Abstract
The adoption of continuously reinforced titanium matrix composites (TMCs) for lightweight components in aerospace applications is currently limited, in part, by a lack of understanding of manufacturing flaws observed in the literature. This study examines the role of groups of broken monofilaments that might be induced during the manufacture of TMCs due to its prevalence in other manufacturing processes.
To quantify the impact of broken monofilament on performance a method of introducing seeded defects was developed. Samples with broken monofilaments were uniaxially tensile tested with 2D digital image correlation (DIC) for strain mapping. The specimens with seeded defects were found to exhibit similar tensile behaviour to that of the non-defect control specimens. DIC indicated that damage occurred (i) near the seeded breaks, (ii) at the coupon edges, and (iii) apparently randomly within the sample. Metallography of tested samples confirmed the three locations of damage clusters corresponding to DIC measurements. Damage initiation at multiple sites indicates that the defect's impact on tensile performance is limited.
Tension-tension fatigue at σmax = 950 MPa, R=0.1, and a frequency of 3 Hz was conducted on specimens with seeded defects. The seeded defects were not the preferred sites for fatigue crack initiation, instead the coupon edges and surface pits acted as the preferred initiation sites.
Microstructural characterisation of Ti-3Al-2.5V was conducted using EBSD. Comparison between composite and monolithic samples showed broadly similar texture, grain size, and grain shape. A moderate transverse texture was observed in both the monolithic material and the composite matrix which could result in slight anisotropy.
FEA was conducted to provide an understanding of the stress redistribution due to the induced broken monofilament defects. The neighbouring intact monofilaments see an 11% increase in stress. This differs significantly from experimental findings, reported in the literature, for single-ply composites where the stress redistribution resulted in an increase of 40% on the nearest neighbour (Mcdonald et al. 2003).
In conclusion, the use of continuous reinforced TMCs are likely to be tolerant of broken monofilaments in multi-ply layup under both quasi-static and fatigue loading. Further research is required to further validate and extend these findings to other matrices and composite architectures.