11 Properties of Materials: Analysis of Key Performance Points

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11 Properties of Materials: Analysis of Key Performance Points 1

1. Mechanical properties of materials under uniaxial static tension

1. Definition:

Craze: Craze is a kind of defect produced in the deformation process of polymer materials. Because of its low density and high reflective ability to light, it looks silver, so it gets its name.

Craze occurs in the weak structure or defect part of polymer materials.

Superplasticity: the material shows a very large elongation (about 1000%) under certain conditions without necking and fracture, which is called superplasticity.

The proportion of strain εg generated by grain boundary sliding in total strain εt is generally between 50% and 70%, which indicates that grain boundary sliding plays a major role in superplastic deformation.

Brittle fracture: Before material fracture, there is basically no obvious macroscopic plastic deformation and no obvious omen, which is often shown as a sudden rapid fracture process, so it is very dangerous.

Ductile fracture: the fracture process that produces obvious macroscopic plastic deformation before and during material fracture.

In ductile fracture, the crack propagation process is generally slow and consumes a lot of plastic deformation energy.

Cleavage fracture: under the action of normal stress, brittle transgranular fracture along a specific crystal plane caused by the destruction of the bonding bond between atoms is called cleavage fracture. (The cleavage step, river pattern and tongue pattern are the basic microscopic characteristics of the cleavage fracture.)

Shear fracture: Shear fracture is the fracture caused by material sliding and separation along the slip plane under the action of shear stress. (Micropore aggregation fracture is a common mode of ductile fracture of materials. Its fracture surface is usually dark gray and fibrous in macro, while the characteristic pattern of micro fracture surface is that a large number of “dimples” are distributed on the fracture surface.)

2. Try to describe the difference between ductile fracture and brittle fracture. Why is brittle fracture the most dangerous?

Type of stress, degree of plastic deformation, presence or absence of omen, and speed of crack propagation.

3. What is the difference between breaking strength σc and tensile strength σb?

If plastic deformation does not occur before fracture or plastic deformation is very small, no necking occurs, and brittle fracture occurs to the material, then σcb.

If necking occurs before fracture, σc and σb are not equal.

4. What scope does Griffith formula apply to and under what circumstances should it be revised?

Griffith formula is only applicable to brittle solids containing microcracks, such as glass, inorganic crystal materials, ultra-high strength steel, etc.

For many engineering structural materials, such as structural steel and polymer materials, the crack tip will produce large plastic deformation, which will consume a lot of plastic deformation work.

Therefore, the Griffith formula must be revised.

2. Mechanical properties of materials under uniaxial static tension

1. Soft coefficient of stress state

The ratio of τmax and σmax is called the stress state soft coefficient, which is expressed by α.

The larger α is, the greater the maximum shear stress component is, which means that the softer the stress state is, and the easier the material is to produce plastic deformation.

On the contrary, the smaller α is, the harder the stress state is, and the more brittle fracture occurs.

2. How to understand the phenomenon of “notch strengthening” of plastic materials?

Under the condition of notch, the yield stress of the specimen is higher than that of the specimen under uniaxial tension due to the occurrence of three-way stress, which is called notch “strengthening” phenomenon.

We cannot regard “notch strengthening” as a means of strengthening materials, because notch “strengthening” is purely caused by the plastic deformation of materials constrained by three-dimensional stress.

At this time, the σs value of the material itself does not change.

3. The characteristics and application scope of uniaxial tension, compression, bending and torsion tests are comprehensively compared.

In case of unidirectional tension, the normal stress component is large, the shear stress component is small, and the stress state is hard.

It is generally applicable to the test of so-called plastic materials with low plastic deformation resistance and cutting resistance.

Compression: the stress state soft coefficient of unidirectional compression is a = 2, and the compression test is mainly used for brittle materials.

Bending: there is no effect of the so-called specimen deflection on the test results during bending loading, such as tension.

In bending test, the stress distribution on the section is also the maximum stress on the surface, so it can sensitively reflect the surface defects of materials.

Torsion test: the stress state softness coefficient of torsion is higher than that of tension, so it can be used to measure the strength and plasticity of materials that are brittle in tension.

In torsion test, the stress distribution of sample section is the largest on the surface, so it is very sensitive to the reaction of material surface hardening and surface defects.

In torsion test, the normal stress and shear stress are approximately equal;

Cut off the fracture surface, and the fracture surface is perpendicular to the axis of the sample. This fracture surface is often used for plastic materials.

