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1、Fracture and Fatigue Control in Steel Structures S. T. ROLFE CONSIDERABLE effort has been devoted to the prevention of brittle fracture* in manufactured structures such as aircraft and pressure vessels, where large numbers of essentially identical structures are fabricated under closely controlled c
2、onditions. For example, the emphasis on safety and reliability of nuclear pressure vessels and the ensuing extensive research, as well as stringent controls, have led to a situation where the probability of a brittle fracture in a nuclear pressure vessel is virtually zero. For other types of manufac
3、tured structures, the causes of field failures usually can be remedied by changes in design of subsequent units. In contrast, other types of structures, such as bridges and buildings, are often individually designed for a specific function and location. The overall service experience of steels in th
4、ese structures has been excellent, so that the designer in the past has seldom concerned himself with notch-toughness as a design parameter. However, the trend in structural design has been such that the following changes have occurred. 1.Structural engineers and architects are designing more comple
5、x structures than in the past. 2.There is increased use of high-strength, thick, welded steel members, as compared with lower-strength, thinner, riveted or bolted steel members. 3.The choice of construction practices has become increasingly dependent on minimum cost. 4.The magnitude and number of ty
6、pes of loadings considered in design have increased. Because of the above noted changes, the increasing number of structures subjected to severe loadings (such as offshore drilling rigs), the use of more precise methods of analysis, and the explicit recognition of inelastic behavior in the design pr
7、ocess, the probability of brittle fracture S. T. Rolfe is Professor of Civil Engineering, University of Kansas, Lawrence, Kansas. *Brittle fracture is a type of catastrophic failure that usually occurs without prior plastic deformation and at extremely high speeds (crack speeds as high as 7,000 fps
8、or possibly more). The fracture is usually characterized by a flat fracture surface (cleavage) with little or no shear lips and at average stress levels below those of general yielding. Brittle fractures are not so common as fatigue, yielding, or buckling failures, but when they do occur they may be
9、 more costly in terms of human life and/or property damage. incidence in structures of many types would appear to be increasing. Therefore, the designer should become more aware of the conditions under which brittle fracture may occur and the available methods for preventing brittle fractures, parti
10、cularly in view of the current AISC Code of Standard Practice, which assigns responsibility for the suitability, adequacy, or legality of a design. Almost all large complex steel structures are designed using structural steels that have yield strengths ranging from 36 to 100 ksi. These steels have i
11、nherent levels of notch toughness that generally are adequate for most structural applications. However, the fracture behavior of these structural steels and weldments can be affected significantly by temperature, loading rate, stress level, and flaw size, as well as by plate thickness or constraint
12、, joint geometry, and workmanship. The effect of temperature on notch toughness is generally well known, but the roles of stress (or strain), flaw size, loading rate, and thickness are less well known. In addition, it is possible for the inherent notch toughness of these steels to vary depending upo
13、n manufacturing variables (thermo-mechanical history), even though the steel may meet an existing chemistry or tensile test specification. From a fracture control viewpoint, therefore, the basic problems are as follows. Is it necessary to specify notch toughness for the steels and weldments used in
14、a particular class of structures, based on the specific design, fabrication, and service conditions to which the structures will be subjected? Furthermore, if notch toughness requirements are necessary, what notch toughness level should be specified to ensure satisfactory performance at reasonable c
15、ost. Also, what joining techniques and fabrication controls are required, consistent with the overall service conditions and consequences of failure. It should be noted that notch- toughness requirements often are developed to be used in conjunction with good design, fabrication, and inspection proc
16、edures, without being specific as to how “good“ procedures are defined. Because the cost of structural steels generally increases with their ability to perform satisfactorily under more severe operating conditions, the designer should not arbitrarily specify more notch toughness than is required. Ho
17、w much notch toughness is sufficient for a particular structural application is a difficult question to answer, and establishing the fracture-toughness requirements and the concomitant quality control and inspection requirements for various 2 ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRU
18、CTION structural applications should be an important design consideration. As with most other aspects of design, it is as much an economic matter as a technical one. Over the years many different tests have been used to evaluate the notch toughness of steels. These include the Charpy V-notch (CVN) i
19、mpact test, the drop-weight nil- ductility transition (NDT) test, the dynamic tear (DT) test, the wide-plate test, the Battelle drop-weight tear (DWTT) test and many others.1-7 Generally, these tests were developed for a specific purpose. The CVN test is widely used as a screening test in alloy stee
20、l development as well as a quality- control test. In addition, because of correlations with service experience, the CVN impact test often is used in specifications for alloy steels for various structural and pressure-vessel applications. The NDT test is used to establish the minimum service temperat
21、ure for various Navy and structural applications, whereas the Battelle DWTT test was developed to relate the fracture appearance of line-pipe steels to temperature. All these tests generally have one thing in common, namely, to produce fracture in steels under carefully controlled laboratory conditi
22、ons. Hopefully, the results of the tests can be correlated with service behavior to establish levels of performance for various steels being considered for specific applications. However, even if correlations are developed for a class of materials and structures, they do not necessarily hold for oth
23、er designs, new operating conditions, or new materials, because the results, which are expressed in terms of energy, fracture appearance, or percentage deformation, cannot be translated into normal structural design and inspection parameters, namely, stress and flaw size. Fortunately, recent advance
24、s in the fracture mechanics field have led to techniques and concepts which permit a more rational approach to fracture as a part of the design process than was possible in the past. FRACTURE MECHANICS AND DESIGN As a general rule the designer must properly proportion his structure to prevent failur
25、e by tensile overload (yielding or ductile fracture), compressive instability, and by stable crack growth (for example, arising from fatigue or stress corrosion) or unstable crack growth (brittle fracture). Design to prevent brittle fracture usually refers to using a relatively low allowable stress
26、level, as well as to the elimination (as much as possible) of those structural details that act as stress raisers that can be potential fracture initiation sites, e.g., certain weld joint details, holes, intersecting plates, arc strikes, etc. Actually, large complex structures (welded or bolted), ca
27、nnot be designed or fabricated without some discontinuities, although good design and fabrication practices can minimize the original size and number of these discontinuities. It is realized that stress concentrations or discontinuities will be present, but the designer assumes that his structural m
28、aterials will yield locally and redistribute the load in the vicinity of these stress concentrations or discontinuities. The selection of structural materials and allowable stress levels is based on the appropriate realization of the fact that crack-like discontinuities in large complex structures m
29、ay be present or may initiate under cyclic loading or stress corrosion, and that some level of notch toughness is desirable. “Fracture mechanics“ is a term commonly used to describe a method of characterizing fracture toughness, fatigue crack growth, or stress-corrosion crack growth behavior in term
30、s of structural design parameters familiar to the engineer, namely, stress and flaw size.7 Fracture mechanics commonly is subdivided into two general categories: linear-elastic and elastic-plastic* fracture mechanics. Although linear-elastic fracture mechanics techniques are established reasonably w
31、ell as compared with elastic-plastic fracture mechanics, most commonly used structural metals do not behave elastically to fracture and thus linear-elastic fracture analysis techniques are not directly applicable to most structural steels. This is good, because obviously the engineer wants his mater
32、ials to exhibit gross structural general-yielding behavior rather than a brittle type (linear-elastic) behavior. Elastic-plastic fracture mechanics approaches are not yet well-defined and, in fact, no widely accepted simple analysis technique for this type of behavior is available to the engineer. C
33、onsiderable research on elastic-plastic fracture mechanics is underway. However, the research based approaches are yet to be simplified to the point where they can be widely used by engineering designers, although Crack-Opening-Displacement (COD) test methods have been used in some areas of fracture
