Biaxial characterization of poly(ether‐ether‐ketone) for thermoforming: A comparison between bulge and in‐plane biaxial testing

Abstract

The biaxial response of extruded PEEK films at conditions relevant to thermoforming has been investigated extensively using a combination of load-controlled (bulge test) and displacement-controlled (biaxial stretcher) experiments. Results from bulge testing yielded average and maximum strain rate ranges of 2.5-5 and 5-18 s −1 , respectively, across the forming temperature range. In-plane biaxial characterization highlighted the anisotropic nonlinear viscoelastic behavior of the films with strong dependence on the yield and strain hardening behavior on the temperature and strain history at conditions equivalent to the forming process. The combined approach to material characterization highlights the pros and cons of each test method, the complementary nature of the data generated, and the need to use both methods to have a complete data set for developing accurate material models and validated numerical simulations of thermoforming. FIG. 2. (a) Annotated photo showing PEEK film mounted in the QBS and (b) EQ deformation of PEEK specimens (i) unstretched and (ii) biaxially stretched to a ratio of 2:2. [Color figure can be viewed at wileyonlinelibrary.com]

Modelling the Temperature Dependent Biaxial Response of Poly(ether-ether-ketone) Above and Below the Glass Transition for Thermoforming Applications

“…The authors [23] found strong correlation between the experimentally observed final part shapes and those predicted in simulations. By observing the biaxial stress–strain data recorded in a previous study [13] parallels can be drawn between the biaxial deformation behaviour observed for aPET and PEEK, thus warranting the evaluation of this material model to capture the nonlinear, viscoelastic behaviour of PEEK.…”

“…Mechanical characterisation of PEEK has been primarily focussed on uniaxial stretching [8,9,10,11,12], with authors detailing the strong temperature and deformation rate dependence of specimens. A lack of research concerning the deformation in more than one axis is evident, and has only been investigated recently by the authors [13]. In our previous study, both load-controlled and displacement-controlled testing of thin extruded PEEK specimens were conducted at conditions comparable to the thermoforming process, drawing similar parallels to the temperature and strain rate dependencies found during uniaxial tests as well as noting profound anisotropy when simultaneously stretching in two axes [13].…”

“…Extensive biaxial testing of PEEK samples has been conducted and presented in previous work [13], and hence a brief summary is presented here. Following Dynamic Mechanical Analysis of the PEEK films, of number average molecular weight equal to 45,000 g/mol, a formable temperature window spanning above and below the T g of 150 °C was revealed (130–155 °C).…”

“…Following Dynamic Mechanical Analysis of the PEEK films, of number average molecular weight equal to 45,000 g/mol, a formable temperature window spanning above and below the T g of 150 °C was revealed (130–155 °C). Upon realization of this forming window, the 12 μm thick extruded PEEK films were biaxially stretched in the machine (MD) and transverse directions (TD) simultaneously at equal strain rates and displacements, detailed in previous work [13] at the conditions shown in Table 1. Samples were stretched using the purpose-built Queen’s Biaxial Stretcher (QBS)—specializing in the replication of temperature, deformation speeds and modes of deformation equivalent to those experienced during thermoforming and stretch blow moulding processes.…”

“…A lack of research concerning the deformation in more than one axis is evident, and has only been investigated recently by the authors [13]. In our previous study, both load-controlled and displacement-controlled testing of thin extruded PEEK specimens were conducted at conditions comparable to the thermoforming process, drawing similar parallels to the temperature and strain rate dependencies found during uniaxial tests as well as noting profound anisotropy when simultaneously stretching in two axes [13]. It is the data from this investigation which serves as the basis for modelling the constitutive biaxial deformation behaviour of PEEK, for the application of thermoforming.…”