Quantification and analysis of coating surface strains in T-bend tests

Abstract

Pre-painted sheet metal (e.g. coil coated with polyester-melamine) undergoes large deformations when formed into architectural cladding or white goods. The coatings provide protection and superior aesthetics, so must withstand failure by cracking or delamination during forming. The T-bend test is an industry standard test used to qualitatively compare the formability of coatings and mimics the conditions experienced during hemming processes. The failure of coatings during forming is strain governed, so understanding the surface strains in the T-bend test is of great interest to manufacturers. For the first time, the maximum surface strains experienced during the T-bend test have been predicted using finite element modelling (FEM) and verified experimentally using digital image correlation. The experimental shapes of the deformed blank are compared with the FEM results for further verification. In addition, a novel analytical model is proposed to determine the maximum surface strains. It is shown that strains of up to ~ 225% are applied during a 0T test (bent around a zero thickness spacer) reducing to ~ 23% at 4T (bent around a four times sheet thickness spacer). The finite element model, experimental data and new analytical model show excellent agreement and indicate that behaviour is independent of the substrate thickness or material used. Understanding the strain behaviour quantifies the formerly qualitative T-bend. This will improve the efficacy of the test, allowing metal formers and coating developers to better understand the performance requirements, to reduce waste and to develop better coatings. Graphical abstract

Study on Press Formability and Properties of UV-Curable Polyurethane Acrylate Coatings with Different Reactive Diluents

“…The t-bending test has been accustomed to evaluate the formability of distinct coating systems in a cooperative way. The T-bend test results are registered in the form ‘xT’ where the x denotes the level of T-bend which a coating can sustain without failure by through-thickness cracking of the coating or loss of adhesion [ 24 , 25 ]. A 0T T-bend relates to a spacer thickness of 0× the sheet metal thickness, a 1T T-bend relates to a spacer thickness of 1× the thickness of the sheet metal, and so on.…”

Processing and Weathering of Sol-Gel Clearcoats for Coil-Coated Steel

“…The coatings are deposited and cured before forming the steel. Therefore, the coatings must have resistance to failure by cracking and delamination during forming to maintain the protection of the steel [42]. The T-bend test is used to assess coating formability and adhesion.…”

“…The T-bend test is used to assess coating formability and adhesion. The highest rating, '0T', corresponds to a 180 • bend of the steel, resulting in the highest potential surface strain [42]. All the coatings had a T-bend rating of '0T', indicating excellent formability and adhesion.…”

Investigation on thickness size effect on ductility of magnetron sputtered Niobium coatings on SS316L substrate for forming of precoated metallic bipolar plates

“…However, considering complex equipment and low deposition rate of existing coating processes, preparing coating for metallic BPPs requires both high economic cost and intolerable time consumption, which is a critical issue to be addressed [11]. Recently, precoating process has been proposed as a promising way to improve the manufacturing efficiency and lower the manufacturing cost of BPPs [12]. The precoated metallic foils prepared with large roll to roll foil coating line could be directly stamped into the expected profiles, which offers IOP Publishing doi:10.1088/1757-899X/1270/1/012111 2 several advantages such as improved product quality, reduced production cost, shorter and automated processing cycle, and reduction of production scrap [13].…”