Internal axial force
Cut the bar conceptually and balance forces. Internal force N stays constant between applied loads when distributed axial loading is absent. For a singly restrained bar, equilibrium determines the reaction.
Check yourself: Does your segment satisfy axial equilibrium?
Normal stress and strain
Away from end disturbances, uniform axial stress is σ = N/A using original area. Engineering strain ε = δ/L uses original length. Stress has force-per-area units; strain is dimensionless and signed.
Check yourself: Have you distinguished total extension from extension per unit length?
Elastic material law
Uniaxial linear elasticity gives σ = Eε within the elastic range. Homogeneity makes material properties spatially constant. This simple uniaxial model requires traction-free lateral surfaces and excludes local end disturbances.
Check yourself: Is the material response assumed linear, and are lateral constraints absent?
Axial displacement and compatibility
For a uniform segment, δ = NL/(AE). Add signed extensions for segments in series: interface displacements match, but strains need not. A second axial restraint introduces another displacement constraint, not a freely extending endpoint.
Check yourself: Which displacement is fixed, and which endpoint may move?
Temperature and model limits
Temperature changes add free thermal strain αΔT; restraint may create stress. Here uniform unchanged temperature eliminates thermal strain. Centric loading excludes bending; neglected end effects exclude local stress concentrations from these calculations.
Check yourself: Are thermal strain or eccentric loading being silently omitted?