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Title: An investigation into the post impact and post indentation behavior of auxetic composites
Author: Aziz, Salman
Awarding Body: University of Bolton
Current Institution: University of Bolton
Date of Award: 2016
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Composites have been extensively used in high performance structural applications due their lightweight and better strength to weight ratio. A common defect in angle ply laminates is caused by the low velocity impacts, due to their poor resistance to accidental impact by foreign objects, and also the defect is barely visible. Actually contact zone between the target and the penetrating object is relatively large and the whole structure is affected even well away from the impact point. This type of interaction can generate large delaminations, which can reduce the strength under compressive loading. At present, allowance for the delamination induced strength reduction is given by maintaining the strain limits to the structures that prevent the failure due to delamination. If any damage is found in the structure, it is not easy to repair due to the larger damage area. Carbon auxetic composites can be considered as good candidate materials with special properties required for today’s modern technology. This thesis presents the study of auxetic laminates and their response to low velocity multiple impact events in order to assess the damage behaviour of the laminates. These materials are of great interest because the damage area created is smaller, which does not affect the whole structure as compared to the conventional carbon laminates. As received unidirectional 12k tow fibre reinforced, high performance (IM7/8552), epoxy resin pre-preg, which one of the stiffest fibre matrix systems, was used to prepare 24-layers auxetic and positive Poisson’s ratio laminates. This work focuses on four stacking sequences; all through-thickness auxetic and positive Poisson’s ratio laminates were prepared with [±30]s and [35/-20/25/40/-85/40/25/-45/35/-15/25/40] s angles, respectively. Stacking angles for the in-plane auxetic and positive Poisson’s ratio laminates were designed as [0/15/75/15]s and [0/-70/10/25]s respectively. These laminates were cured by vacuum bagging technique before testing in order to achieve the highest quality specimens. All the tests presented in this work are conducted on 100mm2 squared size specimens, by using a standard 12.7mm steel hemisphere indentor. In this work multiple indentation and impact tests were conducted both at the initial test site and also away from the initial test site to determine the extent of damage zone. The most important conclusions and findings drawn from the experimental results are as follows. From the low velocity multiple impacts, indentation testing, fractography, residual testing and dynamic analysis it can be concluded that through-thickness auxetic laminates are found to be better than the positive Poisson’s ratio laminates, even though they were tested 20mm away from the vicinity of the initial test site. Their confined damage area can prevent the structure from catastrophic failure because the damage is more concentrated at the test site and is easy to repair due to the smaller damage area. A preliminary study into the high velocity impact on the through-thickness laminates and the low velocity impacts on the in-plane laminates was also carried out in order to study their impact response. Here, auxetic and the positive Poisson’s ratio laminates show almost similar damage response to the high velocity impacts. However, auxetic in-plane laminates were found to have better resistant to an impact event as compared to the positive Poisson’s ratio specimens.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available