When engineers evaluate the material properties, tensile strength and yield strength are two critical indicators among the assessment data. Especially when you need to choose which materials are suitable for your machined parts, these two performance data play a crucial role. So, what is the relationship between the two? Below, I will introduce their differences in detail.
Key Takeaways
- Yield strength indicates when permanent deformation begins; tensile strength indicates the maximum engineering stress a material can withstand under tension. Reaching ultimate tensile strength does not necessarily mean immediate fracture.
- Yield strength matters for both part performance and machining. It helps engineers prevent permanent deformation under service loads and during machining, especially in thin-walled parts and slender shafts.
- Material strength affects CNC machining decisions. Stronger materials can require greater cutting forces, adjusted cutting speeds, and effective cooling to manage tool wear.
- The yield-to-tensile ratio provides context for material behavior after yielding. Consider it alongside ductility when evaluating chip formation and choosing cutting strategies.
- Choose sufficient strength, then balance machinability and cost. Start with yield strength that meets the maximum working stress multiplied by an appropriate safety factor, then evaluate tensile strength and ductility.
What is Yield Strength?
Yield strength is defined as stress required to bring about a transition from elastic to plastic deformation upon application of external tensile or compressive load.
To give an easy-to-understand example, for example, if you gently stretch a spring, it will immediately bounce back to its original shape when you let go. But when you stretch hard, the spring is completely deformed, and even if you let go, it can no longer be retracted.The critical point that causes a material to undergo permanent deformation is the yield strength. The unit of yield strength is typically Pascal (Pa) or Megapascal (MPa).
This property is crucial in the field of design and engineering because it helps you understand the loads that a material can withstand in the event of permanent deformation.
What is Tensile Strength?
Tensile strength can also be referred to as ultimate tensile strength(UTS). This is the maximum stress value that a material can withstand while being stretched before it breaks. It is the highest point on the stress-strain curve. Tensile strength is commonly measured in units of Pascals (Pa) or megapascals (MPa). In addition, tensile strength is a basic mechanical property of metals, polymers and composites.
To use a spring as an example again, imagine pulling a spring with all your strength in both directions until it breaks. The highest amount of tensile force the spring withstood before it broke is called its tensile strength.
Stress-Strain Curve Analysis
If you want to better understand these two properties, it is important to understand the stress-strain curve and its measurement content.
The stress-strain curve is a graph plotted from a standard tensile test. It records the relative change in length (strain, ϵ) of a material under the action of an increasing tensile force (stress, σ).
1. Elastic Region
The curve is a straight line. Stress is directly proportional to strain. At this stage, if you remove the external force being applied to the material, the material will return to its original shape. In addition, the slope of this line is the Young’s modulus(E), which represents the rigidity of the material. The steeper the slope, the harder the material.
2. Yielding
At this stage, the curve begins to bend. At this time, the crystal structure inside the material begins to slip, and irreversible plastic deformation occurs. The stress value corresponding to this point is the yield strength.
3. Strain Hardening
The curve rises again until it reaches its highest point. At this time, although the material has deformed, the internal structure has become stronger due to lattice distortion. You must apply even greater force to continue stretching it. It should be noted that the stress value corresponding to the peak of the curve is the tensile strength.
4. Necking and Fracture
At this stage, the curve begins to decline after passing its peak, until it suddenly breaks off. A localized region of the material begins to thin rapidly, and its load-bearing capacity decreases until it eventually fractures. The strain value at the end of the curve represents the material’s ductility.
Yield Strength vs. Tensile Strength: Key Differences
So what are the differences between the two? Below, I will explain it in detail.
Definition
Through the above detailed introduction, I believe you already understand this.
- Yield strength definition: Yield strength is the stress at which a material exhibits a specified amount of permanent plastic deformation.
- Tensile strength definition: Tensile strength is the maximum engineering stress obtained by dividing the maximum load in a tensile test by the original cross-sectional area of the specimen.
Application
Yield strength helps you determine whether a material will maintain its shape under expected loads and stresses. This is particularly important for structural components, as you need to ensure the material’s structural integrity.
Tensile strength helps you understand the ultimate load-bearing capacity of materials. In this way, you can use suitable materials to manufacture structural components that must not fail under maximum stress conditions.
