An overview of mechanical and
physical testing of composite
materials
1
N. Saba 1 , M. Jawaid 1 , M.T.H. Sultan 1,2,3
1
Laboratory of Biocomposite Technology (BIOCOMPOSITE), Institute of Tropical Forestry
and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Malaysia; 2Aerospace
Manufacturing Research Centre (AMRC), Level 7, Tower Block, Faculty of Engineering,
Universiti Putra Malaysia, Serdang, Malaysia; 3Department of Aerospace Engineering,
Universiti Putra Malaysia, Serdang, Malaysia
1.1
Introduction
The need to improve the mechanical properties of polymers drives the development of
the glass fiberereinforced polymers as first composite parts to be used for radar domes,
boat hulls, and car body sections through pultrusion or vacuum bag molding [1]. Composites express a mechanical behavior significantly different from that of conventional
materials, such as metals, owing to their nature. Therefore, other standards designed for
directly evaluating the bulk mechanical properties of composites are usually preferable
for tensile, compressive, and shear tests [1]. Composite materials especially lightweight
composite structures are being used in an ever-increasing variety of products and applications, such as aerospace, construction, ground transportation, and environmentally
sustainable energy systems that immensely need mechanical and physical testing of
components and structures of composite materials prior their applications. Thus the
full characterization of the properties of anisotropic and inhomogeneous composite materials, for use in demanding structural applications, requires a wide range of mechanical
tests. The mechanical testing of composite materials involves a range of test types and a
plethora of standards (ASTM, ISO, CEN), along with testing conditions in a variety
of different environments: https://www.qualitymag.com/articles/91960-mechanicaltesting-of-composites. In addition, auditing bodies such as NADCAP dictate performance criteria, for example, alignment for the testing equipment. Research studies
claimed that the most common mechanical properties such as modulus of elasticity,
Poisson’s ratio, tensile strength, and ultimate tensile strain for fiber-reinforced polymer
unidirectional composites can easily be obtained from tensile testing along the fiber
direction [2]. In another study, it has been reported that mechanical properties, namely,
tensile modulus, tensile strength, and fracture toughness, are affected by the geometry of
the particles [3]. Modulus of elasticity and Poisson’s ratio were determined
by measuring the strains during the initial stage of the test through strain gauges or
extensometers. Loading is made to continue to ultimate failure and the point at which
Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites and Hybrid Composites
https://doi.org/10.1016/B978-0-08-102292-4.00001-1
Copyright © 2019 Elsevier Ltd. All rights reserved.
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Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites
tensile strength and ultimate tensile strain are determined. The modulus of elasticity and
Poisson’s ratio properties are measured at load levels well below the point of failure,
typically corresponding to strain levels between 0.1% and 0.3%. However, the tensile
strength and ultimate tensile strain values are the challenges, and they become more
difficult to obtain as the specimen’s tensile strength increases [2].
1.2
Mechanical and physical testing
The mechanical and physical testing of polymers and their composites is vital to
determine the material properties for use in design and analysis of the product, quality
control, application performance requirements, and production process. The mechanical
and physical testing ensure the material complies with performance requirements
in accordance with industrial specifications, especially to the aerospace, automotive,
consumer, medical, and defense industries [4]. Mechanical testing of polymer composites involves the determination of mechanical parameters such as strength and stiffness
to investigate its use for the design of a composite’s structure. The most common
standardized mechanical testing of polymer composites includes tensile (tension),
flexural, impact [5], shear, and compression with open and closed holes, and the
physical testing includes water absorption, density, void content, hardness, and scratch
resistance. Researchers also conducted the compression, shear, and interlaminar strength
for the determination of proper parameters of the failure criterion model involving
ultimate strength and failure prediction of composite samples: https://www.aacresearch.at/en/mechanische-pruefung-2. Beside these, many other standardized bearing
strength tests according to ASTM D 5961 and interlaminar fracture toughness tests
to ASTM D 5538 are reported as mechanical tests on composites: https://www.
qualitymag.com/articles/91960-mechanical-testing-of-composites.
