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4D dart reproducibly impacts the center of the clamped portion of the film. 8.2 Check the apparatus periodically to make sure specimen slippage during testing is not occurring. If slippage occurs. this is reason to reject the results. The likelihood of occur- rence of slippage increases with increasing dart weight and with increasing drop height and is greater with some materials than with others. Slippage may be checked conveniently during the course of testing of a routine sample at a missile weight wherein both failures and non - failures are being observed. Either of the fol- lowing procedures is effective. S.2.1 Before dropping the missile on a clamped specimen. draw a circle on the film using a ball point pen in contact w ith the inside wall of the upper clamp. :\,,ply only the pressure of the pen itself to the film. (Caution—For safety reasons. the dart should not be supported by the electromagnet whit drawing the circle.) After the dart is dropped and prior to removing the plastic film, draw another circle using a ball-point pen of anothe color. If the lines are drawn. distinct doubt lines at any point on the circumference sho that slippage has occurred. 8.2.2 If crocus cloth or sandpaper is affixe to the gaskets to effect greater gripping. deter mine slippage simply by inspecting the c)ampe film area after impact for evidence of scratc marks produced as slippage occurred. STAIRCASE TESTING TECHNIQUE 01709 F Add the necessary number of incremental weights onto the dart shaft and put the locking collar into place so that the weights are held securely in place. 9.5 Select a missile weight increment AW appropriate to the impact strength of the sam- ple: The value chosen for A W should be such that three to six (but at least three) missile weights will be employed in the determination. N A W value equal to some 5 to 15 % of WF. the impact failure weight. is usually appropriate. 9.6 Place the first test specimen over the bottom part of the clamp. making sure that it is uniformly flat, free of folds. and that it covers the gasket at all points. Clamp in place with the top part of the annular clamp. . 9.7 Energize the electromagnet and put the dart into position. De -energize the magnet to release the dart. If the dart bounces off the specimen surface. catch the dart after it e bounces to prevent both multiple impact with the specimen surface and damage to the hemi- spherical contact surface of the dart resulting r from impact with metal parts of the apparatus. e 9.8 Examine the test specimen for any e%i- w deuce of slippage. If slippage occurs. this u reason to reject the results. d 9.9 Examine the specimen to determine - whether it has or has not failed. Failure is d defined as any break through the film that can In be observed readily by feeling or by viewing the specimen under back -lighting conditions. Re- cord the result on a form such as that shown in Fig._'. using an 0 to denote non -failure and an ,X to denote failure. 9. Procedure 9.10 if the first specimen failed. decrease the U By this technique. a uniform missile missile weight by A W. if the first specimen did weight increment is empioved during test and not fail. increase the missile weight b% ,ti' t of eachTest the second specimen. tinue te-,ting the missile weight is changed after tessuccessive specimens. decreasing or increasinrl specimen. 9.2 Select Method A or Method B for use. as the missile weight by %W between drops de desired. or as required by the relevant material pending upon whether the preceding specimen specification. Set up the apparatus for testing did or did not fail. as described in 8.1. If desired. carry out a 9.11 After 20 specimens have been tested slippage check as described in 8.3 at some point il ti un t10 at th slpointbtestngos complete. If not' during the course of testing.1. 9.3 'Measure and record the average thick- complete testing a�follows: ness of the test specimens in the area of impact 9.11.1 if .%., < 10. continue testing additional to the nearest 0.0025 mm (0.001 in.). specimens until N - 10, then stop testing. 9.4 For a starting point, select a missile 9. 11.2 If N > 10. continue testing additional weight near the expected impact failure weight. specimens until the total number of non -failures 90