J.M. Cabrero

Assessment of phenomenological failure criteria for wood

Many different phenomenological failure criteria have been proposed. These type of criteria do not explain the mechanism of failure itself. They merely identify failure (yes or no) and are usually regarded in practice as a simple and reliable tool for design. Most of them were developed for composite materials, but are extensively applied for wood. In this study, existing phenomenological strength criteria for orthotropic materials were applied to clear wood data. Instead of fitting the criteria to available experimental data, the criteria were used to predict failure of biaxial tests based on uniaxial strength. This procedure is closer to practice, and hence an answer to the question “is there any reliable failure criterion for wood?” Predictability of the criteria was assessed using normalisation procedure, and statistical significance of the difference of the means analysed. Based on the results, it can be concluded that a general criterion cannot be applied to predict failure, but should be chosen according to the biaxial stress state.ZusammenfassungIn der Literatur werden viele verschiedene phänomenologische Versagenskriterien beschrieben, die jedoch nicht den Versagensmechanismus selber erklären, sondern nur den Bruch identifizieren (ja oder nein). In der Praxis werden diese Kriterien in der Regel als einfaches und zuverlässiges Werkzeug für die Bemessung angesehen. Die meisten der Kriterien wurden für Verbundwerkstoffe entwickelt, werden aber zunehmend auf Holz angewandt. In dieser Studie werden bestehende phänomenologische Versagenskriterien für orthotrope Werkstoffe auf fehlerfreies Holz angewendet. Das Modell wurde nicht an die vorhandenen Versuchswerte angepasst, sondern die Kriterien wurden dazu verwendet, um den Bruch von zweiaxialen Versuchen mittels einaxialen Festigkeitswerten zu bestimmen. Dieses Verfahren ist praxisnäher und liefert somit eine Antwort auf die Frage, „Gibt es ein zuverlässiges Versagenskriterium für Holz?“ Die Vorhersagbarkeit der Modelle wurde anhand normalisierter Abweichungen von den Versuchswerten und statistischer Signifikanz analysiert. Aus den Ergebnissen kann geschlossen werden, dass es für Holz kein allgemeines Versagenskriterium gibt, sondern nach dem Spannungszustand gewählt werden sollte.

Evaluation of Failure Criteria in Wood Members

Multi-axial stress conditions exist in almost all engineering applications, but design procedures, in most cases, assume uniaxial stress conditions. This assumption is not adequate for orthotropic materials such as timber. Initial failure mode in lumber is usually tension perpendicular to the grain or axial tension around natural defects such as knots. Strength properties of lumber have been obtained from clear-wood specimens and in-grade testing of full-scale lumber whereas failure criteria are usually those originally developed for composite materials. Some of the existing failure criteria are relatively easy to use, but prove valid only under special orthotropic conditions. Norris and Tsai-Hill criteria assume the tension and compression strengths to be equal, which is not the case for wood. Other criteria are usually based on quadric surfaces, in which certain constraints are taken strictly from geometrical considerations to achieve a closed envelope. Since these conditions are not defined from a physical stand point, some parts of the resulting curve leads to errors in failure predictions. These quadric criteria also require evaluation of interaction coefficients for bi-axial stress conditions. Evaluation of these interaction coefficients demands extensive experimental testing, which limits their practical application. Some proposals have been made to deal with the interaction approach either by evaluating the interaction factor in terms of uniaxial strength or by proposing a given value in the absence of experimental data. The proposed paper will review the existing failure criteria and the applicable theories of fracture mechanics. Their relevance to timber structural members will be discussed and validated.

Selected mechanical properties of four Central American wood species

In this paper, mechanical properties of the four Central American wood species Amapola (Pseudobombax ellipticum), Manchiche (Lonchocarpus castilloi), Pucte (Bucida buceras) and Santamaria (Calophyllum brasiliense) determined in a preliminary experimental study are presented.

Failure of timber constructions

Abstract Timber is becoming an increasingly important construction material all over the world. However, there is usually a lack of knowledge regarding its usage. This chapter provides background information on the failure of timber structures, and especially of timber connections, which are crucial to structural timber design. Wood is a complicated material from the mechanical point of view. Its heterogeneity, fiber direction, size dependence, and rheological properties are key concepts required to understand its failure behavior. There is a lack of knowledge on how to understand and assess timber failure. This chapter analyzes timber failure from two different perspectives: the global scale of the failure of buildings as a system, and in particular regarding the failure of the material and the structural connections. After introducing the key concepts of timber mechanics, the chapter explains the failure of structural timber connections. Last, the most common failures in timber structures are discussed. To that means, different recent failures of timber buildings are explained and analyzed.