Damage tolerance of composite toecap

Composite Structures 67 (2005) 167–174

www.elsevier.com/locate/compstruct

Damage tolerance of composite toecap

Seung Min Lee, Tae Seong Lim, Dai Gil Lee *

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Mechanical Design Laboratory

with Advanced Material, ME3221, 373-1, Guseong-dong, Yuseong-gu, Daejeon-shi, 305-701, Republic of Korea

Available online 5 November 2004

Abstract

In this study, the glass fiber polyester composite toecap for safety shoes was designed and manufactured to increase the energy

absorption capacity during impact and to reduce the weight of steel toecap. The static compression and drop weight impact tests of

the composite toecap were performed with respect to stacking sequence, and the damage after impact was measured by macrography

and CAI (Compression after impact) test. From the experimental results, it was found that the stacking sequence and fiber types of

the composite toecaps had much influence on the static stiffness and impact damage of the toecap. The weight saving of the com-

posite toecap was about 40% compared with the steel toecap of comparable static and impact characteristics.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Safety shoes; Toecap; CAI (Compression after impact); Impact energy absorption capability

1. Introduction

Since fiber reinforced composites have both high spe-

cific stiffness (modulus/density) and specific strength(strength/density), they have been widely used in light-

weight structures such as robots, machine tools, and

passenger cars [1–4]. Especially, glass fiber reinforced

polymeric composites have been used for structures sub-

jected to static and dynamic loads such as bumpers and

impact beams because of their high impact energy

absorption characteristics and impact damage tolerance.

Nowadays the impact characteristics of glass fiber rein-forced composite materials are being studied much for

the application to structures subjected to impact loads.

Toecaps are installed in the front parts of safety shoes

to protect a worker�s toes from external static and im-

pact loads. The impact energy absorption capability of

conventional steel toecap decreases much when the serv-

0263-8223/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compstruct.2004.09.009

* Corresponding author. Tel.: +82 42 869 3221; fax: +82 42 869

5221.

E-mail address: [email protected] (D.G. Lee).

URL: http://www.scs.kaist.ac.kr.

ice temperature is below nil ductility temperature of the

steel, and the heavy weight of toecap may increase the

worker�s fatigue. Also, safety shoes with steel toecaps

can not pass a security checkpoint without alarmingbells. The best way to overcome these drawbacks of

the steel toecap without sacrificing safety is to employ

glass fiber reinforced composite materials for the toecap

material because of their high specific strength and im-

pact energy absorption characteristics [5–8].

The damage tolerance of composite materials should

be considered carefully to apply them to the toecap, be-

cause the low-velocity impact on the toecap may reducethe residual strength of the composite material of the

toecap, even when the damage due to the low velocity

impact is not detectable [9–11].

In this study, a composite toecap for safety shoes was

developed using glass fiber reinforced polyester compos-

ite. To determine the thickness of the toecap, the finite

element analyses of the steel and the composite toecaps

under static and impact loads were performed. Also thestatic and impact tests of the plate type composite spec-

imens were performed to determine the optimum design

parameters of the composite toecap, such as fiber types,

mailto:[email protected]

168 S.M. Lee et al. / Composite Structures 67 (2005) 167–174

stacking sequences, and stacking angles. Then the com-

posite toecaps with different stacking sequences and

materials were fabricated and tested under static and im-

pact loads to estimate the impact load capability. Also

the variation of the flexural stiffness of the toecap with

respect to the number of impact subjected was investi-gated to evaluate the damage tolerance of the composite

toecap.

2. Design parameters for the composite toecap

Fig. 1 shows the schematic configurations of the

standard test methods of safety shoes required by theindustrial standard. In the static test, a static load is ap-

plied to the top surface of the toecap by the cylinder of

75mm diameter. The height of wet clay (hclay) should

not be reduced more than 15mm under the load of

10.8kN, and the height of the toecap (hcap) whose orig-

inal height is larger than 33mm should not be smaller

than 22mm after unloading. In the impact test, the imp-

actor of 3mm nose radius with the mass of 23 ± 0.5kg isdropped at the height of 300mm. The height of clay and

Fig. 1. Schematic drawings of the industrial standard test methods for

safety shoes: (a) static test and (b) impact test.

the toecap should also satisfy the above mentioned lim-

it-values of the static standard.

The finite element analyses of the composite toecap

under static and dynamic loads were performed using

ABAQUS Standard and ABAQUS Explicit to deter-

mine the required thickness of the composite toecap.The stress distribution in the steel toecap was also ob-

tained and compared with that of the composite toecap.

