Increasing materials research is a major factor in the rise of modern manufacturing. The variety of materials that can be machined is also increasing in CNC machining, and it is beneficial to have a basic understanding of key properties and differences to make the most of the latest developments.
In this article we will focus on the main properties of the material: strength, stiffness and hardness. We will explain each of these values in detail and provide some professional tips for manufacturing processes.
Strength, stiffness, hardness: why are they important?
Material properties are the characteristics of a material that determine its response (how it changes shape and form) to external forces. Based on material properties, materials are classified as soft, hard, elastic, conductive, etc.
Strength, stiffness and hardness are the three most important material properties associated with materials for CNC machining. They define how materials behave under the influence of cutting tools. In fact, it is possible to classify a material as “difficult to cut” just based on these three properties.
What is material resistance?
Strength is probably the most important mechanical property for technical applications. It is the first specification that engineers check when selecting materials for their project. In the simplest sense, it determines how much a material deforms when an external force acts on it. The stronger a material is, the less it will deform.
To explain this in more detail, we use the classic stress-strain curve. As you can see, it contains a lot of information in just one graph. It may be a little difficult to understand at first, but let's break it down into manageable chunks.
First, we will briefly introduce the concepts of stress and strain, properties represented by the axes of the diagram.
Stress is the force per unit area. The greater the force acting on the material, the greater the tension. Elongation, on the other hand, is the deformation of the material per unit length. It is, therefore, a measure of the reaction of the material under stress. Of course, they are directly related: if one of the values increases, the other also increases and vice versa.
It should be noted that there are several types of tension depending on the type of force applied to an object. In our case, we only considered tensile stress since it is the most common type of loading condition in most applications.
As can be seen in the figure, there are points and regions of the stress-strain curve. They represent different types of strengths, which are our next topic of discussion.
Stretch limit
Yield resistance is the resistance of a material before reaching its yield point. The yield point is a point on the stress-strain curve before which the material deforms elastically. That is, it returns to its original shape when the tension is removed. When loaded beyond this point, the material undergoes permanent plastic deformation that does not disappear even when the load is removed.
Understandably, yield strength is the most important property for designers, as plastic deformation has an adverse effect on most products.
Maximum tensile strength
Tensile strength is another point on the stress-strain curve that defines the maximum tensile stress that a material can achieve. It occurs in the plastic deformation zone and represents the point at which the material begins to tend to failure.
From a design perspective, this is an important measure of stress if the part is expected to deform plastically. In general, stress should remain well below UTS to avoid excessive deformation and fracture.
Breaking strength
The name breaking force is self-explanatory. It is the stress value at which a material fails completely and begins to break. This means it breaks. This is an extreme stress point and should be avoided at all costs during the design phase.
A good example in machining is tool breakage. If a cutting tool is overused, the cutting edge breaks and the tool becomes unusable.
Compressive strength
As explained above, there are different ways to apply tension to a part. The previous discussion was based on tensile stresses only. However, most materials behave similarly under other types of stress.
Compressive strength is the resistance of the material under compressive forces. The behavior is the same, the material deforms (initially elastic, then plastic) under pressure. Eventually it starts to break down at a certain level of tension.
In general, compressive strength is greater than tensile strength because it is more difficult to compress materials than to stretch them. In construction, it is an important characteristic when the part is subjected to compression forces. Some examples include hydraulic presses, columns, and vehicle suspension systems.
What is material stiffness?
Stiffness is another material property with great relevance for design and manufacturing. Basically, it is a measure of the rate of deformation of the material under the influence of force. Therefore, a material with high rigidity deforms less than a less rigid material.
In relation to the stress-strain curve, stiffness is the relationship between the change in stress (force) and the strain (strain). In other words, it is the value of the slope of the stress-strain curve. Therefore, the formula for calculating stiffness is:
If only the stiffness of the elastic region is taken into account, it is also called “modulus of elasticity”. This is a constant value that applies to all points in the elastic deformation zone.
