An experimental investigation on mode I delamination of z-pinned double-cantilever-beams (DCB) and corresponding pin pullout behaviour is presented. The effects of loading rate on delamination crack opening and z-pin bridging mechanisms is reported. Optical micrographs of z-pins after pullout show that a higher loading rate causes more splitting damage in the pins. Comparison of fracture load shows that the fracture load rises with increasing loading rate. Z-pin pullout tests were also conducted to give a detailed description of z-pin bridging mechanisms

Liu, H. Y.

Mai, Y. W.atr

Yan, W.

Yu, X. Y.

University of Queensland eSpace

SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
Experimental Study On Z-Pinned DCB Mode I Delamination 
Hong-Yuan Liu1,   Wenyi Yan2,   Xiao-Yan Yu1  and  Yiu-Wing Mai1  
1Centre for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic 
Engineering J07, University of Sydney, Sydney, NSW 2006, Australia 
2Computational Engineering Research Centre, Faculty of Engineering and Surveying, University of Southern 
Queensland, Toowoomba, QLD 4350, Australia 
 
 
 
ABSTRACT:  An experimental investigation on mode I delamination of z-pinned double-
cantilever-beams (DCB) and corresponding pin pullout behaviour is presented. The effects of 
loading rate on delamination crack opening and z-pin bridging mechanisms is reported. Optical 
micrographs of z-pins after pullout show that a higher loading rate causes more splitting damage in 
the pins. Comparison of fracture load shows that the fracture load rises with increasing loading rate. 
Z-pin pullout tests were also conducted to give a detailed description of z-pin bridging mechanisms. 
 
1. INTRODUCTION 
Z-pin, also called Z-FiberTM, (see Aztex website) is a novel technique which was developed to 
increase the strength of laminated composites in the thickness direction, that is, z-direction. Recent 
studies have confirmed that z-pinning greatly improves the fracture toughness of reinforced 
composites against interlaminar delamination. Its performance under typical Mode I, Mode II and 
mixed Mode I/II loading were studied experimentally and numerically [Cartie, 2000; Yan et al 
2003; Yan et al, 2004]. Fig. 1 shows a double-cantilever-beam (DCB) specimen with z-pin 
reinforcement. During Mode I delamination, a reinforcing z-pin provides a closure force to the 
opening crack. The efficiency of z-pinning is strongly dependent on the corresponding bridging 
mechanisms [Liu et al, 2003]. The functional relationship between delamination crack-opening and 
closure force from a single pin is called the “bridging law”, which can be evaluated in principle by a 
z-pin pullout test [Dai et al, 2004]. As reported in earlier studies, the pullout behaviour of a single 
fibre can be affected by the loading rate [Liu et al, 1999]. A high pullout rate can reduce both 
debonding force and friction coefficient [Chai et al, 1996; Povirk et al, 1993]. With rapid expansion 
of applications of composites, composite structures often face rather complex in-service conditions, 
one of which is the effect of loading rate. The bridging mechanisms of z-pin reinforcement under 
different loading rates must hence be critically evaluated before any further development of this 
new technique can be advanced. But, to-date, no rigorous theoretical models and no experiments on 
this special topic have been published. 
 This paper presents an experimental study on the effects of loading rate on z-pinned DCB mode I 
delamination fractures. Loading rates of 1 and 100 mm/min were chosen and the corresponding 
load-displacement curves were obtained. Optical photos of z-pin microstructure after pullout were 
provided to examine the z-pin damage under different loading rates. Tests were carried out for two 
types of samples: small pin reinforcement with an areal density of 0.5% and big pin reinforcement 
with an areal density of 2%. The results show that loading rate has a noticeable effect not only on 
the pullout/fracture load but also on the failure mechanisms. Experimental results of z-pin pullout 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
under the same loading rates were also provided to confirm the conclusion obtained from the DCB 
tests. 
 
 
 
 
 
 
 
Figure 1. DCB specimen with z-pinning reinforcement. 
2. EXPERIMENTAL PROCEDURE AND RESULTS 
2.1 Z-pinned DCB tests 
The experimental configuration for the z-pinned DCB mode I test is shown in Figure 1, in which the 
z-pins were made of carbon fibre (T300) reinforced BMI resin. The laminated beams were made of 
carbon fibre (IMS) reinforced epoxy (924) with dimensions: 150mm in length, 20 mm in width and 
1.5 mm in thickness. The z-pins were vertically inserted into the beams by an ultrasonic insertion 
machine before curing [Cartie, 2000]. A thermal insulation film with a length of 50 mm was 
inserted between the upper and lower beams to create an initial crack between them. Two T-shaped 
tabs were glued to the top and bottom surfaces of the laminates and were firmly gripped for testing 
in an Instron 5567 universal machine at crosshead speeds (V) of either 1 or 100 mm/min. Load-
displacement traces were recorded until the delamination crack propagated along the full length of 
the beams. In all samples, the first column of z-pins was located at 5 mm away from the initial 
delamination tip and the length of pinned region was 25 mm. Two types of samples were tested. 
Figure 2(a) shows load-displacement curves of small pin reinforced DCB delamination, in which 
the z-pin diameter, d, is 0.28 mm and the areal density, D, is 0.5% (7 columns×6 rows). It is shown 
that at a higher loading rate, a larger applied load was needed to spread the delamination. The 
results of big pin reinforced DCB delamination tests are shown in Figure 2(b), where the laminated 
beams were reinforced by 8 columns×6 rows big pins (d=0.51 mm), that is D=2%. It can be seen 
that the results agree with those in Fig. 2(a) from small pin tests, i.e., they confirm the effect of 
loading rate on the delamination crack opening.  
 
