Water's Effect on the Strength of Wood by Kelsey Kress, Bryan Heinzelman, and Cody Corsetti SRJC Engineering 45 December 2009

Purpose To see the effects of moisture on the strength of the wood, by soaking four different types of wood for various amounts of time and then compressing them.

The Test All samples had the same starting dimensions of.75x3.5x5 in. The 4 types of wood used were: –Douglas Fir –Pine –Redwood –Red Oak

The Test The 4 samples included: –one that had been oven dried(the control) –one that had soaked for 2.5 hrs –one that had soaked for a day –one that had soaked for 10 days.

Douglas Fir Most widely used lumber in the framing of houses. –Doug Fir is softwood, the type of wood that is easy to work with. –It's the most plentiful softwood in North America –It is relatively inexpensive

Douglas Fir Douglas Fir has a superior strength-to- weight ratio –This ratio is the stress at failure relative to the density of the wood. Its specific gravity allows for great nail/plate holding ability.

Douglas Fir Douglas Fir has great dimensional stability. –This is the ability of the wood to retain its shape when subjected to various types of temperature, moisture, pressure and other stresses.

Pine Used in high-value carpentry such as flooring, paneling, trim, and furniture. –Finishes nicely with a coat of stain or varnish. –After milling, it has little insect or decay resistances; as a result, it is generally used indoors.

Redwood Used for siding, decking, trim, paneling, and many other applications where the finished product needs to be aesthetically pleasing. –Has a reddish-brown color that darkens with age. Redwood is lightweight and easy to cut and nail/screw into

Redwood Redwood resists shrinking, warping, splitting and insects, which allows it to be used in abusive environments. Its cell structure has thousands of air cavities, which gives it a superior insulating ability. Its lack of resin makes it partially resistant to fire.

Red Oak Used to make wine barrels, flooring, and cabinets; another type of wood that produces a nice-looking finished product It is close grained, heavy, and difficult to cut/penetrate. It is one of the best types of wood used in steam bending. –Red oak is a hardwood and, as such, it has many more pores and capillaries than does softwood. Red Oak was the most expensive of all our woods, coming in at $2.30 a foot.

Microscopic View of Each Wood Type Hardwood (e.g. Red Oak) Softwood (e.g. Doug Fir, Pine and Redwood)

Procedure We oven dried all 16 of our samples over night to ensure that all water content was removed.

Procedure We then weighed all of them to determine the mass before soaking.

The wet samples were soaked for logarithmic time periods (e.g. 2.4 hrs, 24 hrs, and 10 days).

Procedure We ran a compression test on the dry, control sample; increments of compression length were recorded at every thousand pounds of added force.

After soaking for the amount of specified time, we removed the wet samples from the water, recorded their mass after soaking, and then ran a compression test on them. –Again, compression length was recorded at every thousand pounds of added force.

Procedure We then plotted stress vs. strain curves for all four types of the samples to determine the modulus of elasticity, yield strength, and possible tensile strength of the respective woods.

% Water Content vs. Log of Time Soaked Water Content is (Mass H2O absorbed)/ (Final Mass)*100 Here is data: Dry: –D.F.0 –Pine 0 –Rdwd0 –Rdoak0 2.4 Hr: –D.F.12.09% –Pine26.35% –Rdwd2.69% –Rdoak13.70% 1 day: –D.F12.04% –Pine25.97% –Rdwd12.90% –Rdoak11.25% 10 day: –D.F.24.84% –Pine40.12% –Rdwd19.35% –Rdoak22.64%

% Water Content vs. Time Note: The 1-day- soak samples had to be re-dried and then re-soaked. This could have led to incorrect measurements, which cause the unexpected dip in the graph.

Results When testing the red oak, the tensile test machine was turning on and off. This is a possible reason for the inaccurate data points that caused the jump in the graph.

Results

Modulus of Elasticity in lbs/in 2 (Slope of Elastic Region) Dry2 Hrs1 Day10 Days Doug Fir Pine Red Oak Redwood Yield Strength in lbs/in 2 (Top of Elastic Region) Dry2 Hrs1 Day10 Days Doug Fir Pine Red Oak Redwood

Conclusion As soaking time increased, both the modulus of elasticity and yield strength generally declined as well.

References