Susan Zachariah2ND M.Sc BPSCBPST, KOCHI

Introduction Fibre reinforced composite materials have been used for quite a long time in

highly demanding applications, where static and/or dynamic loads are applied in the presence of environmental loading (e.g.temperature, water or other aggressive liquid).

More recently, the use of composite materials was extended for primary structures in aircrafts, automotive applications, infrastructures, rebars and rehabilitation/strengthening of bridges and even complete bridges.

This facts brings the issue of durability, hence prediction of long term properties and residual life, as a determinant factor in the success of the referred applications.

However, when organic fibres are used (e.g. aramidic fibres,UHMWPE), the variation of fibre properties with time is also a factor to be considered in the prediction of long term properties of composite materials.

The long-term behaviour of composite materials may be affected by physical (e.g.changing in Tg) and chemical ageing (change in molecular weight, oxidation, change in density of reticulation).

In fact, the swelling, the plasticization and the slow hydrolysis (with scission) of the resin, and the slow attack of the liquid to the fibre/resin interface correspond to loss of properties with influence in the creep and fatigue behaviour of composite materials.

In the actual service conditions the mechanical loads act in combination with the environmental factors, like moisture, temperature and radiation.

The understanding of those phenomena and the modelling of the time property evolution is a crucial task for longterm durability analysis of a polymer-based composite materials.

Effects of Surface Preparation on the Long-Term Durability of Adhesively Bonded Composite Joints

The surface preparation of a bonded joint is key to its strength and long-term durability.

High strength primary bonds include covalent and ionic bonds that are conducive to long-term durability, especially in the presence of moisture.

They are more difficult to form but stronger. Secondary bonds (polar, Vander Waals and hydrogen bonds) are weak interactions that break and reform easily, resulting in poor adhesion.

The crack propagates along one of the interfaces, an undesirable failure, as the bond is the weak link and the joint will likely have poor long-term durability; these types of cracks often jump from one interface to the other.

Static wedge tests provided long-term durability data in a relatively short period of time.

STATIC WEDGE.

Instead of loading specimens until failure, they are wedged open at a constant displacement and placed in an environment that encourages crack growth.

The static wedge test requires fewer pieces of specialized equipment than any other test. Consequently, it is generally used as a simple pass-fail or comparison test instead of for quantitatively evaluating bonds.

For rapid fracture surface feedback without durability evaluation, the wedge can simply be forced entirely through the sample (dubbed the non-instrumented hammer and wedge test).

A constant imposed strain ε0 results in a drop in stress σ(t) as a function of time.

Stress Relaxation

Stress relaxation in carbon nanotube-based fibers for

load-bearing applicationsTensile stress relaxation tests The specimen fabrication technique and the testing equipment used in the tensile stress relaxation experiments were the same as those used in quasi-static tensile measurements. The ultimate strain values of both pure and composite fibers, obtained from the quasi- static tensile tests, were used as the reference strain level to determine the applied strain in the relaxation tests. To investigate the effects of the initial strain level, strain rate and gauge length on the tensile relaxation behavior of CNT fibers, specimens with two gauge lengths, namely, 7 and 15 mm, were deformed to several strain levels, namely, 0.5%, 1.0%, 1.5% and 2.0% at different strain rates, namely, 5.5 · 105, 5.5 · 104 and 5.5 · 103 s1. Upon reaching the predetermined initial strain (e0), the specimen was then held at this strain level, and the force (F) needed to sustain the constant strain was monitored and recorded by a digital camera for a time span of 1 h. Specimens of single carbon fiber were also tested for comparison. All stress relaxation tests were performed at room temperature.

The results of stress relaxation were plotted with the stress ratio (rt/r0) obtained from the stress at a specific time (rt) divided by the maximum stress (r0) when the required strain was attained versus log time; or the stress relaxation modulus at a particular time (Et = rt/e0) against the logarithm of time.

Both the pure CNT fiber and the CNT/epoxy composite fiber exhibited significant stress decay during the relaxation process, whereas no obvious stress relaxation was

observed in the case of the single carbon fiber

Fig:Comparisons of relaxation behavior of carbon fiber, pure CNT fiber and CNT/epoxy composite fiber

NOTE:

STRESS RELAXATION OF WOOD COMPOSITES

Ken Youssefi Mechanical Engineering Dept. 16

Engineering Applications: Composite materials have been used in aerospace, automobile, and marine applications (see Figs. 1-3). Recently, composite materials have been increasingly considered in civil engineering structures. The latter applications include seismic retrofit of bridge columns (Fig. 4), replacements of deteriorated bridge decks (Fig. 5), and new bridge structures (Fig. 6).

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Figure 1 Figure 2 Figure 3

Figure 4 Figure 5 Figure 6

Medical Applications: Stents are made with steel and more recently with polymers with shape memory effects (Wache, et al.).

The material is deformed within a temperature range of glass transition temperature (Tg) of amorphous phase and melting temperature (Tm) of crystalline phase, then was cooled below Tg. After the material was reheated between Tg and Tm, the original structural shape was recovered. High dosage (up to 35% by weight) and at a high rate of release of medication were noted in this study.

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Application of CompositesApplication of Composites

Ken Youssefi Mechanical Engineering Dept. 20

Pedestrian bridge in Denmark, 130 feet long

(1997)

Swedish Navy, Stealth (2005)

Lance Armstrong’s 2-lb. Trek bike, 2004 Tour de France

Application of Composites in Application of Composites in Aircraft IndustryAircraft Industry

Ken Youssefi Mechanical Engineering Dept. 21

20% more fuel efficiency and 35,000 lbs. lighter

High speed fan blades;

High performance racing body parts;

Hydroxy apatite composite

Nano medicine

Specific advantages of Specific advantages of nanoclays in medical nanoclays in medical devices and packagingdevices and packaging

Controlled permeation rates of therapeutic agents in a device Controlled degradation behaviour of devices, packaging [e.g

tissue scaffolds, shedding of surface biofilms from tubing] Better high-temperature performance and thus improved

performance in sterilisation of packs/devices Extended property range of medical polymers