Term
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Definition
| The phenomenon of material transport by atomic motion. |
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Term
| How is diffusion demonstrated? |
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Definition
| By using a diffusion couple. |
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Term
| Define impurity diffusion/interdiffusion. |
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Definition
| The process whereby atoms of one metal diffuse into another. |
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Term
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Definition
| When the atoms exchange positions in a pure metal. |
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Term
| Define Vacancy diffusion. |
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Definition
| The interchange of an atom from a normal lattice positon to an adjacent vacant lattice site or vacancy. |
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Term
| Define interstitial diffusion. |
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Definition
| The type of diffusion that allows atoms to migrate from an interstitial position to a neighboring one that is empty. |
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Term
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Definition
| The rate of mass transfer from one metal to another during diffusion. |
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Term
| What is steady-state diffusion? |
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Definition
| When the diffusion flux is constant w.r.t. time. |
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Term
| What is fick's first law used to find? |
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Definition
| Fick's first law defines the diffusion flux for a steady-state diffusion process. |
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Term
| What is nonsteady-state diffusion? |
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Definition
| The more practical of the two types, where the diffusion flux and concentration gradient are variables that change w/ time. |
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Term
| What is fick's second law used for? |
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Definition
| Finding the concentration diffused for a certain time with a nonsteady-state diffusion process. |
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Term
| Which variable has the most influence on diffusion? |
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Definition
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Term
| What is the activation energy? |
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Definition
| The energy required to produce the diffusion motion of one mole of atoms. It is the reason temp. is the most influential of all diff. variables. |
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Term
| Where does the interest lie when considering diffusion between polymeric materials? |
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Definition
| The diffusive motion of small foreign molecules (O2,H20,CO2,CH4) between molecular chains. This directly affets how a polymer diffuses. |
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Term
| Describe ionic diffusion. |
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Definition
| Usually occurs in a vacancy mechanism. It also occurs in pairs to maintiain neutrality. The movement of ion's creates electric current, therfore the diffusion flux represents the conductivity of a material. |
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Term
| How are the mechanical properties of materials ascertained? |
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Definition
| They are found by performing carefully designed laboratory experiments that replicate as nearly as possible the service conditions. |
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Term
| What is the difference between structural engineers and materials engineers? |
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Definition
Structural-determine stresses and stress distributions within members that are subjected to well-defined loads.
Materials-concerned with the production/fabrication of materials to meet certain requirements |
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Term
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Definition
| One of the most common stress-strain tests where a specimin is deformed to fracture from a gradually increasing tensile load along the axis of rotation. Measurements of elongation are necessary for data collection. |
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Term
| Define engineering stress |
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Definition
| A.K.A. Sigma, engr. stress is defined by the relationship of the instantaneous load divided by the initial cross section. |
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Term
| Define engineering strain. |
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Definition
| It is the change in length divided by the initial length. This gives the percent change in length |
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Term
| What is the difference between a compression and a tensile test? |
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Definition
| Compression applies a compressive force, while tensile pulls apart. |
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Term
| What is a shear/torsional test? |
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Definition
| Test using a pure shear force that provides a rotational force about a fixed axis. |
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Term
| What is the modulus of elasticity? |
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Definition
| A.k.a young's modulus, E is a constant or proportionality that directly relates stress and strain. It also represents the stiffness of the material. |
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Term
| What must be true for elastic deformation? |
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Definition
| The stress-strain relationship must be proportional, following young's modulus. This may be thought of the stiffness. It also is nonpermanent. |
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Term
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Definition
| This is the time dependant elastic behavior that is due to the microscopic and atomistic processes that cause deformation. Usually too small to recognize. |
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Term
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Definition
| v, it is defined as the ratio of lateral to axial strains, and is used when defining the elastic properties of materials. |
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Term
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Definition
| it is the stress level when plastic deformation begins. |
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Term
| What is the yield strength? |
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Definition
| It is the point when the stress-strain curve goes from elastic to plastic |
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Term
| What is the tensile strength? |
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Definition
| TS is the stress at the maximum on the stress-strain curve |
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Term
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Definition
| It is a measure of the degreee of plastic deformation that has been sustained at a fracture. Level of brittleness. |
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Term
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Definition
| the capacity of a material to absorb energy when it is deformed elastically and then upon unloading to have this energy recovered. |