To make the concept of rigidity more understandable for our readers, we can refer to Hooke's Law. Defines stiffness as the ratio between the force exerted on a spring and its extension in response to that force. Therefore, k in the following figure is the stiffness.
As you can see, the greater the elongation, the lower the spring stiffness. This also corresponds to the general understanding of the term “stiffness”.
Stiffness during machining – deflection of the cutting tool
Let's extend our discussion of stiffness to a real-world example from the machining world. The cutting tool absorbs the cutting forces generated by the cutting process and must be rigid enough not to deform. Small deviations are normal, but excessive deformations can affect the tolerance of the part and even break the tool.
Therefore, engineers apply their knowledge before tool selection to check whether their tool will bend within acceptable limits or not. A highly simplified version of this calculation process can be found below.
In a machining setup, the tool acts as a cantilever beam. For simplicity, we assume that the cutting force is a point load at the tool tip (as shown in the figure). In this case, the stiffness of the tool (point-loaded cantilever beam) results from:
E is the modulus of elasticity of the material from which the tool is made, I is the moment of inertia and L is the length that the tool protrudes from the spindle.
Now that we know the relationship between stiffness, force and deformation, we can calculate the expected deflection of the tool using the cutting force P:
What is material hardness?
Hardness is a measure of a material's resistance to local permanent deformation. Simply put, it is the resistance of a material to surface deformations such as indentations, scratches and abrasion.
The harder the material, the harder it will be to scratch. A popular material with high hardness is diamond, which is often used as a cutting tool.
Hardness is an important material property that must be taken into account when making technical decisions. For example, in machining, only hard cutting tool materials such as carbide or PCN are compatible with difficult-to-cut materials such as titanium. Softer materials, such as high-speed steel, would fail immediately against hard metals.
How is hardness measured?
Hardness is one of the best documented material properties. Professionals often use hardness as a single measure of material quality. There are several tests and scales to measure and quantify hardness. We will briefly discuss the most important resistance tests and scales.
Most hardness tests follow a similar standardized procedure. An indenter of a specific shape and made of a specific material is pressed into the surface of the test material with a known force. The dimensions of the print are then noted. Finally, all these values are used to calculate hardness.
Among the tests, the most common are the Vickers, Brinell and Rockwell tests.
- Vickers Hardness Test: A pyramid-shaped diamond indenter is used in this test. Primary applications include thin sections and smaller parts. Therefore, light penetration forces are used.
- Brinell hardness test: The indenter is a steel ball with a diameter of 10 mm. The standard force is 29.42 kN.
- Rockwell Hardness Test: The Rockwell test is very versatile and uses different indenters, loads and scales. The load application is also a little different in that it applies a load in two stages. First a small load, then a large load.
As you can imagine, there are also numerous hardness scales due to the different hardness tests. Each test has specific applications. For example, the three tests in the previous section are mainly applicable to metals. The Shore hardness scale, on the other hand, is more popular for elastomers and other polymers.
Sometimes engineers need to convert measurements from one scale to another. Conversion tables are an effective method for this. An example of this can be found below. So it makes sense to have these conversion tables on hand.
Strength, stiffness and hardness properties of common materials
Material properties can vary significantly from material to material. Therefore, each material has its specific applications for which no other material is suitable. Now that we have a basic understanding of strength, stiffness, and hardness, it's good to see their different values across the spectrum of materials.
| Material type | material | tensile strength | stiffness | toughness |
| metal | Titanium (Ti-6AL-4V) | 880MPa | 113.8 GPa | 334 Brinell |
| Steel (AISI 4140) | 415MPa | 205 GPa | 197 Brinell | |
| Aluminum (Al 6061-T6) | 276 MPa | 68.9 GPa | 95 Brinell | |
| Bronze (CA 104) | 740MPa | 115 GPa | 210 Brinell | |
| Composite material | Carbon fiber | 3200MPa | 234 GPa | 50HRC |
| Organic | Wood | 120MPa | 20MPa | ~1400 (Janka scale) |
| Inorganic | Glass | 7 MPa | Fragile | 5.5-7 Mohs |
| Diamond | 2800 MPa | 1050 GPa | 10 Mohs | |
| Plastics/polymers | Natural rubber | >27.6MPa | – | 30-90 Coast A |
| Various grades of PVC (molded) | 30-50MPa | Up to 3.25 GPa | 50-99 Coast A | |
| Polyethylene (PET) | 2.69 – 60.7 MPa | 0.483 – 1.75 GPa | 55 – 69 Costa D | |
| Section | 29.6 – 48 MPa | 1.79 – 3.2 GPa | 100 Costa D |
Strength vs. Stiffness vs. Hardness: What are the differences?
