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Time-dependent failure of amorphous polylactides in static loading conditions.

Engels TA, Söntjens SH, Smit TH, Govaert LE - J Mater Sci Mater Med (2010)

Bottom Line: The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow.The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework.The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

View Article: PubMed Central - PubMed

Affiliation: Section Materials Technology (MaTe), Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. l.e.govaert@tue.nl

ABSTRACT
Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

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Left: Compressive true stress versus strain measured at a constant true strain rate. Right: Compressive true strain versus loading time measured under a constant stress
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Fig2: Left: Compressive true stress versus strain measured at a constant true strain rate. Right: Compressive true strain versus loading time measured under a constant stress

Mentions: To illustrate the time-dependent failure of glassy polymers, first the behavior of PLLA in compression under a variety of applied strain rates and stresses is examined. Figure 2 (left) shows the true stress versus true strain response, i.e. the intrinsic behavior, for PLLA as measured under compression at a constant true strain rate, resulting in homogeneous deformation over large strains. Initially the material behavior is linear visco-elastic, but at increasing stress levels it becomes strongly nonlinear, eventually reaching a maximum, i.e. the yield stress (here at 4% strain and a stress of approximately 94 MPa). Subsequently two characteristic phenomena are encountered: (1) strain softening, the initial decrease of true stress with strain and (2) strain hardening, the subsequent upswing of the true stress–strain curve [19]. The interplay between strain softening and strain hardening, to a large extend, determines the toughness of a material, where materials with strong softening and weak hardening behave brittle, and materials with weak softening and strong hardening tough [31, 32]. The strong strain softening and very weak strain hardening found in Fig. 2 (left), is therefore in full accordance with the brittle nature of PLA.Fig. 2


Time-dependent failure of amorphous polylactides in static loading conditions.

Engels TA, Söntjens SH, Smit TH, Govaert LE - J Mater Sci Mater Med (2010)

Left: Compressive true stress versus strain measured at a constant true strain rate. Right: Compressive true strain versus loading time measured under a constant stress
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2805790&req=5

Fig2: Left: Compressive true stress versus strain measured at a constant true strain rate. Right: Compressive true strain versus loading time measured under a constant stress
Mentions: To illustrate the time-dependent failure of glassy polymers, first the behavior of PLLA in compression under a variety of applied strain rates and stresses is examined. Figure 2 (left) shows the true stress versus true strain response, i.e. the intrinsic behavior, for PLLA as measured under compression at a constant true strain rate, resulting in homogeneous deformation over large strains. Initially the material behavior is linear visco-elastic, but at increasing stress levels it becomes strongly nonlinear, eventually reaching a maximum, i.e. the yield stress (here at 4% strain and a stress of approximately 94 MPa). Subsequently two characteristic phenomena are encountered: (1) strain softening, the initial decrease of true stress with strain and (2) strain hardening, the subsequent upswing of the true stress–strain curve [19]. The interplay between strain softening and strain hardening, to a large extend, determines the toughness of a material, where materials with strong softening and weak hardening behave brittle, and materials with weak softening and strong hardening tough [31, 32]. The strong strain softening and very weak strain hardening found in Fig. 2 (left), is therefore in full accordance with the brittle nature of PLA.Fig. 2

Bottom Line: The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow.The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework.The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

View Article: PubMed Central - PubMed

Affiliation: Section Materials Technology (MaTe), Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. l.e.govaert@tue.nl

ABSTRACT
Polylactides are commonly praised for their excellent mechanical properties (e.g. a high modulus and yield strength). In combination with their bioresorbability and biocompatibility, they are considered prime candidates for application in load-bearing biomedical implants. Unfortunately, however, their long-term performance under static load is far from impressive. In a previous in vivo study on degradable polylactide spinal cages in a goat model it was observed that, although short-term mechanical and real-time degradation experiments predicted otherwise, the implants failed prematurely under the specified loads. In this study we demonstrate that this premature failure is attributed to the time-dependent character of the material used. The phenomenon is common to all polymers, and finds its origin in stress-activated segmental molecular mobility leading to a steady rate of plastic flow. The stress-dependence of this flow-rate is well captured by Eyring's theory of absolute rates, as demonstrated on three amorphous polylactides of different stereoregularity.We show that the kinetics of the three materials are comparable and can be well described using the proposed modeling framework. The main conclusion is that knowledge of the instantaneous strength of a polymeric material is insufficient to predict its long-term performance.

Show MeSH
Related in: MedlinePlus