Cracked specimen

Many workers have employed pre-cracked specimens in a number of configurations that permit interchangeable data to be obtained from an analysis derived with fracture mechanics. The pre-crack provides an ideal crevice for... 

Fig. 8.97 Classification of pre-cracked specimens for stress-corrosion testing (after Smith and...

Pre-cracked specimens are sometimes useful for other reasons than the analysis that they afiford in relation to stress-intensity factors. Such applications may be associated with the simulation of service situations, the relative ease with which stress-corrosion cracks can be initiated at pre-cracks or the advantages that sometimes accrue from the propagation of a single crack. The claim that has sometimes been made of pre-cracked specimen tests-that they circumvent the initiation stage of cracking in plain specimens, erroneously assumed invariably to be related to the creation of a corrosion pit that provides a measure of stress concentration approaching that... 

Comparison of the Results from Plain and Pre-cracked Specimens... 

Fig. 8.98 Threshold stress intensities from pre-cracked specimen tests, and threshold stresses from plain specimen tests, for a Mg-7A1 alloy in various structural conditions tested in chromate-chloride solution (after Wearmouth ei al )...

Abstract—The effects of metal alkoxide type and relative humidity on the durability of alkoxide-primed, adhesively bonded steel wedge crack specimens have been determined. Aluminum tri-sec-butoxide, aluminum tri-tert-butoxide, tetrabutyl orthosilicate, and titanium(IV) butoxide were used as alkoxide primers. Grit-blasted, acetone-rinsed mild steel adherends were the substrates bonded with epoxy and polyethersulfone. The two aluminum alkoxides significantly enhanced the durability of the adhesively bonded steel, while the titanium alkoxide showed no improvement in durability over a nonprimed control. The silicon alkoxide-primed samples gave an intermediate response. The failure plane in the adhesively bonded samples varied with the relative humidity during the priming process. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

McDevitt, N. T., et al., Accelerated Corrosion of Adhesively Bonded 7075 Aluminum Using Wedge Crack Specimens, Air Force Materials Laboratory Technical Report 77-184, October... 

For Keff> 0, depending on the loading and crack/specimen geometry conditions, one can draw on known32,33 solutions for the crack shape and Keff. If u(x) is the crack opening at some location, x (see Fig. 10.6), then, in the presence of a remote applied stress, [Pg.347]

Clearly, the function p(u,t) for a stationary crack depends on the crack shape as well as on crack/specimen geometry. 

Strength approach a constant value, we find decreased crack propagation rates in pre-cracked specimens and a continued rise in fatigue lifetimes in unnotched specimens. 

Pre-cracked specimens were tested in three point bending (mode I) at room temperature and at Vo =1 m/s using a falling weight type machine Fractovis 6789 by Ceast. 

Finally, when real load-time curves of pre-cracked specimens are processed, the critical fracture parameters deduced result in good agreement with the ISO protocol [15]. 

This is the identical result to Eq. (20) for the stress at infinity and illustrates that, if a uniform pr sure fe applied to a cracked specimen, we have a stress intensity factor of —ayjita due to the external pressure and one of+cr y/iTa due to that acting in the crack. Thus, the resultant value at the cradc tip is zero. 

A specimen of finite thickness may be viewed simply as a shce taken from the penny-shaped crack specimen (Fig. 4.6). As a penny-shaped crack embedded in a large body, the crack-tip stress field is not affected by the external boundary surfaces and plane strain conditions that prevail along the entire crack front. As a slice, however, the crack in this alternate specimen is now in contact with two free... 

In Eqn. (4.16), F(a/W) is a function of a/lT for single-edge-cracked specimens and depends on the specimen geometry. For a small change in Ad at constant load. 

For the recommended range of values of Uo/W of 0.45 to 0.55, a value for H/50 of 0.05 has been adopted for the testing of single-edge-cracked specimens. This is embodied in the so-called hve percent slope offset method for establishing the pop-in load in ASTM Method E-399 for Plane Strain Fracture Toughness [2]. 

In case of a cracked specimen, the material surrounding the crack tip can be divided into three parts (1) the first plastic zone, (2) the second plastic zone, and (3) the elastic fracture surface [Lu et al., 1996]. The specific energy balance equation for a cracked specimen can be expressed as ... 

The formula contains a quantity PIBw, with units of tensile stress, multiplied by the square root of the crack length a. The dimensionless polynomial in brackets is valid for 0.7 > alw > 0.3. Compendia exist of functions fC(F, a, tv) that relate the stress intensity to the applied force F, the specimen width w and crack length a, for particular cracked specimen geometries. We will see some of them in Section 9.4.4. 

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