Constitutive equations are mathematical models that describe the mechanical behaviour of materials until rupture. They are necessary in the component design and validation processes and, in particular, they are critical to predict failure and assess the structural integrity. Accurate constitutive models of "simple" materials have been developed and validated over the years. Current trends to reduce weight and energy consumption and increase functionality, are, however, leading to the development of "complex" materials with unconventional microstructures (multifunctional composites, nanostructured metals, etc. [1-2]) or materials with evolving microstructures which deform by unconventional mechanisms (TRIP and TWIP steels ). Most existing constitutive models fail to reproduce their mechanical behaviour because they are not able to capture the essential features of the mechanisms of deformation and fracture in the nm-μm range. Therefore, understanding the microstructure evolution under mechanical loading at the microscopic level is essential to improve the structural integrity and reliability of components.
Figure 1. Failure by fibre kinking under compression of a high strength PBO fiber (K. Tamargo, J. M. Molina-Aldareguía, C. González and J. LLorca).
To explore the deformation and fracture mechanisms of materials in the nm-μm range, IMDEA Materials’ research line on Advanced Characterization of Materials has developed the capability to perform in-situ mechanical tests inside scanning electron. Moreover, full-field measurements of the strains in the different phases of complex materials can be obtained from the analysis of the micrographs using digital image correlation. This technique offers the possibility to visualize the evolution of the microstructure under mechanical load, to follow the way the stresses are shared between the different phases at the micro and nanoscale and to capture the dynamics of the deformation and fracture processes. Information obtained with these tests is complemented through instrumented nanoindentation, which provides information about the properties of the phases and interfaces in the material in the critical range from nm to μm.
Figure 2. Deformation and fracture in uniaxial compression of a glass-fibre epoxy matrix composite inside the scanning electron microscope. (a) Secondary electron image showing fibres and matrix distribution. (b) Strain field (superimposed on the previous image), showing the strain concentration in the matrix. (c) Onset of fracture through interface decohesion (J. M. Molina-Aldareguía, L. P. Canal and C. González).
Figure 3. Progressive interface fracture in a glass-fibre reinforced composite during a push-in test to assess interface strength . (M. Rodriguez, J. M. Molina-Aldareguía, C. González and J. LLorca).
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By Dr. Jon Mikel Molina-Aldareguía
Micromechanics and Nanomechanics Group