Critical Study of the Bursting Strength Test for Paper

An extensive investigation has been made of alleged variables in the bursting test. The studjincluded clamping pressure, nature of clamping surfaces, sizo of bursting orifice, types of diaphragms, effect of manner of installation of diaphragms, effect of air entrapped under diaphragm, speed of operation, effect of viscosity of liquid medium in hydraulic chamber, calibration and performance of pressure gauges, relation of bursting to tensile results, and relation of size of bursting orifice to bursting pressure. A device is described for measuring the vertical deflection of the paper in the bursting test. As a result of the investigation it is recommended that a clamping pressure be used corresponding to a total load on the specimen of approximately 1,000 pounds. An all-metal clamping head having a relatively smooth clamping surface and a bursting orifice 1.20 inches in diameter is recommended to replace the rubber-faced clamping head formerly used. No evidence was found that the type of diaphragms ordinarily used, the manner of installation, or the viscosity of the hydraulic medium used have a significant effect upon the test. The results are affected to some extent by the testing rate, being somewhat greater the higher the testing rate. Air in the hydraulic chamber, because of its compressibility, has the effect of reducing the testing rate. The testing rate may conveniently be expressed in terms of the time interval during which the specimen is being stressed. For the official method this corresponds to about 1 second. Because of the difficulty of properly adjusting the pressure gauges of the type commonly used on the bursting tester for operation at high speed, the official testing rate is about the maximum that could safely be used. The equation PR= 2T, in which P is the bursting pressure, R the radius of curvature of the paper at rupture in the bursting test, and Tthe machine direction tensile breaking strength per unit width, relates the bursting and tensile properties and enables a correlation to be made of data obtained with bursting orifices of different sizes.