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63 Cards in this Set
- Front
- Back
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How does one prevent fatigue?
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1. Choose materials with low fatigue crack propagation rates.
2. Reduce stress by thicker sections, design changes. 3. Induce compressive stresses by shot peening. 4. Inhibitors stop corrosive action. 5 Cathodic protection. 6. Coatings, both organic(paints) and inorganic,( zinc, chrome plates etc). |
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Describe initiation phase of fatigue?
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plastic deformate at 45 angle of applied stress
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What happens at stage 11 of fatigue?
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the crack macroscopically propagates at 90o to the stress axis. Multiple slips occur.
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What happens at stage 111 of fatigue?
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the remaining cross section of material is too small to withstand the applied stress and tensile shear overload failure occurs forming a final shear lip at 45o to the surface.
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What are the effects of stage 1 fatigue?
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persistent slip bands (PSB's) will initiate cracks w/out corrosion.
Corrosion mechanisms: a. local galvanic activity between phases. b. dissolution of metal after mechanical disruption of passive films by PSB's. c. metal dissolution simply due to active sites in PSB's. d. dissolution of grain boundary regions due to compositional effects, for example sensitized steels. e. pitting corrosion. |
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What do the graphs for fatigue at stage II look like?
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measure log(da/dn) vs. delta K, Linear increase in delta K and log (da/dn).
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What are some mechanisms for stage II fatigue.
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corrosive environment oxidizes the tip.
the general corrosion of the crack tip in extending the crack length at all times in addition to the increase in crack length from the mechanical stressing. |
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Describe the stress intensity at which a crack will initiate
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A vertical region in which the crack has initiated and grows very rapidly. This is where the K1scc determination is made.
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Describe steady state rate of crack growth is present
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The crack is increasing in length and does so over a reasonable range of stress intensity
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Describe unstable crack growth
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crack growth rate increases very rapidly with increasing stress concentration to failure.
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What is the equation for stress intensity?
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K= s f(Y) (pi*a)^(1/2)
where:- K- stress intensity; s - stress applied:f(Y) - geometric factor a- crack length. |
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What methods are used for stress application?
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a) rising Step Load Test.
b) single cantilever test c) as C ring test d) residual strength tests e) double cantilever bend test |
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What is the rising step load test?
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The load on the samples immersed in solution is increased at regular time periods of between one and four hours. When the load decreases more than 10% then a crack is assumed to have initiated. At this point the load to initiate a crack is known along with the crack length when the crack started to grow. K1scc can then be calculated.
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What is the single cantilever test?
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imilar sample is loaded at one end. The load is a constant static load. After a given time period, often 1000hrs, the crack is measured to see if it has grown. Several samples at different loads must be run concurrently. K1scc is determined as the minimum load at which crack growth occurs.
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What is the C-ring test?
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a C shaped sample is loaded and cracks initiation measured
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What is the residual strength test?
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, a sample is loaded to 75% of the UTS and left in the environment for 30 days. The sample is then fractured and the fracture stress compared to an unexposed sample. The decrease in failure load is a measure of the SCC susceptibility of the metal.
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What is the slow strain rate test?
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A cylindrical sample in an environment is pulled in a tensile test at very slow strain rates. The low strain rate enables the environment to initiate SCC damage. The decrease in mechanical properties between exposed and unexposed samples at the same strain rate is again a measure of the SCC susceptibility
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What is the double cantilever bend test?
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The two bolts threaded into the top are tightened against each other and the material cracked in air. As the crack runs down the sample, the deflection required at the top is met and so the crack then stops running. This then is a measure of K1c for the material. The cracked sample is then placed in the environment of interest and the crack monitored to see if it grows and to what length. After the crack stops growing, the value of K1scc is measured. The value of this is that a single sample provides the measurement of stress corrosion cracking on a quantitative scale.
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Examples of Environments Producing SCC
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Aluminum alloys - chlorides, seawater.