Formula
These two formulas are very similar in form; both are force divided by area, but the timing of their values is different
Yield Strength Formula
$$\sigma_y = \frac{F_y}{A_0}$$
- σy =Yield strength (MPa)
- Fy= Load corresponding to the yield point (N)
- A0= Original cross-sectional area of the specimen (mm²)
Ultimate Tensile Strength Formula
$$\sigma_{UTS} = \frac{F_{max}}{A_0}$$
- σUTS = Ultimate tensile strength (MPa)
- Fmax = Maximum load during the entire tensile process(N)
- A0= Original cross-sectional area of the specimen (mm²)
Measurement Method
For the yield strength of materials, we generally use the engineering determination method, also called the 0.2% offset method. In order to unify the definition, engineers will translate the strain value 0.2% to the right along the straight line of the elastic stage on the stress-strain diagram (i.e. ϵ=0.002).
The method for measuring tensile strength is relatively straightforward. A tensile testing machine continuously applies tensile force to the material, and the machine’s sensors record the force exerted on the material in real time. The maximum tensile force recorded is divided by the original cross-sectional area of the specimen, and the resulting maximum stress value is the tensile strength.
Intermolecular Forces
Yield strength: The local metallic bonds inside the material are broken, the atomic planes slide as a whole along a specific direction by an atomic distance, and then new metallic bonds are re-established.
Tensile strength: Strong external forces directly overcome the metallic bond forces between atoms. Internal micro-voids merge, and metallic bonds completely break.
Engineer’s Perspective
Understanding the yield strength of materials is a safe benchmark to ensure that parts do not deform. Understanding the tensile strength of a material can help engineers evaluate the material’s ultimate load-bearing capacity. It can be said that these two data are crucial for engineers to design parts.
Why Both Matter in CNC Machining?
In reality, many new CNC operators only look at the hardness on the drawing, but ignore the combination of yield strength and tensile strength. In fact, these two indicators are key to whether the part can be successfully machined. During cutting, the stress applied by the tool tip must be greater than the yield strength and ultimately exceed the tensile strength.
Yield Strength
The higher the yield strength, the less likely the material is to undergo plastic deformation. When machining materials with high yield strength, you need to set the machine tool to output greater torque, allowing the cutting tool to withstand greater cutting resistance to cut into the material.
At the same time, it should be noted that when machining thin-walled parts or slender shafts, if the cutting force exceeds the material’s yield strength, the part may undergo permanent deformation during the machining process.
Tensile Strength
You must also know that heat is generated when a cutting tool machines a material. And if this heat cannot be taken away in time, it will all be concentrated on the tool tip, thus affecting the service life of the tool. The higher the tensile strength of the material being machined, the more heat is released at the moment of fracture.
Therefore, when machining high-tensile-strength materials, the most effective way to control temperature is to reduce the cutting speed (Vc). Of course, you can also use high-pressure coolant for cooling.
Yield-to-Tensile Ratio
The yield-to-tensile strength ratio is the ratio of yield strength to tensile strength. It can be used to compare the stress margin between the yield point and the maximum engineering stress, but it does not, on its own, represent ductility, toughness, or chip-breaking performance.
Low Yield-to-Tensile Ratio
Typical Materials: Low-carbon steel, austenitic stainless steel.
The typical characteristic of this type of material is its high toughness. When the tool cuts the material, it will deform but not break, forming continuous long strip-shaped chips. Therefore, to address this issue, you must use a blade with an aggressive chip breakers or increase the feed rate (fz) during programming.
High Yield-to-Tensile Ratio
Typical materials: Certain high-hardness quenched steels.
In CNC machining, the higher strength and hardness of this type of steel may lead to greater cutting resistance and faster tool wear. Your actual machining strategy should be formulated based on the hardness after heat treatment, workpiece rigidity, and recommended tool parameters, rather than solely on the yield-strength ratio to select parameters.
Practical Machining Operations
Of course, let me give you a specific real-life example to clarify. As you know, when machining TC4 thin-walled parts, common problems include inadequate cutting, uneven wall thickness, and even chatter marks appearing on the part surface. Why do these problems occur? And how do we reduce the deformation issues of thin-walled parts?
Reason for Easy Springing Back
In fact, TC4 has a relatively high yield strength, and cutting tools need to apply significant local stress to cause plastic deformation of the material and form chips. Therefore, under the same cutting conditions, cutting resistance is usually greater.
