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INVESTIGATING THE INFLUENCE OF CHANGES IN MOLECULAR STRUCTURE OF POLYAMIDE 12 GRADES ON THE RESISTANCE AGAINST SLOW CRACK GROWTH

 

Mario Messiha, Britta Gerets, Jan Heimink, Andreas Frank, Florian Arbeiter, Gerald Pinter

# 2021 Amsterdam

A majority of field failure of viscoelastic plastic pipes can be attributed to slow crack growth (SCG) and has become crucial in terms of characterization of newly developed materials – particularly, if they are designed for pressurized pipe applications. In this study, investigations in terms of SCG resistances with the aid of two different accelerated test methods, namely the cyclic cracked round bar (CRB) test and the strain hardening (SH) test were completed using four different grades of unplasticized polyamide 12 (PA-U12) – three pure PA-U12 non-pipe grades of varying molecular weight (MW) and one compounded PA-U12 pipe grade. Highly correlating results were found with both methods when it comes to pure PA-U12 non-pipe grades. However, results show significantly higher failure times for the compounded pipe grade in the CRB test, while SH test results do not differ much from results of the non-pipe grades. This can be associated with a change in the nature of guiding failure mechanisms during crack propagation – an observable transition from crazing to shear yielding in the cyclic CRB test. A further key element in cyclic CRB test study was the formation of plastic zones prior to crack growth via crack freezing analysis. Results show relatively large plastic deformations as well as pronounced blunting effects at the vicinity of the crack tip for the PA-U12 compound compared to the other materials, which further indicates a higher capability of the pipe grade to store larger amounts of the crack driving energy by forming deformation zones.

Related keywords : polyamide 12, slow crack growth, cyclic cracked round bar test, strain hardening test, crack tip plasticity.
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