BUTT-JOINT TENSILE-STRENGTH - INTERFACE CORNER STRESS INTENSITY FACTOR PREDICTION

The asymptotic form of the interface corner stress field in a butt joi nt is discussed, and a failure analysis based on the stress intensity factor defining the magnitude of this asymptotic stress field is valid ated. A stress singularity of type Kr-delta(delta < 0) exists at an in terface corner in a butt joint (i.e. where an interface intersects a s tress-free edge). A simple relation defines the stress intensity facto r K for an idealized butt joint composed of a thin elastic adhesive la yer bonded between rigid adherends and subjected to transverse tension and uniform adhesive shrinkage. This stress intensity factor, referre d to here as the free-edge stress intensity factor Kf, is applicable t o both plane strain and axisymmetric geometries. The way that uniform adhesive shrinkage (thermal contraction) during cure alters interface corner stress fields is also discussed. When adhesive shrinkage is pre sent, both constant and singular terms must be included in the asympto tic solution to attain good agreement with full field finite element r esults over a reasonably large interface corner region. Experiments ha ve been carried out to investigate the applicability of a K-f-based fa ilure criterion to butt joints. Butt joints were fabricated by bonding two stainless steel rods together with an epoxy adhesive (Epon 828/T- 403). The measured joint strength increased by a factor of 2 as the bo nd thickness was reduced from 2.0 to 0.25 mm. The observed bond thickn ess effect is accurately predicted when failure is presumed to occur a t a critical K-f value. This fracture criterion suggests that the butt joint tensile strength varies roughly as the reciprocal of the cube r oot of bond thickness when the adhesive's Poisson's ratio is between 0 .3 and 0.4, residual stress levels at the interface corner are negligi ble, the adherends are essentially rigid relative to the adhesive, and small-scale yielding conditions hold at the interface corner.