=0.05m/s 04 08 0.4 Applied load, N Fig. 11. Mean values of the friction coefficients measured on the various materials for the different experimental conditions Ref. [34]. formation of surface cracks, the laminated composite exhibited for mild wear. Under the severest conditions the consistent lower specific wear than the stress free materials. For a material removal of a plastically deformed surface layer and, ith a given flaw size, the critical condition for the onset of consequently, a more consistent abrasion mechanism triggers cracking at the surface occurs when the stress intensity factor a transition to severe wear. However, as the cracking of the Kl, due to the maximum principal tensile stress(oImax), surface(and consequent removal of grains)initially triggered is greater or equal to the local fracture toughness KIc. this avalanche-type mechanism, debris production was The presence of compressive residual stresses increases the hindered from the outset by high toughness in the case of apparent surface fracture toughness and, as it opposes the stressed material(A/Az). tensile stress generated in the wake of the sliding contact, it prevents the formation and propagation of cracks 5.3. Abrasive wec Even under the severest operating conditions, the wear behaviour of the hybrid laminated composite A/AZ was still Abrasive wear resistance is another aspect of the tribole better than that of the other materials micro- and macro- ical behaviour of materials cracking, followed by limited removal of material, and limited This type of wear can be easily encountered in real systems abrasion by a third body were identified as wear mechanisms where a third body, also constituted by the debris produced 2 25 severe Applied load, Fig. 12. Semi-Log plot of the disc specific wear of the various materials as function of the different experimental conditions Ref. [341formation of surface cracks, the laminated composite exhibited lower specific wear than the stress free materials. For a material with a given flaw size, the critical condition for the onset of cracking at the surface occurs when the stress intensity factor KI, due to the maximum principal tensile stress (s1max), is greater or equal to the local fracture toughness KIc. The presence of compressive residual stresses increases the apparent surface fracture toughness and, as it opposes the tensile stress generated in the wake of the sliding contact, it prevents the formation and propagation of cracks. Even under the severest operating conditions, the wear behaviour of the hybrid laminated composite A/AZ was still better than that of the other materials. Micro- and macro￾cracking, followed by limited removal of material, and limited abrasion by a third body were identified as wear mechanisms for mild wear. Under the severest conditions the consistent removal of a plastically deformed surface layer and, consequently, a more consistent abrasion mechanism triggers a transition to severe wear. However, as the cracking of the surface (and consequent removal of grains) initially triggered this avalanche-type mechanism, debris production was hindered from the outset by high toughness in the case of stressed material (A/AZ). 5.3. Abrasive wear Abrasive wear resistance is another aspect of the tribolo￾gical behaviour of materials. This type of wear can be easily encountered in real systems where a third body, also constituted by the debris produced Fig. 11. Mean values of the friction coefficients measured on the various materials for the different experimental conditions Ref. [34]. Fig. 12. Semi-Log plot of the disc specific wear of the various materials as function of the different experimental conditions Ref. [34]. 564 G. de Portu et al. / Composites: Part B 37 (2006) 556–567