However, the direct cause of thin-walled parts springing back is elastic deformation. The elastic modulus of TC4 is about 110 GPa, which is lower than that of steel. Under the same geometric dimensions and loading conditions, TC4 is also more prone to elastic deflection. Additionally, thin-walled structures have low stiffness, so they are prone to yielding under cutting forces and springing back after unloading.
Therefore, you must remember that high yield strength does not equate to high stiffness. Even if the part does not undergo permanent deformation, elastic displacement during cutting can cause it to exceed dimensional tolerances.
Corresponding Machining Solution
In our actual machining cases, after rough machining, we need to finish mill the shell sidewalls to the final wall thickness. To eliminate the above problems, we mainly make adjustments from the following three aspects:
1. First, reduce the radial depth of cut, rather than just reducing the feed rate. When side milling,we first distinguish between radial depth of cut (ae) and axial depth of cut (ap). We reduce the lateral load by decreasing the radial engagement and removing the allowance in multiple passes.
2. Adjust the feed according to the feed per tooth to avoid continuous friction of the tool. You must pay close attention to this because excessive feed may increase cutting force, but too little feed may also cause the cutting edge to repeatedly rub against the material, increasing heat and work-hardening risks.
3. Retain support and remove material evenly. During rough machining stage, we will maintain sufficient wall thickness and, when conditions permit, alternately machine both sides of thin-walled parts to avoid machining one side to final dimensions first.
How to Choose the Right Material Based on Strength?
Choosing the right material for CNC machined parts has always been a major headache for many engineers. Because this is not only related to the performance of the material, but also the cost, use environment, etc. Next, I will sort out a set of material selection logic based on strength indicators for you.
I’m sure you don’t want the parts to deform during operation. Therefore, yield strength is definitely your primary factor when selecting materials. You need to calculate the maximum working stress the parts will endure under extreme operating conditions, then multiply it by a reasonable safety factor, and use the resulting value as your target yield strength.
Secondly, once the yield strength of the material meets the standard, you need to consider its tensile strength next. If the yield strengths of the two materials are similar, you should choose the one with lower tensile strength and moderate ductility. The reason is that the higher the compressive strength of the material, the worse its machinability becomes. This will consume the carbide blades in the CNC workshop, forcing operators to reduce cutting speeds, ultimately leading to an increase in the unit labor cost per part.
Common CNC Material Strength Quick Reference Chart
| Material | Yield Strength | Tensile Strength | Machinability |
| Aluminum 6061-T6 | ≈ 275 MPa | ≈ 310 MPa | Excellent |
| Brass C360 | ≈ 310 MPa | ≈ 400 MPa | Perfect |
| Mild Steel 1018 | ≈ 370 MPa | ≈ 440 MPa | Good |
| Alloy Steel 4140 | ≈ 415 MPa | ≈ 655 MPa | Fair |
| Stainless Steel 304 | ≈ 215 MPa | ≈ 505 MPa | Fair |
| Titanium Ti-6Al-4V (Grade 5) | ≈ 880 MPa | ≈ 950 MPa | Difficult |
* The table data is sourced from MatWeb.
Conclusion
Well, that’s all about the comparison between yield strength and tensile strength. By reading the entire article, I believe you will have a deeper understanding of material evaluation and selection. Of course, choosing the right material is only the first step toward a successful product, and transforming drawings into perfect products is another challenge. Please feel free to entrust your drawings to us. Our team will use advanced machine tools and CNC machining services to manufacture high-quality precision parts for you.
Reference Resources：
- Methods for measuring yield strength and tensile strength: ASTM E8 Standard Test Methods[link]
- Microscopic crystal structure and failure analysis: ASM International[link]
FAQ
Q1: Is UTS the point of failure?
A: Ultimate Tensile Strength (UTS) is the maximum stress a material can withstand, but it is not equivalent to the failure point. After reaching the UTS, the material undergoes necking. At this time, the cross-sectional area of the material shrinks, and the bearing capacity decreases, and it will not fail until the fracture point.
Q2: How to calculate yield strength from tensile strength?
A: In fact, it is not possible to directly calculate the yield strength using a universal mathematical formula based solely on the tensile strength. But in engineering practice, if you do not know the yield strength, you can use some empirical approximation methods: for carbon steel and low-alloy steel, the yield strength is approximately 0.6 to 0.8 times the tensile strength; for aluminum alloys, the ratio is usually between 0.7 and 0.9.
