RECOMMENDATIONS

An investigation into the tensile properties of selected quasi-isotropic layups have been explored and reported in this research. The conclusions of this work could be used as a basis for future investigations into the strength characteristics of this type of laminates, and non-conventional laminates in general. Based on the preceding statement, the following recommendations are proposed for further work on laminated composites with quasi-isotropic stiffness characteristics:

• More precise/higher fidelity methodologies must be applied during laminate manufacturing. In particular, the layup of fibers at desired orientations can be done with the Advanced Fiber Placement (AFP) technology, which guarantees higher layup accuracy when compared to the Manual layup procedure implemented in this research. In addition, the acceptability of the curing procedure should be validated by checking for possible pores and the quality/uniformity of resin distribution in the cured panels. This can be done by

using the method of Thermogravimetric Analysis (TGA), for checking the fiber volume fractions (FVF) of cured panels; in addition to the visual inspection carried out in this work.

• In addition to the previous recommendation, there is also need to pay close attention to the machining phase of the specimen preparation. In particular, a meticulous method of polishing the coupon edges must be implemented, to ensure that all the coupons are uniformly polished.

• Experimental investigations must be conducted with a higher degree of attention

to perfecting the alignment of test specimens with respect to the loading axis of the test machine. Alignment can be verified by using three or more strain gages to check the difference in strain during loading of a rectangular “alignment coupon” similar to the actual specimens to be investigated, as suggested by in [3]. The possibility of having misaligned grips must also be considered and checked.

• Also, the deformation of test coupons under loading could be monitored with more sophisticated devices. One method is to utilize strain gages that not only provide higher precision than the extensometers used, but also has the advantage of being able to measure deformation in both the longitudinal and transverse axes of the coupons. The method of Digital Image Correlation (DIC) can also be used obtain more reliable measures of strain as each coupon is loaded.

• In the current work, there is no monitoring of the progression of failure in test coupons during the experimental procedure. While it is difficult to experimentally monitor the evolution of failure in laminated composites, modern methodologies such as Acoustic Emission (AE) can be explored for this purpose. This will give

more insight into the similarities and differences in the failure of candidate layups, by providing a higher level of intricacy.

• In order to further distinguish the failure of individual candidate laminate, a post- test evaluation of coupons should be carried out in future test programs. A study of tested coupons will aid in identifying the failure modes and determining the sequence of failure events in the specimens. It will in particular aid in monitoring the initiation of failure at the edges and determining the predominant failure modes and events.

• Further work must be done in order to properly compare the stress states obtained

from the three-dimensional FE analysis of the selected laminates. This is rather a difficult problem because of the fact that at the free edges where they are mostly important, the out-of-plane stresses are singular in nature. There must be some phenomenological basis for selecting the location at which stresses would be extracted for comparison.

• In addition to the above, there must also be a good justification for the measures of stress that would be compared to characterize the relative failure behavior of selected laminates. In the work of Sun [1] which serves as a premise to the current research, the resultants of stress components are implemented. The point stress or average stress approaches used for the analysis of notched laminates could possibly be explored for the stress components predominant to the failure characteristics of selected laminates.

• A comparison of the selected measures of stress, with corresponding material allowables is a logical step towards developing failure criteria for the candidate

laminates presented in the current work. This comparison should focus on the effects of 3D stresses firstly, on delamination and ultimately on the failure of the laminates.

• Logically, the comparisons of stress distributions should be followed by the development of definitive failure criteria and corresponding framework for Progressive Failure Analysis (PFA) of the selected quasi-isotropic laminates; and by extension other types of Non-Conventional Laminates with rectilinear fibers. The developed failure criteria and PFA framework should eventually form the basis for the failure analysis of Steered Fiber Laminates (SFLs).

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APPENDIX A:

DISTRIBUTION OF IN-PLANE STRESSES IN NON-CONVENTIONAL

CANDIDATES

Figure A.1 Distribution of 𝝈_𝑥 in the individual plies of candidate A2

Figure A.2 Distribution of 𝝈𝑦 in the individual plies of candidate

Figure A.3 Distribution of 𝝈_𝑥𝑦 in the individual plies of candidate A2

Figure A.5 Distribution of 𝝈_𝑦 in the individual plies of candidate A3

Figure A.7 Distribution of 𝝈_𝑥 in the individual plies of candidate A5

Figure A.9 Distribution of 𝝈_𝑥𝑦 in the individual plies of candidate A5

Figure A.11 Distribution of 𝝈𝑦 in the individual plies of candidate A6