When utilizing adhesive bonding in structures where a failure might have serious consequences, one is generally interested in the possibility of determining the sensitivity of the bond strength to certain types of bond imperfections. It is well known that most adhesive bonds will contain defects such as voids, regions with no or poor bonding, and microcracks. When such defects are located in regions with alternating stresses of sufficiently high amplitude, they might grow to form macrocracks.
To investigate how the static strength of an adhesive spar—wingskin joint is affected by the presence of two straight edge cracks in the interface between the adhesive and the spar flange and located symmetrically with respect to the spar center line, experiments were performed for different crack sizes. Specimens were loaded by a tensile force acting along the spar center line in order to simulate the effect of fuel pressure. To find out whether the usual linear elastic fracture mechanics (LEFM) assumption of a critical elastic energy release rate value at onset of fracture applied in this case also, the elastic energy release rate was calculated analytically by use of a simple beam model and numerically by use of the finite element method (FEM).
The following conclusions were drawn for the specific geometry considered:
1. For crack lengths below 4 mm, the joint was critical with respect to interlaminar separation in the spar flange.
2. For crack lengths above 4 mm, the joint was critical with respect to debonding.
3. The critical energy release rate was approximately constant for crack lengths in the interval of 4 to 15 mm. The influence of the radius in the connection between the spar web and spar flange and the elasticity modulus of the elastic insert in the same region was investigated numerically. From this it was concluded that:
4. For cracks larger than about 10 mm the joint will become more crack sensitive with increasing radius, whereas it will be almost unaffected by changes in the insert elasticity modulus.