Additionally, there is a commonly used engineering estimate—the 0.2% offset method—but this is not a calculation formula.
It is also worth noting that these ratios are only rough empirical values and cannot replace actual test data.
Q3: What is 0.2% yield strength?
A: 0.2% yield strength (also known as 0.2% proof stress) is an engineering agreed definition of yield strength. It is defined as follows: on the stress-strain curve, draw a straight line parallel to the elastic segment from 0.2% strain (i.e. 0.002) on the horizontal axis. The stress value corresponding to the intersection of this straight line and the stress-strain curve is the 0.2% yield strength, recorded as Rp0.2or σ0.2.
Q4: Is a higher yield strength better?
A: It depends on the specific application scenario. A material with high yield strength means that it can withstand greater loads. However, the higher the yield strength, the lower the toughness and ductility of the material, making it more prone to brittle fracture.
Q5: Why do we use 0.2% offset?
A: 0.2% offset is a convention of engineering practicality. The reasons are as follows:
1. It is small enough that it does not affect the functionality or dimensional accuracy of the part.
2. It falls outside the range of measurement errors and curve noise associated with the elastic region, ensuring clear plot intersections and good repeatability.
Q6: What is the most common value of offset yield strength?
A: 0.2% is the most common standard value.
Q7: What does 0.1% proof stress mean?
A: On the stress-strain curve, draw a line parallel to the elastic region starting from the point on the horizontal axis where the strain is 0.1% (i.e., 0.001). The stress value corresponding to the intersection point with the curve is recorded as Rp0.1.
Q8: Is yield strength always lower than tensile strength?
A: For most common metal materials, the yield strength is lower than the tensile strength.
Q9: How does yield strength change as a material is heated?
A: General rule: As temperature increases, the yield strength of the material decreases.
Q10: Is yield point and yield strength the same?
A: Yield strength and yield point are not exactly the same. Yield point is a geometric concept. It is that location on the stress-strain curve where the material begins to deform plastically. Yield strength is an exact stress value.
Q11: Is yield strength and yield stress the same?
A: Yes, yield stress and yield strength are almost identical.
Q12: What is the yield strength of steel?
A: The yield strength of steel ranges from approximately 200 MPa to over 2000 MPa. The max yield strength of steel can reach 2400 MPa.
Q13: What is the yield strength of a36 steel?
A: A36 steel yield strength is generally between 250 and 290 MPa.
Q14: Is ultimate tensile strength the same as yield strength?
A: Yield strength and ultimate strength are completely different. Yield strength is the stress at which a material begins to undergo permanent plastic deformation. Ultimate tensile strength (UTS) is the maximum engineering stress that a material can withstand.
Q15: What is ultimate tensile strength?
A: Ultimate tensile strength (UTS) refers to the maximum stress a material can withstand when subjected to tensile stress. It corresponds to the highest point on the stress-strain curve. The units are MPa or ksi.
Q16: What is the tensile strength of steel?
A: Steel tensile strength range is approximately from 400 MPa to over 2500 MPa.
Q17: What is the tensile strength of carbon fiber?
A: Carbon fiber tensile strength ranges from 3500 MPa to 7000 MPa.
Q18: What is the tensile strength of a36 steel?
A: A36 steel tensile strength ranges from 400 to 550 MPa.
Q19: Is tensile strength the same as breaking strength?
A: Tensile strength is the maximum engineering stress a material experiences during the stretching process. Breaking strength is the engineering stress at the moment of fracture.
Q20: What is the symbol for tensile strength?
A: Rm is the international standard tensile strength symbol. σu or Fu is more common in North America.
Q21: What is fracture point?
A: The fracture point is the endpoint on the stress-strain curve. It corresponds to the stress and strain values at the moment when the material completely separates and fractures.
Q22: What is the UTS point?
A: UTS point is the highest point on the engineering stress-strain curve, corresponding to the maximum engineering stress that the material can withstand.
Q23: What steel has the highest UTS?
A: Maraging steel and ultra-high-strength steel.
Q24: Which is better 550 or 550D?
A: When the strength is the same, steel with higher ductility has more advantages in structural safety. So 550D is a better choice.
Q25: How to test tensile strength of concrete?
A: The following three indirect testing methods are primarily used to test the tensile strength of concrete:
1. splitting tensile test
2. flexural tensile test
3. direct tensile test.
