Chevron notched short bar testing of high toughness steels

Theoretical and Applied Fracture Mechanics 5 (1986) 109-116 North-Holland

109

CHEVRON NOTCHED S H O R T BAR TESTING OF HIGH TOUGHNESS STEELS J. MORRISON and J.P. G O U G H Defence Research Establishment Pacific FMO Victoria, BC, Canada VOS IBO

Chevron notched short bar tests offer a simple, inexpensive method of fracture toughness measurement for both quality control and analytical purposes. They also provide data for thinner specimens than current techniques allow. This report describes an investigation into the applicability of this technique to medium strength, high toughness steel, which cannot easily be tested using plane strain fracture toughness methods. The results show that while the test procedure provides an accurate ranking of these steels in order of toughness, it systematically overestimates the values obtained from alternative test methods. In addition, current plasticity correction analyses are inadequate.

1. Introduction

The most widely used parameter for routine fracture resistance estimation as a basis for quality control is the Charpy 'V' notch impact energy. Although inexpensive, material efficient, and straightforward to carry out, the Charpy test has no direct quantitative significance. Minimum required values are generally established by correlation with other tests or service experience and there is no consistent method of applying such data for the establishment of industry-wide quality control standards. The incorporation of fracture mechanics criteria, including damage tolerance and sub-critical crack growth, into design is generally recognised as offering improved component safety and reliability. However, the basic parameters of such analyses, notably fracture toughness, are more difficult and expensive to determine experimentally by standard methods such as ASTM E399, ASTM [2], and require a distinctly more sophisticated laboratory capability than that needed for routine quality control purposes using, for example, a Charpy test. An additional problem is that valid Ktc measurements require specimens whose dimensions exceed a minimum specified value, such that the toughness can be considered a material property which is not specimen size dependent. In smaller specimens, the loss of elastic constraint at the crack tip leads to deviation from plane strain

conditions and yields K values which may have little physical significance when applied to real structures. Figure 1 shows the toughness yieldstress domain within which a valid plane strain toughness measurement can be made on a 25 mm specimen. Clearly many important structural materials fall outside this linear elastic region. Recent intensive development has led to the standardization of new procedures for fracture characterization of ductile materials. These materials exhibit elastic-plastic, rather than linear elastic behaviour under normal test conditions in the available section thicknesses. Such concepts as J-Integral, ASTM [3], R-Curve, ASTM [4], Albrecht et al. [1], and Equivalent Energy, ASTM [5] are being employed. Unfortunately, these methods tend to be even more demanding of laboratory capability than linear elastic K~c tests. They can, however, be used to provide toughness estimates from relatively small specimens. This capability is indicated by the elastic-plastic region of Fig. 1 which is defined by the specimen size restrictions of the ASTM J-Integral procedure. The short bar test, first introduced by Barker [7], has been proposed as a more widely employable, simple technique for toughness measurement. The chevron notched specimen is said to provide a measure of the energy required per unit area to quasistatically advance a steady state crack under plane strain conditions, and that this corresponds to the property measured by a conventional ASTM E399 fracture test. Due to the increased degree of

016%8442/86/$3.50 1986, Elsevier Science Publishers B.V. (North-Holland)

110

J. Morrison, J.P. Gough / Chevron notched short bar testing

f

,/

2soH |

/

/

'ASTM" ' / 7 "

'

Ea 3

LIMIT

/

~

L

I

SHORT BAR MIT

_

7

ASTM

0 (

E399 LIMIT

150

o n

m ioo

50

0

LINEAR

250

ELASTIC

I

I

I

I

I

I

500

750

I000

1250

1500

1750

2000

YIELD STRENGTH (MPo) Fig. 1. Toughness-yield strength range for 25 mm thick fracture specimens.

crack tip constraint, a 25 mm short bar specimen is said to be capable of providing data up to the limit shown in Fig. 1, whereas a valid ASTM E399 test would require a 70 mm specimen thickness. This is of particular interest for testing elasticplastic materials. There is reasonable agreement that the short bar specimen can provide meaningful estimates of plane strain fracture toughness for relatively brittle materials, Barker [6], including quartz, concrete, and low toughness metals, Barker and Baratta [8], Munz [13]. In these cases, toughness is estimated directly from maximum load, which means that the test is very simple to perform. In 1981, an SAE Aerospace Recommended Practice, Society of Automotive Engineers [14], was issued, aimed principally at aluminum alloy testing. At the time of writing, an inter-laboratory test program is being conducted by ASTM Task Group E24.01.05 using high strength steel, as well as aluminum and titanium alloys. As part of a general assessment of the Short Bar technique, and in particular, its applicability to a wider range of elastic-plastic materials, this paper describes its application for toughness determination in a number of medium strength, high toughness, low alloy steels. The results are compared to previously reported estimates made using ASTM procedures.

