|M.Sc Student||Ben Yaacov Gidon|
|Subject||Effect of Hygrothermal conditions on Strength of phenolic|
|Department||Department of Mechanical Engineering||Supervisor||Professor Emeritus Eli Altus|
|Full Thesis text|
Graphite-Phenolic (GPh) composite is commonly used to fabricate structural parts that function at very high temperatures under mechanical and hygrothermal loads. During production and in its lifetime, GPh can absorb and desorb water. Hygrothermal interlaminar stresses can develop and influence the material’s mechanical properties.
The ability to quantify and establish the material’s strength profile as a function of environmental condition is important when designing high performance structures. Previous researches investigated moisture content influence on the GPh’s flexural strength. During the drying experiment, the flexure strength decreased in the first hours to a minimum value less than half of its original ultimate strength (wet in equilibrium) and then increased back to almost its original ultimate strength in the end of the drying process. A mismatch was noted between the analysis and the experiment at the time of minimum strength. Moreover, the analysis’s minimum strength value was lower and not physical (i.e. negative) compared to the experiment.
The aim of this study is to investigate the drying process of the GPh and to understand the factors that can cause this mismatch in time and value of minimum strength. The main factor suspected is the specimen’s surface boundary condition. The assumption is that during the drying process, a moisture boundary layer develops near the specimen’s surface that decelerates the diffusion process by changing the moisture concentration on the specimen’s surface. Subsequently postponing the minimum strength time and influencing its value.
The research includes experimental and numerical studies. The boundary layer assumption was verified in a controlled drying experiment. Later, the temperature change on the specimen’s outer surface was investigated using thermal camera. A temperature drop of up to 2°C was found on the specimen’s surface. This small change influence on the diffusion coefficient is negligible.
During the numerical study, there were some difficulties to converge to the minimum strength. When other solutions didn’t promote the converging, the compression Young modulus was investigated and obtained. It was found that the modulus value was smaller than previously used. Moreover, it was evident that up to 2% strain, the modulus value changes as function of the strain. Therefore, the Young modulus is an additional factor that influences the minimum strength.
Since obtaining the moisture concentration distribution on the specimen’s surface can be very difficult, a new approach was suggested. Simple and accurate weight measurements of the specimens were obtained during the drying experiment. The weight change function was derivate as a function of time and divided by the specimen’s outer surface in order to extract the diffusion flux function that was later defined as the boundary condition in the analyses. The Uniform distribution flux boundary condition wasn’t suitable to represent the real boundary condition. When using the Nonuniform distribution flux boundary condition, several distributions of fluxes along the specimen’s surface were examined in order to converge to the minimum strength.
Substituting the new Young modulus in the Nonuniform distribution flux boundary condition analysis has achieved a good prediction of the time and the value of minimum strength.