33 b
Tensile Modulus, E
E MPa = Tensile Stress
σ
Tensile Strain
ε
3.3.6 Impact Test
Impact testing is designed to measure the performance of an object under high-rate loading. It can be a method for evaluating the toughness and
notch sensitivity of engineering materials. It is usually used to test the toughness of metals, but it also used for polymers, ceramics and composites.
The notched test specimen is broken by the impact of a heavy pendulum or hammer, falling at a predetermined velocity through a fixed distance. The
test measures the energy absorbed by the fractured specimen. Generally, impact testing can be classified into two categories. There are low
velocity and high velocity impact test. Low velocity impact test is generally done by using a drop-hammer or swinging pendulum involving a relatively
large mass. While for the high velocity impact, it is using a gas gun or some other ballistic launcher which is mainly concern with small masses. For low
velocity impact testing, there have two types of testing commonly used which are the Izod and Charpy test. They have been used to measure the
impact performance of materials particularly with respect to brittle or ductile transition temperature and notch sensitivity. For Izod test, it is consists a
rectangular or square cross section bar of specified dimensions is clamped at one end and struck towards the top of the test piece with a pendulum. For
this project, it will focus on Charpy test only. It was realized that the result of tensile test could not predict a fracture mode
for example under some circumstances normally ductile materials fracture abruptly with very little plastic deformation. So, impact test were chosen to
represent those most severe relative to the potential for fracture namely is
34 deformation at a relatively low temperature, a high strain rate, and a triaxial
stress rate. In addition, the notched test specimen is broken by the impact of a heavy pendulum or hammer, falling at a predetermined velocity through a
fixed distance in impact testing.
3.3.7 Charpy Test
The Charpy impact test is a standardized high strain-rate test of standard notched specimen which determines the amount of energy absorbed by a
material during fracture. This absorbed energy is a measure of a given materials toughness and acts as a tool to study temperature-dependent
brittle-ductile transition. Materials are used to build load-bearing structures. It is important to know if the material will survive the conditions that the
structure will see in service. Important factors that adversely affect the toughness of a structure include low-test temperatures, extra loading and
high strain rates due to over pressurization or impacts and the effect of stress concentrations such as notches and cracks. These all tend to encourage
fracture.
In certain extent, the complex interaction of these factors can be included in the design process by using fracture mechanics theory. The impact test is a
method for evaluating the relative toughness of engineering materials. The Charpy impact test continues to be used nowadays to assess the notch
sensitivity and impact toughness of engineering materials. It is usually used to test the toughness of metals. Similar tests can be used for polymers,
ceramics and composites.
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