The loading cylinder and the impactor were assumed to

be rigid and the toecap was modeled using the eight-

node three dimensional elements of ABAQUS. The fric-

tion coefficient between the toecap and the base plate of

Fig. 2 was assumed to be 0.3.2. For the preliminary

check of feasibility of the composite toecap, the proper-ties of steel and composite toecap were assumed to be

isotropic.

From the static and impact FE analysis results of the

steel toecap as shown in Fig. 3(a), tensile stresses oc-

curred at the front edge of the steel toecap, while com-

pressive stresses occurred at the top surface of the steel

toecap, which caused plastic deformation. In the case

of the composite toecap as shown in Fig. 3(b), the upperpart would be failed by the delamination and fiber

breakage, compared to the strength of the composite

material used. Also peak tensile and compressive stress

concentrations occurred at the front edge of toecap,

which affected much the overall deflection and damage

of toecap. The deflection of composite toecap of

Fig. 2. Finite element modeling: (a) static analysis and (b) impact

analysis.

Table 2

Experimental results of static test using flat plate specimens

Specimen

type

Thickness

(mm)

Fiber volume

fraction (%)

Stiffness

(MN/m)

A-1 [02/chopped2]S 3.2 60 0.71

A-2 [chopped2/02]S 3.2 60 0.32

A-3 [chopped/0/chopped/0]S 3.2 60 0.55

B-1 [(0/90)2/chopped2]S 3.0 54 0.29

B-2 [chopped2/(0/90)2]S 3.0 54 0.17

B-3 [chopped/(0/90)/

chopped/(0/90)]S

3.0 54 0.22

C-1 [±452/chopped2]S 3.0 54 0.11

C-2 [chopped2/±452]S 3.0 54 0.20

C-3 [chopped/±45/

chopped/±45]S

3.0 54 0.16

Fig. 3. Stress distributions of toecaps from finite element analysis: (a) steel toecap and (b) composite toecap.

S.M. Lee et al. / Composite Structures 67 (2005) 167–174 169

3.2mm thickness was similar to that of the steel toecap

of 1.7mm.

In order to obtain the optimum stacking sequence of

the composite toecap, static and impact tests were per-formed using flat plate type specimens rather than

curved toecap specimen for the easy preparation of spec-

imens. Four different ply types of E-glass fibers were

selected as shown in Table 1.

The specimens were fabricated by impregnating poly-

ester resin into the plies and curing in an autoclave by the

vacuumbag degassingmethod at 80 �C for 2h. The length

and width of specimens were 80mm and 30mm, respec-tively. Two loading cylinders of 6mm diameter whose

span was 60mm were used to support the specimen. In

the static test, the load was applied to the specimen by a

cylinder of 6mm diameter through Instron 4469 machine

to the midpoint of plate specimen, from which the load

versus deflection curves were obtained.

The stiffness of the specimens was in the order of A

type > B type > C type as shown in Table 2. The load–displacement curves for A and B type specimens had

abrupt drops from the peak points due to the shear fail-

Table 1

Specifications of E-glass fiber plies (DONG-IL HIMAX, Korea)

Product

Name

Total mass

(kg/m2)

Fiber

orientation

Fiber mass

(kg/m2)

T800-E06 0.821 0 0.813

DB600-E06 0.610 +45/�45 0.301/0.301

DB600-E06 0.610 0/90 0.301/0.301

Chopped mat 0.430 – 0.430

ure and compression failure, respectively. However, the

load for C type specimen increased and decreased grad-ually as shown in Fig. 4.

In the impact test, the impactor of 2.7kg was dropped

at the height of 300mm, and the initial impact speed and

force history data were measured by the photoelectric

sensor (E32-T11L and E3X-F21, Omron, Japan) and

the force transducer (PCB234B, PCB, USA), respec-

tively. Fig. 5 shows the schematic diagram and the pho-

tograph of the drop weight impact tester.During the impact tests, the signals from the photoe-

lectric sensor and the force transducer were obtained as

shown in Fig. 6. The initial speed of the impactor before

impact can be calculated from the time interval and the

distance between the photo detection bars considering

the gravity effect. From the measured force with the

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30 35 40 45 50 55

Time (msec)

Vol

tage

(V

)

Photo (V)

Force transducer(V)

t1 t2

Fig. 6. Signal from the photoelectric sensor and the force transducer.