 
 
 
 
 
 
 
 
     (a)               (b) 
     (a)                (b) 
Figure 2. Load-displacement curve of mode I DCB tests under loading rates of V=1 mm/min and V=100 
mm/min, in which (a) d=0.28 mm, D=0.5% and (b) d=0.51 mm, D=2%. 
0
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Z-pin bridged area
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
 
 
 
 
 
 
 
 
 
 
 
          (a)                    (b) 
Figure 3. Optical photos of z-pin ends after DCB delamination tests in which d=0.51 mm, D=2%; (a) V=1 
mm/min and (b) V=100 mm/min. Splitting along length of pin due to shear failure is apparent at the high 
loading rate. Magnification is 50x. 
 
 Results from the above tests seem to be inconsistent with previous studies [Liu et al, 1999; Chai 
et al, 1996; Povirk et al, 1993] in which it was predicted that the pullout force of a single fibre 
would decrease with increasing loading rate. To understand loading rate effect on the bridging 
mechanisms of z-pins, optical photomicrographs of typical z-pin ends after pullout were taken with 
a WILD Heerbrugg microscope, as shown in Figure 3, for the two different loading rates and 
d=0.51 mm, D=2%. It can be seen that when the loading rate is low, the z-pin pulls out without any 
obvious damage. However, when the loading rate is high, the z-pin is pulled out accompanied by a 
number of splits along the length of the pin, which may be caused by the high rate shearing during 
both pullout and bending of the beams. At a high loading rate, the cross-head displacement of the 
beam was increased at a very high rate which was 100 times higher than that at the low loading rate 
test. However, the delamination crack could not propagate at the same rate due to the resistance 
imposed by the z-pins. At a given crack length, the curvature of the beam under a high loading rate 
was hence larger than that under a low loading rate. That means, z-pins under a high loading rate 
suffered more severe bending before being pullout which caused splitting damage in the z-pins as 
shown in Figure 3(b). Consequently, the resistance to bending of z-pins delayed the delamination 
propagation. A higher applied load was required for further crack growth. In the meantime, when 
the pins were bent, its embedded length applied an additional pressure to the laminates as a reaction 
to the bending. This pressure increased the “snubbing” friction against the pin pullout. As reported 
by Liu et al [Liu et al, 2003], z-pin bridging force due to frictional pullout contributes significantly 
to the delamination resistance. As shown in Figure 2, a higher load is needed for crack growth when 
the loading rate is higher.  
 
 
 
 
 
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
2.2 Z-pin pullout tests 
 
 
 
 
 
 
 
 
 
 
 
Figure 4. (a) Illustration of experimental configuration for 3×3 z-pins pull-out tests; (b) a simplified tri-linear 
bridging law in z-pinned DCB mode I delamination [Liu et al, 2003]. 
 
  Z-pin pullout tests as shown in Figure 4(a) were carried out to study the relationship between the 
bridging force and the pullout displacement under different loading rates. Details of experimental 
procedure and samples were given in our earlier paper [Dai et al, 2004]. Generally, a complete pull-
out procedure includes three stages, which can be described by a tri-linear bridging law as shown in 
Figure 4(b). The bridging law can be characterised by four parameters: maximum debonding force 
Pd, maximum frictional force Pf and corresponding displacements δ1, δ2 [Liu et al, 2003]. In the first 
stage, with increasing applied displacement, the pullout force increases rapidly until it reaches a 
peak value, Pd. Then, in the second stage, the pin debonds from the laminates and the load drops 
very rapidly to a value Pf, with a very small increase of displacement. In the third stage, the load 
gradually reduces to zero with frictional pullout. Figure 5(a) shows the results of maximum 
debonding force Pd and maximum frictional force Pf of 3×3 small pins (d=0.28 mm) pullout tests. 
The pin-to-pin distance is 3.51 mm that represents D=0.5%. The loading rates were set at 1 and 100 
mm/min. The values given in Figure 5 are the total force applied to the nine pins. It can be seen that 
under a high loading rate, the debonding force is reduced. It indicates that the adhesive strength is 
significantly degraded by the loading rate, which is consistent with Chai et al’s results [Chai et al, 
1989, Liu et al, 1999]. However, in a z-pin pullout test, nine pins were inserted into two laminates 
before curing. During the insertion, a small amount of misalignment always occurred in some pins, 
(i.e., some pins were not perfectly perpendicular to the laminates). When the pullout load was 
applied to the laminates, these misaligned pins were bent in addition to being pulled by tension. As 
we discussed in the previous section, a high rate load caused a larger bending on the z-pins. 
Therefore, the resistance from the bent pins and also the corresponding “snubbing” friction were 
larger when the loading rate was higher. Figure 5(a) shows the maximum frictional force to increase 
with increasing loading rate. This result confirms the DCB tests results given in Figure 2(a).  
 
