Question 1. Discuss briefly:1a. Fatigue of welded vis-a-vis virgin material (base metal)Initiation: In base metals fatigue crack initiation occurs under repeated stresses from micro-discontinuities. In welded joints only propagation of crack occurs from the inherent weld defects.Locations and causes of flaws: In base metal the flaws or micro discontinuities are initiated at atomic defects at lattice dislocations, micro defects at grain boundaries or macro defects due to non-metallic inclusions. These flaws initiate and grow under repeated loading. Figure 1 Lattice Dislocation

Figure 2 Nonmetallic inclusions Figure 3 Defects at grain boundaries

In weldmetal the flaws are already present due to fabrication. These flaws are mostly seen at weld toe or weld root. At weld toe flaws occur due to high stress concentration due to change in geometry, high tensile residual stresses and discontinuities due to slag inclusions. At weld root flaws such as lack of penetration and lack of fusion, and porosity due to gases used in welding act as crack initiators. Figure 4Weld DefectsMechanism of propagation: Stage (1) Initiation: With repeated cyclic loading of stresses, the dislocations that appear on the surface or the other material defects act as areas of stress concentrations forming minute cracks. These cracks nucleate nucleates at the discontinuities. Stage (2) Propagation: During the propagation phase the nucleated cracks advance in the plane of maximum tensile stress range. The cracks can grow along the grain boundaries (environment assisted cracking) or across the grains. With each cycle of loading microstriations can be found behind the fatigue crack tip radiating from the origin of the crack. And macro bands called beach marks can be observed by naked eye that represent a period of cyclic loading. These marks are observed only in gradual ductile propagation. In case of brittle cracks, fibrous texture radiating from the cracks can be identified. Stage (3) failure: When the crack advances its critical size rapid tearing occurs.Crack propagation mechanisms are similar in base and weld metals. Effect of residual stresses: Effect of residual stresses in base metals due to manufacturing processes is minimal. Where the effects of residual stresses is significant in weld metal. Fatigue parameters: Weld metal fatigue depends only on stress range. Fatigue in base metals depend on 2 of the fatigue parameters.1b. Brittle Vs. Ductile failure. Ductile failure:1) Ductile failure is usually observed in materials that show ductility after yielding (BCC) in the presence of shear stresses. 2) Plastic deformation is observed before failure that is the member experiences ductility absorbing a high amount of energy before failure.3) The microvoids coalesce and crack propagates normal to the direction of applied tensile stress. 4) Further crack propagates along the maximum shear plane 45 to the tensile stress creating shear lips. This creates a cup and cone shape as shown in the figure. For a highly ductile material only shear lips are formed along with high amounts of necking.

Figure 5 Ductile failure5) The fracture surface has a dimpled texture. 6) Most of the materials experience ductile failure above their transition temperatures.7) In case of cracks blunting of crack tips is observed at crack tip.Brittle Failure: 1) Brittle failure is mostly observed in FCC materials in upper transition temperatures. 2) The failure is sudden3) No plastic deformation is observed, less energy absorbed.4) The fracture surface is flat with no microvoids.5) No blunting of crack tip is observed.

Figure 6 Brittle fracture1c. Mechanisms of residual stresses due to welding:Residual stresses are self-equilibrating stresses that develop after unloading due to plastic deformation. Welding is one of the sources of residual stresses in the fabricated components. Weld residual stresses occur due restraint on the weld to shrinkage during cooling. The order of these residual stresses go up to material yield stresses.

Figure 7 Residual Stresses in a welded plateWeld metal tries to expand due to high input during welding which is restrained by surrounding colder material. As the weldment cools down, the expanded weld metal tries to shrink which is again restrained by the surrounding material. Now the weld metal experiences tensile stresses due to the restraint to shrink as shown in the figure 2. The surrounding base metal experiences compressive forces. 1d. Different approaches to fatigue assessment and their basic premises. Two different approaches are followed to for fatigue life assessment:1. Safe-life approach (Total life )2. Fail safe approach (Damage tolerant approach

