What is strength?
Strength is a material’s ability to withstand a load without failing or permanently deforming. It is measured as stress in megapascals (MPa) or pounds per square inch (psi).
What are the types of 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.
- Tensile strength is the maximum pulling stress a material can withstand before breaking, making it important for fasteners, beams, and cables.
- Yield strength is the stress at which permanent deformation begins, making it critical for precision and load-bearing parts.
- Compressive strength is a material’s ability to resist squeezing forces, making it important for columns, supports, and other parts under compression.
- Shear strength is a material’s ability to resist sliding forces, making it important for fasteners, pins, and joints.
- Fatigue strength is the maximum stress a material can withstand through repeated loading without failing, making it important for aerospace parts, rotating shafts, and springs.
- Impact strength is a material’s ability to absorb energy without fracturing under sudden loads, making it important for automotive crash components and protective gear.
What factors 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 composition affects its strength and other properties. More carbon can increase steel’s strength and hardness but reduce ductility and machinability, while alloying elements may improve toughness and corrosion resistance.
Phase Structure and Distribution
A metal’s phase structure affects its strength, hardness, and ductility. Hard phases like martensite increase strength, while softer phases like ferrite improve ductility; uniform phase distribution supports consistent performance.
Heat Treatment
Heat treatment changes a metal’s strength and ductility: annealing softens it, quenching and tempering increase hardness and tensile strength, and precipitation hardening strengthens aluminum and titanium alloys.
Grain Size
Smaller metal grains generally increase strength because their boundaries limit dislocation movement, while larger grains can improve ductility but reduce strength.
Work Hardening
Work hardening increases a metal’s yield and tensile strength through plastic deformation but reduces ductility. Cold-rolled metals are often stronger than annealed metals.
Residual Stress
Residual stresses from welding, machining, or forming can reduce fatigue strength and cause early failure. Stress-relief annealing and shot peening help improve 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.
How is strength measured?
Measuring a material's strength 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 tensile test measures strength by stretching a dog-bone-shaped sample in a universal testing machine until it breaks while recording the applied load and elongation.
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 testing measures how materials respond to squeezing forces, helping determine compressive strength, modulus, and buckling behavior.
Shear Testing
Shear testing measures a material’s resistance to sliding forces, helping evaluate fasteners, welds, and joints.
Fatigue Testing
Fatigue testing evaluates how a material withstands repeated loading over thousands or millions of cycles. It determines the endurance limit—the maximum stress the material can sustain without failing—and requires precise control of load amplitude, frequency, and test environment.
Impact Testing
Impact testing uses Charpy or Izod tests to measure how much energy a notched specimen absorbs before fracturing, helping assess toughness and brittleness, especially at low temperatures.
Hardness Testing
While not a direct measure of strength, hardness tests 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.
What are the failure modes associated with strength?
Strength-related failures occur when loading, geometry, material condition, or machining causes a part to exceed its strength or lose stiffness.
| Failure mode | What happens | Common CNC-related contributors |
|---|---|---|
| Yielding / plastic deformation | The part permanently bends, stretches, twists, or dents after stress exceeds yield strength | Undersized sections, thin walls, insufficient section modulus, wrong material or heat treatment, underestimated load |
| Ductile overload fracture | The part plastically deforms substantially and then tears when load exceeds ultimate strength | Tensile overload, bending overload, torsional overload, inadequate safety factor |
| Brittle fracture | Sudden cracking with little visible prior deformation | Low-toughness material, low temperature, sharp internal corners, high constraint, poor heat treatment, hydrogen exposure |
| Fatigue fracture | Small crack initiates and grows under repeated loading, then ends in sudden final fracture—often below yield strength | Threads, holes, keyways, sharp fillets, tool marks, burrs, chatter, rough surface, tensile residual stress |
| Buckling | A slender member suddenly bows, wrinkles, or collapses under compression before its compressive material strength is reached | Thin webs, long unsupported spans, thin walls, eccentric loading, poor fixturing assumptions |
| Shear failure | Material shears across a plane, such as at pins, tabs, fastener holes, or thin ligaments | Too-small cross section, inadequate edge distance around holes, high torsion, poor load path |
| Bearing / tear-out | Local crushing around a fastener hole or tearing from a hole to an edge | Thin material, insufficient hole-to-edge distance, oversized fastener loads, weak local geometry |
| Torsional failure | Shaft or feature twists excessively or fractures along a shear-dominated path | Small shaft diameter, stress risers at keyways or cross holes, abrupt diameter transitions |
| Contact fatigue / surface crushing | Repeated concentrated contact causes pitting, indentation, spalling, or cracking | Poor bearing-seat finish, incorrect fit, high Hertzian contact stress, inadequate surface hardness |
| Creep or stress relaxation | Permanent deformation accumulates over time at elevated temperature, or preload falls | Aluminum, polymers, and some alloys used near temperature limits; clamped features and threaded joints are susceptible |
| Corrosion-assisted cracking | Corrosion reduces section thickness or helps cracks initiate and grow | Incompatible material/environment, damaged coating, chloride exposure, tensile residual stress; can combine with fatigue |
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 does strength affect CNC machining?
A metal’s strength affects both part performance and machinability. Stronger metals perform well in service but require more CNC machining planning.
Machinability vs. Strength Trade-Off
Stronger metals require more heat and tool control, increasing cycle times and costs. Aluminum and brass machine faster and cheaper but may not suit high-load applications.
Tooling and Cutting Parameters
High-strength metals need specialized tools and cutting parameters. Carbide or ceramic tooling, optimized feeds and speeds, and advanced cooling or toolpaths help prevent tool failure, part distortion, and chip-removal issues.
Part Geometry and Tolerancing
High-strength metals can deflect the part and cutting tool, especially in thin-walled features, causing dimensional and surface-finish issues. To reduce these risks, 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 need robust fixturing to prevent vibration, chatter, and dimensional errors. Early collaboration with machinists helps ensure secure, efficient workholding.
Post-Machining Considerations
High-strength metals may require post-machining treatments, such as stress relief or aging, to restore their properties. Account for these steps in the schedule and dimensional allowances.
How to choose the right material for strength applications?
Choose a strong metal based on the part’s loads, required yield or tensile strength, and environment. Titanium offers strength and corrosion resistance, while 17-4 PH stainless steel provides durability for demanding medical and aerospace parts.
Stronger metals are harder and more costly to machine, so choose a material compatible with available equipment. A more machinable alloy can often be strengthened through heat treatment, such as quenching, tempering, or precipitation hardening.
Balance cost, availability, and machinability when selecting a strong metal. High-strength aluminum and alloy steels can offer practical alternatives to costly Inconel or titanium, while early manufacturer collaboration can improve manufacturability without sacrificing strength.
The right strong metal balances material performance, design intent, machining needs, and application demands. This helps produce reliable, high-quality parts efficiently while reducing risk and maximizing value.
