Fracture, material

It is very important, from one hand, to accept a hypothesis about the material fracture properties before physical model building because general view of TF is going to change depending on mechanical model (brittle, elasto-plastic, visco-elasto-plastic, ete.) of the material. From the other hand, it is necessary to keep in mind that the material response to loads or actions is different depending on the accepted mechanical model because rheological properties of the material determine type of response in time. The most remarkable difference can be observed between brittle materials and materials with explicit plastic properties. 

Fatigue. Engineering components often experience repeated cycles of load or deflection during their service fives. Under repetitive loading most metallic materials fracture at stresses well below their ultimate tensile strengths, by a process known as fatigue. The actual lifetime of the part depends on service conditions, eg, magnitude of stress or strain, temperature, environment, surface condition of the part, as well as on the microstmcture. 

We now turn to the other end of the stress-strain curve and explain why, in tensile straining, materials eventually start to neck, a name for plastic instability. It means that flow becomes localised across one section of the specimen or component, as shown in Fig. 11.5, and (if straining continues) the material fractures there. Plasticine necks readily chewing gum is very resistant to necking. 

Creep Rupture. When a plastic is subjected to a constant tensile stress its strain increases until a point is reached where the material fractures. This is called creep rupture or, occasionally, static fatigue. It is important for designers... 

Fracture is caused by higher stresses around flaws or cracks than in the surrounding material. However, fracture mechanics is much more than the study of stress concentration factors. Such factors are useful in determining the influence of relatively large holes in bodies (see Section 6.3, Holes in Laminates), but are not particularly helpful when the body has sharp notches or crack-like flaws. For composite materials, fracture has a new dimension as opposed to homogeneous isotropic materials because of the presence of two or more constituents. Fracture can be a fracture of the individual constituents or a separation of the interface between the constituents. 

Brittleness Brittle materials exhibit tensile stress-strain behavior different from that illustrated in Fig. 2-13. Specimens of such materials fracture without appreciable material yielding. Thus, the tensile stress-strain curves of brittle materials often show relatively little deviation from the initial linearity, relatively low strain at failure, and no point of zero slope. Different materials may exhibit significantly different tensile stress-strain behavior when exposed to different factors such as the same temperature and strain rate or at different temperatures. Tensile stress-strain data obtained per ASTM for several plastics at room temperature are shown in Table 2-3. 

In ordinary solids such as crystalline or amorphous glassy materials, an externally applied force changes the distance between neighboring atoms, resulting in interatomic or intermolecular forces. In these materials, the distance between two atoms should only be altered by no more than a fraction of an angstrom if the deformation is to be recoverable. At higher deformations, the atoms slide past each other, and either flow takes place or the material fractures. The response of rubbers on the other hand is almost entirely intramolecular [4,5]. 

Work hardening of our crystal system occurs as the dislocations move and eventually meet and lock. However these are still potential weaknesses and increasing applied stress or strain will again produce motion. Eventually catastrophic failure occurs as the material fractures. Here dislocations can come together to produce a crack. 

The conclusion to be made from this work is that the fiber and its properties as well as the epoxy matrix and its properties are the same for all three cases. Only the interphase has been altered. Strength of materials fracture models would not predict a difference in fracture toughness and yet experimentally alteration of a 200 nm interphase zone changes the composite fracture properties dramatically. 

Alternative approaches, termed indentation thermal shock tests , with pre-cracks of known sizes have been used by several authors to assess thermal shock damage in monolithic ceramics. Knoop (Hasselmann et al., 1978 Faber etal, 1981) or Vickers (Gong etal., 1992 Osterstock, 1993 Andersson and Rowcliffe, 1996 Tancret and Osterstock, 1997 Collin and Rowcliffe, 1999, 2000 Lee et al., 2002) indentations were made on rectangular bars, which were then heated to pre-determined temperatures and quenched into water. Crack extensions from the indentations were measured as a function of quench temperature differential, and the critical temperature for spontaneous crack growth (failure) was determined for the material. Fracture mechanics analyses, which took into account measured resistance-curve (7 -curve) functions, were then used to account for the data trends. 

The most important materials failure to avoid in the design of metal equipment is sudden catastrophic failure. This occurs when the material fractures under impulse instead of bending. Catastrophic failure can cause complete destruction of piping or equipment, and can result in explosions, huge spills, and consequent fires. Causes of some of the more common types of catastrophic failures are ... 

Brittleness—the force with which the material fractures. This is related to hardness and cohesiveness. In brittle materials, cohesiveness is low, and hardness can be either low or high. Brittle materials often create sound effects when masticated (e.g., toast, carrots, celery). 

The addition of SiC whiskers to silicon nitride matrices resulted in only moderate increases in the fracture toughness a compared to monolithic materials. Fracture strengths of composite materials showed increases in some cases and slight decreases in others. Summaries of the variations in toughness and strength are given in Tables 2.3 and 2.4. The moderate increases in... 

Interestingly, the ductile-brittle transition observed for the MIM system provided an opportunity to assess the material fracture toughness, which was not possible using classical fracture mechanics tests due to the intrinsic brittleness of the MIM system. The measurement of the critical crack length, Lc, in the contact plane at the onset of brittle propagation allows estimation of a fracture toughness K C = a x+JnLc in the order of 0.85 MPa m1/2, i.e. much less than that of a poly(methylmethacrylate) homopolymer (1.20 MPa m1/2). 

In a torsion test, a capstan-shaped specimen is twisted in a viscometer, and the generated stress and strain are measured upto the point of material fracture. Torsion produces what is called a pure stress, a condition that maintains sample shape and volume during the test. The material can fail in shear, tension, compression or in a combination mode, and the test does not dictate the mode of failure (Hamann, 1983). The main disadvantages of torsion are (1) specimen shaping and preparation are usually complex and tedious, and (2) the technique is not applicable to soft or sticky... 

Four point bending tests are used to investigate the evolution of the toughness with the loading rate for the two materials (see Fig.l and table 1). We used an Instron servohydraulic tensile test machine in which a force rate was prescribed from 12N/mn to 5200N/mn. We choose to represent the influence of the loading rate with the variable Kj which is derived from Eq. 3. The stress rate CTq is then involved and estimated from the prescribed force rate. This variable K j is preferred to the prescribed force rate to provide data for the material fracture under mode... 

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