Chloe Zhang
Toughness is the ability of a material to absorb energy and deform plastically without fracturing. There are several ways to measure the toughness of plastic materials, including the Charpy and Izod impact tests. These tests involve using a pendulum to impact a secured sample, measuring the amount of energy absorbed by the sample. Another method is to perform a tensile test, where a specimen is pulled apart and the force on the cross-sectional area is plotted relative to elongation. The Gardner impact strength test is also used to measure the impact toughness of thinner sheets of material by dropping a weight on a thin specimen and examining it for penetration.
| Characteristics | Values |
|---|---|
| Toughness | The ability of a material to absorb energy and plastically deform without fracturing |
| Izod impact test | A quantitative test that measures the impact toughness of a thick (~12 mm) specimen of plastic |
| Charpy impact test | A similar test to the Izod impact test but with a slightly different setup; more commonly used with metals |
| Tensile test | A specimen is pulled apart by a pair of grips; the force on the cross-sectional area of the part, or stress, is plotted relative to the elongation, or strain |
| Gardner impact strength test | A test for measuring the impact toughness of a thin sheet of plastic using a dropped weight |
| Flexural strength | How well a material resists deformation under load |
| Tensile strength | How much stress a plastic can withstand without breaking when stretched or pulled |
| Shear strength | N/A |
| Rockwell hardness | N/A |
| Elongation at break | N/A |
| Ductility | A measure of how much something deforms plastically before fracture |
| Temperature | As temperature decreases, ductility and toughness decrease |
| Notch effect | The distribution of stress; a material might not withstand the simultaneous elastic and plastic deformation in various directions when a multiaxial stress state is produced due to the presence of a notch |
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What You'll Learn
Izod impact strength
The Izod impact strength test is a standard method of determining the impact resistance of materials, including plastics. The test is used to measure the toughness of a material, or its ability to resist breaking when subjected to sudden shock.
The test involves notching a bar on the test specimen, clamping one end, and striking the other end with a pendulum so that it breaks off. The energy absorbed by the sample is calculated from the height the pendulum swings to after hitting the sample. The test is named after the English engineer Edwin Gilbert Izod, who described it in a 1903 address to the British Association.
The notch tip radius and notch depth must be accurate to get a correct reading of impact strength. This is because the notch localizes the stress and the sensitivity will vary depending on the material being used. A lower temperature will also influence how brittle a material is and how much strain occurs during the test.
The Izod test is often used for testing plastics, while the Charpy test is more commonly used for testing metals. The two tests are similar, but the Charpy test uses a three-point bending configuration, while the Izod test uses a cantilevered beam configuration.
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Charpy impact strength
The Charpy impact test, also known as the Charpy V-notch test, is a standardised method for determining the impact strength of materials. It measures the amount of energy absorbed by a notched sample when it is hit by a weighted pendulum. The test is widely used in industry to assess the toughness of plastic materials.
The Charpy impact test was developed around 1900 by S. B. Russell (American) and Georges Charpy (French). The test became known as the Charpy test in the early 1900s due to Charpy's technical contributions and standardisation efforts. The apparatus consists of a pendulum of known mass and length that is dropped from a fixed height to impact a notched specimen of material. The sample can be made of plastic or metal and must have either a V or U-shaped notch. The notch in the sample affects the results of the impact test, so it is necessary for the notch to be of regular dimensions and geometry. The size of the sample can also impact the results, as the dimensions determine whether the material is in plane strain.
To perform the Charpy impact test, the operator first raises and locks the pendulum at a fixed height. The tester then places the sample on a support in the striking anvil, ensuring that the sample is well secured. The pendulum is then released, fracturing the sample. The energy absorbed by the sample is read from a dial on the machine, typically graded in joules. The impact strength is then calculated by dividing the energy absorbed by the cross-sectional area of the sample at the notch. The unit of impact strength is J/m2.
The Charpy impact test is a comparative-style test that provides characteristic values for the impact strength of a material at high strain rates. The test is typically performed at ambient or low temperatures, and it can be repeated at multiple temperatures to determine the ductile-to-brittle transition temperature. The Charpy test is simple, quick, and inexpensive to perform, but a disadvantage is that some results are only comparative.
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Tensile toughness
The toughness of a material is its ability to absorb energy and resist fracture upon collision. Toughness is often measured using the Charpy and Izod impact tests, which involve striking a secured sample with a weighted pendulum. These tests are used to measure the impact resistance of plastics and are widely used across industries.
