A combined upper-bound and elastic-plastic finite element solution for a fastener hole subjected to internal torsion

Abstract

Fastener holes used in the mechanical joints are vulnerable to failure due to development of stress concentration at their edges. Inducing compressive residual stresses by different techniques has been the most common method to reinforce the holes to date. In this work, a new reinforcement technique called internal torsion, which can be classified as a localized severe plastic deformation process, is proposed as an alternative to the cold expansion pre-stressing. A special specimen is designed to represent the behavior of a typical fastener hole during the internal torsion process. The deformation of the specimen in the vicinity of its hole surface is studied by introducing a parametric kinematically admissible velocity field (PKAVF) within the deformation affected zone (DAZ). Calibration of the parameters in relation to the deformation of the material during the process is done by an elastic-plastic finite element solution that was performed in ABAQUS for a specimen made of interstitial free (IF) steel. Numerical analysis of the deformation is carried out to understand the process and to estimate the optimum process parameters. Subsequently, the calibrated model is used in an upper-bound solution of the problem to estimate the torque-twist response of the specimen during internal torsion. Finally, the results of upper-bound solution are compared with those of finite element analysis. There is a good agreement between the upper-bound solution and finite element results, which verifies validity of the calibrated velocity field model and the upper-bound solution based on the model for the internal torsion problem.

A New Horizon for Barreling Compression Test: Exponential Profile Modeling

“…As shown in Figure 8c and f, both strain and strain rate are heterogeneously distributed in the sample. In order to find effective stress distribution in the sample, one needs to account for the variations in a upper bound energy rate theorem (e.g., [18] ). As a result, the variations significantly complicate evaluation of effective stress and its distribution in the sample.…”

A comparative numerical study of combined cold expansion and local torsion on fastener holes

“…This is a complicated case of less application which has been dealt with in another investigation. 8 A discussion on the practical conditions responsible for either of the distributions is beyond the scope of this work.…”

“…These processes modify the properties of materials locally and result in significant improvement in the mechanical properties of the processed samples. The local torsion 8 can be considered as a localized SPD process. Analytical study of the local torsion dates back to a work that investigated the elastic-perfectly plastic and elastic-plastic response of an infinite disk with a circular hole loaded concurrently by pressure and twisting along the interior surface of the hole.…”

A new conception for enhancement of fatigue life of large number of fastener holes in aircraft structures

“…For many reasons, the SPD methods are practically inapplicable to fastener holes in aircraft structures. An idea for a modified torsion straining, called internal torsion, was demonstrated, as an alternative to the existing mandrel cold working methods. The same authors offer a combination of mandrel cold working and local torsion of fastener hole, considering an idealized case .…”