Ductile rupture

Fracture often determines the reliability of a material in its practical applications. Brittle fracture of a material is the reason for a sudden catastrophe. The mechanical property ductile or brittle determines, in essence, whether or not a tool can be made from a given material. Let us identify the imperfections of a crystal and the chemical processes which cause ductility and brittleness. We distinguish two limiting cases of failure 1) A crystal, under external stress, deforms by forming a narrowing neck until eventually ductile rupture occurs. Dislocations are the only imperfections involved in this process of failure. 2) Crystals fracture suddenly. A sharp crack propagates and causes the failure. 

Since experimental creep rupture times rarely exceed 10" h, it is necessary to extrapolate the data, using a straight line extension of the ductile rupture line on the log-log graph. The British Gas Specification for polyethylene pipe required the 50 year creep rupture stress cr o > 10 MPa. The International Standard ISO 9080 classifies polyethylene as PE80 if the lower confidence limit of the 50 year creep rupture strength lies between 8.0 and 9.9 MPa, and as PEIOO if it lies between 10.0 and 11.9 MPa. 

Gurson, A. L. (1977) Continuum theory of ductile rupture by void nucleation and growth part I - yield criteria and flow rules for porous ductile media, J. Eng. Mater. Technol., 99, 1-15. 

The temperature is the transition temperature between brittle and ductile rupture. It can be determined by tests on specific toughness specimens or it can be correlated (for increased easiness) with an energy value absorbed in the common Charpy V test (generally 5.1 x 10 or 8.7 x 10 J m , corresponding to 30 or 50 ft lb respectively. 

Bunsell and Hearle (1971) showed that cyclic loading of nylon commonly gave tensile breaks after the same time as for creep rupture at the peak cyclic load. The exception was when the fibres were cycled from zero load to about 50% of break load. In these circumstances, the break showed a tail on one end, which is typically about five fibre diameters long and had stripped off the other end. Fig. 9a,b. The sequence of failure was first the development of a small transverse crack, which then turned and ran along the fibre at an angle of about 5° to the fibre axis. Fig. 9c. When the crack had crossed half the fibre, the stress on the residual cross-section caused final ductile rupture, Fig. 9d. 

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. 

Primary stresses are those that can cause ductile rupture or a complete loss of load-carrying capability due to plastic collapse of the structure upon a single application of load. The purpose of the Code limits on primary stress is to prevent gross plastic deformation and to provide a nominal factor of safety on the ductile burst pressure. ... 

Ductile rupture failure occurs when the plastic deformation, in a part that exhibit ductile behavior, is carried to the extreme so that the member separates into two pieces. Initiation and coalescence of internal voids slowly propagate to failure, leaving a dull, fibrous rupture surface. 

When the material behavior is brittle rather than ductile, the mechanics of the failure process are much different. Instead of the slow coalescence of voids associated with ductile rupture, brittle fracture proceeds by the high-velocity propagation of a crack across the loaded member. If the material behavior is clearly brittle, fracture may be predicted with reasonable accuracy through use of the maximum normal stress theory of failure. Thus failure is predicted to occur in the multi-axial state of stress when the maximum principal normal stress becomes equal to or exceeds the maximum normal stress at the time of failure in a simple uniaxial stress test using a specimen of the sane material. 

Needleman A, Tveigaaid V (1984) An analysis of ductile rupture in notehed bars. J Mech Phys Solids 32 461-490... 

Ductile rupture, collapse or creep rupture caused by load-controlled stress is insignificant. 

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