Study of the polymer material fracture process by the acoustic emission method

Abstract

The studies of the mechanics of the damage of the polymer materials are now extending beyond the question of the load-carrying capability of the articles, constructions, and structures. The control of the damage processes and the knowledge of their relationships are of tremendous importance for engineering and technology in the various industrial fields. For example, the service life of the constructions and structures can be increased by slowing the growth of the cracks, while in the machining of the materials, conversely, it is often advisable to reduce the strength of the material that is being worked. The studies of Kudinov, Poduraev, and Kabaldin [1‐ 3] have shown that the damage of the material is a necessary part of the cutting process. The commonality of the materials damage and cutting processes has been established, specifically the realization of both processes either by the development of the elastoplastic deformation to the critical state or by the formation and growth of the cracks, i.e., the realization of brittle fracture in the microvolumes of the material. Thus, we can represent the working of the various materials, and the polymers in particular, by cutting as a version of a controllable damage process that is accompanied by the dynamic action of the tool cutting edges on the workpiece surface. Considering the foregoing and the fact that the polymer materials have an ensemble of specific properties that is different from the behavior of the metals and alloys in mechanical working by cutting, it is important to study the polymer materials damage mechanism with the use of the modern methods. The study of the kinetics of damage accumulation in the deformation and fracture process has considerable theoretical and practical importance: this study will make it possible to establish the mechanism of the damage of the material in each stage of the deformation process, and determine the influence of the various damage conditions. It is known [4, 5] that tension is the most dangerous form of the stress state of the polymer materials. The tests of the materials are most often performed in tension, since this type of deformation can be realized in nearly the pure form, in contrast with the compression, shearing, and torsional deformations. Therefore the mechanical characteristics that are determined in tension are the primary basic data for the calculations of the strength of the parts and structures that are made of the polymer materials. At the present time the acoustic emission method, based on the analysis of the parameters of the elastic acoustic emission (AE) waves, is used effectively to study the deformation and failure mechanisms of the solid bodies [6]. This method makes it possible to gather a considerable amount of information on the physical processes taking place in the structure of the material, and to evaluate quantitatively the degree of the damage accumulation and the mechanism of the structural changes. The parameters of the AE method that are most informative and are traditionally used include the following: the peak amplitude of the AE signals, the overall signal count, the count rate, and the overall energy of the signals. The peak amplitude of the signals is the energetic index of the level of the damage of the material. The amplitude distribution of the AE pulses can be used to evaluate the type of deformation in the specific loading conditions. The overall signal count is the information on the integral level of the damage of the object in the course of some external action, including the increase of the length of the defect and the change of its area. The signal count rate (the intensity of the pulses) reflects the rate of the development of the damage of the material at the given moment of time. The overall energy of the AE signals is the area below the envelope of the electric signal. It is generally proportional to the dimensions of the cracks that form under load. The studies were performed on the standard flat specimens with the use of the ALA TOO IMASH 20‐ 75 universal testing system as the loading device. The specimens for the tests of the polymer materials were fabricated in the form of double-end blades with 3 × 2 mm rectangular section in the working zone. The loading curve was recorded with the use of the tensile testing system recorder. Along with the loading curve, the AE signals were recorded continuously with the use