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CHAPTER 6
Conclusions
6.1 Conclusions
• The use of a 2% z-pinning density is preferable to 4% for reinforcement in
the manner carried out in these tests. This is quite simply due to a lesser reduction in laminate cross sectional area (which heavily influences the tensile strength) due to the presence of the z-pins and a lesser amount of load bearing fibre disturbance.
• On the whole, these results show no ‘conclusive’ improvement in the tensile load capability of the joint through the use of z-pin reinforcement. It could be argued that z-pins offer a small load capability improvement in the 10 – 30 mm overlap length range however confirmation of this would require further tests.
• This work has shown that the failure mode can be altered, and designed for, through the use of z-pins. The failure mode change is a real difference but it is hard to quantify whether delamination is more favourable than
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failure of the laminate or visa-versa, because ultimately the strength is limited by the design of the structure. For an aerospace structure, a failure mode which is the least catastrophic would probably be the more favourable. It is important also to understand that the failure modes are highly ‘structure specific’. The delamination / laminate failure transition threshold for z-pin reinforcement stated in this work should not be directly transferred to alternative structures without further testing.
• It is seen that the presence of the Z-pin block in the impacted I-sections limits the transverse extent of the impact induced delamination resulting in a limitated impact damaged area. Consequently there is no reduction in post impact tensile strength, similar to the knock-down factor seen here and in the literature for standard tensile strength due to the insertion of the pins. This absence of a knock down in post impacted tensile strength suggests that the addition of z-pins into this type of joint improves the damage tolerance of the structure.
• It is believed that future development can improve these joints still further. There are steps that can be taken which may offer greater energy absorption whilst maintaining the highest load capability like a more intelligent placement of individual z-pins and an improved layout of the patch.
