Effect Of Hygrothermal Ageing On The Mechanical Properties Of CFRP Composites

Low velocity impacts occur at a velocity below 10 m/s.Low-velocity impacts in composite structures have been under study since the beginning of the 1970 s.These impacts are common phenomena observed in normal everyday working situations and in aeronautics,many researchers have attempted to design optimum structures mainly with respect to their weight.The structures may lose up to 50% of their strength when facing low-velocity impact problems which may have resulted due to accidents during manufacturing or maintenance processes.Since current technology used in aerospace is highly dependent on the use of composite materials,predicting their behavior to such events is vital.During usage in their applications,these composite structures will be subjected to a range of environmental conditions.The two most prevalent conditions that these materials must face are temperature and humidity,and more importantly,their combined effect.For this research plan,the effect of low velocity impacts on a CFRP with three distinct types of orientation systems was measured simultaneously and compared to ascertain which orientation shows the best mechanical properties under the given parameters of artificial ageing and low velocity impact.In the aerospace industry,three common CFRP lay-up configurations are generally used,and they are {0/90/45/-45/90/0},{0/-60/-45/90/45/60/0} and {0/45/90/-45/0}.This study focused on these three orientations using T-300 carbon fiber and TDE-85 matrix to form composites.These composite panels were manufactured in panels,cut into the required ASTM standard test specimens and subjected to hygrothermal ageing until moisture saturation at four different combinations of temperature and humidity,which are 60°C/60% RH,60°C/90% RH,90°C/60% RH and finally 90°C/90% RH.The baseline test results showed that for low velocity impacts and the compression after impact tests that orientations with higher quasi isotropic orientations fared better than orientations which were based on a cross ply structure.It could also be ascertained that for bending properties,these same quasi isotropic structures were better suited to handle the stresses in such environments.From the hygrothermal testing,the reaction of each orientation to slightly elevated as well as extreme conditions was observed.It was observed that at lower temperatures and lower humidity levels,the amount of moisture absorbed by each orientation was lower and that as the temperature and humidity levels were increased,the amount of moisture absorbed by the samples also increased.Through the testing,it was seen that the orientation {0/-60/-45/90/45/60/0} showed the least amount of moisture absorption while the orientation {0/90/45/-45/90/0} showed the highest.The last orientation showed absorption behavior which was between the two.The composite structures which use a higher number of cross plies absorb less moisture than the samples which use less.The data provided in this study supports this theory.The low velocity impacts conducted showed that hygrothermal ageing has a negative impact on low velocity impacts.Through the data collected,the pattern that emerges is that the higher the temperature and humidity levels a sample is subjected to,the lower the impact resistance,regardless of the orientation of the sample.However,it was seen that the samples made with {0/-60/-45/90/45/60/0} orientation fared the best and produced the highest resistance to low velocity impacts after hygrothermal ageing in all four environments used.The composite plates also showed indent damage to the strike surface and now damage to the rear end of the plate.However,the samples which were conditioned at extremely high temperatures showed delamination and surface crack propagation.Most of the damage was internal,as was proven by the optical digital microscopy and the SEM analysis.When samples are subjected to hygrothermal ageing,fiber matrix interface degradation causes a loss of mechanical properties which in turn are further weakened when low velocity impacts take place.However,samples with a high quasi isotropic alignment such as {0/-60/-45/90/45/60/0} have shown to perform better than samples with a high cross ply alignment such as {0/90/45/-45/90/0},which showed the lowest residual compressive strength.Tensile and flexural tests were used to consolidate the data which showed a loss in compressive residual strength and low velocity impact resistance.Surprisingly,the orientation {0/90/45/-45/90/0} showed the highest tensile strength and the least percentage loss with ageing while the orientation {0/-60/-45/90/45/60/0} showed the lowest average tensile strength and highest loss in properties.This loss in tensile strength is however,offset by the flexural strength which shows the opposite trend when it comes to orientations.Since during low velocity impacts compressive stresses are more predominant,the flexural strength plays a larger role in maintaining the mechanical properties and therefore the orientations {0/-60/-45/90/45/60/0} shows superior mechanical properties.From the results,it can be seen that both tensile strength and flexural strength are dependent upon composite ply orientation.When it comes to tensile strength,sample with a higher fiber content in the direction of the stress show superior mechanical properties whereas for flexural strength,samples with higher fiber content outside the direction of the applied stress show superior mechanical properties.Finally,through SEM analysis the mechanism of failure was identified.Samples which were aged and the least elevated conditions showed that fracture happened when loaded until failure.These samples showed slight signs of fiber matrix interface failure,with the pullout fibers being short and the matrix debris small and located towards the fracture surface.However,samples which were aged in highly elevated conditions showed very clear signs of fiber matrix interface failure,with long pullout fibers,presence of voids,absence of matrix between fibers and loss of fiber orientation.Correlating these SEM findings with the loss of mechanical properties has lead to the deduction that fiber matrix interface failure is the reason behind the loss of mechanical properties at elevated temperatures.