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Materials Test Two
| Question | Answer |
|---|---|
| For a tensile test specimen, gauge length is the specimen's | an initial length slightly shorter than the initial reduced specimen length |
| A dogbone shaped specimen is used because | deformation is confined to the narrow center region, and the shape reduces the likelihood of fracture at the specimen's end |
| Examples of torsion | machine axles, drive shafts, twist drills |
| Torsional tests are normally performed on | cylindrical solid shafts or tubes |
| Shear | A force applied so as to cause or tend to cause two adjacent parts of the same body to slide relative to each other in a direction parallel to their plane of contact |
| Ductile materials exhibit ___ plastic deformation with ___ energy absorption before fracture | substantial, high |
| A stable crack will extend ___ in the applies tress | an increase |
| Ductile fracture requires ___ strain energy compared to brittle fracture | higher |
| Brittle fracture | Fracture that occurs by rapid crack propagation and without appreciable macroscopic deformation |
| The inclination angle at which shear stress is a maximum | 45 degrees |
| Cup-and-cone fracture | One of the mating surfaces is in the form of a cup and the other like a cone |
| ___ materials show virtually 100% reduction in area at fracture | Highly ductile |
| __ materials show almost no reduction in area and explain why | Brittle, lack of significant plastic deformation |
| The long axis of an elliptical crack during the process of ductile fracture is _____ to the direction of the applied stress | perpendicular |
| A scanning electron fractograph showing parabolic dimples is indicative of fracture from _____ stress | shear |
| Cleavage | Crack propagation corresponds to the successive and repeated breaking of atomic bonds along specific crystallographic planes |
| Transgranular | Transcrystalline, pass through the grains |
| Intergranular | Along grain boundaries |
| Stress raisers | Presence of microscopic flows that always exist under normal conditions at the surface which amplify an applied stress in their locale |
| The magnitude of localized stress is highest ___ the crack tip | at |
| A maximum stress much higher than the applied stress results from a _____ microcrack with a _____ tip radius of curvature | long, small |
| The _____ is a measure of the degree to which an external stress is amplified at a crack tip | stress concentration factor |
| A technique of fracture analysis used to determine the stress level at which preexisting cracks of known size will propagate, leading to fracture | Fracture mechanics |
| The measure of a material's resistance to fracture when a crack is present | Fracture toughness (Kc) |
| For the condition of plane strain, the measure of a material's resistance to fracture when a crack is present | Plane strain fracture toughness (Kcl) |
| A _____ can be used to find internal or surface flaws in in-service components and during quality control in manufacturing | non-destructive test |
| A _____ is used in design to account for flaws which result in lower strength and toughness | safety factor |
| Plane strain fracture toughness is used when the specimen thickness is ____ the crack dimensions | much greater than |
| The plane strain fracture toughness, Klc , decreases with _____ strain rate and _____ temperature. | increasing, decreasing |
| Two standardized tests used to measure impact energy | Charpy and Izod |
| Impact testing is used to determine fracture characteristics at _____ temperature and _____ loading rates | low, high |
| In the _____ impact test, the specimen is supported on both sides with impact on the side opposite the v-notch | Charpy |
| The _____ is calculated from the difference between the initial height, h, and the maximum height, h', of the impact hammer. | energy absorption |
| Ductile-to-brittle transition | The transition from ductile to brittle behavior with a decrease in temperature exhibited by some low-strength steel (BCC) alloys; the temperature range over which the transition occurs is determined by Charpy and Izod impact tests |
| Fatigue is the largest case of failure in metals | True |
| Common fatigue test | Rotating bending beam |
| Three steps of fatigue failure | 1. Crack initiation 2. Crack propagation 3. Crack failure |
| Crack initiation | A small crack forms at some point of high stress concentration |
| Crack propagation | Crack advances incrementally during each stress cycle |
| Final failure | Occurs rapidly once advancing crack has reached a critical size |
| Cracks associated with fatigue failure almost always initiate _____ at a point of _____ stress | on the surface, high |
| Surface markings visible to the unaided eye, called _____, appear on components that experience interruptions in crack propagation | beachmarks |
| Beachmarks and striations are _____ on the region where rapid failure occurs | absent |
| Striations | Microscopic in size, thought to represent the advance distance of a crack front during a single load cycle |
| Fatigue life _____ with increasing mean stress | decreases |
| A(n) _____ corner will result in the longest fatigue life | filleted |
| _____ is a surface treatment that strengthens the surface of a steel component by diffusing carbon or nitrogen | Case hardening |
| Shot peening | Introducing residual compressive stresses |
| Thermal fatigue | A type of fatigue failure in which the cyclic stresses are introduced by fluctuating thermal stresses |
| Corrosion fatigue | A type of failure that results from the simultaneous action of a cyclic stress and chemical attack |
| Corrosion fatigue occurs in a corrosive environment with ____ stress | cyclic |
| Thermal stresses are introduced when thermal expansion or contraction is ____ | restrained |
| Thermal fatigue occurs at elevated temperature with _____ external mechanical stress | zero |
| Creep | The time-dependent permanent deformation that occurs under stress; for most materials it is important only at elevated temperatures |
| Creep tests for brittle materials use a _____ load | compressive |
| Why are creep tests for brittle materials using a compressive load? | There is no stress amplification and crack propagation in compressive tests |
| Creep occurs in metals at temperatures greater than _____ times the melting temperature | 0.4 |
| The rupture lifetime of a material will _____ with increasing temperature | decrease |
| As stress increases, the steady-state creep rate will _____ | increase |
| A material's resistance to creep _____ with increasing melting temperature | increases |
| A material with _____ grains has a high resistance to creep | large |
| A component with _____ has the highest resistance to creep | a single crystal |
