Willie: I do not have a good explanlation for these findings. The only thing I could think of is that hickory maybe has more strectch than boo and thus does not "stress" the belly lamiante like bamboo does. However this idea does not fit into virtual-bow-logic as there is no stretch parameter. My "stretch" must somehow be connected to MOE and here the differences betwween boo and hickory are not so big....yes boo is a little stiffer.
Other - maybe related - question: Why do we have different stress on back and belly if we bend a piece of wood?
I think the main reason is that different wood and grass species have different rheological properties, and therefore their viscoelastic behavior differs. For example, hickory is well known to perform excellently at very low humidity but is generally sluggish at high humidity. Its intrinsic properties vary with conditions (both external and internal). Additionally, viscoelastic material properties are time-dependent. A well-known example of this is that with a fast release we obtain a higher arrow speed than with a long anchor at full draw. Thus, the main reason lies in the intrinsic material properties, which we do not know well enough and which are difficult to model accurately. In practice, we must rely on measured data. For example, the measured difference between hickory and bamboo backing is both interesting and important.
The second question "Why do we have different stress on back and belly if we bend a piece of wood?". In theory, a homogeneous, symmetric bending beam should have identical absolute values of tensile and compressive stresses within the elastic range. However, in a bending bow there is also an axial force (a force in the direction of the bowstring), which affects the stress distribution by introducing additional compression force. As a result, the compressive strain is higher than the tensile strain, and this effect is also modeled in VirtualBow. In reality, there are additional material and geometrical factors that influence the stress distribution, each contributing its own effect, but which are difficult to model.
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