Impact Force-Deflection Diagrams for the FPL Toughness Test

A method is given for producing force-deflection diagrams of: specimens tested in the FPL Toughness Testing Machine. The record is produced on an oscilloscope from the output of force and deflection transducers. The FPL Toughness Testing Machine is a pendulum-type impact tester which yields a single value of total absorbed energy per specimen. Such limited information gives little insight into the relative behavior of wood under impact as compared to static loading. Therefore, in this study the standard FPL machine was modified by adding electric stain gages and a linear potentiometer to measure force and deflection respectively. The design of these components is described in detail. Tests were made using 30 ASTM standard toughness specimens of American beech (Fagus grandifolia Ehrh). Each specimen was 0.79 inch by 0.79 inch by 11.0 inches. Specimens were made and tested in the green condition for Young’s Modulus only by three means–dynamically by Kitazawa’s vibrational technique, a modified static bending test, and a destructive toughness test. Results indicated that the fundamental method of using electrical transducers and an oscilloscope for obtaining force-deflection diagrams of an impact test is correct. However, refinements are needed to obtain accurate curves. The most serious source of error is the deflection-measuring system. Effects of the chain that applies the load are another problem since proper testing requires either constant rates of loading or deflection. One effect of the chain is to superimpose an oscillating load upon the constantly increasing load produced by the pendulum, causing variations in the deflection rate. Another effect stems from the energy stored in the loaded chain. In the case of a brash specimen which fails suddenly and completely, the chain will do little work in breaking the specimen, but the machine will indicate that this energy has been used to break the specimen. Thus, it is doubtful that the machine can present a true and consistent picture of the work to rupture, which is the purpose of the toughness test.

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