Normal fracture, the fracture surface and the axis of the sample form an angle of about 45 °, which is the result of normal stress, and brittle materials often have this fracture surface.

4. Try to compare the similarities and differences between Brinell hardness test and Vickers hardness test principles, and compare the advantages and disadvantages of Brinell hardness test, Rockwell hardness test and Vickers hardness test and their application scope.

The test principle of Vickers hardness is basically similar to Brinell hardness, and the hardness value is calculated according to the load borne by the unit area of the indentation.

The difference is that the indenter used in Vickers hardness test is a diamond pyramid with an angle of 136 ° between two opposite sides.

The Brinell hardness adopts hardened steel ball or hard alloy ball.

Advantages of Brinell hardness test:

The indentation area is large, its hardness value can reflect the average performance of each constituent phase in a large area, and the test data is stable and has high repeatability.

Therefore, Brinell hardness test is most suitable for measuring the hardness of gray cast iron, bearing alloy and other materials.

Disadvantages of Brinell hardness test:

Due to the large indentation diameter, it is generally inappropriate to directly inspect the finished product;

In addition, for materials with different hardness, the indenter diameter and load need to be replaced, and the measurement of indentation diameter is also troublesome.

Advantages of Rockwell hardness test:

Easy and fast operation;

The indentation is small, and the workpiece can be inspected directly;

Disadvantages: 

Poor representation due to small indentation;

The hardness values measured with different scales can neither be directly compared nor exchanged.

The Vickers hardness test has many advantages:

Accurate and reliable measurement;

You can select any load.

In addition, Vickers hardness does not have the problem that the hardness of different scales of Rockwell hardness cannot be unified, and the thickness of the test piece is thinner than that of Rockwell hardness.

Disadvantages of Vickers hardness test: 

Its measuring method is troublesome, its working efficiency is low, the indentation area is small, and its representativeness is poor, so it is not suitable for routine inspection of mass production.

Related reading: Metal Hardness: The Definite Guide

3. Impact toughness and low temperature brittleness of materials

1. Low temperature brittleness, ductile brittle transition temperature.

When the test temperature is lower than a certain temperature tk (ductile brittle transition temperature), the materials of bcc or some closely packed hexagonal crystal metals and alloys, especially the medium and low strength structural steels commonly used in engineering, change from the ductile state to the brittle state, and the impact absorption energy decreases significantly.

The fracture machine changes from micropore aggregation to transgranular cleavage, and the fracture characteristics change from fibrous to crystalline, which is called low-temperature brittleness.

2. The physical essence of low temperature brittleness and its influencing factors are explained.

Below the ductile brittle transition temperature, the fracture strength is lower than the yield strength, and the material is brittle at low temperature.

A. Influence of crystal structure: Body centered cubic metals and their alloys have low temperature brittleness, while face centered cubic metals and their alloys generally have no low temperature brittleness.

The low temperature brittleness of BCC metals may be closely related to the late yielding phenomenon.

B. The influence of chemical composition: the content of interstitial solute elements increases, the higher energy decreases, and the ductile brittle transition temperature increases.

C. Influence of microstructure: refining grain and structure can increase the toughness of materials.

D. Influence of temperature: It is relatively complex, and brittle (blue brittle) occurs within a certain temperature range.

E. Effect of loading rate: Increasing the loading rate is like lowering the temperature, which increases the brittleness of the material and increases the ductile brittle transition temperature.

F. Influence of specimen shape and size: the smaller the curvature radius of notch, the higher tk.

3. The reason for improving toughness by refining grains?

Grain boundary is the resistance of crack propagation;

The number of dislocations in the pre grain boundary packing is reduced, which is conducive to reducing the stress concentration;

The increase of the total area of the grain boundary reduces the concentration of impurities on the grain boundary to avoid intergranular brittle fracture.

4. Fracture toughness of materials

1. Low stress brittle fracture

When the working stress of large parts is not high, even far below the yield limit, brittle fracture often occurs, which is called low stress brittle fracture.

2. Explain the names and meanings of the following symbols: KIc; JIc; GIc; δc.

KIC (stress strain field intensity factor at the crack tip in the crack body) is the plane strain fracture toughness, which represents the ability of materials to resist the unstable propagation of cracks under the plane strain state.

J Ic (strain energy at crack tip) is also called fracture toughness, but it represents the ability of materials to resist crack initiation and propagation.

GIc refers to the energy consumed per unit area when the material prevents the unstable propagation of cracks.

δ C (crack opening displacement) is also called the fracture toughness of the material, which indicates the ability of the material to prevent the crack from starting to expand.

