The Problem with Cracks

The past few decades have seen outstanding advances in the use of composite materials in structural applications. When it comes to the structural integrity, a single material by itself isn’t strong enough, when there are flaws or cracks. But when you combine two different materials, that’s when things get interesting.

Bonded bimaterials are materials used in engineering, and are a combination of two different materials. They find a wide range of applications such as in pressure vessels, ceramic coatings, solid rocket propellants, electronic encapsulations, etc.

However, cracks tend to appear at the bonding interface due to fabrication flaws, residual stresses, and cyclic fatigue loading.

To improve the structural integrity and safety, it is important to study the stress distributions in the vicinity of these cracks. Stress intensity factor (SIF) at the crack tip is the governing factor which dictates the start of a crack propagation. It is important to understand the variation of SIF for various bimaterial combinations under a variety of different loading conditions.

There have been several studies on crack lying along the bimaterial surface. However, there is limited literature on experimental studies on crack terminating at the bimaterial interface.

In this study, the authors Mr. A. Vivekanandan and Prof. K. Ramesh from the Department of Applied Mechanics, Indian Institute of Technology Madras, Chennai, India, have studied a crack terminating perpendicular to the interface and oriented at different angles to the applied load. SIF variation under different mode mixities was evaluated experimentally using digital photoelasticity.

A crack can fail under three different modes, namely, the opening mode, shearing mode and tearing mode, which are referred to modes 1, 2 and 3 respectively. A crack is in general subjected to varying mode mixities, that is, the combination of these three modes when a crack is located at different orientations with reference to the point of loading. Photoelastic method is a proven method to visualize and crack tip stress field to evaluate the crack tip SIF.

In this study, a novel bimaterial Brazilian disc specimen was chosen. This is one of the widely used compression testing specimens. Several studies have been done using cracked Brazilian disc specimen made up of different materials. 

Two different bimaterial pairs were chosen, namely, a metal-polymer pair (Aluminum-Epoxy), and a polymer-polymer pair (PMMA (polymethyl methacrylate) -Epoxy).  

The complex crack tip stress field equation for a crack terminating to the bimaterial interface was fed into an in house developed PSIF software. This software, along with another in house software called DigiTFP® are developed at the Digital Photomechanics Lab at IIT Madras. These software packages are used to process the images taken at the crack tip and evaluate crack tip SIF with sophisticated image processing and mathematical algorithms.

A variety of mode mixities was applied on the specimens. The interfacial SIF values were determined. It was found that the interfacial SIF values for the metal-polymer pair were found to be much higher than that of the polymer-polymer pair. The peak values of interfacial SIFs was observed at a crack orientation angle of 30 degrees. At angles close to 0 degrees and 90 degrees, the SIF values were less pronounced.

Article by Akshay Anantharaman
Click here for the original link to the paper


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