Question

a.Sub-laminar tearing is a common failure pattern; explain why this is particularly dangerous for commercial aircraft.

b.Why does the specification of direction of the fibers in a composite improve its load bearing capacity?

c.Use the Hunt Library or a preferred source to research composite assembly. Why are curing times and pressures critical?

Answer 1

1. Sub-laminar tearing is a common failure pattern:

Lamellar tearing can occur beneath the weld especially in rolled steel plate which has poor through-thickness ductility.

t is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur:

1. Transverse strain - the shrinkage strains on welding must act in the short direction of the plate ie through the plate thickness
   
2. Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions
   
3. Material susceptibility - the plate must have poor ductility in the through-thickness direction

Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen.

2. Specification of direction of the fibers in a composite improve its load bearning capacity:

The strength and stiffness of a composite buildup depends on the orientation sequence of the plies. The practical range of strength and stiffness of carbon fiber extends from values as low as those provided by fiberglass to as high as those provided by titanium. This range of values is determined by the orientation of the plies to the applied load. Proper selection of ply orientation in advanced composite materials is necessary to provide a structurally efficient design.Because the strength design requirements are a function of the applied load direction, ply orientation and ply sequence have to be correct. It is critical during a repair to replace each damaged ply with a ply of the same material and ply orientation.The fibers in a unidirectional material run in one direction and the strength and stiffness is only in the direction of the fiber. Pre-impregnated (prepreg) tape is an example of a unidirectional ply orientation