Jg Kaufman

Notch-Yield Ratio as a Quality Control Index for Plane-Strain Fracture Toughness

The ratio of notch-tensile strength to tensile-yield strength (the notch-yield ratio) is correlatable with plane-strain fracture toughness, K I c . As a result of that fact and relative simplicity of notch-tension testing,it is suitable for plant quality control testing for fracture toughness. The test procedures and quality control testing practices are described, and data showing the correlation between K I c and notch-yield ratio from both 1 / 2 and 1 1 / 1 6 -in.-diameter notched tension specimens for several aluminum alloys are presented. The 1 1 / 1 6 -in.-diameter specimen provides a more discriminating correlation at high toughness levels than does the 1 / 2 -in.-diameter specimen. It is emphasized that this is not a procedure for estimating K I c directly, but for providing assurance that K I c equals or exceeds a stated value.

Creep Cracking in 2219-T851 Plate at Elevated Temperatures

The susceptibility of 2219 to time-dependent (creep) crack growth under sustained load has been evaluated, and, while no crack growth was observed at room temperature, it was observed at elevated temperatures at stress intensities (K I c c ) well below K I c . It appears that creep cracking will take place at stress intensities down to a threshold, designated K I c c t , about 40 percent of K I c at 300°F. and preliminary check tests suggest that similar behavior would be in evidence at 212 and 350°F in both the L-T and T-L orientations, though much work remains to be done in defining the extent of the temperature dependence. The rate of crack growth is controlled primarily by the instantaneous stress intensity factor and can be described by the following relationship: log da/dt = 0.085K - 4.14.

More on specimen size effects in fracture toughness testing

A new study of the effect of specimen size on the results of plane-strain fracture toughness tests of a relatively tough aluminum alloy, 2219-T851, suggests that an increase in specimen size requirements may be necessary to assure size-independent test results. Specifically, it appears that the crack length limit should be increased to 5(K I c /σ Y S ) 2 , which has the effect of keeping the maximum nominal net-section stress below two thirds of the yield strength. The current limit on thickness could be maintained at 2.5 (K I c /σ Y S ) 2 , although there is some evidence that it might be relaxed further. Additional work is needed to check the generality of these suggestions for other materials, as well as the possibility that if W/B=2 and a=B 5(K I c /σ Y S ) 2 , K m a x may be useful as an engineering estimate of K I c .

Fracture Toughness of plain and welded 3-in.-thick aluminum alloy plate

The plane-strain fracture toughness, K I c , of 3-in. 5083-H321, 5086-H32, 6061-T651, and 7005-T6351 plate and welds in the 5083, 6061, and 7005 plate have been evaluated in tests carried out in accordance with ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399-72). Valid values of K I c could be obtained only for the 7005-T6351 plate in all orientations. Both 5083-H321 and 5086-H32 are so tough that even 3-in.-thick specimens did not provide adequate restraint to permit valid measurements of K I c , and in most tests of 6061-T651 there was excessive plasticity masking a K I c measurement. From criteria that consider the load-carrying capability of the precracked fracture-toughness specimen and the yield strength, the alloys and tempers rate in the following order: 5086-H32 (toughest) 5083-H321 7005-T6351 6061-T651 However, the 7005-T6351 appears to provide the best combination of strength and toughness. Valid values of K I c could not be obtained for most welds, primarily because of their very high toughness, and because of the problem in precracking resulting from the residual stresses from welding. Overall comparisons indicate that (a) welds of the recommended filler alloys are as tough as or tougher than the parent material, and (b) 5039 and 5356 welds are generally tougher than 4043 welds, either as-welded or heat-treated and aged after welding.

Sharply Notch Cylindrical Tension Specimen for Screening Plane-Strain Fracture Toughness. Part I: Influence of Fundamental Testing Variables on Notch Strength. Part II: Applications in Aluminum Alloy Quality Assurance of Fracture Toughness

