Compact-tension

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

Fig. 6. Fracture toughness test specimens (a) single-edge notch (b) center notch (c) compact tension and (d) three-point bend. Terms are defined in text.

Compact tension samples (Fig. 7.1) were prepared from cast plates. Pressure injection moulding was used for Phenoxy PKHJ. The toughness tests were performed at ambient temperature according to ASTM-E399 [77]. The results are given in Table 6.1. 

Fig. 7.1. Compact tension test-piece [77], W = 20 mm, B = 4 mm the crack (length a) was introduced by tapping with a razor blade after cooling of the polymer with liquid nitrogen, if necessary...

For comparison, the following relationship has been found for an X70 pipeline steel (slow strain rate test using compact tension specimens with the precrack located in the heat-affected zone of a weld) ... 

An appropriate cure cycle was established based on the results obtained from the thermal analysis and cure rheology studies of the resin and cured BCB bar and dogbone shaped samples were fabricated for testing. Bar shaped specimens had the dimensions of 3.5 x 0.5 X 0.125 and were used to stake compact tension specimens for fracture toughness studies and for dynamic mechanical analysis of a torsion bar. Dogbone shaped specimens for tensile tests had a gauge area of 1 x 0.15 and were approximately 0.040 thick. 

In the study of crack growth in epoxy polymers the double-torsion and compact-tension specimens have been the most widely used. The values of KIc may be obtained from 1,2)... 

Pig. 1 a and b. Fracture toughness test specimens a three-point bend and b compact tension. Pre-crack length denoted by a. Movement of crosshead is vertical for both... 

A limited number of KIC values were obtained with a compact tension geometry for which KIC is calculated with Eq. (3)... 

The fracture toughness results presented thus far were derived from a fairly small 3PB specimen. In limited work with a proportionately larger 3PB sample (B 6.4 mm), a similar K1C value was found for the control. A second geometry, compact tension (CT), was utilized for alternate determinations of K,c. The K t values found with the CT specimens agreed within error with those obtained with the 3PB geometry, as shown in Table 4. 

Fig. 15 Craze-like defect formed in the stress-whitened zone in front of the crack in a POM compact tension specimen loaded at 100 °C (from [65])...

Fig. 9 Specimen geometries for fracture measurements a compact-tension, b three-point bending...

Specimen geometry for the compact-tension (CT) test is shown in Fig. 9a. 

Single-edge-notched (SEN) or compact-tension (CT) specimens may be used. Shape factors are tabulated (Williams and Cawood, 1990). 

Fig. 10.3 Curves showing the crack extension as a function of time at temperature 1200°C.46 The material used in the study was Coors AD998 polycrystalline alumina with grain size ranging from 2 im to 10 /im. The data was obtained by testing two compact tension samples, ADCT12 and ADCT17.

The compact tension technique could be readily adapted to elevated temperature use with a little creative engineering, but may not be as informative, or as robust, as the indentation strength methods. 

Fig. 12.11 Schematic of a compact tension specimen and in situ observation apparatus.

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