18.4: Stress-Strain Diagram – Brittle Materials

Brittle materials, including glass, cast iron, and stone, exhibit unique characteristics. They fracture without considerable change in their elongation rate, indicating that their breaking and ultimate strength are equivalent. Such materials also show lower strain levels at the point of rupture. The failure in brittle materials predominantly results from normal stresses, as evidenced by the rupture created along a surface perpendicular to the applied load. These materials do not display significant necking. Necking is the local reduction in cross-sectional area under stress. An interesting aspect of most brittle materials is their higher ultimate strength in compression than in tension, primarily due to microscopic defects like cracks or cavities that can weaken the material under tensile stress but have minimal impact on its compressive strength.

Concrete, a common brittle material, behaves differently under tension and compression. The stress-strain diagram under tension reveals a linear elastic range up to the yield point, followed by a rapid increase in strain until rupture. In contrast, concrete shows a larger linear elastic range under compression, and rupture does not occur even at peak stress. Instead, stress decreases as strain increases until rupture. Importantly, the modulus of elasticity, indicated in the stress-strain curve by the slope of the linear section, remains consistent in both tension and compression for most brittle materials.

Brittle materials, under tensile stress, do not elongate much before rupturing, meaning their ultimate and breaking strengths are identical. Compared with ductile materials, they possess lower strain during rupture.

It occurs along a surface perpendicular to the load, indicating that normal stresses primarily cause the failure.

Brittle materials do not undergo noticeable necking under stress.

Most brittle materials have higher ultimate strength in compression than tension, mainly due to microscopic flaws like cracks or cavities, that weaken the material under tension.

Consider a stress-strain diagram for the concrete slab, an example of a brittle material.

Under tension, a linear elastic range is observed until the yield point, and then strain increases more rapidly than stress until the slab ruptures.

Under compression, concrete displays a larger linear elastic range, and rupture doesn't occur as stress hits its peak. Instead, stress decreases while strain continues to increase until rupture.

The modulus of elasticity, represented by the slope of the linear region, is equal in both compression and tension for most brittle materials.