Abstract

This practice covers a procedure for measuring the ultrasonic velocities in the outer wall of polyethylene storage tanks. The practice is intended for application to the outer surfaces of the wall of polyethylene tanks. An angle beam lateral longitudinal (LCR) wave is excited with wedges along a circumferential chord of the tank wall. A digital ultrasonic flaw detector is used with sending-receiving search units in through transmission mode. The observed velocity is temperature corrected and compared to the expected velocity for a new, unexposed sample of material, which is the same as the material being evaluated. The difference between the observed and temperature corrected velocities determines the degree of UV exposure of the tank.

This abstract is a brief summary of the referenced standard. It is informational only and not an official part of the standard; the full text of the standard itself must be referred to for its use and application. ASTM does not give any warranty express or implied or make any representation that the contents of this abstract are accurate, complete or up to date.

Significance and Use

Measuring the velocity of ultrasound in materials is a unique method for determining nondestructively the physical properties, which can vary due to both manufacturing processes and environmental attack. Velocity is directly related to the elastic moduli, which can vary based on environmental exposure and manufacturing process, The L _CR method described herein is able to measure the velocity between two adjacent points on a surface and therefore is independent of the conditions on the opposite wall. Applications of the method beyond polymer tanks will undoubtedly be developed and examination may occur in the production line as well as in the in-service mode.

1. Scope

1.1 This practice covers a procedure for measuring the ultrasonic velocities in the outer wall of polyethylene storage tanks. An angle beam lateral longitudinal (L _CR ) wave is excited with wedges along a circumferential chord of the tank wall. A digital ultrasonic flaw detector is used with sending-receiving search units in through transmission mode. The observed velocity is temperature corrected and compared to the expected velocity for a new, unexposed sample of material which is the same as the material being evaluated. The difference between the observed and temperature corrected velocities determines the degree of UV exposure of the tank.

1.2 The practice is intended for application to the outer surfaces of the wall of polyethylene tanks. Degradation typically occurs in an outer layer approximately 3.2-mm (0.125-in.) thick. Since the technique does not interrogate the inside wall of the tank, wall thickness is not a consideration other than to be aware of possible guided (Lamb) wave effects or reflections off of the inner tank wall. No special surface preparation is necessary beyond wiping the area with a clean rag. Inside wall properties are not important since the longitudinal wave does not strike this surface. The excitation of Lamb waves must be avoided by choosing an excitation frequency such that the ratio of wavelength to wall thickness is one fifth or less.

1.3 UV degradation on the outer surface causes a stiffening of the material and an increase in Young's modulus and the longitudinal wave velocity.

1.4 The values stated in SI units are to be regarded as standard. The values given in parentheses are mathematical conversions to inch-pound units that are provided for information only and are not considered standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.

ASTM Standards

E494 Practice for Measuring Ultrasonic Velocity in Materials

E543 Specification for Agencies Performing Nondestructive Testing

E1316 Terminology for Nondestructive Examinations

E2373 Practice for Use of the Ultrasonic Time of Flight Diffraction (TOFD) Technique

ASNT Documents

AIA Document

Keywords

Longitudinal waves; Polyethylene (PE) tank walls; Ultrasonic velocity;

ICS Code

ICS Number Code 23.020.10 (Stationary containers and tanks); 83.080.01 (Plastics in general)

DOI: 10.1520/E2479-11

ASTM International is a member of CrossRef.

ASTM E2479