We provide new approaches for the evaluation of local damage (by micromechanics and microtomography), global and local ductility, delayed fracture, H embrittlement, and crack propagation assessment (in terms of fracture toughness) with the aim to optimise the development and production of advanced metal sheets.
NOVEL METAL CHARACTERISATION SERVICES
Sheet material development and production
- Edge cracking and stretch-flangeability
- Micro-mechanical characterisation
- Fracture toughness
- Crashworthiness
- Delayed fracture and H embrittlement
- Fatigue resistance
Edge cracking and stretch-flangeability
Optimised Hole Expansion Test (HET)
A non-standardised HET methodology has been developed through the investigation of the main parameters affecting the hole expansion ratio (HE) as clearance, tool wear, burr position and time from punch to expand. An experimental test configuration developed to objectively measure crack initiation and propagation and obtain reliable and repetitive values of HER.
Half Specimen Dome Test (HSDT)
The Half Specimen Dome Test (HSDT) permits to determine edge cracking behaviour during flanging in metal sheets. Compared to hole flanging tests, HSDT is a more time and cost-saving test for evaluating edge cracking. Another advantage is that is unaffected by aspects related to the tools manufacturing for cutting the holes, as the coaxiality between punch and die.
Local damage evaluation of shear edges
Micromechanical techniques as nanoindentation tests and Electron Back-Scatter Diffraction (EBSD) allows to assess the local damage and properties of sheared areas. The extension of local damage in sheared areas can be evaluated measuring the Young’s modulus variation by a methodology developed using the nanoindentation technique. EBSD permits to evaluate the damage by the quantification of phase transformations, grain distributions and grain size variations, recrystallised grains and texture due to the cutting and forming processes.
Micro-mechanical characterisation
Micro-tensile tests
Micro-Tensile Tests (MTT) permit to locally assess basic tensile properties as yield stress, ultimate tensile strength, total elongation, reduction of area and uniform elongation.
Furthermore, MTT can be used to study strain rate sensitivity in a wide range of temperatures.
These miniaturised testing techniques can be successfully applied when insufficient volume of material is available, for the development of new alloys, for the evaluation of the local mechanical properties of in-service components and as an input data in numerical simulation for advanced material modelling.
X-Ray microtomography
3D scanning via X-Ray microtomography and quantity analysis of sheet material internal defects (voids, cracks, inclusions), including position and defect size. This service can be complemented by compression or tensile testing to study how defects evolve with increased load.
Fracture toughness
Fracture toughness evaluation
Fracture toughness evaluation of thin metal sheets (1-3 mm of thickness), using mechanically notched or fatigue pre-cracked DENT specimens, and thick sheets (>3mm of thickness) by bending tests of fatigue pre-cracked SENB specimens, following fast and affordable methodologies based on the Essential Work of Fracture (EWF). The EWF is the most suitable fracture parameter to rationalise crack-related problems in high-strenght metal sheets, such as edge fracture or crash behaviour
Crashworthiness
Dynamic Component Test
The crash behaviour of components can be evaluated by two different tests: dynamic 3-point bending tests on omega profiles and dynamic compression tests on box beams.
Dynamic 3-point bending tests are performed on an improved setup of Drop Weight Tower with two high speed cameras (assisted by DIC analysis), application of strain gauges on the component and high-speed thermography. Uniaxial compression tests of crash boxes can be performed, applying 3D DIC for measuring deformations fields around wrinkles and potential cracks.
Intrinsic material crashworthiness
A small-scale laboratory test using the Essential Work of Fracture (EWF) methodology for the fracture toughness evaluation at high strain rates allows to successfully reproduce real crash conditions, where strain rate abruptly increases. The EWF tests are conducted in a high-speed tensile testing machine, combining Digital Image Correlation (DIC) and high-speed photography for the evaluation of the local deformation in the fracture zone.
Fatigue resistance
Fatigue resistance test
A novel fatigue test based on damage control allows to accurately determine the fatigue limit of materials in a fast, cheap and easy way, using only one specimen. The methodology consists in monitoring the compliance or displacement evolution during the fatigue test.
Delayed fracture and H embrittlement
H diffusion testing
Evaluation of the effective hydrogen diffusible coefficient by electrochemical technique, using HELIOS 2 PERMEATION equipment. Tests can be performed on bare sheets or coated sheet metal, and can also be performed at controlled temperature.
Slow Strain Rate Test (SSRT)
Uniaxial tensile slow strain rate tests on notched and/or unnotched specimens based on the guidelines contained in SEP 1970:2011 standard. Critical diffusible hydrogen concentration is determined in agreement to ASTM STP 962. The test detects the critical diffusible hydrogen concentration and tensile strength and/or the elongation at fracture against diffusible hydrogen content curve.
Four Point Bending Test (FPBT)
Incremental step loading four-point bending test on notched specimens to evaluate the critical diffusible hydrogen concentration determined in agreement with ASTM STP 962 and ASTM F1624-12, as well as the material’s flexural strength versus the diffusible hydrogen content curve.
Small Punch Test (SPT) of thin sheets
Novel small punch tests permit to characterise the hydrogen embrittlement effect on high-strength steels sheets in a rapid and simple way. The test method consists in a spherical punch pressed against a disc specimen along its axis under displacement control. Circular plates with and without H are extracted from thin metal sheets, according to CWA 15627 and EN 10371 standards, to obtain the material yielding, tensile strength and/or values on the elongationvv at fracture. This test allows to accurately detect the effect of small quantities of H in the embrittlement of metal sheets.