Low-Velocity Impact and Compression after Impact (CAI) Behaviors of Carbon-Aramid/Epoxy Hybrid Braided Composite Laminates

CAO HongxueSUN YingTANG MengyunDING XuCHEN Li

1 School of Textile Science and Engineering,Tiangong University,Tianjin 300387,China 2 Key Laboratory of Advanced Textile Composite Materials of Ministry of Education,Institute of Composite Materials,Tiangong University,Tianjin 300387,China

Abstract:Low-velocity impact and in-plane axial compression after impact (CAI) behaviors of carbon-aramid/epoxy hybrid braided composite laminates were investigated experimentally. The following three different types of carbon-aramid/epoxy hybrid braided composite laminates were produced and tested:(a) inter-hybrid laminates,(b) sandwich-like inter-hybrid laminates,and (c) unsymmetric-hybrid laminates. At the same time,carbon/epoxy braided composite laminates were used for comparisons. Impact properties and impact resistance were studied. Internal damages and damage mechanisms of laminates were detected by ultrasonic C-scan and B-scan methods. The results show that the ductility index (DI) values of three kinds of hybrid laminates aforementioned are 37%,4% and 120% higher than those of carbon/epoxy laminates,respectively. The peak load of inter-hybrid laminates is higher than that of sandwich-like inter-hybrid laminates and unsymmetric-hybrid laminates. The average damage area and dent depths of inter-hybrid laminates are 64% and 69% smaller than those of carbon/epoxy laminates. Those results show that carbon-aramid/epoxy hybrid braided composite laminates could significantly improve the impact properties and toughness of non-hybrid braided composite laminates.

Key words:composite;braided composite;carbon/epoxy;carbon-aramid/epoxy hybrid laminated composite;low-velocity impact;in-plane axial compression;compression after impact (CAI)

Introduction

Braided composites,as with other composites,possess high specific stiffness and strength.At the same time,they provide superior usability,desirable and tailorable stiffness and strength.Braided composites have been used for braided air ducts,jet engine stator vanes,overbraided fuel lines,fan blades and automotive shafts[1],and their application has been explored for orthopedic treatments[2].Recently,Airbus has focused on using carbon fiber braided composites for frames of the A350-1000[3-4].Those exciting developments in the aerospace and engineering structures demonstrate the future of braided composites.The basic threat to the more widespread use of braided composites in the aircraft and automotive primary structure is the problem of residual strength in the presence of impact damage.

To improve toughness and energy absorption while maintaining specific stiffness and strength for braided composites[5-9],many specific problems related to hybrid combinations of carbon and aramid fibers should be considered and addressed.Doreyetal.[10]tested the residual tensile,bending and shear strength of carbon-aramid/epoxy hybrid laminates after drop-weight impact.The results conclude that K/C/K (two Kevlar layers/eight Carbon layers /two Kevlar layers) hybrid laminate is a good compromise.Threshold energy for the onset of damage is up to four times that of carbon/epoxy laminates.Fracture energy is twice that of carbon/epoxy laminates.Zhangetal.[11]studied the impact strength with four hybrid ratios of carbon/aramid hybrid 3D braided reinforced nylon composites.The results reveal that the impact strength of carbon/aramid hybrid 3D braided reinforced nylon composites increases with the increase of aramid fiber volume fraction.Gustinetal.[12]researched the impact and the compression properties of carbon-aramid/epoxy sandwich-like inter-hybrid composites.The results confirm that the addition of aramid to the facesheet improves the maximum absorbed energy and the average maximum impact force by approximately 10% compared to the carbon/epoxy laminates.Wanetal.[13]investigated the impact properties of carbon-aramid/epoxy braided composites with four kinds of hybrid ratios.The absorbed energy and the residual flexural strength increase with Kevlar fiber volume fraction content.Kimetal.[14]studied the hybridization effect of carbon and aramid fibers on the interlaminar fracture toughness and crack propagation behaviors.The results show that the average crack propagation lengths of hybrid composites are 15.7% smaller than those of carbon/epoxy laminates.Baoetal.[15]tested the impact and compression after impact (CAI) properties of carbon-aramid/epoxy hybrid laminates.The results show that the residual strength values of hybrid laminates are 16.5% higher than those of carbon/epoxy laminates.The maximum depth of impact damage pits of hybrid composites is 71.4% smaller than that of carbon/epoxy composites.

