Wood Industry Guide for Material Testing
Wood is a unique construction material because it is organic. For this reason there will always be a slight variability in performance from tree to tree. Deviations of about 10% are common. Different species of trees have different material strength levels depending on the exact criteria being tested.
Modulus of Elasticity (MOE) is a common measure of how flexible a piece of wood is before it ruptures. A common measurement of MOE would be between 1,000,000 to 1,500,00 psi.
The Modulus of Rupture (MOR) is the ultimate bending stress than can be achieved before a rupture occurs. A common value for MOR could be around 5,000 psi.
Introduction to ASTM Testing for Wood
ASTM D198 is the most widely used specification for testing wood. It covers Compression, Flexure, Tension, Torsion and Shear testing on full sized wood specimens. 2x4 timber boards are tested under this standard. Universal Grip offers a customizable wood testing package which utilizes a Galdabini universal testing machine. By changing the grips, different tests can be performed including bend testing.
The dimensions of a wooden plank are listed as length, width and depth. The length of a board is pretty straight forward. The width and the depth are often confused with each other. In the example of a 2x4” plank, the width is the 2” and the depth is the 4”. 2x4’s actually measure 1.5”x3.5”. The reason for the difference is because they are cut at 2” width and a 4” depth but then are sent through the drying and planing process. Once the boards are dried and planed they lose a little bit of size.
Testing the flexural strength of wooden beams requires a
large bend fixture. The span of the fixture must be 18 times
the depth of the board. With a standard 2x4” board this is
about 5¼ feet. Universal Grip recommends a 6 foot long bend
fixture to perform both MOE and MOR analysis. The width
of the supports should cover the entire depth (3.5”) so supports
with 4 inches of width are most common.
Wood has a moderately high tensile strength and requires
a fixture with high amount of clamping strength in order to
secure it. Wood is also somewhat soft, which poses challenges
for standard mechanical vise grips. The Universal Grip
Company offers a self-tightening Scissor Grip design which
is specifically designed for testing softer materials such as
timber. The scissor grip is spring loaded and the grip jaw faces
are covered with a serrated pattern for extra gripping force.
Wooden beams can be tested in compression. There are a few different tests, mostly dependent on the direction of the compression force in relation to the direction of the wood grain. Both fixed compression platens and spherically seated platens are available. Galdabini can supply a clear Lexan safety cage to protect from any wood splinters or abnormal ruptures.
Torsion Test Fixture for Wood Testing
Universal Grip has designed a unique fixture for converting a Universal Testing Machine into a torsion testing system. The fixture uses a gear system to convert the pulling motion of the crosshead into a rotational force. A Pair of high capacity vise grips are used to secure and twist the sample until failure. As the machine crosshead moves upwards, the series of gears twists the vise grips to create the torsional force. Please contact Universal Grip directly if you are interested in this solution.
The advanced material analysis of wood requires a high precision universal testing machine. Because of the way wood specimens behave during failure analysis, they should be tested at very slow speed rates. Wood has a tendency to crack and fracture all at once, so slow-rate testing is ideal. Galdabini offers an industry leading 0.0005 mm/min speed 100kN machine. Of course the speed can also be increased to 500 mm/min as well. This system is a benchtop testing machine and can also be configured for tensile and compression testing. All machines come standard with on-site installation and training.
Graphworks 6 is the flexible and easy to use software that powers every Galdabini testing machine. The software has many features and is highly configurable based on the exact test being run. Universal Grip can preload your machine with a standard library of test methods, or help customize a test based on your standards. Multi-phase tests and even slow-rate cyclic fatigue testing can be performed using Graphworks 6. Both Galdabini and Universal Grip are available for software and system support through a specialized version of the TeamView screen sharing program.