Mechanical properties of Plastics
Toughness of a plastic is measured by its resistance to impacts. It is the ability of a material to resist both fracture and deformation. One common way to discuss the toughness of a polymer is to examine the area underneath the stress - strain curve for the particular polymer.
In general, ‘Hardness’, ‘Toughness’ and ‘Strength’ are similar terms to use, but in material science they are three distinct properties yet also share some overlap. Here are the key differences:
- Hardness is how well material holds together when friction is applied
- Strength is how much force is required before the material deforms. It tells us about the amount of load a material can bear
- Toughness is the ability of a material to resist breaking when force is applied
Toughness is the combination of strength and ductility
To be tough, a material must exhibit both fairly good strength and ductility to resist cracking and deformation under impact loading.
Impact test signifies toughness, or impact strength, of a material that is the ability of material to absorb energy during plastic deformation. This energy absorption is directly related to the brittleness of the material. Results of impact tests are expressed in terms of either:
- Amount of energy absorbed (Nm) or
- Amount of energy absorbed per unit cross sectional area (Nm/cm2)
Applications include:
- Measure of the energy required to crack a material
- Screen materials for impact developments
- Define uses of materials in automotive applications
Check out more on toughness:
» How to Measure the Toughness at Low Temperature of Plastic
» Toughness at Low Temperature Values of Several Plastics
How to Measure Toughness of a Polymer?
The energy absorption, or toughness, of a material is measured by various techniques and two most commonly used methods are: Izod and Charpy Test.
The two methods are based on common principle of applying the load at high rate and measuring the amount of energy absorbed (Kg/m or Joule) in breaking the sample due to impact. However, there are some difference also in these two methods in terms of:
- Sample size and shape,
- Method of holding of the sample and
- Maximum energy content of pendulum that hits the sample during test
| Toughness Test |
Sample |
Holding |
| Izod |
Held vertically on anvil as cantilever |
Cantilever type and notch faces the pendulum |
| Charpy |
Held horizontally on anvil as simply supported beam |
Simply supported type and notch is opposite side of pendulum impact (not facing the pendulum) |
Since most of the engineering components are invariably designed with notch and stress raisers, therefore, it becomes important to know the behavior of material with notch under impact loading.
- Hence, toughness test is usually conducted using sample with notch.
- Moreover, un-notched samples can also be used for the toughness test and the results are expresses accordingly.
These tests can be used as a quick and easy quality control check to determine if a material meets specific impact properties or to compare materials for general toughness.
Values of toughness are not directly used for design purpose, but these only indicate the ability of the material to withstand against shock/impact load. These tests are useful for comparing the resistance to impact loading of different materials or the same material in different processing conditions such as heat treatment, procedure and mechanical working etc.
The toughness of polymers, or resistance to impact, varies with the:
- Molecular structure,
- surrounding temperature and
- type of stress applications
Case must be taken in relating flexibility to toughness, but generally, a more rubbery character gives higher elongation at break and better impact resistance values, although such materials would have lower stiffness.
Factors Affecting Toughness of Plastics
» Degree of Crystallinity - Greater the crystallinity, the harder the polymer
» Temperature - Change in behavior at ductile - brittle transition temperature
» Long Chain Branches - Long chain branches may increase the polymer toughness
Toughness Values of Several Plastics at Low Temperature
Among numerous impact tests available, we have selected Notched Izod Impact test performed at low temperature (-40°F or -40°C). It measures the energy to be applied to a notched standardized sample to break it at low temperature.
Since many materials (especially thermoplastics) exhibit lower impact strength at reduced temperatures, this test is often conducted at lower temperatures to simulate the intended end-use environment of the material. To conduct this test at low temperatures, the specimens are conditioned at the specified temperature in a freezer, quickly removed, and impacted one at a time. (Neither ASTM or ISO specify a conditioning time or elapsed time from freezer to impact - typical values from other specifications are 6 hours of conditioning and 5 seconds from freezer to impact.)
The result of the Izod test is reported in energy lost during the impact per unit of specimen thickness (such as ft-lb/in or J/cm) at the notch. Tests results, especially in Europe, may be reported as energy lost per unit cross-sectional area at the notch (J/m2 or ft-lb/in2).
Test methods used to measure Notched Izod Impact (Or notch sensitivity) in plastics are ASTM D256 and ISO 180.
Click to find polymer you are looking for:
A-C |
E-M |
PA-PC |
PE-PL |
PM-PP |