The influence of fungal decay on the properties of timber I—The effect of progressive decay by Polyporus hispidus , Fr., on the strength of English ash ( Fraxinus excelsior , L.)

Abstract

The wood of English ash ( Fraxinus excelsior , L.) is frequently found to break under a strain much below that which it can normally withstand, and a peculiar “brash” fracture is produced. This can, in some instances, be explained on anatomical grounds, for example, when the timber contains a large proportion of weak springwood. Cases have arisen, however, where ash wood, indistinguishable anatomically from material of good quality, is lacking in toughness and fractures suddenly under load in a “brash” manner. Nutman (1929) has found that in such timbers fungal hyphae are invariably present and suggests that Polyporus hispidus , Fr., which is one of the chief fungi attacking ash in this country, is mainly responsible for this “brashness”. Little detailed work has, however, been carried out on the effect of this fungus on the strength of ash wood. Baxter (1925), who made a limited number of tests on infected black ash ( Fraxinus nigra , Marsh), found that incipient decay lowered the compressive strength of the wood. The lack of further and more comprehensive data prompted the present investigation on English ash specimens infected with P. hispidus under controlled conditions. In particular, it was considered that the effect of this fungus on the characteristic mechanical properties of ash wood, namely, toughness and elasticity, should be examined. Previous work at this laboratory (Cartwright and others, 1931) has shown that reliable results can be obtained in the testing of small specimens of wood after exposure to fungal attack, and that the progressive loss in mechanical strength which occurs can be closely correlated with the advance of the attack as evinced by chemical changes and loss in dry weight. A log of English ash wood of good quality was sawn into baulks of approximately 4 inches by 3 inches. These baulks were marked off into lengths and divisions suitable for the preparation of small specimens for static bending, compression, and impact bending (toughness) tests. This allocation, which supplied specimens for fungal inoculation and for controls, was similar to that used in the previous investigation (Cartwright and others, 1931). A conversion diagram is shown in fig. 1. The short lengths were taken in pairs from each baulk, namely, 100-200, 300-400, etc., so as to ensure correct end- and side-matching of the control testpieces with those for inoculation. The specimens were carefully selected for straightness and evenness of grain and weighed before sterilization. The average moisture content of the test-pieces at conversion, obtained from control specimens, was 37·9%, based on oven-dry weight.