4C2 Tripos 2015 Flashcards
Describe briefly what is meant by a laminate and explain why laminates are commonly employed in preference to a unidirectional lamina.
A laminate is made up of a stacked assembly of unidirectional plies, each having its fibre axis lying at a specified angle to a reference direction. They are used in preference to unidirectional plies because they are more isotropic, which can be desirable in some circumstances where loading is in more than one direction, or to avoid splitting failure.
What is a symmetric laminate and how does this symmetry affect the laminate’s elastic response?
A symmetric laminate is one possessing a mirror plane lying in the plane of the laminate i.e. the stacking sequence in the top half reflects that in the bottom half. A symmetric laminate does not exhibit bending-stretching coupling (the coupling stiffness [B]=0), i.e. in-plane loading will not generate any out-of-plane distortion and vice versa.
Name features of GFRP that might make it suitable for certain applications
-Good corrosion resistance
-Low thermal expansion
-High stiffness/strength
-Low weight
(-High cost)
Need to consider the application in question and determine which of the above are and are not relevant.
When using tsai hill failure criteria, what value of SL and ST do you use?
Use the one corresponding to the type of σ1/σ2. ie if σ1 is tension, use SL+ (also tension)
What’s important to remember about σ1/σ2 in the tsai hill failure criteria?
They are in the material axes, so any loading criteria will have to first be converted to these axes before they can be used in the equation.
Discuss the applicability of LEFM (Linear elastic fracture mechanics) to fracture in fibre composites.
LEFM is applicable when the process zone is small compared to the crack/specimen dimensions. In composites, the size of the process zone can be large compared to the gaps between the fibres, leading to a difference in the crack propagation in the matrix and through the composite.
Why do woven composites tend to have lower stiffness and compressive strength than an equivalent composite made of prepreg with the same fibres and matrix?
Woven materials have fibres which do not run parallel to the loading direction. This means that their contribution to the stiffness is less. Moreover compressive strength is sensitive to fibre misalignment or load misalignment, as this induces shear in the matrix causing microbuckling. This is a cause of reduced compressive strength.
Why is barely visible impact damage an important issue in aerospace design?
Barely visible damage is caused by relatively low impact events such as dropping of tools or stones impacting the structure. A key problem is that, by definition, this damage is difficult to see so may be present without the operator knowing it. So this means that the structure needs to be designed to cope with such damage. Often the damage is in the form of sub-surface delamination, which will be difficult to see but may lead to premature failure, either due to fatigue growth or due to buckling of the delaminated region under a compressive loading.
Why are septic tanks (underground tanks used as part of a small-scale sewage treatment system) commonly made of GFRP?
Light so easy transport + installation
Relatively low cost
Can make curved sections easily
Good corrosion resisance
Such tanks are quite large (several metres in length) so the relatively low weight of GFRP will make transport and installation in the remote locations easier. While the deadweight loading of covering soil will not be negligible requiring reasonable structural properties. In addition GFRP is relatively low cost for this commodity item and can be formed into the required curved shapes easily. Finally GFRP has good corrosion resistance against water and other chemicals in the sewage. All of the above indicate GFRP as a good choice.
