Lecture 11 Flashcards
(9 cards)
What are the 5 failure modes?
Longitudinal tensile failure
Transverse tensile failure
Shear failure
Longitudinal compressive failure
Transverse compressive failure
Give 4 examples of failure criteria used for composites.
Limit criteria
- Maximum stress
- Maximum strain
Interactive criteria
- Tsai Hill
- Tsai Wu
The interactive criteria allow for an interaction of multiaxial stresses.
Explain the maximum stress criterion.
It consists of 5 sub-criteria corresponding to the strength in the 5 fundamental failure modes.
If any one of these limits are exceeded by the corresponding stress in the principal material axis (1,2) then the material is deemed to have failed.
σ1 ≥ σ1T
σ1 ≤ σ1C
σ2 ≥ σ2T
σ2 ≤ σ2C
τ12 ≥ τ’12
Note that compressive stress is taken to be a negative so failure would occur if the compressive stress is more negative than the compressive strength of the material.
Explain the maximum strain criterion.
The material is deemed to fail if any one of the principal strains exceed the failure strains of the material:
ε1 ≥ ε1T
ε1 ≤ ε1C
ε2 ≥ ε2T
ε2 ≤ ε2C
γ12 ≥ γ’12
Note that compressive strain is taken to be a negative so failure would occur if the compressive strain is more negative than the compressive strain of the material.
What is the difference between limit and interactive criteria?
Limit criteria are simple to use but do not consider interaction between different failure modes.
In intermediate values of orientation and multi-directional laminates, there is a combination of longitudinal, transverse and shear stresses.
The interactive criteria assumes a stress component affects strength of other directions.
Explain interactive criteria.
They take account of stress interactions and allow for the fact that failure loads for multi axial stress state is often different than uniaxial stress state.
Explain Tsai-Hill criterion.
There are many interactive criteria available, the most common is Tsai-Hill, which was developed from Hill’s anisotropic failure criterion derived from Von Mises yield criterion.
The criterion takes account of all 5 failure modes and their interaction. Only one value for failure stress is obtained making only one criterion to be satisfied. Using the criterion, if the value is greater than or equal to one, the composite material has deemed to have failed.
What would be the difference in failure stress between maximum stress and tsai-hill for a UD composite loaded in uniaxial stress parallel to a principal direction?
They would give the same failure stress. As the expression for tsai-hill will be simplifed.
What happens when one composite layer fails?
Failure in a composite laminate can propagate catastrophically, it initiates from one layer and if other layers cannot tolerate the load, the whole component will fail.
Final failure can be at higher loads than the first failure strength if other layers take the total load after removal of the first ply.
After failure in different plies, stiffness will be reduced in certain directions, but those layers may still partially contribute to the overall stiffness. The magnitude of stiffness contribution depends on multiple factors and is rather complicated to calculate.
