Strength in Metals

Strength is a foundational consideration in material selection and mechanical design. Components need sufficient strength to support expected loads without permanent deformation or catastrophic failure, while also balancing factors like weight, cost, manufacturability, and fatigue life.

Strength is the ability of a metal to withstand an applied load without failure or plastic deformation.

It reflects the material’s capacity to resist forces that attempt to deform or fracture it and is typically measured in terms of stress, which is force per unit area. Stress is typically represented in megapascals (MPa) or pounds per square inch (psi).

What is Strength?

Strength is not a single mechanical property, but rather a general term that encompasses several specific types of resistance to different loading conditions, and is determined through mechanical testing such as tensile, compression, shear, or impact tests.

Factors that Affect Strength

In addition to understanding the specific types of strengths found in metals, it's important to note the other factors that influence strength and why one material might be better than the other for your project.

Composition

A metal’s strength is strongly influenced by its chemical composition. The elements present in a metal or alloy — such as carbon, chromium, nickel, and molybdenum — play a critical role in determining its mechanical behavior. For instance, increasing the carbon content in steel generally boosts its tensile strength and hardness, but this comes at the cost of reduced ductility and machinability. Alloying elements like nickel and chromium can enhance toughness and corrosion resistance, while trace impurities may introduce brittleness or weaken the material.

Phase Structure and Distribution

A metal phase structure and distribution also significantly affect its strength. Metals can contain multiple phases, such as ferrite, austenite, and martensite, each with distinct mechanical properties. Hard phases like martensite contribute to increased strength and hardness, while softer phases like ferrite enhance ductility. A uniform and well-controlled distribution of these phases ensures consistent performance across the material.

Heat Treatment

Heat treatment is another important factor. Techniques like annealing, quenching, tempering, and precipitation hardening can significantly change a metal’s strength profile. Annealing softens the metal and improves ductility, while quenching followed by tempering increases hardness and tensile strength. Precipitation hardening, commonly used in aluminum and titanium alloys, introduces fine particles that obstruct dislocation movement, thereby enhancing strength.

Grain Size

Grain size also plays a pivotal role in determining strength. Metals are composed of microscopic crystals or grains, and the size of these grains affects how the material responds to stress. Smaller grains create more grain boundaries, which impede the movement of dislocations — the primary carriers of plastic deformation. This phenomenon, known as the Hall-Petch relationship, means that finer grain structures typically result in higher strength. Conversely, larger grains may improve ductility but reduce overall strength.

Work Hardening

Work hardening is the process of strengthening a metal through plastic deformation. When a metal is cold-worked, its internal dislocation density increases, making further deformation more difficult. This results in higher yield and tensile strength, although it can also reduce ductility. Cold-rolled metals, for example, are often stronger than their annealed counterparts due to the effects of work hardening.

Residual Stress

Residual stresses, which are stresses locked into a material after manufacturing processes like welding, machining, or forming, can influence strength as well. These stresses may reduce fatigue strength and lead to premature failure if not properly managed. Techniques such as stress-relief annealing or shot peening are often used to mitigate these effects and improve the material’s durability.

Higher strength doesn’t always translate to better performance across the board; it often comes with trade-offs like reduced ductility, increased brittleness, or poor machinability.

If your component calls for a metal with high strength, consider all the types of strength and determine what is most appropriate and relevant based on the part’s end use, the product’s overall application, the loading environment, and failure risks.

Common Failure Modes in Strong Metals

Work hardening is the process of strengthening a metal through plastic deformation. When a metal is cold-worked, its internal dislocation density increases, making further deformation more difficult. This results in higher yield and tensile strength, although it can also reduce ductility. Cold-rolled metals, for example, are often stronger than their annealed counterparts due to the effects of work hardening.

Yielding

One of the most prevalent failure modes is yielding, which occurs when a metal is stressed beyond its yield strength. At this point, the material undergoes plastic deformation — meaning it won’t return to its original shape even after the load is removed. Yielding isn’t necessarily dangerous, but in precision applications, even small deformations can compromise performance or fit.

Fracture

Fracture is a more severe form of failure, and it typically happens when a metal reaches its ultimate tensile strength. The material stretches until it can no longer resist the applied force, and then it breaks. Fractures can be ductile — where the metal stretches significantly before breaking — or brittle, where the break occurs suddenly with little warning. Brittle fracture is especially dangerous because it often happens without visible signs of damage.