Many studies on the impact and CAI behaviors of carbon-aramid/epoxy hybrid laminates have been conducted.Our previous research results revealed that the addition of aramid fibers changed the CAI damage mode of carbon/epoxy laminates from brittle fracture to plastic damage[16-18].This paper would offer references for the compromise optimization of braided composites to meet specific design requirements.

1 Experiments

1.1 Preparation of specimens

In this experiment,2D braided tubes of carbon fiber and aramid fiber were 2×2 2D biaxial regular braided.The fiber bundles in two braided directions crossed once every two bundles,and then were flattened,cut and laminated according to the experimental requirements.Resin transfer molding (RTM) process was used to prepare the required laminates.The curing agent and the accelerant of epoxy system were methyltetrahydrophthalic anhydride and dimethyl benzylamine,respectively.The performance parameters of carbon fiber,aramid fiber and epoxy resin system are given in Table 1.

Table 1 Performance parameters of reinforced fibers and resin system

The single-layer fabric designed for laying was single-carbon braided fabric and the single-aramid braided fabric.The structural parameters of two kinds of single-layer fabrics used in carbon-aramid/epoxy hybrid laminates are shown in Table 2.The lamination mode and structural parameters are shown in Table 3.In Table 3,C represents carbon/epoxy laminate;IE represents inter-hybrid laminate;SA represents sandwich-like inter-hybrid laminate;US represents unsymmetric-hybrid laminate.The coupons with dimensions of 150 mm×100 mm×(5±0.02) mm (length×width×thickness) for testing were cut from the 380 mm×180 mm×(5±0.02) mm (length×width×thickness) rectangular laminates,as shown in Fig.1.

Table 2 Structural parameters of single-layer fabrics

Table 3 Structural parameters of carbon-aramid hybrid braided composite laminates

Fig.1 Cutting diagrams of specimen:(a) before cutting;(b) cutting;(c) after cutting

1.2 Composite tests

1.2.1Low-velocityimpacttest

Low-velocity impact tests on laminates were conducted according to ASTM D7136/D7137M-05 using an Instron Dynatup 9250HV equipped with a 6.5 kg steel hemispherical impactor with a diameter of 12.7 mm (shown in Fig.2).The Instron Dynatup 9250HV is equipped with a device to prevent the secondary impact,and the pneumatic device can prevent the impactor from impacting the specimen for many times.The impact energy was 33.5 J.The carbon layer was chosen as an impact surface in the experiment.According to ASTM D7136/D7137M-05,the thickness of the specimens is 5 mm,and the impact energy formula is

E=CE·t,

(1)

whereEis impact energy (J),CEis impact energy per unit thickness (6.7 J/mm),andtis specimen thickness (mm).

Fig.2 Instron Dynatup 9250HV impact testing machine

1.2.2C-scanandB-scantests

In this experiment,the BSN-C3409 water-immersed ultrasonic scanning detection system (shown in Fig.3) was used to place the test specimen in the water tank.The test specimen was scanned.The reflected wave of each detection point was extracted.After processing the signal by a computer,the ultrasonic C-scan imaging and the ultrasonic B-scan imaging were output.

Fig.3 BSN-C3409 water-immersed ultrasonic scanning detection system

1.2.3In-planeaxialcompressionandCAItests

In-plane axial compression and CAI tests were performed according to ASTM D7136/D7137M-05 using an AG-250kNE testing machine [shown in Fig.4(a)] with a special fixture [shown in Fig.4(b)].And the compression speed of the testing machine was 1.25 mm/min.As the same as the low-velocity impact test,five identical specimens were selected for each type of laminates.

