cryogenics,mechanical properties
LOW-TEMPERATURE PROPERTIES OF
ENGINEERING MATERIALS
ENGINEERING MATERIALS
Familiarity with the properties and behavior of materials used in any system is essential to the design engineer. At first thought, one might suppose that by observing the variation of material properties at room temperature he could extrapolate this information down through the relatively small temperature range involved in cryogenics (some 300°C) with fair confidence. In some cases, such as for the elastic constants, this may be done with acceptable accuracy. On the other hand, there are several significant effects that appear only at very low temperatures. Some examples of these effects include the vanishing of specific heats, superconductivity, and ductile-brittle transitions in carbon steel. None of these phenomena can be inferred from property measurements made at near-ambient temperatures.
In this chapter, we shall investigate the physical properties of some engineering materials commonly used in cryogenic engineering. The primary purpose of the chapter is to examine the effect of variation of temperature on material properties in the cryogenic temperature range and to become familiar with the properties and behavior of materials at low temperatures.
MECHANICAL PROPERTIES
2.1. Ultimate and yield strength
For many materials, there is a definite value of stress at which the strain of the material in a simple tensile test begins to increase quite rapidly with increase in stress. This value of stress is defined as the yield strength Sy of the material. For other materials that do not exhibit a sharp change in the slope of the stress-strain curve, the yield strength is defined as the stress required to permanently deform the material in a simple tensile test by 0.2 percent (sometimes 0.1 percent is used). The ultimate strength Su of a material is defined as the maximum nominal stress attained during a simple tensile test. The temperature variation of the ultimate and yield strengths of several engineering materials is shown in Figs. 2.1 and 2.2
Fig.2.1. Ultimate strength for several engineering materials: (I) 2024-T4 aluminum; (2) beryllium copper; (3) K Monel; (4) titanium; (5) 304 stainless steel; (6) CI020 carbon steel; (7) 9 percent Ni steel; (8) Teflon; (9) Invar-36 (Durham et al. 1962).
Many engineering materials are alloys, in which alloying materials with atoms of different size from those of the basic material are added to the basic material; for example, carbon is added to iron to produce carbon steel. If the alloying-element atoms are smaller than the atoms of the basic material, the smaller atoms tend to migrate to regions around dislocations in the metal. The presence of the smaller atoms around the dislocation tends to "pin" the dislocation in place or make dislocation motion more difficult (Wigley 1971). The yielding process in alloys takes place when a stress large enough to pull many dislocations away from their "atmosphere" of alloying atoms is applied. Plastic deformation or yielding occurs because of the gross motion of these dislocations through the material.
As the temperature is lowered, the atoms of the material vibrate less vigorously. Because of the decreased thermal agitation of the atoms, a larger applied stress is required to tear dislocations from their atmosphere of alloying atoms. From this line of reasoning, we should expect that the yield strength for alloys would increase as the temperature is decreased. This has been found to be true for most engineering materials.
Fig. 2.2. Yield strength for several engineering materials: (1) 2024- T4 aluminum: (2) beryllium copper; (3) K Monel; (4) titanium; (5) 304 stainless steel; (6) CI020 carbon steel; (7) 9 percent Ni steel; (8) Teflon; (9) Invar-36 (Durham et al. 1962).
2.2. Fatigue strength
There are several ways to express the resistance of a material to stresses that vary with time, but the most common method is a simple reversed bending test The stress required for failure after a given number of cycles is called the fatigue strength Sf. Some materials, such as carbon steels and aluminum-magnesium alloys, have the property that the fatigue failure will not occur if the stress is maintained below a certain value, called the endurance limit Se, no matter how many cycles have
elapsed. The temperature variation of the fatigue strength at 106 cycles for several materials is shown in Fig. 2.3.
Because of the time involved to complete a test, fatigue data at cryogenic temperatures are not as extensive as ultimate-strength and yield strength data; however, for the materials that have been tested, it has been found that the fatigue strength increases as the temperature is decreased.
Fig. 2.3. Fatigue strength at 1()6 cycles: (I) 2024- T4 aluminum; (2) beryllium copper; (3) K Monel; (4) titanium; (5) 304 stainless steel; (6) CI020 carbon steel (Durham et al. 1962).
Fatigue failure generally occurs in three stages for the case of more than about 10) cycles: microcrack initiation, slow crack growth until a critical crack size is achieved, and the final rapid failure either by ductile rupture or by cleavage. Microcrack initiation usually occurs at the surface of the specimen as a result of inhomogeneous shear deformation or at small flaws near the surface. The growth of the microcracks occurs as the material fails at the high-stress region around the tip of the crack. As the temperature of a material is decreased, a larger stress is required to extend the crack; therefore, we should expect to observe that the fatigue strength increases as the temperature is decreased.
For aluminum alloys, it has been found (De Money and Wolfer 1961) that the ratio of fatigue strength to ultimate strength remains fairly constant as the temperature is lowered. This fact may be used in estimating the fatigue strength for nonferrous materials at cryogenic temperatures if no fatigue data are available at the low temperatures.
