Hardwood makes a great material. People like its colour, weight and its air of luxury, so I was shocked to learn that in the UK we burn 84% of the hardwood we harvest. With my research I hope to make it easier for people to use hardwoods, because it gives them more information about the wood properties of species like oak, ash, beech, birch and many more.
At the moment we don’t have a lot of information about the properties of UK hardwoods. Some research on the topic has been done in the 1930s and 40s at the Forest Products Research Laboratory (FPRL). But this was almost 100 years ago, and the methods we use for testing have changed since then. One important test is the three-point bending test in which we measure the strength and stiffness of small specimens, commonly to compare the properties of different species. But before 1949 the “small” specimens were not even that small – the bending test was done using the “2-inch” standard, which means specimens were 2in by 2in wide and 40 in long. (In the science world we tend to use SI units, so that’s 51 x 51 x 1016 mm.) Nowadays we usually test according to the “2-centimetre” standard, with much smaller specimens of 20 x 20 x 300 mm.
Of course there are differences in the strength of a specimen that is the size of your standard ruler and one that is the size of a yard stick. You can easily imagine that it takes more force to break a larger piece of wood. But shouldn’t make a difference, because we calculate strength properties depending on cross-section area. Strength, for example, is measured in N/mm2 – force per area. Still, there are probably differences between the two sizes, and we would actually expect the larger specimen to be weaker than the smaller one. This is because there is a higher chance of defects being present in the larger specimen. Even though we try to cut specimens in a way so they don’t contain large defects like knots or splits, there are probably irregularities in the wood structure that we cannot see, and there are probably more irregularities the larger the specimen is.
Of course, the FPRL researchers realised that there is a difference also in 1949 when the test standard changed. Back then they carried out an investigation to see which difference the size makes. Unfortunately, I could not find their original report, even though I went looking in the National Archives. I could only find a document that tells us the result: apparently the larger specimens only reach 95%of the strength of smaller specimens. You might think this result is all we need, but, as a researcher, I am sceptical of one simple number. Things are usually more complicated, especially with wood. Also, the research was done using mostly softwoods and none of our UK hardwoods at all, so of course I am wondering if the number even applies to hardwoods.
I wanted to see for myself and so I gathered specimens in the two different sizes of seven UK hardwood species. Unfortunately I haven’t actually managed to do the testing yet – the conference is still more than two months away and life as a PhD student is busy!I am expecting to see numbers that are close to the FPRL findings, but probably slightly different for different species.
Keywords: hardwood, mechanical properties, BS 373, size effect
Authors
Marlene Cramer
Edinburgh Napier University, UK
Luis Hidalgo Lomba
Edinburgh Napier University, UK
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