1.2.1
Tensile test
Tensile testing is a destructive test process that provides information about the tensile
strength, yield strength, and ductility of the metallic material. It measures the force
required to break a composite or plastic specimen and the extent to which the specimen
stretches or elongates to that breaking point. Tensile testing of composites is generally in
the form of basic tension or flat-sandwich tension testing in accordance with standards
such as ISO 527-4, ISO 527-5, ASTM D 638, ASTM D 3039, and ASTM C 297.
Such tests produce stress-strain diagrams used to determine tensile modulus. Tensile
testing is presented in Fig. 1.1.
Tensile testing also provides tensile strength (at yield and at break), tensile
modulus, tensile strain, elongation, and percent elongation at yield, elongation, and
elongation at break in percent http://www.intertek.com/polymers/tensile-testing/.
In-plane tensile testing of plain composite laminates is the most common test. Tensile
tests are also performed on resin-impregnated bundles of fibers (“tows”), through
thickness specimens (cut from thick sections of laminates), and sections of sandwich
core materials: https://www.qualitymag.com/articles/91960-mechanical-testing-of-
An overview of mechanical and physical testing of composite materials
3
Figure 1.1 Tensile testing of plastics and composites.
composites. Alignment is critical for composite testing applications because composites are anisotropic and generally brittle, as the anisotropy means that the properties
and strength of the material differ depending on the direction of the applied force or
load. Thus, the tensile strength of a composite material is very high in the direction
parallel to the fiber orientation, while the tensile strength is much lower if tested
in any other direction. Interestingly, to determine maximum tensile strength in the
direction parallel to the fiber direction, the tensile test must have superior axialload-string alignment, primarily critical in the aerospace industry, where composites
are often applied in high-tensile-stress structures. Currently, a range of proven gripping
mechanisms including manual, pneumatic, and hydraulic actuation is available for
ambient, subambient, and high-temperature testing, ranging between 269 and
600 C: https://www.aac-research.at/en/mechanische-pruefung-2/. Obtained test data
specify the optimal materials, design parts to withstand application forces, and provide
key quality control checks for materials.
1.2.2
Flexural test
As the physical properties of many materials (especially thermoplastics) can
vary depending on ambient temperature, it is sometimes appropriate to test materials
at temperatures that simulate the intended end use environment. The flexural test
measures the force required to bend a beam under three-point loading conditions,
and it is generally applicable to both rigid and semirigid materials, resins, and
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Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites
laminated fiber composite materials [6]. The data is often used to select materials for
parts that will support loads without flexing. The most common flexural testing of plastics, polymer composites, and large fiber-reinforced plates involves three-point and
four-point bend testing according to ISO 14,125, ISO 178, ASTM D 790, and
ASTM D 6272 to ensure suitability under various conditions for better insight into
their properties and to ensure that they are suitable for the intended application. A variety of specimen shapes can be used for this test, but the most commonly used specimen size for ASTM is 3.2 mm 12.7 mm 125 mm (0.12500 0.500 5.000 ), the
rectangular samples of dimension 160 mm 20 mm 8 mm 10 mm 1 mm for
ASTM D790 [7], and for ISO, it is 10 mm 4 mm 80 mm.
Most commonly in the flexural test, the specimen lies on a support span, and the
load is applied to the center by the loading nose producing three points bending at
a specified rate. The parameters for this test are the support span, the speed of the
loading, and the maximum deflection for the test. These parameters are based on the
test specimen thickness and are defined differently by ASTM and ISO. For ASTM
D790, the test is stopped when the specimen reaches 5% deflection or the specimen
breaks before 5%, but for ISO 178, the test is stopped when the specimen breaks. If
the specimen does not break, the test is continued as far as possible, and the stress
at 3.5% (conventional deflection) is reported.
