Durable Wood Product Evaluations

Studying the Accuracy of Small Specimens in Testing

Synopsis:

• In an effort to reduce the amount of time required to obtain meaningful durability data for wood products, it has become widely accepted to use smaller specimens in field tests.
• Smaller specimens generally yield faster results but tests comparing small specimens with matched larger lumber specimens showed that the relationship of the failure rates in the matched samples varies greatly.
• This research shows that more testing is needed before we can be confident that results from accelerated methods reflect the durability of commercial wood products.

Overview: The development of improved durable wood products involves years of testing to ensure long-term durability. The most rigorous and meaningful evaluations place test specimens (stakes) in soil contact in humid climates that may require many years to yield meaningful results. Recently researchers have turned to the use of smaller specimens in an attempt to accelerate these stake tests. It has become widely accepted that these smaller specimens provide meaningful results several times faster than the lumber specimens traditionally used in stake tests. Although the smaller specimens do generally fail more quickly than larger specimens, it is unclear how their durability relates to much larger commercial wood products. Forest Service and university scientists are beginning to provide answers to this question with research in field plots at Harrison Experimental Forest (Saucier, Mississippi). Scientists compared the durability of small specimens to that of matched larger lumber specimens for 64 treatment groups. This comparison revealed that although smaller specimens showed obvious evidence of decay 2.1 times sooner than the lumber specimens, this relationship varied greatly. In some cases the smaller specimens provided little or no acceleration. A similar trend was noted when time to failure was compared for the two specimen sizes. The results indicate that there is substantial uncertainty in the use of small specimens to predict the durability of larger specimens or commercial wood products. Further research is underway to develop models to better define the relationship between accelerated stake test results and the durability of commercial products.

• A Comparison of the Durability of Two Stake Sizes in Southern Mississippi
• Challenges in accelerated testing of durable wood products
• Observations on the Predictive Value of Short-term Tests
• The Need for Performance Criteria in Evaluating the Durability of Wood Products