2.3. Impact strength
The Charpy and the Izod impact tests give a measure of the resistance of a material to impact loading. These tests indicate the energy absorbed by the material when it is fractured by a suddenly applied force. Charpy impact strength of several materials is shown in Fig. 2.4.
Fig. 2.4. Charpy impact strength at low temperatures: (I) 2024·T4 aluminum; (2) beryl· lium copper; (3) K Monel; (4) titanium; (5) 304 stainless steel; (6) CI020 carbon steel; (7) 9 percent Ni steel (Durham et al. 1962).
A ductile-brittle transition occurs in some materials, such as carbon steel, at temperatures ranging from room temperature down to 78 K, which results in a severely reduced impact strength at low temperatures. The impact behavior of a metal is largely determined by its lattice structure. The face-centered-cubic (FCC) lattice has more slip planes available for plastic deformation than does the body-centered-cubic (BCC) lattice. In addition, interstitial impurity atoms interact only with edge dislocations to retard slipping in the FCC structure; whereas, both edge and screw dislocations can become pinned in the BCC structure. The metals with a FCC lattice or hexagonal lattice tend to fail by plastic deformation in the impact test (thereby absorbing a relatively large amount of energy before breaking) and retain their resistance to impact as the temperature is lowered. The metals with a BCC lattice tend to reach a temperature at which it is more energetically favorable to fracture by cleaving (thereby absorbing a relatively small amount of energy). Thus these materials become brittle at low temperatures.
Most plastics and rubber materials become brittle upon cooling below a transition temperature also. Two notable exceptions are Teflon and Kel-F.
2.4. Hardness and ductility
The ductility of materials is usually indicated by the percentage elongation to failure or the reduction in cross-sectional area of a specimen in a simple tensile test. The accepted dividing line between a brittle material and 'a ductile one is 5 percent elongation or a strain of 0.05 cm/cm at failure. Materials that elongate more than this value before failure are called ductile; those with less than 5 percent elongation are called brittle. The ductility of several materials as a function of temperature is shown in Fig. 2.5.
For materials that do not exhibit a ductile-to-brittle transition at low temperatures, the ductility usually increases somewhat as the temperature is lowered. For the carbon steels, which do have a low-temperature transition, the elongation at failure drops from 25 to 30 percent for the softer steels down to 2 or 3 percent during the transition. Obviously, these materials should not be used at low temperatures in any applications in which ductility is important.
Hardness of metals is measured by the indention made in the surface of the material by a standard indenter. Common hardness tests include (1) Brinell (ball indenter), (2) Vickers (diamond pyramid indenter), and (3, Rockwell (ball or diamond indenter with various loads). In general, the hardness of metals as measured by any of these means is directly proportional to the ultimate strength of the material; therefore, the hardness increases as the temperature is decreased. This proportionality is to be expected because a penetration test is essentially a miniature tensile test.
Fig. 2.5. Percent elongation for various materials: (I) 2024-T4 aluminum; (2) beryllium copper; (3) K Monel; (4) titanium; (5) 304 stainless steel; (6) C1020 carbon steel; (7) 9 percent Ni steel (Durham et al. 1962).
2.5. Elastic moduli
There are three commonly used elastic moduli: (1) Young's modulus E. the rate of change of tensile stress with respect to strain at constant temperature in the elastic region; (2) shear modulus G. the rate of change of shear stress with respect to shear strain at constant temperature in the elastic region; and (3) bulk modulus B, the rate of change of pressure (corresponding to a uniform three-dimensional stress) with respect to volumetric strain (change in volume per unit volume) at constant temperature. If the material is isotropic (many polycrystalline materials can be considered isotropic for engineering purposes), these three moduli are related through Poisson's ratio v, the ratio of strain in one direction due to a stress applied perpendicular to that direction to the strain parallel to the applied stress:
Fig. 2.6. Young's modulus at low temperatures: (I) 2024- T 4 aluminum; (2) beryllium copper; (3) K Monel; (4) titanium; (5) 304 stainless steel; (6) ClO20 carbon steel; (7) 9 percent Ni steel (Durham et al. 1962).
The variation of Young's modulus with temperature for several materials is given in Fig. 2.6.
As the temperature is decreased, interatomic and intermolecular forces tend to increase because of the decrease in the disturbing influence of atomic and molecular vibrations. Because elastic reaction is due to the action of these intermolecular and interatomic forces, one would expect the elastic moduli to increase as the temperature is decreased. In addition, it has been found experimentally that Poisson's ratio for isotropic materials do not change appreciably with change in temperature in the cryogenic range; therefore, all three of the previously mentioned elastic moduli vary in the same manner with temperature.
References
- Cryogenic Systems - Barron R. F
- Cryogenic Engineering - Scot R. W.
- Cryogenic Engineering - Bell J.H.
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