What is tensile strength
What is tensile strength?
Table of Contents
- What is tensile strength?
- Nomenclature of Tensile Strength
- Example Values for the Tensile Strength of Metallic Materials
- Types of tensile strength
- The difference between yield strength and tensile strength
- How do I test the tensile strength of a material?
Tensile strength is the metal from the uniform shape of plastic deformation to the local concentration of plastic deformation of the critical value of the transition, but also the metal in the static tensile conditions of the maximum load-bearing capacity. Tensile strength Rm (also known as tear strength) is the evaluation of strength properties of the material characteristic value. Tensile strength is the maximum mechanical tensile stress to which a test piece can be loaded. If the tensile strength is exceeded, the material fails: force absorption is reduced until the specimen finally tears. Tensile strength that characterizes the maximum uniform plastic deformation of the material resistance, tensile specimens in the maximum tensile stress before the deformation is uniform, but beyond that, the metal began to appear necking phenomenon, that is, produce a concentration of deformation; for the brittle materials with no (or very small) uniform plastic deformation, which reflects the material’s resistance to fracture. The symbol for Rm (GB/T 228-1987 old national standard tensile strength symbol for σb), the unit is MPa. When the steel yielded to a certain extent, due to internal grain rearrangement, its resistance to deformation and re-improvement of the ability to resist deformation, at this time, the deformation, although the development of a very fast, can only be increased with the increase in stress until the stress reaches its maximum value. After that, the ability of steel to resist deformation is significantly reduced, and in the weakest point of large plastic deformation, where the specimen cross-section rapidly shrinks, necking phenomenon, until fracture damage. The maximum stress value of steel before fracture in tension is called the strength limit or tensile strength.
Nomenclature of Tensile Strength
Tensile strength is an important property of materials used in engineering and manufacturing applications. It is a key factor in the design of structures (e.g., buildings, bridges, and airplanes) that can withstand forces due to tension or stretching. It is also important when designing products subjected to tensile or stretching forces, such as ropes, cables, and wires.
Tensile strength is usually determined by performing a tensile test, which involves applying an axial load to a specimen until it breaks. The load is applied using a tensile testing machine, which measures the force required to break the sample. The tensile strength is the maximum load the specimen can withstand before it breaks.
Tensile strength is usually reported in units of force per unit area, such as pounds per square inch (psi) or megapascals (MPa). The tensile strength of a material depends on its composition, structure, and processing history. For example, steels with higher carbon content tend to have higher tensile strength than those with lower carbon content.
The tensile strength of a material is an important factor in determining its suitability for a particular application. For example, in the aerospace industry, the tensile strength of materials used in aircraft structures is critical to ensuring the safety and reliability of the aircraft. Similarly, in the construction industry, the tensile strength of materials used in building structures, such as steel and concrete, is critical to ensuring that the structure can withstand the forces of wind, earthquakes, and other environmental factors.
The tensile strength can also be affected by various factors such as temperature, strain rate, and fatigue. For example, the tensile strength of certain materials may decrease at high temperatures, while the strength of other materials may increase. Similarly, the tensile strength of a material may decrease as the rate of deformation increases. Fatigue also affects the tensile strength of a material, causing it to decrease over time with repeated loading.
Ultimate tensile strength is a strength property, which depends on sample size, which measures the amount of stress a material can withstand before it changes from experiencing uniform plastic deformation to experiencing locally concentrated deformation. Necking begins at this point.
Since necking can lead to failure and can be life-threatening, it is important to consider this parameter when selecting the right material for the application.
Design Ultimate Tensile Strength means that a portion of the asset will permanently deform once it is subjected to the load for which it was designed. This deformation may change the material’s crystal structure, making it no longer functional.
In the case of cold-drawn bars, the tensile strength is increased by cold-drawing, where the bar is drawn through a die that reduces its outer diameter. This is critical for shafts used in a variety of applications.
Definition and symbolic representation of tensile strength
Tensile strength is a structural property that resists failure under load due to excessive stress or deformation.
Tensile strength is usually the maximum tensile strength and is generally calculated by dividing the peak tensile force by the cross-sectional area of the sample.
Tensile strength is measured using a tensile machine/tensiometer. Tensile machines/tensiometers are equipped with a load cell. They are commonly used in addition to tensile testing for measuring other material properties, including Young’s Modulus of Elasticity, Yield Strength, Elongation, Strain, and Maximum Tensile Strength.
