F15_IA_2018AIndustrial Application Proposal Report
http://support.spring8.or.jp/Report_JSR/PDF_JSR_30B/2018B1789.pdf

2018B1789

BL14B2

Crack Growth in Directly Bonded Carbon Fibre Reinforced Thermoplasticand Nano-Structured Aluminium Monitored by Time-Lapse Synchrotron
X-ray Computed Tomography
Kristine Munk Jespersena,b, Hikaru Abeb, Hiroki Otab, Kei Saitob, Keita Wadab,Atsushi Hosoib,c, Hiroyuki Kawadab,caKanagawa Institute of Industrial Science and Technology (KISTEC), bWaseda University,cKagami Memorial Research Institute for Materials Science and Technology

Abstract
The current study examines the 3D damage mechanisms of an aluminium alloy directly bonded to acarbon reinforced thermoplastic using two different types of surface treatments by time-lapse X-ray
Computed Tomography (CT). Down-sized double cantilever beam specimens are gradually subjected toincreased load by using a simple clamping solution and X-ray CT scans are performed for each load step.
The damage mechanisms along with the crack opening relative to the crack length is observed to differ
significantly for the two considered bonding cases. The presence of a nanostructure on the aluminium surface
is found to divert the crack propagation path and thus strengthening the interface properties of the bond line.
Keywords:

time-lapse X-ray CT, dissimilar bonding, carbon reinforced thermoplastic, crack growth

Background and Objective
The use of carbon fibre reinforced thermoplastics (CFRTPs) for industrial application is increasing,
however it is rarely feasible to replace an entire structure with CFRTPs and thus bonding to other materials
such as aluminium is required. However, it is challenging to join CFRTPs to other materials by adhesives
due to their chemically inactive nature, and mechanical joining adds weight to the structure and introduces
stress concentrations. Alternative bonding methods such as ultrasonic welding [1,2], friction spot joining[3,4], and induction heating [5] among others [6,7] have been proposed, however limitations to the adherend
thickness and limited bonding area still exist. In response to these challenges, the authors proposed analternative bonding technology [8,9] where CFRTP and aluminium can be strongly bonded directly in a hotpress during the manufacturing process by modifying the aluminium surface. The proposed bonding
technology can both be applied for regular joining of parts but can also applied to large areas and therefore
is well suited for mass-production of multi-material parts e.g. for the automotive industry. By allowing thetechnology to be used in industry, it will be possible to use CFRTPs in more applications providing lighter
structures with the possibility for recycling of the materials at the end of the structural life. Nevertheless,further understanding of the strengthening and damage mechanisms is necessary.
Thus, the current work studies the damage progression of a crack in a bonded CFRTP and aluminium
specimen by monitoring the region around the crack tip in three dimensions (3D) using X-ray Computed
Tomography (CT). Visualising the crack progression in 3D makes it possible to obtain an enhanced
understanding of the damage and strengthening mechanisms of the proposed bonding technology. Such anunderstanding is required to establish realistic strength and life-time prediction models, which is necessary
for industrial application. Furthermore, it is possible to visualise strong and weak points in the bond bymonitoring the crack growth, and thus the current work also provides valuable input on how to improve thebonding technology even further.
Experiments
The experiments of the current study monitored the progression of an interface crack between
aluminium and CFRTP by two different bonding methods using time-lapse X-ray CT. The crack progressionof the bonded surface was monitored by carrying out repeated X-ray CT scans of the same region andgradually progress the crack using a simple loading clamp device. The crack length was also monitored by acamera with digital image correlation software to monitor the approximate location of the crack. To be able
to scan a larger region without compromising the resolution, four overlapping scans were carried out per load
step and subsequently stitched into one volume. The time-lapse experiment for each specimen was terminatedwhen the crack-tip had progressed outside field of view of the X-ray CT scan, resulting in a different number

