Sofia Teixeira de Freitas

Adhesion Properties of Bonded Composite-to-Aluminium Joints Using Peel Tests

In this research, the adhesion properties of bonded composite-to-aluminium joints are evaluated using floating roller peel tests. Tests were performed using two different adhesives and different adherend lay-ups: composite-to-aluminium, composite-to-composite and aluminium-to-aluminium. The results show that floating roller peel tests, widely used in metal bonding, can also be used to assess adhesion properties of composite bonding and composite-to-aluminium bonding. However, attention should be paid on which results are important to take from the peel tests. In adhesion tests, the failure mode is more important than the failure load. The peel load can only be compared when using exactly the same type of flexible adherend. Even when the adhesion properties are good (cohesive failure), the peel load value can decrease up to a factor of 10 when peeling off a composite flexible adherend instead of an aluminium flexible adherend.

Sandwich system for renovation of orthotropic steel bridge decks

A sandwich system of two steel faces and a polyurethane core is studied as a renovation system for orthotropic steel bridge decks. An experimental program has been carried out aiming to better understand the sandwich beam flexural behavior. The temperature significantly affects the sandwich flexural behavior. Increase in the temperature decreases the sandwich stiffness and strength. The stiffness is more difficult to predict at high temperatures due to the viscoelastic behavior of the core. Stiffer and stronger renovation solutions can be achieved by putting the extra weight on the core thickness rather than on the faces thickness. Stresses on the deck plate can be reduced by 60—95% using this renovation system.

Test method to assess interface adhesion in composite bonding

This paper introduces a new type of peel tests dedicated to composite bonding: Composite Peel Tests. This test is inspired on the standard floating roller peel test widely used for metal bonding.The aim of this study is to investigate the potential of the Composite Peel Test to assess interface adhesion in composite bonded structures. To this end, peel tests were performed with nine different types of adhesives and at two environmental temperatures, room temperature and +80°C. The results were compared with the standard floating roller peel tests with Aluminium adherends.The results show that when using the Composite Peel Test good interface adhesion results either in cohesive failure of the adhesive or intra-laminar failure of the composite, while bad adhesion results in adhesive failure. In most cases of good interface adhesion, increasing the temperature favors cohesive failure of the adhesive in detriment of intra-laminar failure of the composite.Peel strengths can be used as a quality indicator of interface adhesion only if using exactly the same type of flexible adherend (peeling-off member). Nevertheless, if cohesive failure is the dominant failure mode, the comparison between adhesives’ peel strength is consistent disregarding of the type of peel-off adherend. Composite Peel Tests are suitable to assess interface adhesion of composite bonded structures.

Structural monitoring of a strengthened orthotropic steel bridge deck using strain data

Orthotropic steel bridges have experienced early fatigue cracks at several welded connections in the steel deck plate. One of the possible strengthening systems to enlarge the fatigue life of the existing decks consists of bonding a second steel plate to the existing deck. This renovation technique was for the first time applied on the orthotropic deck of the movable bridge Scharsterrijn. This article describes the results of the structural monitoring carried out to evaluate the short-term and long-term performance of the strengthening system. Static and dynamic controlled load tests were carried out using a calibrated truck. Strain history measurements were recorded continuously during 1 year from the normal traffic running on the bridge. The short-term measurements show significant decrease of the stress level at the bridge deck after the renovation, especially at the deck plate details. The stresses at the welds between the deck plate and the stiffener web reduce approximately 55% at the deck plate side and 35% at the stiffener web side. Due to this reduction, the fatigue life of these welds is expected to increase 11 times at the deck plate side and 3.6 times at the stiffener side. The long-term measurements do not show significant changes in the stress level at the bridge deck during the year of monitoring. The strengthening system has demonstrated good performance reliability to prolong the life span of the movable orthotropic bridges.

Parametric Study on the Interface Layer of Renovation Solutions for Orthotropic Steel Bridge Decks

Abstract:i¾?The article presents renovation solutions for orthotropic steel bridge decks consisting of bonding a second steel plate to the existing steel deck in order to reduce the stresses and enlarge the life span of the orthotropic bridge deck. Two solutions for the interface layer between the existing deck plate and the second steel plate are presented: thin epoxy bonded system and thick polyurethane sandwich system. A parametric study based on analytical solutions is carried out on flexural behavior of beams representing the renovation solutions. The influence of geometrical, mechanical and structural parameters on the stiffness and stress reduction factor of the system is studied. Maximum values of stiffness and stress reduction are achieved when maximizing the interface layer thickness and minimizing the second steel plate thickness with in certain practical limits. Based on the weight restrictions one can choose the most efficient interface layer regarding thickness and mechanical properties.

Out-of-autoclave manufacturing of GLARE panels using resistance heating:

Autoclave manufacturing of fibre metal laminates, such as GLARE, is an expensive process. Therefore, there is an increasing interest to find cost-effective out-of-autoclave manufacturing processes without diminishing the laminate quality. The aim of this study is to evaluate the quality of fibre metal laminate panels adhesively bonded and cured using resistance heating. Three manufacturing processes are compared for different layups with an embedded steel mesh at the mid-plane: autoclave curing, resistance bonding of two (autoclave-cured) panels and complete out-of-autoclave resistance curing of panels. Interlaminar shear strength tests and optical microscopy analysis showed that resistance bonding is a promising technique, leading to results comparable to autoclave curing. Resistance curing led to an interlaminar shear strength decrease of 30–60%. A study of the correlation between degree of cure and distance from the mesh revealed the potential of resistance bonding to be used for flexible embedded mesh geometries and on-site repairs.

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.

The use of acoustic emission and composite peel tests to detect weak adhesion in composite structures

ABSTRACT Adhesive bonding is one of the most promising joining technologies for composite aircraft. However, to comply with current aircraft certification rules, current safety-critical bonded joints, in which at least one of the interfaces requires additional surface preparation, are always used in combination with redundant mechanical fasteners, such as rivets and bolts. This lack of trust in bonded structures is mostly linked to the fear of lack of adhesion or a “weak bond”. The aim of this paper is to tackle this challenge by assessing the ability to use composite peel tests and acoustic emission (AE) technique to assess adhesion quality and distinguish a good bond quality from a “weak bond”. Composite Bell Peel (CBP) tests and Double-Cantilever-Beam (DCB) tests were performed on contaminated and non-contaminated CFRP bonded specimens. The results show that peel strength drops significantly at the location of the contaminated interface that has led to weak adhesion, as a result from adhesive failure. The AE signals obtained during DCB tests show different features for cracks growing at the interface (“weak bonds”) and inside the adhesive layer (cohesive failure). In addition to this, scattering of the AE signals were observed in the contaminated specimens with “weak bonds”.