Standard fracture toughness tests are designed to allow reproducible determination of the relevant fracture characterising parameter, be it plane strain fracture toughness, K_1C, the J-integral or crack tip opening displacement, CTOD.  Hence certain conditions have to be met in the tests regarding specimen and crack geometry, loading parameters and shape of load-displacement curve, before a valid result can be reported.  Bodies like the British Standards Institution (BSI) and the American Society for Testing and Materials (ASTM) have developed and published standards for testing of metallic and other materials, and some specialist organisations have published standards for particular components, e.g. uPVC water pipes.   Some of the extant fracture toughness standards are referenced below:

BSI, BS 7448 : Part 1 : 1991, Fracture mechanics toughness tests, Part 1.  Method for determination of K_1C, critical CTOD and critical J values of metallic materials - this combined, extended and replaced BS 5447 (K_1C) and BS 5762 (CTOD).

BSI, BS 3505 : 1986, Unplasticised polyvinyl chloride (PVC-U) pressure pipes for cold potable water.

ASTM, E 399, Standard test method for plane-strain fracture toughness of metallic materials.

ASTM, E 813, Standard test method for J_1C, a measure of fracture toughness.

ASTM, B 771, Standard test method for short rod fracture toughness of cemented carbides.

Foundation for Water Research, Water Industry Specification No. 4-31-06, 1990, Specification for blue unplasticised PVC pressure pipes, integral joints and post-formed bends for cold potable water (underground use).

This theory box is concerned with K_1C testing and will give only a brief synopsis of the conditions to be met to obtain a valid plane strain fracture toughness value.  In BS 7448 : Part 1, one first makes a choice between K_1C/CTOD and K_1C/J-integral tests.   This is done because the J-integral test excludes certain geometries.    In both cases, after completing the test, the data is examined to see if it will provide a valid K_1C value.  If not, one then has the option of determining a yielding fracture mechanics parameter.

Achievement of a valid K_1C result depends on the shape of the force vs displacement record, specimen size and crack geometry, and the 0.2% proof strength and toughness of the material at the test temperature.  In terms of specimens dimensions, a valid result requires that:

Essentially, these conditions ensure that the plastic zone size is less than 1/15 of the relevant dimension, which ensures that plane strain conditions prevail and that LEFM is applicable.

During the fatigue pre-cracking (which is done to ensure that a sharp enough defect is present to give a lower bound value of the toughness), there are requirements on:

• the fatigue loading during the final 1.3 mm or 50% of pre-crack extension (whichever is lower)
• crack length (0.45 < a/W < 0.55 - this ensures that the K-calibration equation is at its most accurate)
• difference between crack length measurements on the specimen surfaces (when measured to +/-0.05 mm this shall not exceed 15% of the average of the two measurements).  When measured at 9 stations after the test, the difference between any two measurements must be < 10% of the initial crack length.
• the plane of the crack, which must always be within 10^o of the plane of crack extension

During the fracture toughness test, load is applied within a set rate, corresponding to K changing in the range 0.5 MPam^½s^-1 to 3.0 MPam^½s^-1, while a trace of load versus displacement is recorded.  The appearance of a trace suitable for  K_1C determination will conform to one of three types shown below.

In case 1, fracture occurs with no previous significant plasticity, in case 2 some plasticity occurs prior to fracture, while in case 3 a pop-in occurs, followed by some plasticity prior to final fracture.  Pop-ins giving both force and load changes of < 1% are ignored, while all others are considered significant.  The load-displacement traces are interpreted to obtain a provisional test result, K_Q, which is then checked against the plasticity requirements mentioned above.  This interpretation requires construction of a line through the origin of the axes with a slope generally 5% less than the slope of the initial part of the record.  This is equivalent to crack extension of around 2%, due to the increase in specimen compliance, and ensures that the provisional toughness value does represent crack initiation.

The final check on validity is to ensure that the ratio of F_max/F_Q < 1.10.  If this is not the case, it is possible that there was too much plasticity prior to the fracture occurring and a CTOD analysis should be performed.

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