Structural analysis

Structural analysis (also called stress analysis or strength calculation) is the analytical proof that a component, assembly or load-bearing structure can carry its expected loads with an adequate margin. It checks that no permanent plastic deformation, fracture, loss of stability (buckling) or fatigue failure occurs, and that deflections stay within what the function allows.

The process starts with the load cases: operating and extreme forces, moments, pressures and temperatures and their combinations, including assembly, transport and foreseeable misuse. Stresses and deformations are then calculated and compared with allowable values. For ductile metals, the equivalent stress (usually von Mises, based on the distortion energy theory) is compared with the yield strength; for brittle materials, the maximum principal stress is compared with the ultimate strength. The result is a factor of safety for each criterion.

Depending on the complexity, engineers use an analytical calculation based on strength of materials, calculations to standards for machine elements and structures (for example bolted joints to VDI 2230, gears to ISO 6336, steel structures to Eurocode 3 or, in the US, AISC 360) or a numerical calculation using finite element analysis for complex shapes. In practice the approaches are combined: a hand calculation gives a quick estimate and a sanity check, while FEA shows the stress distribution in detail.

The output is a calculation report with the input assumptions, load cases, material properties, results and conclusion. For machinery, it is usually part of the technical documentation the manufacturer needs for the risk assessment and to demonstrate safety.

Structural analysis is a must for load-bearing and safety-relevant parts: lifting and handling equipment, machine frames, shafts, bolted and welded joints, pressure parts, mounts for heavy components and any part whose failure could injure someone. It also pays off when you want to cut weight or cost, because it shows where there is more material than needed.

Ideally it is done during design, before a prototype is built, and repeated after every significant change of shape, material or load. For mass-produced and safety-critical products, the calculation is usually backed by a test that confirms its assumptions.

Most errors come from the inputs, not the calculation. A missing load case (impact, drop, vibration, assembly preload, thermal expansion) or an underestimated load leads to a part that passes on paper and cracks in service. Check cyclically loaded parts for material fatigue as well, because a static calculation is not enough for them. Also watch notches, welds and holes, where the actual stress is much higher than the nominal stress.

Take material properties from standards or datasheets for the specific stock form and condition (heat treatment, thickness and, for 3D-printed parts, print orientation). When you outsource the analysis, provide a description of the function, the magnitude and nature of the loads, the required service life and, if applicable, the required factor of safety, and ask for a calculation report that makes every assumption clear.