01. Mechanical properties of materials under uniaxial static tension
1. Explanation of terms:
Crazing: crazing is a defect produced in the deformation process of polymer materials.
Because of its low density and high reflection ability to light, it looks silver, so it is named.
Crazing occurs in the weak structure or defective 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 strain εg caused by grain boundary sliding generally accounts for 50% ~ 70% of the total strain εt, which indicates that grain boundary sliding plays a major role in superplastic deformation.
Brittle fracture: the material basically does not produce obvious macroscopic plastic deformation before fracture, and there is no obvious omen.
It often shows a sudden and rapid fracture process, so it is very dangerous.
Ductile fracture: the fracture process that produces obvious macroscopic plastic deformation before and during the fracture of materials.
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, the brittle transgranular fracture along a specific crystal plane caused by the destruction of the bonding bond between atoms is called cleavage fracture.
(cleavage step, river pattern and tongue pattern are the basic microscopic characteristics of cleavage fracture.)
Shear fracture: shear fracture is the fracture caused by the sliding separation of materials along the sliding surface under the action of shear stress.
(micropore aggregation fracture is a common mode of ductile fracture of materials.
The fracture is often dark gray and fibrous in macro view, and the characteristic pattern of micro fracture is that a large number of “dimples” are distributed on the fracture.)
2. Explain the difference between ductile fracture and brittle fracture. Why is brittle fracture the most dangerous?
Stress type, degree of plastic deformation, presence or absence of omen, speed of crack growth.
3. What is the difference between breaking strength σc and tensile strength σb?
If there is no plastic deformation before fracture, or the plastic deformation is very small, there is no necking, and the material has brittle fracture, then σc= σb.
If necking occurs before fracture, then σc and σb unequal.
4. What scope does Griffith formula apply and under what circumstances does it need to be modified?
Griffith formula is only applicable to brittle solids with microcracks, such as glass, inorganic crystal materials, ultra-high strength steel, etc.
For many engineering structural materials, such as structural steel and polymer materials, large plastic deformation will occur at the crack tip, which will consume a lot of plastic deformation work.
Therefore, the Griffith formula must be modified.
02. Mechanical properties of materials under uniaxial static tension
1. Soft coefficient of stress state
The ratio of τmax to σmax is called the soft coefficient of stress state, which is expressed by α.
The larger the α, the larger the maximum shear stress component, indicating that the softer the stress state, the easier the material is to produce plastic deformation.
On the contrary, the smaller α, the harder the stress state, the easier the material is to produce brittle fracture.
2. How to understand the “notch strengthening” phenomenon of plastic materials?
Under the condition of notch, due to the three-dimensional stress, the yield stress of the sample is higher than that under uniaxial tension, that is, the so-called notch “strengthening” phenomenon occurs.
We cannot regard “notch strengthening” as a means of strengthening materials, because notch “strengthening” is purely due to 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 unidirectional tension, the normal stress component is large, the shear stress component is small, and the stress state is hard.
It is generally suitable for the test of so-called plastic materials with low plastic deformation resistance and cutting resistance.
Compression: the stress state softness coefficient of unidirectional compression is a = 2.
The compression test is mainly used for brittle materials.
Bending: there is no influence of the so-called specimen deflection on the test results during bending loading, such as tension.
During the bending test, the stress distribution on the section is also the largest on the surface, so it can sensitively reflect the surface defects of the material.
Torsion test: the soft coefficient of torsion stress state is higher than that of tension stress state, so it can be used to determine the strength and plasticity of materials that are brittle in tension.
During torsion test, the stress distribution of the sample section is the largest, so it is very sensitive to the reflection of material surface hardening and surface defects.
In the torsion test, the normal stress and shear stress are approximately equal;
Cut off the fracture, which is perpendicular to the axis of the sample.
This fracture is often used in plastic materials.
Normal fracture: the angle between the section and the axis of the sample is about 45 °, which is the result of normal stress. Brittle materials often have this kind of fracture.
4. Try to compare the similarities and differences between Brinell hardness and Vickers hardness test principles, and compare the advantages, disadvantages and application scope of Brinell, Rockwell and Vickers hardness tests.
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 indentation.
The difference is that the indenter used in Vickers hardness test is a diamond pyramid with an included angle of 136 ° between the two sides.
Brinell hardness adopts quenched steel ball or cemented carbide ball.
Advantages of Brinell hardness test: the indentation area is large, and its hardness value can reflect the average performance of each constituent phase of the material in a large area, with stable test data and 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 not suitable to test directly on the finished parts;
In addition, the indenter diameter and load need to be changed for materials with different hardness, and the measurement of indentation diameter is also troublesome.
