1. Mechanical properties of materials under uniaxial static stress
1. Definition:
Craze: Craze is a type of defect that occurs in the deformation process of polymeric materials. Appears as a silver color due to its low density and high light reflectivity. The craze occurs in weak or defective parts of polymeric materials.
Superplasticity: Under certain conditions, the material presents a very large elongation (around 1000%) without strangulation or rupture, which is called superplasticity. The proportion of strain generated by grain boundary slip, εg, to the total strain, εt, is typically between 50% and 70%, indicating that grain boundary slip plays an important role in superplastic deformation.
Brittle fracture: Before the material fractures, there is no obvious macroscopic plastic deformation or warning signs. This process is often sudden and quick, which makes it very dangerous.
Ductile Fracture: The fracture process that shows obvious macroscopic plastic deformation before and during fracture. In ductile fracture, the crack propagation process is generally slow and consumes a large amount of plastic strain energy.
Cleavage Fracture: Brittle fracture along a specific crystal plane, caused by the destruction of bonds between atoms under normal stress, is called cleavage fracture. The 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 and separation of material along the sliding plane under shear stress. Micropore aggregation fracture is a common mode of ductile fracture in materials. The fracture surface is generally dark gray and fibrous in macro, while the microfracture surface presents a characteristic pattern of many “ripples” distributed on the 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 crack propagation speed.
3. What is the difference between breaking strength σ c and tensile strength σ b ?
If the material undergoes no plastic deformation or very little plastic deformation before fracture, and brittle fracture occurs without necking, then the critical stress, σc, is equal to the rupture stress, σb.
However, if necking occurs before fracture, σc and σb are not equal.
4. To what scope does Griffith's formula apply and under what circumstances should it be revised?
Griffith's formula is only suitable for brittle solids containing microcracks, such as glass, inorganic crystalline materials, and ultra-high-strength steel.
For many structural engineering materials, such as structural steel and polymeric materials, the crack tip undergoes significant plastic deformation, which consumes a large amount of plastic deformation energy.
Therefore, Griffith's formula must be modified to accurately reflect this phenomenon.
2. Mechanical properties of materials under uniaxial static stress
1. Smooth stress state coefficient
The ratio between the maximum shear stress, τmax, and the maximum normal stress, σmax, is called the stress state smoothness coefficient, denoted by α.
The higher α is, the greater the maximum shear stress component becomes, indicating a softer stress state and greater ease of plastic deformation in the material.
On the other hand, the smaller α is, the more difficult the stress state becomes, leading to a more brittle fracture.
2. How to understand the phenomenon of “notch reinforcement” of plastic materials?
When a sample has a notch, its yield stress is greater than that of a sample under uniaxial tension due to the presence of triaxial stress, which is called the “notch reinforcement” phenomenon.
However, this “notch reinforcement” cannot be considered a method of material reinforcement, as it is merely a result of plastic deformation of the material constrained by three-dimensional stresses.
In this case, the σs value of the material itself remains unchanged.
3. The characteristics and scope of application of uniaxial tension, compression, bending and torsion tests are comprehensively compared.
Reviewed:
In unidirectional stress, the normal stress component is large while the shear stress component is small, resulting in a strong stress state.
This test is typically applied to materials with low resistance to plastic deformation and cutting resistance, known as plastic materials.
Unidirectional compression has a stress state smoothness coefficient of a = 2 and is primarily used to test brittle materials.
Flexural tests do not suffer from the sample deflection that occurs during tensile tests.
In bending, the cross-sectional stress distribution reaches its maximum at the surface, making it an effective way to reflect surface defects in materials.
Torsion Test: The coefficient of smoothness of the stress state in torsion is greater than that in tension, making it an effective method for evaluating the strength and plasticity of materials that are brittle under tension.
In torsion testing, the stress distribution in the sample section is highest at the surface, making it highly sensitive to material surface hardening and surface defects.
The torsion test results in approximately equal normal stress and shear stress.
The fracture surface in the torsion test is perpendicular to the sample axis and is often used to evaluate plastic materials.
In normal fracture, the angle between the fracture surface and the sample axis is approximately 45 degrees, due to normal stress. Brittle materials often have this type of 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 scope of application.
The principle of the Vickers hardness test is similar to the Brinell hardness test in that both methods calculate hardness values based on the load per unit area of the indentation.
The main difference between the two tests is the type of indenter used. In the Vickers hardness test, a diamond pyramid indenter with an angle of 136 degrees between opposite sides is used. In contrast, the Brinell hardness test uses a hardened steel ball or hard alloy ball as the indenter.
Advantages of Brinell Hardness Test:
The large indentation area of the Brinell hardness test makes it capable of reflecting the average performance of each constituent phase over a large area, and the test results are stable and highly repeatable.
As a result, the Brinell hardness test is particularly suitable for measuring the hardness of materials such as gray cast iron and bearing alloys.
Disadvantages of Brinell Hardness Test:
The large indentation diameter of the Brinell hardness test makes it generally unsuitable for direct inspection of finished products.
Furthermore, the need to substitute the indenter diameter and load for materials with varying hardness, as well as the inconvenience of measuring the indenter diameter, are additional disadvantages of the test.
Advantages of Rockwell hardness test:
Easy and fast operation;
The indentation is small and the part can be inspected directly;
Disadvantages:
Poor representation due to small recoil;
Hardness values measured with different scales cannot be directly compared or 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 Rockwell hardness scales cannot be unified, and the thickness of the test piece is thinner than that of Rockwell hardness.
Disadvantages of Vickers hardness test:
Its measurement method is problematic, its working efficiency is low, the indentation area is small, and its representativeness is low, so it is not suitable for routine inspection of mass production.
Related reading: Metal Hardness: The Ultimate Guide
3. Impact resistance and fragility of materials at low temperatures
1. Low temperature brittleness, ductile and brittle transition temperature.
When the temperature during testing falls below a certain temperature, tk (the ductile-brittle transition temperature), materials such as CCC or close-packed hexagonal crystal metals and alloys, particularly medium- and low-strength structural steels commonly used in engineering , undergo a transition from a ductile state to a brittle state, resulting in a significant decrease in impact absorption energy.
This transition is characterized by a change in fracture mode from micropore aggregation to transgranular cleavage and a change in fracture appearance from fibrous to crystalline, a phenomenon known as low-temperature brittleness.
2. The physical essence of low-temperature brittleness and its influencing factors are explained.
At temperatures below the ductile-brittle transition temperature, the fracture strength is lower than the yield strength, resulting in brittle behavior at low temperatures.
A. Influence of crystal structure: Body-centered cubic metals and their alloys exhibit brittleness at low temperatures, while face-centered cubic metals and their alloys generally do not exhibit brittleness at low temperatures.
The low-temperature brittleness of CCC metals may be closely related to the late yield phenomenon.
B. The influence of chemical composition: the content of interstitial solute elements increases, the highest energy decreases, and the ductile and brittle transition temperature increases.
C. Influence of microstructure: refining the grain and structure can increase the toughness of materials.
D. Influence of temperature: It is relatively complex and brittle (brittle blue) occurs within 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 increases the ductile and brittle transition temperature.
F. Influence of sample shape and size: the smaller the radius of curvature of the notch, the higher the tk.
3. The reason for improving toughness through grain refinement?
Grain boundaries serve as resistance to crack propagation.
Reducing the number of dislocations in the pre-grain boundary packing helps reduce stress concentration.
An increase in the total grain boundary area reduces the concentration of impurities along the grain boundaries, thus reducing the likelihood of intergranular brittle fracture.
4. Fracture resistance of materials
1. Low tension brittle fracture
When the working stress of large parts is not high, even far below the yield strength, 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 (the stress-strain field intensity factor at the crack tip in the crack body) is a measure of plane strain fracture toughness and represents the ability of a material to resist unstable crack propagation under plane strain conditions.
JIc (the strain energy at the crack tip) is also known as fracture toughness and represents the ability of a material to resist crack initiation and propagation.
GIc represents the energy consumed per unit area to prevent the unstable propagation of cracks in a material.
δC (crack opening displacement), also known as fracture toughness of the material, indicates the ability of the material to prevent the initiation of crack expansion.
3. Explain the similarities and differences between KI and KIc.
KI and KIC are two distinct concepts.
KI is a mechanical parameter that represents the strength of the stress-strain field at the crack tip in the crack body and depends on the applied stress, sample size and crack type, but is independent of the material.
On the other hand, KIC is an index of mechanical property of the material that depends on internal factors such as composition and structure, but is not affected by external factors such as applied stress and sample size.
The relationship between KI and KIC is similar to that between σ and σs, where KI and σ are mechanical parameters, and KIC and σs are indices of mechanical properties of materials.
5. Fatigue property of materials
1. What are the characteristics of fatigue failure?
(1) This type of failure is a sudden and unexpected failure that occurs without noticeable plastic deformation before fatigue failure and is characterized by brittle fracture.
(2) Fatigue failure is a type of fracture delayed by low stress cycling.
(3) Fatigue is highly sensitive to defects such as notches, cracks and structural defects.
(4) Forms of fatigue can be classified in several ways.
According to the state of stress, the forms of fatigue include bending fatigue, torsional fatigue, tension and compression fatigue, contact fatigue and compound fatigue.
Based on the stress level and fracture life, fatigue can be classified into high-cycle fatigue and low-cycle fatigue.
2. How many characteristic areas of the fatigue fracture surface?
Fatigue source, fatigue crack growth zone and transient fracture zone.
3. Try to describe the similarities and differences between 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;
ΔKth (fatigue crack growth limit value) represents the infinite life fatigue performance of the crack specimen, which is suitable for the design and fatigue resistance verification of cracked parts.
6. Wear resistance of materials
1. How many types of wear are there? Shows the morphology of the surface damage.
Adhesion wear, abrasive wear, corrosion wear and pitting fatigue wear (contact fatigue).
Adhesion wear: The wear surface is characterized by crusts of different sizes on the surface of the parts.
Abrasive wear: Groove formed by scratching or obvious groove on the friction surface.
Contact fatigue: There are many holes (marks) 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 greater the wear resistance” correct? Why?
Correct. Because wear is inversely proportional to hardness.
3. From the point of view of improving material fatigue resistance, contact fatigue resistance and wear resistance, the issues that need attention in chemical heat treatment are analyzed.
The residual compressive stress of the surface layer increases while the surface strength and hardness increase.
7. High temperature performance of materials
1. Explain the following terms:
Approximate specific temperature: T/T l
Creep: Refers to the gradual plastic deformation of a material under the influence of constant temperature and load over a long period of time.
Strength: This term refers to the maximum stress that a material can withstand without undergoing creep fracture, under a specific temperature and within a designated period of time.
Creep limit: represents the resistance of a material to creep deformation at high temperature.
Relaxation Stability: The term used to describe the ability of a material to resist stress relaxation is called relaxation stability.
2. The creep deformation and fracture mechanism of materials is summarized.
The main mechanisms of creep deformation in materials include dislocation slip, atomic diffusion, and grain boundary slip.
For polymeric materials, elongation of the molecular chain under external force is also a factor contributing to creep.
Intercrystalline fracture is a common form of creep fracture, particularly at high temperatures and low stress levels. This is because the strength of polycrystalline grains and grain boundaries decreases with temperature, but the latter decreases more quickly, leading to lower grain boundary strength relative to grain strength at high temperatures.
There are two models to explain grain boundary fracture: the grain boundary slip and stress concentration model and the vacancy aggregation model.
3. The differences between the mechanisms of creep deformation and plastic deformation of metals at high temperatures are discussed.
The plastic deformation mechanism of metals is based on sliding and twinning.
The creep deformation mechanism of metals is mainly driven by dislocation slip, diffusion creep and grain boundary slip.
At high temperatures, the elevated temperature provides thermal activation to atoms and vacancies, allowing dislocations to move and continue to cause creep deformation.
