Ioannis K. Giannopoulos and colleagues have presented impact damage characteristics and the compression-after-impact strength of a conceptually applied, fire retardant woven composite laminate. The new findings are published in journal, Composites Part B.
Low velocity impact into woven carbon fiber reinforced plastic CFRP laminates via various damage mechanisms employed to absorb impact in which its damaging effect is shown to be more favorable than the one caused upon similar fiber and matrix unidirectional material system.
However, analytical prediction of impact damage and post impact performance of woven composites laminated structures is more difficult task to perform than unidirectional materials. Despite the fracture mechanisms and failure sequences documents from observations, parametric analytic formulations for predicting the impact performance have not yet attained the maturity level of unidirectional ones. Hence new research method needs to be materialized for improvement of numerical model efficiency and accuracy in order to develop computer based tools for material selection in structural design.
Mode-I and mode-II fracture toughness fracture toughness, GIC and GIC respectively are one of the most important material influencing the impact and compression-after-impact CAI process. They are known to depend on matrix material features such as fiber types, fiber volume fraction, manufacturing process and interphase regions between matrix and fiber etc. Hence, fracture toughness values are simply tested by composite layered specimen and not using method that test purely matrix materials.
Successful testing procedures in order to quantify inter-laminar fracture toughness for unidirectional composites under mode-I and mode-II have been achieved but that isn’t the case of woven fibers as run –arrest type of propagation was experienced most of the time due to split surface morphology. However, test other than unidirectional laminates along major fiber direction are conducted by slightly violating applicability of existing unidirectional testing periods.
Results after impact test showed that a high peak force is expected for thicker laminates, smaller transverse displacement, increased damage tolerance and shear failure under compression-after-impact.
Two layup configurations were put to the test, for assessing the mechanical performance and help in the design decision process. Configuration C1 was a quasi-isotropic layup configuration where configuration C2 was more directional in stiffness and strength. Ultrasonic detected delamination damage for configuration C1 was bigger than those of C2 although maximum diameter was similar at each energy level. It was also evident that bigger damage was incurred into the quasi-isotropic layup C1 for the same amount of impact energy.
Maximum impact force attained from C1 configuration is somewhat larger at least for impact levels of 8 and 15J and it was seen that quasi-isotropic C1 configuration is stiffer than C2 in terms of transverse deflection, hence the stiffer in terms of transverse deflection quasi-isotropic layup resist impact loading more and a bigger damage was inflicted onto it.
Critical threshold values was observed at vicinity of 4.2KN at first load. At a higher impact levels of 8J, response is more or less the same and most probably other damage modes are present besides delamination.
Microscopic images revealed that failure mechanisms for impact energy level below 15J was mainly due to internal delamination and in case of impact energy beyond 15J, more damage modes where observed which confirmed transition region captured at least for configuration C2.
Impacts below 15J, C1 configuration had lower compression-after-impact strength because it softened larger impact damage. Beyond 20-25J mark, compression-after-impact strength values of the two configurations were virtually the same despite C1 specimen exhibiting large impact damage area at higher energies of 35J and 50J.
Overall the testing survey showed that the fracture toughness of the pigmented matrix material with the fire retardant particles exhibited lower toughness values than currently used aerospace grade toughened matrices. Hence the impact and post impact performance was inferior to similar types of woven composites used in aerospace sector. The design decision on the most favorable configuration was the one that showed the smaller percentage in the CAI strength decrease after impact and not the one with the higher performance prior to impact.
This study prove that compression-after-impact strength of a woven carbon fiber reinforced plastic material system presented would be ideal in non-critical, non-primarily loaded structural components whose probable failure during service will not result in loss of the aircraft.
Ioannis K. Giannopoulos1 , Efstathios E. Theotokoglou2,Xiang Zhang1,3. Impact Damage and CAI Strength of a Woven CFRP Material with Fire Retardant Properties. Composites Part B: Engineering, Volume 91, 2016, Pages 8–17.Show Affiliations
- Centre of Excellence for Aeronautics, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, MK43 0AL, UK
- School of Applied Mathematical and Physical Sciences, Department of Mechanics Laboratory of Testing and Materials, The National Technical University of Athens, Zographou Campus, Theocaris Bld., GR-157 73, Athens, Greece
- Faculty of Engineering and Computing, Coventry University, Coventry, CV1 5FB, UK
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