Estimation of wall strength of the aortic aneurysms is necessary for the prediction of their rupture risk. We previously found a significant correlation between their tensile strength σ
MAX and a yielding parameter τ
σ, which is the stress when tangent elastic modulus reaches at 63% of the plateau level. This may indicate that the wall strength is estimated from their pressure-diameter relationship. Here we show a possible mechanism of the correlation between τ
σ and σ
MAX by focusing on alignment of collagen fibers. Thin (150-µm) slices of porcine thoracic aortas were uniaxially stretched in circumferential direction until failure under a microscope, and a retardance, a phase shift when polarized light passes through a birefringent material, was measured as the degree of collagen fiber alignment. Strength σ
MAX correlated significantly with τ
σ as obtained previously. The retardance increased with the increase in the stress and reached a plateau at the stress σ
Ret-plateau, indicating that σ
Ret-plateau is the stress at which most of the intramural collagen fibers have aligned. The stress σ
Ret-plateau correlated significantly with τ
σ and both parameters has similar values. This may indicate that the aortic wall yields when all of collagen fibers become straight. Smaller σ
Ret-plateau means that most collagen fibers are stretched and loaded at smaller stress, resulting in failure at smaller stress. This seems to be a reason for the significant correlation between τ
σ and σ
MAX.
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