B: when stretching bends
B is zero for a symmetric stack and non-zero otherwise: [0/90]T has B11 = −B22 (stretch makes it bend, cooling makes it curl, lesson 3.3); a balanced but unsymmetric [−45/+45]T has only B16 and B26, so pulling it makes it twist. NASA RP-1351 example 9 works an unsymmetric [+45/0/90]T: B11 = 131.2 lb. AddPly's stiffness tool shows B = 0 for its symmetric example and fills B when one ply is changed.
Predict · at 0:42 in the video
Now two plies, minus forty-five and plus forty-five. Balanced, but not symmetric. Pull it straight. Does it stay flat?
No. It twists. Its only non-zero B terms are B one six and B two six: they turn a stretch into a twist.
This course is our own. These books go deeper.
- Basic Mechanics of Laminated Composite Plates, A. T. Nettles, NASA Reference Publication 1351, 1994
- Mechanics of Composite Materials, R. M. Jones, Taylor & Francis, 2nd ed., 1999
- Engineering Mechanics of Composite Materials, I. M. Daniel & O. Ishai, Oxford University Press, 2nd ed., 2006
- Design and Analysis of Composite Structures, C. Kassapoglou, Wiley, 2nd ed., 2013
- Composite Materials Handbook (CMH-17), Volume 3, Rev. G, CMH-17, SAE International, 2012
The course designs in AddPly, our browser app for laminate design: regions, ply stacks with live stacking-rule checks, thickness, mass and the ply book. It does not do strength, failure or draping; where a lesson covers those, the simulation is our own.
Simulations in this lesson are our own. Ply data come from the datasheet or textbook named on screen; where a number rests on an assumption, the video says so.
