Ultrastructural analysis of softwood fracture surfaces

Abstract

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:

Data reduction scheme for measuring G IIc of wood in end-notched flexure (ENF) tests

“…In fact, from the current and initial compliance, the flexural modulus and equivalent crack length were calculated wEqs. The mode I fracture surface created ahead of the cutting blade exhibits the characteristic appearance of a fracture in perpendicular-to-grain tension through the middle lamella of tracheids (Zink et al 1994). Considerable scatter in initial specimen stiffness can be observed in Figure 8a, which reveals high modulus variability between different specimens.…”

Fracturas en madera de Eucalyptus nitens: efecto de las propiedades mecánicas a nivel ultraestructural y de la anatomía celular

“…Estas fracturas o microgrietas han sido clasi cadas en tres tipos: Fracturas que ocurren entre las células a nivel de la lamela media (LM) separando las células adyacentes; fracturas en la pared celular o dentro de la pared secundaria, generalmente en la interfase S1/S2 y fracturas en la pared celular o ruptura completa de la pared, incluyendo todas las capas (Putoczki et al 2007, Smith et al 2003, Zink et al 1994, Côte y Hanna 1983. Putoczki et al (2007) mencionan que las fracturas se producen por la diferencia que existe entre los módulos de elasticidad, o por diferencias en la resistencia de los componentes de la pared celular.…”

Changes in Toughness and Fracture Characteristics of Wood Attacked by the Blue Stain FungusLasiodiplodia theobromae

“…12, double headed arrow). Transwall failure in latewood and intrawall failure normally at the S,-S 2 interface in thin early wood cells are common features in tensile fractured sound wood (Zink et al 1994). This type of tension forces are also created during impact in toughness; features which were commonly observed in the present study.…”