The specimen in the tensile process, the material after the yielding stage into the strengthening stage with the transverse cross-section size is significantly reduced in the maximum force sustained at the time of pull-off (F), divided by the original cross-sectional area of the specimen (A) the resulting stress (σ), known as the tensile strength or strength limit (σ), the international system, the tensile strength unit expressed in pascals or megapascals, which is equivalent to the newton / square meter (N / m²). In the American system, tensile strength is expressed in pounds per square inch (lbf/In² or psi). It indicates the maximum ability of a metallic material to resist damage under tension. The formula for calculating tensile strength is:
σ = F/A
In the formula:
- F – the maximum force that the specimen is subjected to when it is pulled off, N (Newton);
- A – original cross-sectional area of the specimen, mm².
Unit: N/(kg force per unit area)
For brittle materials and plastic materials that do not form necking, the highest tensile load is the breaking load, and therefore the tensile strength also represents the breaking resistance. For the formation of necking plastic materials, its tensile strength represents the maximum uniform deformation resistance and the material in the static tensile conditions of the ultimate load-bearing capacity. For parts such as wire rope, tensile strength is a more meaningful performance indicator. Tensile strength is easy to determine, and good reproducibility and other mechanical property indicators such as fatigue limit and hardness; there is a certain relationship as one of the conventional mechanical properties of materials used to evaluate product quality and process specifications.
Necking phenomenon and significance
Necking is a special phenomenon in which the deformation of ductile metallic materials is concentrated in a localized area during the tensile test, resulting from the joint action of strain hardening (physical factor) and cross-section reduction (geometric factor).
Example Values for the Tensile Strength of Metallic Materials
|Tensile strength of metallic materials – Example Values|
|Material mane||Material No.||Old designation||Rm||Rp0.2|
Types of tensile strength
There are three types of tensile strength:
All weld metal testing
This involves only testing the weld metal of the sample welded test plate. This is a standard test during the qualification process of filler metal and certain procedures. Please note that all weld metal stretching rods may contain weld areas where dilution of the base metal occurs; This may not apply to all welded joint configurations.
Transverse tensile test
During this process, the stretching rod is extracted from the transverse axis of the plate (perpendicular to the axis of the weld seam). As a result, both the welding metal and the base metal were tested, capturing the interaction between the two. This is a touchstone: if the failure occurs in the base metal, it can be determined that the weld metal is stronger than the base metal, and vice versa. However, this type of testing has limitations. The ultimate tensile strength can be extracted with high precision, but yield strength or percentage elongation (a measure of ductility) cannot be extracted.
Longitudinal tensile test
This sample configuration includes testing samples taken from the longitudinal axis of the weld seam. According to the joint design, the longitudinal tensile rod may contain both base and weld metal.
The specifications followed specify the types of tensile tests required for specific applications. For example, welds used for seismic applications typically have stricter tensile testing requirements (additional sample requirements) than average welds manufactured according to AWS D1.1/D1.1M: Structural Welding Code – Steel. Always checking code requirements to ensure compliance before conducting any tensile testing is important.
The difference between yield strength and tensile strength
To discuss these two concepts, let’s start with how materials are destroyed. Any material subjected to continuously increasing or constant external forces will eventually exceed a certain limit and be destroyed. Many types of external forces can cause damage to materials, such as tension, pressure, shear force, torsion, etc. The tensile strength and yield strength are obtained through tensile tests only for tensile purposes.
The material is continuously stretched at the specified loading rate until fracture, and the maximum force borne during the fracture process is the maximum load-bearing capacity of the material under static tensile conditions. The ultimate tensile load is a force expression representing a material’s maximum ability to resist failure under tensile force. For brittle materials with no (or very small) uniform plastic deformation, it reflects the fracture resistance of the material.
The tensile strength is obtained through tensile testing and is tested using a tensile testing machine (usually a universal testing machine that can perform various tensile, compressive, and bending tests). The material is continuously stretched at a specified constant loading rate (i.e., the increase in tensile force per unit time) until it breaks or reaches the specified degree of damage (such as some butt weld strength tests that do not break), which is the force that causes the final failure of the material, it is the ultimate tensile load of the material. The ultimate tensile load is a force expression expressed in Newton (N), as Newton is a very small unit, so in most cases, it is more commonly used in thousands of Newton (KN). Due to the varying sizes of various materials, it is difficult to evaluate the strength of materials under tensile ultimate load. So, dividing the tensile ultimate load by the cross-sectional area of the experimental material yields the tensile ultimate load per unit area. The force exerted per unit area is an expression of strength, expressed in Pascal (Pa). Similarly, Pascal is a small unit, usually expressed in megapascals (MPa).
So, the ratio of the ultimate tensile load to the cross-sectional area of the experimental material is the tensile strength. Tensile strength is the limit at which a material can withstand external forces per unit area. Beyond this limit, the material will undergo dissociative damage.
Yield strength only applies to materials with elasticity, and materials without elasticity do not have yield strength. For example, various metal materials, plastics, rubber, and so on all have elasticity and yield strength. Glass, ceramics, bricks, and other materials generally have no elasticity, and even if they have elasticity, they are very small, so there is no mention of yield strength.
Elastic materials are subjected to constantly increasing external forces until they break. What changes have occurred?
The material undergoes elastic deformation under external forces, following Hooke’s law. What is elastic deformation? The external force is eliminated, and the material will return to its original size and shape. The material will enter the plastic deformation period when the external force increases to a certain value. Once the material undergoes plastic deformation, its original size and shape cannot be restored due to external forces! And the strength of the critical point that causes two types of deformation is the yield strength of the material! Regarding the applied tension, the tension value at this critical point is called the yield point. From a crystal perspective, only when the tensile force exceeds the yield point can the crystal bonding of the material begin to be disrupted! The destruction of materials starts from the yield point, not from the moment of fracture!
It is correct to clarify how these two strengths come about, so materials with high yield strength can withstand greater destructive forces.
Differences between yield strength and tensile strength
1. Strength difference: The yield strength corresponds to the yield point, which refers to the point at which the material undergoes plastic deformation. The corresponding strength is yield strength, while tensile strength refers to the material’s ability to resist external forces and the maximum strength at break during tensile testing.
2. Different deformation capabilities: yield strength reflects the ability of materials to resist deformation; tensile strength reflects the ability of a material to resist tensile failure.
3. Different meanings
- Tensile strength: indicates the actual bearing capacity of ductile materials. Still, this bearing capacity is only limited to the loading conditions of smooth specimens under uniaxial tension, and the tensile strength of ductile materials cannot be used as a design parameter because the corresponding strain is far from what needs to be achieved in actual use.
- Yield strength: Yield strength has direct practical significance and serves as a rough measure of certain mechanical behaviors and process properties of materials in engineering. For example, an increase in material yield strength is sensitive to stress corrosion and hydrogen embrittlement; The material has low yield strength, good cold working formability and welding performance, and so on. Therefore, yield strength is an indispensable and important indicator of material properties.
But no matter which strength, just using one to describe the matter cannot indicate whether this material is safe or sturdy!
Let’s talk about steel here, nothing else. There is another parameter about yield and tensile strength, which many people may need to learn. What exactly does it matter? Even fewer people may know. This parameter is the yield strength ratio! The yield strength ratio is the ratio of yield strength to tensile strength. The range is between 0 and 1. The yield strength ratio is one indicator for measuring steel’s brittleness. The larger the yield strength ratio, the smaller the difference between the steel’s yield strength and tensile strength. The worse the plasticity of the steel, the greater its brittleness!
Why do you say that? Here we need to introduce a new indicator – elongation. Simply put, it means how long the steel has been stretched after being pulled apart compared to the original. This is an important indicator for testing the plasticity of steel. The larger this value, the better the ductility of the steel. As I mentioned above, when the steel is stretched beyond the yield point, it is no longer possible to return to its original size, and it continues to be stretched until it breaks. The larger the yield-strength ratio, the smaller the yield and tensile strength difference. Therefore, while the loading rate remains constant, the shorter the time the steel is stretched, the lower the elongation.
According to the energy conservation law, energy can only be transformed or transmitted. When steel is stretched, it is ultimately the conversion and absorption of energy. Before the yield point, the steel is in the elastic deformation period, where almost all external tensile force is counteracted by the elastic force (converted into elastic potential energy), and little external energy is absorbed or converted; only a small amount is converted into thermal energy. After passing the yield point, the elastic force counteracts some external force (converted into elastic potential energy), while some are converted into thermal energy. The energy exerted by the external force on the steel is mainly absorbed during the plastic deformation period!
As I mentioned earlier, the failure of materials starts from the yield point. The lower the yield ratio, the longer the time from initial failure to fracture of the material. The higher the yield ratio, the shorter the time from initial failure to fracture of the material. Energy is extensively converted into thermal energy between the yield and fracture points.
So, if the yield or tensile strength is high, this material is better or safer. Not necessarily! Only steel with high yield strength and low yield ratio is safer! Unfortunately, the cost of such steel is too high, and it is unlikely to be used in civilian vehicles.
Nowadays, in addition to strength, there is also an important indicator of steel: toughness! So far, I have yet to see that car company describe the toughness of the steel used! It’s all about exaggerating the strength of steel! On the contrary, in the vast majority of cases, increasing the strength of steel often reduces its toughness! Reducing toughness means increasing brittleness! The toughness of steel is an important indicator related to its safety.
One indicator may have been intentionally or unintentionally forgotten by car companies – impact toughness or power.
Using the same force, push, or hit you hard, which will do the most damage to you? The answer is very clear! The impact resistance of steel is an important factor in the safety of the relationship! Haven’t you seen that car accident where the force was slowly applied until the car broke? It’s all an instant impact! How much tensile strength do you have if you can’t handle the instantaneous force?
The current steel shows that a strength greater than 1000Mpa is mostly tensile, and a yield strength exceeding 800Mpa is a simple task. For example, 40Cr, a common “universal steel” (suitable for anything), can yield close to 800Mpa and tensile strength above 900MPa through general modulation processes.
But balancing the high yield, high elongation, and good impact resistance is relatively difficult!
Almost all steel materials have the same problem: improving their strength while reducing their impact resistance! For example, high-strength bolts with a tensile strength of 1040-1240MPa in grade 10.9 are considered qualified, with a yield strength greater than 940Mpa, an elongation greater than 10%, and impact toughness of 59J/CM2; However, high-strength bolts of the same material grade 8.8 (one lower grade) are considered qualified if their tensile strength is between 830-1030MPa, yield strength is greater than 660Mpa, elongation is greater than 12%, and impact toughness is 78J/CM2.
Therefore, for the vast majority of metal materials, it is achieved at the cost of reducing certain technical performance indicators while improving certain technical performance indicators. It cannot be balanced. The steel industry is one of the most mature industrial technologies of humanity, and there are not many secrets. The technical indicators of steel materials are not necessarily better as they are higher or lower, but rather adjusted to a range that can be balanced according to needs. For people in our industry, except for structural problems (referring to product defects), there is no distinction between good and bad technical indicators of steel. It depends on where you use it. Only use the wrong place without saying the wrong thing.
How do I test the tensile strength of a material?
The tensile test is the most basic and widely used material mechanical property test. On the one hand, the mechanical property index of tensile test can be used as the basis for engineering design, evaluation of materials, and preferred process, which has important engineering practical significance. On the other hand, the tensile test can reveal the basic mechanical behavior of the material and is also the study of the mechanical properties of the basic test method.
A tensile test, usually carried out under axial loading conditions, is characterized by the test machine loading axis and the specimen axis of coincidence; the load is applied slowly.
In a tensile test on the tensile testing machine, the specimen in the load increases smoothly under the deformation until fracture, which can be derived from a series of strength indicators (tensile strength and yield strength), amount of plasticity indicators (elongation and section shrinkage).
Creep data can also be obtained from tensile tests conducted at elevated temperatures.
- Wire specimens are suitable for wire diameters less than 4mm;
- Bar specimens are suitable for bar and thick plate, thick pipe;
- Plate specimens are suitable for thin plate, thin pipe;
- The whole pipe specimen for the outer diameter ≤ 30mm and wall thickness ≤ 2mm pipe.
Main test items
- Tensile strength (Rm): the specimen after yielding to the maximum force in the process of pulling off the corresponding stress, characterizing the material’s ability to withstand the maximum stress;
- Yield strength: metal materials present yielding phenomenon during the test to achieve plastic deformation, and force does not increase the stress point, should distinguish between the upper yield strength and the lower yield strength, characterize the material to withstand plastic deformation ability, for no obvious up and down yielding material, usually measured Rp0.2 instead of yield strength;
- On the yield strength (ReH): the specimen yielding and the first decline in force before the highest stress;
- The lower yield strength (ReL) in the yielding period, not counting the initial transient effect of the lowest stress;
- The provisions of non-proportional elongation strength (Rp): non-proportional elongation is equal to the specified percentage of the scale distance of the extensometer when the stress, such as Rp0.2 indicates that the provisions of non-proportional elongation of 0.2% of the stress;
- Elongation at break (A): After the specimen is pulled off, the residual elongation of the post-break scale distance and the original scale distance of the ratio of the percentage, characterizing the material plasticity;
- Section shrinkage (Z): after the specimen is pulled, necking at the cross-sectional area of the maximum shrinkage and the original cross-sectional area of the ratio of the percentage, characterizing the material plasticity;
- Modulus of elasticity: the size of the stress required to produce a unit strain, characterizing the size of the material’s ability to resist deformation.
