Moisture-induced Cracking Research Articles

Moisture-induced cracking in a flexural bilayer with application to historical paintings

Crack channelling in a brittle, flexural bilayer, composed of a coating adhering to a substrate of finite thickness, is addressed for the case of a uniform moisture content (or temperature) variation. The linear stress profile within the coating may lead to three different fracture mechanisms, which are a channelling crack with delamination absent, a channelling crack with delamination of finite length, and a channelling crack with delamination of unbounded extent. Failure mechanism maps illustrate the dependence of the active crack channelling mechanism, and the corresponding critical crack channelling stress, upon the stiffness mismatch between the layers and upon the ratio of interfacial toughness to coating toughness. Although the results are applicable to bilayers in a wide range of applications, the study focuses on the prediction of crack channelling in historical paintings. The significance of bending of the bilayer upon crack channelling is explored by comparing the fracture characteristics of the simply-supported, flexural bilayer with those of a rigidly supported bilayer that is constrained against bending. These two bilayer systems are representative of the different framing techniques commonly used for historical paintings, with the simply-supported bilayer reflecting a regular wooden panel that can bend freely, and the rigidly supported bilayer characterizing a wooden panel that does not bend as a result of cradle additions composed of several horizontal members and corresponding orthogonal cross-pieces. Independent of the type of framing technique applied, it is shown that crack channelling with delamination can be avoided in historical paintings when the delamination toughness exceeds the mode I toughness of the paint layer. Under these conditions paint flaking does not occur and the visual appearance of the painting is preserved. The failure maps constructed in this study provide a useful tool for museum conservators to identify acceptable indoor humidity and temperature variations such that historical paintings do not degrade by a combination of crack channelling and delamination.

A study of moisture-induced cracking during a short-term rain event in a pre-cracked asphalt concrete pavement with an expansive base layer

ABSTRACT A model is developed herein for predicting moisture-induced crack growth during a short-term rain event, considered herein to be of duration not greater than 2 hr, in a pre-cracked asphalt concrete pavement built on an expansive base layer. The model is in two parts, the first being the prediction of moisture transport into the pavement, and this part is accompanied by an experimental component focused on measuring the diffusivity of moisture in asphalt concrete. The second part of the model deals with the prediction of the effect of moisture transport on the subsequent mechanical behaviour of the pavement, with emphasis placed on the initiation and propagation of an additional surface crack due to expansion of the base layer. Both components of the model are developed computationally via the use of an in-house one-way coupled finite element code, and a description of this algorithm is briefly reviewed herein. The model developed suggests that the pre-existing crack must extend completely through the asphalt concrete layer into the highly permeable and expansive base layer for there to be significant moisture transport during a short-term rain event. In the case wherein the initial crack extends into the base layer, the stress concentration induced due to dilatation of the base layer upon moisture absorption is predicted to cause additional cracking rapidly.

Influence of sorption hysteresis on moisture transport in wood

The independent domain theory is used to analyze the influence of sorption hysteresis on the behavior of wood exposed to environmental conditions. Due to hysteresis, sorption history of wood has an impact on its performance under varying moisture conditions. An integrated Preisach–Mayergoyz (IPM) approach that is phenomenologically sound and insightful while mathematically and computationally straightforward to implement is applied, where the parameters of the IPM space are determined from experiments. The implementation of the IPM approach in a heat and moisture transport model is validated based on independent measurements of dynamic vapor sorption in spruce wood. The transport model, including sorption hysteresis, is further used to simulate the response of a wood beam to weekly humidity variations and hourly climatic humidity and temperature variations. The analysis shows that hysteresis of wood is strongly dependent on the magnitude of moisture changes. As moisture penetrates the material faster in longitudinal than in radial direction, due to a lower vapor resistance factor, the effect of hysteresis is more pronounced deeper into the wood in longitudinal than transversal directions. Furthermore, the simulations imply that errors in moisture content of more than 20 % (or 30 %) can be made when using the main adsorption curve (or main desorption curve) instead of the hysteresis model for wood sorption behavior. Sorption hysteresis must thus be accounted for in heat and mass transport models to assess moisture-related damage risks such as mold growth, rot or moisture-induced cracking, and the IPM approach offers such an appropriate avenue.

A hybrid interface-element for the simulation of moisture-induced cracks in wood

Moisture-induced cracking of wood is a well-known phenomenon. Nevertheless, until now, no numerical approaches are at hand to appropriately capture this feature. Hence, in the framework of this paper, a novel multiphysical hybrid interface-element and a respective interface material formulation are derived in order to be able to coincidentally simulate anisotropic moisture transfer through and along the crack plane and the mechanical response. Hygro-mechanical coupling is considered by means of moisture-dependent input quantities and swelling/shrinkage of the adjacent bulk material. The hybrid element is applied for a comprehensive numerical parameter study on the influence of wood species, material direction and initial moisture content on moisture-induced cracking.

On the physics of moisture-induced cracking in metal-glass (copper-silica) interfaces

Environmentally dependent subcritical crack growth, or stress-corrosion cracking, along ceramic-metal interfaces is studied for the silica glass-copper system. Tests were conducted in various gaseous and liquid environments in order to determine their relative effects on stress-corrosion cracking and to gain some insight into the mechanisms that control interfacial crack growth. In agreement with previous studies, interfacial crack-growth rates were found to vary by orders of magnitude depending on the moisture content in gaseous environments. Water and several organic liquids, namely n-butanol, methanol, and N-methylformamide, were also found to promote stress-corrosion cracking. Specifically, crack-growth behavior was found to be largely dependent on the molecular structure of the test environment. Crack growth at high velocities was limited by either transport of the reactive species to the crack tip or by viscous drag contributions. Results are discussed in the context of the current mechanistic models proposed for the stress corrosion of bulk silica.

Moisture sensitivity and reliability of plastic thermally enhanced QFP packages

Standard plastic quad flat packs have difficulty meeting the thermal performance requirements of devices with high power dissipation. The development of plastic thermally enhanced quad flat packs (TEQFP), which incorporate heat slugs or heat spreaders in the leadframes, provide a cost effective solution. Because the final packages are of molded plastic, TEQFP's are susceptible to moisture-induced cracking and other plastic reliability problems. This paper discusses the impact of moisture on package cracking during the board mounting process and the overall reliability of TEQFP's. Based on our study using scanning acoustic tomography and reliability stress tests, our findings reveal that TEQFP's have comparable reliability to standard PQFP's if proper storage and drying guidelines are followed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>