In most cases, strength, stiffness and hardness are sufficient to determine the suitability of a material for a technical application. This does not mean that other material properties are somehow unnecessary, but they only become relevant in certain cases.
These three properties also have certain relationships with each other. For example, it is often proven that tensile strength and hardness are directly related. High hardness generally means high strength.
A similar argument applies to the relationship between strength and stiffness. Solid materials are generally more rigid. This is why we choose solid materials when we want small deformations, as was the case in the cutting tool deflection example above.
However, this is not a hard and fast rule either. Brittle materials like glass, for example, are rigid but not solid. This means that they do not deform as much under load, but rather quickly enter the plastic deformation zone and break.
It is best to evaluate these material properties separately. Typically, underlying relationships such as those discussed can be derived directly, but in doing so, engineers must be careful and always double-check the accuracy of their assumptions for specific material selection.
Technical design best practices
Technical design is not child's play. It requires skill and experience, as well as creativity, to design technical components. Considering material properties is an important part of this process.
In this section, we list some of the standard engineering practices relevant to material properties that professionals adapt to optimize the value of their projects.
Understand the properties of materials
We know this is obvious, but it's good to revisit engineering principles and theories in textbooks every now and then, as this can help solidify the fundamentals of what to do when there are project bottlenecks, which can be of great benefit.
Identify critical components and geometric features
Every project has certain parts or areas that are more prone to errors than others. These may be major structural components, such as the hinges on a door, or they may be special features, such as sharp corners with high stress concentrations. If designers identify them in advance, they can pay special attention to them.
Safety factor
It is common practice to consider a safety factor when designing parts. In other words, the part was designed to withstand loads greater than its nominal load. For example, a pulley may be designed to support a load of 1 ton, but is designed to fail at 1.5 tons. The safety factor in this case is 1.5. This is a precautionary measure to compensate for unexpected overloads.
Avoid sharp corners
Sharp corners are often concentrations of tension. Therefore, it is advisable to provide the corners with curves or chamfers. In this case, the choice of material is important. Hard, solid materials can withstand higher stresses and may not require large bends. Softer materials require an appropriate rounding scheme to avoid problems.
Take temperature into account
Temperature causes the properties of the material to change and thermal expansion/contraction occurs. Plastics, for example, become extremely soft at high temperatures. Therefore, high temperature applications are almost entirely dominated by metals and composites.
Consider dynamic loads
Dynamic loads are forces that change over time. Different materials have different resistance to such loading conditions. Some fail very quickly, while others can withstand millions of cycles of dynamic voltage fluctuations.
Concluding
In mechanical and structural engineering, the fundamental principles of material properties play a critical role in defining the performance, reliability, and safety of various products and structures. Strength, stiffness and hardness are the most important properties that must be considered when designing load-bearing components and selecting materials.
Common questions
What is more important: strength, rigidity or hardness?
While each of these features is important, the application determines which one the designer should focus on most. For example, strength is arguably more important in a vehicle chassis, as its main function is to support loads.
How do you find the material properties of a material?
Organizations such as ASTM have material property standards for different types and qualities of materials. Material properties can be found in their publications. Additionally, most material suppliers also provide a data sheet for their specific brand/batch of materials.
Does temperature change the properties of the material?
Material properties depend on temperature. In general, strength, stiffness and hardness decrease as temperature increases and vice versa.