Fig.4 Photographs of a compression test system using digital image correlation (DIC) for (a)AG-250kNE and (b) clamp for holding specimens

2 Results and Discussion

2.1 Low-velocity impact test results

2.1.1Load-timecurves

The load-time curves can well reflect the damage process of the specimen.Typical load-time curves for the four kinds of laminates at the same energy level are shown in Fig.5.In the initial stage of loading,the first oscillation point appears in all laminates.The main damage form is delamination and is accompanied by matrix cracks[19-22].The oscillation points of load-time curves of carbon/epoxy laminates are the most obvious.During the expansion of impact damage,some carbon fibers will suddenly fracture and pull out.At the same time,internal delamination occurs in the carbon/epoxy laminates.A similar result has been obtained in Ref.[23].Due to the good toughness and the high elongation at break of aramid fibers,the damage of hybrid laminates during impact damage expansion is relatively slow.After reaching the peak loadPm,the load-time curve of unsymmetric-hybrid laminates decreases seriously.The load-time curves descending part of inter-hybrid and sandwich-like inter-hybrid laminates are smooth.And there are no serious load failures.It shows that inter-hybrid and sandwich-like inter-hybrid laminates have good impact resistance.A similar result has been obtained in Ref.[24].

Fig.5 Typical impact load-time curves of four kinds of laminates

Pmis an important index to measure the load-bearing capacity of laminates,which represents the maximum force that laminates can bear before they suffer serious damage.And it is related to the initial stiffness of materials[25].ThePmof four kinds of laminates is shown in Fig.6.At the same impact energy,thePmof carbon/epoxy laminates is the highest.And the initial stiffness of carbon/epoxy laminates is the highest from Fig.5.In the three kinds of hybrid laminates,the in-plane stiffness andPmof inter-hybrid laminates are the highest.The in-plane stiffness andPmof unsymmetric-hybrid laminates are the lowest.It can be concluded that inter-hybrid laminates can effectively resist the impact of drop-weight.

Fig.6 Pm of four kinds of laminates

2.1.2Energy-timecurves

The impact energy-time curves (shown in Fig.7) show that the impact absorbed energy of the specimens can be divided into three different regions[26-28].In zone I,the values of absorbed energy are relatively low.The main way of absorbing energy is to produce pits and deformation in thickness direction.In zone II,the energy-time curves increase rapidly at first and then slowly and reach the maximum value with the increase of time,mainly because of the rapid increase of the contact area between the impactor nose and the specimens.In zone III,the curves are basically straight and fall back are not obvious.At this stage,the impactor nose rebound ends and separates from the specimen.The change rule of the impact energy-time curves of the four kinds of laminates is the same,and the slope of the curves is the same as that of the curves in Fig.5.The higher the stiffness is,the faster the absorption energy increases.

Fig.7 Typical impact energy-time curves of four kindsof laminates

The toughness of laminates is also one of the important parameters to evaluate their mechanical properties.In order to evaluate the toughness of laminates,two dimensionless parameters [ductility index (DI) value and damage degree] are usually used to analyze the impact energy of laminates.The DI is the ratio of damage propagation energyEpto energy at point of the maximum loadEm,and the damage degree is the ratio of total absorbed energyEtto impact energyE[29].DI value and damage degree of four kinds of laminates are shown in Table 4.The DI value of carbon-aramid/epoxy hybrid laminates is higher than that of carbon/epoxy laminates,because the elongation at break of aramid fibers is higher,and the fiber absorbs more energy when it is stretched and pulled out during impact.In hybrid laminates,although the DI value of unsymmetric-hybrid laminates is the highest,the impact resistance of unsymmetric-hybrid laminates is weaker because of the smallest in-plane stiffness,the smallestPmand the smallestEm,while thePmof inter-hybrid laminates is 2% higher than that of sandwich-like inter-hybrid laminates,and the DI value is 31.5% higher than that of sandwich-like inter-hybrid laminates,showing good impact toughness.