Advantages of Rockwell hardness test:
Simple and rapid operation;
The indentation is small, and the workpiece can be inspected directly;
Disadvantages:
Poor representativeness due to small indentation;
The hardness values measured with different scales can neither be directly compared nor exchanged with each other.
Vickers hardness test has many advantages:
Accurate and reliable measurement;
The load can be selected arbitrarily.
In addition, Vickers hardness does not have the problem that the hardness of different scales can not be unified, and the thickness of the test piece is thinner than that of Rockwell hardness.
Disadvantages of Vickers hardness test:
Its determination method is troublesome, low efficiency, small indentation area and poor representativeness, so it is not suitable for routine inspection of mass production.
03. 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 material changes from ductile state to brittle state, the impact absorption energy decreases obviously, the micro pore aggregation of the fracture machine changes to transgranular cleavage, and the fracture characteristics change from fibrous to crystalline, which is low-temperature brittleness.
2. This post tries to explain the physical essence of low temperature brittleness and its influencing factors.
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 do not have low-temperature brittleness.
The low temperature brittleness of BCC METALS may be closely related to the phenomenon of late yield.
B. Effects of chemical composition:
The content of interstitial solute elements increases, the high-order energy decreases and the ductile brittle transition temperature increases.
C. Effect of microstructure:
Grain refinement and microstructure can increase the toughness of the material.
D. Effect of temperature:
It is complex, and brittleness (blue brittleness) occurs in a certain temperature range.
E. Effect of loading rate:
Increasing the loading rate is like decreasing the temperature, which increases the brittleness of the material and the ductile brittle transition temperature.
F. Effect of specimen shape and size:
The smaller the radius of curvature of the notch, the higher the tk.
3. Why grain refinement improves toughness?
Grain boundary is the resistance of crack propagation;
The number of dislocations accumulated in front of grain boundary is reduced, which is conducive to reducing stress concentration;
The increase of the total area of grain boundary reduces the impurity concentration on the grain boundary and avoids intergranular brittle fracture.
04. Fracture toughness of materials
1. Low stress brittle fracture
Brittle fracture often occurs in large parts when the working stress is not high or even far below the yield limit, which is the so-called low stress brittle fracture.
2. Describe the name and meaning 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 indicates the ability of the material to resist the unstable propagation of crack in the plane strain state.
JⅠc (strain energy at the crack tip) is also called fracture toughness, but it represents the ability of a material to resist crack initiation and propagation.
GIC refers to the energy consumed per unit area when the material prevents the unstable propagation of crack.
δCc(crack opening displacement), also known as the fracture toughness of a material, indicates the ability of a material to prevent crack propagation.
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-strain field at the crack tip in a cracked body.
It depends on the applied stress, sample size and crack type, but has nothing to do with the material.
However, KIc is the mechanical property index of materials, which depends on the internal factors such as material composition and microstructure, but has nothing to do with external factors such as applied stress and sample size.
The relationship between KⅠ and KⅠC is the same as that between σ and σS.
Both KⅠ and σ are mechanical parameters, while both KⅠC and σs are mechanical property indexes of materials.
05. Fatigue performance of materials
1. Characteristics of fatigue failure?
(1) This failure is a kind of latent sudden failure. Before fatigue failure, there will be no obvious plastic deformation and 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 has a high degree of sample selection for 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. Several characteristic areas of fatigue fracture?
Fatigue source, fatigue crack propagation zone and instantaneous fracture zone
3. Try to describe σ- 1 and ΔKth.
σ- 1 (fatigue strength) represents the infinite life fatigue strength of smooth samples, which is suitable for traditional fatigue strength design and verification;
△ Kth (threshold value of fatigue crack growth) represents the infinite life fatigue performance of cracked samples, which is suitable for the design and fatigue strength verification of cracked parts.
06. Wear performance of materials
1. How many types of wear are there? Explain their surface damage morphology.
Adhesive wear, abrasive wear, corrosion wear and pitting fatigue wear (contact fatigue)
Adhesive wear: the wear surface is characterized by scabs of different sizes on the surface of the parts.
Abrasive wear: there are scratches or grooves formed by obvious plough wrinkles on the friction surface.
Contact fatigue: there are many pits (hemp pits) on the contact surface, some of which are deep, and there are traces of fatigue crack propagation lines at the bottom.
2. Is the saying “the harder the material, the higher the wear resistance” right? Why?
Correct. Because wear is inversely proportional to hardness.
3. From the viewpoint of improving material fatigue strength, contact fatigue strength and wear resistance, this article analyzes the matters needing attention in chemical heat treatment.
