Progress on investigation on damage analysis in bonded polymer composites under fatigue

• Published in: International journal of fatigue 2017-03, Vol.96, p.224-236
• Main Author: Olajide, Sheriff O.
• Format: Article
• Language: English
• Subjects: Adhesive bonding; Adhesive joints; Aging aircraft; Aircraft components; Aircraft control; Anomalies; Bonded joints; Bonding strength; Brittle fracture; Composites; Control surfaces; Crack propagation; Damage assessment; Defects; Degradation; Extrapolation; Fatigue failure; Fatigue life; Life prediction; Materials fatigue; Polymer; Polymer matrix composites; Polymers; Scattering; Stiffness; Strain hardening; Structural damage; Variable amplitude fatigue
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2016.12.016

•An important step towards enabling realistic modelling in the fatigue of bonded joints is presented.•A rationale for a general acceptable standard in the fabrication of bonded joints for fatigue is presented.•Residual strain effect “within the adhesive layer only” in bonded joints is presented.•Evidence of none existence fatigue cycle pattern in bonded joints fracture surface is presented. Several unexpected fatigue failures of polymer bonded joints used to repair damaged critical structures such as aircraft control surfaces have been reported in-service. The fatigue life and fatigue life scatter in polymer bonded joints was found to be highly dependent on the rate of stiffness degradation of the polymer adhesive layer under fatigue cycle. Since stiffness is a function of shape and boundary condition and not just material, this makes the static strength of bonded joints unsuitable for assessing and predicting fatigue life. The stiffness degradation of the polymer bonded joint led to in-homogenous strain hardening of the adhesive layer leading to eventual brittle fracture. The in-homogenous regional strain hardening in the polymer joint adhesive layer was caused by fabricated defects and anomalies. Residual strain effect was found in bonded joints after fatigue failure. This was extrapolated to reason that there may be a mean strain effect in adhesively bonded joints under uninterrupted fatigue leading to further research in this area. This research article completes knowledge created in two earlier ones, completing a necessary step in many steps towards addressing fatigue of bonded joints.