3. Explain the similarities and differences between KI and KIc.

KI and KIC are two different concepts.

KI is a mechanical parameter that represents the strength of the stress and strain field at the crack tip in the crack body.

It depends on the applied stress, sample size and crack type, but is independent of the material.

However, KIC is the mechanical property index of the material, which depends on the internal factors such as the composition and structure of the material, but has nothing to do with external factors such as external stress and sample size.

The relationship between KI and KIC is the same as that between σ and σs.

KI and σ are mechanical parameters, while KIC and σs are mechanical property indexes of materials.

5. Fatigue property of materials

1. What are the characteristics of fatigue failure?

(1) This failure is a hidden sudden failure, which will not occur obvious plastic deformation before fatigue failure, and is brittle fracture.

(2) Fatigue failure belongs to low stress cycle delayed fracture.

(3) Fatigue is very sensitive to defects (notch, crack and structure), that is, it is highly selective to defects.

(4) Fatigue forms can be classified according to different methods.

According to the stress state, there are bending fatigue, torsion fatigue, tension and compression fatigue, contact fatigue and composite fatigue;

According to the stress level and fracture life, there are high cycle fatigue and low cycle fatigue.

2. How many characteristic areas of fatigue fracture surface?

Fatigue source, fatigue crack growth zone and transient fracture zone.

3. Try to describe the similarities and differences between σ-1and ΔKth.

σ-1 (fatigue strength) represents the infinite life fatigue strength of smooth specimens, which is suitable for traditional fatigue strength design and verification;

ΔKth (threshold value of fatigue crack growth) represents the infinite life fatigue performance of the crack sample, which is suitable for the design and fatigue strength check of the cracked parts.

6. Wear resistance of materials

1. How many types of wear are there? It shows their surface damage morphology.

Adhesion wear, abrasive wear, corrosion wear and pitting fatigue wear (contact fatigue).

Adhesion wear: The wear surface is characterized by scabs of different sizes on the surface of the parts.

Abrasive wear: groove formed by scratch or obvious furrow on friction surface.

Contact fatigue: there are many pits (pockmarks) on the contact surface, some of which are deep, and there are traces of fatigue crack growth lines at the bottom.

2. Is the statement “the harder the material, the higher the wear resistance” correct? Why?

Correct. Because the wear is inversely proportional to the hardness.

3. From the point of view of improving material fatigue strength, contact fatigue strength and wear resistance, the matters needing attention in chemical heat treatment are analyzed.

The residual compressive stress of the surface layer is increased while the surface strength and hardness are increased.

7. High temperature performance of materials

1. Explain the following terms:

Approximate specific temperature: T/Tm

Creep: It refers to the phenomenon that the material slowly produces plastic deformation under the action of constant temperature and load for a long time.

Endurance strength: refers to the maximum stress of materials without creep fracture under a certain temperature and within a specified time.

Creep limit: it represents the resistance of materials to high temperature creep deformation.

Relaxation stability: The ability of materials to resist stress relaxation is called relaxation stability.

2. The creep deformation and fracture mechanism of materials are summarized.

The creep deformation mechanism of materials mainly includes dislocation slip, atomic diffusion and grain boundary slip.

For polymer materials, there is also the stretch of molecular chains along the external force.

Intercrystalline fracture is a common form of creep fracture, especially under high temperature and low stress.

This is because the strength of polycrystalline grains and grain boundaries decreases with the increase of temperature, but the latter decreases faster, resulting in lower relative strength of grain boundaries at high temperature.

There are two models of grain boundary fracture: one is grain boundary sliding and stress concentration model; The other is vacancy aggregation model.

3. The differences between creep deformation and plastic deformation mechanisms of metals at high temperatures are discussed.

The plastic deformation mechanism of metal is slip and twinning.

The creep deformation mechanism of metal is dislocation slip, diffusion creep and grain boundary slip.

At high temperature, due to the rise of temperature, it provides the possibility of thermal activation for atoms and vacancies, so that dislocations can overcome some obstacles to move and continue to produce creep deformation;

Under the action of external force, uneven stress field is generated in the crystal, atoms and vacancies have different potential energy at different positions, and they will diffuse directionally from high potential energy potential to low potential energy potential.

8. Thermal properties of materials

1. Try to analyze the factors affecting the heat capacity of materials?

For solid materials, the heat capacity has little relationship with the structure of the materials;

First order phase transition, the heat capacity curve changes discontinuously, and the heat capacity is infinite.