Table 4 Summary of average DI and damage degree for four kinds of laminates

2.1.3Damageevaluationofspecimens

Figure 8 shows the typical damage morphology of laminates after impact.Visible impact craters and resin cracks occur on the front of laminates.The craters are circular.As shown in Fig.8,the average damage area of all specimens can be ranked from large to small in the following order:unsymmetric-hybrid laminates >sandwich-like inter-hybrid laminates >inter-hybrid laminates >carbon/epoxy laminates.The bending stiffness of laminates is the main factor affecting the area of pits.The greater the flexural rigidity of the laminates,the better the ability to resist the drop of the impactor nose,the less the contact area,and the smaller the pit area is.The results show that the bending stiffness of unsymmetric-hybrid laminates is the lowest.The flexural stiffness of carbon/epoxy laminates is relatively high,but there is fiber fracture damage on the impact back.The impact backs of inter-hybrid,sandwich-like inter-hybrid and unsymmetric-hybrid laminates all exhibit slight arching deformation,but there are no matrix cracks and fiber fracture damage.It shows that carbon-aramid/epoxy hybrid laminates obviously improve the fracture toughness of carbon/epoxy laminates,especially that inter-hybrid laminates well balance bending stiffness and fracture toughness.

As shown in Fig.8,dotted arrows indicate the propagation direction and length of cracks.Carbon/epoxy laminates have four directions of cracks.There is no delamination on the side of the laminates.Inter-hybrid laminates have V-shape cracks,and the cracks are long.And there is no delamination on the side of the laminates.This is because carbon fibers and aramid fibers are arranged at intervals and show good synergy.Sandwich-like inter-hybrid laminates have short cracks and serious delamination on the side of the laminates.Unsymmetric-hybrid laminates have the most cracks.And there is delamination on the side of the laminates.

Fig.8 Typical damage morphology photos of four kinds of laminates after impact

Figure 9 shows 3D super-depth photos of four kinds of laminates after impact.There are circular pits on the front of the laminates caused by the contact between the impactor nose and specimens.The pits are accompanied by resin cracks and fiber breakage.Fiber pull-out and fracture occur on the impact back of carbon/epoxy laminates,which are due to the weak shear resistance and the low elongation at break of carbon fibers.There is no visible damage on the impact back of carbon-aramid/epoxy hybrid laminates,but there is a certain degree of arch deformation.This is due to the high elongation at break of aramid fibers,which consumes energy in the process of plastic deformation,delays the occurrence of damage and increases the toughness of specimens.

Fig.9 Typical 3D super-depth photos of four kinds of laminates after impact

The results of C-scan imaging of four kinds of laminates are shown in Fig.10.TheX-axis represents the scanning length of the specimen and theY-axis represents the scanning width of the specimen.The damage area shows dark blue.When encountering material internal defects such as resin-rich zone,mirror reflection may occur.At this time,the scanning color is red or yellow.The uninjured area of the specimen is normal light blue or green.

As shown in Fig.10,the C-scan imaging of the four kinds of laminates is different.It can be seen that the hybrid laminates have a great influence on the internal damage characteristics of impact.Although the external damage morphology of carbon/epoxy laminates is not serious (shown in Figs.8 and 10),serious damage occurs in the interior of the specimens.The area of damage zone of inter-hybrid laminates is about 254.34 mm2,which is much smaller than that of carbon/epoxy laminates(about 706.5 mm2).For sandwich-like inter-hybrid laminates,a wide range of bright yellow and red spots appear on the C-scan imaging (shown in Fig.10),because of serious cracks on the side of the specimen (shown in Fig.8).The internal damage area of unsymmetric-hybrid laminates is the largest.It may be concluded that inter-hybrid laminates could optimize the impact performance of laminates.

