Fatigue and Fracture - Free

1 Dexter, and Fisher, Fatigue and Fracture structural engineering HandbookEd. Chen Wai-FahBoca Raton: CRC Press LLC, 1999 Fatigue and FractureRobert J. Dexter andJohn W. FisherDepartment of Civil engineering ,Lehigh University,Bethlehem, Design and Evaluation of Structures for FatigueClassification of structural Details for Fatigue Scale Effects inFatigue Distortion and Multiaxial Loading Effects in Fatigue Low-Cycle Fatigue Due to Seismic Evaluation of structural Details for FractureSpecification of Steel and Filler Metal Fracture Defining TermsReferencesFurther IntroductionThis chapter provides an overview of aspects of Fatigue and Fracture that are relevant to design orassessment of structural components made of concrete, steel, and aluminum. This chapter is intendedfor practicing civil and structural engineers engaged in regulation, design, inspection, repair, andretrofit of a variety of structures ,including buildings; bridges; sign, signal, and luminaire supportstructures; chimneys; transmission towers ;et c.

2 Established procedures are explained for design andin-service assessment to ensure that structures are resistant to Fatigue and Fracture . This chapter isnot intended as a comprehensive review of the latest research results in the subject area; therefore,many interesting aspects of Fatigue and Fracture are not design and assessment procedures outlined in this chapter maybe applied to other similarstructures, even outside the traditional domain of civil engineers, including offshore structures,cranes, heavy vehicle frames, and ships. The mechanical engineering approach, which works wellfor smooth machine parts, gives an overly optimistic assessment of the Fatigue strength of structuraldetails. There are many cases of failures of these types of structures, such as the crane in the vehicle frame in , which would have been predicted had the structural engineeringapproach been possibility of Fatigue must be checked for any structural member that is subjected to cyclicloading.

3 Among the few cases where cracking has occurred in structures, the cracks are usually onlya nuisance and may even go unnoticed. Only in certain truly non-redundant structural systemscan cracking lead to structural collapse. The loading for most structures is essentially under fixed-can cracking lead to structural collapse. The loading for most structures is essentially under fixed-connections in redundant structures are essentially under displacement-control boundary other words, because of the stiffness of the surrounding structure, the ends of the member have toc 1999byCRCP ressLLC The Effective Stress Range for Variable-Amplitude LoadingAnalysisFIGURE : Fatigue cracking at welded detail in crane in a way that is compatible with nearby members. Under displacement control, a membercan continue to provide integrity ( , transfer shear) after it has reached ultimate strength andis in the descending branch of the load-displacement curve.

4 This behavior under displacementcontrol is referred to as load shedding. In order for load shedding to be fully effective, individualcritical members in tension must elongate to several times the yield strain locally without short-term performance should not lead to complacency, because Fatigue and stress-corrosion cracking may take decades to manifest. Corrosion and other structural damage can pre-cipitate and accelerate Fatigue and Fracture . Also, fabrication cracks may be built into a structure andnever discovered. These dormant cracks can Fracture if the structure is ever loaded into the inelasticrange, such as in an cracking in steel bridges in the has become a more frequent occurrence since the a large crack that was discovered in 1970 at the end of a coverplate in one of theYellow Mill Pond multibeam structures located at Bridgeport, Connecticut.

5 Between 1970 and 1981,c 1999 by CRC Press LLCFIGURE : Fatigue cracking at welded detail in vehicle Fatigue cracks were discovered at the ends of coverplates in this bridge [19].Fatiguecrackinginbridges, , assumption of a Fatigue limit at two million cycles proved to be incorrect. As a result of extensivelarge-scale Fatigue testing, it is now possible to clearly identify and avoid details that are expectedto have low Fatigue strength. The Fatigue problems with the older bridges can be avoided in newconstruction. Fortunately, it is also possible to retrofit or upgrade the Fatigue strength of existingbridges with poor Fatigue is a possible failure mode for structural members or connections that are cycledinto the inelastic region for a small number of cycles. For example, bracing members in a bracedframe or beam-to-column connections in a welded special-moment frame (WSMF) may be subjectedto low-cycle Fatigue in an earthquake.

