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·Materials Characterization and testing
Ultrasonic Characteristics of Plastic bonded explosive (PBX) JOB-9003 in Thermal shock TestsTian Yong, Zhang Weibing, Hao Yin, Li Jingming, Wen Maoping
Institute of Chemical Materials, China Academy of Engineering Physics,
P.O.Box 919-312, Mianyang 621900 China; 0086-0816-2485371; Fax 0086-0816-2281339
*Sponsored by the fund of China Academy of Engineering Physics.
As one kind of mixed polymeric systems composed by explosive crystal, binder and some additives, and shaped by granulating, pressing and machining, PBX parts are played simultaneously as special functional materials and structural parts in weapons. Not only may PBX damage occur on account of some technological facts, but also occur or evolve due to affection of environmental facts. In the course of transportation, using, and storage, PBX will mainly subjected to mechanical pre-tightened force and thermal stress caused by temperature gradient shift or even simultaneously, then damage in PBX may initiate, accumulate or expand, and finally may cause PBX to be broken [5-6].
In order to obtain some information about the damage behavior of PBXs, we promoted an original ultrasonic testing observation to PBX JOB-9003 samples during thermal shock tests (TSTs) and acquired some useful ultrasonic parameter data with response to the damage or destruction of the samples.
PBX material: JOB-9003; size: 65mm×25mm×25mm, shaped by machining thermal pressed cylinder; sample quantity: 7.
Thermal Shock Test Method
Equipment: constant temperature box, 2 sets.
High temperature: 50°C, low temperature: 0°C shift time of conveying samples from high temperature box into low one or conversely: <1min; stable time of samples staying in high or low temperature box for temperature balance: about 30min.
8 round or 30 cycle TSTs totally. From the first to the eighth, each round contains 1, 1, 2, 2, 4, 4, 8, 8 thermal shock cycles (TSCs) respectively and a cycle includes one time of thermal shock from high temperature to low temperature and one time from low to high. Before TSTs and after every round, the sizes, densities, and ultrasonic characteristics of the samples are measured.
Ultrasonic Testing Method
Principal: based on the relationship between ultrasonic velocity and elastic modulus of materials, and damage or cracks existed in materials will cause the attenuation to ultrasonic to be intensified.[1,7]
Parameters to be tested: bottom-echo height (gain) and longitude wave velocity, and mainly tested along the long axis of samples.
Ultrasonic detector: Model CTS-36, digital.
Probe: 2MHz, 20mm in diameter, water coupling.
Detecting conditions: sensitivity, 80%; threshold, 20%.
Facade, Size and Density Variations of Samples in TSTs
Three of the samples numbered 93028-2, 93028-6,93029-4 respectively appear apparent cracks on the facades, and the densities descend from normal value 1.848g/cm3, 1.849g/cm3, and 1.847g/cm3 before TSTs to 1.846g/cm3, 1.846g/cm3, and 1.843g/cm3 after the first round of TSTs. But no apparent dimensional variation of the three samples observed.
Also no apparent variation of the other four samples observed on the facade, size, and density until 8 round TSTs finished.
Ultrasonic Longitude Wave Velocity Variations of Samples in TSTs
Table 1 gives the ultrasonic velocity of the seven samples tested along the long axis before and after TSTs.
The data shows that the ultrasonic velocity of the seven samples appears different evolution in a degree during TSTs. Besides for the descending of the ultrasonic velocity of all samples after 1st round of TSTs, three of samples, 93028-2, 83028-6,and 93029-4, display apparent cracks on the facade. And after 2nd round TSTs, two of the other samples, numbered 93028-7 and 93029-7, their bottom waves turn to be undetectable. The other two samples, numbered 93028-8 and 93028-9, their ultrasonic velocity appears some featured variations during all TSTs. Ultrasonic velocity descends a little about 20m/s at 1st round TSTs and keeps constant nearly at 2nd and 3rd rounds. Starting from 4th round of TSTs, with TSCs to be increased rapidly, ultrasonic velocity descends slowly and step by step. At 7th round, the ultrasonic velocity of sample 93028-9 descends to 2954m/s, descending amplitude near up to 3%, and at the following 8th round, the bottom-echo disappeared. Sample 93028-8, after the 8 rounds of TSTs, the ultrasonic velocity descends to the level of 93028-9 sample at 7th round, in which that the 6th round ultrasonic velocity looks like abnormal might concern with the testing error.
|Number of Sample||round number of TSTs (and accumulated TSCs)|
|Table 1: The ultrasonic velocity of 7samples tested along the long axis during TSTs (m/s)|
Ultrasonic Bottom-Echo Height (Gain) Variations of Samples inTSTs
Table 2 gives the bottom-echo heights (gains) of the seven samples tested along the long axis before and after TSTs.
The data show that the bottom-echo heights of the seven samples appear seemingly more apparent variation features during TSTs. The five samples respectively numbered as 93028-2, 83028-6, 93029-4, 93028-7, and 93029-7, after 1st round of TSTs, their bottom-echo heights are all apparently lowered, and their gains are all significantly increased and reach the observed maximum about 50dB with almost the same ascending amplitude. For sample 93028-9, the gain varies matched to the ultrasonic velocity somewhat. The gain of sample 93028-9 increases a few about 2dB at 1st round of TSTs, and almost keeps constantly at 2nd and 3rd rounds of TSTs. But starting from 4th round of TSTs, the gain increases rapidly, and at 7th round, the gain gets up to 52.8dB with an ascending amplitude of 70%, which agrees to the other five samples cracked or bottom-echo disappeared. But after the last round of TSTs, sample 93028-8 has not been observed variation in the bottom-echo height.
|Number of Sample||round number of TSTs (and accumulated TSCs)|
|Table 2: The bottom-wave gains of 7samples tested along the long axis during TSTs (dB)|
The seven samples behaved distinguishable different evolution courses during TSTs. Sample 93028-2, 93028-6, and 93029-4,after 1st round (only one cycle) of TSTs, coordinated with the responding density descending and ultrasonic gain ascending significantly, they appeared noticeable cracks and broken. Sample 93028-7 and 93029-7, their ultrasonic gains varied as the previewed three samples after 1st round of TSTs, and after 2nd round their bottom waves disappeared. The two samples did not show density variation. And these five samples did not play differently on the aspect of ultrasonic velocity variation. Sample 93028-9 almost showed a completed variation of both ultrasonic velocity and gain, however the ultrasonic gain variation is quite more considerable than the velocity variation. Sample 93028-8 performed ultrasonic velocity variation just like sample 93028-9, but no such company of ultrasonic bottom-echo height varied significantly as sample 93028-9. It implies that the sample might resist thermal shocks furthermore.
The evolution tracks of the seven samples in TSTs are so different, most probably it might be due to their different original state. There is a very big possibility the five samples cracked or bottom-echo disappeared at 1st or 2nd round of TSTs have more severe initial damage or cracks in their original states than the other two samples. This also shows that the initial damage or cracks are very dangerous to PBX JOB-9003 as structure materials.
Responding to the thermal shock damage of PBXs, there seems to be some coordination between ultrasonic gain and velocity. Sample 93028-9 is a example which undergoes a complete damage evolution course with their ultrasonic gain and velocity varies typically and inter-relevantly. The ultrasonic gains of all samples nearly reached to the same level before cracked or bottom-echo disappeared, shows that the destruction of PBX JOB-9003 in TSTs might has a variation limitation of ultrasonic parameters especially ultrasonic gain.
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