Fatigue

Fatigue failure is another critical concern, especially in components subjected to cyclic loading. Even if the stress is below the yield strength, repeated loading and unloading can initiate microscopic cracks that grow over time. Eventually, these cracks lead to sudden failure, often without any prior deformation. Fatigue is a silent killer in aerospace, automotive, and rotating machinery applications.

Impact

Impact failure occurs when a metal is struck with a sudden force, such as in a collision or drop. Even strong metals can fail under high-speed or high-energy impacts, especially if they are brittle or have internal flaws. Impact testing helps engineers evaluate a material’s toughness — its ability to absorb energy without fracturing.

Creep

Another subtle but dangerous failure mode is creep, which happens when metals are exposed to high temperatures and constant stress over long periods. The material slowly deforms, even if the stress is below the yield point. Creep is common in turbine blades, exhaust systems, and other high-temperature environments.

Stress Corrosion Cracking

Stress corrosion cracking (SCC) combines mechanical stress and a corrosive environment to cause unexpected failure. Metals that are strong in dry conditions may crack when exposed to moisture, chemicals, or salt. SCC is especially problematic in stainless steels and high-strength alloys used in marine or chemical processing applications.

Understanding these failure modes helps engineers select the right material, apply appropriate safety factors, and design parts that perform reliably under real-world conditions.

How Strength is Measured

Measuring the strength of metals involves standardized testing methods that quantify how a material responds to applied forces. These tests are typically performed in controlled lab environments using specialized equipment, and the results are used to generate stress-strain curves that reveal critical mechanical properties.

Tensile Testing

The most common method for measuring strength is the tensile test, which evaluates how a metal behaves under pulling forces. A sample — usually a machined dog-bone-shaped specimen — is placed in a universal testing machine (UTM) and stretched until it fractures. During the test, sensors record the applied load and the elongation of the specimen.

From this data, engineers extract key values:

• Yield Strength: The stress at which permanent deformation begins.
• Ultimate Tensile Strength (UTS): The maximum stress the material can withstand.
• Elongation: A measure of ductility, expressed as a percentage of original length.
• Modulus of Elasticity: The slope of the elastic portion of the stress-strain curve.
• Tensile testing follows standards such as ASTM E8 (for metals) and ISO 6892, ensuring consistency across labs and industries.

Compression Testing

Compression tests are similar to tensile tests but apply force in the opposite direction — squeezing the specimen. These tests are used for materials that are more likely to experience compressive loads, such as cast iron or ceramics. The results help determine compressive strength, modulus, and buckling behavior.

Shear Testing

Shear strength is measured by applying force parallel to the surface of the specimen, often using a punch or double-shear setup. This test is critical for evaluating fasteners, welds, and joints. Standards like ASTM B831 guide shear testing procedures.

Fatigue Testing

Fatigue strength is assessed by subjecting a specimen to cyclic loading — repeated application of stress over thousands or millions of cycles. The goal is to determine the endurance limit, or the maximum stress a material can withstand indefinitely without failure. Fatigue testing is time-intensive and requires precise control of load amplitude, frequency, and environment.

Impact Testing

Impact strength is measured using tests like the Charpy or Izod method, where a pendulum strikes a notched specimen to evaluate how much energy it absorbs before fracturing. These tests are especially useful for assessing toughness and brittleness, particularly in cold temperatures. Standards include ASTM E23 and ISO 148.

Hardness Testing

While not a direct measure of strength, hardness tests (like Rockwell, Brinell, and Vickers) provide useful insights into a metal’s resistance to deformation. Hardness often correlates with tensile strength, making it a quick and practical method for quality control.

Strength in Commonly Machined Metals

Strength is a key attribute to assess when evaluating materials for a CNC machining project. Stronger metals can withstand higher mechanical loads, resist deformation, and maintain structural integrity under stress — making them ideal for components that must perform reliably in demanding environments.

While selecting a stronger material typically enhances load-bearing capacity and durability, it often comes at the expense of machinability. For example, titanium alloys offer excellent strength-to-weight ratios but are notoriously difficult to machine due to their toughness and low thermal conductivity. Conversely, aluminum alloys are easier to machine but generally offer lower strength compared to steels and nickel-based alloys.

The table below ranks the most commonly used metals in CNC machining from lowest to highest tensile strength, with ranges accounting for the variety of alloys you see in a given metal (e.g., Aluminum 6061, Aluminum 7075, and Aluminum 2024).

