Toughness -

What is toughness?

Toughness is the ability of a material to resist both fracture and deformation. The area underneath the stress-strain curve depicts the toughness of a polymer. The larger the area occupied higher will be the toughness of the material. To be tough, a material must exhibit both good strength and ductility.

Which materials show good toughness?

Thermoplastics with Good Toughness
TPEs/TPVs with Good Toughness
Rubbers with Good Toughness
Thermosets with Good Toughness

How do hardness, toughness, and strength differ?

In general, ‘Hardness’, ‘Toughness’, and ‘Strength’ are similar terms to use. But, in material science, they are three distinct properties that also share some overlap. Here are the key differences:

Hardness is how well material holds together when friction is applied. Discover more about hardness property here »
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.

How does the toughness of polymers vary?

The toughness of polymers, or resistance to impact, varies with the:

Molecular structure,
Surrounding temperature, and
Type of stress applications.

A 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.

Which factors affect the toughness of plastics?

The factors that affect the toughness of plastics are:

» 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.

How to measure the toughness of plastics?

The toughness of plastics is measured by their resistance to impacts. The impact test is the ability of a material to absorb energy during plastic deformation. It signifies toughness or impact strength of a material. This energy absorption is 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 of impact test

The measure of the energy required to crack a material
Screen materials for impact developments
Define uses of materials in automotive applications

What are the types of impact tests?

The two most common methods to determine toughness include: Izod and Charpy Test.

Key principle

These impact methods are based on the common principle of:

applying the load at a high rate, and
measuring the amount of energy absorbed (Kg/m or Joule) in breaking the sample due to impact.

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)

Description of Charpy and Izod Impact Tests
(Source: ResearchGate)

Differences between izod and charpy tests

Some differences between these two methods are in terms of:

Sample size and shape,
Method of holding the sample, and
The maximum energy content of the pendulum that hits the sample during the test.

Notched and unnotched test

Most of the engineering components are designed with notch and stress raisers. Thus, it becomes important to know the behavior of the material with a notch under impact loading.

Hence, a toughness test is usually conducted using a sample with a notch.
Moreover, un-notched samples can also be used for the toughness test and the results are expressed accordingly.

a) Notched and Unnotched Charpy Test; b) Notched and Unnotched Izod Test
(Source: ScienceDirect)

These tests can be used as a quick and easy quality control check. They determine if a material meets specific impact properties or compare materials for toughness.

Key advantages of impact tests

Values of toughness are not directly used for design purposes. They only indicate the ability of the material to withstand 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. These conditions can be heat treatment, procedure, mechanical working, etc.

Note: The result of the impact test is reported in energy lost during the impact per unit of specimen thickness (such as ft-lb/in or J/cm). Test results, especially in Europe, may be reported as energy lost per unit cross-sectional area (J/m² or ft-lb/in²).

What are the test methods to measure toughness?

Test methods used to measure Notched Izod Impact (or notch sensitivity) and Charpy Impact in plastics are:

ASTM D256-10(2018): Plastics — Determination of Izod pendulum impact resistance
ISO 180:2019: Plastics — Determination of Izod impact strength
ISO 179-1:2010: Plastics — Determination of Charpy impact properties — Part 1: Non-instrumented impact test
ISO 179-2:2020: Plastics — Determination of Charpy impact properties — Part 2: Instrumented impact test