Ductile and brittle behavior

The most common type of stress-strain tests is that in which the response (strain) of a sample subjected to a force that increases with time, at constant rate, is measured. The shape of the stress-strain curves is used to define ductile and brittle behavior. Since the mechanical properties of polymers depend on both temperature and observation time, the shape of the stress-strain curves changes with the strain rate and temperature. Figure 14.1 illustrates different types of stress-strain curves. The curves for hard and brittle polymers (Fig. 14.1a) show that the stress increases more or less linearly with the strain. This behavior is characteristic of amorphous poly-... 

Geotechnical Earthquake Engineering Damage Mechanism Observed, Fig. 4 Schematic illustration of ductile and brittle behaviors... 

Carpick, R.W., Enachescu, M., Ogletree, D.F. and Salmeron, M., Making, breaking, and sliding of nanometer-scale contacts. In Beltz, G.E., Selinger, R.L.B., Kim, K.-S. and Marder, M.P., (Eds.), Fracture and Ductile vs. Brittle Behavior-Theory, Modeling and Experiment. Materials Research Society, Warrendale, PA, 1999, pp. 93-103. 

Material behavior have many classifications. Examples are (1) creep, and relaxation behavior with a primary load environment of high or moderate temperatures (2) fatigue, viscoelastic, and elastic range vibration or impact (3) fluidlike flow, as a solid to a gas, which is a very high velocity or hypervelocity impact and (4) crack propagation and environmental embrittlement, as well as ductile and brittle fractures. 

The main considerations of mechanical properties of metals and alloys at low temperatures taken into account for safety reasons are the transition from ductile-to-brittle behavior, certain unconventional modes of plastic deformation, and mechanical and elastic properties changes due to phase transformations in the crystalline structure. 

Significant variation of the ultimate mechanical properties of poly(hexamethylene sehacate), HMS, is possible by con-trol of thermal history without significant variation of percent crystallinity. Both banded and unbanded spherulite morphology samples obtained by crystallization at 52°C and 60°C respectively fracture in a brittle fashion at a strain of r O.Ol in./in. An ice-water-quenched specimen does not fracture after a strain of 1.40 in./in. The difference in deformation behavior is interpreted as variation of the population of tie molecules or tie fibrils and variation of crystalline morphological dimensions. The deformation process transforms the appearance of the quenched sample from a creamy white opaque color to a translucent material. Additional experiments are suggested which should define the morphological characteristics that result in variation of the mechanical properties from ductile to brittle behavior. 

Recently Moore and Petrie (5) have demonstrated that control of sample thermal history can result in transition from ductile to brittle behavior for polyethylene terephthalate. This transition in behavior was related to volume relaxation of the glassy state. 

Volume 539— Fracture and Ductile vs. Brittle Behavior—Theory, Modelling and... 

Some properties of materials can vary according to the rate at which stress is applied some materials are plastic and ductile if the stress is applied slowly but can be elastic or brittle if the stress is applied by impact. The deformation mechanisms occurring during compaction of fines and agglomerated foods depend on the elastic and viscous flow, in addition to ductile yielding and brittle behavior common in pharmaceutical and food compaction processes (Barletta et al., 1993b). 

Ductility is more commonly defined as the ability of a material to deform easily upon the application of a tensile force, or as the ability of a material to withstand plastic deformation without rupture. Ductility may also be thought of in terms of bendability and crushability. Ductile materials show large deformation before fracture. The lack of ductility is often termed brittleness. Usually, if two materials have the same strength and hardness, the one that has the higher ductility is more desirable. The ductility of many metals can change if conditions are altered. An increase in temperature will increase ductility. A decrease in temperature will cause a decrease in ductility and a change from ductile to brittle behavior. Irradiation will also decrease ductility, as discussed in Module 5. 

Ductile fracture dok-Cl [MF L, MF, fr. ductilis, from ducere] (14c) (ductile rupture) adj. The breaking or tearing, most commonly in tension, of a test specimen or part after considerable unrecoverable stretching (plastic strain) has occurred. Since the mode of fracture depends on conditions as well as material, the distinction between ductile and brittle fracture, which latter occurs after relatively little, recoverable strain, is not always clear. Low temperatures, especially below the glass transition (Tg), and high rates of strain favor brittle behavior, while the opposites favor ductile behavior. 

Rice, J. R. and Thomson, R. (1973), Ductile versus brittle behavior of crystals,, , 73-97. 

Depending on the behavior of materials under mechanical actions, they can be classified, vis-a-vis mechanosynthesis or mechanical alloying processes, into two major groups ductile and brittle materials. The former are essentially the materials showing plastic deformation, that is, metals and alloys, and the latter are basically the materials fracturing under mechanical action without noticeable plastic deformation, that is, ceramics. Most of the perovskites are considered in the latter group. 

The transition from ductile to brittle behavior is related to the relaxation enthalpy of the free volume. Figure 5.200 shows the increase in relaxation enthalpy with increasing exposure temperature and longer exposure times yield strain decreases simultaneously [773]. 

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