Standard for tensile testing
|Test items||Testing standards||Standard name|
|Tensile strength test||ASTM E8/E8M-13a||Metallic materials – Tensile testing methods|
|ASTM A370-14||Standard Test Method for Mechanical Properties Testing of Steel Products|
|ISO 6892-1:2009||Metallic materials – Tensile testing methods|
|GB/T 228-2010||Metallic materials – Tensile testing methods|
|JIS Z2241-2011||Metallic materials – Tensile testing methods|
|AS 1391:2007||Metallic materials – Tensile testing methods|
|ASTM B557-14||Standard Test Method for Tensile Testing of Wrought and Cast Aluminum and Magnesium Alloy Products|
Operating steps of tensile strength testing machine
The tensile strength testing machine is mainly suitable for testing the mechanical properties such as tensile strength, tensile strength, and tensile deformation of non-metallic materials, metal materials, and components. How to operate a tensile strength testing machine? We will take you through the operation process of the tensile strength testing machine:
- 1. After turning on the power and turning on the power switch, the power indicator light will light up. We need to wait for 15 minutes, and the tensile strength testing machine can only be connected to the computer after preheating and stabilizing.
- 2. Check whether the experimental instrument works properly, input the measured sample size, and select the detection plan. Measure the sample’s length, width, and thickness, take the average of each parameter, and record the data.
- 3. If each spline is not pulled apart within the specified time, the test will fail and be repeated. Repeat the experiment several times, calculate the results, generate a test report, and calculate the elongation at break.
- 4. Install the fixture, select a sample clamp that meets the testing specifications, and install it on the tensile testing machine.
- 5. Adjust the fixture, clamp the upper end of the sample, and then move the upper fixture down to reset the displacement value, force value, and large deformation value. Clamp the lower fixture, reset the force value, and run the experimental program.
- 6. After the test, clean the used samples, remove the upper and lower fixtures, and perform daily maintenance. Exit the program, turn off the power switch, and cut off the power.
The above is the operation process of the tensile strength testing machine.
Test steps for testing the tensile strength of materials
- 1) Prepare the test piece. Perform tensile tests on plain carbon steel and aluminum alloy specimens of the same size and shape. Use a graduated machine to mark the circumference within the original gauge range. Divide the gauge length into 10 equal squares. The original diameter was measured to be 10mm, and the original gauge length was 100mm.
- 2) Adjust the testing machine. Manually control the upper clamp to the appropriate clamping position. Choose a suitable force measuring plate. Start the testing machine and raise the workbench by about 10mm to eliminate the influence of the self-weight of the workbench system. Adjust the active pointer to align with the zero point, move the driven pointer closer to the active pointer, and adjust the automatic drawing device.
- 3) Clamp the test piece. Firstly, clamp the specimen into the upper clamp, move the lower clamp to the appropriate clamping position, and finally, clamp the lower end of the specimen. (Aluminum alloy materials without significant yield phenomenon need to be reproduced with an electronic extensometer).
- 4) Inspection and commissioning. Check the completion of the above steps. Start the testing machine, pre-apply a small amount of load (the stress corresponding to the load should not exceed the proportional limit of the material), and then unload to zero to check whether the testing machine is working properly.
- 5) Conduct experiments. Start the testing machine, slowly and evenly load, and carefully observe the rotation of the force measuring pointer and the drawing device drawing the diagram. Please capture and record the yield load value and record it for calculating the yield point stress value. Pay attention to observing the slip phenomenon during the yielding stage. After the yield stage, the loading speed can be faster. When reaching the maximum value, pay attention to the phenomenon of “necking”. Immediately stop the test piece after fracture and record the maximum load value. (There is no obvious yielding phenomenon in the aluminum alloy sample).
- 6) Remove the test piece and record the paper.
- 7) Measure the gauge distance after fracture using a vernier caliper.
- 8) Measure the minimum diameter at the necking point with a vernier caliper.
Four stages in the stretching process of metal materials
The tensile curve obtained from the experiment is a load elongation curve, which has four stages: elastic stage, yield stage, strengthening stage, and necking stage.
- 1) Elastic stage: As the load increases, the strain increases proportionally with the stress. If the load is removed, the specimen will return to its original state, exhibiting elastic deformation. During this stage, the elastic modulus E of the material can be measured.
