Slide 2 Failure criteria for laminated composites Defining
failure is a matter of purpose. Failure may be defined as the first
event that damages the structure or the point of structural
collapse. For composite laminates we distinguish between first ply
failure when the first ply is damaged and ultimate failure when the
laminate fails to carry the load. Ultimate failure requires
progressive failure analysis where we reduce the stiffness of
failed plies and redistribute the load. Slide 3 Failure criteria
for isotropic layers Failure is yielding for ductile materials and
fracture for brittle materials. Every direction has same properties
so we prefer to define the failure based on principal stresses.
Why? We will deal only with the plane stress condition, which will
simplify the failure criteria. Then principal stresses are What
about the third principal stress? Slide 4 Maximum normal stress
criterion For ductile materials strength is same in tension and
compression so criterion for safety is However, criterion is rarely
suitable for ductile materials. For brittle materials the ultimate
limits are different in tension and compression Slide 5 Maximum
strain criterion Similar to maximum normal stress criterion but
applied to strain. Applicable to brittle materials so tension and
compression are different. What is wrong with the figure? Slide 6
Maximum shear stress (Tresca) criterion Henri Tresca (1814-1885)
French ME Material yields when maximum shear stress reaches the
value attained in tensile test. Maximum shear stress is one half of
the difference between the maximum and minimum principal stress. In
simple tensile test it is one half of the applied stress. So
criterion is Slide 7 Distortional Energy (von Mises) criterion
Richard Edler von Mises (1883 Lviv, 1953 Boston). Distortion energy
(shape but not volume change) controls failure. Safe condition For
plane stress reduces to Slide 8 Comparison between criteria Largest
differences when principal strains have opposite signs Slide 9
Maximum difference between Tresca and von Mises