The strong and the weak: nucleating insight into the role of mechanical heterogeneity on material failure

Material failure is both undesirable and unavoidable. Therefore, ideally, this catastrophic failure event is forecasted or delayed by controlling and understanding this event. Elucidating the conditions under which failure occurs is challenging, due to the interplay between localized damage processes and long-ranged mechanical interactions. For polymeric materials that are being used in, e.g., transportation or packaging, understanding the conditions under which failure occurs is even more challenging due to large deformations, viscoelasticity, and heterogeneity at the microscale. This work focuses on the latter, by controlling and measuring the heterogeneity of soft materials and trying to relate this to material failure. To do so, the first part of this thesis, Chapters 2-4, focus on synthesis of materials with controlled heterogeneity and mechanical characterization of this heterogeneity. The second part of this thesis, Chapters 5-6, explore methods to extract detailed information about localized mechanics and bond rupture required to determine the role of heterogeneity in material failure, using mechanosensors.

In Chapter 2 we develop a methodology to create a material with controlled heterogeneity, obtained with photolithography. Subsequently, we mechanically characterize this material by means of a home-built multi-point indentation setup which indents a material in a grid-fashion to create a mechanical map. The obtained mechanical map resembles the intended mechanical heterogeneity, although some feature blurring is present. Therefore, in Chapter 3, we use finite elements method simulations to explore the length scales involved in this mechanical blurring. The simulations show that a characteristic blurring length can be used as a correction. However, experimental verification is not obtained due to a height dependence of the elastic modulus.

In Chapter 4, we use the developed multi-point indentation setup to find similarities between mechanical maps of meat analogues, in an attempt to present the food industry with a quantitative method to compare the texture of meat analogues with animal meats. By using the commonly used spatial autocorrelation technique Moran’s I we find a decay length that resembles the length scales over which the mechanics in meat samples correlate. This decay length can be used to quantify the resemblance between meat analogues with animal meats.

In the second part of the thesis, Chapter 5 shows the mechanoluminescent crosslinking polymer 1,2-dioxetane being used to quantify bond rupture events in polymer networks during puncture tests. A more detailed understanding of the microscopic processes occurring during indentation fracture is obtained by simultaneously recording the bulk force exerted on the puncture probe, and relating this to the bond rupture events. We find that a significant amount of bonds rupture before the probe penetrates the surface, known as puncture, at which a sharp drop in the force is observed. Further probe penetration proceeds by the propagation of a ring-shaped crack. By quantifying the number of rupturing bonds during this propagation stage and showing that this number is higher than the number of bonds for a cylinder of the probe size (i.e. in case of brittle failure), the damage zone is associated with diffuse failure of a significant size. Furthermore, puncture in double networks shows that the majority of the bonds rupture in the first network, as is generally assumed but had not yet been confirmed in other studies. Comparison with MD computer simulations shows that the latter is due to delocalization of the stresses in a large zone around the indenter.

In Chapter 6, we continue previous work performed by our lab, in which laser speckle imaging (LSI) showed that delayed failure of elastomer is preceded by small differential strains well before crack nucleation. Here, we incorporate the stress sensor spiropyran to study the stress accumulation during these moments before crack nucleation. We find that the timescale at which spiropyran onsets is much shorter to the crack nucleation than the small differential strains observed with LSI.

In the final Chapter, Chapter 7, we reflect on our findings and presents two additional experimental projects that were started during this project that specifically attempted to address the goal of the project; find the role of heterogeneity in material failure. Even though these projects were not finished completely at the end of the project, they do pave the way towards finding the project goal. Lastly, a perspective on how to achieve this goal experimentally is presented.