While increasing the surface strength and hardness, the residual compressive stress of the surface layer is increased.
07. High temperature performance of materials
1. Explain the following nouns:
Approximate specific temperature: T / Tm
Creep: it refers to the phenomenon that materials slowly produce plastic deformation under the action of constant temperature and constant load for a long time.
Endurance strength: it is the maximum stress that the material will not creep fracture under a certain temperature and within the specified time.
Creep limit: it indicates the resistance of the material to high temperature creep deformation.
Relaxation stability: the ability of a material to resist stress relaxation is called relaxation stability.
2. The creep deformation and fracture mechanism of the material 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 stretching of molecular chain segments along external forces.
Intergranular fracture is a common form of creep fracture, especially at high temperature and low stress.
This is because the strength in polycrystal and grain boundary decreases with the increase of temperature, but the latter decreases faster, resulting in the lower relative strength of grain boundary 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 difference between creep deformation and plastic deformation mechanism of metal at high temperature is described.
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, the increase of temperature provides the possibility of thermal activation for atoms and vacancies, so that dislocations can overcome some obstacles 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.
08. Thermal properties of materials
1. Try to analyze the factors affecting the heat capacity of materials?
For solid materials, the heat capacity has little to do with the structure of the material;
In the first-order phase transition, the heat capacity curve changes discontinuously, and the heat capacity is infinite.
The second-order phase transition is gradually completed in a certain temperature range, and the heat capacity reaches a finite maximum accordingly.
2. Try to explain why the thermal conductivity of glass is often several orders of magnitude lower than that of crystalline solids.
The thermal conductivity of amorphous materials is small because the amorphous state is a short-range ordered structure, which can be discussed as a crystal with small grain size.
With small grain size and more grain boundaries, phonons are more vulnerable to scattering, so the thermal conductivity is much smaller.
09. Magnetic properties of materials
1. Why does antimagnetism occur in matter?
Under the action of magnetic field, the orbital movement 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 law that the paramagnetism of single-phase solid solution is higher than that of two-phase mixed structure, and there is a linear relationship between the paramagnetism of the mixture and the alloy composition, the maximum solubility and gold solubility curve of the alloy at a certain temperature can be determined.
Study the decomposition of aluminum alloy;
The order disorder transformation, isomerism transformation and recrystallization temperature of materials were studied.
3. Try to explain the conditions under which the material produces ferromagnetism.
For a metal to be ferromagnetic, it is not enough for its atoms to have only the non offset spin magnetic moment.
It must also make the spin magnetic moment arrange 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 and has the characteristics of high magnetic conductivity and low Hc.
The hysteresis loop of hard magnetic materials is hypertrophic and has the characteristics of high Hc, Br and (BH) m.
10. Electrical properties of materials
1. Try to explain the similarities and differences between quantum free electron conduction theory and classical conduction theory.
The electric field formed by positive ions in metal is uniform, there is no interaction between valence electrons and ions, and 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 metals increase with the increase of temperature, while the resistance of semiconductors decreases with the increase of temperature?
The increase of temperature will aggravate the ion vibration, increase the amplitude of thermal vibration, increase the degree of disorder of atoms, reduce the free path of electron motion, increase the scattering probability and increase the resistivity.
The conductivity of semiconductors is mainly caused by electrons and holes.
With the increase of temperature, the kinetic energy of electrons increases, resulting in the increase of the number of free electrons and holes in the crystal, which increases the conductivity and decreases the resistance.
3. What are the three main indicators to characterize the properties of superconductors?
(1) Critical transition temperature Tc
(2) Critical magnetic field Hc
(3) Critical current density Jc
4. This paper briefly discusses the application of resistance measurement in metal research.
The change of resistivity is measured to study the change of microstructure of metals and alloys.
(1) Measure the solubility curve of solid solution.
(2) Measurement of shape memory alloy transformation temperature.
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 properties and the basic parameters are briefly described.
Linear optical performance: when the light with a single frequency is incident into the non absorbing transparent medium, its frequency does not change;
When light with different frequencies is incident into the medium at the same time, there is no mutual coupling between light waves and no new frequency;
When two beams of light meet, if it is coherent light, interference will occur.
If it is incoherent light, there is only light intensity superposition, that is, it obeys the principle of linear superposition.
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 under full band, which will be in the excited state after absorbing photons.
There is no need to transition to the conduction band to collide and generate heat.
3. The conditions for producing nonlinear optical properties are briefly described.
The incident light is strong light.
Crystal symmetry requirements.
Phase matching.
