Characteristics of Energy Evolution and Constitutive Model of Damage in Freeze–Thaw Ancient Building Grey Brick Materials under Loading

Freeze–thaw cycles are a major factor affecting the durability of ancient Chinese building walls in harsh winter climes. This study focuses on the grey brick walls of historical structures in Longshengzhuang, Inner Mongolia, to investigate the damage evolution law of ancient building walls under hard cold conditions. This work analyzes many cycles of thawing and freezing on the curves of stress-strain and strength parameters of grey bricks from ancient buildings under uniaxial compression utilizing digital image correlation (DIC) technology. Simultaneously, using the principle of thermodynamics to investigate the energy evolution characteristics of frozen melting bricks, along with combining the theory of energy depletion and damage mechanics principle, this work establishes the fragmented damage conceptual model of freezing melted bricks. The results reveal that, as the number of cycles of freeze–thaw increases, the compressive stress-strain curve flattens, compressive peak stress and elastic modulus decrease, and compressive peak strain rises. Based on the dissipated strain energy during the deformation process, the load energy development curve of grey bricks may be classified into four phases: initial damage; elastic damage; plastic damage; and postpeak failure. The total energy, elasticity strain energy, and dissipating strain energy at the peak all fall exponentially as the number of freezing–thawing operations increases. Further, the segmental damage constitutive model for freeze–thawed grey bricks, built utilizing energy dissipation theory and damage mechanics principles, more accurately depicts the damage evolution process of freeze–thawed grey bricks.