The ultrastructural characteristics of fracture surfaces from southern pine and Douglas-fir specimens tested in tension at various angles to grain were examined. The fracture surface morphology was inspected using scanning electron microscopy. Three anatomical failure types were recognized: intercell failure, transwall failure, and intrawall failure. Certain failure characteristics were ascribed as a function of the magnitudes of parallel-and perpendicular-tograin tension and parallel-to-grain shear present in the specimen. In specimens tested in parallel-and perpendicular-to-grain tension, the thick-walled latewood cells were found to fail in a combination of transwall and intrawall failure. The intrawall failures were usually at the $1-$2 interface. The more thin-walled earlywood cells were more likely to exhibit abrupt, transwall failures. At intermediate angles of load to grain, surfaces indicative of the type found in pure shear tests were predominant. Perpendicular-to-grain tension failures resulted in mostly intercell failures. Ray cells consistently exhibited transwall failures. The failure surface frequently changed planes in all loading modes. This path transfer was inevidably associated with material discontinuities in the wood. When the path did transfer, all three failure types were observed. No significant species effect was observed. IntroductionBasic to the understanding of the mechanical behavior of a material is the appreciation of the mechanical behavior of its individual units and their interaction. For wood, this is not a simple matter. The relationship between the structural organization of wood and its mechanical behavior is extremely complex. Under external stress, the response of wood is a function of material interaction on several levels of complicated inhomogeneous structure. IOu (1964) summarized it as follows:

Young-growth ponderosa pine on two different sites in northern New Mexico were evaluated for differences in fibril angle. Measurements were made radially at the butt end, mid-length, and top end of the merchantable stem divided into multiple 8-foot log lengths. Results indicated that the butt ends have larger fibril angles than the other two locations. Fibril angle did not vary significantly with either site index from 55 to 100 or d.b.h. from 9 to 14 inches. Keywords: Ponderosa pine, fibril angle, young growth, site index Management Implications Loss of lumber grade from excessive warp usually results from abnormal and/or asymmetrical shrinkage when lumber dries. In conifers, fibril angle, juvenile wood, compression wood, grain orientation, and knots have been associated with excessive lumber warpage. Longitudinal shrinkage typically ranges from 0.1% to 0.3% when wood is dried from green to ovendry conditions. Longitudinal shrinkage greater than 0.3% from green to overdry conditions for straight grain material is considered abnormal and is related to deviation of the microfibrils (fibril angle) from the longitudinal axis of the tracheid. Study results indicated that the fibril angle of young-growth ponderosa pine did not significantly vary with site index from 55 to 100 or d.b.h. from 9 to

Growing site index did not affect either the grade or the amount of warp. 2. Smaller trees and logs had lower grade recovery and more warp than larger trees and logs. 3. Log position in tree had little effect on grade recovery and warp when log diameter was accounted for. 4. Logs 8 inches or less in diameter, regardless of location in tree, had similar grade and warpage, and were affected only by moisture content. These research results should help the buyers and sellers to determine the stumpage value of timber. Height to 4-inch top d.i.b. (feet) 36.2 ±6.8 47.4 ± 10.9 Height to 6-inch top d.i.b. (feet) 30.5 ± 7.3 39.5 ±10.8 Volume, gross scale of logs (cubic feet) 10.8 ±5.5 13.6± 7.0 No. of 8-foot logs per tree 3.1 ±0.9 4.0 ± 1.2 Age at stump (years) 76 ±19 78 ±24 Lean (degrees) 2.2 ±1.9 2.2 ± 1.9 ''Each mean and standard deviation is based on a sample of 41 trees. U.S. Department of Agriculture Forest Service Rocky Mountain Forest and Range Experiment Station The Rocky Mountain Station is one of eight regional experiment stations, plus the Forest Products Laboratory and the Washington Office Staff, that make up the Forest Service research organization RESEARCH FOCUS Research programs at the Rocky Mountain Station are coordinated with area universities and with other institutions. Many studies are conducted on a cooperative basis to accelerate solutions to problems involving range, water, wildlife and fish habitat, human and community development, timber, recreation, protection, and multiresource evaluation.