2. Experimental 2.1. Chemical composition and mechanical properties

Eight quenched and tempered low alloy steels were used in these experiments. Although they all belong to essentially the same class of Ni-Cr-Mo-V low alloy steels, and almost all fall within the elastic-plastic region of Fig. 1, there are significant differences in strength and toughness, arising mainly from the different fabrication histories. Table 1 shows the chemical compositions and mechanical properties of each steel. 2.2. Fracture testing in accordance with A S T M standards E399 and E813

During the earlier studies, estimates of the fracture toughnesses were made using standard ASTM procedures using 25 mm thick arc tension specimens. The load at crack initiation was estimated by means of a 5% change in compliance in accordance with a Kt~ determination of ASTM E399. Unfortunately, with the exception of steels A and B, the 25 mm maximum specimen thickness limited an E399 test to an invalid Kc~ estimate of toughness, which is specimen size dependent. For these high toughness steels, therefore, frac-

J. Morrison. J.P. Gough / Chevron notchedshort bar testing

111

Table 1 Basic properties Chemical composition (wt.%) a Steel C

Si

Mn

Cr

Ni

Mo

V

A B C D E F G H

0.21 0.26 0.20 0.27 0.22 0.25 0.25 0.26

0.65 0.73 0.60 0.56 0.58 0.48 0.49 0.73

1.03 0.91 1.00 0.82 1.00 0.94 1.20 0.94

2.73 2.53 2.06 2.91 2.02 3.14 3.12 2.65

0.52 0.47 0.52 0.55 0.42 0.60 0.44 0.40

0.15 0.10 0.15

0.32 0.30 0.30 0.38 0.32 0.38 0.37 0.32

Tensile and impact properties Steel Oy b (M Pa) A B C D E F G H

ou (M Pa)

1230 1330 1250 955 1215 1070 1100 950

Eiong. (%)

1295 1340 1295 1080 1275 1250 1185 1025

15 16 17 18 16 18 18 19

Oy/%

0.17 0.15 0.14

Charpy V-notch (Joules)

0.95 0.99 0.97 0.88 0.95 0.86 0.93 0.93

- 40oC

27°C

17 18 27 28 28 39 28 56

24 26 38 41 41 43 49 60

a bal. Fe+O.Ol% P+O.Ol% S b 0.2% offset.

ture resistance was also assessed b y m e a n s of the r e c o m m e n d e d p r o c e d u r e of A S T M E813 for J - I n tegral m e a s u r e m e n t using the single specimen unl o a d i n g c o m p l i a n c e technique. T a b l e 2 shows the fracture toughness d a t a from earlier studies using these techniques. F o r these q u e n c e d a n d t e m p e r e d steels, there is a well docum e n t e d e m p i r i c a l c o r r e l a t i o n between u p p e r shelf C h a r p y ' V ' n o t c h energy and toughness, Barsom Table 2 Fracture toughness test data Steel

A B C D E F G H a Valid Kit.

K(MPafm) E399

E813

From Charpy

120 a 113 a 151 121 138 129 140

108

101 107 146 144 153 153 168 184

148 131 128 141 149 180

a n d Rolfe [11], a n d the p r e d i c t e d K~¢ values are shown for c o m p a r i s o n , together with Kt~ values e s t i m a t e d from J,¢ using the expression

f

Kl¢ = ~[ V

JicE

1 -/-

(1)

It can b e seen that, with increasing toughness, K o (E399) e s t i m a t e s fall b e l o w those o f the o t h e r two m e t h o d s . O n the o t h e r h a n d , C h a r p y p r e d i c t ions fall b e l o w valid Ktc values for low toughness materials, where the u p p e r shelf energy c o r r e l a t i o n is n o t strictly a p p l i c a b l e . T h e d a t a e s t i m a t e d f r o m the J~c tests t e n d to fall b e t w e e n those for the o t h e r two m e t h o d s , with n o s y s t e m a t i c d e v i a t i o n with increasing toughness. A l l the d a t a fall within 10% of the mean.

