Week 2- Mechanical Properties Flashcards
What are mechanical properties of materials?
Mechanical properties determine a material’s behavior when subjected to mechanical stresses, including:
* Elastic modulus
* Ductility
* Hardness
* Various measures of strength
These properties are crucial in design and manufacturing, as they influence material selection and processing.
What is the dilemma faced by designers regarding mechanical properties?
Desirable mechanical properties, such as high strength, often make manufacturing more difficult.
This trade-off can impact the feasibility and cost-effectiveness of production.
What are the three types of static stresses materials can be subjected to?
- Tensile - stretching the material
- Compressive - squeezing the material
- Shear - causing adjacent portions of the material to slide against each other
Understanding these stresses is essential for analyzing material behavior under load.
What is a tensile test?
The most common test for studying stress-strain relationships, especially in metals, involving elongating the material and reducing its diameter.
This test provides critical data on material strength and ductility.
What does engineering stress represent?
Engineering stress is defined as force divided by original area:
* σ = F/A₀
Where σ is engineering stress, F is the applied force, and A₀ is the original area of the test specimen.
What is Hooke’s Law?
Hooke’s Law relates stress and strain in the elastic region:
* σ = Eε
Where E is the modulus of elasticity and ε is engineering strain.
What is the yield point in a stress-strain curve?
The yield point marks the transition from elastic behavior to plastic deformation, identified by a change in slope in the curve.
It is also known as yield strength, yield stress, or elastic limit.
What does tensile strength refer to?
Tensile strength (TS) is the maximum engineering stress reached during a tensile test.
It is also referred to as ultimate tensile strength.
What is ductility in a tensile test?
Ductility is the ability of a material to plastically strain without fracture, measured as elongation (EL).
EL = (L_f - L_o) / L_o, where L_f is the length at fracture and L_o is the original length.
What is true stress?
True stress is calculated by dividing the instantaneous area by the applied load:
* σ = F/A
Where A is the actual (instantaneous) area resisting the load.
What is strain hardening?
Strain hardening is the increase in true stress as strain increases in the plastic region of the stress-strain curve.
This occurs before necking starts.
What is a compression test?
A compression test applies a load that squeezes the ends of a cylindrical specimen, measuring changes in height and diameter.
This test is essential for evaluating materials under compressive loads.
How is engineering stress defined in a compression test?
Engineering stress in compression is defined as negative due to the reduction in height and increase in cross-sectional area:
* σ = -F/A₀
A₀ is the original area of the specimen.
What is the transverse rupture strength?
Transverse rupture strength is the strength value derived from testing brittle materials using a bending test.
This test is necessary as brittle materials cannot be tested effectively using conventional tensile tests.
What is hardness in materials?
Hardness is the resistance to permanent indentation, indicating a material’s resistance to scratching and wear.
Hard materials are crucial for tooling in manufacturing.
What are common methods for testing hardness?
- Brinell hardness test
- Rockwell hardness test
- Vickers hardness test
- Knoop hardness test
- Scleroscope
- Durometer
These methods are chosen based on the material’s hardness level.
What is shear stress?
Shear stress is defined as the applied force divided by the area over which deflection occurs:
* τ = F/A
τ represents shear stress, F is the applied force, and A is the area.
What is the relationship between shear stress and shear strain in the elastic region?
In the elastic region, shear stress and shear strain are related through the shear modulus (G):
* τ = Gγ
Where γ is shear strain.
What is the Brinell Hardness Test?
A test that uses a hard ball pressed into a specimen’s surface with a load of 500, 1500, or 3000 kg
Widely used for testing metals and nonmetals of low to medium hardness.
Describe the Rockwell Hardness Test process.
A cone-shaped indenter is pressed into the specimen using a minor load of 10 kg, followed by a major load of 150 kg
The additional penetration distance is converted into a Rockwell hardness reading.
What type of indenter does the Vickers Hardness Test use?
A pyramid-shaped indenter made of diamond
Various load amounts are applied depending on the hardness of the material.
What is unique about the Knoop Hardness Test?
It uses a pyramid-shaped indenter with a length to width ratio of 7:1 and applies lighter loads than the Vickers test
It is suitable for small and thin specimens.
How does the Scleroscope Test measure hardness?
It measures the rebound height of a hammer dropped from a certain distance
Less rebound indicates that the material is softer.
What does a durometer measure?
The elastic deformation of rubber and similar flexible materials by pressing an indenter into the surface
Resistance to penetration indicates hardness.
What is hot hardness?
The ability of a material to retain hardness at elevated temperatures
Typical hardness varies with temperature for different materials.
What happens to metals during recrystallization?
New grains form that are free of strain, behaving as a perfectly plastic material
This occurs when metals are heated to high temperatures and deformed.
What is the recrystallization temperature of a given metal?
About one-half its melting point (0.5 Tm) as measured on an absolute temperature scale
It is the temperature at which new grains form in about one hour.
What is hot working in manufacturing?
Forming a metal at temperatures above its recrystallization temperature
This allows for greater straining with lower forces.
Define fatigue in materials.
The failure of components subjected to rapidly fluctuating loads
Can occur due to cyclic stresses from mechanical loads or thermal stresses.
What is creep?
The permanent deformation of a component under a static load maintained for a period of time
It can occur in metals and some nonmetallic materials at any temperature.
What is stress relaxation?
The decrease in stresses resulting from external loading over time, even with constant dimensions
Example: Decrease in tensile stress of a wire in tension between fixed ends.
Describe the impact test.
A test to determine impact properties involving breaking a notched specimen with a swinging pendulum
Includes Charpy and Izod tests, measuring energy dissipated in breaking the specimen.
What are the two types of fractures in ductile materials?
- Necking and fracture
- Buckling under compressive load
- Complete ductile fracture
Schematic illustrations help to visualize these types.
What is transgranular fracture?
Fracture cracks pass through grains, resulting in a faceted texture on the fracture surface
This occurs in brittle materials.
How do residual stresses develop in materials?
Develop when workpieces are subjected to non-uniform plastic deformation
These stresses remain after external forces are removed.
What is the effect of temperature gradients on residual stresses?
Residual stresses can develop during the cooling of a casting or hot forging
Tensile residual stresses are generally undesirable.
How can residual stresses be reduced or eliminated?
- Stress-relief annealing
- Further plastic deformation
- Relaxation over time at room temperature
Raising the temperature can speed up relaxation.
Define viscosity.
The resistance to flow characteristic of a given fluid
It is a measure of internal friction when velocity gradients are present.
What is shear stress in the context of fluids?
The frictional force exerted by the fluid per unit area
It is related to the motion of an upper plate in a fluid.
What is shear rate?
The change in velocity relative to the change in distance
It is expressed as velocity gradient perpendicular to flow direction.
What defines shear viscosity?
The relationship between shear stress and shear rate
For Newtonian fluids, shear viscosity is constant; for non-Newtonian fluids, it is not.
What behavior do thermoplastic polymer melts exhibit?
Non-Newtonian behavior, with viscosity affected by flow rate
This complicates analysis of polymer shaping processes.
What is viscoelastic behavior?
The material property that determines strain when subjected to stress and temperature over time
It is a combination of viscosity and elasticity.