1. Safe-life approach: a. Conservative approachb. It is assumed that fracture does not occur in the service life of the structure. So service life of the structure = total initiation and propagation time of a fracture in the structure. N=Ni+Npc. Presence of inherent discontinuities is not accepted.d. The structure is designed that no fracture occurs in the structures life time. e. This kind approach is generally suitable for non-redundant structures where a single fracture can lead to the failure of structure. f. The effects of fatigue assessment parameters such as, mean stress, variable amplitude, stress concentration, environment etc. are addressed through reduction in fatigue life of the structure. g. Under low stresses, material deforms only elastically and the fatigue life goes up to large number of cycles leading to low cycle fatigue. In this case the fatigue life is assessed in terms of stress and number of cycles. h. Whereas under high stresses the material reaches its yield stress within a few cycles leading to high strains and failure. In this case the fatigue life assessed in terms of number cycles and strain range.2. Fail-safe approach:a. This approach allows discontinuities in the structure and assumes that all Engineering components are flawed.b. Inherent flaws are detected using nondestructive testing methods and the flaw sizes are estimated.c. Fatigue life of the structure is calculated as the number of cycles required for the propagation of flaw as a function of flaw size and loading conditions using fracture mechanics. N=Np.d. This approach is most suitable for the structures that have a number of alternate load paths. That is structure with high redundancy can be designed with this approach where load redistribution occurs between components in case of fatigue damage.e. In such a case a strict inspection regime is required for monitoring the flaw.

Question 2. Bryte Bend Bridge FailureThe Bryte Bend Bridge is a twin parallel structure that runs across river Sacramento, California. The bridge experienced failure in the erection stage in 1970, due to low material toughness and poor fabrication.

Figure 7 Bryte bend bridge Details of the Bridge:The bridge has two structures that run parallel across the river for an overall length of 4050 ft with a vertical clear from mean high water as 55ft. The section bridge just above the river has 4 spans that were connected by hinges. The superstructure is a steel trapezoidal box shaped section and was supported by reinforced concrete piers. As can be seen in figure 2, the exterior webs of the cross section are sloped to reduce the width of the section over concrete pier. Centrally, a web longitudinally stiffened the box and conventional girder flanges were welded to the sides of the box and the central girder. The bottom plate of the box was longitudinally stiffened by a series of vertical plates. The super structure was fabricated by A36 steel in all low stress areas and A414 steel for web and compression members. The tension steel flanges were made of A517 steel that had lower notch toughness than expected. This material was adopted from pressure vessel steels. The failure and its analysis: The bridge failed during the erection itself in terms of brittle fracture in across one of the outer flanges near the concrete pier. The fracture initiated at the intersection of a in thick cross frame member attached to the 2 in thick outer top flanges of the box when concrete was poured. The brittle fracture propagated across the 30 in. wide flange and was arrested in at 4 in down into the web. The fracture surface had a herring bone type texture as shown in figure 3. The 100 ksi yield stress material, designed for 45 ksi failed at static loading of 28 ksi (poured concrete).

Figure 8 Superstructure of Bryte Bend Bridge under constructionWhen the steel from the flange was tested for KIc under similar and loading (very slow loading rate) and service conditions (60 oF) as that of the failure, the KIc value was found to be 55 ksi, which was less than that expected from the material. Further investigation showed the presence of an already initiated weld crack of 0.2 in. in tensile residual stress region of the flange and arrested at 1.3 in as it entered the compression residual stresses. For the given flaw depth and flange thickness, crack propagation can be expected under residual stresses 60- 80 ksi. Upon further loading of 28 ksi due to concrete, along with residual stresses brittle fracture occurs with very thin shear lips. This fracture propagated in to the web for 4 inches and was arrested due to higher toughness the web (lower thickness).

Figure 9 Fracture surface of the flange at pier 12 in Bryte Bend BridgeCause for the failure: The two main reasons for the failure of Bryte Bend Bridge were as follows: (i) Fabrication did not follow the design details: The design Details were mis-interpreted by the fabricator. The horizontal cross bracings were welded to the upper flanges of the box which were designed to take only horizontal forces from the slanting web. In addition to this the bracing member of 24 in. X in. was welded to the flange for an entire width of 24in. developing high residual stresses which further boosted the crack growth. (ii) Improper material selection: A pressure vessel quality steel A517 instead of A514 was specified for the flanges. It was believed that the two direction rolling in the manufacture of A517 steels made them superior to A514. As a result not impact testing was performed to check the notch toughness of the steel. The material supplied turned out to be out of specification according to ASTM standard. Repairs:It was decided that complete replacement of plates and field welding were not a feasible option. The following repairs were done:(1) The entire structure was jacked to zero stress condition(2) Later additional plates were added to the flanges as shown in figure 4. This reduced the design stress in the original plates significantly.(3) The redundancy of the structure was increased establishing multiple load paths to carry the loads in case of any further fractures.

