S. Budhe

Influence of mechanical surface treatment on fatigue life of bonded joints

ABSTRACT Adhesively bonded joints can support a longer fatigue life if compared to conventional joining techniques, provided that a set of requirements is fulfilled. One of the most important requirements is the mechanical preparation of the bonded joint surface, which improves the joint interface adhesion. The aim of this work is to investigate the influence of surface roughness of mild steel substrates on fatigue behavior in adhesive bonded plates. To accomplish this objective, three different surface treatments were used on A36 steel substrate specimens, namely sand blasting, grit blasting, and bristle blasting. Bonded plate specimens, using end-notched flexure format, with a thin adhesive epoxy layer were manufactured and tested, under mode II loading condition, in both static and dynamic tests. The results confirm the importance of surface treatment of the substrate on the fatigue life, confirming that adhesively bonded joints have significant performance differences when subjected to static and dynamic loadings.

INFLUENCE OF ADHEREND SURFACE ROUGHNESS ON THE ADHESIVE BOND STRENGTH

SURFACE TREATMENT OF THE ADHERENDS PRIOR TO ADHESIVE BONDING PLAYS AN IMPORTANT ROLE IN THE ENHANCING OF STRENGTH AND DURABILITY OF BONDED JOINTS. IN THIS WORK, AN INVESTIGATION ON EFFECT OF ADHEREND SURFACE ROUGHNESS ON ADHESIVE BOND STRENGTH WAS PERFORMED. SINGLE STRAP JOINTS WITH DIFFERENT ADHERENDS (MILD STEEL AND ALUMINIUM) BONDED WITH AN EPOXY RESIN (ARALDITE® 2015) WERE TESTED. THE ADHEREND SURFACE WAS TREATED BY MECHANICAL ABRASION PROCESS USING AN EMERY PAPER. CONTACT ANGLE MEASUREMENT AND SEM ANALYSIS TO UNDERSTAND THE WETTABILITY AND THE FAILURE MECHANISM OF THE JOINTS WERE PERFORMED. IT WAS FOUND THAT AN OPTIMUM SURFACE ROUGHNESS EXISTS FOR A MAXIMUM BONDING STRENGTH AND THE ROUGHNESS RANGE DEPENDS ON THE ADHEREND MATERIAL. THE JOINT STRENGTH CHANGES ARE ASSOCIATED NOT ONLY SIMPLY BY THE INCREASED BONDING AREA, SURFACE TEXTURE OR MECHANICAL INTERLOCKING, BUT ALSO BY THE CHEMICAL CHARACTERISTICS OF THE SURFACE AND THE CHEMICAL BOND BETWEEN THEM.

Experimental analysis of metal-composite repair of floating offshore units (FPSO)

ABSTRACT This work presents the analysis of the fatigue behaviour of a repaired metal-composite panel under mixed-mode loading conditions and also discusses a composite repair of the damaged tube on an offshore unit application. A carbon steel plate (A36) as a parent substrate and an epoxy resin was used. The composite material for the repair was a biaxial +45°/−45° non-crimp carbon fabric. Static and dynamic three-point bending tests were performed. It was found that the fatigue load must be lower than 60% of the static load for fatigue life >105, as recommended by Det Norske Veritas (DNV). It is likely that the findings of this study will be a boost to improve the understanding of the long-term performance of bonded repair in an offshore environment.

Interlaminar adhesion assessment of carbon-epoxy laminates under salt water ageing using peel tests

The aim of this study is to assess the interlaminar adhesion of carbon-epoxy laminates under salt water condition. Carbon-epoxy laminate specimens were immersed in a salt water tank for 60 days. Some specimens were then dried at room temperature for 280 days, until recovering their initial weight. Specimens were tested using the composite peel test, an adaptation of the floating roller peel tests for composite materials. The results showed a degradation of peel strength in some areas due to the ageing process. The drying process did not affect the test results. A scanning electron microscopic analysis carried out on the fracture surface of the specimens revealed a typical mode I failure microstructure. A mixture of matrix failure and fibre/matrix interfacial failure was observed in non-aged specimens. Finally, a chemical characterization of the fracture surfaces with energy-dispersive spectroscopy confirmed the penetration of salt water in regions near the edge of the specimens. A degradation of the fibre/matrix interface adhesion was observed in affected areas. Floating roller peel tests proved to be a fast and effective method to access the interlaminar adhesion performance of composite laminates.

An assessment of composite repair system in offshore platform for corroded circumferential welds in super duplex steel pipe

The main aim of this study is to assess the effectiveness of a composite repair system in severely corroded circumferential welds in super duplex stainless steel pipes as a preventive measure against the premature corrosion damage at the welds. Artificial defects were fabricated on the super duplex steel tube in order to reproduce the localized corrosion damage defects found in real welded joints. Three kinds of through thickness defects were considered: 25%, 50% and 96% of the perimeter of the pipe. The performance of the repaired pipe was assessed by hydrostatic tests as per ISO 24817 standard. The results showed that the composite repair system can sustain the designed failure pressure even for the pipe damaged with through-wall defect up to 96% of the perimeter of the pipe. Hence, the composite repair system can be used as a preliminary tool to protect the unexpected or premature failure at the welds and maintain an adequate level of mechanical strength for a given operating pressure. This composite repair system can assure that the pipe will not leak until a planned maintenance of the line. Nevertheless, further work is still desirable to improve the confidence in the long-term performance of bonded composite.

Bonded repair of composite structures in aerospace application: a review on environmental issues

Over the last two decades, the repair of existing engineering structures using fiber reinforced polymer composites has attracted a great attention by aerospace industry, as it is more economical than replacing new. With an increased use of composite material in aerospace field, it is thus essential to restore the structural integrity by repair of damaged part. Concerns regarding the long term durability of composite repair bonded joints have been a major obstacle for critical component of aerospace structures. This paper reviews the current research on the environmental durability of adhesive bonded repair of composite structures to focus on the durability concerns and suggestion on the research needed in this area. The most important environmental factors (moisture and temperature) are reviewed thoroughly and also combined environmental effect. Finite element methods used to predict the environmental influence on the composite bonded joints are briefly reviewed. Finally, the paper concludes with key findings, opportunities and future research topics in order to develop cost effective, better quality and reliable composite repair bonded joints.