Measuring tenacity tests - Mechanical fracture

Overview

ABSTRACT

Shock testing cannot predict the fracture of cracked parts. Yet, this is a major concern in the industry. To respond to this, new tests, including fracture toughness tests have been developed. Indeed, fracture mechanics establishes a quantitative relationship between the load that a part is subject to, and the dimensions of a crack and the material property called tenacity. The tests for determining the tenacity include those based on fracture mechanics in linear elasticity, those based on fracture mechanics in elasto-plasticity and those based on the local approach.

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AUTHOR

Dominique FRANÇOIS : Honorary Professor at École Centrale Paris

INTRODUCTION

As we pointed out in the introduction to [M 4 165] ("Rupture testing. Impact testing"), ruptures in service are extremely costly. We cited a few examples of disasters caused by ruptures.

Numerous tests have therefore been proposed to optimize the resistance of materials to the risk of fracture. Since sudden stresses are particularly dangerous, impact tests play a very important role. They are covered in the [M 4 165] dossier. However, they cannot be used to quantitatively predict the fracture of parts containing a crack of a given size. This is a major concern, particularly in the aerospace and nuclear industries. The growth of these industries has been accompanied by theoretical developments and the development of new tests, which are described in this dossier.

If we need to know precisely the loads that parts containing defects, such as fatigue cracks, can withstand, we need to turn to fracture mechanics [1] [2] [3] [4] [5] [6]. Fracture mechanics and the tests derived from it enable us to calculate the size of critical defects under a given loading, or the critical load leading to failure for a defect of assumed or measured dimensions. Fracture mechanics has been widely developed over the last fifty years, particularly in the nuclear, aeronautical, space and petrochemical industries. It is now widely applied in other fields. Fracture mechanics establishes a quantitative relationship between the load to which a part is subjected, the dimensions of a crack and a material property called toughness (figure 1 ).