Identification of plasticity-controlled creep and fatigue failure mechanisms in transversely loaded unidirectional thermoplastic composites

Abstract

In continuous fiber-reinforced thermoplastics, the macroscopic failure mode in transverse long-term failure is dominated by a brittle crack-growth mechanism. Neat thermoplastic matrices, on the other hand, generally display also a plasticity-controlled mechanism in long-term loading at elevated stress levels and/or temperature. This failure mechanism requires a different approach to lifetime prediction than crack growth; hence, it is important to identify it in the long-term performance of composites. In this study, we demonstrate the presence of the plasticity-controlled failure mechanism in long-term failure of transversely loaded unidirectional (UD) thermoplastic composites made of glass/iPP, carbon/PEEK and carbon/PEKK. The main method used is to compare the lifetime in cyclic loading to that in static loading at the same level of maximum stress, where an increase in lifetime is characteristic for plasticity controlled failure, and, vice versa, a decrease is indicative for fatigue crack growth. In addition, the applicability of a lifetime prediction method common to plasticity-controlled failure of neat thermoplastics is evaluated for the composites investigated. The results of this study indicate that the plasticity-controlled failure was present in composites, although the extent to which the effects are present varied depending on the materials investigated. Glass/iPP showed the most explicit evidence of the plasticity-controlled failure over the entire load range experimentally covered. Its long-term failure was delayed with a decrease in the stress ratio and lifetime was predicted well using the principles of plasticity-controlled failure.

Evaluation of the Viscoplastic Strain of High-Density Polyethylene/Multiwall Carbon Nanotube Composites Using the Reaction Rate Relation

“…In the transition Region II-III, the deformation is still governed by plasticity-controlled mechanisms, although its rate is lower than in Region II. In the general case, Region II-III with a "close to zero" slope is related to the apparent transition from the secondary to tertiary creep [22,23]. The latter one appears in Region III, which ends with failure of the material.…”

“…The rate of creep strain as a function of strain in semi-logarithmic coordinates, known as the Sherby-Dorn diagram [22], is widely used to predict the creep lifetime of metals and polymer composites under plasticity-controlled failure conditions [23,24]. The minimums on Sherby-Dorn diagrams -"critical strain" -depends neither on the applied stress nor on the magnitude of creep.…”

Modelling of Environmental Ageing of Polymers and Polymer Composites—Durability Prediction Methods

“…According to Eyring’s thermal activation flow theory, the strain rate (or the characteristic time) is given by the relationship where A 1 (s − 1 ) is a material constant and is the coefficient linked to the activation volume; σ is the applied stress. Eyring’s activated flow theory is widely used to assess plasticity-controlled failure in thermoplastic polymers and composites [ 31 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 ]. A detailed discussion is given in Section 2.3 .…”

“…A methodology for prediction of long-term failure under the plasticity-controlled mechanism has been proposed by Erp et al for oriented polypropylene [ 70 ] and further validated in a series of studies of Govaert and coworkers for various engineering thermoplastic polymers [ 73 ] and their fibre reinforced composites [ 71 , 72 ] and nanocomposites [ 69 ]. The method is based on the “critical strain” concept and the Eyring thermal activation theory for viscoplastic flow.…”

Production of PP Composites Reinforced with Flax and Hemp Woven Mesh Fabrics via Compression Molding

“…Among polymer families, the use of thermoplastic matrices sparked an increasing interest due to their advantages over thermoset in terms of short processing time [1][2][3][4], potential recyclability and the possibility to be remodeled at high temperatures. Indeed, thermoplastic composites are used in many industrial fields, such as in automotive, naval and sports [5][6][7][8].…”