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Zero-span strength

Introduction

The zero-span test is where paper tensile strength is tested with the jaws initially touching.  The test is usually done with specially designed testers manufactured by Pulmac instruments.  A short span test is done with a small separation between the jaws. Pulmac zero-span testers measure the pressure that is applied to force the jaws apart which is converted into load on the sample.  The instruments only give the breaking strength. The commercial tests are load controlled.  The load in the sample is increased at an approximately constant rate until the sample fails.

Displacement

Every zero-span test produces a displacement between the jaws, which increases with the load.  This was first seen by Cowan (1).  He suggested the jaw movement came from slippage when the load was transferred into the sample by friction.  This was later disputed (2).

Most of my work has been to develop a technique to obtain more information from the zero-span test by measuring displacement. This work was done in collaboration with SCA Graphic Research and Mid-Sweden University in Sundsvall.  This was done by instrumenting a zero-span tester to measure displacement and load during each test. 

Subtraction technique

The subtraction technique (3) developed in collaboration with SCA  was to average 24 load-displacement measurements at zero-span and at a short span. Typically a short span of 400 microns was chosen. The short-span displacement comes from the strain in the free span and the displacement under the jaws.  The subtraction technique works by subtracting the short span displacement from the zero-span displacement.  Given that the free span is known this subtracted displacement can be converted to strain.  In June 2006 I gave a review paper on the zero-span test at the Euromech Colloquium in Sundsvall (4).  The strain and stress at break measured by subtraction were shown to be comparable with single fibre tests, but the elastic modulus is too low.

References

My complete publications in zero-span testing.

  1. Cowan, W.F., - Short span tensile analysis, Pulmac Instruments, Montreal, Canada (1975).
  2. Hagglund, R., Gradin, P.A., and Tarakameh, D., - Some aspects on the zero-span tensile test, Exp. Mech., 44(4):365 (2004).
  3. Batchelor, W.J. and Westerlind, B.S., - Measurement of short span stress-strain curves of paper, Nordic Pulp & Paper Research Journal, 18(1):44 (2003).
  4. Batchelor, W., - The zero-span test-what are we measuring?, Euromech Colloquium 486 Deformation and Fracture Processes in Paper and Wood Materials,p (2006).