Quasi-ductility

C/SiC composites show a quasi-ductile fracture behaviour, derived from mechanisms like crack deflection and fibre pullout. Figure 14 shows exemplarily these effects within a C/C-SiC composite. The linear-elastic behaviour of C/SiC is less pronounced than for example SiC/SiC composites due to the inherent microcracks in the matrix which occur during cooling-down from processing to room temperature because of the high thermal mismatch between C-fibres and SiC-matrix. 

Work of fracture is considerably increased when a brittle matrix is reinforced by a system of inclusions in the form of grains or fibres. An internal structure created purposefully transforms a brittle behaviour into a quasi-ductile one, characterized by large deformations and high fracture toughness. This transformation into a composite material is described here on many occasions. 

Previous work indicates that the indentation response of mica glass-ceramic materials is a function of the macroscopic uniaxial compressive yield stress, Y and a strain-hardening characteristic. A range of behavior, from quasi-ductile to nominally brittle, could be attained by simply varying the size and shape of the grains through... 

A. C. Fischer Cripps and B. R. Lawn, Indentation stress strain curves for quasi ductile ceramics, Acta Mater. 44(2), 519 527 (1996). 

The fracture surface of an as-received sample is shown in Figure 2. The fracture surface exhibits the characteristic fiber pull-out typical of quasi-ductile glass matrix composite materials [13,15]. However, inspection of the fracture surfaces reveals that the average pull-out lengths are not uniform across the composite section but depend on the relative orientation of the fibre bundles and the fracture propagation plane. Areas exhibiting fewer fibres are observed when these were oriented parallel to the fracture surface. This behaviour explains qualitatively the lower Kic values determined in this material in comparison to unidirectional fibre reinforced composites, as mentioned above, where all fibres contribute equally to toughening by the pull-out mechanism [13,15]. 

Measured or default values of strengths of existing materials may be used with their mean values for quasi-ductile elements and with partial safety factors equal to unity (unless in case of lower level of reliability of data, point (c) in section Documenting Is (Part of) Designing, when higher factors should be used). 

LGMs of the AT/alumina and AT/ZTA displayed some very interesting properties which include excellent machinability, low thermal expansion coefficient, improved thermal shock resistance, low hardness (about 5 GPa), low Young s modulus (E) (250 GPa) and excellent flaw tolerance [Pratapa, 1997 Pratapa Low, 1998 Skala, 2000 Manurung, 2001], These materials appeared to display a large degree of near-surface quasi-plasticity under the Hertzian or the Vickers indenter which effectively inhibits the formation and propagation of cracks. The ductile behaviour of these materials was... 

Some of the most important early experimental observations were of transitions from the quasi-brittle crazing deformation mode to the ductile shear deformation mechanisms with changes in the experimental conditions, such as temperature and strain rate, as well as in polymer variables, such as polymer backbone architecture, blend composition, crosslinking and physical aging state of the polymer glass. One of the strengths of the model of craze growth outlined above is that it allows one to make sense out of some experimentally observed craze-to-shear transitions that had previously defied explanation . The idea behind this explanation is quite simple One writes an expression for the shear yield stress, viz ... 

The present study is restricted to a quasi-static formulation and focusses on the beginning of crack propagation. Therefore, our attention has been devoted to exploring possible causes for the onset of the brittle to ductile transition for such loading conditions. Therefore, arguments based on inertia effects are not invoked, nor is crack branching at high crack speeds [15]. 

Jic is the critical value of the near-tip quasi-static fracture parameter J, i.e. the crack initiation toughness. As for ductile materials a stable crack growth can occur at J values several times the value J c, the non-dimensional tearing modulus Tr was proposed for defining the crack growth toughness [26] ... 

Also included in Table I are the true fracture stress (of) calculated from the cross section of the fractured specimen, and the fracture strain (ef). The fracture stress dropped significantly, from 171 MPa for LLDPE to 100 MPa and 35 MPa for 10% and 25% PS, respectively. The fracture stress of the 37.5% PS blend was even lower, 8.5 MPa, but was still higher than the oy value of 5.9 MPa, so the blend deformed in a ductile manner. The blend with 50% PS fractured in a quasi-brittle manner at a stress of 7.0 MPa, which was slightly lower than cry for this composition. The large decrease in Of with increasing PS concentration was consistent with debonded PS particles that were not load-bearing during plastic deformation. 

Ductile-to-Quasi-Britde Transition. The transition from Type I to Type II and Type III stress-strain behavior was accompanied by a sharp drop in f. The large decrease in fracture strain with a relatively small change in composition has been called a ductile-to-quasi-brittle transition rather than a ductile-to-brittle transition, because even when the fracture strain is low,... 

The ductile-to-quasi-brittle transition occurs because the true fracture stress, as determined by P, decreases with increasing PS more rapidly than the yield stress, determined by P (Figure 9). The critical PS content (Vf>s) at the ductile-to-quasi-brittle transition can be determined from the condition that the true yield stress is equal to the true fracture stress. From equations 8 and 13,... 

Figure 9. Schematic of the effect of PS content on yield stress (atJ) and true fracture stress (ef) to give the ductile-to-quasi-brittle transition.

The close-up of dimpled areas is shown in Figure 5.61. The dimples are shallow and inclined characteristic of ductile fracture. No sign of cleavage or quasi-cleavage is observed. The absence of river pattern excludes the possibility of brittle fracture. 

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