2.3. Short bar testing procedure Short b a r s p e c i m e n s of each steel were machined from the b r o k e n halves o f the arc tension s p e c i m e n s used in the earlier tests, Laforce a n d M o r r i s o n [12]. T h e d e t a i l e d short b a r g e o m e t r y is

J. Morrison, J.P. Gough /Chevron notched short bar testing

112

B

shown in Fig. 2 and the relatively small size of the specimen can be appreciated by noting that it weighed only 7% of the same thickness arc tension specimen. The test procedure followed was essentially that of SAE ARP1704 and the ASTM task group. The specimens were loaded in stroke control in a 220 kN servohydraulic test machine. Specimen mouth opening along the load line is monitored by means of an external clip gauge (Fig. 3). The clip gauge was fabricated from titanium in accordance with a design provided by the ASTM task group. A similar gauge design is contained in the SAE document. This assembly, complete with 500 ~ foil strain gauges epoxied to each arm, gives an output of 10 m V / V / m m over a range of 2.5 mm from a nominal 25 mm initial separation.

r

V

-~ I0

/I

ii

Geometry parameter B 0

Io/B L/B H/B S/B T/B "r/B R/B

\\

\\

Value

Tolerance

> (KlcSB/Oys) 2 55.2 ° 0.531 1.500 0.870 0.130 0.313 0.015 > 2.0

+0.5 ° + 0.005 + 0.010 + 0.005 +0.010 ±0.005 ± 0.015 -

Fig. 2. Short bar specimen dimensions and tolerances.

2.4. Data analysis Consideration of the stability criteria for a crack growing from the 'V' notch of a short bar specimen indicates that there is a range of crack length over which stable crack extension occurs. An increasing load is required to advance the crack until, at the critical crack length, the applied force reaches a maximum value, Barker [7]. Low toughness materials exhibit predominantly elastic behaviour, characterized by sudden crack jumps and sharp discontinuities in the load displacement curve. In these cases, l~lane strain fracture toughness is estimated directly from the maximum load, FMAx, the critical crack length is defined by

bG,¢= TEST

PIECE

I

CLIPtGAUGE

FM2AX

2

( d-d~-C] Qa: ....

=

bKI2c(1 E

p2)

'

(2)

where b is the width of the crack front, Gl¢ is the plane strain critical strain energy release rate, C is the specimen compliance and a is the crack length. Hence

K,¢ = FMAxEt/2 ( dC ]1/2 (2b)'/2

(3)

~,d--a'a] . . . .

AFMAx

= B3:(a _ :),/2,

(4)

where A2 Fig. 3. Schematic short bar testing arrangement.

eS 3

-- -2"b- ( da ] . . . .

,

(5)

J. Morrison, J.P. Gough / Cheoronnotchedshort bar testing

113

p may also include a component resulting from the presence of residual stresses near the crack tip which will change the slope of the unloading/reloading curves.

where A is a geometric calibration parameter for the specific test piece dimensions. Published data are available for estimating values for the calibration coefficient A, Barker [10]. For the standard specimen of Fig. 2, A has been estimated at between 22.0 and 23.9, with an uncertainty of +5%. For conservatism, the recommended value is 22.0. More ductile specimens give smooth loaddisplacement curves without large discontinuities. Non-linearity due to local plasticity at the point of the chevron notch can cause errors in K estimates based on FMAx. In this case, a test procedure has been developed to permit the calculation of a plasticity correction factor based on the deviation of specimen compliance from an ideal elastic value. This approach requiries a number of unloading-reloading cycles to be carried out during the test to determine the load at which the specimen compliance indicates that the specified critical crack length, equivalent to the peak load position in an ideal linear elastic test, has been reached. In the case of the 25 mm specimen of Fig. 3, the critical crack length occurs after an increase in compliance of 138%. As shown in Fig. 4, the critical load, FCRIT, is estimated by extrapolation between unloading slopes which straddle the required value denoting the critical crack position. The plasticity correction factor, p, is estimated from the unloading slopes as shown in the figure and the term FCRIT(1 + p ) replaces FMAx in eq. (4). When p is zero, the change in compliance between adjacent unloadings is entirely due to crack growth (ie, linear-elastic behaviour), when p is unity, it is entirely due to plasticity. The test method is considered valid for p values less than 0.2. The term

