Stiffness reduction effect

Although the stiffness reduction curves offer a reasonable basis on which to establish damage curves for constant amplitude loading at different stress levels, these damage curves may not reflect all physical effects which are important for the development of fatigue damage. That, however, would mean that the development of realistic cumulative damage laws for composite materials would require considerable further research and development work. 

Fig. 10. The construct stiffness was effected by the slip reduction and rod preloads in distraction or compression...

Matrix cracking results in reduction in the laminate stiffness and strength. Stiffness reduction in CMCs is greater than that observed in polymer matrix composites due to lower ratio between fibre and matrix moduli. It also affects coefficients thermal expansion and vibration frequencies. A number of models have been suggested to estimate the effect of transverse macrocracks in the 90° plies and matrix cracks bridged by fibres in the 0° plies on the mechanical properties of cross-ply CMC laminates (Pryce and Smith, 1994 Daniel and Anastassopoulos, 1995 Lu and Hutchinson, 1995 Erdman and Weitsman, 1998 Birman and Byrd, 2001 Yasmin and Bowen, 2002 Birman and Weitsman, 2003). 

SR characterizes the well-known Mullins effect which associates a strong stiffness reduction (stress-softening) to the enhancement of the residual strain. The more stable the material, the lower %stabilization is. The aim for our materials is a low %SR, it means that the elastomer does not lose its initial properties and so that the Mullins effect is low. 

Molecular CN reduction effects oppositely to that of compression on the H-bond length-stiffness relaxation and the associated properties of water ice. 

These values are determined by experiment. It is, however, by no means a trivial task to measure the lamina compressive and shear strengths (52,53). Also the failure of the first ply of a laminate does not necessarily coincide with the maximum load that the laminate can sustain. In many practical composite laminates first-ply failure may be accompanied by a very small reduction in the laminate stiffness. Local ply-level failures can reduce the stress-raising effects of notches and enhance fatigue performance (54). 

As a class the aliphatic polyalkenamers have low values due to a combination of low chain stiffness and low interchain attraction. The presence of double bonds has the effect of increasing the flexibility of adjacent single bonds (see Chapter 4) and overall this leads to a reduction in. Thus in the sequence from polydecenamer down to polypentenamer an increase in the double bond concentration leads to a lowering of Tg. On the other hand the Tg of polybutenamer, i.e. poly butadiene, is somewhat higher than that of polypentenamer, presumably because the proportion of stiff links, i.e. double bonds, becomes sufficiently high to override the flexibilising effect on adjacent chains. Consequently the polypentenamers have the lowest Tg values known for hydrocarbon polymers (cis- -114°C, trans- -97°C). 

Reduction of exhaust emissions is being tackled in two ways by engineers, including precombustion and postcombustion technology. One of the most effective methods now being researched and adopted includes use of synthetic fuel made from natural gas. This fuel is crystal clear, and just like water, it has no aromatics, contains no sulfur or heavy metals, and when used with a postcombustion device such as a catalytic converter any remaining NO, or other emissions can be drastically reduced. Estimates currently place the cost of this fuel at 1.50 per gallon, with availability in 2004 to meet the next round of stiff EPA exhaust emission standards. 

At very low frequencies the movement of the panel will be controlled by the stiffness, as inertia is a dynamic force and cannot come into effect until the panel has measurable velocity. Stiffness controls the performance of the panel at low frequencies until resonance occurs. As the driving frequency increases, the resonance zone is passed and we enter the mass-controlled area. The increase in the sound-reduction index with frequency is approximately linear at this point, and can be represented by Figure 42.8. 

Gold suppresses or prevents but, but does not cure, arthritis and synovitis. The therapeutic effects from gold compounds occur slowly. Early improvement is often limited to reduction in morning stiffness. The full effects of gold therapy are not known for 6 to 8 weeks or in some cases after 6 months of therapy. 

A development reported recently [519] involves reduction of the cystine disulphide bonds in wool with either thioglycolic acid or tetrakis(hydroxymethyl)phosphonium chloride to form thiol groups, followed by crosslinking with bifunctional reactive dyes. This gave improved insect resistance but had adverse effects on physical properties such as strength, shrinkage and stiffness, thus limiting the potential of the process for commercial use. 

The moduli of model polyurethane networks clearly show reductions below the values expected for perfect networks, with the reductions increasing with pre-gel intramolecular reactlon(5-7). The reductions can be shown to be too large to come solely from pre-gel loop forma-tion( ), some must occur post-gel. In addition, extrapolation to conditions of zero pre-gel intramolecular reaction, by increasing reactant concentrations, molar masses of reactants or chain stiffness, still leaves a residual proportion of inelastic chains due to gel-gel intramolecular reaction. It is basically a law-of-mass-action effect( ). The numbers of reactive groups on gel molecules are unlimited. Intramolecular reaction occurs, and some of this gives Inelastic chains. Only a small amount of such reaction has a marked effect on the modulus. 

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