Starter crack

The EDT specimen shown in Fig 3.34(a) has been used to characterize the interlaminar failure of the composite in opening mode I (Whitney and Knight, 1985). The straight-sided tensile specimen has starter cracks placed along the free edges at the laminate mid-plane (Fig 3.34(a)). Due to the low interlaminar shear... 

Specimens without a fillet were also tested in which starter cracks had been deliberately introduced into the bond line. Figure 7 (a) and (b) shows, respectively, the tensile and shear components of strain for an example of such a specimen with a crack of length 0.5mm. 

Although we do not measure crack velocity pier se, friilure during the rather short tip>-particle contact (typically 35 ms) requires a minimum crack velocity and thus a minimum K. The exponential dep>endence of crack velocity on crxy in Eq. (2) ensures that the majority of crack growth will occur when axy is close to the p>eak shear stress, rtional to c. ... 

Plots of Gc as a function of the nominal prarticle/substrate contact area f are show in Fig. 3(a) for several values of relative humidity. TTie data were initially modeled the data with a two parameter curve fit (gc A ") for each value of relative humidity. Each fit yielded n = 0.5 to within the uncertainty of the curve fitting procedure (typically 0.1). For simplicity the curve fits in Fig. 3(a) represent one parameter least squares fits with n = 0.5, i.e., Gc A-. Since we expect Gc this dep>endence suggests that the initial flaw size is p)ropx)rtional to p>article area (c A). Interfricial flaws are exp)ected to serve as starter cracks, and may be respionsible for the particle size dependence of the shear stress required for detachment. 

Specimens were milled to the dimensions just indicated, and a semicircular groove was machined across the bottom of each specimen, parallel to the crack plane, to accommodate a pin that provided vertical support during testing. To promote planarity of crack growth, side grooves were cut with a 0.5-mm-thick slitting saw to a depth of 6.3 mm, and a reverse-chevron-tipped starter crack was cut to a... 

A starter crack is introduced in a specimen, and a load is applied, as shown in Fig. 12.8. As a result, the starter crack will grow with time. 

The methodology just described, while useful in predicting the lifetime of a sample with a well-defined starter crack (tf cannot be calculated without a knowledge of A i or c,), is not amenable to a probabilistic analysis such as... 

Starter Crack. From Which Crack Grows... 

A natural crack initiated in the center of the adhesive layer in a symmetric joint between symmetric isotropic (metal) adherends will tend to propagate through the center of the adhesive layer. However, in wood joints, there is a strong tendeney for the crack to travel in the wood near the joint. This condition should be expeeted in joints made with the lower-density species or in species with the low-density earlywood such as the Southern pines (Finns spp.). However, wood failure is not uneommon in joints made with high-density species even when there is a starter crack in the adhesive layer before testing. There seem to be some rational explanations for this behavior. 

Use of fracture mechanics test methods for failure analysis of FRP composite elements is feasible, but to the best knowledge of the author, no examples of this have been published. The challenge here is mainly the preparation of suitable test specimens from structural elements and the creation of the starter cracks. ISO 15024 does provide some information on mode I starter crack preparation for nonstandard specimens in an informative appendix (B.8, Guidelines for wedge precracking ), and that procedure can be adapted for other specimen geometries. 

Figure 8.11 Delamination resistance of symmetric cross-ply carbon fibre—epoxy (IM7/977-2) under quasi-static mode I loading, average initiation (init), average propagation (prop) and maximum prop values (averages from two to five laboratories and standard deviations) for testing from a starter crack thin-film insert and from a mode I precrack.

Figure 1 shows a compact tension specimen in which a starter crack propagated at a controlled rate such that the test could be stopped prior to specimen failure. Whitened areas above and below the fracture surfaces are a result of particle cavitation. This particular material was a flexibilized vinyl ester containing a rubber particulate phase. The large volume of material involved in the fracture process resulted in high toughness and a ductile tearing mode of fracture as opposed to the brittle fracture mode of the base resin. 

Of particular interest is whether there is a lower limit to the applied stress below which no environmental attack is observed and this has been investigated in some detail using a fracture mechanics approach [36,45,136,137]. The TDCB geometry (see Table 7.1), consisting of aluminium alloy beams bonded with an epoxy adhesive, has been widely employed and the commonly used experimental technique has been to place this specimen under an applied load in the environment of interest and monitor the rate and locus of crack growth. The typical observation is that the cohesive in adhesive starter crack does not... 

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