WAVELET ANALYSIS BASED ULTRASONIC NONDESTRUCTIVE TESTING OF POLYMER BONDED EXPLOSIVE

Polymer Bonded Explosives (PBX) are well-known explosives for the weapons, especially for the parts of some important weapon. Generally, they first are modelled in cylinder. One of the most important issues in PBX cylinder properties is the detection of its flaw. Each PBX cylinder may have flaws that influence the security, machine-performance and other important physical or chemical performance. Its detection can be made by ultrasonic testing techniques. The ultrasonic (US) testing is based on the fact that the ultrasonic wave is able to distinguish different acoustic impedances when there is the different medium inside the cylinder. The special purpose sensors ranging from 1.25MHz to 5MHz were used to emit/receive the ultrasonic signals. When the US pulse wave propagates from the first surface in the PBX, part of the pulse is propagated back to the transducer owed to the flaw interface. The other part of US pulse wave goes forward and reaches the next surface, this signal eventually goes back to the transducer. These signals containing the flaw information are recorded for analysis. Based on processing the signals by means of the wavelet transform, we can distinguish the flaw with the frequency change of flaw echo and verify the central frequency of the transducer, obtain the accurate part frequency of the signals with which is concerned, and also accurately measure the signal absolute value (amplitude) depending on the effective frequency. The accuracy obtained will encourage its implementation for nondestructive testing of the PBX’s flaw. Introduction: Polymer bonded explosives (PBXs) are the main charge explosives used in nuclear warheads and they are designed to detonate reliably to bring about nuclear implosion. PBXs are highly filled composite materials of crystalline high explosive in a polymeric binder matrix. Generally, they first are molded in cylinder or other shapes. The polymer binder provides structural integrity and mechanical strength. One of the most important issues in PBXs’ cylinder properties is the detection of its flaw. The facts implicate that defects (or change of the location density) or flaws in PBX’s cylinder will affect its security, machine-performance and other important physical performances. Its detection can be made by ultrasonic testing techniques [1]. This work presents an ultrasonic pulse-echo method based on wavelet analysis, which is developed for determining possible defects or flaws in each JOB-9003 PBX’s cylinder or others. Ultrasonic Methodology Based on Wavelet Analysis: The ultrasonic pulse-echo technique is based on the fact that the ultrasonic wave is able to distinguish different acoustic impedances when there is the different medium inside the JOB-9003 explosive cylinder. When the US pulse wave propagates from the first surface in the JOB-9003 cylinder, part of the pulse is back propagated to the transducer owed to the flaw interface, the other part of US pulse wave goes forward and reaches the next surface, this signal eventually goes back to the transducer. These signals containing the flaw information are recorded for analysis. The utilization of wavelet transform (WT) is widespread and an attractive signal processing technique for nondestructive evaluation of material characterization. The WT has already been shown as a useful tool for the interpretation and the enhancement of ultrasonic data [2,3,4]. In this paper, the WT theory is briefly introduced and its application to ultrasonic testing (UT) of JOB-9003 cylinder is explained. The continuous wavelet transform (CWT) is defined as the sum over all time of the signal multiplied by scaled, shifted versions of the wavelet function Ψ(t). Scaling a wavelet simply means stretching (or compressing) it, shifting a wavelet means delaying (or hastening) its onset. The CWT or continuous-time wavelet transform of function S(t) with respect to a wavelet Ψ(t) is defined as follows[5]: