## 4 Overview of Experimental Research in the Literature
### Stress-Strain Behaviour  
The compressive stress-strain relationship can be used to estimate the material properties of masonry, i.e., its strength, elastic moduli, and stress distribution under compressive loads [9]. Since masonry structures are constructed with different and non-homogenous elements, they can display typical behaviour that is non-elastic and anisotropic [23; 36; 102; 103]. Prior investigations [9; 85; 104; 105] indicated that the stress-strain profiles of unreinforced masonry constructed with stiff unit-soft mortar combination tend to exhibit greater nonlinear behaviour up to the point of maximum load compared to masonry constructed with stiff mortar-soft unit combination. These studies suggest that the compressive strength of the unit is the primary factor influencing the overall strength of the masonry, while the other component properties may merely alter the deformation path of the structure. For example, [PERSON] et al. [14] observed that the individual properties of mortar and growth only had minimal influence on the shape of the stress-strain curve of full-scale walls constructed with three different unit strength (\(f_{\text{b}}\) = 18 MPa, 27.3 MPa, and 34.5 MPa); mortar strength (\(f_{\text{m}}\) = 12 MPa, 18 MPa, and 24 MPa); and growth strength (\(f_{\text{g}}\) = 31.3 MPa, 34.9 MPa, and 42.4 MPa). Observations from the test results indicate that for walls with bond beams at a middle section, the stress-strain curve is linear up to approximately 70% of the failure load. In grouted wall panels without bond beams at the middle section, the stress-strain curve of these test specimens was linear up to about 60% of the ultimate failure load (e.g., see Figure 12). Similar results were also noted in the study conducted by [PERSON] and [PERSON] [81], which investigated the stress-strain response of URM wallettes constructed using bricks with compressive strength of 3.8 MPa and 15.8 MPa, along with two different mortar types: a stiff mortar (mix ratio 1:3) with compressive strength of 6.48 MPa and a weaker mortar (mix ratio 1:5). Two cases of clay and compressed earth brick wallettes (particularly specimens CLB1 and CLB2) are shown in Figure 12. The authors noted that the masonry specimens built using the 1:3 mortar displayed significantly lower compressibility, particularly beyond the linear elastic regions of the curves, compared to masonry specimens constructed with 1:5 mortar. However, it is worth highlighting that, regardless of the unit-mortar combination, the authors did not verify the complete descending portions in the stress-strain profiles. Figure 12: Stress–strain response of URM walls produced with different combinations of unit strength (\(f_{\text{b}}\)) and mortar strength (\(f_{\text{m}}\)) as derived from the experimental literature [14; 24; 76; 81]. Accordingly, [PERSON] et al. [76] verified that the existing prediction models prescribed in the masonry standards do not consider the various failure modes of unreinforced masonry or the onset of the nonlinear stress-strain curves. Two cases of clay brick wallettes (particularly CL90s and M6 specimens) are also shown in Figure 12. [PERSON] et al. [76] explained that unreinforced masonry wallettes constructed using stiff unit-softer mortar combinations exhibit significant deformations at the start of the compression loading regime due to the early plastic deformation of the mortar. This was subsequently followed by an upward, relatively linear segment occurring between 30% and 60% of the peak load level. Beyond this point, typically above 60% of the peak load, the curve ceases to be linear, and the material reaches its maximum stress, ultimately leading to failure. In contrast, the masonry built with a combination of stiff mortar and softer units exhibited a different behaviour. Initially, it absorbed high stress with minimal deformation, followed by abrupt failure with very limited plastic deformation. These findings emphasize the significant influence of mortar properties on masonry deformation.