Under sufficiently adverse combinations of tensile stress, temperature, loading rate, geometric discontinuity (notch), and restraint, a steel member may experience a brittle fracture. All these factors need not be present. In general, a brittle fracture is a failure that occurs by cleavage with little indication of plastic deformation. In contrast, a ductile fracture occurs mainly by shear, usually preceded by considerable plastic deformation.
Design against brittle fracture requires selection of the proper grade of steel for the application and avoiding notchlike defects in both design and fabrication. An awareness of the phenomenon is important so that steps can be taken to minimize the possibility of this undesirable, usually catastrophic failure mode.
An empirical approach and an analytical approach directed toward selection and evaluation of steels to resist brittle fracture are outlined below. These methods are actually complementary and are frequently used together in evaluating material and fabrication requirements.
Charpy V-Notch Test. Many tests have been developed to rate steels on their relative resistance to brittle fracture. One of the most commonly used tests is the Charpy V-notch test, which specifically evaluates notch toughness, that is, the resistance to fracture in the presence of a notch. In this test, a small square bar with a specified-size V-shaped notch at its midlength (type A impact-test specimen of ASTM A370) is simply supported at its ends as a beam and fractured by a blow from a swinging pendulum. The amount of energy required to fracture the specimen or the appearance of the fracture surface is determined over a range of temperatures. The appearance of the fracture surface is usually expressed as the percentage of the surface that appears to have fractured by shear.
A shear fracture is indicated by a dull or fibrous appearance. A shiny or crystalline
appearance is associated with a cleavage fracture.
The data obtained from a Charpy test are used to plot curves, such as those in Fig. 1.11, of energy or percentage of shear fracture as a function of temperature. The temperature near the bottom of the energy-temperature curve, at which a selected low value of energy is absorbed, often 15 ft lb, is called the ductility transition temperature or the 15-ft lb
transition temperature. The temperature at which the percentage of shear fracture decreases to 50% is often called the fracture-appearance transition temperature. These transition temperatures serve as a rating of the resistance of different steels to brittle fracture. The lower the transition temperature, the greater is the notch toughness.
Of the steels in Table 1.1, A36 steel generally has about the highest transition temperature.
Since this steel has an excellent service record in a variety of structural applications, it appears likely that any of the structural steels, when designed and fabricated in an appropriate manner, could be used for similar applications with little likelihood of brittle fracture. Nevertheless, it is important to avoid unusual temperature, notch, and stress conditions to minimize susceptibility to brittle fracture.
In applications where notch toughness is considered important, the minimum Charpy V-notch value and test temperature should be specified, because there may be considerable variation in toughness within any given product designation unless specifically produced to minimum requirements. The test temperature may be specified higher than the lowest operating temperature to compensate for a lower rate of loading in the anticipated application.
(See Art. 1.1.5.)
It should be noted that as the thickness of members increases, the inherent restraint increases and tends to inhibit ductile behavior. Thus special precautions or greater toughness, or both, is required for tension or flexural members comprised of thick material. (See Art. 1.17.)
Fracture-Mechanics Analysis. Fracture mechanics offers a more direct approach for prediction of crack propagation. For this analysis, it is assumed that a crack, which may be defined as a flat, internal defect, is always present in a stressed body. By linear-elastic stress analysis and laboratory tests on a precracked specimen, the defect size is related to the applied stress that will cause crack propagation and brittle fracture, as outlined below. Near the tip of a crack, the stress component ƒ perpendicular to the plane of the crack (Fig. 1.12a) can be expressed as
of crack and to applied loading. The factor KI can be determined from elastic theory for given crack geometries and loading conditions. For example, for a through-thickness crack of length 2a in an infinite plate under uniform stress (Fig. 1.12a),
If a specimen with a crack of known geometry is loaded until the crack propagates rapidly and causes failure, the value of KI at that stress level can be calculated from the derived expression. This value is termed the fracture toughness Kc.
A precracked tension or bend-type specimen is usually used for such tests. As the thickness of the specimen increases and the stress condition changes from plane stress to plane strain, the fracture toughness decreases to a minimum value, as illustrated in Fig. 1.12c. This
value of plane-strain fracture toughness designated KIc, may be regarded as a fundamental material property.
Thus, if KIc is substituted for KI, for example, in Eq. (1.15) or (1.16) a numerical relationship is obtained between the crack geometry and the applied stress that will cause fracture.
With this relationship established, brittle fracture may be avoided by determining the maximum-size crack present in the body and maintaining the applied stress below the corresponding level. The tests must be conducted at or correlated with temperatures and strain rates appropriate for the application, because fracture toughness decreases with temperature and loading rate. Correlations have been made to enable fracture toughness values to be estimated from the results of Charpy V-notch tests.
Fracture-mechanics analysis has proven quite useful, particularly in critical applications.
Fracture-control plans can be established with suitable inspection intervals to ensure that imperfections, such as fatigue cracks do not grow to critical size.
(J. M. Barsom and S. T. Rolfe, Fracture and Fatigue Control in Structures; Applications of Fracture Mechanics, Prentice-Hall, Inc. Englewood Cliffs, N.J.)