Polymers and Composite Materials
Polymer materials in their basic form exhibit a range of characteristics and behavior from elastic solid to a viscous liquid. These behavior and properties depend on their material constituents, their structure, temperature, frequency and time scale at which the material or the engineering component is analyzed. The viscous liquid polymer is defined as by having no definite shape and flow. Deformation under the effect of applied load is irreversible. Elastic materials such as steels and aluminum deform instantaneously under the application of load and return to the original state upon the removal of load, provided the applied load is within the yield limits of the material. An elastic solid polymer is characterized by having a definite shape that deforms under external forces, storing this deformation energy and giving it back upon the removal of applied load.
polymer resins consist of long polymer molecules which may or may not have side chains attached to them. The
side chains are not linked to other polymer molecules as shown in Figure(1).
Thus there is an absence of cross-links in the thermo- plastic structure.
Thermoplastic resins in a granular form can be repeatedly melted or solidified by heating and cooling. Heat softens or melts the material so that it can be molded.
Cooling in the mold solidifies the material into a given shape. There are two
types of thermoplastic polymers,
Crystalline and Amorphous. Following list enumerates the features and
properties of both the polymer types.
Figure 1: Chains in Thermoplastic Polymers
- Crystalline solids break along
particular points and directions.
- Crystalline solids have an ordered structural pattern of
- Crystalline solids flow well at
a higher temperature.
- Reinforcement with fibers in crystalline
polymers increases the load-bearing capabilities.
- Crystalline polymers tend to
shrink more than amorphous.
- The molecular structure of crystalline polymers
makes them more suitable for opaque
parts and components.
- Examples: Polyethylene,
Polypropylene, Nylon, Acetal, Polyethersulfone, etc.
- Amorphous solids break into uneven parts with ragged edges.
- Amorphous solids have
a random orientation of molecules with no proper
geometrical or pattern formation.
- Amorphous solids do not flow as easily
and can give problems
in mold filling.
- Examples: ABS, Polystyrene, Polycarbonate, etc.
Figure (2) shows the general types and classification of polymers.
Figure 2: Types of Polymers and Their Classification
The need to improve the mechanical properties of polymers drives the development of various composites. Composites express a mechanical behavior signiﬁcantly different from that of conventional materials. They provide high load carrying capability, high stiffness to weight ratio and tolerance to damage from water, specific industrial oils, greases etc.
Composite materials are
engineered or naturally occurring materials made from two or more constituent
materials. The properties of the constituent materials are mostly significantly
different. The physical, mechanical and chemical properties remain separate and
distinct within the finished material structure. Most composites are made with
stiff and tough fibres in a polymer matrix. The polymer matrix is weaker and acts
more as a binder and parent material. The objective is usually to come up with
a material structure which is strong and stiff able to carry heavy loads. Commercial
grade composite materials mostly have glass or carbon fibres in a matrix of
thermosetting polymers like epoxy, nylons and polyester based resins. Glass
fibres are the most frequently used reinforcing fibres in reinforced polymers.
The mechanical characteristics which are predominantly improved by these fibres
are tensile and compressive strength. In addition, thermal dimensional
stability also increases. Thermoplastic
polymers are preferred as the matrix material where the end goal is to make
moldable parts and components. Glass filled nylon and other polymers offer good
mechanical, chemical at a lower cost. Fibre-Reinforced Polymer (FRP), is a
composite material made of a polymer matrix reinforced with fibres. These
fibres are usually glass or fibres. FRPs are commonly used in the aerospace,
automotive, marine, and construction industries.
materials also employ continuous ﬁber reinforcements in the form of a ply.
Figure 3 shows two types of such plies where unidirectional ﬁbers and woven
fabric bundles are laid out. These plies are impregnated by a polymer resin to
form a ply structure. For most composites, the ply is the basic building block
as a lamina structure. This lamina may be a unidirectional prepreg, a fabric, or
a strand mat.
3: Unidirection and Woven Fabric Composites
Mechanical and Physical
The mechanical and physical testing of polymers and their composites is important to determine the material properties. These properties help us understand the deformation characteristics and failure modes which can further be used in design and analysis of end products. The mechanical and physical testing ensure that material complies with performance requirements in accordance with industrial speciﬁcations, especially to the demanding aerospace, automotive, consumer, medical industries. Mechanical testing of polymeric composites involves the determination of mechanical parameters such as strength, stiffness, elongation, fatigue life etc., to facilitate its use in the design of structures.
The mechanical testing of composite materials involves a range of test types and standards like ASTM, ISO, EN etc., along with testing conditions in different environments.
most common mechanical properties such as Modulus of Elasticity, Poisson’s
ratio, Tensile strength, and Ultimate tensile strain for composites are
obtained from tensile testing and these properties are affected by the geometry,
size and properties of the reinforcements.
The Modulus of Elasticity and Poisson’s ratio are determined by
measuring the strains during the elastic deformation part of the test,
typically below the strain levels of 0.5%.
