K_Ic is the ASTM designation for the value measured in accordance with ASTM standard E 399, "Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials." K_Iv is the ASTM designation for the value measured in accordance with ASTM standard E 1304, "Standard Test Method for Plane-Strain (Chevron-Notch) Fracture Toughness of Metallic Materials," commonly referred to as the Short Rod test method. A subject of some discussion since the introduction of the Short Rod test method in 1976, has been whether or not these two tests measure the same material property. A closely related question is: Are the values obtained by these two tests numerically equivalent? This Technical Note addresses these two important questions.

The two tests in question have many similarities and some differences. The significant similarities and differences are summarized below. More detailed descriptions of the tests is given in the ASTM standards themselves and in Technical Notes 501 and 502.

Test Similarities

• Both tests measure fracture toughness in specimens subjected to Mode I loading.
  
• Both test methods require the use of a minimum size test specimen to assure that plane-strain conditions predominate along the crack front at the time of fracture toughness measurement.
  
• Both tests measure the load required to advance a naturally created crack of known length.

Test Differences

• Crack length measurement in E 399 is done by making a physical measurement of the tested specimen half. A compliance technique is used in E 1304 to determine crack length at any time during the test.
  
• Most commonly, E 399 tests are performed with a compact tension (CT) specimen geometry. E 1304 uses a much smaller short rod (SR) geometry with a thin chevron notch to aid in maintaining plane-strain conditions along the crack front.
  
• E-399 uses a fatigue pre-cracking method to generate the natural crack in the test specimen. E 1304 creates a natural crack by growing a quasi-steady state crack with a monotonically increasing load on the two test specimen halves.

Both tests measure mode I, plane-strain fracture toughness of a naturally created crack in a test specimen.

ASTM Standard Bias
ASTM standard methods E 399 and E 1304 are both test methods which, when standard validity criteria are met, result in ASTM-acceptable values for plane-strain fracture toughness, K_Ic and K_Iv respectively. The Bias section of each of these standards contains the statement, "There is no accepted standard value for the fracture toughness of any material." Thus, from a standards point of view, both test methods measure the same material property.

Does K_Iv Equal K_Ic?
Both ASTM E 399 and E 1304 are based on the same LEFM principles. When LEFM assumptions are met, both tests should generate the same value for plane-strain fracture toughness. For many materials, this is true. Real materials, however, do not always strictly follow LEFM principles. Both E 399 and E 1304 have provisions to protect test results from excessive bias due to plasticity in the fracture behavior of the test specimen. When the test specimen exhibits some plastic behavior in the fracture toughness tests, K_Iv and K_Ic values may be different for the same material. Technical Note 507 discusses the effect of plasticity on fracture toughness test results.

K_Ic and K_Iv are indeed measures of the same material property, and, when LEFM conditions prevail, the two values are essentially equivalent. This conclusion is supported by experimental data where the two tests produce essentially equal results on the same material.

Comparative Test Results

In 1980, a multi-laboratory blind testing program (2) was conducted to compare plain-strain fracture toughness results obtained by ASTM standard E 399 with those obtained by the Short Rod method (E 1304). In this test program, six independent laboratories obtained valid K_Ic values on a variety of materials using Compact Tension (CT) specimens. The tested CT halves were then machined into Short Rod (SR) test specimens and tested in accordance with procedures now designated as E 1304. The results from this comparative test program are summarized in the table below. Note that the original data for the Short Rod tests has been modified to reflect data reduction in accordance with the current ASTM E 1304 standard.

Excellent agreement between the two test methods over a wide range of materials and a wide range of toughness values supports the arguments above that the two test methods do indeed measure the same material property. In some cases K_Iv may produce a slightly larger numerical value than K_Ic . More conservative values are sometimes obtained by E 399 because of the 2% crack growth allowed before load measurement.

Rising R-Curve Effects

An R-Curve is a plot of the stress intensity required to advance a crack, K_R , versus the distance of crack extension from some starting point, usually a fatigue pre-crack. The R-curve results from the development of the crack-tip plastic zone as the crack advances. Brown (1) shows test results that indicate that for some higher toughness aluminum alloys, the R-Curve effect may result in K_Iv values that are higher than corresponding K_Ic values. Barker (2) argues that in most cases, differences between K_Iv and K_Ic values result from effects other than R-Curve effects. One such effect is the non-homogeneous distribution of toughness through the thickness of an aluminum plate (see Technical Note 505).

Despite the excellent correlation between K_Iv and K_Ic values shown above, data should be obtained for the particular material of interest before values are used interchangeably.

References

1. Brown, K. R., Chevron-Notched Specimens: Testing and Stress Analysis, ASTM STP 855, 1984, pp. 237-254.
   
2. Barker, L. M., Journal of Testing and Evaluation, Vol. 17, No. 4, July 1989, pp. 218-223.