Flexural testing also gives a semiqualitative idea of the fiber/matrix interfacial
strength of a composite [6]. Flexural properties testing provides editable and raw
data on flexural stress at yield, flexural strain at yield, flexural stress at break, flexural
strain at break, flexural stress at 3.5% (ISO) or 5.0% (ASTM) deflection, flexural
modulus, and stress/strain curves. Flexural modulus is used as an indication of a
material’s stiffness when flexed: http://www.intertek.com/polymers/testing/flexuralproperties/.
1.2.3
Impact test
The impact test is designed to determine how a specimen of a known material such as
polymers, ceramics, and composites will respond to a suddenly applied stress.
The impact test is explicitly used for evaluating the toughness, brittleness, notch
sensitivity, and impact strength of engineering materials to resist high-rate loading
[8,9]. The ability to quantify the impact property is a great advantage in product liability and safety. Impact test specimen types include notch configurations such as
V-notch, U-notch, and keyhole notch. Impact testing most commonly consists of
Charpy and Izod specimen configurations. The Izod impact test differs from Charpy
impact test in the way that the notch is positioned facing the striker. Thus in the
Charpy test, the test piece is held horizontally between two vertical bars, but in the
Izod test, the specimen stands erect, like a fence post.
However, in the keyhole impact test, the notch, which is machined to look like a
keyhole, is usually applied by the steel casting industries and is tested in the same
manner as the “V” and “U” notch. The V, U, and key notch are presented in Fig. 1.2.
Keyhole impact testing is usually performed where the material thickness is restricted
and is tested down to cryogenic temperatures: http://www.wmtr.com/en.charpy.html.
An overview of mechanical and physical testing of composite materials
5
Figure 1.2 Different notches of impact test: http://www.wmtr.com/en.charpy.html.
1.2.3.1
Charpy impact
The Charpy impact test was invented in 1900 by Georges Augustin Albert Charpy
(1865e1945), and it is regarded as one of the most commonly used test to evaluate
the relative toughness of a material in a fast and economic way. The Charpy impact
test measures the energy absorbed by a standard notched specimen while breaking
under an impact load. This test continues to be used as an economical quality control
method to determine the notch sensitivity and impact toughness of engineering materials such as metals, composites, ceramics, and polymers. The standard Charpy impact
test specimen is of dimension 55 mm 10 mm 10 mm, having a notch machined
across one of the larger dimensions, as illustrated in Fig. 1.3. The Charpy impact
test measures the energy absorbed by a standard notched specimen while breaking
under an impact load [10]. This test consists of striking a suitable specimen with a
hammer on a pendulum arm while the specimen is held securely at each end. The
hammer strikes opposite the notch. The energy absorbed by the specimen is determined
precisely by measuring the decrease in motion of the pendulum arm. The important
factors that affect the toughness of a material include low temperatures, high strain rates
(by impact or pressurization), and stress concentrators such as notches, cracks, and
voids: http://www.wmtr.com/en.charpy.html.
1.2.3.2
Izod impact
The Izod impact test was named after English engineer Edwin Gilbert Izod. The Izod
impact test is like the Charpy impact test and is used to test materials at low temperature:
Figure 1.3 Charpy impact test.
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Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites
Figure 1.4 V-notch maker and impact testing machine [8].
http://www.wmtr.com/en.izod.html. In this test, a specimen is machined to a square
or round section, with either one, two, or three notches that have a dimension
of 70 mm 15 mm 3 mm [8]. The Izod impact test consists of a pendulum with a
determined weight at the end of its arm swinging down and striking the specimen while
it is held securely in a vertical position [11]. The V-notch maker and impact test machine
are displayed in Fig. 1.4 [8]. The impact strength is determined by the loss of energy of
the pendulum as determined by precisely measuring the loss of height in the pendulum’s
swing [9]. Researchers also defined impact strength as the tendency of polymer composites to endure high-energy impact without breaking or fracturing. They also reported that
in fiber-reinforced polymer composites and hybrid composites the impact properties are
governed by the properties of the individual fibers used for hybridization, interlaminar,
and interfacial adhesion between the fiber and the matrix [8].
