High strength sheet materials, such as advanced high strength steels (AHSS) or high strength aluminium alloys, are fundamental for the development of new automotive lightweight concepts. Such materials are characterised by their unique mechanical properties combining high strength and excellent impact performance, which makes them especially suitable for structural and safety-related auto body parts.
However, they have also introduced new challenges to sheet metal stampers and automotive components manufacturers related to their cracking susceptibility. Because of their limited ductility, high strength sheet materials are more prone to the occurrence of cracks during cold forming (edge cracking) or under impact conditions (Figure 1).
This kind of fractures cannot be described by conventional fracture and ductility criteria based on the deformation limits defined by the Forming Limit Curves (FLCs) or the elongation values from uniaxial tensile tests. Therefore, aimed at improving material selection and optimising new material design, there is an increasing need for the identification of the material properties that best describe these crack-related failures.
Fracture toughness has shown to be a relevant material property to understand the cracking behaviour of high strength metal sheets. It has been applied to select the most appropriate material in sheet metal forming and to rank their fracture and crash performance. Thus, the evaluation of fracture toughness in high strength sheets has become relevant for steelmakers, part producers and carmakers to optimise material selection. However, the measurement of the fracture toughness of thin metal sheets is experimentally complex and there are no affordable standard methods. It hampers the use of this property in the automotive industry, and the knowledge of the fracture properties of high strength sheet materials is limited.
Figure 1: Examples of crack-related fractures in high strength steel sheets.
Fracture toughness characterisation of metal sheets
There are different standard testing procedures to characterise the fracture toughness of metallic materials within the frame of fracture mechanics, such as the ASTM E1820 or the ASTM E2472. However, these testing procedures involve exhaustive specimen preparation, crack growth monitoring and complex data post-processing. Furthermore, the size requirements described in the ASTM E1820 are not satisfied by the thin sheets commonly used in the automotive industry (thickness: 1-4 mm). These characteristics hamper the industrial implementation of fracture toughness measurements as routine testing for material screening.
Alternative 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 characterize the fracture resistance of sheet materials for engineering applications: polymers, aluminium alloys, steel, etc.
During the last years, Eurecat 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 these high strength sheet materials and the results can be used to understand their cracking behaviour during forming or in crash situations.
The main advantage of the EWF method is its relative simplicity compared to standard testing procedures. The experimental procedure is schematized in Figure 2. It 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.
The method is quite easy because there is no need for crack growth monitoring and data post-processing is rather simple. However, specimen preparation still being one of the main drawbacks since, as recommended by fracture mechanics standards, it is necessary the use of pre-cracked specimens. The preparation of these pre-cracks requires the application of expensive and time-consuming fatigue pre-cracking operations and the use of specialized equipment, which significantly increases the cost and time of the tests.
Figure 2: Schematic representation of the experimental procedure for the EWF determination
A new notching procedure for rapid fracture resistance characterisation. The way to extend the use of fracture toughness in materials characterisation.
In order to overcome the experimental difficulties in fracture toughness specimen preparation, Eurecat has developed and patented an innovative tool for sheet specimens notching. The tool consists of a modular cutting die, equipped with a bevelled punch (Figure 3 left) designed to introduce crack-like sharp notches in rectangular sheet specimens. The obtained geometry is a rectangular Double Edge Notched Tension (DENT) specimen (Figure 3 right).
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.
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.
This is a clear progress in sheet materials testing and will allow extending the use of the fracture toughness of metal sheets for accurate material selection and optimisation. Moreover, the new tool converts the fracture toughness test into an easy and fast method, as the property can be routinely measured and implemented in many materials laboratories.
More information about the new tool, the experimental procedure and its potential applications can be found here.
Figure 3: Left.Tool for introducing sharp notches in sheet metal specimens and detail of the bevelled punch. Right. Schematic representation of the notching procedure and detail of the sheared notch.
David Frómeta
PhD student in Materials Science and Engineering. He is a researcher at the Unit of Metallic and Ceramic Materials of Eurecat. His research interests are formability and fracture characterisation of sheet metal, with special focus on fracture mechanics testing of high strength metal sheets.
