Sabina Alessi

Review of photoelastic image analysis applied to structural birefringent materials: glass and polymers

Abstract. Photoelasticity is particularly suitable for the analysis of the stress state in structural materials that are transparent and birefringent. Some techniques of digital photoelasticity (phase shifting and RGB) are applied to the analysis of stress field in two classes of structural materials. The first one consists of tempered glasses, such as those used in the automotive and architectural fields. The second one consists of thermoset polymers, typically used as matrices in fiber reinforced plastic structural composites. The birefringence of such resins is, in particular, exploited to investigate the development of swelling stresses and changes in fracture toughness as induced by water uptake aging.

Photoelastic stress analysis assisted evaluation of fracture toughness in hydrothermally aged epoxies

The present work has investigated the fracture toughness of a model DGEBA epoxy system subject to Hidro-Thermal aging. A Photoelastic Stress Analysis technique has been implemented, showing the evolution of stresses arising throughout the water uptake process due to the non-uniform swelling of the material. Gravimetric and Dynamic Mechanical Thermal Analyses have further complemented the characterization, showing the onset of plasticization effects with aging. The correlation of all previous characterizations has allowed to conclude that an increase of KIC fracture toughness is obtained at the fully saturated condition. In particular Photoelasticity has also revealed the onset of relevant swelling induced stresses during the first stages of water absorption, leading to an increase of fracture toughness due to compressive stresses settling near the crack tip. A stress free condition is instead reestablished at the later stages of absorption, suggesting that the increased toughness of the saturated material is an effect of the modifications induced by aging on the polymer structure.

Xyloglucan-based hydrogel films for wound dressing: Structure-property relationships

Thin xyloglucan-based hydrogel films have been synthetized and characterized in the prospect of producing wound dressings. Polyvinyl alcohol (PVA) and glycerol (Gro) were added to have an optimal combination of softness, conformability and resilience. Physical hydrogels have been transformed into permanent covalent hydrogels by reaction with glutaraldehyde (GA). Network structure-process-property relationships are discussed on the account of the results of several complementary characterizations: FTIR, rheology, thermal analysis, morphological analysis, moisture retention and swelling measurements. Selected formulations were also subjected to preliminary in vitro cytotoxicity tests. The physical and mechanical properties of some of the xyloglucan-based hydrogel films produced, combined with absence of cytotoxicity, make them suitable candidates for integration with sensors to monitor the wound healing process and further biological investigations in animal models.

Mode I fracture toughness behavior of hydro-thermally aged carbon fibre reinforced DGEBA-HHPA-PES systems

In this work the Mode I fracture toughness behavior of unidirectional CFRP laminates is investigated by means of Double Cantilever Beam (DCB) tests. The composite samples were manufactured by thermal curing after impregnation of a Carbon fabric with a DGEBA epoxy and anhydride HHPA curing agent. One resin batch was also mixed with a PES thermoplastic monomer to enhance the matrix toughness. Two lots of samples, toughened and untoughened, were then left to soak in hot water to achieve various degrees of aging. The influence of matrix toughening and hydrothermal aging on the delamination behavior of the composite have then been assessed and correlated with characterization data from Dynamic Mechanical Thermal Analysis (DMTA) and Scanning Electron Microscopy (SEM).

Solid state NMR spectroscopy investigation of the molecular structure of epoxy based materials cured in different conditions

In this work two epoxy resin model systems, whose monomers are typically used in structural composites, were thermally cured in different cure conditions in order to obtain different cross-linking densities. Their molecular structures were investigated through solid state NMR spectroscopy in order to correlate them to the cure process conditions used and the results were discussed in the light of the dynamical mechanical thermal analysis (DMTA) performed.

Radiation Curing of Thermosetting-Thermoplastic Blends as Matrices for Structural Carbon Fibre Composites

Radiation processing indicates all the processes based on the use of ionizing radiation that cause chemical changes in the matter. Practical application of radiation processing have been evolved since the introduction of this technology nearly fifty years ago. The earliest developments are represented by the sterilization of disposable medical products, preservation of food and crosslinking of plastic materials, while the curing of monomeric coatings was developed somewhat later (Woods, 2000; Cleland et al. 2003). In the last years the use of these and other processes has grown more and more and they are widely practiced today to produce heat–shrinkable plastic films for packaging foods or insulation on electrical wires and jackets on multi-conductor cables in order to increase heat tolerance and to improve the resistance to abrasion and solvents. Other important applications include the reduction of the molecular weight by scissoring of polymers, the grafting of monomers into polymers in order to modify their surface properties and the curing of fibrereinforced polymer composite materials, whose main applications are in automotive and aeronautic/aerospace industries (Clough, 2001; Singh et al. 1996; Lopata et al. 1999; Goodman & Palmese, 2002; Jhonson, 2006; Berejka, 2010). Due to its behaviour regarding the possibility to involve very small particles, like electrons or heavy ions, radiation processing is the ideal way to produce nano-structured systems. Different examples are reported in literature, such as nano-litography devices (Woods, 2000) and nano-hydrogels especially in the biomedical area (Singh & Kuma, 2008; Chmielewski, 2010). The radiation sources generally used in radiation processing applications can be divided into two main groups, that one regarding the use of natural or artificial isotopes and the other one in which particle accelerators are employed. In the first group artificial radioisotopes, like Cobalt-60 and Cesium-137, are included and the second group comprises electron accelerators, accelerators for the production of positive ions and x-ray generators (Woods & Pikaev, 1990; Spinks & Woods, 1990). Currently the most widely used sources of ionizing radiations in the industrial processes are Cobalt-60 and electron accelerators.

IDROTHERMAL AGING OF IONIZING RADIATION CURED EPOXY MATRICES FOR CARBON FIBER COMPOSITES

A poly(ethersulfone) (PES) engineering thermoplastic was used as toughening agent for an epoxy resin. Different percentages of PES in the resin were used, then cured by electron beam irradiation and some samples were thermally post‐cured. The influence of the hydrothermal ageing on all types of samples was studied, measuring the water absorption percentage, the thermal properties, by dynamical mechanical thermal analysis (DMTA), and the toughness, by K|c measurements.

CARBON FIBRE COMPOSITE MATERIALS PRODUCED BY GAMMA RADIATION INDUCED CURING OF EPOXY RESINS

It is well known that ionizing radiation can initiate polymerization of suitable monomers for many applications. In this work an epoxy difunctional monomer has been used as matrix of a carbon fibre composite in order to produce materials through gamma radiation, for aerospace and advanced automotive applications. Radiation curing has been performed at different absorbed doses and, as comparison, also thermal curing of the same monomer formulations has been done. Furthermore some irradiated samples have been also subjected to a post irradiation thermal curing in order to complete the polymerization reactions. The properties of the cured materials have been studied by moisture absorption isotherms, dynamic mechanical thermal analysis and mechanical flexural tests.