3. Results

Figure 5 shows typical load-deflection curves (no unloading interruptions) for each of the steels tested. FMAx and the displacement at which FMAx occurs increase with increasing toughness. A measure of the nonlinearity due to plasticity effects is indicated in Fig. 6, where displacement is plotted as a function of the dimensionless compliance, E B C , taken from the unloading/reloading slopes. At a given value of compliance, the spread in displacement is a measure of the plasticity differences between the samples. Typically 5 or 6 unloading/reloading interruptions were carried out during each test, rather than the two or three recommended, in order to gain further insight into specimen plasticity behaviour. As can be seen in Fig. 7, as toughness increases and plasticity effects become more pronounced, FCRIT is displaced to larger mouth openings and both the hysteresis in the unloading curves and the A Xo values increase. For low toughness material, A X0 is negative at small mouth opening displacements, implying crack closure due to residual stress effects at some positive applied load. A model analogous to that for plasticity correction has been used, Barker [9], to demonstrate that the factor (1 + p) includes residual stress effects. The materials used in this study had been subject to various degrees of cold work during fabriI I

2

~..~CRIT

F

(ie t a n o

iI

/ 0

q

b

C o

MOUTH OPENING

MOUTH OPENING

AXo

MOUTH OPENING

Fig, 4. Plasticitycompliancecorrection; (a). Linear elastic, (b). Elastic plastic, (c). Plasticityfactor estimation.

= 0.42 )

tanoo

114

J. Morrison, J.P. Gough /Chevron notched short bar lesting

40

a a

2OL

//

0t /

, A ~B ~C "D\F ,

0

1

2

20

MOUTH OPENING (mm)

Fig. 5. Typical short bar load displacement curves.

o

cation to provide beneficial residual stresses. A large component of these stresses would be released during specimen preparation, however, the different levels of residual stress will result in different degrees of relaxation. This will account,

b o MOUTH

OPENING (ram)

40 500

o-A x-B

400

+ _C A _D I-E v-F

o

ZJ¢

o -G

300

cI

o-H

20

O .J

÷ 200 a

x

v 100

•

%

C

a o,. -o" I

0.4

I

<>

o I

0.8

I

I

I

1.2

I

1.6

M O U T H OPENING (ram)

Fig. 6. Compliance as a function of mouth opening.

I 2.0

0

1 MOUTH OPENING (ram)

Fig. 7. Unloading responses for steels of different toughness; (a) Steel A, (b) Steel F, (c) Steel H.

J. Morrison, J.P. Gough / Chevron notched short bar testing Table 3 Short bar test results Steel

KIcSa (MPa~fm)

KIc (Estimate from Table 2) (MPavrm)

% Difference

A B C D E F G H

128 128 149 159 165 162 184 194

120 113 146 144 153 153 168 184

+7 +13 +2 +10 +8 +6 + 10 +5

a a b b b b b b

" Valid Kic. b Charpy correlation.

to some extent, for the observed differences in A X0 at small extensions. It is clear that these steels behave in an elasticplastic manner and therefore merit toughness estimation using the (1 + p ) correction. In practice, it is found that AX0 cannot be determined sufficiently accurately to render an accurate value of the parameter p. In the region of the critical unloading slope for the determination of Fcm x, A Xo has a value of 0.002-0.010 mm with an uncertainty of + 0.002 ram. Also, A X0 is a rapidly increasing function of the displacement in this vicinity, thus the p value may vary dramatically according to the exact location of the unloading points between which it is measured. Given the relatively flat curves and the tendency for FCRIT tO fall close to FMAx for all but the toughest steel, the K estimates have been based solely on FMAx according to eq. (4) and the average values are shown in Table 3.

115

A similar trend, with more extreme deviations, has been reported, Munz [13] for aluminum alloys, the discrepancy increasing with increasing toughness; a trend not observed in Table 3. Since the plasticity correction was intended to compensate for a loss in apparent toughness (ie, a lower KIcSB), the use of FMAx would also be expected to yield conservative results. A similar result would be expected from ASTM E399 results, where plasticity effects are ignored subject to adequate specimen thickness and a limit on the ratio of maximum load to critical stress intensity load. The latter restriction frequently invalidates otherwise acceptable data. The trend toward non-conservative K~csB results at high toughness reflects the fact that the critical stress intensity is reached at greater crack lengths. In any short bar specimen, the crack has already propagated a relatively long distance compared to the 2% crack growth measurement point of a standard KI¢ test. Some deviation between the two test results is therefore expected. In the short bar tests, the relatively flat loaddisplacement records indicate increasing resistance to crack growth. As in cases where there is a rising crack growth resistance curve, K increases as the crack extends, leading to greater discrepancies between short bar and ASTM E399 test results. A rising resistance curve is associated with increasing material toughness or a loss of plane strain constraint due to, for example, inadequate specimen thickness. The stress intensity necessary for crack growth thus increases with crack extension.

