Mechanical testing Flashcards
What is mechanical testing?
The evaluation of parameters representing the mechanical properties of materials
What are 4 fundamental mechanical properties?
– Strength
– Hardness
– Ductility
– Toughness
What are the 4 purposes of mechanical testing?
– Engineering design
– Materials selection
– Materials development
– Investigation of failures
Describe the mechanical property of strength.
Usually considered as the tensile strength, which is defined as the maximum force required to fracture per unit cross sectional area in tension. In most cases however, the yield strength, the force at which the material begins to permanently deform, is the limiting factor
What is the limiting factor of strength?
In most cases however, the yield strength, the force at which the material begins to permanently deform, is the limiting factor
Describe the mechanical property of ductility.
This is considered to be the capacity to undergo deformation
(generally under tension) without rupture.
Describe the mechanical property of toughness.
This is the ability to withstand bending or deflection, or absorb energy,
without fracture.
Define hardness of material
Resistance of plastic deformation and also resistance to abrasion or scratching
What are properties of the tensile tests? (4)
– Tensile strength, ductility, yield stress, toughness – Destructive, standard specimens – Slow & expensive -- One of the most valuable and commonly used of the mechanical tests for materials.
What are properties of the hardness tests (3)
– Simple numbers: scale
divisions or ratios
– Any specimen with a flat and smooth surface
– Quick and cheap
What is the loading method of the tensile tester?
mechanically or hydraulically
What are the three steps that occur in tensile testing? ***
- When a tensile load is applied to a metal specimen it stretches elastically up to the elastic limit. Following this, plastic strain occurs which is permanent in nature with the cross‐section of the metal steadily being reduced in size.
- Finally, the specimen will ‘neck’ down in one location and break soon afterwards.
- Load is measured by the force in newtons; Specimen extension measured by the change in the length ; Gauge length is the length of the specimen
What is the elastic region of a stress-strain curve? (2)
– in this region the stress is proportional to strain and the material will return to its original length when the load is released.
– Elastic limit, yield stress&proof stress, young’s modulus
What is the plastic region of the stress-strain curve? (4)
– The Plastic Region – in this region the linear relationship between stress and strain disappears, generally the rate of increase in stress falls away with respect to strain.
– Tensil strength, % elongation & % area reduction
– Region for strain hardening
– Region of necking
What is plastic deformation from an atomic perspective?
From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbours and then reforming bonds with new neighbours as large numbers of atoms or molecules move relative to one another, upon removal of the stress they do not return to their original position
What is plastic deformation in crystalline solids?
PD is accomplished by means of a process called slip, which involves the motion of dislocations
What is plastic deformation in non-crystalline solids?
PD occurs by viscous flow mechanism
What happens to materials in general due to plastic deformation? (2)
- Most structures are designed to ensure only elastic deformation willresult
- Materials experienced a permanent deformation may not be capable of functioning as intended.
What is proof stress?
Proof Stress (stress at the offset yield point), same parameter as yieldstress. This is the stress required to produce a specified permanent strain, usually 0.2% of the original gauge length.
What is tensile strength?
the maximum load sustained
What is percentage elongation?
Percentage elongation, a measure of the ductility of the material. A gauge length, (L0) usually 50 mm, is
marked on the specimen before testing. After failure, the ends are placed together and the new length (Lf)
is measured.
What is percentage reduction in area?
a more reliable measure of ductility for round bars. After testing, the minimum cross section at or near the fracture is measured.
What is young’s modulus?
may be calculated from the slope of the straight‐line part of the curve.
What does the difference between proof stress and T.S give?
The difference between the proof stress and the T.S. gives an indication of the amount of cold work that may be performed on the material. When the difference is small the metal will fail with little visual warning.
Why is the elastic limit and limit of proportionality seldom recorded?
• The elastic limit and limit of proportionality is seldom recorded as they are often difficult to measure accurately.
What is the yielding section of a stress and strain graph?
the stress level at which plastic deformation begins, or where the phenomenon of yielding occurs
What is the yielding section of a stress and strain graph like in mild steels and other materials?
