Test methods precracked specimens

Static tests with precracked specimens Principle of the method... 

As was cited in the case of immersion testing, most SCC test work is accomplished using mechanical, nonelecdrochemical methods. It has been estimated that 90 percent of all SCC testing is handled by one of the following methods (1) constant strain, (2) constant load, or (3) precracked specimens. Prestressed samples, such as are shown in Fig. 28-18, have been used for laboratory and field SCC testing. The variable observed is time to failure or visible cracldng. Unfortunately, such tests do not provide acceleration of failure. 

Method for the preparation and use of precracked specimens Method for slow strain rate testing... 

After precracking, specimens were kept in environment of DT tests for about 20 hours, and then DT tests were carried out. In this study, in order to avoid hysteresis of the load-relaxation method (Sano, 1988), all of... 

J. Sedtiks, Making and using precracked cantilever beam specimens, in Stress Corrosion Cracking Test Methods, National Association of Corrosion Engineers (NACE), Houston, TX, 1990, pp. 37 5. 

Use of fracture mechanics test methods for failure analysis of FRP composite elements is feasible, but to the best knowledge of the author, no examples of this have been published. The challenge here is mainly the preparation of suitable test specimens from structural elements and the creation of the starter cracks. ISO 15024 does provide some information on mode I starter crack preparation for nonstandard specimens in an informative appendix (B.8, Guidelines for wedge precracking ), and that procedure can be adapted for other specimen geometries. 

B K Neale and R H Priest, The unloading compliance method for crack length measurement using compact tension and precracked Charpy specimens, Elastic-Plastic Fracture Test Methods The User s Experience, ASTM STP856, F J Loss and E T Wessell, eds, American Society for Testing and Materials, Philadelphia, PA, 1985,375-393. 

The time to failure measured on non-precracked specimens represents the sum of the initiation and propagation times of a crack. Unfortunately, crack initiation, which often is a slow step, is a random phenomenon. As a result, the reproducibility of corrosion fatigue tests earried out with non-precracked specimens is usually mediocre. Furthermore, to determine a Wohler eurve, a large number of specimens is needed. Statistical methods are usually applied to determine the endurance limit in corrosion fatigue tests. 

Precracked specimens and linear elastic fracture mechanics analysis methods have also been widely used in SCC testing. As was noted earlier, this t5 of specimen and technique was originEilly anticipated to result in revolutionary improvements in SCC understanding and development of quantitative information for use in selection and design. 

Lisagor, W. B., Influence of Precracked Specimen Configuration ind Starting Stress Intensity on the Stress Corrosion Cracking of 4340 Steel, Environment Sensitive Fracture Evaluation and Comparison of Test Methods, ASTM STP 821, S. W. Dean, E. N. Pugh, and G. M. UgiEmslQf, Eds., ASTM International, West Conshohocken, PA, 1984, pp. 80-97. 

This test method can be used to measure the relative susceptibility of materials by "Determining the Threshold Stress Intensity Factor for Environment-Assisted Cracking EAC of Metallic Materials. Constant load specimens, both the fatigue precracked single edge beam SE(B) and precracked compact specimens are tested in bending. The SE(B) was originally referred to as the Cantilever Beam Test. 

The test method was initially developed as the Slow Strain Rate (SSR) smooth bar Tensile Test (SSRTT), but more recently, it has incorporated the use of notched and precracked or fracture mechanic types of specimens. The SSR is conducted in tension at a constant displacement rate in the strain range of 10 to 10 s and always produces fracture of the test specimen. The degree of susceptibility is measiured as a SSR-ratio in terms of the rupture properties, being more susceptible as the value of the SSR-ratio decreases from unity until it reaches a threshold (Fig. 23). 

The use of precracked SCC specimens is necessary for some metals in which an SCC flaw will not initiate, but in which other types of flaws such as weld cracks can propagate by SCC. This is not necessary for aluminum alloys, which will initiate a SCC flaw on an unflawed, smooth specimen. Most SCC testing of aluminum has been done using smooth specimens because these specimens and test methods are faster and less expensive. Although the data from the two types of testing are not interchangeable, both methods tend to rank aluminum alloys in the Scune relative order of increasing resistance to SCC [20]. 

Category 2 tests are fracture mechanics tests that use notched and precracked specimens that are statically or dynamically loaded, such as cantilever beam, compact tension, and double cantilever beam specimens, and are conducted per ASTM E 399, Test Method for Plane-Strain Fracture Toughness of Metallic Materials. 

The CNB test specimen does not require crack length measurement, thereby providing some convenience to the user. However the SEPB and SCF test methods require quality measurements if accurate fracture toughness estimates are expected. For the SEPB test specimen, the precrack front is very difficult to delineate unless it leaves a crack arrest line. An arrest line is easily detected if the crack tip turns out of plane or is hooked, thereby making the precrack and fast fracture plane non-coplanar, as shown in Figure 7. In one study, some cracks were so planar that the front could not be detected with conventional optical microscopy [12]. In such cases, the use of dye penetrants have been successful as shown in Figure 8, however, in some cases penetrants tend to bleed on the fracture surface and the measurements must be made immediately. Dye penetrants must be used with caution as they may cause environmentally assisted crack growth. 

One of the advantages of the three methods in C 1421 is that they all use a simple bend bar. Each 3 mm x 4 mm x 45 mm to 47 mm bar test specimen was prepared in accordance with the generic ( standard ) preparation procedure specified in the flexural strength standards MIL STD 1942 MR," ASTM C 1161," and ISO 14704. ° One 4 mm wide face was finished with a 900 grit wheel (finer than is normally required) to produce a flat and smooth surface for the Knoop indentation in the SCF test method. The four long edges of the test specimens were chamfered in accordance with requirements for the surface crack in flexure (SCF) and precracked beam (SEPB) test methods. 

Precracked specimens and linear elastic fiacture mechanics (LEFM) methods of analysis have also been widely used for SCC testing in recent years. It was anticipated that this new technique would provide... 

Testing aluminum alloys with precracked specimens, especially the bolt-loaded double-beam type, has received widespread use in recent years, and the ranking of materials by this method is generally in good agreemoit with that established with smooth specimen tests. However, a number of problems in the interpretation of test results must be taten into account... 

C. Micheletti and M. Buratti, New Testing Methods for the Evaluation of the Stress-Corrosion Behavior of High-Strength Aluminum Alloys by the Use of Precracked Specimens, Symposium Proceedings, Aluminum Alloys in the Aircrcft Industry, (Turin, Italy) Oct 1976, Technicopy Ltd., 1978, p 149-159... 

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