This paper is concerned with the use of the sharply notched cylindrical specimen as an index of plane-strain fracture toughness in quality assurance of aluminum alloy products. Specifically, information is presented that relates to the use of the ASTM Tentative Method for Sharp-Notch Tension Testing with Cylindrical Specimens (E 602-76T) in quality assurance programs. The first part of this paper describes the results of an investigation into the influence of fundamental testing variables on the sharp notch strength of several high-strength aluminum alloys. The results indicate that variations in the notch root radius and eccentricity of loading (expressed in terms of the percent bending in a verification specimen) within the range permitted by the Tentative Method can contribute significantly to the scatter observed in relations between the sharp notch to yield strength ratio (NYR) and K I c . The results also show that the upper limit of K I c beyond which the NYR loses useful sensitivity to further increases in K I c decreases with decreasing specimen size (diameter). It appears that the notch strength of the smaller of the two specimens (½ and 1 1/16 in. diameter; 13 and 27 mm diameter) specified in the Tentative Method will have rather limited application as an index of K I c for the tougher high-strength aluminum alloys. However, the upper limit of 1.3 presently placed on the NYR appears to be overly conservative for high-strength aluminum alloys. The second part of the paper describes the statistical analysis of correlations between the NYR and K I c for various lots of 2124-T851 aluminum alloy plate. The purpose of the analysis was to demonstrate how the sharp-notch cylindrical specimen could be used in a quality assurance program for high-strength aluminum alloy products based on a minimum acceptable value of K I c . The results indicate that the NYR from the larger of the two specimens specified in the Tentative Method provides a better correlation with K I c than does the NYR from the smaller specimen. A modified regression analysis is introduced which establishes tighter tolerance limits for the correlations than can be obtained using conventional procedures. The consequence is an improvement in the cost effectiveness of quality control procedures using the sharply notched cylindrical specimen. A review of existing data shows that crack orientation and product thickness can influence the correlations but that for practical purposes of quality assurance, correlations based on the T-L orientation will ensure that the minimum value of K I c is exceeded in all three orientations. Thickness effects can be handled by establishing separate correlations depending on whether the plate product is greater or less than 4 in. thick. Employing the modified regression analysis, a simple quality assurance plan for fracture toughness guarantee of aluminum alloy products was developed and shown to be cost effective based on available data for the aluminum alloy 2124-T851.

Large-scale fracture toughness tests of thick 5083-0 plate and 5183 welded panels at room temperature, -260 and -320°F

The exceptionally high toughness of thick 5083-0 plate and 5183 welded panels at room temperature, -260 and -320°F has been demonstrated by large-scale fracture toughness testing. No evidence of a fracture instability at elastic stresses was observed, even in K I c tests of 7.7-in.-thick notched bend specimens and in a K c test of a 4-in.-thick and 44-in.-square edge-notched tension panel. Ductile tearing was observed in all cases with panels as thick as or thicker than those used in critical structures such as liquefied natural gas tanks. The toughness at -260 and -320° F was as high as or higher than at room temperature by all available indexes. While these conclusions indicate that the application of fracture mechanics to the design of 5083-0 tanks is not very appropriate and may be unduly conservative, some engineering estimates of K I c and K c were made from the results of the fracture tests.

Influence of Stress Intensity Level during Fatigue Precracking on Results of Plane-Strain Fracture Toughness Tests

One of the more troublesome parts of conducting a plane-strain fracture toughness test in accordance with ASTM Test for Plain Strain Fracture Toughness of Metallic Materials (E 399-72) is producing a satisfactory fatigue crack in the specimen prior to fracturing it. The 1968 version of ASTM E 399 indicated that the stress intensity during the last stage of fatigue cracking should not exceed 0.0012 in. 1 /2 E (modulus of elasticity) or 0.5 K I c but that has been gradually eased so that the current limits are 0.002 in. 1 /2 E or 0.6 K I c . Data for four aluminum alloys support the fact that it is satisfactory to use stress intensities up to 0.002 in. 1 /2 E and suggest that the other limit could be increased to 80 percent of K I c . This may be unique to aluminum alloys, and additional data should be generated for other metals to determine whether or not this is generally true.

Effects of Some Variables in Specimen Selection and Procedures on the Results of Shear Tests

Studies of the effect of variations in test method and specimen orientation and location on the results of shear tests of aluminum alloys have shown that, in double-shear tests, (a) specimen size and lubrication-even thatfrom the human skin-affect shear strength values by 3 to 4 percent, and (b) shear strength varies with location of shear plane through the thickness of aluminum alloy products. In blanking-shear tests, clearance between punch and dies up to 12 percent of the sheet thickness has no significant effect on test values.

An Automated Method for Evaluating Resistance to Stress-Corrosion Cracking with Ring-Loaded Precracked Specimens

This paper describes a ring-loading method for evaluating stress-corrosion resistance with precracked specimens and presents representative data for compact specimens of some high-strength aluminum alloys. The deflection of the ring was large relative to that of the specimen, simulating deadweight loading. This method provides several advantages over techniques previously used. Unlike most constant-displacement loading systems, the load and crack length and thus the stress intensity are known accurately throughout the life of each test, and the initiation and growth of stress-corrosion cracking (SCC) lead to a definitive end point (failure). Also, the effects of corrosion product wedging are minimal. The ring loading system is more compact than most deadweight systems, and because it is readily automated, data can be collected with the expenditure of few manhours.