01. Mechanical properties of materials under uniaxial static tension
1. Explanation of terms:
Madness: cracking is a defect produced in the deformation process of polymeric materials.
Due to its low density and high light-reflecting ability, it appears silvery, which is why it is called.
Cracks occur in the weak structure or defective part of polymeric materials.
Superplasticity: the material presents a very large elongation (around 1000%) under certain conditions, without necking and fracture, which is called superplasticity.
The strain εg caused by grain boundary slip generally accounts for 50% ~ 70% of the total strain εt, which indicates that grain boundary slip plays an important 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 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, brittle transgranular fracture along a specific crystal plane caused by the destruction of the 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 usually dark gray and fibrous in macro view, and the characteristic pattern of microfracture is that a large number of “ripples” are distributed in the fracture.)
2. Explain 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, speed of crack growth.
3. What is the difference between breaking strength σ c and tensile strength σ b ?
If there is no plastic deformation before fracture, or if the plastic deformation is very small, there is no necking and the material presents brittle fracture, then σc = σb.
If necking occurs before fracture, then σc and σb are unequal.
4. What is the scope of application of Griffith's formula and under what circumstances does it need to be modified?
Griffith's formula is only applicable to brittle solids with microcracks, such as glass, inorganic crystalline materials, ultra-high-strength steel, etc.
For many structural engineering 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, Griffith's formula must be modified.
02. Mechanical properties of materials under uniaxial static tension
1. Smooth stress state coefficient
The ratio between τmax and σmax is called the smooth stress state coefficient, which is expressed by α.
The higher the α, the higher the maximum shear stress component, indicating that the softer the stress state, the easier it is for the material to produce plastic deformation.
On the contrary, the smaller α, the harder the stress state and the easier it is for the material to produce brittle fracture.
2. How to understand the phenomenon of “notch reinforcement” of plastic materials?
Under the notch condition, due to three-dimensional tension, the yield stress of the sample is greater than that under uniaxial tension, that is, the so-called notch “strengthening” phenomenon occurs.
We cannot consider “notch reinforcement” as a means of reinforcing materials, because notch “reinforcement” is purely due to the plastic deformation of materials constrained by three-dimensional tension.
At this time, the σs value of the material itself does not change.
3. The characteristics and scope of application of uniaxial tension, compression, bending and torsion tests are comprehensively compared.
In unidirectional stress, the normal stress component is large, the shear stress component is small, and the stress state is difficult.
It is generally suitable for testing so-called plastic materials with low plastic deformation resistance and cutting resistance.
Compression: The stress state smoothness coefficient of unidirectional compression is a = 2.
Compression testing is mainly used for brittle materials.
Bending: There is no influence of the so-called sample deflection on the test results during bending loading, such as stress.
During the bending test, the stress distribution in the section is also the largest on the surface, which can sensitively reflect the surface defects of the material.
Torsion test: The smooth coefficient of the torsional stress state is greater than that of the tensile stress state, so it can be used to determine the strength and plasticity of materials that are brittle under stress.
During the torsion test, the stress distribution of the sample section is larger, 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 the fracture, which is perpendicular to the sample axis.
This fracture is often used in plastic materials.
Normal fracture: the angle between the section and the sample axis is about 45°, which is the result of normal stress. Brittle materials often present this type of fracture.
4. Try to compare the similarities and differences between the principles of Brinell and Vickers hardness tests, and compare the advantages, disadvantages and scope of application of Brinell, Rockwell and Vickers hardness tests.
The testing principle of Vickers hardness is basically similar to that of Brinell hardness, and the hardness value is calculated according to the load supported by the unit indentation area.
The difference is that the indenter used in the Vickers hardness test is a diamond pyramid with an included angle of 136° between the two sides.
Brinell hardness adopts hardened steel ball or 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 over a large area, with stable test data and high repeatability.
Therefore, the Brinell hardness test is more suitable for measuring the hardness of gray cast iron, bearing alloys and other materials.
Disadvantages of Brinell hardness test: due to the large diameter of the cutout, it is generally not suitable to test directly on finished parts;
Furthermore, the diameter and load of the indenter need to be changed for materials with different hardness, and the measurement of the indenter diameter is also problematic.
Advantages of Rockwell hardness test:
Simple and fast operation;
The indentation is small and the part can be inspected directly;
Disadvantages:
Weak representation due to small indentation;
Hardness values measured with different scales cannot be directly compared or exchanged with each other.