In the instance where a traditionally low-strength metal won’t suit your project, there are options for making it stronger, especially if that particular material fits your application better. For example, solution-annealed stainless steel 17-4 PH has moderate strength, but after precipitation hardening, it can reach tensile strengths over 1100 MPa.

Mild steels, carbon steels, alloy steels, and aluminum alloys are commonly treated to increase strength. See the table below:

Understanding the strength profiles of these metals — and how they can be modified — helps engineers select materials that balance performance, cost, and machinability. This ensures that machined parts meet both functional and durability requirements, especially in industries where failure is not an option.

How Strength Affects CNC Machining Operations

A metal’s strength doesn’t just influence how a part performs in the field — it also has a direct impact on how that part is machined. Stronger metals typically offer better in-service performance, but they also introduce challenges during CNC machining that engineers must account for during the design phase.

Machinability vs. Strength Trade-Off

In general, the stronger the metal, the more difficult it is to machine. High-strength materials like titanium, Inconel, and hardened steels resist cutting forces, generate more heat, and cause greater tool wear. This means longer cycle times, higher tooling costs, and tighter process controls. On the other hand, lower-strength materials like aluminum and brass are easier to machine, allowing for faster production and lower costs — but they may not meet the mechanical demands of high-load applications.

Tooling and Cutting Parameters

High-strength metals require specialized tooling and optimized cutting parameters. Carbide or ceramic tools are often necessary to withstand the heat and stress of machining strong alloys. Feed rates, spindle speeds, and depth of cut must be carefully adjusted to avoid tool failure or part distortion. In some cases, high-pressure coolant systems or adaptive toolpaths are used to manage heat and chip evacuation.

Part Geometry and Tolerancing

When designing parts from high-strength metals, engineers must consider how the material’s resistance to deformation affects dimensional accuracy. Stronger metals can cause deflection in both the part and the cutting tool, especially in thin-walled or high-aspect-ratio features. This can lead to tolerance stack-up issues or surface finish problems. To mitigate this, engineers may need to:

• Increase wall thicknesses or add support features
• Loosen non-critical tolerances
• Specify secondary operations like grinding or honing for tight-tolerance features

Fixturing and Workholding

Strong metals often require more robust fixturing to prevent movement or vibration during machining. The cutting forces involved can be significant, especially in roughing operations. Poor workholding can lead to chatter, dimensional inaccuracies, or even part rejection. Engineers should collaborate with machinists early in the design process to ensure the part can be securely and efficiently fixtured.

Post-Machining Considerations

Because high-strength metals are often used in critical applications, post-machining processes like stress relief, heat treatment, or surface finishing may be required to restore or enhance mechanical properties. For example, machining a precipitation-hardened stainless steel may require a final aging treatment to achieve full strength. Designers should account for these steps in both the timeline and the dimensional allowances of the part.

Choosing the Right Metal for Strength Applications

Selecting a strong metal for a machined part begins with defining the component’s main functional requirements and mechanical stresses, such as static loads, impacts, or cyclic forces. Prioritize properties like yield or tensile strength based on the part’s intended use, and consider environmental demands—high temperatures, corrosion, or abrasion may call for specialized metals. For example, titanium alloys excel when both strength and corrosion resistance are needed, while precipitation-hardened stainless steels like 17-4 PH combine durability and corrosion resistance for demanding medical and aerospace applications.

Machinability is a critical factor in material selection. Stronger metals are generally harder to machine, leading to increased tooling costs, longer cycle times, and greater complexity. Engineers should assess if the chosen material can be efficiently machined with current equipment. In some cases, selecting a more machinable alloy and then enhancing its strength through heat treatments—such as quenching, tempering, or precipitation hardening—can be more practical. These processes can significantly improve the mechanical properties of materials like 4140 steel or aluminum 7075.

Cost and availability are key factors in material selection. High-performance alloys such as Inconel or titanium provide excellent strength but are costlier and more complex to machine. When budget or lead time is a concern, engineers often choose accessible options like high-strength aluminum or alloy steels, which offer a balance of performance and manufacturability. Early collaboration with manufacturing partners allows machinists to recommend material and design adjustments that improve manufacturability while maintaining required strength.

Ultimately, selecting the right strong metal is about aligning material capabilities with design intent, machining feasibility, and application demands. A thoughtful, strength-focused strategy ensures that machined components perform reliably, meet quality standards, and are produced efficiently — all while minimizing risk and maximizing value.