Margins of safety
Ultimate load is limit load × 1.5 (14 CFR 25.303); the margin of safety is allowable ÷ (limit × factor) − 1, so zero is a pass with no reserve. For our quasi-isotropic laminate the margin flips sign with the criterion (+0.06 by max stress, −0.03 by Tsai-Wu at an assumed 250 MPa limit stress), and real allowables are statistical (14 CFR 25.613), lower than the typical strengths used here.
Predict · at 0:54 in the video
A margin of exactly zero: pass or fail?
Pass, just. At zero, the part carries exactly its ultimate load, with nothing to spare. And the criterion matters.
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.