conclusions were taken from the results of these tests. The results indicate that with the use of Teflon as an impact tip, pulse length can be reliably predicted without any testing iteration and the measured amplitude will always be lower than the predicted amplitude within this range of accelerations. The average duration of the best-fit half-sine has a 2.76% error when compared to the predicted pulse. The measured maximum amplitudes have an average error of 27.2% when compared to the predicted pulse. The error becomes greater as the acceleration becomes higher. To correct the amplitude, it is theorized that the drop height could just be increased until the desired amplitude is achieved because pulse length is unaffected by drop height. This is consistent with the empirical tuning method mentioned above.

The results of the scale model impact tests can be seen in full detail in the appendix of the full report. An image of the scale model test set up can be seen below.

Teflon Impact Tip Test Data, Targeting Pulse Profile 2 (Unfiltered)

Teflon Impact Tip Test Data, Targeting Pulse Profile 2 (Filtered)

Rubber Impact Tip Test Data, Targeting Pulse Profile 3

Proof of Repeatability using Rubber impact test data

1^st Trial: 9.70 g, 16.6 milliseconds

2^nd Trial: 10.25 g, 16.5 milliseconds

3^rd Trial: 10.32 g, 16.5 milliseconds