Summary
Stress relaxation tests were performed with six tropical American species. Stress relaxation was not found to be a linear function of strain at any level of strain. At qual low levels of strain, stress relaxation in compression was much greater than in tension.
A mechanical model consisting of an isolated spring in parallel with a spring and dashpot in series was used as an aid in the derivation of equations describing stress relaxation.
An attempt to apply Newtonian viscous theory to the model was unsuccessful in accounting for rate of relaxation. However, when the hyperbolic sine law of viscous flow was applied, mathematically derived curves fitted the data very well.
Stress relaxation appears to be related to “departure strain” which may be obtained readily from static stress strain diagrams.
Zusammenfassung
Versuche über die Spannungsrelaxation wurden mit sechs tropischen Holzarten Amerikas durchgeführt. Es erwies sich, daß die Spannungsrelaxation nicht in jedem Bereich der Dehnung eine lineare Funktion dieser Dehnung ist. In vergleichbar niedrigen Dehnungsbereichen zeigte sich zum Beispiel, daß die Spannungsrelaxation bei Druck größer ist als bei Zugbeanspruchung. Mit Hilfe eines mechanischen Modells, bestehend aus einer einzelnen Feder in Parallelschaltung zu einer Feder mit Dämpfungselement wurden Gleichungen zur Beschreibung der Spannungsrelaxation abgeleitet.
Der Versuch die Newtonschen Viskositätsgesetze auf dieses Modell anzuwenden, schlug aufgrund der Relaxationsgeschwíndigkeit fehl. Bei Anwendung des hyperbolischen Sinussatzes für viskoses Fließen stimmten jedoch die ermittelten Werte recht gut mit den mathematisch berechneten Kurvenwerten überein.
Die Spannungsrelaxation scheint mit der sogenannten “Anfangsdehnung” zusammenzuhängen, wie man sie stets bei statischen Spannungs-Dehnungsschaubildern erhält.
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A condensation of a dissertation submitted to the faculty of the Yale School of Forestry as partial fulfillment of the requirements of the D. For. degree.
This research is part of a comprehensive study being conducted at the Yale School of Forestry in cooperation with the Office of Naval Research, Department of the Navy, under Contract Nonr 609 (13), Project NR 330-001, Properties of Tropical Woods. The author acknowledges the fellowships granted by the Organization of American States, and the Instituto Nacional de la Investigación Científica de México. The author wishes to thank Professor Frederick F. Wangaard for his counsel and assistance, and Professors Robert M. Kellogg and Robert P. Vreeland for encouragement and assistance.
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Echenique-Manrique, R. Stress relaxation of wood at several levels of strain. Wood Science and Technology 3, 49–72 (1969). https://doi.org/10.1007/BF00349984
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DOI: https://doi.org/10.1007/BF00349984