Integrity 6.11.2 Crack Fix
Single edge notched tension (SENT) specimens prepared in accordance with DNV RP F108 are usually surface-notched with an 'over-square' or 2BxB geometry. Since the crack extends over a crack front equal to the specimen dimension of 2B, growing a uniform, straight-fronted fatigue pre-crack is difficult, especially when testing weld metal or heat affected zone (HAZ). There are no standardised crack front straightness requirements for SENT specimens, so equivalent requirements for SENB specimens are applied, but not often met. TWI has reviewed its SENT test data to compare crack front straightness requirements from several testing standards. The qualification pass rate for parent metal specimens ranges between 13.8% and 100% depending on the standard used. Recommendations are made on suitable crack shape validity requirements for SENT specimens notched into parent, welds and HAZs.
Integrity 6.11.2 Crack
The established methods for carrying out fracture toughness testing using lower constraint single edge notched tension (SENT) specimens employs surface-notching of an 'over square' or 2BxB geometry specimen, Fig.1. This wide pre-crack front with relatively little specimen width to grow and stabilise the fatigue pre-crack means that achieving a uniformly straight fatigue pre-crack can be difficult, especially in specimens notched into weld metal or heat affected zone (HAZ). It has been observed that SENT specimens in thicker material are more likely to meet the validity requirement than those from thinner material. [Pisarski, 2010]
The only current specification for SENT testing, DNV-RP-F108,[DNV, 2006] specifies the use of over-square specimens but does not give explicit validity requirements for crack front straightness. However, many test houses apply the equivalent requirements for SENB weld specimens such as those given in BS 7448 Part 2,[BSI 1997a] and have struggled to meet them. In 2010, BS 7448 Part 2 was superseded by BS EN ISO 15653 [ISO, 2010] which applies slightly different criteria for valid crack front straightness.
TWI has carried out a review of its existing SENT test data to compare the crack front straightness requirements given in a number of testing standards, and to quantify the effect of specimen thickness on crack front straightness in SENT specimens. The findings will help form part of a future British Standard for SENT testing.
A total of 412 SENT test results were reviewed, consisting of 70 parent metal specimens, 130 weld metal specimens and 212 fusion line (HAZ) specimens. All the specimens were of 2BxB design (that is, the specimen thickness dimension being twice the material thickness and the specimen width being equal to the specimen thickness, shown in Fig.1), with material thickness of between 6 and 28mm (and therefore the SENT specimen thickness was between 12 and 56mm). None of the specimens were side-grooved, as this is not a requirement of DNV RP F108. The fatigue pre-crack measurements reported for each test specimen were tabulated and the average crack length determined in accordance with the appropriate standard. The data was then used to perform the crack shape qualification checks as detailed in standards including BS 7448 Parts 1, 2 and 4,[BSI, 1991, 1997a and 1997b] ISO 12135 [ISO, 2002], BS EN ISO 15653 [ISO, 2010], ASTM 1820 and ASTM 1290.[ASTM, 2008 and 2009] Of these, only BS7448 Part 2 and BS EN ISO 15653 specifically address specimens notched into weld and HAZs; the others are intended for plain, or parent, material. It should be noted that the standards listed above do not include the 'over-square' 2BxB design that is used for SENT tests. The crack shape criteria given in these standards is intended for BxB or Bx2B SENB bend specimens instead.
The definition of initial crack length, a0, is consistent across all these standards. The initial crack length, a0, is obtained by firstly averaging the two measurements at the outer points and then averaging this value with the seven inner points. It should be noted that BS 7448 (all parts) and ISO 12135 (ISO 15653 refers the user to ISO 12135) state that the two outer measurements should be taken 0.01B from the surface; whilst, by contrast, ASTM E1820 and E1290 state that the two outer measurements should be taken 0.005W from the surface. Whilst these two statements are identical for Bx2B specimens, they differ for BxB specimens or specimens of W/B not equal to 2. All the standards define the dimensions B and W in accordance with the orientations given in Fig.1, it is only the use of either B or W to define other dimensions that differs. The different methods used by each standard to determine the fatigue pre-crack front straightness are summarised below.
It should be noted that Part 4 is used to define a fracture toughness resistance curve and the SENB and CT specimens employed are usually side-grooved, so crack front straightness requirements are applied to the fatigue pre-crack front after side-grooving.
