CT-Specimen

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CT-specimen or written in full Compact Tension (CT) specimen

The Anglo-Saxon abbreviation CT stands for ‘Compact Tension’ and the CT-test specimen is referred to in German as a compact tensile test specimen.

Requirements for test specimen geometry

When experimentally determining fracture mechanical values (see: fracture mechanical testing), the following basic conditions must be observed:

1. Under the respective test conditions, the test specimen dimensions must be significantly larger than the extent of the plastic zone at the crack tip.
2. The force, notch expansion (see: crack opening) and load-load application point displacement must be continuously measurable.
3. To calculate the stress intensity factor K at the moment of unstable crack propagation, the stress on the test specimen and the critical crack length must be precisely determinable.
4. For the corresponding test specimen geometry, the determining equation, i.e. the relationship between stress and crack length, must be known.

In order to fulfill these requirements, a series of specifications were established based on ASTM standard E 399 [1] and incorporated into the existing standards.

Test specimen shape

Fig.:

Schematic illustration of the CT-specimen

Dimension (according [1, 2]):
W = 2 B, special shape: W = B to 4 B
s = 0.55
H = 1.2 W
a = (0.35–0.55) W
D = 0.25 W
G = 1.25 W

Typical dimensions for plastics [3, 4]:
Example 1: Designation: 48 mm x 50 mm-specimen
W = 40 mm, B = 10 mm, H = 48 mm, G = 50 mm, D = 10 mm, L = 12 mm, a = 18 mm, s = 22 mm, N = 2 mm

Example 2: Designation: 96 mm x 100 mm-specimen
W = 80 mm, B = 3...10 mm, H = 96 mm, G = 100 mm, D = 20 mm, L = 36 mm, a = 38 mm, s = 44 mm, l = 2 mm

Determination equation [1]

$K_{I}={\frac {F}{B\cdot W^{1/2}}}f(a/W)$

$f(a/W)=29.6(a/W)^{1/2}-185.5(a/W)^{3/2}+655.7(a/W)^{5/2}-1017(a/W)^{7/2}+638.9(a/W)^{9/2}\!\$

Designation according thickness B:
CT 10, CT 15, CT 20, CT 30

Geometry criterion for metals:

$B,a,(W-a)\geq 2.5{\bigg (}{\frac {K_{I}}{R_{e}}}{\bigg )}^{2}$

Geometry criterion for plastics:

$B,a,(W-a)\geq \beta {\bigg (}{\frac {K}{\sigma _{y}}}{\bigg )}^{2}$

It is valid: R_e = y = yield stress (yield point)

The geometry constant β depends on the material (see also: geometry criterion, fracture toughness).

A comprehensive summary of suitable test specimens for fracture mechanics investigations on plastics and composite materials is included in fracture mechanics test specimens.

See also

• Specimen for fracture mechanics tests
• Compact tension specimen
• Geometry function
• Laser double-scanner
• Hybrid methods, examples
• Laser multi-scanner
• Toughness temperature dependence

References

[1]	ASTM E 399 (2024): Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness of Metallic Materials
[2]	Blumenauer, H., Pusch, G.: Technische Bruchmechanik. Deutscher Verlag für Grundstoffindustrie, Leipzig Stuttgart (1993) 3rd Edition, (ISBN 3-342-00659-5; see AMK-Library under E 29-3)
[3]	Grellmann, W., Seidler, S. (Eds.): Deformation and Fracture Behaviour of Polymers. Springer, Berlin Heidelberg (2001) (ISBN 978-3-540-41247-6; e-Book: ISBN 978-3-662-04556-5; see AMK-Library under A 7)
[4]	Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 233/234 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)