For example, the molecular weight of the polymer chains is critical to tensile strength. Polymers with higher molecular weights have higher tensile strength because they form stronger intermolecular bonds. Additionally, temperature and pressure can affect tensile properties. During manufacturing, stretching or orienting the plastic in a specific direction can increase tensile strength in that direction, but it may reduce strength in other directions.
Other factors that can influence tensile toughness include the use of additives such as fillers and plasticizers. Fillers can increase the stiffness and strength of plastic but may reduce its elongation at break by creating stress concentrations. On the other hand, plasticizers can improve flexibility and elongation but may reduce tensile strength by making the polymer chains more susceptible to breaking under tension.
In summary, tensile toughness is a critical aspect of plastic materials, and its measurement is influenced by various factors related to the material's composition and manufacturing processes. By understanding and testing these factors, manufacturers can ensure that their plastic products meet the required toughness standards for their intended applications.
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Gardner impact strength
The Gardner Impact test is a traditional method for evaluating the impact strength or toughness of a plastic material. It is also used to specify appropriate materials for applications involving impact or to evaluate the effect of secondary finishing operations or other environmental factors on plastic impact properties. The Gardner Impact test was developed in conjunction with the Society of the Plastics Industry (SPI) to assess the impact resistance of rigid sheets of PVC and other plastics.
The test involves placing the sample on a base plate with an opening of a specified diameter. An "impactor" is placed on top of the test sample, with a nose of a specified radius in contact with the centre of the test sample. A weight is then raised inside a guide tube to a predetermined height and released, forcing the nose through the test sample. The drop height, drop weight, and the test result (pass/fail) are recorded. The most common method to analyse this data is the "Bruceton Staircase" method. A number of samples are used to bracket the pass/fail energy level, and then a series of 20 impacts are conducted. If a test sample passes, the drop height is increased by one unit, and if it fails, the height is decreased by one unit. The results from the 20 impacts are used to calculate the Mean Failure Height, i.e. the point at which 50% of the test samples will fail under impact.
The Gardner Impact test is a pass/fail test conducted at a specific drop height and with a designated mass to assess compliance with particular specifications. It can also be used as a classification test, where the drop height and/or mass are gradually increased to determine the minimum mass and/or height at which the sample cracks or peels. The Gardner Impact test can be used to evaluate the resistance of a dry film of paint, varnish, or related products to cracking or peeling from a substrate under deformation caused by a falling weight.
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Rockwell hardness
The Rockwell hardness test is a popular and non-destructive method used to determine the relative hardness of plastic materials. It is one of the most commonly used tests for plastics, especially polyolefins. The test uses a diamond cone or hardened steel ball, deployed against the sample with both a minor and major load. The resulting indentation on the material is used to calculate the sample's hardness value. The Rockwell hardness number is directly related to the indentation hardness of the plastic material, meaning that the higher the reading, the harder the material.
The Rockwell hardness test is carried out using a Rockwell hardness tester. A standard specimen of 0.25 inches (6.35 mm) minimum thickness is used, which can be either moulded or cut from a plastic sheet. The specimen must have flat and parallel surfaces, and be free from sink marks, burrs, or other protrusions.
The Rockwell hardness test measures hardness in progressive numbers on different scales corresponding to the size of the ball indenter used. The R scale, for example, uses a 60.00 kg major load and a 0.50-inch (12.70 mm) diameter indenting ball. The M scale, meanwhile, uses a 100.00 kg major load and a 0.25-inch (6.35 mm) diameter ball. The Rockwell hardness number is then calculated using the distance "R-B", as shown in Fig. 5.5.
The ASTM D785 and ISO 2039 standards describe methods for determining the hardness of plastics using a loaded ball indenter. The ISO 2039-1 standard, for example, specifies a ball indentation test, where the hardness value is determined by measuring the surface of the indentation and then calculating the hardness from the measured indentation depth. The Rockwell method, on the other hand, measures the indentation depth at a defined preload, with different indenters available depending on the respective Rockwell scale.
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Frequently asked questions
Toughness is the ability of a material to absorb energy and plastically deform without fracturing.
There are several methods to measure the toughness of plastic materials. One way is to perform a tensile test like ASTM D638, where a specimen is pulled apart by a pair of grips. Another method is to use the Charpy impact test, which uses a pendulum to impact a secured sample. The toughness of the material is determined by how much energy it absorbs before it fractures.
Other methods include the Izod impact test, Gardner impact strength test, and tensile impact test. The Izod impact test is more commonly reported because it is quick and easy to get a quantitative value. However, it is tested on a thick specimen (~12 mm), which may not be representative of the geometries used in plastic parts. The Gardner impact strength test is better for measuring the impact toughness of thinner sheets of material.