In mild steels and other materials that show yield point phenomena there is a defined change in slope at the onset of plastic deformation. The elastic limit is coincident with the upper Y.P. ‐ this is the maximum stress up to which return to original length occurs. The yield strength, σy , is determined by the stress at the lower yield point.
What is the yielding section of a stress and strain graph like in ductile materials? (3)
In ductile materials, it is difficult to define the change in slope corresponding to the elastic limit. The point of yielding may be determined as the initial departure from linearity of the curve, this is called the proportional limit, at point P, representing the onset of plastic deformation on a microscopic level.
– The yield strength, σy , is determined using the 0.002 strain offset method.
– This point is found by drawing a line offset by 0.2% of the gauge length parallel to the line of proportionality. The yield strength, is the value of the stress point of intersection.
What is the index of hardness?
- The index of hardness is a manifestation of several related properties of a material, which may include a combined effect of yield points, tensile strength, ductility, work hardening characteristics and resistance to abrasion.
- Depending upon the manner by which the hardness index of a material is secured, different combinations of these properties are effective in influencing the hardness.
What are the 4 types of hardness of a metal or an alloy?
– Indentation hardness: the resistance offered by metals to indentation i.e. mainly measure of its plasticity and strength.
– Cutting hardness: resistance to various cutting operations.
– Abrasive hardness: resistance to wear when subjected to rotating or
sliding motion.
– Rebound hardness: resistance to strike and rebound, i.e.measure of its elastic properties. For example the Shore Scleroscrope.
What do you require in hardness testing?
Require: a penetrator of known geometry, a known load, and a flat surface
What do you need to provide in hardness testing? (2)
Provide a hardness number depending on the test, is expressed as:
– An arbitrary number which is inversely proportional to the depth of impression.
– A calculated number proportional to the mean load per area of the surface produced by the indentation.
What is hardness?
• Resistance to permanently indenting the surface.
What is large hardness?
• Large hardness means:
- resistance to plastic deformation or cracking in compression.
- better wear properties.
What is the brinell hardness test? (2)
This consists of applying a known load to the surface of a specimen to be tested through the application of a hardened steel ball of known diameter.
The diameter of the resulting impression on the specimen is determined by an estimation of the diameter of the impression (d) made to the nearest 0.05 mm, using a microscope.
What is the vickers hardness test?
The standard Vickers indenter is a square based diamond pyramid, the angle between opposite faces being standardised at 136
What is the rockwell test?
The resistance to penetration is measured in terms of the depth of impression. A depth gauge graduated in arbitrary units is attached to the machine.
How is the rockwell test measurement? (3)
For measurement, 1) a minor load is first applied to seat the specimen and to remove backlash; 2) the major load is applied, 3) the major load is removed, leaving the minor load.
What is impact testing for fracture toughness?
- The impact testing is to break completely a standard specimen with a fast moving striker having a standard amount of energy at impact.
What are the testing conditions for impact testing for fracture toughness? (3)
Testing conditions
– Deformation at relatively low temperature
– A high strain rate
– A triaxial stress state
Why is tensile test not used for impact testing for fracture toughness?
- The tensile test may give an indication of toughness by indicating the extent of elongation or reduction in area. However the rates of loading are slow
What is the charpy test?
uses a square cross‐ sectioned specimen supported at both ends being struck behind the notch.
What is the izod test?
uses a round or square specimen mounted vertically being struck with the notch facing the striker
What happens in the charpy and izod test?
The hammer is released from a fixed height h and strikes the specimen; the energy expended in fracture is reflected in the difference between h and the swing height h’.
What is simple fracture?
Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static and at temperatures below the melting temperature of the material.
How can fractures also occur?
Fracture can also occur from fatigue when cyclic stresses are imposed, and from creep, the time dependant deformation at elevated temperature
What are the 4 types of stresses?
Stresses can be tensile, compressive, shear or torsional
What are 4 factors affecting impact test results?
crystal structure
fibre structure
composition of metal
heat treatment
As temperature increases for a BCC what happens to the impact energy?
Low-strength steels, impact energy increases alot from a low impact energy to a higher impact energy as temperature increases
As temperature increases for a FCC/HCP what happens to the impact energy?
