Static bending

The first reported study of a reaction of wood with an epoxide appears to be that of McMillan (1963). This involved the use of gaseous ethylene oxide (Figure 4.9, R=H) at a temperature of 93 °C and a pressure of 3 atmospheres (0.3 MPa). In some cases, the wood was diffusion pre-treated with trimethylamine vapour as the catalyst. A 65 % ASE at 20 % WPG was obtained, attributed to a bulking effect due to in situ polymerization of the epoxide. There was no effect on the static bending strength of samples, and the modified wood became distinctly brown at higher levels of treatment. 

Microcantilevers can be coated with DNA or antibodies to respond to biological molecules or even a single virus.1-2-3 Bound material can be detected by the change in resonant frequency, as above, or by measuring nanometer-scale static bending, shown at the left, caused by stress on the surface of the cantilever when molecules bind. 

Fig. 4.22 Schematic of cantilever operating in (a) resonant and (b) static bending mode, (c) Photograph of microfabricated cantilever array (adapted from Battiston et al., 2001, p. 122)...

Static Bending Untreated MC -7.2% Treated (Based on untreated wood) Untreated Treated... 

Limited tests on strength properties of wood acetylated with thioacetic acid showed that MOR in static bending, impact strength, maximum crushing strength in compression and density improved. It is, therefore, evident that thioacetic acid has either no adverse effect on wood components and their interlinks or the improvements affected by modification far exceed these effects, so as to be of little consequence and ending in net gains. 

There have been several studies on the changes in mechanical properties of fiberboards made from chemically modified wood fiber. Hardboards made from control and acetylated hemlock fiber using 7% phenyl-formaldehyde adhesive were tested. In static bending, MOR was reduced by 23% and MOE reduced by 16% in acetylated boards as compared to control boards [38]. Tensile strength parallel to the surface was reduced by 5% but there was no... 

Fiberboards made from control and acetylated aspen fiber were made using 8% phenol-formaldehyde resin and tested in static bending. MOR increased by 15% and MOE increased by 40% in acetylated fiberboards compared to controls [39]. The acetylated boards were reported to have a more uniform density and a more consolidated surface as compared to controls. 

Strength in static bending, MPa Charpy impact ductility, kJm Rockwell hardness, scale M Shrinkage, cm/cm Water absorption, %... 

The value of the coefficient r is about two for the modulus of elasticity, and four for the modulus of rupture in static bending. The maximum value of about six applies to compressive strength parallel to the grain (42). 

Most of the mechanical properties of propylene oxide-modified wood are reduced (116). The modulus of elasticity is reduced 14%, modulus of rupture is reduced 17%, fiber stress at proportional limit is reduced 9%, and maximum crushing strength is reduced 10% (116). Ethylene oxide-modified wood showed no reduction in static bending tests (107). 

Strength properties are time dependent. The load that timber can sustain without failure decreases with time. If the short-term ultimate load in the 5-minute static bending test is taken as the reference point, then wood will fail, on average, at about 66% of that load after 1 year, at 62% after 10 years and at 56% after about 27 to 200 years depending on the curve used to fit the data there is a dearth of long term experimental evidence (>10 yr). 

Table V. Compression and Static Bending Strengths of Buried Wood...

Dynamic tests were conducted on bend specimens using an instrumented drop-weight test machine. The specimens were prepared and the results evaluated, as recommended in ASTM E399 for static bend specimens. The Ko (dynamic fracture toughness) values were calculated from the maximum load on the instrumented drop weight tup. A soft steel half-round pad was used between the specimen and the tup, so that load rise times were 1 to 2 msec and inertial effects were not significant. 

The flexural properties characterize the strength, deformation and stiffness behaviour of plastics under quasi-static bend loading. For these tests commercial universal test systems equipped with three-point and four-point fixtures are used. The most important standard for the bend test of plastics is the ISO 178 Plastics - Determination of flexural properties, which favors the three point bend test (Fig. 4.39). The data collected include also values determined according DIN 53452 and DIN 53457 as well as ASTM D 790. The specimen of preference exhibits dimensions of 80 x 10 x 4 mm and can be produced directly by injection moulding or cutting the shoulders from multipurpose specimens. 

Figure 2.17 Effect of OP-10 (1—3) and MDI (4, 5) content in cured PN 609-2IM resin on temperature of the maximum tan 6 for the dipole-grouped relaxation process at failure stress (1, 5), on the failure stress <7 at static bending of the resin (2,4), and on the optical density /3o of the resin (3).

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