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What is Fracture Toughness

Fracture toughness is defined as the energy spent in the creation of two surfaces at the crack tip that give rise to crack propagation. The ASTM E1820 describes the standard procedure to evaluate the fracture toughness of ductile engineering materials, as AHSS, by means of the traditional J-integral and Crack Tip Opening Displacement (CTOD) measurements.

Nonetheless, it is intended to plane strain fracture toughness characterisation and the defined specimen thickness requirements are not satisfied for thin sheets as AHSS thin sheets (1-3 mm). Alternative standards were developed later for the evaluation of the resistance to stable crack extension of thin-gauge materials, the ASTM E2472 and the ISO 22889.

However, those standards methodologies are complex and involve exhaustive specimen preparation, rigorous data treatment and the measurement of the crack advance during the tests, which is one of the main difficulties in fracture toughness measurement.

Optimised methodologies to evaluate fracture toughness

An alternative simpler and faster experimental approach to satisfy the growing need of knowing the fracture properties of thin metallic sheets

Non-standard tests, such as the Essential Work of Fracture (EWF) methodology, offer a simpler solution for measuring the fracture toughness of thin metal sheets. The EWF method has been extensively used to characterise the fracture resistance of sheet materials for engineering applications: polymers, aluminium alloys, steel, etc.

During the last years, Eurecat Technology Center has worked intensively on the application of this method to evaluate the fracture toughness of thin AHSS sheets and high strength aluminium alloys. The EWF methodology has shown to be suitable to readily measure the fracture toughness of high-strength sheet materials and the results can be used to understand their cracking behaviour during forming or in crash situations.

Novel procedure to evaluate fracture toughness

This novel method consists of testing up to fracture a series of notched specimens with different ligament lengths (l0, the uncracked area between the notches) and record the load-displacement curves. Then, the energy under the load-displacement curves (Wf) is calculated, divided by the initial cross-section area (l0·t0, where t0 is the sheet thickness) and plotted as a function of the ligament length. Making a linear regression of the data, the specific essential work of fracture, we is obtained in the intercept. This parameter is equivalent to the standard fracture toughness value Jc.

This approach will contribute to boost a wide usage of AHSS for high-added value sheet products (as safety related automotive parts) while reducing production costs and time-to-market for sheet products in automotive and other industrial end-use sectors.


Schematic representation of the experimental procedure for the EWF determination

No need for crack growth monitoring

The method is quite easy because there is no need for crack growth monitoring and data post-processing is rather simple.

No need of fatigue pre-cracked specimens

Eurecat has developed and patented a tool to avoid fatigue pre-cracking of specimens. The tool consists of a modular cutting die, equipped with a bevelled punch designed to introduce crack-like sharp notches in rectangular sheet specimens. The obtained geometry is a rectangular Double Edge Notched Tension (DENT) specimen.

Cost-efficient notching procedure

The new notching procedure permits obtaining a large number of specimens in a few minutes. Furthermore, the tool can be equipped in a universal testing machine and there is no need for special equipment. This supposes a great time-saving in specimen preparation and represents a fast and economic alternative to conventional fatigue pre-cracking procedures.

Method validated in industrial scenarios

Within the FormPlanet project, the results obtained with the new tool have been successfully validated in several AHSS, aluminium alloys and stainless-steel sheets. The process is robust and reliable,and permits to prepare ready-to-test specimens for fracture toughness evaluation.