Austentitic stainless steels - neutral halides, hot halides, concentrated caustics above 120C. Ferritic stainless steels - Ammonium nitrates and chlorides, hypochlorites. Carbon ferritic steels - caustic NaOH solutions greater than 50C, calcium, ammonium and sodium nitrate solutions, seawater. Nickel chrome alloys - chlorides above 200C, low pH (4) with oxidizers. Monels - HF acids Cu-Zn (brasses) - ammonia vapors and salts. Titanium alloys - organic solvents, methanol. |
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What do the nomenclatures L,S, and T represent?
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L or longitudinal direction is parallel to the rolling direction.
S is the material thickness T direction - material width |
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How is sample orientation deterimined?
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The first letter indicates the direction which is 90o to the plane of the crack and the second letter indicates the direction of crack growth
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What is corrosion fatigue?
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cyclic stress and a corrosive environment to decrease the number of cycles to failure.
Corrosive environment decreases failure lifetime. |
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What does an S - N curve measure?
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It is a stress against the log of the number of cycles to failure curve,
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How does one prevent fretting?
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1. Reduce the normal load.
2. Lubricate to stop the friction between the two surfaces - sometimes called anti fretting compounds. 3. Increase the hardness of the metal by solution hardening. 4. Generally materials that oxidize easily and form tenacious oxides are problems. These include aluminum and its alloys and titanium. 5. Increase amplitude of motion. 6.Use gaskets or sealants between parts. |
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What is the test for fretting?
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e test technique is to clamp pads of metal onto a sample being cyclically stressed. The clamped region is immersed in solution. The stressed sample stretches in a tension tension or other cycle
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What are the three mechanisms of fretting?
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1. The wear oxidation mechanism.
2. The oxidation wear mechanism. 3. The Delamination Theory of Wear. |
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Describe The wear oxidation mechanism in fretting?
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small particles of debris are produced. Oxidization is hard and creates rapid wear by mircomachining.
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Describe The oxidation wear mechanism as it relates to fretting.
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Surface is oxidized first. Debris removed and oxidizes again
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Describe The Delamination Theory of Wear as it relates to fretting.
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Platelets are detached from the surface of the material. Voids form 10-20 micro-m below surface. eventually come to the surface when the friction between the two surfaces overcomes the stress at the thin neck of the plate. Void cracks grow from stress.
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What is fretting corrosion?
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the combined action of an aggressive environment and relative, small amplitude motion between two surfaces, one of which is metallic.
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What are the conditions for fretting to occur?
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1. A high normal load between the two surfaces.
2. Relative motion between the two surfaces, For fretting this value is around 5m (125 mils) or less so it is small amplitude. 3. An aggressive environment to promote corrosion. |
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How does one prevent SCC?
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1. Remove or reduce stress - stress relief anneal, thicker sections.
2. Change environment - add inhibitors. 3. Change material - look at data to find better material. 4. Cathodic protection will avoid the problems but remember that hydrogen embrittlement is initiated by cathodic conditions. |
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What is the mechanism that causes graphitization?
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The graphite flakes lead to an ever present cathode site on gray cast irons. The ferrite is dissolved by anodic reaction, leaving the graphite network behind.
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How does one prevent graphitization?
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1. Use malleable, white cast irons or nodular cast irons. These all avoid having a continuous graphite cathode.
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When does EROSION CORROSION occur?
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velocity of fluid around the corrosion. The removal of the ionic species promotes the rapid dissolution of the anode material. Can also remove protective film
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What effects erosion corrosion?
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-velocity
-PH -oxygen -turbulence Impingement -caviation |
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How does turbulence effect erosion corrosion?
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It creates inlet corrosion. Greatest effect near inlet and turns.
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How does pH effect erosion corrosion?
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Depends on material. Some materials are active or passive at different pHs
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Hows does oxygen effect erosion corrosion?
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Can form a stable film layer. Some film layers created can be unstable
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How does impingement impact erosion corrosion?
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particles in liquid act as small machines and remove the surface material. Elbows and bends most prone to this damage.
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How is cavitation erosion formed in erosion corrision?
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Bubbles or droplets impact surface and once they pop or removed, then portion of material is removed.
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How does one prevent erosion corrosion?
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1. Choose erosion resistant metals.
