Crack initiation energy

Other factors which can affect impact behaviour are fabrication defects such as internal voids, inclusions and additives such as pigments, all of which can cause stress concentrations within the material. In addition, internal welds caused by the fusion of partially cooled melt fronts usually turn out to be areas of weakness. The environment may also affect impact behaviour. Plastics exposed to sunlight and weathering for prolonged periods tend to become embrittled due to degradation. Alternatively if the plastic is in the vicinity of a fluid which attacks it, then the crack initiation energy may be reduced. Some plastics are affected by very simple fluids e.g. domestic heating oils act as plasticisers for polyethylene. The effect which water can have on the impact behaviour of nylon is also spectacular as illustrated in Fig. 2.80. 

Impact tests may be performed with notched or unnotched specimens, but the results cannot be compared. The presence of a notch induces a great part of the crack initiation energy. The speed of the striker or of the crosshead may be varied from 1ms-1 to several kms-1 for ballistic tests strain rates may vary in the range of 10-1 up to 100 s-1. Very often the energy for failure is divided by the effective cross-sectional area or the deformed volume, in order to calculate, respectively, the surface or the volume resilience. Using the compliance formula and simple assumptions, GIc may be directly calculated (Williams, 1984). 

The fracture toughness of a semi-crystalline polymer (PP) filled with mineral submicron and micron scale particles is investigated according to the J-integral method determination of the crack initiation energy (J ), and the crack propagation resistance dJ/d(Aa). Ultrafine mineral... 

Other environmental factors besides temperature may also affect impact behavior. For example, if the material is in the vicinity of a fluid which attacks it, then the crack-initiation energies may be reduced, resulting in lower impact strength. Some materials, particularly nylon, are significantly affected by water, as illustrated in Figure 3.40. The absorption of water produces a spectacular improvement in the impact behavior of nylon. 

The crack initiation energy can readily be determined from a mode I cleavage test using a double cantilever beam specimen [1,2]. This method has been applied to wood-adhesive joints by many researchers [3 17]. The test requires the beam compliance (C), load at crack initiation (P ), crack length at initiation (a), specimen thickness (/), and change in beam compliance = change in displacement of load points/change in... 

Whereas the fracture of bonded joints is due to the presence of critical discontinuity in a field of stress, the stress intensity or crack initiation energy of bonded joints and materials at which fracture occurs is also a function of the properties of the wood and the adhesive, the environment at a given time, changes in the environment, and external forces on the joint or bonded material. These relationships are explored in the following sections. 

Brittle polymers, for which < 0.15 mmol/cm and Coo > 7.5. They fracture by a dominant crazing mechanism and additionally exhibit a low crack initiation energy and a low crack propagation energy (resulting in low, both uimotched and notched, toughness). Examples are PS or PMMA. 

Hristov [123] studied the impact fracture behavior of WPG modified by MAPP as compatibilizer and poly (butadiene styrene) rubber as impact modifier using Gharpy impact testing. They demonstrated that the crack propagation energy compared to the crack initiation energy is much more influenced by the morphology. 

Crack propagation Crack initiation Energy ratio s... 

In view of the small depth of the wet tip zone, most of the crack growth occurs within a dry region. Consequently, the immersed fracture data relate crack initiation energy. 

Impact fatigue tests have also been conducted on some polymers namely Nylon 66, HIPS acrylonitrile butadiene styrene (ABS), and rubber-toughened Nylon. The impact energy was 98% of the crack initiation energy. Specimens withstood 100-200 cycles of loading before failure. 

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