Industrial Application Proposal Report

F15_IA_2018Aof steps for each specimen, since the cracks progressed at different speeds.
Samples: The samples were manufactured as down-sized double cantilever beam (DCB) specimens withthe dimensions as shown in Fig. 1(a). For all the specimens, an A5052 aluminium (0.11%Si, 0.3%Fe,0.03%Cu, 0.06%Mn, 2.57%Mg, 0.22%Cr, 0.03%Zn, 0.02%Ti, 96.66%Al) was bonded to a CFRTP with apolyamide 6 (PA6) matrix using plain woven T300 carbon fibres as reinforcement. First larger specimens
were bonded in a hot press at 300C° for 3 minutes, which then subsequently were cut out into the 5x50mmmini DCB specimens used for the X-ray CT experiments. A polyamide insert film was used to create thepre-crack. Experiments were carried out on a total of four specimens, two specimens using as rolled (AR)A5052 aluminium only subjected to a silane coupling treatment, and two specimens using aluminium witha nanostructured (NS) surface combined with a silane coupling treatment. Holes with internal thread were
drilled in the pre-cracked region of all the specimens to firmly attach the loading clamp (see Fig 1(b)). The
considered specimens were referred to as AR1, AR2, NS1, and NS2. Two load steps were carried out for
AR1, three for AR2, six for NS1, and three for NS2.

Fig. 1. Illustration of (a) specimen dimensions and (b) load clamp principle for all the samples
Measuring conditions: The X-ray CT experiments were carried out on beamline BL14B2 with an X-ray
energy of 30 keV, direct expose interval of 50, rotation step degree of 0.2, camera length of 110mm, andexposure time of 500ms over a 180° rotation. In general, the scan time for the combined four scans was 2.53 hours per load step. Due to time limitations, the experiment for the last specimen was carried out with arotation step degree of 0.5 (lower number of projections). The voxel size for all the X-ray CT scans was 2.86µm.
Results and Discussion
Fig. 2 illustrates the location of the scanned region of a specimen, along with an example of themonitored crack progression for the NS1 specimen. It is seen that the crack path is diverted away from thebonded interface and into the CFRTP resulting in intralaminar failure. In other words, the high strength of
the bond resulted in failure elsewhere, which is generally desired of a joint.

Industrial Application Proposal Report

F15_IA_2018AFig. 2. Example of scanned region on loaded specimen along with crack path for NS1 casePrevious studies using single-lap joint tests to compare the AR and NS cases obtained similar bondstrengths [8], since the bond for both cases was sufficiently strong leading failure in the matrix rather thanthe bond. However, a clear difference is observed in the opening mode of the crack examined in the current
study using the mini DCB specimen s. Fig. 3 compares the 2D views of the reconstructed data of the same
region for three of the load steps of the AR1 (Fig. 3(a)) and NS1 (Fig. 3(b)) cases. It can be seen that theopening of the specimen relative to the crack length is significantly smaller for the AR case than for NS.
Thus, it takes significantly higher force to progress the crack of the NS case than the AR case, showing asignificant difference in bonding strength. Furthermore, it is seen that intralaminar failure is induced for theNS case, which was not observed for the AR case. Hence, the presence of the nanostructure was observed todivert the fracture path, which is desired in general, however also should be considered when establishingmodels to predict the bonding behaviour. In addition, it should also be noted both cases include voids near
the bond line. These voids were found to appear in the resin rich regions of the plain weave fabric and only
locally close to the bond line and is therefore likely an artefact of the bonding method. Thus, these voids
could be avoided by carrying out the bonding procedure under vacuum, which is believed to further improvethe bonding properties. If the voids can be avoided, less scatter in the strength data is also expected, which isrequired for industrial application of the proposed bonding method.
Acknowledgements
This paper is based on results obtained from a project of Kanagawa Institute of Industrial Scienceand Technology (KISTEC) and was supported by JSPS KAKENHI Grant Number 18H01342. Furthermore,
support from the Amada Foundation is kindly acknowledged.

Industrial Application Proposal Report

F15_IA_2018AFig. 3. Reconstructed 2D views of X-ray CT data for the four stitched scans of (a) AR1 and (b)NS1 at different load steps
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