The second order phase transformation is completed gradually in a certain temperature range, and the heat capacity reaches a finite maximum.

2. Try to explain why the thermal conductivity of glass is often several orders of magnitude lower than that of crystalline solid.

The thermal conductivity of amorphous materials is small, because amorphous is a short-range ordered structure, which can be approximately considered as a crystal with very small grains.

With small grain size and many grain boundaries, phonons are more vulnerable to scattering, so the thermal conductivity is much smaller.

9. Magnetic properties of materials

1. Why is diamagnetism produced in materials?

Under the action of magnetic field, the orbital motion of electrons in matter produces diamagnetism.

2. What are the main applications of diamagnetic and paramagnetic susceptibility in metal research?

Determine the maximum solubility curve in the alloy phase diagram:

According to the rule that the paramagnetism of single-phase solid solution is higher than that of two-phase mixed structure, and the relationship between the paramagnetism of mixture and alloy composition is linear, the maximum solubility of alloy at a certain temperature and the alloy solubility curve can be determined.

Study the decomposition of aluminum alloy;

The order disorder transition, isomerism transition and recrystallization temperature were studied.

3. Explain the conditions under which ferromagnetism occurs.

If a metal is to have ferromagnetism, it is not enough only that its atoms are not offset by the spin magnetic moment, but also that the spin magnetic moment must be arranged spontaneously in phase to produce spontaneous magnetization.

4. Try to explain the main performance marks of soft magnetic materials and hard magnetic materials.

The hysteresis loop of soft magnetic materials is thin, with high magnetic conductivity and low Hc. The magnetic hysteresis loop of the hard magnetic material is hypertrophy, with high Hc, Br and (BH) m.

10. Electrical properties of materials

1. Explain the similarities and differences between the quantum free electron conduction theory and the classical conduction theory.

The electric field formed by positive ions in the metal is uniform, and there is no interaction between valence electrons and ions.

It is owned by the whole metal and can move freely in the whole metal.

According to the quantum free electron theory, the inner electrons of each atom in the metal basically maintain the energy state of a single atom, while all valence electrons have different energy states according to the quantization law, that is, they have different energy levels.

The energy band theory also believes that the valence electrons in metals are public and the energy is quantized.

The difference is that it believes that the potential field caused by ions in metals is not uniform, but changes periodically.

2. Why does the resistance of metal increase with temperature, while that of semiconductor decreases with temperature?

The increase of temperature will intensify the ion vibration, increase the amplitude of thermal vibration, increase the disorder of atoms, reduce the free path of electron movement, and increase the scattering probability, which will lead to the increase of resistivity.

The conduction of semiconductors is mainly caused by electrons and holes.

The increase of temperature increases the kinetic energy of electrons, causing the increase of the number of free electrons and holes in the crystal, thus increasing the conductivity and decreasing the resistance.

3. What are the three main indicators of superconductor performance?

(1) Critical transition temperature Tc

(2) Critical magnetic field Hc

(3) Critical current density Jc

4. The application of resistance measurement in metal research is briefly discussed.

The change of microstructure of metals and alloys is studied by measuring the change of resistivity.

(1) Measure the solubility curve of solid solution

(2) Measure the transformation temperature in shape memory alloy.

5. What are the conductive sensitive effects of semiconductors?

Thermal effect, photosensitive effect, pressure sensitive effect (voltage sensitive and pressure sensitive), magnetic sensitive effect (Hall effect and magnetoresistance effect), etc.

6. What are the main damage forms of insulating materials?

Electrical breakdown, thermal breakdown and chemical breakdown.

11. Optical properties of materials

1. The concept of linear optical performance and its basic parameters are briefly described.

Linear optical performance: when light of a single frequency is incident into a non absorbable transparent medium, its frequency does not change at all;

When light of different frequencies is incident into the medium at the same time, there is no mutual coupling between the light waves and no new frequency is generated;

When two beams of light meet, if it is coherent light, interference will occur;

If it is incoherent light, only light intensity superposition will occur, that is, the principle of linear superposition will be obeyed.

Refraction, dispersion, reflection, absorption, scattering, etc.

2. Try to analyze the feasibility of preparing transparent metal products?

It is not feasible because the metal absorbs visible light very strongly.

This is because the valence electrons of the metal are in the incomplete band. After absorbing photons, they are in an excited state.

They can collide and heat without jumping to the conduction band.

3. The conditions for producing nonlinear optical properties are briefly described.

The incident light is strong;

Symmetry requirements of crystals;

Phase matching.

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