Ductile failure, importance

The distinction between brittle and ductile failure is also manifested in two other ways (1) the energy dissipated in fracture and (2) the nature of the fracture surface. The energy dissipated is an important consideration for practical applications and forms the basis of the Charpy and Izod impact tests (discussed in Section 12.8 below). At the testing speeds under which the practical impact tests are conducted it is difficult to determine the stress-strain curve, so impact strengths are customarily quoted in terms of the fracture energy for a standard specimen. 

Criteria of Elastic Failure. Of the criteria of elastic failure which have been formulated, the two most important for ductile materials are the maximum shear stress criterion and the shear strain energy criterion. According to the former criterion, from equation 7... 

There are a number of fracture modes, the most important of which are ductile overload, which is fairly well understood and can be predicted reasonably accurately, and brittle fracture, which is less predictable from an engineering viewpoint and can cause catastrophic failures due to the speed of the fracture. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

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. 

Blast loaded structures produce high reaction loads at column supports. This usually requires substantial base plates as well as high capacity anchor bolts. Achieving full anchorage of these bolts is of primary importance and will usually require headed bolts or plates at the embedded end of the bolts to prevent pullout. When anchor bolts are securely anchored into concrete, the failure mechanism is a ductile, tensile failure of the bolt steel. Insufficient edge distance or insufficient spacing between bolts results in a lower anchorage capacity and a brittle failure mode. 

Other important parameters for the correlation between GJJ and GJ include the ductility or the failure strain, particularly the non-linear strain (Jordan and Bradley, 1988 Jordan et al., 1989) of the matrix resin, the bond strength of the fiber-matrix interface (Jordan and Bradley, 1987 Bradley 1989a, b), and the fiber V and their distributions in the composites (Hunston et al., 1987). A high failure strain promotes the intrinsic capacity of the resin to permit shear deformation, and is shown to increase the G and G. values almost linearly, the rate of increase being steeper for G j than for Gf. ... 

At higher temperatures, the failure occurs with yielding, which is now the predominant deformation mechanism. From an experimental point of view, domains define what is called the ductile-brittle transition temperature, TB, which is a very important characteristic for polymers. The ductile-brittle transition is also associated with a stiffness-toughness balance. Note that it is also possible to determine a ductile-brittle strain rate transition varying k at a given temperature. 

Ductile fracture is characterized by considerable plastic deformation prior to and during propagation of the crack. An important amount of gross deformation is usually present at the fracture surfaces. Figure 7.63 is an example of a ductile fracture of a Swellex bolt. Note the failure plane at 45° and localized reduction of the surface. 

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