The 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 cannot be unified and the thickness of the test piece is thinner than Rockwell hardness.
Disadvantages of Vickers hardness test:
Its determination method is problematic, low efficiency, small indentation area and low representativeness, so it is not suitable for routine inspection of mass production.
03. Impact resistance and fragility of materials at low temperatures
1. Low temperature brittleness; Ductile and brittle transition temperature.
When the test temperature is lower than a certain temperature tk (ductile and brittle transition temperature), the material changes from the ductile state to the brittle state, the impact absorption energy decreases obviously, the micropore aggregation of the fracture machine changes for transgranular cleavage, and the fracture characteristics change from fibrous to crystalline, which is low-temperature brittleness.
2. This post attempts to explain the physical essence of brittleness at low temperatures and its influencing factors.
Below the ductile and brittle transition temperature, the fracture strength is less than the yield strength and the material is brittle at low temperature.
A. Influence of crystal structure
Body-centered cubic metals and their alloys exhibit low-temperature brittleness, while face-centered cubic metals and their alloys generally do not exhibit low-temperature brittleness.
The low-temperature brittleness of BCC METALS may be closely related to the late yield phenomenon.
B. Effects of chemical composition:
The content of interstitial solute elements increases, the higher order energy decreases, and the ductile and 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 over a certain temperature range.
E. Effect of charging rate:
Increasing the loading rate is like decreasing the temperature, which increases the brittleness of the material and the ductile and brittle transition temperature.
F. Effect of sample shape and size:
The smaller the radius of curvature of the notch, the higher the tk.
3. Why does grain refinement improve toughness?
The grain boundary is the resistance to crack propagation;
The number of dislocations accumulated in front of the grain boundary is reduced, which contributes to reducing stress concentration;
Increasing the total grain boundary area reduces the concentration of impurities at the grain boundary and prevents intergranular brittle fracture.
04. Fracture resistance of materials
1. Low tension brittle fracture
Brittle fracture generally occurs in large parts when the working stress is not high or even far below the yield strength, 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 unstable crack propagation in the plane strain state.
JⅠc (crack tip strain energy) is also called fracture toughness, but 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 unstable crack propagation.
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 K EU and KI c .
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 fissured body.
It depends on the applied voltage, the size of the sample and the type of crack, but it has nothing to do with the material.
However, KIc is the index of mechanical properties of materials, which depends on internal factors such as material composition and microstructure, but has nothing to do with external factors such as applied voltage 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 KⅠC and σs are indices of mechanical properties of materials.
05. Fatigue performance of materials
1. Characteristics of fatigue failure?
(1) This failure is a kind of sudden and latent failure. Before fatigue failure, there will be no obvious plastic deformation and brittle fracture.
(2) Fatigue failure belongs to the delayed fracture of the low stress cycle.
(3) Fatigue is very sensitive to defects (notch, crack and structure), that is, it has a high degree of sample selection for defects.
(4) Forms of fatigue can be classified according to different methods.
According to the state of stress, there are bending fatigue, torsional fatigue, tension and compression fatigue, contact fatigue and compound fatigue;
According to the stress level and fracture life, there are high cycle fatigue and low cycle fatigue.
2. Various characteristic areas of fatigue fracture?
Fatigue source, fatigue crack propagation zone and instantaneous fracture zone
3. Try to describe σ- 1 and ΔKº .
σ- 1 (fatigue strength) represents the infinite life fatigue strength of smooth samples, which is suitable for traditional fatigue strength design and verification;
△Kth (fatigue crack growth limit value) represents the infinite life fatigue performance of cracked samples, which is suitable for the design and verification of fatigue resistance of cracked parts.
06. Material wear performance
1. How many types of wear are there? Explain the morphology of surface damage.
Adhesive wear, abrasive wear, corrosion wear and pitting fatigue wear (contact fatigue)
Adhesive wear: the wear surface is characterized by crusts of different sizes on the surface of the parts.
Abrasive wear: There are scratches or grooves formed by obvious wrinkles on the friction surface.
Contact fatigue: there are many holes (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 greater the wear resistance” right? Why?
Correct. Because wear is inversely proportional to hardness.
3. From the perspective of improving material fatigue resistance, contact fatigue resistance and wear resistance, this article analyzes the matters that need attention in chemical heat treatment.
