Those of you who work with composite materials will be familiar with the Classic Laminate Theory. According to this theory, from the properties of the material, the stiffness matrix can be easily obtained. Then, using very tedious expressions, the coefficients of the reduced stiffness matrix can be calculated. The thing is that, based on my own experience, those expressions are introduced without any explanation, so most of the people just use them, ignoring where they come from. So, if you’re curious or you just want to understand a bit more about this theory, keep reading this post!
The first thing that we have to consider is the rotation of our coordinate system . We need to know how to express our new coordinate system in terms of the original one. Following Figure 1 and using basic trigonometry, the relationship can be found.