  The results of big pins pullout are given in Figure 5(b), in which 3×3 big z-pins (d=0.51 mm) were 
used. The pin-to-pin distance is 3.10 mm that represents D=2%. The loading rates were also set at 1 
and 100 mm/min. The load-displacement curves were taken until the pins were completely pulled 
out. Since the load-drop due to debonding rarely appeared in the big pin pullout tests [Dai et al, 
 
z-pins 
laminates 
T-shaped tab 
displacement δ 
pu
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ou
t f
or
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  P
 
(δ 2, P f )
(δ 1, P d )
h0
pu
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ou
t f
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  P
 
pu
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t f
or
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  P
 
pu
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  P
 
SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
2004], only the results of maximum frictional load are shown. It is also proven that the friction load 
increases with rising loading rate. These results are in agreement with those of DCB tests shown in 
Fig. 2(b) and the optical photos shown in Figure 3. 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5. (a) Maximum debonding force and maximum frictional force of 3×3 small pin pullout, in which, 
d=0.28 mm and D=0.5%; and (b) maximum frictional pullout force of 3×3 big pin pullout, in which, d=0.51 
mm and D=2%.  
 
 
3. CONCLUSION 
Mode I delamination tests on z-pinned DCB laminates were performed to evaluate the effects of 
loading rate on the z-pin bridging mechanisms. It is found that under different loading rates, the 
bridging mechanisms are different. Under a high loading rate, the z-pins experience both tension 
and bending due to a rapid increase of DCB curvature. As a result, the pins were always pulled out 
accompanied by serious splitting along their lengths due to shear failure. Moreover, bending of the 
pins against the laminates provided an additional “snubbing” friction to deter delamination growth. 
Thus, the fracture load increased when the loading rate was high. Results of pullout tests showed 
that the adhesive strength between z-pins and laminates was degraded by loading rate. As a result, 
the debonding force was significantly decreased when the loading rate was increased. But a higher 
loading rate caused more splitting damage inside the pin and an additional snubbing friction against 
the laminates so that the bridging force was increased. 
 
  It should be noted that in this study only two rates were considered: 1 and 100 mm/min. Under 
these rates, the kinetic energy effect on crack-opening was small compared to that of the strain 
energy and can be ignored. At higher loading rates, considerable increase of kinetic energy may 
weaken the effect of interfacial friction on the delamination behaviour and accelerate delamination 
growth. In contrast, a very low pullout rate may cause stick-slips during z-pin pullout [Povirk et al, 
1993], which may cause an extra resistance to delamination growth. To provide a complete study of 
loading rate effects on z-pin bridging mechanisms, further work must be conducted.  
 
 
0
100
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SIF2004 Structural Integrity and Fracture. http://eprint.uq.edu.au/archive/00000836 
 
ACKNOWLEDGEMENTS 
The authors would like to thank the Australian Research Council (ARC) for the continuing support 
of this project by a Federation Fellowship and a Research Fellowship to Y-WM and H-YL, 
respectively, both tenable at the University of Sydney. Professor Ivana Partridge and Dr Denis 
Cartié of Cranfield University kindly provided all the z-pinned composite samples for testing in this 
study. Their assistance is most appreciated.  
 
REFERENCES 
Cartie, D D R, Effect of Z-FibreTM on the Delamination Behaviour Carbon Fibre/Epoxy Laminates. PhD 
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Dai, S-C, Yan, W, Liu, H-Y and Mai, Y-W, Composites Science and Technology, accepted, 2004. 
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Liu, H-Y, Yan, W and Mai, Y-W. Z-fibre bridging stress in composite delamination. In: Fracture of 
Polymers, Composites and Adhesives II, ESIS Publication 32. Editors: B. R. K. Blackman, A. Pavan and J. 
G. Williams, (2003) 491-502. 
Liu, H-Y, Zhang, X, Mai, Y-W and Diao, X-X. On steady-state fibre pull-out part II: computer simulation. 
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Materials and Technology, 115, 286-291 (1993). 
Yan, W, Liu, H-Y and Mai, Y-W, Numerical study on the mode I delamination toughness of z-pinned 
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Mode II delamination toughness of z-pinned laminates, Composites Science and Technology, in press (2004). 


Experimental Study On Z-Pinned DCB Mode I Delamination

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Experimental Study On Z-Pinned DCB Mode I Delamination

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