Structural fracture resistance

The magnitude and nature of the load are considered in formulating the design. The load may be essentially quasistatic, cyclic, or impact. Many structural failures, for example, have been caused by supposedly innocuous structural details welded in place without any consideration given to their effect on fatigue properties. The service temperatures are also important, since they affect the fracture resistance of a material. 

From a practical point of view, the main consequence of physical ageing by structural relaxation is embrittlement (decrease in fracture resistance Chapter 12). For the other aspects of mechanical behavior, ageing has either no effect or a favourable effect (increase of relaxation times, leading to a decrease of creep or relaxation rates). This is the reason why, in most thermoset applications, the knowledge of short-term properties is considered to be sufficient for engineering design, as far as fracture and durability are not concerned. 

The basis for the relationship between fracture and yielding is related to crack-tip blunting which causes a decrease of the local stress concentration. So, higher applied loads are required to produce failure. A decrease of cry will increase the values of KIc and GIc. This is the general trend that is observed therefore, it is not possible to pretend to increase both the yield stress and the fracture resistance of a neat thermoset. However, this may be accomplished by including a modifier in the formulation (Chapters 8 and 13). The resulting heterogeneous structure promotes new deformation mechanisms that increase the fracture resistance. 

The effect of structural parameters on KIc and GIc can be summarized as follows any change in the chemical structure (use of monomers with different molar masses, use of nonstoichiometric formulations, etc.), will produce a variation in Tg this will directly affect the value of fracture resistance. An increase in Tg will lead to an increase in constant temperature, and to a decrease in the fracture resistance. This is why high-Tg epoxy networks exhibit very low values of fracture resistance. 

The assessment of relationships between fracture resistance and structural parameters would require analyzing experimental results obtained at constant (Tg — T). Unfortunately, most of the results reported in the literature for materials of different Tg were obtained at room temperature. Conclusions regarding the influence of structural parameters on the fracture... 

In the range where bicontinuous morphologies are obtained, KIc may attain a maximum local value (but it then increases again for phase-inverted structures). The presence of this maximum seems to be related to the formation of an incipient phase-inverted structure that exhibits a lack of adhesion between both phases. In these cases, bicontinuous or double-phase morphologies lead to a better fracture resistance (Girard-Reydet et al., 1997). 

Novel, toughened one-component epoxy structural adhesives based on epoxy-terminated polyurethane prepolymer incorporating an oxolidone structure were developed to provide improved toughness, fracture resistance and adhesive properties with good chemical and moisture resistance.21 The hybrid resin cures with a standard latent curing agent/accelerator. 

Under defined conditions, the toughness is also driven by the content and spatial distribution of the -nucleating agent. The increase in fracture resistance is more pronounced in PP homopolymers than in random or rubber-modified copolymers. In the case of sequential copolymers, the molecular architecture inhibits a maximization of the amount of the /1-phase in heterophasic systems, the rubber phase mainly controls the fracture behavior. The performance of -nucleated grades has been explained in terms of smaller spherulitic size, lower packing density and favorable lamellar arrangement of the /3-modification (towards the cross-hatched structure of the non-nucleated resin) which induce a higher mobility of both crystalline and amorphous phases. 

Elastomeric Modified Adhesives. The major characteristic of the resins discussed above is that after cure, or after polymerization, they are extremely brittle. Tims, the utility of unmodified common resins as structural adhesives would be very limited For highly cross-linked resin systems to be useful structural adhesives, they have to be modified to ensure fracture resistance. Modification can be effected by the addition of an elastomer which is soluble within the cross-linked resin. Modification of a cross-linked resin in this fashion generally decreases the glass-transition temperature but increases the resin flexibility7, and thus increases the fracture resistance of the cured adhesive. Recently, structural adhesives have been modified by elastomers which are soluble within the uncured structural adhesive, but then phase separate during the cure to form a two-phase system The matrix properties are mostly retained the glass-transition temperature is only moderately affected by the presence of the elastomer, yet the fracture resistance is substantially improved... 

Finally, as for the effect of detergent concentration in the water solution for HDPE-1 at 50°C, an decrease in the fracture resistance was observed for increasing detergent concentration until about 40%. This may be attributed to micellar structures that are formed in the solution around small hydrocarbon impurities, as suggested by Tonyali... 

Indeed, as obvious from both exemples given in Fig. 2, the transition could thus be determined accurately within 0.1-0.2 decades of test speeds with few samples in a relative short time frame. Moreover, as the apparent values (Kimax) are always lower than the effective parameters (Keff), none of the material descriptor would be overestimated. In addition, since Kjmax-values have been shown to provide a semi-quantitative evaluation (in terms of test speed or temperature) of fracture resistance parameters, a coherent material comparison would be possible over the whole investigated range. This remark remains true as long as the grades have similar rp. For iPP grades, it should be checked (and considered with more caution) when materials exhibit different particle and matrix melt flow rates, or different crystalline structures. It should also be investigated in detail when different polymer families (ABS versus HIPS or rubber modified iPP) are compared. 

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