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Term
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Definition
| the measure of the ability of a material to absorb energy up to fracture. |
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Term
| What is true stress, and what is different from true stress and engr. stress? |
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Definition
| True stress represents the force per unit area but uses the instantenous cross section instead of the initial cs. |
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Term
| What is true strain and what is different from true strain and engr. strain? |
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Definition
| True strain represents the percent change in length but uses the instantenous length instead of initial length |
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Term
| Define flexural strength. |
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Definition
| the stress at the fracture during a flexure test, which is what we did in lab. |
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Term
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Definition
| type of polymers with a rubber-like elasticity |
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Term
| What is hardness, and how is it measured? |
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Definition
| A measure of a material's resistance to localized plastic deformation (scratch or dent). Measured by the depth or size of the resulting indentation. |
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Term
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Definition
| Redefining stress measurements to allow for unanticipated failures. |
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Term
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Definition
| Aka working stress, it is used instead of design stress because it includes an added factor of safety to further decrease the # of failures. |
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Term
| describe an edge dislocation. |
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Definition
| localized lattice distortion exists along the end of an extra half-plane of atoms along a dislocation line. |
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Term
| describe an screw dislocation. |
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Definition
| the result of a shear distortion, where its dislocation line is actually a spiral atomic ramp. |
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Term
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Definition
| The process by which plastic deformation is produced by dislocation motion. The slip plane is the plane along the dislocation line |
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Term
| what is dislocation density? |
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Definition
| the number of dislocations in a material, expressed as the total dislocation length per unit volume. |
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Term
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Definition
The slip plane defines directions (called slip directions) along which dislocation motion occurs.
Slip systems are a combination of slip directions and slip planes. The slip plane usually has the most dense atomic packing. |
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Term
| What is unique about deformation in plastics? |
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Definition
| The dislocation motion occurs along the slip system that has the most favorable orientation. |
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Term
| How does the grain size in a polycrystalline metal influence its mechanical properties? |
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Definition
| The creation of grain boundaries creates a permenant dislocation of different orientations where the motion of grains must change direction because of an angled grain boundary. Therefore the smaller the grains, the stronger and harder it is. |
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Term
| Describe solid solution strengthening. |
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Definition
| this involves alloying with impurity atoms that go into either substitutional or interstitial solid solutions. By increasing the concentration of impurity results in higher strengths. |
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Term
| What is strain hardening? |
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Definition
| It is the phenomenon whereby a ductile metal becomes harder and stronger as it is plastically deformed. Aka cold working, because the temp. at deformation is relatively "cold" compared to its melting temp. |
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Term
| Describe the process of recovery. |
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Definition
| It is when some of the stored internal strain energy is relieved by virtue of dislocation motion. There is a decrease in the # of dislocations, and physicla properties are recovered to their precold-worked states. |
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Term
| Describe the process of recrystalization. |
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Definition
| this is the formation of a new set of strain-free and equiaxed grains (i.e. having approx. equal dimentions in all directions). |
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Term
| How is recrystallization temperature used? |
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Definition
| It describes recrystallization behavior of a particular metal alloy, and is the temp at which recrystall. just reaches completion. |
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Term
| Describe the phenomenon of grain growth. |
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Definition
| After recry. is complete, the strain free grains will continue to grow if left at elevated temp. This phenomenon is known as grain growth. It can occur in any type of material. |
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Term
| Describe the mechanism of elastic deformation. |
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Definition
| It is the elongation of the chain of molecules from their stable conformations, in the direction of the applied stress. |
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Term
| Describe the deformation of elastomers. |
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Definition
| Since they are so rubbery, it is hard to define any deformation of elastomers. The driving force is entropy, which is a measure of disorder in a system. When returning to its natural state, the entropy will increase as the chains become tangled and less orderly in the system. |
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Term
| Name the causes for failure |
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Definition
1.)improper material selection/processing
2.)inadequate design/misuse
3.)Understanding the environment |
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Term
| What are the two modes of fracture, and what makes them different? |
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Definition
Ductile- substantial plastic deformation w/ high energy absorbtion
brittle- very little plastic deformation and low energy absorbtion |
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Term
| What are the two steps of fracture |
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Definition
| Crack formation, and crack propogation |
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Term
| What is a stable crack? Which kind of fracture is it associated with? |
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Definition
| A crack that resists extension unless further stress is applied. It is associated with ductile cracks. |