1.2.4
Compression test
Composite compression testing methods provide a means of introducing a compressive load into the material while preventing it from buckling. Compression tests are
performed for composite materials that are in the form of a relatively thin and flat rectangular test specimen such as laminate panels. Compression testing was conducted for
polymers, composites, and elastomers and can also be conducted on plain or “open/filled hole” specimens. Remarkably, compression testing determines behavior of materials under a crushing load, and the compression and deformation at various loads is
recorded to calculate compressive stress and strain. In general, there are three methods
of introducing a compressive load into a test specimen:
•
•
End loading: all of the load is introduced into the flat end of the test specimen.
Shear loading: the load is introduced into the wide faces of the test specimen.
An overview of mechanical and physical testing of composite materials
•
7
Combined loading: a combination of shear and end loading is used: https://www.qualitymag.
com/articles/91960-mechanical-testing-of-composites.
The most common testing standards include ASTM D 695, ASTM D 3410, and
ISO 14,126. The resulting stress-strain diagram provides information on elastic limit,
proportional limit, yield point, yield strength, and compressive strength. Compression
fixtures are designed to meet the unique requirements of composite materials by
providing precise alignment and precision guidance to prevent buckling.
Another type of compression called “compression after impact (CAI)” is gaining
attention for showing considerable advances in damage-tolerant composites and also
the repeatability of composite performance such as the process of adding sheets
between plies and additives to the resin. CAI requires a drop tower to provide the
impact before a compression test is conducted on a testing machine. Most common
standards for CAI include Airbus AITM 1.0010, ASTM D 7136D 7137, SACMA
2R-94, and Boeing BSS 7260.
1.2.5
Bending test
The three-point as well as four-point bending tests are conducted for measuring the
deflection and bending strength of fiber-reinforced polymer plastics. The mechanical
parameters for the bending tests lies between 269 and 600 C, using the moving
coil extensometer or cross-head movement of the machine: https://www.aacresearch.at/en/mechanische-pruefung-2.
1.2.6
Interlaminar shear strength test
The interlaminar shear strength (ILSS) is another important mechanical test that provides information about the quality of the resin-fiber bond. The ILSS of unidirectional
laminates and carbon fiberereinforced plastics are usually determined through a threepoint bending test, where the resistance to interlaminar shear stress is kept parallel to
the layers of the laminate and is measured in accordance with DIN EN 2563: https://
www.aac-research.at/en/mechanische-pruefung-2.
1.2.7
Cryogenic test
Cryogenic tests are performed on materials that are chiefly designed for application in
space technology. In this test tensile, compression, shear, bending, and ILSS tests of
fiber-reinforced plastics are conducted in cryostat in liquid helium temperature close
to absolute zero (down to 4 degrees Kelvin): https://www.aac-research.at/en/
mechanische-pruefung-2.
1.2.8
Shear test
In some instances, physical properties of materials can vary depending on ambient temperature, so it is appropriate to test materials at temperatures that simulate the intended
8
Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites
end use environment: https://www.qualitymag.com/articles/91960-mechanical-testingof-composites. Shear tests are used to determine the attributes such as shear strain, shear
stress, shear modulus, and failure mode since the awareness of the "deformable"
mechanical properties of plastics and polymer composites is essential to extend their
applications. Shear testing can be used for quality control, comparative testing, and finite
element (FE) analysis of new materials. Shear strength results are important to design a
wide variety of materials like adhesives, plastics, films, and sheet products that tend to
be subjected to various "shear loads," or in applications where factors such as crushing
loads are a risk, and it is one of the key values used in FE analysis, such as by aerospace,
transportation, defense, and material manufacturers. Different materials such as homopolymers and polypropylene-based composites will behave differently in shear tests,
and at times, unpredictable results may occur: http://www.intertek.com/shear-testing/.