Uniaxial Tension Test
4: Uniaxial Tension Test on a Material Sample as per ASTM D638
The stress (σ) in a uniaxial tension test is calculated from;
σ = Load / Area of the material
The strain(ε) is calculated from;
ε = δl (change in length) / l1
(Initial length) ……………………………………..(2)
slope of the initial linear portion of the curve (E) is the Young’s modulus and
E = (σ2- σ1) / (ε2- ε1) ……………………………………..(3)
3 Point Bend Flexure
Test (ASTM D790)
point bending testing is done to understand the bending stress, flexural stress
and strain of composite and thermoplastic materials. The specimen is loaded in
a horizontal position, and in such a way that the compressive stress occurs in
the upper portion and the tensile stress occurs in the lower portion of the
cross section. This is done by having round bars or curved surfaces supporting
the specimen from underneath. Round bars or supports with suitable radius are
provided so as to have a single point or line of contact with the specimen.
Figure 5: 3 Point Bend Test Setup
at AdvanSES as Per ASTM D790
load is applied by the rounded nose on the top surface of the specimen. If the
specimen is symmetrical about its cross section the maximum tensile and
compressive stresses will be equal. This test fixture and geometry provides
loading conditions so that specimen fails in tension or compression. For most
composite materials, the compressive strength is lower than the tensile and the
specimen will fail at the compression surface. This compressive failure is
associated with the local buckling (micro buckling) of individual fibres.
4 Point Bend Flexure
Test (ASTM D6272)
four-point flexural test provides values for the modulus of elasticity in
bending, flexural stress, flexural. This test is very similar to the
three-point bending flexural test. The major difference being that with the
addition of a fourth nose for load application the portion of the beam between
the two loading points is put under maximum stress. In the 3 point bend test
only the portion of beam under the loading nose is under stress.
Figure 6: 4 Point Bend Test Setup
at AdvanSES as per ASTM D6272
arrangement helps when testing high stiffness materials like ceramics, where
the number and severity of flaws under maximum stress is directly related to
the flexural strength and crack initiation in the material. Compared to the
three-point bending flexural test, there are no shear forces in the four-point
bending flexural test in the area between the two loading pins.
Poisson’s Ratio Test as
per ASTM D3039
ratio is one of the most important parameter used for structure design where
all dimensional changes resulting from application of force need to be taken
into account. For this test method, Poisson’s ratio is obtained from strains
resulting from uniaxial stress only. ASTM D3039 is primarily used to evaluate
the Poison’s ratio.
Figure 7: Poisson’s Ratio Test
Setup as per ASTM 3039 at AdvanSES
is performed by applying a tensile force to a specimen and measuring various
properties of the specimen under stress. Two strain gauges are bonded to the
specimen at 0 and 90 degrees to measure the lateral and linear strains. The
ratio of the lateral and linear strain provides us with the Poisson’s ratio.
The compressive properties
of materials are important when the product performs under compressive
Figure 8: Flatwise Compression
Test Setup as per ASTM C365 at AdvanSES
conditions. The testing is carried out in the direction normal to the plane of
facings as the core would be placed in a structural sandwich construction.
test procedures pertain to compression call for test conditions where the
deformation is applied under quasi-static conditions negating the mass and
D6641 is the testing specification that determines compressive strength and
stiffness of polymer matrix composite materials using a combined loading
compression (CLC) test fixture. This test procedure introduces the compressive
force into the specimen through combined shear end loading.
Figure 9: Combined Loading
Compression Setup with Unsupported Gauge Length
ASTM D6641 includes two
procedures; Procedure A: to be used with untabbed specimens such as fabrics,
chopped fiber composites, laminates with a maximum of 50% 0° plies. Procedure
B: is to be used with tabbed specimens having higher orthotropic properties
such as unidirectional composites. The use of tabs is necessary to increase the
load-bearing area at the specimen ends.
ASTM D7791 describes the determination of dynamic
fatigue properties of plastics in uniaxial loading conditions. Rigid or
semi-rigid plastic samples are loaded in tension (Procedure A) and rigid plastic samples are loaded in
compression (Procedure B) to determine the effect of processing, surface
condition, stress, and such, on the fatigue resistance of plastic and
reinforced composite materials subjected to uniaxial stress for a large number
of cycles. The results are suitable for study of high load carrying capability
of candidate materials. ASTM recommends a test frequency of 5 hz or lower.The
tests can be carried out under load or displacement control.
Figure 10: Axial
Fatigue Samples under Test at AdvanSES as per ASTM D7791
The test method allows generation of a stress or
strain as a function of cycles, with the fatigue limit characterized by failure
of the specimen or reaching 107 cycles. The 107 cycle
value is chosen to limit the test time, but depending on the applications this
may or may not be the best choice. The maximum and minimum stress or strain
levels are defined through an R ratio. The R ratio is the ratio
of minimum to maximum stress or displacement that the material is cycled
through during testing. For this standard, samples may be loaded in either
tension or compression.
variety of standardized mechanical tests on composite materials including
tension, compression, flexural, shear, and fatigue have been discussed. These
mechanical properties of polymers, fiber-reinforced polymeric composites immensely
depend on the nature of the polymer, fiber, plies, and the fiber-matrix
interfacial bonding. Advanced engineering design and analysis applications like
Finite Element Analysis use this mechanical test data to characterize the
materials. Second part of the paper will show the use of these mechanical
characterization tests in FEA software like Ansys, Abaqus, LS-Dyna, MSC-Marc
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