| Three usual causes of failure | Improper materials selection and processing Inadequate component design Component misuse |
| Cracks in ductile materials are said to be | stable |
| Stable meaning | resist extension without an increase in applied stress |
| The fatigue _____ is the stress level at which failure will occur for a specified number of cycles in nonferrous alloys | strength |
| The fatigue _____ is the number of cycles to cause failure at a specific stress level | life |
| The fatigue _____ is the stress level below which fatigue failure will not occur for some ferrous and titanium alloys | limit |
| Fatigue testing involves applying a _____ stress in rotating-bending, torsion, or uniaxial tension-compression | cyclic |
| Plastic deformation | net movement of large numbers of atoms in response to an applied stress |
| In crystalline solids, plastic deformation most often involves | the motion of dislocations |
| Process by which plastic deformation is produced by dislocation motion | slip |
| The crystallographic plane along which the dislocation line traverses | slip plane |
| Total dislocation length per unit volume of material | dislocation density |
| The measured strength of a material is _____ the theoretical strength | less than |
| Why is the measured strength of a material less than the theoretical strength | The theoretical strength assumes perfect crystals without dislocations. Materials with dislocations have a lower measured strength |
| Dislocation motion is similar to the movement of a | Caterpillar |
| 5% of deformation energy that is stored internally | Strain energy associated with dislocations |
| The extra half-plane of atoms in an edge dislocation causes a _____ lattice strain below the half-plane | tensile |
| For a screw dislocation, lattice strains are | pure shear only |
| The magnitude of the strains around a dislocation line _____ with radial distance from the dislocation | decreases |
| Two edge dislocations of the same sign and lying on the same slip plane will exert _____ on each other | repulsive |
| The number of dislocations in a metal _____ during plastic deformation | increases dramatically |
| Slight displacements of atoms relative to their normal lattice positions, normally imposed by crystalline defects such as dislocations, and interstitial and impurity atoms | lattice strain |
| Slip system includes | The slip plane with the highest planar density and the slip direction with the highest linear density |
| resolved shear stress | An applied tensile or compressive stress resolved into a shear component along a specific plane and direction within that plane |
| critical resolved shear stress | The shear stress, resolved within a slip plane and direction, required to initiate slip |
| Shear components of an applied tensile or compressive stress exist at _____ alignments to the stress direction | all but parallel or perpendicular |
| Slip is more complex in polycrystalline materials because of the _____ slip direction of adjacent grains | random |
| Polycrystalline metals require _____ stress to initiate slip and yielding compared to equivalent single-crystal metals | higher |
| An individual grain's change in shape during deformation is constrained by the _____ | neighboring grains |
| Deformation by _____ is homogeneous | twinning |
| Mechanical twinning occurs in BCC and HCP metals at _____ temperatures and _____ rates of loading | low, high |
| For body-centered cubic metals, the twinning plane is _____ and the twinning direction is _____ | (112), [111] |
| A material with _____ grains is hardest and strongest | small |
| The grain size can be modified by _____ | plastic deformation and heat treatment |
| Grain size reduction improves | strength and toughness |
| Solid-solution strengthening | Hardening and strengthening of metals that result from alloying in which a solid solution is formed. The presence of impurity atoms restricts dislocation mobility |
| A substitutional atom smaller than the host atoms will tend to move to a position _____ the slip plane near an edge dislocation | above |
| Increasing the concentration of impurity atoms in an alloy will _____ the alloy's strength | increase |
| A substitutional atom larger than the host atoms exerts a _____ strain on the surrounding crystal lattice | compressive |
| ___ is the relief of some internal strain energy of a previously cold-worked metal | recovery |
| Physical properties such as electrical and thermal conductivities _____ during recovery | return to a precold-worked state |
| Recovery of a cold-worked metal involves dislocation motion due to _____ | enhanced atomc diffusion |
| The amount of recrystallization depends on _____ | time and temperature |
| The recrystallization temperature of a metal alloy is typically _____ the absolute melting temperature | less than (1/3 to 1/2) |
| For metals and metal alloys there is a critical degree of cold work _____ which recrystallization cannot occur | below (2%-20% cold work) |
| Explain if it is possible for ceramic materials to experience recrystallization | Not expected because ceramics don't store energy from cold work and because atomic mobility is low |
| Why do some metals (e.g., lead, tin) not strain harden when deformed at room temperature | Lead and tin have low melting points so room temperature is already a large fraction of their melting temperature (effectively hot-working) |
| Anelastic | Time-dependent elastic (nonpermanent) deformation |
| Elastic | Time-independent and nonpermenant deformation |
| Viscoelastic | Time dependent and nonpermanent deformation |
| Plastic | Permanent deformation |
| A material's ability to absorb energy and plastically deform at a high strain rate with stress raisers present. | Notch toughness |
| A material's resistance to fracture with a crack present. | Fracture toughness |
| The area under an engineering stress-strain curve up to fracture at a low strain rate. | Strain toughness |
| The capacity of a material to absorb energy when it is elastically deformed | Resilience |
| A measure of a material's ability to undergo appreciable plastic deformation before fracture; it may be expressed as percent elongation (%EL) or percent reduction in area (%RA) from a tensile test | Ductility |
| The stress required to produce a very slight yet specified amount of plastic strain; a strain offset of 0.002 is commonly used | Yield strength |
| The maximum engineering stress, in tension, that may be sustained without fracture. Often termed ultimate (tensile) strength | Tensile strength |
| The onset of plastic deformation | Yielding |
| The point on a stress-strain curve at which the straight-line proportionality between stress and strain ceases | Proportional limit |
| The measure of a material's resistance to deformation by surface indentation or by abrasion | Hardness |
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