In-plane shear properties can be easily measured on a tensile test specimen with
a 45 degree fiber orientation. The specimen’s axial and transverse strain is
measured using either strain gauges or a biaxial extensometer. Standards for shear
test include ASTM D3518 and ISO 14,129. The ASTM C 273, ASTM D 5379,
ASTM D 4255, ASTM D 2344, and ISO 14,130 can also be met with an interlaminar
flexural and shear testing fixture: https://www.qualitymag.com/articles/91960mechanical-testing-of-composites.
1.2.9
Fatigue test
Fatigue and fracture testing is another important mechanical test that accommodates the
dynamic loads <1 up to 2500 kN. This test is an essential requirement of composite
materials, especially in demanding applications such as aerospace and wind power.
In this test the load frames need to provide the high stiffness along with exceptional
alignment that composite testing demands.
1.3
Physical test
1.3.1
Water absorption test
Water absorption, often called water absorption 24 hour/equilibrium, is the most
important physical test for the materials to be used for exterior applications under
ASTM D570: http://www.intertek.com/polymers/testlopedia/water-absorption-astmd570/. It is used to determine the amount of water absorbed under specified conditions.
Factors affecting water absorption behavior include the type of plastic, additives used,
temperature, and length of exposure. The results obtained give an insight on the performance of the materials in water or humid environments. Percent of water absorption
was calculated from Eq. (1.1) using ASTM D570 [10].
Water absorptionð%Þ ¼
Wn Wd
100
Wd
(1.1)
An overview of mechanical and physical testing of composite materials
9
where Wn is the weight of composites samples after immersion, and Wd is the weight of
the composite samples before immersion.
For the water absorption test, the specimens are dried in an oven for a specified time
and temperature and then placed in a desiccator to cool. Immediately upon cooling,
the specimens are weighed. The material is then emerged in water at agreed upon
conditions, often 23 C for 24 h, or until equilibrium, and then removed, patted dry
with lint-free cloth, and weighed carefully. The water absorption test is expressed as
an increase in weight-percent and is continued for several days until the constant
weight of the samples is obtained.
1.3.2
Density
Density is one of the most important factors in determining the properties of polymer
composites material [13] and is defined as mass of the material per unit volume,
measured as per standard ASTM D792-75 or ASTM D792-91 [14]. In the case of
fiber-reinforced polymer composites, its value mainly depends on the relative proportion of reinforcement and matrix [15]. The density of composite constituents (fibers
and matrices) is determined by weighing the specimen in air and then weighing it while
suspended on a wire and immersed in water, and the difference in water is noted. In the
case of the specimen having the density lower than that of water, a sinker is allowed to
attach with a wire to facilitate immersion. The density r is then calculated from Eq.
(1.2)
r¼
ð0:9975Þa
ða þ w bÞ
(1.2)
where a is the weight of the specimen in air, b is the total weight of specimen and
sinker completely immersed, while the wire is partially immersed, and w is the weight
of a fully immersed sinker but partially immersed wire. The density of a composite is
also determined in a similar way. In some cases, composite density is measured by
using ASTM D1895 standard, calculated by using Eq. (1.3)
m
Density g cm3 ¼
v
(1.3)
where m is the mass of the composites, and v is the volume of composites.
1.3.3
Void content
Void content is another physical property that needs to be analyzed in composites
according to ASTM D2734. The void content is determined from the theoretical
and experimental density of the composites through Eq. (1.4) [16,17].
Void Contentsð%Þ ¼
rtheoretical rexperimental
rtheoretical
(1.4)
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Mechanical and Physical Testing of Biocomposites, Fibre-Reinforced Composites
where
rtheoretical ¼ 1
W
f =r
f
þ Wm=r
m
Wf ¼ fiber weight fraction, Wm ¼ matrix weight fraction, rf ¼ fiber density, and
rm ¼ matrix density.
