Z-pin blocks

Standard Mode I Double Cantilever Beam specimens for delamination testing of a unidirectional (UD) IM7/977-2 composite were Z-pinned with two separate blocks of Z-Fiber reinforcement. The reinforced beam configuration was such as to provoke an unstable delamination, propagating between the two Z-pin blocks. Crack resistance curves for these specific geometry specimens of IM7/977-2 indicate that the unstable delamination cracks are arrested by the second Z-pin block, with the crack propagation resistance being dictated primarily by the Z-pinning density within a block. Acoustic emission analysis is used to interpret visual observations and other test data. 

KEYWORDS Z-Fiber , Acoustic Emission (AE), delamination, unstable crack propagation, crack arrest, Z-pin blocks, IM7/977-2... 

Several extra long SO.5/0 specimens were used to find out how far an unstable delarriination would propagate from the back end of the first Z-pin block in a 4 mm thick UD beam of... 

The S2/2 specimens tested at a loading rate of 2 mm/min show a rising R-curve from 4500 J/m to 6500 J/m through the pinned areas. One S2/2 specimen was loaded at 5 mm/min and found to exhibit an apparent R-curve dropping from 4500 J/m to 3500 J/m within the first Z-pin block. At present, it is not clear whether this behaviour has to be expected for this type of specimen, and if confirmed by additional tests, why it occurs. Visual inspection of all S2/2 specimens after the test showed that during tests carried out at 5 mm/min, the Z-pins located in the first block failed in tension, whereas they pulled-out from the specimens tested at a loading speed of 2 mm/min. 

Preliminary trials in complex aerospace type structures suggest that selectively placed blocks of Z-pins, of dimensions similar to those used in the laboratory specimens, have a significant capacity to resist or even stop the propagation of an inter-laminar crack in the structure. The success of even relatively simple FE models in predicting the behaviour of any specific block of Z-pins in the laboratory specimens would seem to indicate that the same approach will be successful in more sophisticated local-global models of structures containing such reinforcement. 

The only significant difference between both measurements occurs when the delamination approaches the end of the first pinned block. There, AE source location shows relatively high activity throughout the remainder of the pinned area and considerable activity between the first and second block of Z-pins. This can, quite likely, be attributed to the large stresses exerted on the few remaining Z-pins. Except for the time when the delamination is approaching the last rows of Z-pins, the width of the dense band of AE source locations remains roughly constant with time. 

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