Fig.10 Typical ultrasonic C-scan imaging results of four kinds of laminates

In the ultrasonic B-scan imaging (shown in Fig.11),the transverse coordinate (X-axis) of the specimen length and the longitudinal coordinate (Y-axis) of the acoustic wave propagation in the specimen are used to display the image.The results of ultrasonic B-scan imaging along the central axis of the specimen were selected to analyze the cross-section damage.The damage range of carbon/epoxy laminates is shorter in the impact front length (X-axis) direction (about 50 mm),but the thickness (Y-axis) direction is the largest (6.5 mm),which indicates that the specimen has bending deformation and the arch height is 1.5 mm.The damage range of inter-hybrid laminates is 75 mm in the direction of impact front length (X-axis),50% larger than that of carbon/epoxy laminates,and 2 mm in the direction of thickness (Y-axis),69% shorter than that of carbon/epoxy laminates.The signal intensity of the reflected wave on the scanning surface (impact back) is high and uniform.The specimen bends and deforms.The arch height is 1.5 mm,but the slope of the arch is milder than that of the carbon/epoxy laminates.For sandwich-like inter-hybrid laminates with serious stratification,the B-scan imaging shows that the intensity of reflected wave signal is high.This is consistent with the results of C-scan imaging of sandwich-like inter-hybrid laminates in Fig.10.The damage range of unsymmetric-hybrid laminates is 95 mm in the direction of impact front length (X-axis),90% lager than that of carbon/epoxy laminates,and 7.5 mm in the direction of thickness (Y-axis),15.4% lager than that of carbon/epoxy laminates.It shows that the internal damage range of unsymmetric-hybrid laminates is the largest,and the bending deformation is the most serious.This is consistent with the results of the C-scan imaging of unsymmetric-hybrid laminates.And the inter-hybrid laminates have good impact resistance.

Fig.11 Typical ultrasonic B-scan imaging results of four kinds of laminates

2.2 Comparisons of in-plane axial compression and CAI

Figure 12 shows the comparison diagram of in-plane axial compression and CAI load-displacement curves of four kinds of typical laminates.It can be seen from Fig.12 that under the impact energy of 33.5 J,the maximum compression load of the four kinds of laminates decreases to different degrees.Compression load-displacement curves of carbon/epoxy laminates after impact coincide basically with those of non-impact damaged laminates.Both showed a linear rise until sudden damage,and the load value of the curve decreased slightly.After the laminates are impacted,only the maximum compression load and the displacement at the maximum load decrease.It is indicated that the impact damage has a certain influence on the compressive bearing capacity of the carbon/epoxy laminates,but has little influence on the stiffness of the carbon/epoxy laminates.Therefore,it can be determined that the carbon/epoxy laminates are slightly damaged by low-velocity impact without serious internal structural damage.Comparing the CAI load-displacement curves of inter-hybrid and sandwich-like inter-hybrid laminates with their corresponding non-impact damage load-displacement curves,both the maximum load and the displacement at the maximum load decrease in various degrees,and the initial slope of the curve is almost unchanged.However,the load-displacement curves before and after impact increase linearly and then decrease nonlinearly.From the load-displacement curves before and after impact of unsymmetric-hybrid laminates,it can be seen that the maximum compression load decreases.The displacement at the maximum load increases,and the initial slope of the curve decreases,indicating that unsymmetric-hybrid laminates are seriously damaged by low-velocity impact.

