## 4 Overview of Experimental Research in the Literature
### Stress-Strain Behaviour
This aligns with findings reported in [106], which indicated that the stress-strain relationship becomes progressively nonlinear as mortar strength decreases. [PERSON] et al. [76] further noted that for the stress-strain curve prescribed in Eurocode 6 [24], the descending portion of the curve can only be applied to mortars with \(f_{\text{m}}\) value greater than 7 Mpa. The peak strain is an important parameter that characterizes the behaviour of masonry when subjected to axial compression. Despite its significance, the topic of indirectly estimating the peak strain corresponding to the masonry's compressive strength has received limited attention. The Eurocode 6 [24] recognizes the nonlinear nature of the stress-strain relation of unreinforced masonry under compression. It assumes the curve to be linear up to 0.33, the characteristic compressive strength of the masonry (i.e., 0.33\(f_{\text{m}}^{\prime}\)), or can be considered to be a parabolic curve, rising up to a strain of 0.002, or as a horizontal plateau in the stress-strain curve up to a strain of 0.0035 (e.g., see Figure 12). Based on regression analysis of experimental results, other authors have proposed expressions to deduce the peak strain in relation to the strength of full-scale masonry, its mortar strength and elastic modulus (see a selection in Table 3). A novel rational model for a nonlinear stress-strain curve for HCB masonry wall structures subjected to axial compressive loading was proposed by [PERSON] et al. [80]. Testing was carried out on 23 URM wallettes--590 mm \(\times\) 990 mm \(\times\) 190 mm, their nominal sizes--using three different block/mortar strength combinations, i.e., HCB (\(f_{b}\) = 14.08 mPa, 25.73 mPa, and 31.18 mPa) and mortars (\(f_{m}\) = 6.31 mPa and 15.55 mPa). Results from the study indicated that a reduction in mortar strength produced a corresponding increase in the peak strain of the wall panels, confirming that the mortar properties greatly influence the deformation characteristics of URM walls, contradicting the observations noted in [9; 105]. The model proposed in [80] produced results comparable to the stress-strain curve observed in all tested specimens from the early stages, via the upward curve and up to 20% of the ultimate load in the descending curve. However, in the latter stage of the downward curve, the proposed model failed in predicting the response of the downward curve since the \begin{table}
\begin{tabular}{l c c} \hline \hline
**Reference** & **Model** & **Masonry Type** \\ \hline [PERSON] et al. [75] & \(\epsilon_{pp}=\frac{0.27}{E_{m}^{\prime}}\frac{\epsilon_{\text{B}}}{\mu_{ \text{B}}^{\prime}}\) & Calibrated for clay brick wallette \\ [PERSON] et al. [76] & \(\epsilon_{pp}=\frac{0.34}{f_{\text{m}}^{\prime}}\frac{\{f_{m}^{\prime}}}{\mu_{ \text{m}}^{\prime}}\) & Calibrated for fired-clay brick wallette \\ [PERSON] et al. [80] & \(\epsilon_{pp}=\frac{0.21}{f_{\text{m}}^{\prime}}\frac{\epsilon_{\text{B}}^{ \prime}}{\mu_{\text{B}}^{\prime}}\) & Calibrated for HCB wallette \\ [PERSON] and [PERSON] [81] & \(\epsilon_{pp}=\frac{0.001}{f_{\text{m}}^{\prime}}\frac{\epsilon_{\text{B}}^{ \prime}}{\mu_{\text{B}}^{\prime}}\) & Calibrated for clay brick wallette \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of existing literature studies showing the estimation of the masonry peak strain, \(\epsilon_{\text{pp}}\), using the elastic modulus, \(E^{\prime}\)m, compressive strength of masonry, \(f_{\text{m}}\), compressive strength of mortar, \(f_{\text{m}}\), and the compressive strength of unit, \(f_{\text{b}}\). strain continues to infinity with larger ultimate stress. Despite this, the proposed model is presumed adequate for use in the design and numerical analysis of HCB masonry walls. A new model for correlating the compressive stress-strain curves of masonry prism to that of wallette was also proposed by [PERSON] and [PERSON] [81], based on axial compression tests conducted on clay and compressed earth bricks.