2. Design make parts that will corrode easy to replace. Avoid elbows. Thicken sections. 3. Change environment Use filters to remove particles, slow flow by wider diameters 4. electrochemical protection -use impressed current. |
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What are some Testing Techniques for erosion corrosion?
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Cavitation is tested by spinning a disc in a fluid, the disc having holes in it which causes a pressure drop and the nucleation of bubbles on the surface behind the holes. Particle impingement is usually done by a fluid with particles suspended in them. Erosion test are conducted by placing samples in tube walls.
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What are some STRESS INVOLVED CORROSION MECHANISMS?
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stress corrosion cracking (SCC), corrosion fatigue, fretting and hydrogen assisted environmental cracking or hydrogen embrittlement (HE). Result in diminishing mechanical properties
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What is required for stress corrosion cracking?
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an aggressive environment and tensile stress. Tensile stress can be applied or residual
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What are two types of stress corrosion cracking and how are they distinguished?
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intergranular and transgranular.For intergranular corrosion the density crack decreases but for transgranula the density increases.
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What can cause crack initiation?
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a. Mechanical Features.
ex. Dents. local environemnt and local stress. b. Local galvanic cells initiating dissolution c Pitting type crack initiation. d. Initiation at stress induced phenomena. Slip line provides local anode and rupture passive layer. |
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What is the film cleavage mechanism?
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the surface film grows and may increase in internal stress with thickness. This combined with the applied tensile stress induces brittle failure in the film which propagates across into the metal and provides a period of crack growth.
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What is Adsorption Induced Cleavage?
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During the electrochemical process atoms are absorbed on the surface that weakens the bonds. The stress to initiate a crack then decreases and a crack grows until it is blunted by plastic deformation or grows out of the adsorbed region.
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What is Adsorption Induced Plasticity?
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The adsorption of specific ions in this cases reduces the critical resolved shear stress for dislocation mobility. Dislocations can then move locally under the influence of the tensile stress.
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What is Atomic Surface Mobility?
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At the crack tip atoms move away from the tip and vacancies move in by a surface diffusion process coupled with electrochemical activity.
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What is the Corrosion Tunnel Model?
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corrosion tunnels are formed by active corrosion alone emerging slip lines. When sufficient metal is removed in the tunnels, then the undissolved regions between them fracture by ductile overload and some crack growth occurs until it is plastically blunted and the process starts again.
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What is K1SCC?
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the plain strain fracture toughness under stress corrosion cracking condition
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What are the three distinct regions of crack growth?
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1. the stress intensity at which a crack will initiate
2. Stable Crack Growth 3. Unstable Crack growth |
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How does velocity effect erosion corrosion?
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-cathodic reaction components transport rate is increased
-reaction products are removed -stresses on surface films increase |
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What can cause
Crack Growth Mechanisms? |
a. Film Rupture Mechanism.
b. Film Cleavage Mechanism. c. Adsorption Induced Cleavage d. Adsorption Induced Plasticity. e. Atomic Surface Mobility f. Corrosion tunneling |
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What is the film rupture mechanism?
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The tensile stress ruptures films at the crack tip and the crack grows rapidly from the bare metal exposed until the crack tip can repassivate in some cases or grow slowly to failure in other cases.
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What is selective leaching?
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The preferential dissolution of one element from an alloy.
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Name two types of selective leaching
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de-zincification and graphitization
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What are the two types of de-zinification and what environments favor them?
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Uniform type - high zinc content and acid or aggressive environments.
Plug type- low zinc contents and less aggressive environments |
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What is graphitization?
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It is in gray cast iron. The carbon is the cathode site and is ferritic matrix is the anode which dissolves.
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What is the de-zinification mechanism?
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both elements dissolve, for example in brass both the copper and the zinc dissolve in the environment, but the copper re-deposits in a cathodic galvanic reaction.
Less accepted theory, only zinc disssovles leaving behind porous substance |
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How does one prevent de-zincification?
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1. Use low zinc brasses less than 15% wt Zn.
2. Addition of As(0.4%) retards the process. 3. Use cupronickels, Cu-Ni alloys. |
