Tensile, Yield and Shear Strength: Differences & Use

Tensile, yield and shear strength are the three numbers that describe how a material resists breaking — and using the wrong one in a calculation is how parts fail in the field. Tensile strength (UTS) is the maximum stress a material can withstand before it fractures; yield strength is the stress at which it permanently deforms (the elastic limit); shear strength is its resistance to forces acting parallel to the surface, like a bolt being cut across. Every structural design needs all three, in the right places.

The relationships matter: for most metals, yield strength is 40-90% of tensile strength, and shear strength is roughly 50-60% of tensile strength. For aluminum 6061-T6: tensile ~310 MPa, yield ~276 MPa, shear ~207 MPa. For steel (A36): tensile 400-550 MPa, yield 250 MPa, shear ~0.6×UTS. For plastics the picture changes completely: ABS has tensile strength around 40 MPa and no real yield point — it stretches and creeps instead. The units and the test standards (ASTM D638 for plastic tensile, ASTM E8 for metals) should be on every material data sheet you specify.

When to Design With Each Number

Yield strength governs most engineering design: a part that permanently deforms is failed, even if it does not break. Design to yield with a safety factor — typically 1.5-2× for static loads, more for fatigue — and check that the operating stress stays below the yield stress. Tensile strength matters where ultimate capacity is the question: proof tests, safety-critical components, and materials (like plastics) that fail without a clear yield. Shear strength governs fasteners, pins, rivets and welded joints: the classic bolt-shear calculation uses the shear area and the material's shear strength, and the failure mode (bolt shear vs thread stripping) decides whether the joint holds.

The plastic-specific warning: plastics do not behave like metals. Their "tensile strength" is measured at a specific strain rate and temperature, they creep under sustained load (a plastic bracket at 60% of its tensile strength will slowly stretch and fail over months), they weaken with UV and temperature, and their properties vary with molding quality — weld lines, voids and knit lines reduce local strength dramatically. The molded part is only as strong as its weakest internal feature, which is why the DFM review (gate placement, weld-line avoidance) is a strength issue, not a cosmetic one — our DFM checklist covers weld-line placement explicitly.

Reading the Data Sheet Correctly

The data sheet traps: strength values quoted for the dry, ideal state (nylon absorbs moisture and loses stiffness); values at 23°C that drop by 30-50% at 80°C; "tensile strength at break" vs "at yield" for the same material; and anisotropy in glass-filled grades (stronger along the flow direction). When you specify a material, note the condition: dry-as-molded (DAM) vs conditioned, the temperature, and the test standard. The honest supplier asks what your load case actually is — the materials section of our site maps grades to typical applications.

For reference, material property tables at full-service manufacturers list tensile and yield values per grade alongside recommended applications (firstmold.com).

If a part of yours is failing — deforming, cracking, or a fastener joint letting go — send us the part, the load case and the material. We will check the numbers against the design and tell you whether the fix is material, geometry or process. Contact us to start.

The strain-rate effect is the detail most data sheets hide: plastics and many metals are stronger under fast loading and weaker under sustained load. A nylon bracket that holds a 100 N load for a day may fail after a year of the same load — the creep failure that no static calculation predicts. The design rules that protect against it: keep sustained stress well below the yield strength (25-50% is the common guideline for plastics), avoid stress concentrations at holes and corners, and for load-bearing plastic parts prefer glass-filled grades whose creep resistance is dramatically better than unfilled. If your part holds a load over time, the creep question belongs on the design review before the material is specified.

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