Strength at Break (Tensile) -

What is the definition of tensile strength?

Tensile strength is the ability of plastic material to withstand a maximum amount of tensile stress without failure. The stress occurs while the material is being pulled or stretched. It is the point when a material goes from elastic to plastic deformation.

While removing the stress, the plastic material undergoes:

Elastic deformation: The material returns to its original dimensions. Deformation is reversible and non-permanent. Valid for small strains.
Plastic deformation: The material does not return to its original dimensions. This process is permanent and irreversible.

What are the three types of tensile strength?

There are three types of tensile strength:

Yield strength (A) – The stress a material can withstand without permanent deformation. Know more »
Ultimate strength (B) – The maximum stress a material can withstand
Breaking strength (C) – The stress coordinate on the stress-strain curve at the point of rupture

Stress-strain curve showing tensile strength at points Yield (A), Ultimate Strength (B) and Break (C)
In other words, materials first deform elastically – when you release the stress they return to their original shape. Then with more force they deform plastically, this is yield – when you release the stress they have permanently been stretched into a new shape. Finally, they break; this is ultimately tensile stress or breaking point.

What is tensile strength at break?

Tensile strength (TS) at break measures the maximum stress a plastic specimen can withstand while being stretched before breaking. Some materials can break sharply (brittle failure). While others will deform or elongate before breaking. It is measured as the tensile or compressive load required to fracture something. It is also known as break strength.

Hence, it is one of the important mechanical properties for:

Material evaluation
Quality control
Structure design
Modeling, and
Failure analysis

What are the units of tensile strength?

Tensile strength is measured in units of force per cross-sectional area. In the International System, the units of Tensile Strength are:

Pascal (Pa)
Megapascals (MPa)
Gigapascals (GPa)

In the US, the commonly used units for convenience when measuring tensile strengths are:

Pounds-force per square inch (lbf/in² or psi)
Kilo-pounds per square inch (kpsi)

Which materials show high tensile strength?

How stress-strain curve measures the tensile strength?

When a stretching force or tensile force is applied to an object, it extends. Its behavior can be obtained using a stress-strain curve in the elastic deformation region. This is known as Hooke’s Law. The extension that a force produces depends on the plastic material and dimensions of the object.

Definition of stress

Stress is defined as the force per unit area of plastic. The units of stress are N/m² or Pa.

σ = F/A where,

σ is the stress,
F is the force, and
A is the cross-sectional area of the sample.

Definition of strain

Strain is defined as extension per unit length. And, since it is a ratio of lengths, the strain has no units.

ε = ΔL/L0; ΔL = L-L0 where,

ε is the strain,
L0 is the original length of a bar being stretched,
L is the length after it has been stretched, and
ΔL is the extension of the bar, the difference between these two lengths.

Stress-Strain Plots for a Typical Elastomer, Flexible Plastics, Rigid Plastic, and Fiber
(Source: Principles of Polymerization, Fourth Edition, George Odian)

What is the significance of tensile properties?

Tensile properties provide useful data for plastics engineering design purposes.
They are frequently included in material specifications to ensure quality.
They are often measured during the development of new materials and processes. This is done to compare different materials and processes.
They predict the behavior of a material under various forms of loading other than uniaxial tension.

What are the factors affecting the tensile strength of plastics?

The strength of polymers is further governed by:

Molecular weight

The strength of the polymer rises with the increase in molecular weight. It reaches the saturation level at some value of the molecular weight.

At lower molecular weight: The polymer chains are loosely bonded by weak van der Waals forces. The chains can move easily and are responsible for low strength. Although crystallinity is present.
At higher molecular weight: The polymer chains become large. Hence, crosslinking takes place giving strength to the polymer.

Crosslinking between polymer chains

The crosslinking restricts the motion of the chains. This increases the strength of the polymer.

Crystallinity

The crystalline phase of the polymer increases the tensile strength. Hence the intermolecular bonding is more significant. Therefore, the polymer deformation can result in higher strength leading to oriented chains.

Other factors that impact the tensile strength values of plastics

Velocity of testing,
Orientation level of fibers,
Temperature,
Filler content, etc.

What are the test methods to measure tensile properties of plastics?

In general, “tensile test methods” measure the modulus of elasticity of materials. The common methods used are:

ASTM D638 – Standard Test Method for Tensile Properties of Plastics
ISO 527-1:2012 – Determination of tensile properties. General principles

These methods determine the tensile properties of plastics and plastic composites. This is done under defined conditions that can range from:

pretreatment,
temperature,
humidity, and
machine speed

The test specimens are in the form of a standard dumbbell shaped.

For ASTM D638, the test speed is determined by the material specification. For ISO 527, the test speed is typically 5 or 50 mm/min for measuring strength and elongation, and 1 mm/min for measuring modulus.

Apart from Tensile strength at break, the tensile test results can also calculate:

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