Understanding the scale dependent ductility of epoxies
Topic(s) : Material science
Co-authors :
Janina MITTELHAUS (GERMANY), Suprit BHUSARE (FINLAND), Gaurav MOHANTY , Matti ISAKOV (FINLAND), Essi SARLIN (FINLAND), Pekka LAURIKAINEN , Turkka SALMINEN , Nazanin POURNOORI (FINLAND), Bodo FIEDLER (GERMANY)Abstract :
In general, it is considered that in fiber reinforced polymers (FRPs), the matrix, e.g. an epoxy system, is a large scale homogenous bulk mass. However, between the filaments the matrix volume tends to microscale and therefore differs in properties compared to standard bulk samples. It is known that a size effect exists, for epoxy also in terms of plasticity. However, to date, there is no physical, mechanochemical or molecular explanation for the correlation of plasticity and gauge volume, especially for an archetypal brittle epoxy matrix. By using mechanical bulk parameters as inputs for microscale modeling of composites [1-2], an insufficient agreement between the modeled and observed properties can occur [3]. Therefore, a detailed insight into the micromechanical properties and deformation behavior of epoxy matrix as a microcomponent of composites is significant for the improvement and optimization of FRPs.
A new manufacturing method to produce thin epoxy films with a thickness of 30 µm (similar to the dimensions of resin-rich zones between fibers/layers) was developed. These films (EPIKOTE™ Resin MGS RIMR 135 + Curing Agent MGS RIMH 137) were investigated by DSC regarding the degree of cross-linking [4]. Dog-bone shaped samples (gauge volume: 1.8 mm3) were created by punching or laser cutting and investigated by tensile loading. The film samples exhibited a significantly higher plastic deformation ability in from of shear bands and necking compared to standard bulk samples (gauge volume: 250 mm3) [4]. Further reducing the gauge volume to 0.6 mm³ significantly increased the global elongation at break up to 60 % (standard bulk: 5-12 %). Local strains up to 100% were determined by DIC (Figure 1).
The mechanical properties of a material are the manifestation of the way in which individual bonds within the material respond to the applied macroscopic stress. Infrared (IR) and Raman spectroscopy can be used to study these bonds and thus the molecular deformation mechanisms on a microstructural level.
That is why the manufactured film samples were investigated by ex situ/in situ IR and Raman spectroscopy to analyze the molecular mechanisms caused by load introduction. Various pre-processing and interpretation methods for the determination of spectral changes, such as peak shifts, were used to identify the specific molecular changes related to the mechanical stress from the raw spectral data. This way, it was possible to detect molecular changes of the highly cross-linked macromolecules as a result of plastic deformation caused by tensile load. Atomistic simulations have been shown to provide insights into the origin of specific mechanical responses, such as strain hardening coinciding with hydrogen bonding [5] and π-π interactions. The π-π interactions indicate closely packed aromatic structures in the epoxy backbone and therefore support the molecular orientation observed in (polarized) IR and Raman spectra (Figure 2).
A new manufacturing method to produce thin epoxy films with a thickness of 30 µm (similar to the dimensions of resin-rich zones between fibers/layers) was developed. These films (EPIKOTE™ Resin MGS RIMR 135 + Curing Agent MGS RIMH 137) were investigated by DSC regarding the degree of cross-linking [4]. Dog-bone shaped samples (gauge volume: 1.8 mm3) were created by punching or laser cutting and investigated by tensile loading. The film samples exhibited a significantly higher plastic deformation ability in from of shear bands and necking compared to standard bulk samples (gauge volume: 250 mm3) [4]. Further reducing the gauge volume to 0.6 mm³ significantly increased the global elongation at break up to 60 % (standard bulk: 5-12 %). Local strains up to 100% were determined by DIC (Figure 1).
The mechanical properties of a material are the manifestation of the way in which individual bonds within the material respond to the applied macroscopic stress. Infrared (IR) and Raman spectroscopy can be used to study these bonds and thus the molecular deformation mechanisms on a microstructural level.
That is why the manufactured film samples were investigated by ex situ/in situ IR and Raman spectroscopy to analyze the molecular mechanisms caused by load introduction. Various pre-processing and interpretation methods for the determination of spectral changes, such as peak shifts, were used to identify the specific molecular changes related to the mechanical stress from the raw spectral data. This way, it was possible to detect molecular changes of the highly cross-linked macromolecules as a result of plastic deformation caused by tensile load. Atomistic simulations have been shown to provide insights into the origin of specific mechanical responses, such as strain hardening coinciding with hydrogen bonding [5] and π-π interactions. The π-π interactions indicate closely packed aromatic structures in the epoxy backbone and therefore support the molecular orientation observed in (polarized) IR and Raman spectra (Figure 2).