Crack critical load

Figure 34 shows the critical load of all the samples. For the monolayer samples, Sample 1 has a higher critical load than Sample 2. The multilayers Samples 4, 5, and 6 have higher critical loads than monolayer Samples 1 and 2. Samples 5 and 6 have excellent scratch resistant properties. Only extremely small cracks are found in the scratch tracks of Samples 5 and 6. Therefore, there is no sudden change found in the force and penetration depth curves. Sample 7 has the lowest critical load, similar to the monolayer Sample 2. 

Hard layer and soft layer combined together can reduce the intrinsic stress of the whole coating [17,18,22-27]. Samples 4, 5, and 6 have higher critical load than that of monolayer A and B. For Samples 5 and 6, no obvious crack occurs during the scratch test. Sample 5 has the highest hardness and reduced elastic modulus among the multilayer samples, and the interfaces in Sample 5 also contribute to scratch resistance. So it has the best micromechanical properties here. 

The double torsion test specimen has many advantages over other fracture toughness specimen geometries. Since it is a linear compliance test piece, the crack length is not required in the calculation. The crack propagates at constant velocity which is determined by the crosshead displacement rate. Several readings of the critical load required for crack propagation can be made on each specimen. 

An elastic field, although complex, remains well defined up to critical loading, at which point a cone-shaped crack suddenly develops in the sample. Cracking always starts just beyond the contact edge where surface defects occur and where the stress is highest. 

Once the critical load is exceeded (P > Pc), the crack starts to grow substantially (normally R > 2a, Fig. 6.2.7). For the boundary case of a true cone (i.e., R- 0), crack mechanics becomes independent of the developments in the contact region (i.e., r and Cf°), which is given by the equilibrium equation derived by Roesler (1956)... 

At a certain critical load, the crack spontaneously breaks through to the free surface (the cut-through ), usually with additional crack depth increase D, and changes into a well developed semicircle (solid-line semicircle). This transformation may also be induced in another, stable, manner, e.g., by the action of rosette type stresses around the strain region while loading the indenter before the cut-through . The model given in Fig. 6.2.8 is described by the equation... 

A diagram of the three stages of crack growth up to the critical loading point is given by Fuller et cil. (1980), Fig. 6.2.13. 

Critical loads and critical crack length for some materials calculated from equations (6.2.15)—(6.2.18) (Hagan, 1979)... 

Swain (1978) analysed in detail the critical load inducing crack formation around a scratch in scratch hardness test, and Veldkamp et ah (1978) determined this critical point from the equation... 

The key point in this method is the determination of the critical load where first irreversible cracks or fractures are generated and which therefore indicates the transition from plastic deformation to significant/lasting damages. For that, the normal load is constantly increased and indentation depth and tangential load are simultaneously recorded. The transition from plastic deformation to the fracture range is indicated e.g. by unsteadiness or fluctuations in the detected load flow and the indentation depth. This transition range can also be evaluated by additional... 

Besides impact, fatigue is the most critical loading mechanism for a material, especially under stress cracking environments (Fig. 1). 

Some difficulties also arise for the interpretation of scratch tests carried out at progressively increasing normal load or indentation depth. Figure 3 indicates, for example, that a transition from ductile deformation to brittle cracking can occur when increasing the normal load whilst the contact strain is nominally fixed by the conical indenter angle. This is indeed observed in many polymer systems and the notion of a critical load at the ductile-brittle transition is largely used to characterize the scratch response. This depth... 

Linear Elastic Fracture Mechanics (LEFM) describes the brittle behaviour of a material in term of the critical value of the stress intensity factor at the crack tip, Kq, at the onset of propagation at a critical load value Pc ... 

Nanoscratch tests have been used to simulate the effect of third-body particulate wear debris on component surface scratching during use. The load at which the co-efficient of friction or friction force suddenly increases is identified as the critical load, and is used to evaluate scratch resistance and adhesion strength. The depth-sensing nanoindenter, usually equipped with a conical indenter, can elucidate the mode of failure, whether elastic/plastic deformation, cracking, or delamination. 

The total energy U(a,) available to the specimen for an initial crack length, a, and a critical load, P, is given by... 

We may thus conclude that the fracture process is determined by crack formation and crack propagation. Griffith crack theory is essentially a static conception of critical crack formation. Crack growth, however, also depends on dissipative processes. Below the critical load, crack propagation may advance very slowly. In such a case there is a dissipation of energy due to creep processes. Therefore, fracture is a time-dependent process. This aspect is neglected in the Griffith-Irwin theory of fracture. 

The wear mechanism of multilayer coatings is assumed to occur layer by layer and the cracks are deflected by the interfaces between the sublayers. Besides cutting test the performance of coatings can be tested (at least compared) by scratch tests, in which a critical load is obtained at which the coatings are removed. The critical loads are typically 50-100 N and are influenced by the type of the deposition process. Also the obtained hardness is dependent on the preparation method magnetron sputtered Ti(C, N) layers reach up to about 37GPa [108]. 

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