By increasing the surface strength and hardness, the residual compressive stress of the surface layer increases.
07. High temperature performance of materials
1. Explain the following nouns:
Approximate specific temperature: Contact me
Creeping: 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: is the maximum stress that the material will not fracture under a given temperature and within the specified time.
Displacement limit: indicates the material's resistance to creep deformation at high temperature.
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 polymeric materials, there is also stretching of the molecular chain segments along external forces.
Intergranular fracture is a common form of creep fracture, especially at high temperatures and low stresses.
This is because the resistance in the polycrystal and the grain boundary decreases with increasing temperature, but the latter decreases more quickly, resulting in lower relative grain boundary resistance at high temperature.
There are two grain boundary fracture models: one is the grain boundary slip and stress concentration model; The other is the vacancy aggregation model.
3. The difference between creep deformation and the plastic deformation mechanism of metal at high temperature is described.
The plastic deformation mechanism of metal is sliding and twinning.
The mechanism of metal creep deformation is dislocation creep, diffusion creep and grain boundary sliding.
At high temperature, increasing 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 an external force, an unequal stress field is generated in the crystal.
Atoms and holes have different potential energy at different positions and will diffuse directionally from high potential energy potential to low potential energy potential.
08. Thermal properties of materials
1. Try to analyze the factors that affect the heat capacity of materials?
For solid materials, 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 a small grain size.
With small grain size and more grain boundaries, phonons are more vulnerable to scattering, so the thermal conductivity is much lower.
09. Magnetic properties of materials
1. Why does antimagnetism occur in matter?
Under the action of the magnetic field, the orbital movement of electrons in matter produces diamagnetism.
2. What are the main applications of diamagnetic and paramagnetic susceptibility in Metallurgical Research?
Determine the maximum solubility curve in the phase diagram of the alloy: according to the law that the paramagnetism of the single-phase solid solution is greater than that of the two-phase mixed structure, and there is a linear relationship between the paramagnetism of the mixture and the composition of the alloy, the maximum solubility and solubility curve of gold from the alloy at a certain temperature can be determined.
Study the decomposition of aluminum alloy;
The order disorder transformation, the isomerism transformation and the recrystallization temperature of the 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 undisplaced spin magnetic moment.
It must also cause the spin's magnetic moment to spontaneously organize itself into 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 high Hc, Br and (BH)m characteristics.
10. Electrical properties of materials
1. Try to explain the similarities and differences between the quantum theory of free electron conduction and the classical theory of conduction.
The electric field formed by positive ions in the metal is uniform, there is no interaction between electrons and valence ions, it belongs to the whole metal and can move freely throughout the metal.
According to the quantum free electron theory, the inner electrons of each metal atom basically maintain the energy state of a single atom, while all the valence electrons have different energy states according to the quantization law, i.e. , have different energy levels.
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 increasing temperature, while the resistance of semiconductors decreases with increasing temperature?
Increasing temperature will aggravate ionic vibration, increase the amplitude of thermal vibration, increase the degree of disorder of atoms, reduce the free path of electron movement, increase the probability of scattering, and increase resistivity.
The conductivity of semiconductors is mainly caused by electrons and holes.
As the temperature increases, the kinetic energy of electrons increases, resulting in an increase in the number of free electrons and holes in the crystal, which increases conductivity and decreases 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 article briefly discusses the application of resistance measurement in metal research.
Resistivity change is measured to study the change in microstructure of metals and alloys.
(1) Measure the solubility curve of the solid solution.
(2) Measurement of the transformation temperature of the 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 forms of damage to insulating materials?
Electrical breakdown, thermal breakdown and chemical breakdown.
11. Optical properties of materials
1. The concept of linear optical properties and basic parameters are briefly described.
Linear optical performance: when light with a single frequency falls on the transparent non-absorbing medium, its frequency does not change;
When light with different frequencies falls on the medium at the same time, there is no mutual coupling between the 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 superposition of luminous intensity, that is, it obeys the principle of linear superposition.
Refraction, dispersion, reflection, absorption, dispersion, etc.
2. Do you try to analyze the feasibility of preparing transparent metallic products?
It is not viable because metal strongly absorbs visible light.
This is because the metal's valence electrons are in the subcomplete band, which will be in the excited state after absorbing the 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.
Incident light is strong light.
Crystal symmetry requirements.
Phase matching.
