Method of Tension Stress Testing of Rubber

Abstract

In a method of tension stress testing of rubber of the type wherein a rubber sample is elongated continuously at a predetermined speed and the tension stress is measured when the distance between a pair of reference positions set on the surface of the sample shows a predetermined elongation, prior to the actual test, a plurality of rubber samples having different hardness are elongated continuously under predetermined tension test conditions, the tension stress measuring time between the time of commencing the continuous elongation and a time at which the distance between the reference positions is measured, and by utilizing the data thus obtained and the data regarding the hardness of respective rubber samples, the measuring time function of the tension stress measuring time regarding the hardness is determined by a statistical method. In the actual test, the hardness of a rubber sample is measured, the tension stress measuring time is determined from the measured value of the hardness in accordance with the measuring time function, then the rubber sample is elongated continuously under said tension test conditions, and the stress is measured at the tension stress measuring time to determine the tension stress. SU This invention relates to a method of tension stress testing of rubber and more particularly to an improved method of determining the tension stress of rubber by tension test. Usually the tension stress of rubber is determined by selecting a pair of reference positions, usually established by marking a pair of reference lines on the surface of a rubber sample shaped to have a definite configuration, for example a dumb-bell shape, continuously elongating the sample at a constant or regularly varying speed and measuring the stress (the stress per unit sectional area of the rubber sample before elongation) when the distance (hereinafter termed a "reference distance" and where reference lines are used a "distance between reference lines") shows a predetermined elongation. Various methods of testing for determining the tension stress of rubber have been proposed and there are many regulations such as Japanese Industry Standard (JIS), American Standard of Testing Materials (ASTM), British Standard (BS), Deutsche Industrie-Norm (DIN) and so forth. In the tension stress testing of rubber, usually a rubber sample marked with reference lines is mounted on a stress-electro conversion type tension tester for continuous elongation. Where a pendulum type tension tester is used an operator who watches the reference lines directly reads by himself the value of the stress which varies from time to time or causes a cooperator to read. In the former case, the reading is difficult and the accuracy is low whereas in the latter case two operators are necessary. Alternatively, a recording meter interlocked with the continuous elongation is provided in which the operator is requested to apply a signal to the recorder at a measuring point. Where a stress-electro converting type elongation tester is used, it is usual to employ a system that requests the operator to apply a signal to the recording meter. In any one of these systems it is necessary for the operator to follow up the movement of the reference line by holding a scale for measuing the distance between the reference lines, and the accuracy of measurement greatly depends upon the response speed of the operator. For this reason, the accuracy of measurement differs greatly by the skill of the operator and measurement over a long time causes the operator to tire. Although a number of automatic apparatus for performing tension test of rubber have also been proposed, all of them relate to automatic following up of the reference position on a rubber sample. In other words, how to eliminate the follow up scale has been the important problem. Most of the automatic follow up devices presently available on the market can be classified into a rubber sample contacting type and a rubber sample noncontacting type. In a typical example of the former, a movable element is fixed at a reference point so as to detect the increase in the distance between reference lines, whereas in a typical example of the latter type a light beam is projected upon a reference line and the reflected light beam is detected by a photoelectric detector or a permanent magnet is mounted on a reference position and the flux of the magnet is detected by a detection coil. However, these automatic follow up devices have some disadvantages as follows. Thus, where a reference line is used, as the rubber sample elongates and deforms the width of the reference line increases proportionally so that it becomes difficult or impossible to detect it as a line. Where a detection element is secured to a reference position, the element is displaced due to the deformation of the rubber sample. In the contacting type is is necessary to mount the detection element at a correct reference position before starting of the test, and moreover since the detecting element is in contact with the sample concentrated stress may affect the elongation of the sample. In the noncontacting type the accuracy is generally low although the accuracy is determined by the characteristics of the follow up detecting element. Furthermore, as there is a limit for the response speed of the follow up driving device this type is not suitable for high speed tension test. Further, the non-contacting type automatic follow up devices generally utilize complicated mechanism and require troublesome operations. As above described, the efforts for automation have been limited to the development of mechanical means that can be replaced for the operators action for following up the movement of the reference line by watching the same so that the type of automation as well as the accuracy thereof have been limited. During the course of our investigation regarding the development of new automatic method of tension test of rubber we have endeavored to find out a unique relationship between various physical characteristics of rubber and the elongation thereof and found a remarkable fact that "there is a definite relationship between the hardness of rubber and a time required for the distance between reference lines reaches a predetermined value due to elongation". The invention is based on unique utilization of this fact. Accordingly, it is an object of this invention to provide a novel method of testing tension of rubber capable of obviating various defects of the prior art method. According to this invention there is provided a method of tension stress testing of rubber of the type wherein a rubber sample having a predetermined shape is elongated continuously at a predetermined speed and the tension stress is measured when the distance between a pair of reference positions set on the surface of the sample shows a predetermined elongation, wherein (1) prior to the actual test, a plurality of rubber samples having said predetermined shape and different hardness are elongated continuously under predetermined tension test conditions, the tension stress measuring time between the time of commencing the continuous elongation and a time at which the distance between the reference positions is measured, and by utilizing the data thus obtained and the data regarding the hardness of respective rubber samples, the measuring time function of the tension stress measuring time regarding the hardness is determined by a statistical method, and (2) in the actual test, the hardness of a rubber sample is measured, the tension stress measuring time is determined from the measured value of the hardness in accordance with the measuring time function, then the rubber sample is elongated continuously under said tension test conditions, and the stress is measured at the tension stress measuring time to determine the tension stress.

Patent Drawings (3 sheets)

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