Ductile failure, stress concentration

At temperatures below 10°C the mild steels may lose ductility, causing failure by brittle fracture at points of stress concentrations (especially at welds) [8,9]. The temperatures at which the transition occurs from ductile to brittle fraction depends not only on the steel composition, but also on thickness. 

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

In all blast-resistant structures (steel, concrete, or masonry) special attention should be given to the integrity of connections between structural elements up to the point of maximum response. For example, it is important to prevent premature brittle failure of welded connections to avoid stress concentrations or notches at joints in steel structures and to provide ductile reinforcement detailing in concrete/masonry structure connections. For all materials, it is recommended that connections be designed to be stronger than the connected structural members such that the more ductile member will govern the design over the more brittle connection. 

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

This equation indicates that under isothermal conditions in instances where AHa < U0, w < y, and the polymer will experience ductile failure. Recognizing that the brittle stress concentration coefficient y is nearly constant (4), the requirements for brittle amorphous polymers, whose Affa > UQ and w > y, to fail in a ductile fashion are to minimize (AH — U0)/[Pg.130]

By the first decade of this century it was established that material failures occur at such low stress levels, because real materials do not usually have a perfect crystalline structure and almost always some vacancies, interstitials, dislocations and different sizes of thin microcracks (having linear structure and sharp edges) are present within the sample. Since the local stress near a sharp notch may rise to a level several orders of magnitude higher than that of the applied stress, the thin cracks in solids reduce the theoretical strength of materials by similar orders, and cause the material to break at low stress levels. The failure of such (brittle or ductile) materials was first identified by Inglis (1913) to be the stress concentrations occurring near the tips of the microcracks present within the sample. 

Another failure mode for a material under mechanical load is the plastic strain. The stress concentration at the tip of a crack increases the probability that dislocations will nucleate and move at the head of the crack tip. However, unlike in brittle fracture, plasticity dissipates a lot of energy, thus reducing the stress concentration by blunting the crack. This type of ductile behavior, typical in metals, leads to robust structural materials the initiation of failure does not necessarily extend catastrophically through the entire specimen, and a lot of energy is dissipated during the process of the material strain. 

Flowever, should the elastic hmit be reached, the lack of an appreciable ductile range in the stress—strain curve for typical FRP laminates is likely to cause failure by direct rupture or tearing, particularly at locations of stress concentration or where free movement of the laminate is prevented, such as at joints or points of connection. In stmctures which rely on flexible dynamic response to attenuate the blast loading, joints should be designed ... 

So far no account has been taken of stress distributions. The experimental evidence, de.scribed in the 3rd paper in this volume (fig. 3), is that there is ductile fracture with a crack which progressively opens into a V-notch until catastrophic failure occurs when the notch covers about half the fibre cross-section. If there is a defect, usually on the surface but sometimes internally (when the V-notch becomes a double cone), the stress concentration will lead to the start of the rupture, although it has a negligible effect on the mean fibre stress at which this occurs. If there is no defect, the evidence is that an initial crack will form by a coalescence of voids that form under high stress. Variation in the degree of orientation across a fibre may well play a part. If the skin of the fibre is more highly oriented, it will reach its limiting extension before the core. 

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