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Frequency reduction in elastic beams due to a stable crack: numerical results compared with measured test data

Andreaus U., Casini P., Vestroni F.

Engineering Transactions

2003

Vol. 51, iss.1

81-101

The presence of a crack could not only cause a local variation in the stiffness, but it could affect the mechanical behaviour of the entire structure to a considerable extent. The frequencies of natural vibrations, amplitudes of forced vibrations and areas of dynamic stability change due to the existence of such cracks. The vibration characteristics of cracked structures can be useful for non-destructive testing. In particular, the natural frequencies and mode shapes of cracked beams can provide insight into the extent of damage. The beam has been schematized as a 2-D continuous medium and discretized by means of quadrilateral finite elements. The lowest three natural frequencies (and the associated mode shapes) of the cracked cantilever beam, were obtained via both the modal and spectral analyses, and were compared with experimental data from literature in order to assess the reliability of different models of crack state, namely open crack and contact crack. Both the experimental and numerical results reveal the significant influence of the opening and closing conditions of the crack on the frequency reduction; namely this reduction decreases as more realistic contact phenomena are considered at crack interfaces.

Rocking-sliding of a rigid block: friction influence on free motion

Andreaus U., Casini P.

Engineering Transactions

1998

Vol. 46, iss. 2

143-164

Many technical problems involve contact and impact between bodies. The contact problem is locally characterized by normal and shear forces; this contact may represent different types of phenomena such as rolling, sliding, impact and shock, unilateral contact, etc. The aim of this paper is to study the 'contact-impact' problem of a rigid block colliding on a frictional base, by means of a numerical simulation, and to compare numerical results with analytical responses known from the literature. Frictional contact is taken into account by means of shear forces concentrated at a discrete number of points. Attention has been paid to two-dimensional free motion of the block with three degrees of freedom. Wide ranges of block slenderness and friction angle have been investigated and numerical results have been synthesized by means of the energy reduction ratio, which accounts for contact phenomena occuring during impact.