1.3.4
Rockwell hardness
The Rockwell hardness test is the most frequently used hardness testing method and is
considered to be more accurate and easier to perform than other hardness tests: [18]
http://www.wmtr.com/en.rockwellhardness.html. This test can be performed on all
metals and composites unless the size, shape, or surface conditions of the specimens
are prohibitive: http://www.wmtr.com/en.rockwellhardness.html. The hardness measurement based on the net increase in depth of impression as a load is applied and
is measured according to the ASTM D785 or ISO 2039. In this test a standard
specimen of dimension 6.4 mm in thickness is either molded or cut from a sheet
and is placed on the surface of the Rockwell hardness tester. In this test method, a minor load is applied, and the gauge is set to zero. The major load is applied by tripping a
lever. After 15 s the major load is removed. The specimen is allowed to recover for
15 s, and then the hardness is read off the dial with the minor load still applied. The
hardness of the specimen is read directly from the dial with the unitless R, L, M, E,
and K scales. These scales are called Rockwell hardness numbers and are directly
related to the indentation hardness of the plastic material (i.e., the higher the reading,
the harder the material). Notably, R and M scales are commonly used with plastics:
http://www.intertek.com/polymers/testlopedia/rockwell-hardness-astm-d785/.
Softer polymeric materials will give a wider range of variation in the surfaces, such
as molded surfaces that will give a higher reading than machined surfaces.
1.3.5
Scratch test
Scratch testing is carried out to get an insight into materials to determine the resistance
to abrasion and wear of modern composite materials and automotive paints varnishes,
as well as coatings. Single asperity scratching allows the mechanisms behind abrasive
or ductile damage of samples under investigation and the morphology of the scratches
to be examined with a combination of microscopy and profilometry techniques.
Scratch testing applications include commercial polymers, paint, and varnishes for
the automotive industry, multilayered systems, and application-related performances:
http://www.intertek.com/analytical-laboratories/scratch-testing/.
1.3.6
Industrial applications of mechanical and physical tests
Remarkably, in the polymer and polymer compositeebased industries, mechanical
and physical testing are regarded as one of the most accepted, dominant, and widely
An overview of mechanical and physical testing of composite materials
11
preferable tools by academics and researchers to evaluate strength, stiffness, rigidity,
density, shear, and water absorption tendency. A wide range of high-performance,
advanced industrial applications, including textiles, packaging, construction, bridges,
architecture, railways, aerospace, spacecraft avionics cooling systems, automotive,
ship buildings, sporting goods, flooring, paneling, insulating materials, refractory
lining, kitchen worktops, infrastructure, military, leisure boats, furniture, medicinal
products, plastics, and synthetic leatherebased products, involve mechanical and
physical testing of materials.
1.4
Conclusions
Mechanical and physical properties characterization has a great impact, as it is an
essential requirement for analyzing the strength and stiffness of polymer composites.
These properties follow specific standardization and guidelines. Mechanical tests
including tensile, flexural, and impact properties provide bright reflection on the
material’s ability to resist a sudden rupture or cleavage under applied stress or load.
Moreover, the mechanical properties of fiber-reinforced polymeric composites
immensely depend on the nature of the fiber, polymer, and fiber/matrix interfacial
bonding. Physical properties including water absorption, hardness, shear, void content,
and density also direct composite applications. The diverse advanced and engineering
applications based on mechanical and physical testing such as construction and building, furniture, automobile, railway coach interiors, electrical appliances, and pipes, as
well as storage devices including post boxes, grain storage silos, and bio-gas
containers were reported. This chapter highlights the importance of mechanical and
physical testing to provide valuable data and information of polymers and polymer
composites in several industries to the use of steel, cement and concrete-based polymer
composites.
Acknowledgments
The authors are grateful to Malaysian Industry-Government Group for High Technology
(MIGHT) for financial support of this work Under Newton-Ungku Omar Fund Grant No:
6300873 and 6300896.
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