- 2) Yield stage: Plain carbon steel: After exceeding the elastic stage, the load remains almost unchanged but fluctuates up and down within a small range, resulting in a sharp increase in the elongation of the sample. This phenomenon is called yield. If the small fluctuations in load readings are omitted, this stage can be represented by horizontal line segments on the tensile diagram. Plastic deformation starts suddenly, and the number of loads decreases suddenly. If the load is completely removed, the specimen will not return to its original length and will exhibit permanent deformation. For aluminum alloys, the end point of the elastic region is not accompanied by a sudden decrease in load or other significant changes. The transition from the elastic to the plastic stage is a smooth and gradual curve.
- 3) Strengthening stage: The curve shows an upward trend after the sample passes through the yield stage. Due to the continuous strengthening of the material during plastic deformation, the material’s deformation resistance is enhanced, called strain hardening. If the deformation does not completely disappear when the load is unloaded to zero at this stage, the residual strain when the stress decreases to zero is called plastic strain or residual strain.
- 4) During the necking and fracture stages, the load reading gradually decreases when the specimen elongates to a certain extent.
Elastic zone (from 0 to yield strength YS)
- Lesser elongation: Corresponds to the percentage.
- Elastic elongation: If the stress stops, the sample returns to its initial length.
- Yield strength: When the permanent elongation value reaches 0.2%.
- Yield strength = force at yield (N)/original area of the tensile specimen (mm2) or the same MPA value.
- Longitudinal modulus: ratio of force to elongation (depends on the metal).
- Transverse modulus: ratio of elongation to interfacial shrinkage (applied to all metals -0.3).
Plastic zone (from tensile strength UTS to yield strength YS)
- Greater elongation: a few percent for metals in general, can be as high as 50 to 60 percent.
- Inelastic elongation: If the stress ceases, the specimen remains permanently strained.
- Breaking Force: Tensile strength (UTS) record.
- Tensile Strength = Maximum force before rupture in a tensile test (N)/Original area of the tensile specimen (mm2) or the same MPA value.
- Due to the cold work effect, the strain will keep increasing with the test.
The overall length needs to be measured to calculate the ductility.
- E% = (Lu-L0)/L0x100 (Lu is final length, L0 is initial length).
Young’s Modulus – Important Parameters
- Modulus of elasticity E: unit N/mm2.
- Yield strength YS0.2: unit N/mm2 or MPa.
- Transverse modulus: Poisson’s coefficient, always around 0.3.
- Breaking force: UTS, in N/mm2 or MPa, except in special cases.
- Elongation at break E%: ductility, E < 5% is easy to break (brittle).
- Implementing standards: NF EN 10002 and ASTM E8, the difference between the two is the difference in measurement of different elongation values (L0).
Tensile specimen diagram
The whole process of tensile testing can be illustrated through tensile diagrams for metals, shown in Fig. 1a and b for metals without significant yielding and with significant yielding, respectively. According to the diagram, an axial tensile load is applied to the specimen, and the metal elastically elongates according to Hooke’s law up to point p, which corresponds to the proportional limit. Continue to increase the load over the elastic limit of the e point, the specimen along the es curve continues to elastic, plastic deformation to the s point that is to reach the residual elongation of 0.2% of the specified yield strength. As shown in Figure 1b, when some metals, such as soft steel and individual non-ferrous metals, produce a significant yield phenomenon, it should be measured by the provisions of its σs, upper or lower yield strength (σsu or σsl). From point s, continuous load increase to the maximum load before pulling off point b, the specimen is still uniformly elastic, plastic deformation and produce work hardening, point b load divided by F0 is called tensile strength. After the test reaches point b, the specimen in a weak cross-section begins to shrink locally, the load continues to fall, and the system is destabilized. At this point, the specimen’s local plastic deformation continues to proceed rapidly until the necking is reduced to a certain extent, k point, the specimen is fractured. The test is terminated (see Characterization of Mechanical Properties of Metals).
For materials with obvious yielding phenomena:
- Upper yield strength ReH=FeH/S0 (S0 represents the original cross-sectional area, FeH represents the axial force corresponding to the upper yield point).
- Lower yield strength ReL=FeL/S0 (S0 represents the original cross-sectional area, FeL represents the axial force corresponding to the lower yield point).
- Tensile strength Rm=Fmax/S0 (Fmax refers to the maximum axial force).
For materials with insignificant yield phenomena, the stress value that produces 0.2% residual deformation is specified as the yield limit, called the conditional yield limit or yield strength. External forces greater than this limit will cause permanent failure of the parts and cannot be restored.