Drucker-Prager model for approximating the tensile and compressive behaviour of chocolate at quasi-static strain rates

The mechanical properties of solid chocolate have attracted research interest recently due to the design of new innovative shapes of chocolate products, the need for appropriate handling and transportation, and the manufacturing of healthier chocolate products with different microstructures. However, relevant comprehensive studies remain very limited. The present study examines the mechanical behaviour of milk chocolate at 0.001 m/s and 0.01 m/s in compression and tension and for the first time characterises chocolate as a bimodular material exhibiting an ultimate strength 1.2-1.8 times higher in compression with a Young’s modulus 0.3-0.6 times higher in tension. Also novelly, this macroscopic behaviour of chocolate is associated with its microstructure, by advocating that the higher compressive strength can be attributed to the particulate part of chocolate, while the higher tensile stiffness can be linked to the quasi-polymeric part of chocolate. Furthermore, it is illustrated that yielding in chocolate does not occur under maximum shear stress (Tresca or Von Mises criterion), but due to a combination of shear and normal stresses (Mohr-Coulomb or Drucker-Prager criterion). Cyclic testing of chocolate shows that the Young’s modulus initially decreases with the number of cycles followed by an increase as the number of cycles further increases, while the Young’s modulus is higher when calculated from the unloading part of the stress-strain curve. An attempt to model the behaviour of chocolate using a single constitutive model for both compression and tension is also conducted with adequate agreement between numerical simulations of compression and tension tests and the respective experiments. • Chocolate is bimodular, exhibiting higher strength in compression and higher stiffness in tension. • Compressive strength is governed by the particulate phase and tensile stiffness by the polymeric phase. • Yielding follows Mohr–Coulomb/Drucker–Prager criteria. • Young’s modulus from unloading is higher than that from loading. • A single constitutive model captures chocolate behaviour in both compression and tension.