Issue 50

A. Marinelli et alii, Frattura ed Integrità Strutturale, 50 (2019) 438-450; DOI: 10.3221/IGF-ESIS.50.37 448 Figure 16: Average values ± STDEV for (a) Flexural strength vs. span length for all span/depth ratios; (b) Fracture energy vs. span length for all span/depth ratios. Most of the specimens failed in a similar manner for all shapes, with cracks appearing to propagate from the tip of the notch and then veering slightly off-center. With the span/depth ratio of 6, there were occasions when the specimens failed off-center, with the crack starting at a random location between the notch and roller supports. Most irregular failure modes observed were again attributed post-failure to the presence of relatively large mineral build-ups (Fig.10). DISCUSSION AND CONCLUSIONS he development of suitable methodologies of experimental investigation for two natural building stones used widely in construction projects in Edinburgh was approached through an experimental study comprising both non-destruc- tive (for apparent density, real density, open porosity, total porosity and sound speed propagation) and destructive testing (for compressive and flexural strengths). Fundamental mechanical properties for Corsehill sandstone and Portland limestone were derived, compared and contrasted, with the intention to point out characteristic behaviour patterns of applicability especially to restoration projects. From the study on the fracture resistance of prismatic specimens carrying machined notches of different lengths, and following data processing and interpretation of the results, it seems that the critical COD under specific conditions could be further investigated as a fracture criterion, independently of the method used for its determination. After a critical limit, it is the presence of the notch itself that plays a crucial role and not its relative length. Alternative expressions for one-dimensional fracture criteria (a single point - critical value - in the space of a criterion with parameters: δ crit , angle of notch inclination and angle of anisotropy direction) have been proposed as functions of stresses and δ crit . Such an approach based on a single geometry does not allow for general conclusions although it seems that a criterion based on stresses is relatively stable and convenient for engineering applications. The further objective of this research was to examine the effect that test specimen size and shape have on several key properties of Portland limestone, including deflection at mid-span at peak load, CMOD at peak load, flexural strength and, in particular, fracture energy. For the specimens tested in 3PB, containing three different shapes (span over depth ratio) that came in three different sizes (spans), the relationship between the studied deflection and specimen span length was identified for span/depth ratios of 5/2 and 4, characterised by a gentle positive gradient existing between span lengths of 200 mm and 400 mm, before a dramatic increase between span lengths of 400 mm and 800 mm. For test specimens with a span/depth ratio of 6, this deflection appeared to increase linearly with span length. It was also observed that the deflection seemed independent of the span/depth ratio of the specimens for span lengths 200 mm and 400 mm but for span lengths 800 mm, it increased as the span/depth ratio decreased. A similar pattern of observations was made for the CMOD at peak load, leading to the conclusion that it appears likely for a critical specimen size to exist for Portland limestone, where the effect of specimen size and shape becomes more apparent. A negative correlation between the flexural strength of Portland limestone specimens and their span lengths for all three shapes was observed, supporting previous findings [23]. This was most evident for span/depth ratio of 6, with a significant decrease in flexural strength recorded between the span lengths of 200 mm and 400 mm. An important finding of this experimental study was the positive correlation between fracture energy and test specimen size. Between span lengths of 200 mm and 400 mm there is a gradual increase of fracture energy, before a significant rise when the span length gets to 800 mm. For the span/depth ratio of 5/2, clearly larger values of fracture energy were recorded, indicating the dependence of fracture energy on test specimen size and geometry as supported by other studies [22]. (a) (b) T