Understanding fatigue behaviour of CFRC
The use of multilayer unidirectional Carbon Fibre reinforcement is widely used in the aerospace industry for structural application. The manufacturing of composites structures this type of reinforcement is appealing because it allows the design of materials with quasi-isotropic properties (where strength is given direction is desired).
Because of modern aircraft are often fabricated using an array of materials (aluminium, steel alloys, composites etc), the use of mechanical fasteners and adhesives to join the various part of the aircraft is necessary.
The aim of this demonstration is tomeasure the flexural properties of a unidirectional Carbon Fibre Reinforced Composites (CFRC) and also assess any loss in properties after insertion of a mechanical fastener (i.e. rivet).
Stacking sequence (0, +45, 90, -45) in a composite structure (held together by an auxiliary material and stiches).
Governing equations:
From lecture:
σ_f=3FS/(2bd^2 ) ε_f=6Yd/S^2
With F the load at a given point on the load deflection curve (N), S the support span (mm), b the width of test beam (mm), d the thickness of the test beam (mm), Y the deflection of the center of the beam (mm)
In this demo, use b= 12 mm, d= 2.5 mm and S=40 mm.
Questions
Using a simple 3 point bending configuration, measure the load required to fail a 0 degree, 90 degree oriented CFRC samples. Report the data (width and thickness of the sample, load in N and deflection in mm).
Convert load to stress and deflection in flexural strain. Using the gradient (i.e., slope) of the initial straight-line portion of the stress-strain curve, report the modulus and flexural strength for each type of composite (i.e. 0 and 90 degrees). Comment on the results and particularly the effect of fibre direction.
Using a simple 3 point bending configuration again, measure the load required to fail a 0 degree oriented CFRC samples containing two rivets inserted after drilling a 4.5 mm holes.Comment on the results.
Considering that a 4.5 mm hole was drilled to insert the rivet, approximate the stress concentration factor around the rivet at maximum stress.
Comment on the development and use of structural adhesive for this particular type of application. What are the benefits and weaknesses of using adhesive for such purpose?