Fig.12 Typical in-plane axial compression and CAI load-displacement of four kinds of laminates

Table 5 gives the in-plane axial compressive mechanical properties of the non-impact specimens and the residual compressive mechanical properties of the after impact specimens,where“I”represents in-plane axial compression.The percentage decrease of compressive mechanical properties is obtained.After comparison,it is found that the strength and modulus of the four kinds of after impact laminates all decrease to different degrees compared with the non-impact damage specimens,and the displacement variation trend at the maximum load is different.In terms of the percentage of CAI strength decrease and the percentage of modulus decrease,the carbon/epoxy laminate has the least decrease.It could be said that it is the least sensitive to low-velocity impact damage.Among the three hybrid structural laminates,inter-hybrid laminates have the lowest decrease.In contrast,percentages of the strength and the modulus decrease for sandwich-like inter-hybrid laminates and unsymmetric-hybrid laminates is more than twice as much as inter-hybrid laminates,indicating that inter-hybrid laminates can provide better resistance to elastic deformation and damage.In terms of the percentage of displacement change at the maximum load of CAI,except that the displacement of unsymmetric-hybrid laminates increases by 3.57%,carbon/epoxy laminates,inter-hybrid laminates and sandwich-like inter-hybrid laminates all show different degrees of decline.The displacement at the maximum load decreases,indicating that the resistance of the laminate to elastic deformation decreases after impact.

Table 5 In-plane axial compression and CAI properties of four kinds of laminates

(Table 5 continued)

In the in-plane axial compression and CAI,the DIC measurement system was used to obtain the strain field of the specimen[30].In-plane axial compression failure strain nephogram and failure morphology of four typical laminates are shown in Fig.13.CAI failure strain nephogram and failure morphology of four typical laminates are shown in Fig.14.Epsilon Y represents strain in the direction of in-plane axial compression load.

Fig.13 Typical in-plane axial compression failure strain nephogram and failure morphology of four kinds of laminates

Under CAI and in-plane axial compression test,compression strain and tensile strain appear simultaneously.The difference is that for the in-plane axial compression test,the strain distribution of four kinds of laminates is mainly tensile strain,and that for the CAI test is compression strain.Three kinds of hybrid laminates are mainly compressive strain.

Under the in-plane axial compression test,it can be seen from the failure morphology of four kinds of laminates that cracks along the fiber braiding direction appear at the same position on the front and the back of the specimens,accompanied by failure modes such as fiber fracture,matrix cracking,internal delamination,surface bulge and fiber pull-out.Under the CAI test,it can be seen from the damage morphology of four kinds of laminates that the crack propagates along the fiber braiding direction with the impact point as the center.The back of the specimen bulges and delamination occurs.The back of the carbon/epoxy laminate is damaged seriously.In the hybrid laminates,the damage degree of the inter-hybrid laminates is small,which has strong impact resistance and in-plane axial compression performance.

Fig.14 Typical CAI failure strain nephogram and failure morphology of four kinds of laminates

In conclusion,among the three kinds of hybrid laminates,inter-hybrid laminates have the strongest damage resistance and elastic deformation resistance.That is,they have the strongest impact resistance under low-velocity impact.

3 Conclusions

The low-velocity impact and the in-plane axial CAI behaviors of carbon-aramid/epoxy hybrid braided composite laminates and non-hybrid braided composite laminates have been investigated experimentally.The following conclusions are drawn.

(1) Carbon-aramid/epoxy hybrid braided composite laminates have better impact toughness,and the inter-hybrid laminates have the best hybrid effect.

(2) Hybrid structure has a great influence on the damage characteristics of impact interior.It shows that inter-hybrid laminates can effectively balance the unbalanced bending stress between layers.

(3) By comparing the impact damage morphology,it can be concluded that the damage degree on the front of the specimen is more serious than that on the back of the specimen.In hybrid laminates,inter-hybrid laminates can effectively reduce stress transfer.

(4) The percentage of decrease in compressive strength and modulus after impact is considered.Among the three kinds of hybrid laminates,the decrease in compressive strength and modulus of inter-hybrid laminates is the smallest,indicating that inter-hybrid laminates can provide better resistance to elastic deformation and damage.