The document discusses the long-term durability of composite materials. It notes that composite materials are increasingly being used in demanding applications that subject them to environmental factors over long periods of time. The document examines how physical and chemical aging processes like swelling, plasticization, hydrolysis, and oxidation can degrade the resin and fiber-resin interface, leading to losses in properties like creep and fatigue resistance over time. It also discusses how surface preparation affects the long-term durability of adhesively bonded composite joints.

1. Susan Zachariah
2ND
M.Sc BPS
CBPST, KOCHI

2. 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.

3. 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.

4. 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.

5. Static wedgetestsprovided long-term durability datain 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).

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

7. 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.

8. Both thepureCNT fiber and theCNT/epoxy compositefiber exhibited significant
stressdecay during therelaxation process, whereasno obviousstressrelaxation was
observed in thecaseof thesinglecarbon fiber
Fig:Comparisons of relaxation behavior of carbon fiber, pure CNT fiber and
CNT/epoxy composite fiber
NOTE:

10. STRESS RELAXATION OF WOOD COMPOSITES

16. Ken Youssefi Mechanical Engineering Dept. 16

18.  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).
18
Figure 1 Figure 2 Figure 3
Figure 4 Figure 5 Figure 6

19.  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.
19

20. 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

21. Application of Composites inApplication of Composites in
Aircraft IndustryAircraft Industry
Ken Youssefi Mechanical Engineering Dept. 21
20% more fuel efficiency
and 35,000 lbs. lighter

22. High speed
fan blades;

23. High performance racing body parts;

24. Hydroxy apatite
composite

34. Nano medicine

35. Specific advantages ofSpecific advantages of
nanoclays in medicalnanoclays 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