Tensile Strength

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Tensile strength

Definition of tensile strength

In conventional tensile tests on plastics, stress and strain-related characteristic values are determined in accordance with ISO 527-1, whereby the material parameters correspond to selected points on the stress–strain diagram. By definition, the tensile strength σ_m or σ_M corresponds to the stress value on the vertical axis of the σ–ε diagram at the first stress maximum during the tensile test and is calculated according to Eq. (1) [1].

$\sigma _{m}={\frac {F_{m}}{A_{0}}}={\frac {F_{max}}{A_{0}}}$

(1)

Depending on the type of diagram (a to d) of the plastic being tested, the tensile strength σ_m can be identical to the yield stress σ_y or the tensile stress at breaking σ_b (Fig. 1). Although the tensile stress at break is specified as a parameter in the standard, this value should not be used as a comparative parameter for plastics, as it is highly dependent on the switch-off criterion of the material testing machine.

Strain at tensile strength and tensile strain at break

The associated characteristic value on the horizontal axis marks the location where the first maximum occurs and is referred to as the strain of the tensile strength or, better, the yield strain ε_m. This characteristic value can also correspond to the tensile strain at yield ε_y or the tensile strain at break ε_b. In principle, the strain up to the yield strength ε_y (diagrams b and c in Fig. 1) must be measured with strain gauges as a normative value or, in the case of diagrams without yield strength up to break (diagrams a and d in Fig. 1), also determined using clip-on extensometers (see: tensile test, path measurement technique).

$\varepsilon _{t}=\varepsilon _{y}+{\frac {\Delta L_{t}}{L}}$

(2)

Fig. 1:

Typical stress–strain curves and associated parameters in the tensile test [1]

The characteristic values tensile stress at break or strain at tensile strength are determined for diagram types b and c from the sum of the normative strain up to the yield point and the subsequent nominal strain from the traverse path according to equation (2). L is the clamp distance and L_t corresponds to the extension ΔL or the increase in the distance between the clamping clamps. The normative strain, which is always determined as a deformation measured directly on the test specimen, is calculated according to Eq. (3). L₀ is the initial distance of the strain sensor on the test specimen and L₀ is the measured extension of the sensor distance. The strain can be specified dimensionless (Eqs. (2) and (3)) or as a percentage when multiplied by a factor of 100.

$\varepsilon ={\frac {\Delta L_{0}}{L_{0}}}$

(3)

Stress–strain diagram for PA6

The evaluation according to the standard [1] is difficult if a stress–strain diagram is determined in the tensile test, e.g. for PA 6 according to Fig. 2. Due to the definition that the tensile strength corresponds to the first stress maximum, the absolute maximum according to this standard cannot be assigned a parameter, as all parameters have already been assigned. In contrast to the usual evaluation procedure of testing machine manufacturers and a previous revision of this standard [2], the absolute maximum is designated here as tensile strength σ_m and the relative maximum only as yield stress σ_y. The definition of tensile strength here was that this is the maximum stress that the test specimen can withstand during the tensile test.

Fig. 2:

Nominal and normative strain of a test specimen in the tensile test

The tensile strength depends very much on the test speed and the test temperature, and is only comparable for different materials under identical test conditions and test specimens.

A comprehensive literature analysis of the mechanical properties of the E_t, σ_M and ε_B tensile test is compiled for numerous plastics in [3].

See also

• Tensile test
• Tensile test uniform elongation
• Yield stress
• Crosshead speed
• Test speed
• Material testing machine

Reference

[1]	ISO 527-1 (2019-07): Plastics – Determination of Tensile Properties – Part 1: General Principles
[2]	ISO 527-1 (1993-06): Plastics – Determination of Tensile Properties – Part 1: General Principles
[3]	Bierögel, C., Grellmann, W.: Quasi-Static Tensile Test. In: Grellmann, W., Seidler, S.: Mechanical and Thermomechanical Properties of Polymers. Landolt-Börnstein, Volume VIII/6A3, Springer Berlin (2014) 76–143, (ISBN 978-3-642-55165-9; see AMK-Library under A 16) DOI: http://dx.doi.org/10.1007/978-3-642-55166-6_16