Additional plateAdditional plateCrack removed

Figure 10 Strengthened flange plate after crack was removedThe Bryte Bend Bridge was open to traffic in October 1971 and was in continuous use since that time. References: 1. Barsom JM and Rolfe ST, Fracture and Fatigue Control in Structures: Applications of Fracture Mechanics, Third Edition, 1999.2. http://www.bphod.com/2013/07/yolo-county-california-bridges-i-street.html3. http://rebar.ecn.purdue.edu/fatigue/inventory.aspx?dir=085

3a. Elements of steel and their FunctionsTable 1. Alloying Elements in SteelElements that do not form carbidesElements that form stable carbides

ElementsNi, Si, Co, Al, Cu and NCr, Mn, Mo, W, V, Ti, Zr, and Nb

State of presence in steel Form Solid solution with IronForm compounds with Iron and Carbon

Effects on Transformation Diagram (austenite to pearlite, bainite and martensite.)Except Cobalt (Co) the rest of non-carbide forming elements quantitatively slowdown the transformationInfluence the austenite transformation differently at different temperatures: i. At 700-500 C (pearlite formation), they slow the transformation ii. At 500-400 C, they dramatically slow the transformation iii. At 400-300 C (bainite formation), they speed up the transformation

Table 2. Functions of Elements in SteelSteel making HardenabilityStrength Others

Aluminum (AL)Common Deoxidizer Results in fine grain structure and controls grain growth

Boron (B)Steel more responsive to heat treatmentImproves hardenabilityImproves strength

Cobalt (Co)Improves Red hardenability ( in heated cutting tools)

Chromium (Cr)Steel more responsive to heat treatmentIncreases the depth of hardness Resistance to oxidation and corrosion

Coper (Cu)Improves yield and tensile strengths, reducing ductilityCorrosion Resistance

Iron (Fe)Primary element; Non responsive to heat treatment without alloying elementsLow strength-Highly ductile and soft

Manganese (Mn)Resists hot shortness or thetendency to tear while being forged or rolled; Increases the response to heat treatment

Molybdenum (Mo)Raises hot strengthGood creep resistance

Nickle(Ni)Improves hardenabilityImproves strengthImproves fatigue toughness

Silicon (Si)Deoxidizing agentImproves strengthImproves toughness; Electrical resistance

Tungsten (W)Raises hot strength

Vanadium (V)Is a strong deoxidizerImproves strength at elevated temperaturesPromotes fine grain structure

3b. what is weldability and how is it defined?Weldability is the capacity of a material to be welded under a specific set of fabrication and design conditions and to perform as expected during its service life. Weldability of metal is not an intrinsic property as it is influenced by a) all steps related with welding procedure, b) purpose of the weld joints and c) fabrication conditions in in avoiding any kind of defects due to cracking, hardening and softening of HAZ, oxidation, evaporation, structural modification and affinity to gases. Qualitatively increase in carbon content increases the hardenability of the steel material and decreasing its weldability. In addition to Carbon other alloying elements that are added to iron improve its strength and hence hardness. A carbon equivalent value (CE), which considers the effects of alloying elements can be is used to quantitatively estimate the weldbility of a steel. CE = %C + %Mn/6 + (%Cr+%Mo+%V)/5 + (%Si+%Ni+%Cu)/15Special precautions such as preheating, controlling heat input, and postweld heat treating are normally required for steelwith higher carbon equivalent to maintain the required weldbility. The following table shows gives an overview of how CE effects weldability and the additional treatments required for that. Table 3 Heat Treatment Required for Different CECE% (base metal composition)Heat treatments required