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Term
| What is a unstable crack? Which kind of fracture is it associated with? |
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Definition
| When crack propogation continues after being initiated. It is assoc. with brittle cracks. |
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Term
| Explain a ductile fracture. |
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Definition
| It is stable, and the cracking grows in the direction parallel to the major axis. |
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Term
| Describe a brittle fracture. |
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Definition
| They are unstable, and they take place w/o any deformation. Cracking occurs in the direction of the crack perpendicular to the axis of tensile stress. |
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Term
| What are transgranular fractures? |
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Definition
| They are fractures that pass through grains, and break atomic bonds. |
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Term
| What are intergranular fractures? |
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Definition
| It is when the fracture takes place along the grain boundaries. |
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Term
| Describe the wonderful world of fracture mechanics. |
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Definition
| The subject that allows one to quantify the relationship between material properties, stress level, pressure of cracking producing flaws, and crack propogation mechanics |
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Term
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Definition
| When the flaws that exist in the brittle materials amplify an applied stress in their locale. |
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Term
| What is the stress concentration factor? |
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Definition
| K, it represents the ratio of the maximum stress at the crack tip compared to the magnitude of the nominal applied tensile stress |
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Term
| Define fracture toughness. |
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Definition
| A property that is a measure of a material's resistance to brittle fracture when a crack is present. |
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Term
| When is plane strain used, and what does it mean? |
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Definition
| Plane strain is used when the material the specimen thickness is much greater than the crack dimentions. This gives a new fracture toughness, which accounts for no strain components in the perpendicular faces to the front and back. |
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Term
| Why can you not test the plastic deformation in both crystalline and noncrystalline ceramics? |
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Definition
| Because almost no plastic deformation occurs before fracture. |
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Term
| Do brittle ceramics have higher strengths in the compression tests or the tension tests? |
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Definition
| Compression tests, by a factor of 10. |
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Term
| What is crazing when referring to thermoset polymer failure? |
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Definition
| The inhomogeneous mixing; in low yield stress regions. |
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Term
| What is shear yielding when referring to thermoset polymer failure? |
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Definition
| The homogenius flow of plastic material that also includes covalent bond cessation |
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Term
| List the service environment, specimen geometry factors. |
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Definition
1.)flaws/scratches
2.)thermal history
3.)temperature
4.)geometry
5.)molecular weight
6.)strain rate
7.)processing orientation |
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Term
| Why were impact fracture testing procedures a bad source of information when gathering data for plastic deformation? |
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Definition
| They tested the material under conditions that were poor for testing purposes. The conditions tended to represent the most severe. |
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Term
| What are the two impact testing technuqies, and what are they used for? |
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Definition
| The two tests are the Charpy and the Izod, and they are used to measure impact energy. |
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Term
| What is ductile-to-brittle transition? |
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Definition
| A process that with decreasing temperature, and is one of the primary functions of the Charpy and Izod tests. |
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Term
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Definition
| the form of failure that occurs in structures subjected to dynamic and fluctuating stresses (eg bridges, aircrafts, machine coponents.) |
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Term
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Definition
| it represents the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles |
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Term
| What is fatigue strength? |
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Definition
| The stress level at which failure will occur for some specified number of cycles. |
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Term
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Definition
| The number of cycles to cause failure at a specified stress level |
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Term
| What is the purpose of the s-n curve? |
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Definition
| It is a plot of stress versus the number of cycles. |
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Term
| Describe fatigue in polymers |
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Definition
| fatigue occurs at stress levels that are low relative to the yield strength. Much lower than metals. More sensitive to loading frequency than metals. Failure actually usually do to softening of material rather than typical fatigue process. |
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Term
| What are the three steps for fatigue failure? |
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Definition
1.)crack initiation
2.)crack propogation
3.)final failure |
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Term
| What are some factors that affect fatigue life? |
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Definition
1.)Mean stress-the average of the stress applied
2.)Surface effects- its where the maximum stress is for many common loading situations.
3.)Geometric design |
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Term
| What is primary/transient creep? |
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Definition
| continuously decreasing creep rate |
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Term
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Definition
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Term
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Definition
| The rate of creep is accelerating |
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Term
| What do all creep behavior lead to? |
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Definition
| A decrease in the cross sectional area, and an increase in the strain rate. |
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