0

0.5

1

1.5

2

2.5

0 2 4 6 8 10 12 14 16 18 20

Displacement (mm)

Fo

rce

(kN

)

A

B

C

Shear failure

Compression failure

Fig. 4. Static test result of the flat plate specimens.


force transducer, the acceleration, the speed and the dis-

placement of the tup, and the impact energy can be cal-

culated as follows:

a ¼ F =mþ ag; ð1Þ

v ¼Z t2

t1

adt þ vinitial; ð2Þ

s ¼Z t2

t1

vdt; ð3Þ

E ¼Z t2

t1

Fds ¼Z t2

t1

Fvdt; ð4Þ

where

F: measured force (negative value)

m: mass of the impactor

Fig. 5. Drop weight impact tester for toecap: (a

ag: tup acceleration due to gravity and friction in

the guide

a: acceleration of the impactor

v: speed of the impactor

s: displacement of the impactor

E: energy

From the impact test results using the plate type spec-

imens as shown in Table 3, it was found that the shear

failure occurred when the chopped strand mat was

placed at the mid-plane. The impact energy absorption

capacity was highest when the chopped strand mat

was placed at the impacted surface of the specimen as

shown in Fig. 7.

) schematic diagram and (b) photograph.

Fig. 8. Photograph of the mold assembly.

Fig. 9. Static test results of toecap: (a) Load–displacement curves, (b)

maximum displacements under the applied load of 10.8kN and (c)

photograph after static tests.

Fig. 7. Experimental result of the impact test: (a) force and impact

energy histories and (b) photo of failed specimens.

Table 3

Impact test results of the plate specimens

Specimen

type

Thickness

(mm)

Fiber volume

fraction (%)

Energy

absorption (J)

A-1 3.2 60 21.8

A-2 3.2 60 25.8

A-3 3.2 60 22.5

B-1 3.0 54 14.0

B-2 3.0 54 15.3

B-3 3.0 54 14.9

C-1 3.0 54 16.6

C-2 3.0 54 15.5

C-3 3.0 54 14.6


From the results of the finite element analysis and

static and impact tests with the plate specimens, it wasfound that the stacking sequence, stacking angle and

fabric types influenced much the flexural stiffness and

impact energy absorption capacity of composite struc-

ture. Especially, stacking sequences of A-1, B-1, and

C-1 types were not suitable for fabricating the composite

toecap, because they were failed in the shear failure

mode. Therefore, the composite toecap specimens except

A-1, B-1 and C-1 types were fabricated. Then the staticand dynamic tests were performed with the industrial

test standard for safety shoes.

3. Static and impact test characteristics of composite

toecap

The glass fiber polyester composite toecaps were

manufactured with various stacking sequences and

stacking angles. Fig. 8 shows the mold assembly for

manufacturing the composite toecap.

Fig. 10. Impact test results of steel toecap and composite toecap: (a)

impact force-displacement histories of toecaps, (b) impact energy–time

histories of toecaps, (c) comparisons of maximum displacement and

impact energy absorption capability and (d) photographs of perma-

nent deformation after impact.


Themold set consisted of an outer steel mold and inner

mold made of silicon rubber. After cutting the fiber pre-

form with the determined stacking sequence to the near

net shape, the fiber preform was placed on the outside

of the inner mold. Then the unsaturated polyester resin

was pasted on the preform by a brush, followed by tightlyclosing the inner silicon mold to compact the preform.

The composite performs for toecap were cured at 80 �Cfor 2h in an autoclave by a vacuum degassing method

to remove voids and volatiles generated during the curing

of polyester. The composite toecaps with six different

stacking sequences (A-2, A-3, B-2, B-3, C-2, C-3) were

manufactured and tested. The test results of the compos-

ite toecaps were compared with those of the steel toecap.The static test was performed under the same condi-

tions of the industrial standard test method for toecaps

using the INSTRON 4469. The degree of damage of

toecap was measured by the method of macrography

because the glass fiber polyester composite was translu-

cent. Fig. 9 shows the static test results of the composite

toecap and the steel toecap. From the static test results,

it was found that the displacements of B and C types un-der the applied load of 10.8kN were smaller than that of

the steel toecap. The fiber breakages occurred in the toe-

caps of A and B types, but the C type had the smallest

damage as shown in Fig. 9(c).

The impact test of the toecaps of A, B and C types

were performed through the industrial standard test

method for safety shoes. When the mass and height of

the impactor were 23kg and 300mm, respectively, theinitial speed of the impactor was 2.4m/s. Fig. 10 shows

the impact test results. The deflection and maximum

force of composite toecaps were smaller than that of

steel toecap due to the energy absorption in the initial

stage as shown in Fig. 10(a). The composite toecaps

had a higher impact energy absorption capacity than

that of the steel toecap as shown in Fig. 10(b) and (c)

because the composite toecap absorbed much energythrough the matrix cracking, delamination and fiber

breakage during impact test.

