The Journal of Strain Analysis for Engineering Design

This article presents a new experimental test for determining fracture toughness, in plane stress, in crack opening mode II based on the utilization of double-notched circular test specimens loaded in plane torsion. The proposed methodology for determining fracture toughness involves characterization of the evolution of torque with the degree of rotation for a number of test cases performed with specimens having different lengths of the ligaments between the notches. The work is supported by measurement of the in-plane and gauge length strains in aluminium AA1050-H111, and the overall experimental results show that the new proposed test provides an easy and effective way of evaluating the ability of a sheet metal to resist cracking under in-plane shear loading conditions.

Friction stir welding is a joining process developed in 1991 by The Welding Institute. This welding technique is a solid-state joining process leading to joints with good mechanical performance and low residual stresses. In all welding techniques, the clamping systems have an important role in determining the quality of the welds and mechanical characteristics. Even more in friction stir welding, the position of the clamps plays a critical role because it is mainly a mechanical process with high forces involved. In this article, the correlation between the residual stress field and configurations of clamps has been established numerically. For this purpose an uncoupled thermo-mechanical finite element analysis has been carried out. The mechanical loads due to the tool have been also implemented into the model. The thermal and mechanical models have been validated on temperature field recorded by an infrared camera and residual stress field measured by X-ray diffraction analysis. The friction stir welding test was conducted on 6-mm-thick 5754 H111 aluminium alloy plates.

A semi-analytic method for elastic/plastic shrink-fit analysis and design is developed. In contrast to many available semi-analytic methods, it is assumed that the outer disc obeys the von Mises yield criterion and its associated flow rule. The inner component of the assembly is purely elastic. The complete solution consists of three principal steps. First, the elastic/plastic solution in the outer disc is outlined. The only output of this solution required for the next step is the circumferential strain at the inner radius. It is shown that this strain can be found without having the strain distribution in the plastic region of the disc. This significantly simplifies the design of shrink fits. Moreover, only two parameters related to the outer disc (Poisson’s ratio and the dimensionless inner radius) are involved in numerical part of the elastic/plastic solution in the outer disc. Second, the found circumferential strain at the inner radius is used in conjunction with an analytic solution in the inner component of the assembly to match the two solutions. Any conventional design criterion can be adopted at this stage to determine optimal conditions. The complete solution involves several independent parameters. However, this does not cause any difficulty for design since these parameters are involved in analytic expressions. The final step is only necessary if the distribution of strains in the plastic region of the outer disc should be found.

By integrating the white light photoelasticity and spectrometry, a novel method called transmissivity extremities theory of photoelasticity to determine the state of stress has been recently developed. The key of transmissivity extremities theory of photoelasticity is to establish the systematic relationship of transmissivity with stress and wavelength and further derive the stress quantifying formula. Based on the high resolution and sensitivity of the spectrometer and the high measurement accuracy of the white light photoelasticity, transmissivity extremities theory of photoelasticity can determine the low-level stress even in low birefringence materials. Before extending the application of transmissivity extremities theory of photoelasticity to determine the higher level stress, the correlation between different transmissivity extremities linear equations in the systematic relationship and the relationship between parameters in the stress quantifying formula and thicknesses of specimens need to be derived and investigated. Therefore, in this article, a regression analysis of the constant term and first-order harmonic terms of the Fourier series function was employed to obtain the transmissivity extremities linear equations and parameters in the stress quantifying formula from the database. Both PSM-1 and glass were used to confirm the derived correlation and relationship. The effectiveness and merits of transmissivity extremities theory of photoelasticity were demonstrated by comparing simulation results obtained by transmissivity extremities theory of photoelasticity and the conventional white light photoelasticity, comparison between theoretical and experimental results of a diametrically loaded circular disk as well as the determination of the residual stress distribution of a thin glass substrate used for liquid crystal display.

A new version of the well-known Brazilian disk specimen weakened by a dumbbell-shaped slit with two key-shaped ends, called key-hole notched Brazilian disk specimen, made of polymethylmethacrylate, was utilized to measure experimentally the notch fracture toughness for key-hole notches of various radii and different lengths under pure mode I and pure mode II loading conditions. The experimental values of the mode II notch fracture toughness/mode I notch fracture toughness ratio were theoretically estimated by means of two brittle fracture criteria, namely, the maximum tangential stress and the mean stress criteria. Two groups of critical distances were used in the criteria. The first one was equal to that for sharp crack and the second one was directly determined from the mode I fracture tests. It was found that while both the criteria could estimate the experimental results satisfactorily, the agreement between the theoretical and the experimental results is excellent when one deals with larger notch lengths. Also found in the research was that choosing any of the critical distance groups did not affect the theoretical predictions significantly, meaning that the critical distances of sharp crack could be used in fracture prediction of key-hole notches.