Notch yielding

Fig. 2. Notch strength ratios and notch-yield ratios for Al-Zn-Mg alloys at room and cryogenic temperatures 0.063-in.-thick sheet, transverse.

Alloy and temper Tensile Yield Elongation Temperature, strength, strength, in 2 in, °F psi psi % Notch tensile strength, psi Notch strength ratio Notch yield ratio... 

The results of the tests of the smooth and sharply notched Kt >17) specimens are shown in Table II. The notch strength ratios (ratio of notch tensile strength to tensile strength) and notch yield ratios (ratio of notch tensile strength to tensile yield strength) are shown in Fig. 2 as a function of temperature. 

With decreasing temperature the alloys exhibited some decrease in notch strength and notch yield ratios the amount of the decrease varied with composition and temper. The plots of the ratios as functions of temperature in Fig. 2 are smooth curves, with no suggestion of abrupt transitions in fracture behavior. The data for those alloys that were tested only at temperatures to —320°F may be compared readily with those for alloys tested down to —423°F, with little likelihood of misinterpretation. 

The complete comparison of these alloys cannot be gained from the curves in Fig, 2. Considerably more information is gained from a plot of notch yield ratio as a function of the tensile yield strength, as in the example in Fig. 3 for —320°F also shown are data points for other 7000 series alloys, 2014-T6 and 2219-T87, and a band for data from tests of other alloys [ ]. This plot shows that as yield strength increases, some increase in sensitivity to notches is experienced, and that the data points for most aluminum alloys fall into a narrow range. 

At room temperature, all of the alloys are relatively insensitive to notches, as indicated by relatively high ratios (notch-yield ratios above 1.06), and ratios that are nearly the same for all compositions in -T6-type tempers. For the —W temper, the notch yield ratio is appreciably higher, but this is accomplished at considerable sacrifice in yield strength. 

Alloys X7006-W, X7005-T6, X7006-T63, and X7106 in the -T6 and -T63 tempers are relatively insensitive to notches over the entire temperature range from room temperature to —423°F. Notch yield ratios of some of these materials with sharply notched specimens decrease somewhat, but are about equal to or greater than about 1.0 at —320°F and about 0,95 at —423°F,... 

The notch toughness of X7002-T6, X7006-T6, and X7039-T6 decreases with decreasing temperature, but even at —423 F, the notch yield ratios of the latter two alloys... 

The notch toughness of aluminum alloys is generally related to the level of yield strength with increase in yield strength, notch toughness, as indicated by notch yield ratio, decreases. 

J. G. Kaufman and E. W. Johnson, The Use of Notch-Yield Ratio tq Evaluate the Notch Sensitivity of Aluminum Alloys, Am. Soc. Testing Materials Proc., 62 (1962). 

The ratio of averaged stress on the notched section at maximum load, ay,net (hereafter referred to as fracture strength), to 0.2% proof stress, o-o.2, at the same temperature, also called the notch-yield ratio (oy net/cro,2), is plotted against specimen width in Fig. 2. The data points for the 1000- and 1600-mm-wide specimens... 

The notch-yield ratio was hardly affected by temperature and decreased with an increase in specimen width. The fracture strength of a very wide plate, with a notch such as a notched structure, can be evaluated using a deep-notch test specimen 400 mm wide the notch-yield ratio was found to be about 1.3. The mean line in Fig. 2 may thus be approximated by the following hyperbolic equation ... 

The notch yield ratio, ctbh/o o.i has been accepted as a criterion for evaluating a material s ability to deform plastically in the presence of a sharp notch. This ratio was >2 (Tables III through VI) for all materials, indicating a high level of... 

An indication of the inherent ability of a material to plastically deform locally in the presence of a severe stress-raiser is provided by the notch-yield ratio (NYR)—the ratio of the notch-tensile strength to the tensile yield strength. 

---