6 In sections that are cyclically buckling, the low-cycle Fatigue islinked to the buckling behavior. This emerging area of research is briefly discussed in primary emphasis in this chapter is on high-cycle Fatigue . Truck traffic causes high-cyclefatigue of bridges. Fatigue cracking may occur in industrial buildings subjected to loads from cranesor other equipment or machinery. Although it has not been a problem in the past, Fatigue crackingcould occur in high-rise buildings frequently subjected to large wind loads. Wind loads have causednumerous Fatigue problems in sign, signal, and luminaire support structures [32], transmissiontowers, and cracks can form in structures cycled in compression, they arrest and are not structurallysignificant. Therefore, only members or connections for which the stress cycle is at least partially intension need to be assessed.

7 If a Fatigue crack forms in one element of a bolted or riveted built-upstructural member, the crack cannot propagate directly into neighboring elements. Usually, a rivetedmember will not fail until a second crack forms in another element. Therefore, riveted built-upstructural members are inherently redundant. Once a Fatigue crack forms, it can propagate directlyinto all elements of a continuous welded member and cause failure at service loads. The lack ofc 1999 by CRC Press LLCFIGURE : Fatigue crack originating from the weld toe of a coverplate end detail in one of theYellow Mill Pond redundancy in welded members is one reason that Fatigue and Fracture changed from anuisance to a significant structural integrity problem as welding became widespread in the structures are not inferior to bolted or riveted structures; they just require more attention todesign, detailing, and structures such as bridges and ships, the ratio of the Fatigue -design load to the strength-designloads is large enough that Fatigue may control the design of much of the structure.

8 In long-spanbridges, the load on much of the superstructure is dominated by the dead load, with the fluctuatinglive load relatively small. These members will not be sensitive to Fatigue . However, the deck, stringers,and floorbeams of bridges are subjected to primarily live load and therefore may be controlled , fractureisalmostalwaysprecededbyfatiguec racking; therefore,the primary emphasis should be on preventing Fatigue . Usually, the steel and filler metal haveminimum specified toughness values (such as a Charpy V-Notch [CVN] test requirement). Inthis case, the cracks can grow to be quite long before Fracture occurs. Fatigue cracks grow at anexponentially increasing rate; therefore, most of the life transpires while the crack is very Fracture toughness, greater than the minimum specified values, will allow the crack togrow to a larger size before sudden Fracture occurs.

9 However, the crack is growing so rapidly at theend of life that the additional toughness may increase the life only , Fracture is possible for buildings that are not subjected to cyclic loading. Several largetension chords of long-span trusses fractured while under construction in the 1980s. The tensionchords consisted of welded jumbo shapes, , shapes in groups 4 and 5, as shown in [22].These jumbo shapes are normally used for columns, where they are not subjected to tensile sections often have low Fracture toughness, particularly in the core region of the web and flangejunction. The low toughness has been attributed to the relatively low rolling deformation and slowcooling in these thick shapes. The low toughness is of little consequence if the section is used asa column and remains in compression.

10 The fractures of jumbo tension chords occurred at weldedc 1999 by CRC Press LLCFIGURE : (a) View of jumbo section used as tension chord in a roof truss and (b) closeup viewof Fracture in web originating from weld access holes at welded 1999 by CRC Press LLCsplices at groove welds or at flame-cut edges of cope holes, as shown in In both cases thecracks formed at cope holes in the hard layer formed from thermal cutting. These cracks propagatedin the core region of these jumbo sections, which has very low toughness. As a consequence ofthese brittle fractures,AISC(American Institute of Steel Construction) specifications now have asupplementalCVNnotch toughness requirement for shapes in groups 4 and 5 and (for the samereasons) plates greater than 51 mm thick, when these are welded and subject to primary tensile stressfrom axial load or bending.