Fibres interfaces

In the macrocomposite model it is assumed that the load transfer between the rod and the matrix is brought about by shear stresses in the matrix-fibre interface [35]. When the interfacial shear stress exceeds a critical value r0, the rod debonds from the matrix and the composite fails under tension. The important parameters in this model are the aspect ratio of the rod, the ratio between the shear modulus of the matrix and the tensile modulus of the rod, the volume fraction of rods, and the critical shear stress. As the chains are assumed to have an infinite tensile strength, the tensile fracture of the fibres is not caused by the breaking of the chains, but only by exceeding a critical shear stress. Furthermore, it should be realised that the theory is approximate, because the stress transfer across the chain ends and the stress concentrations are neglected. These effects will be unimportant for an aspect ratio of the rod Lld> 10 [35]. 

The resistance to moisture and hot water is good without hydrolysis but the resistances of glass fibre reinforced compounds can decrease significantly because of polymer/fibre interface alterations. For example, after 1 year in water at 120°C, tensile strength retention is 50% for a given compound. However, special grades with improved hot-water resistance are marketed and are successfully used in hot-water pumps. 

In addition, as the PVC tubes contain a plasticizer and stabilizers of which producers do not give information this needs to be taken into account, particularly, when PVC additives may give problems in analytical methods. In this case, other materials may constitute a better choice. The supplier also informs the users that decaying organic material may influence N-NH4 analysis in auto-analyser systems. Another important consideration on polysulfone fibres application for soil solution sampling is the apparent retention of colloidal Fe at the fibre interface (Jones and Edwards, 1993b) which is not entirely clear and should not to be overlooked, because of the high retention of colloidal Fe and its role in the translocation of PTE in soil. 

Penetration of soil-fibre interface by wash liquid... 

Of these mechanisms, solubilisation and emulsification of soils are controlled by detergent composition, hydrodynamic flow is controlled by washing machine design and fibre flexing is controlled by fabric construction. The textile chemist can only influence the mechanisms that involve the fibre surface, i.e. rollup of oily soil, penetration of soil-fibre interface, surface abrasion and finish swelling. Finishes have been developed that provide soil release performance by taking advantage of all of these mechanisms. 

In terms of nanocomposite reinforcement of thermoplastic starch polymers there has been many exciting new developments. Dufresne [62] and Angles [63] highlight work on the use of microcrystalline whiskers of starch and cellulose as reinforcement in thermoplastic starch polymer and synthetic polymer nanocomposites. They find excellent enhancement of properties, probably due to transcrystallisation processes at the matrix/fibre interface. McGlashan [64] examine the use of nanoscale montmorillonite into thermoplastic starch/polyester blends and find excellent improvements in film blowability and tensile properties. Perhaps surprisingly McGlashan [64] also found an improvement in the clarity of the thermoplastic starch based blown films with nanocomposite addition which was attributed to disruption of large crystals. 

Furthermore, the absorption of water by the interphasal polymer can reduce its yield strength below the interfacial bond strength. Thus, the apparent interfacial shear strength will be reduced, and a yield front, rather than a debond, will propagate along the fibre interface modifying the stress transfer micromechanics at a fibre break. A consequence is that the stress concentrations in adj acent fibres to the fibre break will be reduced, and the probability of the formation of a flaw of critical dimensions is also reduced. The number of interacting fibre breaks associated with a flaw of critical dimensions will increase. 

In spite of the many years of practice, the composites with cement matrix are still the subjeet of numerous investigations, because their properties ate worsening with time, and the main cause ate the detrimental changes occurring in the paste-fibre interface [60]. The extensive smdies to create a theoretical model which would allow predicting their properties are carried out simultaneously [61-63]. 

Effect of water and salt on the adherend. Deterioration of materials such as metals and concrete is often more rapid with salt solution than with water, for example by the action of electro-chemical corrosion. Water itself may be responsible for a number of changes in the adherends concrete is likely to get stronger with further hydration of the cement, and plastic may become weaker by plasticisation. The resin/fibre interface in composite materials is also susceptible to degradation by water. With metals, water may attack... 

Mechanisms of creep in FRP materials are related to the progressive changes in the internal balance of forces within the materials resulting from the behaviour of the fibre, adhesion and load transfer at the resin—fibre interface, and from the deformation characteristics of the matrix. Thus any factors which either directly or indirectly cause changes to any of these key areas will affect the creep process. 

For higher number of thermal shock cycles two typical features arise on the load deflection traces. The first one is a marked pop-in behaviour at loads of about 10 N (marked by circles in Figure 10). The second phenomenon observed is increasing specimen deflection at fracture (maximum) load, as evident when comparing the curves for 10 and 24 cycles in Figure 10. This can be caused by the change of crack tip behaviour affected by damage development at the matrix/fibre interfaces. 

The transverse and shear properties of a unidirectional composite are principally dependent upon the behaviour of the matrix and fibre interface. Consequently they can be orders of magnitude less than their longitudinal counterparts. For GRP the shear modulus and strength are typically 5GPa and 60 MPa and for ultra high modulus carbon fibre composite 4GPa and 50 MPa, respectively. 

In the aim of using such structures for compression resistant or impact resistant materials, it would be interesting to optimize the fibre organization so as to obtain the most efficient fibre interfacings, so as to obtain the highest modulus for a given fibre diameter and a given nonwoven or composite density. 

Thomson, C. 1. Lowe, R. M. Ragauskas, A. J. Imaging cellulose fibre interfaces with fluorescence microscopy and resonance energy transfer. Carbohydr. Polym. 2007, 69, 799-804. 

Modification of the cementitious binder with poiymers The use of polymer dispersions (latex) as an additive to cementitious binders is well established. The polymer particles are smaller by orders of magnitude than the cement grains, and they coalesce to form a continuous film. The use of water dispersed acrylics and PVA polymers (which can more readily be placed at the fibre interface and later on coalesce into a film which could be interlaced within the gel particles) resulted in a fine interfacial structure with much higher bond strength. 

In many of the monofilament and bundled systems this interaction results in a deviation of the crack path as it crosses the fibre, and leads to its separation into a number of microcracks [77-82], as seen in Figure 3.39(a). Observations at higher magnification show that in many instances the crack seemed to be arrested in the matrix, just ahead of the fibre, which it never quite reached rather, it typically turned 90" on a new course parallel to the fibre, about 10-40 /u.m in front of the actual paste-fibre interface (Figure 3.39(b)). In such instances the debonding was not at the actual interface. 

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