Resistance to crack initiation

Highly ductile materials tend to be more resistant to thermal fatigue and also seem more resistant to crack initiation and propagation. 

While a new tire may have excellent resistance to crack initiation and propagation between the steel belts, an aged tire of the exact same construction can exhibit dramatically reduced crack growth resistance, which in some cases may contribute to tire failure. To further underscore the point, the United States National Highway Traffic Safety Administration (NHTSA) determined that tire aging was a main contributor to the Firestone Wilderness AT tire recall [1]. Figure 34.1 shows the failure rate of Wilderness AT tires as a function of tire age (determined from date of tire manufacture). 

The fracture behaviour of polymers, usually under conditions of mode I opening, considered the severest test of a material s resistance to crack initiation and propagation, is widely characterised using linear elastic fracture mechanics (LEFM) parameters, such as the plane strain critical stress intensity factor, Kic, or the critical strain energy release rate, Gic, for crack initiation (determined using standard geometries such as those in Fig. 1). LEFM... 

Equation (15.24) shows that, for high resistance to crack initiation, high strengths combined with low stiffness and CTE are required. Under mild thermal shock conditions (e.g. in a boiling water quench) the thermal conductivity also becomes important, and (15.24) is modified to give ... 

Polymerized epoxy adhesives are amorphous and highly crosslinked materials. This microstructure results in many useful properties such as high modulus and failure strength, low creep, and good chemical and heat resistance. However, the structure of epoxy resins also leads to one undesirable property—they are relatively brittle materials. As such, epoxy adhesives tend to have poor resistance to crack initiation and growth, which results in poor impact and peel properties. In sealant formulations, epoxy resins do not often provide the degree of elongation or movement that is required for many applications. 

Long-term exposure of composites to oxidative environments can have deleterious effects on short-term mechanical behavior, such as resistance to crack initiation. This is particularly true in the case where the composite oxidizes to form an oxide surface scale. Although such reactions can be beneficial in limiting oxidation reactions, when the composite is subsequently cooled to room temperature, the reaction product can be a source of flaws and increase the composite s susceptibility to crack initiation. The following example illustrates this point. 

Heat-Affected Zones. The COD results are listed in Table III. At -170°C, the 9 Ni/Inconel 625 HAZ showed a high resistance to crack initiation. At -196°C, an appraisal of the 9% Ni performance is complicated by the occurrence of pop-ins. If the COD value at first pop-in is used, the 0.10 mm COD criteria was not met. If the maximum COD value is used, ignoring the pop-ins, then the level obtained is above the minimum. 

The individual fibres within unidirectional composites have few defects, and are consequently resistant to crack initiation. Additionally, any crack that does form travels through the matrix and is not transmitted through adjacent fibres. These toughness and crack-arresting properties contribute to good fatigue performance in FRP materials. 

Impact Strength - The impact behaviour of PES is good it is not as outstanding as that of Polycarbonate but it shows better impact perfoimance than that of nylons or acetals. PES cannot be broken in the unnotched Izod test, and a notched Izod (ASTM D 237) has a value of 1 6 ftlb/in notch it is particularly resistant to crack initiation and PES is ev a. tougher after it has absorbed its equilibrium amount of water. 

Some of the more common mechanical properties are provided in Table 10.1 from IAEA (2009). Relative to various measures of toughness, some tests use a blunt notch, while others use a sharp crack to determine material resistance to crack initiation, crack propagation or both, and are conducted under either quasi-static (slow) loading or dynamic (fast) loading conditions. Of all these tests, the Charpy V-notch (CVN) impact test is the most commonly used. In addition to those mechanical properties, there are common reference fracture toughness indices used for RPV steels and many of those are also shown in IAEA (2009). These indices are important because they are used in various ways to normalize fracture toughness of RPV steels. 

In short-fiber composites, areas around fiber ends, areas of poor adhesion, and regions of fiber-fiber contacts may reduce the resistance to crack initiation by acting as stress concentrators. However, fibers may also reduce crack propagation by diverting cracks around the fibers or by bridging cracks. Materials with high-impact... 

One of the shortcomings most adversely affecting the utilization of PP filled with flame retardant magnesium hydroxide (Mg(OH)2) is the steep reduction of subambient fracture toughness compared with that of neat polypropylene. Generally, an increase in the resistance to crack initiation and growth can be achieved... 

According to the German BWRs, field experience, the two following practices during manufacturing seem to have a beneficial effect on the resistance to crack initiation and crack growth a controlled grinding of the weld areas and an appropriate post weld heat treatment... 

It is believed that a bimodal, or a broad distribution of rubber particles resulting from such rubber mixtures, helps in the craze or crack blunting because the larger particles and the small particles provide resistance to crack initiation through increased local shear deformation of the matrix. 

The resistance to crack initiation (critical stress intensity factor K c) was defined from F(, Ao> beam span L, thickness b and depth h ... 

The resistance to the stable crack propagation may be characterized by the energy parameter RG, as well as the resistance to crack initiation by the critical energy release rate related to K c- RG-value is... 

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