Flexural work is used to assess the resistance or bending properties of a material. In a test referred to as a transverse beam test but also referred to as a three-point bend or four-point bending test, the sample is positioned between two points or supports. The composite’s interlaminar strength gives very high resistance to the bending load applied during a bending operation. The ASTM D790 model is used for flexural testing of polymer-based composites .
Flexural strength is the property of the composites that reflects the interlaminar shear strength. This research can be conducted on a UTM computer like that used for the tensile testing. The flexural strength and modulus can be calculated using standard relations. The flexural stress, of is calculated by :
Equations 8.6 and 8.7 are used for rectangular and circular cross sections, respectively.
The flexural strain, ef is calculated by:
The flexural modulus of elasticity (MPa), E, is calculated by: where,
F is the load at a given point on the load deflection curve (N)
L is the support span (mm)
b is the width of the test beam (mm)
d is the depth or thickness of the test beam (mm)
D is the maximum deflection of the center of the beam (mm) m is the gradient (i.e., the slope) of the initial straight-line portion of the load deflection curve (N/mm)
R is the radius of the beam (mm)
Impact strength is the capability of the material to withstand a suddenly applied load and is expressed in terms of energy. It represents the material’s behavior during impact at high speed. Impact behavior is the most important mechanical property of a material. Impact strength is tested through ASTM D256 using a testing sample of dimensions 127 mm x 13 mm x 3 mm. The unit of impact strength is J/cm. There are two types of impact test, Izod and Charpy methods .
Owing to its low cost and robustness Charpy has been used for several years to test composite material. In the Charpy method, the specimen is put in a vise so that at the ends it serves as a beam. Likewise, the specimen is put in a vise in the Izod test, so that its one end is free and behaves like a cantilever plate[ 14].
Composite materials may fail if the energy absorption capacity of the material is not sufficient to withstand the energy being transferred through the application of the impact charge. This test shows the ability of a material to absorb energy during impact on load on the specimen. Although the toughness of a material can be evaluated by area under the stress-strain diagram, an indication of relative toughness comes from the impact test. Usually notch-type specimens are used for impact studies. Two types of notches are used to test bending effect: V-note and keyhole note.
Hardness is another essential mechanical property to note. It measures part resistance to localized plastic deformation; for example, a slight dent or scrape. Hardness testing is widely used to inspect and verify composites. The idea of any hardness test method is to force an indenter into the sample surface, then measure the indentation (depth or actual indentation area). Hardness is not a fundamental property and the combination of yield force, tensile strength, and elasticity modulus determines its value. Depending on the magnitude of the loading force and the dimensions of the indentation, toughness is defined as macro-, micro-, or nano-hardness .
The macro-hardness tests (Rockwell, Brinell, Vickers) are the most widely used methods for fast, routine hardness measurements. The indenting forces for macrohardness measurements are in the range from 50 to 30,000 N .
Plastic hardness is most measured via the Shore® (Durometer) or Rockwell hardness test. Both methods measure the indentation resistance of a plastic and have an objective hardness value that does not inherently fit well with certain properties or fundamental characteristics. Shore A or Shore D is used, which is also commonly used for softer plastics such as polyolefins, fluoropolymers, and vinyl. The Shore A scale is used for the softer rubbers whereas Shore D is used for firm ones. There are many other Shore hardness levels, such as Shore О and Shore H, but most plastics engineers rarely encounter them. The Shore hardness is measured using a Durometer proven tool, the indenter foot of which penetrates the sample, and is thus also known as Durometer hardness . The indentation reading may change over time, so the indentation time and the hardness number are often registered. During hardness testing of composite materials based on polymers, the ASTM D785 standards are typically followed. Compared with other mechanical examinations, some advantages of the hardness test are as follows:
They are simple and cheap to perform - usually, no special specimen needs to be prepared .
They are non-destructive - the specimen is neither fractured nor excessively deformed.
The various hardness tests may be divided into three categories:
Resistance to cutting or abrasion.
Resistance to indentation.
Industrial Application of Mechanical Characterization Techniques
Mechanical characterization testing is considered by academicians and researchers in the polymer and polymer composite-based industries as one of the most prominent, and popularly preferred methods for evaluating strength, rigidity, etc. Product research includes a wide range of high-performance, specialized industrial applications including textiles, packaging, building, aerospace, automotive, and many other products .
The strength, rigidity, and resilience of polymer composites is important to evaluate. All mechanical characterization services meet standardizations and common requirements. Mechanical testing includes tensile, flexural, and impact properties, giving a simple insight into the ability of materials to withstand sudden failure under the load or stress applied . Nevertheless, the mechanical properties of the polymer composite materials depend on the quality of the fiber and polymer and the interfacial bond between the fiber and the matrix. This chapter covers various technical examinations available.
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