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Fracture Toughness

Essentially, a distinction is made between three types of mode in the fracture mechanics:

Mode I:     Crack opening
Mode II:    In-plane shear
Mode III:   Out-of-plane shear

Mode I

Mode I

There are various test arrangements for the individual modes. Mode I and Mode II are usual test procedures, as is Mixed Mode Bending

The test is normally performed with the DCB (Double Cantilever Beam) specimen. 

Here, the crack opening is measured as crosshead travel. 

The crack's growth is visually tracked on both sides of the specimen. 

The test procedures and result evaluations differ depending on the standard applied.  

Standards are: ISO 15024, ASTM D 5528, AITM 1-0005, AITM 1-0053, Boeing BSS 7273, CRAG method 600, NASA method RP 1092 ST-5, ESIS TC 4, prEN 6033 (withdrawn)

 

Mode II

Mode II

Mode II loads can be generated and measured both in the flexural test, as well as in the tensile and compression test with notched specimens. 

The measurement of the Mode II energy release rate is standardized as a flexural test. 

The specimens are designated with SENB (Single End Notch Bending), but ENF (End Notch flexure) is another common term. 

Deflection is measured by the crosshead travel (with stiffness correction), or by means of a displacement transducer, which is applied centrally. 

The crack initiation point is characterized by a force maximum. 

At the end of the measurement, the specimen is cooled in liquid nitrogen and then completely broken to measure the fracture surfaces.

Mixed Mode Bending (MMB)
"Mixed Mode Bending" can be measured on unidirectional laminates. This involves combining Mode I and Mode II. 

 

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