The 10 Mechanical Properties of Materials: A CNC Shop Floor Guide

• 1
  Key takeaways
• 2
  Strength, and why one number is never enough
• 3
  Elasticity and plasticity: the two sides of deformation
• 4
  Hardness and toughness: the trade you make on every alloy
• 5
  Creep, fatigue limit and wear resistance
• 6
  How to pick the right property for your part
• 7
  Which property decides which design question

Tensile strength is the peak stress a coupon carries before it breaks. On a datasheet it appears as ultimate tensile strength, and for 6061-T6 aluminum it sits around 310 MPa. That number tells you when the material fails outright. It does not tell you when the part starts to bend permanently, which is usually the design limit.

Yield strength is that second number. It marks the point where deformation stops being elastic. For the same 6061-T6 the yield strength is roughly 276 MPa, so the useful working stress is far below the ultimate value. Engineers sizing a bracket against a 200 N load care about yield, not ultimate. A part that has yielded is scrap even if it never cracked.

Fatigue strength is the third member of the family and the one most often ignored. A cyclic load at 30% of yield can still fail a part after a million cycles. Aluminum has no true endurance limit, so every aluminum bracket on a vibrating machine will eventually crack unless you keep stress low and eliminate sharp internal corners.

Compressive strength rounds out the group. It matters for spacers, bushings and anything loaded in pure compression. For most metals compressive yield tracks tensile yield closely, so a single value often serves both. For plastics and cast irons the two diverge and you must read the correct curve.

• 1
  Use yield for static sizingApply a safety factor of 1.5 to 2.0 against yield, not against ultimate.
• 2
  Use fatigue data for anything that movesIf the part sees more than 10,000 cycles, get an S-N curve.
• 3
  Watch sharp cornersA 0.5 mm internal radius can cut fatigue life by half versus a 2 mm radius.

Elasticity is the material's ability to return to shape after the load comes off. Young's modulus quantifies it. Steel sits near 200 GPa, aluminum near 69 GPa, titanium near 116 GPa. These values barely change with heat treatment, which surprises people. You cannot heat treat 6061 into a stiffer part. You can only change its strength.

That gap between steel and aluminum is why a steel shaft and an aluminum shaft of identical geometry deflect differently. If deflection drives your design, switching from steel to aluminum means redesigning the section, not just swapping the material. This is a frequent mistake in weight-reduction projects.

Plasticity is the opposite behavior: permanent deformation without fracture. It is what lets a press brake bend a 3 mm steel plate into a 90° flange without cracking. Ductility measures how far that goes. A material with 25% elongation in 50 mm can take tight bend radii; one with 4% elongation will crack if you push it.

For machining, plasticity is a mixed blessing. Ductile materials like 1018 steel and 304 stainless form long chips, generate heat and work-harden at the cut. Brittle materials like cast iron and 440C stainless break chips cleanly but can chip at the tool edge. Both behaviors drive your choice of insert geometry and coolant strategy.

• 1
  Modulus does not heat treatStiffness is set by alloy family, not temper.
• 2
  Ductility sets minimum bend radiusRule of thumb: minimum radius equals one material thickness at 20% elongation.
• 3
  Work hardening bites twiceIn 304 stainless, a dull tool raises hardness at the cut and accelerates wear.

Hardness measures resistance to local indentation. It shows up as Brinell (HB), Rockwell (HRC) or Vickers (HV). For machining, hardness is the single best predictor of tool life. Below 30 HRC you can run coated carbide at aggressive feeds. Between 30 and 45 HRC you slow down and expect shorter insert life. Above 45 HRC you move to ceramic or CBN tooling and often to grinding for the final dimension.

Hardness also correlates loosely with strength. A rough conversion puts 30 HRC near 950 MPa tensile in steel. That is useful when a drawing calls out hardness but no strength value, which happens often on legacy aerospace and automotive prints.

Toughness is the energy a material absorbs before fracture. It combines strength and ductility, so a hard, brittle material can have low toughness even with high strength. Impact toughness is measured with a Charpy or Izod test and reported in joules. A 4140 steel at 28 HRC has good toughness; the same steel at 50 HRC is strong but will shatter under a sudden load.

The engineering consequence is simple. Dynamic loads want toughness. Wear surfaces want hardness. A single part often needs both, which is why you see case-hardened gears: a tough core with a hard skin. When you specify a material, ask which failure mode dominates and pick the property that resists it.

• 1
  Under 30 HRCStandard coated carbide, high speeds, long tool life.
• 2
  30–45 HRCReduced speed, rigid setup, expect insert changes.
• 3
  Above 45 HRCCeramic or CBN inserts, or grind to final size.

Creep is slow deformation under a constant load, and it only becomes visible at elevated temperature. Aluminum starts to creep above roughly 150 °C. Steel holds until 400 °C or higher depending on alloy. If your part sits in an engine bay or near a heater, creep can drift a critical dimension over months. No amount of room-temperature testing will reveal it.

Fatigue limit is the stress below which a steel part survives an effectively infinite number of cycles. Carbon and alloy steels have a real limit, often around 40–50% of tensile strength. Aluminum and most composites do not. This is why steel springs last indefinitely and aluminum ones do not. For aluminum, design to a finite life and state the cycle count on the drawing.

Wear resistance covers abrasion, adhesion and erosion. It is not a single lab value; it depends on the pair of surfaces in contact. Hardness helps, but so does surface finish and lubrication. A hard-chromed or nitrided shaft resists adhesive wear far better than a bare one of the same base hardness.

Together these three properties decide the maintenance interval of a machine. A part that creeps loses alignment. A part that fatigues cracks without warning. A part that wears loses clearance gradually. Each failure mode asks for a different fix, and reading the right property is how you choose it.

• 1
  Creep needs temperatureSpecify creep data only if service temperature exceeds 40% of melting point in Kelvin.
• 2
  Fatigue limit is a steel advantageAluminum has none, so always state a design cycle count.
• 3
  Wear is a system propertyHardness alone does not predict it; surface finish and lubrication matter equally.