SENB-Specimen

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SENB-specimen

The English abbreviation SENB stands for "single-edge notched bend" and the SENB test specimen is referred to in German as a three-point bending test specimen.

Requirements for the test specimen geometry

In the experimental determination of fracture mechanical values, the following basic conditions must be observed:

1. The test specimen dimensions must be substantially greater than the extent of the plastic zone at the crack tip under the respective test conditions.
2. The load, the crack-tip-opening displacement, and the load-load line displacement must be continuously detectable.
3. For the calculation of the stress intensity factor K at the moment of the unstable crack propagation, the load on the test specimen and the critical crack length must be exactly determinable.
4. For the corresponding test body geometry, the determination equation, e.g. the relationship between loading and crack length should be known.

In order to meet these requirements, a number of specifications have been adopted that have been adopted from the ASTM standard E 399 [1] into the existing standards.

Test specimen

W – specimen width B – specimen thickness L – specimen length s – support span N – notch width a – notch depth F – load

Fig. 1:

Schematic representation of the SENB specimen

Dimensions (according to [1, 2]):
W = 2 B, special case: W = B up to 4 B
s = 4 W $\rightarrow$ s/W = 4, s = 40 mm
L = 4,5 W
a = (0,45–0,55) W
N $\geq$ 1,5 mm for U- und V-notch for metals

Typical dimensions for plastics (according to [3, 4]):
W = 10 mm
B = 4 mm (variation B = 2...10 mm)
L = 80 mm
s = 40 mm (variation s = 40...70 mm)
a = 2 mm (variation a = 0,5...7,5 mm)
N $\geq$ 1,5 mm
l $\geq$ 1,3 mm (razor blade, notch length)
r $<\!\$ 0,25 mm (notch radius)
r $\approx$ 0,125 µm (razor blade, notch radius)

Conditional equation

$K_{I}={\frac {F\cdot s}{B\cdot W^{3/2}}}f(a/W)$

$f(a/W)\!\$ für $s/W=4\!\$

Tada [5]:

$f_{1}(a/W)=2,9(a/W)^{1/2}-4,6(a/W)^{3/2}+21,8(a/W)^{5/2}-37,6(a/W)^{7/2}+38,7(a/W)^{9/2}\!\$

Srawley und Gross [6]:

$f_{2}(a/W)={\frac {3}{2}}(a/W)^{1/2}\cdot {\frac {[1,99-a/W\cdot (1-a/W)\cdot (2,15-3,93a/W+2,7(a/W)^{2})]}{(1+2a/W)\cdot (1-a/W)^{3/2}}}\!\$

for s/W = 4
f₂(a/W) shows correspondence with f₁ in the range 0 < a/W < 0,6, then lower values

Geometry criterion for metals:

$B,a,(W-a)\geq 2,5{\bigg (}{\frac {K_{I}}{R_{e}}}{\bigg )}^{2}$

Geometry criterion for plastics:

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

where: R_e = $\sigma$_y = tensile yield stress (yield strength)
The geometry constant $\beta$ is material-dependent. (see geometry criterion, fracture toughness)
A comprehensive assortment of suitable test specimens for fracture mechanics investigations in plastics and composites is contained in fracture mechanics test specimens.

See also

• notch
• notch sensitivity
• notch geometry
• specimen

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) 3. Auflage, (ISBN 3-342-00659-5; see under AMK-Library E 29-3)
[3]	Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser Munich (2022) 3. Edition, (ISBN 978-1-56990-806-8; see under AMK-Library A 22)
[4]	MPK-Procedure MPK-ICIT (2016): Testing of Plastics – Instrumented Charpy Impact Test: Procedure for Determining the Crack Resistance Behaviour Using the Instrumented Impact Test
[5]	Tada, H., Paris, P. C., Irwin, G. R.: The Stress Analysis of Cracks Handbook, 3rd Ed., ASME Press, New York (2000) DOI: https://doi.org/10.1115/1.801535
[6]	Srawley, J. E., Gross, B.: Stress Intensity Factors for Bend and Compact Specimens. Engineering Fracture Mechanics (1972) 587–589. DOI: https://doi.org/10.1016/0013-7944(72)90069-0