Under the influence of external forces, an unequal stress field is generated within the crystal, leading to differences in potential energy between atoms and holes. This results in directional diffusion from high potential energy to low potential energy.
8. Thermal properties of materials
1. Try to analyze the factors that affect the heat capacity of materials?
For solid materials, the heat capacity is not significantly affected by the structure of the material.
In a first-order phase transition, the heat capacity curve changes abruptly and has an infinite value.
In a second-order phase transformation, the change occurs gradually over a specific temperature range and results in a finite maximum heat capacity.
2. Try to explain why the thermal conductivity of glass is often several orders of magnitude lower than that of a crystalline solid.
Amorphous materials have low thermal conductivity because their short-range ordered structure can be considered as a crystal with extremely small grains.
Due to the small grain size and numerous grain boundaries, phonons are easily scattered, leading to significantly reduced thermal conductivity.
9. Magnetic properties of materials
1. Why is diamagnetism produced in materials?
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 metal research?
Determining the maximum solubility curve in the alloy phase diagram:
Using the rule that single-phase solid solutions exhibit greater paramagnetism than two-phase mixed structures and the linear relationship between mixture paramagnetism and alloy composition, the maximum solubility of an alloy at a specific temperature and the solubility curve of the alloy can be determined.
Investigating the decomposition of aluminum alloys:
The order-disorder transition, isomerism transition and recrystallization temperature were studied to better understand the decomposition of aluminum alloys.
3. Explain the conditions under which ferromagnetism occurs.
For a metal to exhibit ferromagnetism, it is not only necessary that its atoms have non-zero spin magnetic moments, but also that these moments spontaneously align and generate spontaneous magnetization.
4. Try to explain the main performance marks of soft magnetic materials and hard magnetic materials.
Soft magnetic materials have a narrow hysteresis loop and are characterized by high magnetic conductivity and low Hc. In contrast, hard magnetic materials have a thick hysteresis loop, high Hc, Br and (BH)m.
10. Electrical properties of materials
1. Explain the similarities and differences between the quantum theory of free electron conduction and the classical theory of conduction.
In a metal, the electric field created by positive ions is uniform and there is no interaction between the valence electrons and the ions. This field is considered a property of all metal and allows the free movement of electrons throughout the metal.
According to quantum free electron theory, the inner electrons of each metal atom retain the energy state of a single atom, while the valence electrons have different energy states due to quantization and have distinct energy levels.
Energy band theory also recognizes that valence electrons in metals are shared and quantized in energy, but suggests that the potential field created 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?
Increasing temperature intensifies ionic vibration and increases the amplitude of thermal vibration, leading to increased atomic disorder, reduced electron movement and increased probability of scattering. These factors result in an increase in resistivity.
In semiconductors, conduction is driven primarily by electrons and holes. An increase in temperature increases the kinetic energy of electrons, leading to an increase in the number of free electrons and holes in the crystal and thus an increase in conductivity and a decrease in resistance.
3. What are the three main performance indicators of superconductors?
(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 in microstructure of metals and alloys is studied by measuring the change in resistivity.
(1) Measure the solubility curve of solid solution
(2) Measure the transformation temperature in 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 performance and its basic parameters are briefly described.
Linear optical properties: When light of a single frequency falls on a transparent medium that does not absorb light, its frequency does not change. When light of different frequencies falls on the medium at the same time, there is no interaction between the light waves and no new frequencies are produced.
When two beams of light cross, if they are coherent light, interference will occur. If they are incoherent light, only the intensity of the light will combine, following the principle of linear superposition.
Other optical properties include refraction, dispersion, reflection, absorption and dispersion.
2. Do you try to analyze the feasibility of preparing transparent metallic products?
It is not practical to use metals for visible light optics because they strongly absorb visible light. This occurs because the valence electrons in metals occupy an incomplete band and, after absorbing photons, are in an excited state. They can transfer energy through collisions and produce heat, but they do not transition to the conduction band.
3. The conditions for producing nonlinear optical properties are briefly described.
The incident light is strong;
Crystal symmetry requirements;
Phase matching.