4. Discussion

5. Conclusions

The reproducibility of the short bar test was typically + 2%, significantly better than would be expected from the other procedures. The ranking of the steels in terms of toughness closely follows that of Table 2, particularly with respect to the Charpy predictions which are shown for comparison (except where valid KI¢ data are available) in Table 3. Although the calibration constant, A, is expected to yield conservative results, the short bar fracture toughness, KxcsB, values are always higher than the Kk's predicted from the other techniques.

Short bar toughness tests employ easily prepared samples, are simple to perform, and, in principle, require straightforward data analysis. While the results provided a good ranking of the steels investigated in terms of toughness, they consistently overestimated K~c relative to other techniques. Further work is required to clarify the discrepancy in measurement point relative to a plane strain test and the plasticity correction procedure.

116

J. Morrtson. J.P. Gough /Chet,ron notched short bar testing

Acknowledgment T h e a u t h o r s g r a t e f u l l y a c k n o w l e d g e the assist a n c e given b y C.H. L a f o r c e of the D e f e n c e Research E s t a b l i s h m e n t V a l c a r t i e r w h o p r o v i d e d the s p e c i m e n s used in this study.

References [1] P. Albrecht et al., "'Tentative Procedure for Determining the Plane Strain Jt-R Curve", J. Test and Eval. 10, 245 (1982). [2] ASTM, "Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials", Annual Book of ASTM Standard, ASTM, Philadelphia (1981 ). [31 ASTM, "Standard Test for Jic, a Measure of Fracture Toughness", Annual Book of ASTM Standards, ASTM, Philadelphia (1981). [4] ASTM, "Standard Practice for R-Curve Determination", Annual Book of ASTM Standards, ASTM, Philadelphia (1981). [5] ASTM, "Practice for Determining a Fracture Toughness of Steel Using Equivalent Energy Method", Working Document of ASTM Subcommittee E24.03 on Alternative Fracture Test Methods, ASTM, Philadelphia (1983). [6] L.M. Barker, "A Simplified Method for Measuring Plane Strain Fracture Toughness", Engrg. Fracture Mech. 9, 361 (1977).

[7] L.M. Barker, "Theory for Determining Kic from Small Non-LEFM Specimens, Supported by Experiments on Aluminum", lnternat. J. Fracture 1.5, 515 (1979). [8] L.M. Barker and F.J. Baratta, "Comparison of Kic Measurements by the Short Rod and ASTM Standard Method of Test for Plane Strain Fracture Toughness of Metallic Materials (E399-78)", J. Test and Eval. 8. 97 (1980). [9] L.M. Barker, "Residual Stress Effects on Fracture Toughness Measurements', in: D. Francois, ed., Advances in Fracture Research." Proc. 5th lnternat. Conference on Fracture, Vol. 5 Pergamon, New York. 2563 (1981).

[10] L.M. Barker. "Compliance Calibration of a Family of Short Rod and Short Bar Fracture Toughness Specimens", Engrg. Fracture Mech. 17, 289 (1983). [11] J.M. Barsom and S.T. Rolfe, "Correlation Between Ktc and Charpy V-Notch Test Results in the Transition Temperature Range". in: D.E. Driscoll, ed.. Impact Testing of Metals, ASTM STP 466, ASTM, Philadelphia, 281 (1970). [12] C.H. Laforce and J. Morrison, "Jl,- Determination Using the Proposed Standard C-Shaped Specimen", J. Eng. Mat. Teeh. 100. 248 (1978). [13] D. Munz, "Determination of Fracture Toughness of High Strength Aluminum Alloys with Chevron Notched Short Rod and Short Bar Specimens", Engrg. Fracture Mech. 15, 231 (1981). [14] Society of Automotive Engineers. "Determination of Short Bar Fracture Toughness of Metallic Materials. Aerospace Recommended Practice ARP 1704", SAE, Warrendale, PA (1981).