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Ultrasonic Characteristics of Plastic bonded explosive (PBX) JOB-9003 in Thermal shock Tests Tian 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 Contact

Abstract

Such ultrasonic characteristics as the longitude wave velocity and bottom-echo height of plastic bonded explosive (PBX) JOB-9003 samples before and after thermal shock tests (TSTs) has been measured. The samples appear increased damage or destruction and some typical variations in ultrasonic characteristics at different phrases of TSTs. The initial states such as damage or cracks of PBX JOB-9003 samples may play a critical role for their behavior in TSTs is shown. And it is basically feasible to employ ultrasonic characterization method to monitor damage and predict destruction of PBX JOB-9003 samples in TSTs, and using the bottom-echo height as an indicating parameter of damage may be more sensitive and more valuable than the longitude wave velocity.
Keywords: Plastic Bonded Explosive, Ultrasonic Characterization, Thermal Shock Test

^*Sponsored by the fund of China Academy of Engineering Physics.

INTRODUCTION

The studies on plastic bonded explosives (PBXs) from the viewpoint of damage mechanics were initiated in China about several years ago[1]. This is because the promotion of PBXs used as structural materials and the development of damage mechanics providing a theoretical and practical foundation [2-4].

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.

SAMPLES AND METHOD

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.

Condition:
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.

Program:
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%.

PHENOMENA, RESULTS AND DISCUSSION

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 1^st round of TSTs, three of samples, 93028-2, 83028-6,and 93029-4, display apparent cracks on the facade. And after 2^nd 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 1^st round TSTs and keeps constant nearly at 2^nd and 3^rd rounds. Starting from 4^th round of TSTs, with TSCs to be increased rapidly, ultrasonic velocity descends slowly and step by step. At 7^th round, the ultrasonic velocity of sample 93028-9 descends to 2954m/s, descending amplitude near up to 3%, and at the following 8^th 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 7^th round, in which that the 6^th round ultrasonic velocity looks like abnormal might concern with the testing error.

Number of Sample	round number of TSTs (and accumulated TSCs)
	0(0)	1(1)	2(2)	3(4)	4(6)	5(10)	6(14)	7(22)	8(30)
93028-2	3042	3021	x*
93028-6	3066	3014	x
93028-7	3043	3014	o**	o	o	o	o	o	o
93028-8	3044	3018	3026	3024	3017	2999	2957	2973	2956
93028-9	3036	3014	3019	3019	3005	2988	2960	2954	o
93029-4	3048	3038	x
93029-7	3043	3014	o	o	o	o	o	o	o
Table 1: The ultrasonic velocity of 7samples tested along the long axis during TSTs (m/s)

* x: such sample appearing apparent cracks at one ahead round of TSTs was not arranged in the following TSTs.,
** o: such sample after this round of TSTs tested no bottom wave.

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 1^st 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 1^st round of TSTs, and almost keeps constantly at 2^nd and 3^rd rounds of TSTs. But starting from 4^th round of TSTs, the gain increases rapidly, and at 7^th 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)
	0(0)	1(1)	2(2)	3(4)	4(6)	5(10)	6(14)	7(22)	8(30)
93028-2	32.0	50.0	x*
93028-6	30.2	49.4	x
93028-7	30.2	49.4	o**	o	o	o	o	o	o
93028-8	31.0	33.0	32.8	31.4	32.0	31.8	31.2	29.0	32.0
93028-9	31.0	33.4	33.0	31.8	36.6	49.8	48.0	52.8	o
93029-4	29.0	48.6	x
93029-7	29.6	48.0	o	o	o	o	o	o	o
Table 2: The bottom-wave gains of 7samples tested along the long axis during TSTs (dB)

* As Tab 1., ** As Tab 1.

Result Discussion
The seven samples behaved distinguishable different evolution courses during TSTs. Sample 93028-2, 93028-6, and 93029-4,after 1^st 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 1^st round of TSTs, and after 2^nd 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 1^st or 2^nd 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.

CONCLUSION

In the course of TSTs of PBX JOB-9003 and ultrasonic characteristic testing before and after TSTs, the damage or destruction of the samples occurred at the different phases of TSTs and some typical and coordinated variations of ultrasonic characteristics of the samples are observed and tested.
The initial states such as damage or cracks of PBX JOB-9003 samples may play a critical role for their behavior in TSTs.
It is basically feasible to employ ultrasonic characterization method to monitor damage and predict destruction of PBX JOB-9003 samples in TSTs, and using the bottom-echo height as an indicating parameter of damage may be more sensitive and more valuable than the longitude wave velocity.

Conferences

1. Tian Yong, Zhang Weibing, Hao Ying, etc. Explosive damage and nondestructive testing. 1998 Annual Reports of China Academy of Engineering Physics. Sichuan S&T Publisher, Sichuan, 1999(in Chinese)
2. Krajicinovic D. Damage mechanics. Elsevier Science Publishers, North-Holland, 1996
3. Yang Guangsong. Damage mechanics and composite materials damage. National Defense Industry Publisher, Beijing, 1995(in Chinese)
4. Yu Shouwen, Feng Xiqiao. Damage mechanics. Tshing Hua university publisher, Beijing, 1997(in Chinese)
5. Skidmore C B, et al. The evolution of micro-structural changes in pressed HMX explosives,11^th International Detonation Symposium, 1998
6. Seaman L, et al. Development of a viscous internal damage model for energetic materials based on the BFRACT micro-fracture model, 11^th International Detonation Symposium, 1998
7. Yu Hongfa, Pei Rui. Research on nondestructive testing of the strength of long-age concrete. Chinese Journal of NDT, 1999,21(11): 484-485,502

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