Test MethodAnalysis of Mullins effect in polyethylene using ultrasonic guided waves

AbstarctSmall strain deformations below yielding can cause plastic deformation in semicrystalline polymers by a process similar to what is described for filled rubber-like materials known as the Mullins effect. Inter-lamellae chains contribute predominantly in deformations at this level, and the residual plastic strain can be attributed to permanent damage to the tie chains, affecting the long-term mechanical resistance of a molded part. With little detectable alteration of the polymer crystallinity at this early onset of plastic deformation, the primary characterization method applied to date is cyclic tensile loading, which provides information of stress-softening by monitoring the unloading path or relaxed stress behavior. An alternative method for monitoring the development of Mullins effect is proposed that can examine a molded part by using selected modes based on ultrasonic guided waves analysis. The technique was examined to determine if it could follow this effect induced by cyclic strain-controlled tensile deformations since it does not require sample preparation and could ultimately be applied while a part was in-service. Results for different polyethylene grades agree in trend with relaxed stress values over four cycles for tests of increasing applied tensile strain, demonstrated by an increase in the attenuation of ultrasonic guided waves. The correlation reveals a good promise in applying this method to structural health monitoring of plastic parts, while in use, to follow the initiation and progress of early service damage.