The other advantages of the composite toecap were

the excellent restoring ratio and the weight saving. The

restoring ratio of composite toecap was about 90% with

the weight saving about 40%, while the restoring ratio of

the steel toecap was 56% as shown in Table 4.

Table 4

Permanent deformation after impact test and toecap weight

Steel toecap Composite toecap

Maximum displacement 16mm 15mm

Restoring displacement 9mm 13.7mm

Permanent deformation 6.9mm 1.3mm

Restoring ratio 56% 91%

Weight 75g 40g

Fig. 11. Photograph of impact test result.


From the static and impact test results, it was found

that the deflections of B-2, C-2, and C-3 type toecaps

were smaller than that of steel toecap. Also, the impact

energy absorption capacities of these composite toecaps

were higher than that of steel. In the view of damage, the

toecap of C-2 type was excellent for both the static and

impact cases as shown in Fig. 9(c) and Fig. 11.

Fig. 12. Impact test results with respect to number of impact: (a)

impact load–displacement histories of composite toecap, (b) impact

energy–time histories of toecap and (c) photograph of damage with

respect to impact number.

4. Damage tolerance of composite toecap

The impact damage mechanism in composite struc-

tures is a very complex process, because various failure

modes occur simultaneously during impact: fiber break-

age, matrix deformation and cracking, fiber debonding,

fiber pullout, etc. Also, the delamination caused by the

difference of modulus at each ply may occur [12,13].

Considering the previous test results, the composite

toecap of C-2 type was selected for the damage tolerancetest. In this test, the degree of damage and the flexural

stiffness were investigated with respect to the number

of impact (0, 1, 3, 5, 10, 15, 20, 40). The mass of the imp-

actor and drop height were 2.7kg and 800mm (the

height of people waist), respectively. The impact tester

has a catcher to capture the impactor, which prevents

the impactor from impacting the test specimens more

than once after the first impact. As the number of im-pact was increased, the displacement of toecap and the

area of damage increased as shown in Fig. 12, because

the multiple impacts onto the composite toecap weak-

ened the upper surface of toecap, which caused the front

edge of composite toecap to carry impact load largely.

Consequently the force-displacement graph became

narrower as shown in Fig. 12(a). The damage due to

the fiber pull-out and breakage occurred at the frontedge of composite toecaps, where the tensile and com-

pressive stress concentrations occurred.

The composite toecap may still be able to carry some

mechanical load, after damaged on the internal or outer

surface. Therefore, CAI (compression after impact) test

was performed to investigate the equivalent flexural stiff-

ness after impact damage. The ratio of flexural stifness

of damaged composite toecap was determined as

Ratio of flexural stiffness ¼ðEIÞdamage

ðEIÞo¼

ðP=dÞdamage

ðP=dÞo;

ð5Þwhere (P/d)damage: Stiffness of the damaged toecap ob-

tained from compressive test, (P/d)o: Stiffness of the

undamaged toecap obtained from compressive test.

The ratio of flexural stiffness decreased abruptly at

the first impact and then decreased slowly with respect

to the number of impact as shown in Fig. 13.From the CAI tests, it was found that the low impact

energy of 12J did not cause catastrophic fracture in the

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35 40 45Number of impact

Rat

io o

f F

lexu

ral

stif

fnes

s

Fig. 13. Ratio of flexural stiffness with respect to impact number when

the impact energy was 12J.


composite toecap. However, the multiple impacts in-

creased the damage area on which the impact tup hit,

consequently the impact energy absorption capacity

and the ratio of flexural stiffness of composite toecap de-

creased as the number of impact increased.

5. Conclusion

In this study, the composite toecap for safety shoes

was developed using the glass fiber polyester composite.

From the static and impact tests of toecaps, it was found

that the composite toecap with the stacking sequence of

[chopped2/±452]S had better deflection, damage toler-ance, and impact energy absorption capabilities as well

as restoring capability than those of the steel conven-

tional toecap. Also, the composite toecaps not only re-

duced the weight by more than 40% but also had

excellent static and impact characteristics. Conse-

quently, it was concluded that the glass fiber composite

materials had high potential to substitute composite toe-

caps for steel ones.

Acknowledgment

This work has been supported by National Research

Laboratory Project of Ministry of Science and Technol-

ogy and in part by BK21 Project and Kores Co. of Kor-

ea. Their supports are gratefully acknowledged.

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Damage tolerance of composite toecap