Shear strength of composite plate girders with web cutouts

An experimental investigation of the shear strengths of composite plate girders, with centrally placed rectangular web cutouts, is described. A series of tests is conducted on short‐span girders having conventional welded stud shear connectors, connecting the composite concrete slabs to the top flanges of the plate girders. These tests indicated that it is the tensile or pullout capacity of the connectors that is primarily responsible for sustaining the composite action under predominantly shear loading. Subsequently, a further series of tests is conducted on short‐span girders with bolted tension connectors, designed to offer negligible resistance to horizontal shear forces at the interfaces between the concrete slabs and plate girders, which confirmed the previous conclusion. Both series of tests indicate that if adequate connectors are provided between a plate girder and a composite concrete slab, the shear strength of the composite girder is significantly higher than that of the plate girder alone. A simple analytical model for predicting the shear strengths of composite plate girders is also presented, which shows satisfactory correlation with the test results.

A novel approach to functionalise pristine unsized carbon fibre using in situ generated diazonium species to enhance interfacial shear strength

Complex molecules have been successfully grafted onto the surface of unsized carbon fibre, a heterogeneous material which is a challenge to functionalise. The in situ generation of highly reactive phenyldiazo-species from their corresponding anilines was employed to achieve this task. The success of an initial proof-of-concept study (bearing a nitro moiety) supported by X-ray Photoelectron Spectroscopy (XPS) and physical characterisation, led to the design and synthesis of a more complex compound possessing a pendant amine moiety which could theoretically react with an epoxide based resin. After attachment to unsized oxidised fibres, analysis by XPS of the resulting fibres (fluorine used as an XPS tag) indicated a marked difference in functionalisation success which was attributed to steric factors, shown to be critical in influencing the attachment of the phenyldiazo-intermediate to the carbon fibre surface. Analysis of key fibre performance parameters of these fibres showed no change in elastic modulus, strength, surface topography or microscopic roughness when compared to the control unsized oxidised fibres. The functionalised fibres did however show a large increase in coefficient of friction. Single fibre fragmentation tests indicated a marked increase in interfacial shear strength, which was attributed to the pendent amine functionalities interacting with the epoxy resin.

A systematic study of carbon fibre surface grafting via in situ diazonium generation for improved interfacial shear strength in epoxy matrix composites

A recently established means of surface functionalization of unsized carbon fibres for enhanced compatibility with epoxy resins was optimised and evaluated using interfacial shear stress measurements. Interfacial adhesion has a strong influence on the bulk mechanical properties of composite materials. In this work we report on the optimisation of our aryl diazo-grafting methodology via a series of reagent concentration studies. The fibres functionalised at each concentration are characterised physically (tensile strength, modulus, coefficient of friction, and via AFM), and chemically (XPS). The interfacial shear strength (IFSS) of all treated fibres was determined via the single fibre fragmentation test, using the Kelly-Tyson model. Large increases in IFSS for all concentrations (28-47%) relative to control fibres were observed. We show that halving the reagent concentration increased the coefficient of friction of the fibre and the interfacial shear strength of the composite while resulting in no loss of the key performance characteristics in the treated fibre.

Rapidly cured epoxy/anhydride composites: effect of residual stress on laminate shear strength

The drive towards rapid cure thermosetting composites requires a better understanding of the residual stresses that develop during curing. This study investigates the impact of residual stresses on the interlaminar shear strength of resin-infused epoxy/anhydride carbon-fibre laminates. The magnitude of the residual stress was varied by changing the initial injection cure temperature between 75 °C and 145 °C. The corresponding cycle times and the final glass transition temperature of the resin were also measured. The experimentally measured chemical shrinkage and thermal expansion properties of the resin after vitrification were used as inputs to a finite element analysis to calculate the peak residual stresses in the composite. An increase in the initial cure temperature from 85 to 135 °C resulted in an increase of 25% in the residual stress, which led to an experimentally measured reduction in the composite's short beam shear strength of approximately 16% (8 MPa), in good agreement with model prediction.