טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGil Steinbrecher
SubjectHygromechanical Coupling and Failure of Laminated
Composite Materials
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Altus Eli
Full Thesis textFull thesis text - English Version


Abstract

The use of carbon-fiber reinforced polymers (CFRP) is well established in demanding environments, especially where high strength and low weight are needed. In some of these applications, liquid or gas molecules may diffuse into or out of the composite structure and change the mechanical state of the material. A good example is Graphite Phenolic (GPh) based thermal insulations which serve under mechanical and hygrothermal loadings. During the polymerization of these composites, high moisture content is produced and the composite is subjected to a variety of hygrothermal effects, which may include a buildup of interlaminar and near edge residual stresses. These stresses may cause cracking of the structural part. In order to prevent such failures, there is a need to characterize and control the mechanical state of the composite material.

The classical theory of diffusion assumes that when a liquid or gas penetrates a solid, the latter does not suffer large deformation. In most cases, Fick's law is adequate and is widely used to characterize the mass transfer of the penetrating constituent, but it does not give any information concerning the effect of the stresses and strains of the solid in which the liquid diffuses. Fick's law assumes that the diffusion coefficient of the material is constant and uncoupled with stress, which leads to a linear governing differential equation. This assumption is contradicted by many empirical studies which show that diffusion rate is affected by external stress. In addition, the composite's absorbance capacity also changes when external loading is applied which again, points at a coupling between the material's mechanical state and diffusion governing equations. Moreover, while changes in the diffusion coefficient are relevant to the transient stage, a change of the absorbance capacity relates to equilibrium stage in which diffusion does not occur. This implies that there is more than one mechanism involved.

The objective of the current research was to study, analytically and experimentally, the hygromechanical coupling, defined as the mutual interaction between the diffusion process and the mechanical state of the composite. Constitutive coupled equations for both water concentration and stress were derived by using generalized chemical potential. An existing model was generalized to include dependence of the coefficient of moisture expansion (CME) on the local water content.

The suggested model was experimentally validated by measuring transient weight and strain between various equilibrium states of humidity, obtaining relevant material parameters. Finally, sets of three point bend specimens were used to obtain the strength profile during drying, which revealed a minimal value at a characteristic time. Using a non-local failure criterion it was possible to associate the temporal strength profile with the build-up of hygromechanical stresses.