34、 analysis for large structures, for example the Alaska pipeline. Although research has shown that numerous factors can contribute to brittle fractures in large welded structures, the recent development of fracture mechanics has shown that there are three primary factors (conceptually) that control t
35、he susceptibility of a structure to brittle fracture. These three primary factors are: 1.Material Toughness Material toughness can be defined as the resistance to unstable crack propagation in the presence of a notch. For linear-elastic behavior the material toughness is measured in terms of a stati
36、c critical stress-intensity factor under conditions of plane stress (Kc), of plane strain (KIc), or for dynamic loading (KId). For elastic-plastic fracture behavior, the material toughness may be measured in terms of ductility related parameters such as JIc, R-curve, COD, and Equivalent Energy Appro
37、aches as defined below: J-Integral TechniqueA path-independent integral which is an average measure of the elastic-plastic stress/strain field ahead of a crack. For elastic conditions, JIc = KE Ic 2/ (1 v2). A test method for this approach is currently in development. * Sometimes referred to as “gen
38、eral yielding“, particularly in the British literature. The term “elastic-plastic“ connotates the situation where a significant yield zone relative to plate thickness of inelastic straining occurs near the crack tip such that the linear-elastic analyses are not applicable. 3 FIRST QUARTER / 1977 Res
39、istance-Curve (R-Curve) AnalysisA procedure used to characterize the resistance to fracture of materials during incremental slow-stable crack extension, KR. At instability KR = Kc, the plane stress fracture-toughness which is dependent upon specimen thickness, as well as temperature and loading rate
40、. Crack-Opening Displacement (COD) Technique Toughness evaluation in terms of the pre-fracture deformation at the tip of a sharp crack that shows considerable potential as a fracture criterion; a proposed test method has been developed by the British Standards Institution. Equivalent Energy Approach
41、An energy approach based on using test results to predict failure, primarily of thick walled pressure vessels. 2.Flaw SizeBrittle fractures initiate from flaws or discontinuities of various kinds. These discontinuities can vary from extremely small cracks, for example, from within a weld arc strike
42、(as was the case in the brittle fracture of a T-2 tanker during World War II), to much larger weld or fatigue cracks. Even though only small flaws may be present initially, repeated loading (fatigue), or stress corrosion can cause them to enlarge, possibly to a critical size where brittle fracture c
43、an occur. 3.Stress LevelTensile stresses (applied, residual, or both) are necessary for brittle fractures to occur. Engineers have known the foregoing facts for many years and have reduced the susceptibility of structures to brittle fractures by applying these concepts to their structures, qualitati
44、vely. That is, good design (the use of lower stress levels and the minimizing of discontinuities) and sound fabrication practice (decreased flaw size through use of proper welding procedures and control), as well as the use of materials with good notch toughness levels (e.g., as measured with a Char
45、py V-notch impact test), have minimized the probability of occurrence of brittle fractures in structures. However, the engineer has not had techniques available to permit evaluation of the relative performance and economic trade-offs between design, fabrication, and materials in a quantitative manne
46、r prior to the development of fracture mechanics. The fundamental concept of linear-elastic fracture mechanics is that the stress field ahead of a sharp crack can be characterized in terms of a single parameter, KI, the stress intensity factor for flat crack propagation (usually referred to as openi
47、ng mode), having units of ksiin . This single parameter KI is related to both the stress level, , and the flaw size, a. When the particular combination of and a leads to a critical value of KI, called KIc or Kc, unstable crack growth occurs. The equations that describe the elastic-stress field in th
48、e vicinity of a crack tip in a body subjected to tensile stresses normal to the plane of a simple crack are presented in Fig. 1. These stress-field equations define the distribution of the elastic-stress field in the vicinity of the crack tip, and can be used to establish the Fig. 1. Elastic-stress-
49、field distribution ahead of a crack relation between KI, , and a for different structural configurations, as shown in Fig. 2.8 Other crack geometries have been analyzed for different structural configurations and are published elsewhere.9,10 If the critical value of KI at failure (Kc, KIc, or KId) can be determined for a given metal of a particular thickness and at a specific temperature and loading rate, the designer can determine theoretically the flaw size that can be tolerated in structural members for a given design stress level. Conversely, he can determine the design stress