Low strength metals
The impact energy is already high and slowly increases a bit more as temperature increases
As temperature increases for a high-strength materials what happens to the impact energy?
the impact energy starts low and slowly increases as temperature increases
How does fibre structure affect impact test results? (2)
- All working processes e.g. forging, rolling and extrusion produce an
orientation of grains and inclusions referred to as a fibre structure
– The path of the crack is greatly influenced by this structure.
How does the composition of metal affect impact test results? Eg. steel
Considering steel, increasing the carbon content decreases the impact strength at room temperature and raises the transition temperature.
How does the composition of metal affect impact test results? Eg. silicon
Silicon, in amounts of 0.3% lower the transition temperature and improves notch impact toughness
How does the composition of metal affect impact test results? Eg. nickel
Nickel is the most effective alloying addition for increasing toughness at low temperature.
How does heat treatment affect impact test results? (3)
– Heat treatment, such as hardening, may also have a large influence on impact properties such as notch toughness.
– The best microstructure for low temperature toughness is that of tempered martensite.
– Impact test could be used to check the effect of heat treatment.
What is plastic behaviour?
This permanent deformation behavior occurs when the tensile (or compressive)
uniaxial stress reaches σy.
What is elastic behaviour?
This reversible behavior often shows a linear relation between stress and strain. To minimize deformation, select a material with a large elastic modulus (E or G).
What is a wrought structure?
a metallic structure that has undergone some work or plastic deformation
From a basic tensile stress-strain curve for a material, how would you determine the value for yield strength?
The point or stress value at which stress/strain curve starts to deviate from a linear relationship
What are two possible causes of polymer degradation?
thermal degradation, UV or other radiation, biodegradation
Name two different types of composite materials, and identify their basic difference.
Particulate - randomly shaped reinforcing particles in the matrix
Fibrous - fibre like reinforcing particles in the matrix
Lamellar - layered material held together with adhesive (matrix)
Natural - naturally occurring materials eg. Wood, bone
What is lamellar composite materials?
Lamellar - layered material held together with adhesive (matrix)
What are fibrous composite materials?
Fibrous - fibre like reinforcing particles in the matrix
What are particulate composite materials?
Particulate - randomly shaped reinforcing particles in the matrix
Describe the principle, process and outcome of the strengthening of glass by thermal tempering.
Thermal tempering of glass is performed by the heating of the glass to temperature above the glass transition region, but below the softening point. It is then rapidly cooled such that the surface layers contract, but, due to the poor thermal conductivity of the material, the subsurface layers remain hot. The glass is then allowed to cool slowly so that the inner layers, as they contract, draw in the surface layers, at leave them in a state of residual compressive stress. This state results in the suppression of any crack growth from the surface; hence a toughened material
Describe the jominy bar test.
The test is known as the Jominy Bar test, in which the selected steel is produced into a round bar as per the Figure 2.1. The bar is the heated to an elevated temperature
(~900°C) so as to ensure the austenitization of the microstructure. The bar is then removed from the heat and placed in a rig , as shown in Figure 2.2, so as to preferentially quench one end. After complete cooling the sample is machined so as to have a flat surface along its length. Along this flat surface hardness tests are performed to obtain a results profile as shown in Figure 2.3. The hardenability characteristics are demonstrated by the hardness levels from the quenched end. For example, the alloy steel is harder to a greater distance (depth) from the quenched end and therefore has higher hardenability.
What is the distinction between cement and concrete?
Concrete is a particulate reinforced composite material consisting of an aggregate (sand or gravel), water (until fully cured), pores, and cement. The cement is the binding medium that holds the concrete together.
Briefly describe three techniques that are used to strengthen concrete by reinforcement.
Reinforcement methods
1. Reinforcement with rebar  steel wires or rods
2. Reinforcement with fine fibres of polymer or other ceramic fibres
3. Introduction of residual compressive stresses by prestressing with loaded
reinforcing bar.
Give important limitations that restrict the use of concrete as a structural material. (3)
Concrete is susceptible to fracture (brittle) due to the presence of pores
Concrete has limited use in tensile or loading that causes bending, due to the presence of pores
Concrete experiences large thermal expansions and contractions Concrete may crack when exposed to freeze/thaw cycles
Describe the stress/strain curve for
(a) Isotactic and linear polypropylene having a weight-average molecular weight of 120,000 g/mol; atactic and linear polypropylene having a weight-average molecular weight of 80,000 g/mol.
