4.1: Generalities

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Figure 4.1: Normal stress. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

It is the task of the designer to consider all possible loads. The combination of materials and design of the structure must be such that it can support loads without failure. In order to estimate such loads one can take measurements during the flight, take measurements of a scale-model in a wind tunnel, do aerodynamic calculations, and/or perform test-flights with a prototype. Aircraft structures must be able to withstand all flight conditions and be able to operate under all payload conditions.

A force applied lengthwise to a piece of structure will cause normal stress, being either tension (also refereed to as traction) or compression stress. See Figure 4.1. With tensile loads, all that matters is the area which is under stress. With compressive loads, also the shape is important, since buckling may occur. Stress is defined as load per area, being \(\sigma = F/A\).

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Figure 4.2: Bending. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

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Figure 4.3: Torsion. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

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Figure 4.4: Shear stress due to bending. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

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Figure 4.5: Shear stress due to torsion. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

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Figure 4.6: Stresses in a plate. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

If a force is applied at right angles (say perpendicular to the lengthwise of a beam), it will apply shear stress and a bending moment. See Figure 4.2. If a force is offset from the line of a beam, it will also cause torsion. See Figure 4.3. Both bending and torsion causes shear stresses. Shear is a form of loading which tries to tear the material, causing the atoms or molecules to slide over one another. See Figure 4.4 and Figure 4.5. Overall, a prototypical structure suffers from both normal (\(\sigma\)) and shear (\(\tau\)) stresses. See Figure 4.6 in which an illustrative example of the stresses over a plate is shown.

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Figure 4.7: Normal deformation. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

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Figure 4.8: Tangential deformation. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

Structures subject to normal or shear stresses may also be deformed. See Figure 4.7 and Figure 4.8.

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Figure 4.9: Behavior of an isotropic material. Adapted from \(F_{\text{RANCHINI}}\) et al. [2].

Strain, \(\epsilon = \tfrac{\Delta l}{l_I} = \tfrac{l - l_I}{l_I}\) is the proportional deflection within a material as a result of an applied stress. It is impossible to be subjected to stress without experiencing strain. For elastic deformation, which is present below the elastic limit, Hooke’s law applies: \(\sigma = E \epsilon\), where \(E\) is refereed to as the modulus of Young, and it is a property of the material. The stresses within a structure must be kept below a defined permitted level, depending of the requirements of the structure (in general, stresses must no exceed the elastic limit, \(\sigma_y\)). See Figure 4.9.