A compact tension specimen (CT) is a type of standard notched specimen in accordance with
ASTM[ASTM E647-00 ''Standard Test Method for Measurement of Fatigue Crack Growth Rates''. ASTM International, 2000.] and
ISO
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* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
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[ISO 7539-6 ''Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and use of pre-cracked specimens for tests under constant load or constant displacement''. 2nd Ed. 2003.] standards. Compact tension specimens are used extensively in the area of
fracture mechanics and
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
testing, in order to establish
fracture toughness and
fatigue crack growth data for a material.
The purpose of using a notched sample is to create a
fatigue crack by applying cyclic loading through pins inserted into the holes on the sample using a laboratory
fatigue test machine. The fatigue crack will begin on the point of the notch and extend through the sample. The length of the crack is typically monitored by measuring the compliance of the coupon which changes as the crack grows, or direct measurement using an optical microscope to measure the position of the crack tip or indirectly from either extensometer readings of the crack mouth opening or attaching strain gauges to the backface of the coupon.
According to the standards, the constraining dimension of the specimen is the thickness of the material. Compact tension specimens are used for experiments where there is a shortage of material available due to their compact design. For rolled materials the notch should be aligned with the roll direction where the material is weakest. This will allow the user to insure that all results achieved are conservative (worst-case scenario).
Stress intensity factor
The
stress intensity factor
In fracture mechanics, the stress intensity factor () is used to predict the stress state ("stress intensity") near the tip of a crack or notch caused by a remote load or residual stresses. It is a theoretical construct usually applied to a h ...
at the crack tip of a compact tension specimen is
:
where
is the applied load,
is the thickness of the specimen,
is the crack length, and
is the effective width of the specimen being the distance between the centreline of the holes and the backface of the coupon. The above equation has been fitted using numerical computations for various specimen geometries.
Crack length
The length of the crack is often measured indirectly during testing by calculating the crack length from the compliance of the coupon. The compliance can be determined from either a crack mouth opening displacement (CMOD) gauge or from strain measurements on the back-face of the coupon.
Crack mouth opening displacement
Crack length can be found using a displacement gauge attached to the mouth of the coupon to measure displacements
using the equation
:
:
This equation is applicable in the range
.
Back-face strain
The crack length can be determined using the back-face strain
with the following equation
:
where
,
and
is the Young's modulus of the coupon material. This equation is applicable in the range
.
Electric Potential Difference
The crack length can also be determined from voltage measurements of the electric potential difference (EPD) at points at each side of the mouth of the machined slot at opposite sides of the coupon using
:
where
is the measured EPD voltage and
is the reference crack voltage corresponding to
on an identical specimen. This equation is requires the electrical excitation current is injected along the load line of the specimen. This equation is applicable in the range
.
References
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Fracture mechanics