The two polypropylene materials will display the stress-strain behavior of a normal plastic. However, the isotactic/linear will have a higher degree of crystallinity (since isotactic are more likely to crystallize than atactic/linear), and therefore, will have a higher tensile modulus and strength. Furthermore, the isotactic/linear also has a higher molecular weight which also leads to an increase in strength.
Describe the stress/strain curve for:
Branched polyvinylchloride having a degree of polymerization of 2000; heavily cross-linked polyvinylchloride having a degree of polymerization of 2000
The branched PVC will probably display the stress-strain behavior of a plastic. However, the heavily crosslinked PVC will undoubtedly have a higher tensile modulus, and, also a higher strength, and will most likely fail in a brittle manner
Describe the stress/strain curve for:
Polystyrene-butadiene random copolymer having a number-average molecular weight of 100,000 g/mol and 10% of the available sites cross-linked and tested at 20 C; polystyrene-butadiene random copolymer having a number-average molecular weight of 120,000 g/mol and 15% of the available sites cross-linked and tested at -85 C. (Hint: polystyrene-butadiene copolymers may exhibit elastomeric behavior).
The copolymer tested at 20 C will display elastomeric behavior in much as it is a random copolymer that is lightly crosslinked; furthermore, the temperature of testing is above its glass transition temperature. On the other hand, since -85 C is below the glass transition temperature of the poly(styrene-butadiene) copolymer, the stress-strain behavior under these conditions.
What is the difference between cast structure and wrought structure?
A cast material is one that has been allowed to solidify from a molten or liquid form. A wrought material is cast material (metal or alloy) that has been subsequently plastically deformed with applied load
Give two actions that you may perform to reduce or prevent an incidence of fretting corrosion. (8)
- avoid vibration transmitting design
- introduce barrier between metals that slip (allow slip)
- increase load (but do not overload) to stop motion
- specify protective coating of a porous (lubricant absorbing material)
- isolate moving components from stationary ones
- increase abrasion resistance between surfaces, by treating one or both of them
- design for exclusive of oxygen on bearing surfaces
- select compatible materials
What are the two elements that are the main constituents of common solder?
Lead and tin
Name two parameters that affect polymer chain stiffness.
Bulky side groups
Polar groups or side-groups
Chain double bonds and aromatic chain groups
How is vulcanising used to modify engineering properties of elastomers?
chemical process (addition of sulphur compounds) used to induce and increase the cross linking process in elastomers, thereby increasing the modulus and tensile strength of materials
What happens to the ductility of a material after cold working?
Ductility is reduced by cold working due to the increase in dislocation density produced by cold working deformation process. Increased dislocation density leads increased dislocation and entanglement and restriction in further mobility of dislocations therefore reducing ability for material to deform further- hence reduced ductility.
How is the modulus of elasticity of a material affected by grain refinement?
Modulus of elasticity is dependent on interatomic bond strength and spacing. It is effectively a measure of the resistance to seperation of adjacent atoms. It is insensitive to grain structure and so is unaffected by changes in grain size such as grain refinement.
What happens to the yield strength of a material after substitutional solid solution strengthening?
Solid solution strengthening is a process by which defects are introduced into the crystal lattice in the form of solute atoms. These defects disrupt the slip planes in the crystals, creating barriers to smooth dislocation movement. If dislocation movement is inhibited then material is strengthened, therefore the yield strength is increased.
What happens to the toughness of a material after annealing?
When a metal is annealed it is softened, which means that its yield strength is reduced and thus is able to deform more easily. Toughness is a measure of resistance to fracture. If it is able to deform under load it is less likely to fracture, therefore annealing is likely to increase toughness.
What happens to the hardness of a material after interstitial solid solution strenghtening?
It is a process by which slip planes in a crystal are disturbed due to the presence of atoms packed into the spaces or holes between atoms in the crystal lattice – causing elastic strains in the lattice. This disturbance is effectively an obstacle to dislocation movement and thus a strengthening or hardening mechanism. Thus the hardness is increased.