0.55Both preheating and post heating

3c. Different heat treatments in steel and their purposes. There are three different types of heat treatment processes in steels. But not all steels respond to the heat treatment processes. 1. Softening processes:a. Annealing b. Normalizing2. Hardening Processes:a. Hardening b. Tempering3. Thermochemical Processes:a. Carburizingb. Nitridingc. Boronizing1a. Annealing: Annealing is performed to soften the steel to increase toughness and ductility, to relieve manufacturing residual stresses, to improve machinability and obtain a specific microstructure. The steel alloy is heated to above Austenitic temperature and held there for temperature equalization. Then the steel is slowly cooled down. This slow controlled cooling avoids the high amounts of martensitic formations. Curve 2 in figure 1 shows the cooling rate for an annealing processes. 1b. Normalizing: This method is used to soften and relive internal stresses in cold worked steels. A fine grain structure can be obtained. This process is similar to annealing except that steel is air cooled which is a higher cooling rate than annealing. Curve 3 in figure 1 shows the cooling rate for an annealing processes.2a. Hardening: The steels which contain high amounts of carbon or other alloying elements are heated to transformation temperature and the rapidly cooled (quenched) to reach the martensitic phase which very hard microstructure. The curve 1 in Figure 1 depicts hardening process. Quenching in oil and water gives different rates of cooling.2b. Tempering:After effects of quenching or hardening include hard, brittle and internally stresses steel. This steel should be heat treated again to improve its toughness and machinability and lower the internal stresses. For this purpose the steel I heated again according to the prescribed tempering curves to obtain particular final properties. Thermochemical Processes:Carbon, Nitrogen and less commonly Boron are diffused into required depth of steel in order to obtain desired properties compared to the bulk steel.

Figure 9. Cooling rates for heat treatment processes

Question 4.Provide a brief overview of High Performance Steels, highlighting their specialty.High Performance Steel (HPS) grades were developed through a collaboration between Federal Highway Administration, the U.S. Navy, and the American Iron and Steel Institute with goal to achieve higher weldability and toughness at a higher strength. HPS 70W (70 ksi) and HPS 100W (100 ksi) respectively replaced steel grades of respective strength due to their efficiency in base metal weldability and higher toughness. Whereas HPS 50W is an as rolled steel with same composition as HPS 70W. It has higher toughness but questionable weldability. All three of these are permitted only in the manufacturing plates for bridges.HPS 50 W: High strength low alloy steel. HPS 70 W: Heat treated High strength low alloy steel. HPS 100 W: Quenched & Tempered Copper-Nickel Steel. Due to higher strength of HPS steels, their efficient usage can reduce the first cost of bridges in addition to better performance. These superior qualities of HPS steels are achieved by lowering the carbon content which is the primary strengthening element that causes hardness. Other alloying elements are carefully added to compensate for the carbons hardness.

Question 5: Friction Stir Welding (FSW):Introduction: FSW is a solid-state joining process that creates extremely high-quality, high-strength joints with low distortion. The process uses no external consumables and filler materials other than the materials being welded themselves. It has no harmful by products such as gaseous emissions and shielding gases. The bonds at the welding are very strong with no scope for weld defects. FSW process has a wide variety of applications in industries such as aerospace, offshore, railways, automobiles, machinery and infrastructure. The process can fabricate either butt or lap joints in a wide range of materials, thicknesses and lengths.Process: The members to be butt or lap welded a firmly clamped in position. A non-consumable profiled spinning tool bit is inserted into a work piece under pressure. The length of the tool bit is less than the depth of the joint. The rotation of the tool creates friction that adiabatically heats the material to a plastic state without melting. At this stage the rotating tool mixes the plastic metal, and as tool traverses the weld joint, it extrudes material in a distinctive flow pattern and forges the material in its wake. The resulting solid phase bond joins the two pieces into one.

Figure 10 Stir Friction welding (www.esabna.com)Features and Benefits of FSW: (1) The solid phase bond between the two pieces in the weldment is entirely made of parent material. (2) Due to lower energy input and lower temperatures, the grain structure in the weld zone is finer than that of the parent material and has similar strength, bending, and fatigue characteristics.(3) Continuous welds of long lengths are possible irrespective of the position. (4) Weld defects are eliminated due to low distortion, high precision, no trapped materials, homogeneity and continuity in the weldments(5) The process is highly energy efficient and is a green process.No grinding, brushing or pickling are required in mass production