Part 3 of BS 7448 [BSI, 2005] for dynamic fracture toughness testing states that for parent material, the difference between any two of the nine crack length measurements shall not exceed 10% of a0 (which is the same definition as BS 7448 Part 1), and for weld/HAZ material the difference between any two of the nine crack length measurements shall not exceed 20 % of a0. [clause 9.7.2b]
Figs 1 to 5 show the crack curvature against specimen size, expressed as the B dimension, for the various standards. Trendlines have been plotted to show the relationship between curvature and thickness. In addition the validity limits of 10% and 20% of a0 for parent and welds respectively are shown where appropriate. The full set of 412 data points are analysed to each standard, but the data points corresponding to parent, weld and fusion line are separately identified, since not all are applicable to every standard.
Table 1 shows the mean values of crack front curvature calculated to the validation criterion for each standard, while Table 2 shows the percentage of specimens which qualified to each validation criterion. These results are given separately for parent, weld and fusion line specimens.
Fig.1 shows the crack front straightness versus specimen thickness for crack front straightness as defined from BS 7448 Part 1 (and ASTM E1290). The linear trend lines show that there is a relationship between crack front straightness and specimen thickness. The results indicate that crack front straightness improves with increasing specimen thickness. Crack front straightness improves by approximately 5% when a 45mm thick specimen is compared with a 25mm thick specimen, for parent material.
The average crack front straightness for parent material in all thicknesses calculated to BS7448: Part 1 is 17.7%. The crack front straightness must be less than 10% of a0 to fully qualify to BS 7448 Part 1, and only 13.8% of the parent metal specimens in the data set pass this criterion.
Fig.2 shows the crack front straightness versus specimen thickness for straightness as defined from BS 7448 Part 2. This differs from Part 1 in that the qualification check only considers the inner seven initial crack length measurements and the criterion limit is increased to 20% for weld or HAZ notched specimens. Whilst Figure 2 shows similar trends to Figure 1, it is clear that all of the points have shifted down the ordinate, such that more specimens meet the qualification requirements. For parent metal specimens, the average crack front curvature reduces to 9.2% according to the definition in BS 7448 Part 2 compared to Part 1, meaning that the percentage of specimens in the data set which have straightness variation less than 10% of a0 increases to 70.7%. For weld and HAZ specimens the average straightness variation is 18 to 20% meaning that between 50 and 65% of specimens are qualified with variation less than 20% of a0.
Fig.3 shows the crack front straightness versus specimen thickness, for straightness as defined from BS 7448 Part 4. The data points are grouped a lot closer than in Figures 1 and 2, and the effect of specimen thickness on crack front straightness is reduced. If straightness values of fatigue pre-cracks for parent metal specimens are calculated to the criteria given in Part 4, the average crack front straightness is 12.4% and the percentage of specimens in the data set which qualify is 34.5%. 72 to 84% of Weld and HAZ specimens meet the crack straightness requirements in Part 4.
Figs.4 and 5 show charts of crack front straightness versus specimen thickness for crack front straightness defined as per ISO 12135 and BS EN ISO 15653 (for welds and HAZs), and ASTM E1820 respectively. The crack front straightness calculated to ISO 12135 would result in an average of 5.5% and a pass rate of 89.7% for parent metal specimens. The weld and HAZ specimens have an average pre-crack straightness of 10%, and when permitted up to 20% crack straightness in BS EN ISO 15653 means that 96% of these specimens are qualified. ASTM E1820 defines crack front straightness using specimen thickness and sets a 5% criterion limit. Under this regime, 100% of parent metal specimens in the data set would qualify, and over 92% of weld and HAZ specimens.
The relationship between ASTM E1820 crack front straightness and specimen thickness for parent metal specimens in the data set is actually the opposite of that exhibited under the other standards. The data shown in Figure 5 suggest that crack front straightness of parent material gets slightly worse with increasing specimen thickness.
The various methods to determine crack curvature given in the different standards give a corresponding wide range of qualification rates, but does it matter which test standard method is used? The objective of setting limits within a testing standard is to avoid validating test results from specimens with crack curvature that is sufficiently excessive to affect the fracture toughness result. Before a particular method can be recommended for a testing standard for SENT testing, it is necessary to make sure that any curvature limits are neither too lenient nor over-restrictive. A reasonable level of crack curvature should not cause the fracture toughness to vary by more than 10% from a perfectly straight crack front, based on other validity limits in Standards being set to give a similar level of error. To quantify the effect of crack curvature on the fracture toughness a series of numerical models were carried out for SENT specimens of similar dimensions and average crack depth, varying only in the amount of crack curvature.