What are the two yield points on the stress-strain curve called?
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Upper yield point (C) and lower yield point (D).
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What are the two yield points on the stress-strain curve called?
Upper yield point (C) and lower yield point (D).
What are the two standardized tests for measuring impact strength?
The IZOD test and the Charpy test.
What is the relationship between yield stress and ultimate tensile strength in practice?
Yield stress is usually two-thirds of the UTS.
How is strain defined?
Strain (ε) is the change in length (δ) divided by the initial length (L₀).
What are the three distinct zones in a fatigue fracture?
What are materials considered in the context of technological revolutions?
The driving force behind technological revolutions and key ingredients for manufacturing.
What is the indentor used in the Rockwell hardness test?
A diamond cone called brale.
What is fatigue failure?
Failure at a stress level far below the material's strength due to alternating or varying stress.
What happens to a material under creep conditions over long periods?
The effect of creep can become appreciable, resulting in ultimate failure of the component.
What is the yield-point?
The stress at which a material begins to deform plastically.
Define hardness.
The resistance of a material to deformation, particularly permanent deformation, scratching, cutting, or abrasion.
What are the three most popular hardness tests?
Brinell hardness test, Rockwell hardness test, and Vicker’s hardness test.
What is ductility?
The ability of a material to undergo plastic deformation under tensile loads.
What is the load applied in the Rockwell hardness test?
First a minor load of 10 kg followed by a major load of 150 kg.
Why is the yield point more important from a design engineer's perspective?
Because structures should withstand forces without yielding.
What is the formula for calculating Brinell hardness number (BHN)?
BHN = Load on ball (kg) / Area of ball impression in mm².
What do toughness and impact strength represent?
The ability of a material to absorb energy before failure or fracture occurs.
How does the stress-strain curve for brittle materials differ from that of ductile materials?
Brittle materials do not exhibit a yield point and break suddenly, while ductile materials show necking before breaking.
What does the ultimate tensile strength (UTS) represent?
The maximum load applied divided by the original cross-sectional area of the test specimen.
What are creep-failures?
Failures that occur when a material continues to deform plastically under stress at a very slow rate over time.
Which material is considered the hardest known material?
Diamond.
What is ultimate tensile strength?
The maximum stress a material can withstand while being stretched or pulled before necking.
Describe the Brinell hardness test.
A test where a hard steel or carbide ball is pressed into the material under a specific load, and the diameter of the indentation is measured to determine hardness.
What is toughness in materials?
Toughness is the ability of a material to withstand both elastic and plastic strains, resulting from a combination of strength and percentage elongation.
Why does the breaking point (F) occur at a lower stress level than the UTS point (E)?
Due to the reduction in cross-sectional area as plastic deformation increases.
What does the stress-strain diagram help to understand?
The behavior of a material when subjected to a force causing deformation.
What was the criterion used to establish the hardness scale?
A simple scratch test, where a material that could scratch another was considered harder.
What happens to the material up to the proportionality point A in the stress-strain diagram?
The stress-strain variation is linear, and Hooke's law holds.
Define toughness.
The ability of a material to absorb energy and plastically deform without fracturing.
What additional test can most tensile testing machines perform?
Compressive strength test.
What are the main categories of material properties mentioned?
Mechanical, thermal, optical, and electrical properties.
What is an example of alternating stress?
An axle under load that experiences compressive stress on top and tensile stress on the bottom, alternating with rotation.
What is the limit of proportionality?
The maximum stress at which stress is directly proportional to strain.
What is resilience in materials?
The ability of a material to deform elastically and return to its original shape.
What is the purpose of standardizing the test piece used for tensile tests?
To ensure identical test results across different laboratories.
How is impact strength related to toughness?
Higher impact strength goes with higher toughness.
Which properties of materials will be focused on in this chapter?
Mechanical properties.
What is the significance of the 0.2% proof-stress in brittle materials?
It indicates the stress at which the specimen suffers a permanent elongation of 0.2% of the initial gauge length.
What is a characteristic of brittle fracture?
A sudden reduction in the cross-sectional area near the fractured portion.
How is percentage elongation measured?
By comparing the gauge length before and after a tensile test.
What happens when a specimen is stressed beyond point B on the stress-strain curve?
Permanent set takes place, entering the plastic deformation region.
What type of indentor is used in the Brinell hardness test?
A hardened steel ball of 10 mm diameter.
What types of fractures are mentioned apart from ductile and brittle?
Fractures caused by fatigue and creep.
What happens when a specimen is subjected to high stress beyond its strength?
It fails and ultimately fractures into two or more parts.
Which materials are typically considered brittle?
Glass and other ceramics.
What can cause creep failure?
Components remaining under steady loads for a long time, particularly in high-temperature conditions.
What is the principle behind hardness tests?
They measure the resistance of a material against penetration by an indentor under a specified load.
Describe the IZOD test setup.
A standardized test specimen is fixed in a vertical position in the IZOD testing machine, and a pendulum strikes the specimen above a notch.
How is the Vicker’s hardness number (VPN) calculated?
VPN = 100 - t, where t is the depth of indentation.
What is the endurance limit?
The level of alternating and varying stress that a material can withstand without failure, even under infinite cycles.
What does a high modulus of elasticity indicate about a material?
That the material is stiff.
Differentiate between failure due to fatigue and creep.
Fatigue failure occurs due to repeated loading and unloading cycles, while creep failure occurs due to prolonged exposure to constant stress at elevated temperatures.
What phenomenon occurs with further straining after reaching the yield points?
Strain hardening or work hardening.
What occurs during brittle fracture?
A small crack grows, resulting in a complete fracture with little plastic deformation.
How is the impact strength calculated in the IZOD test?
It is calculated by measuring the difference between the original energy in the pendulum and the energy left after breaking the test specimen.
What is brittleness?
The inability of a material to withstand shock loads, leading to shattering.
What is the relationship between brittle materials and impact strength?
Brittle materials have low impact strength and poor toughness.
What does the area under the force-elongation curve represent?
The energy required to fracture the material.
What does a stress-strain curve for brittle material indicate?
It shows no yield point and the specimen breaks suddenly without appreciable necking.
What does malleability refer to?
The ability of a material to undergo plastic deformation under compressive loads.
What does hardness indicate in materials?
Hardness indicates wear-resistance and resistance against abrasion or scratching.
What does Hooke's law state?
Stress (σ) is proportional to strain (ε), or σ = Eε, where E is Young's modulus.
What occurs to the material between points A and B in the stress-strain diagram?
The material remains elastic and returns to its original condition when the forces are removed.
What does a high percentage elongation value signify?
It indicates that the material is ductile and can undergo significant plastic deformation before failure.
How is stress defined in the context of materials?
Stress (σ) is the force (F) per unit area (A₀).
What characterizes a ductile fracture?
It occurs after considerable plastic deformation.
What is creep failure?
Failure of material under steady loads over a long time, especially at high temperatures.
What is percentage elongation?
A measure of ductility, calculated as the increase in length of a material divided by its original length, expressed as a percentage.
What does a high percentage elongation indicate?
That the material is very ductile.
What is the significance of the neck formation in the stress-strain curve?
It indicates the point where the specimen begins to break as the cross-sectional area decreases.
Name three common hardness tests.
Brinell, Rockwell, and Vickers hardness tests.
Define stiffness.
The resistance of an elastic body to deformation.
