Nondestructive testing of contaminated CFRP surfaces with the BonNDTinspect® system
© The Author(s) 2017
Received: 22 May 2017
Accepted: 22 July 2017
Published: 1 August 2017
Composite materials are already being used in the mass production of structural components in the automotive industry, particularly at the BMW Group. Adhesive bonding is generally considered to be the best technique for joining CFRP (carbon fiber reinforced plastic) light-weight structures. The conventional NDT (nondestructive testing) methods currently being used focus on the detection of material defects, e.g. debonding. These methods give little information about the surface properties or bond quality. A new ENDT (extended nondestructive testing) method is the BonNDTinspect system, based on a patent held by the Fraunhofer IFAM. An ultrasonic atomizer nozzle creates a water-aerosol and the small water droplets are sprayed onto the surface. Depending on the surface properties (surface energy or contamination state), the aerosol will form wide or narrow drops on the surface. We determined a test for certain contaminations, including release agents, oil, and fingerprints, the detection of which is critical to ensure the performance of adhesively joined CFRP structures. The BonNDTinspect system is an inline-capable NDT technique that is suitable for distinguishing surface states for adhesive bonding of CFRP. We verify this statement with destructive tests, including the single-lap shear test. It was found by using an extension of the evaluation criteria that it is possible to detect contamination such as water-soluble release agent, CFRP dust and fingerprints. The investigated contamination with hydraulic oil allows only a clear differentiation between cleaned and contaminated. A contamination with corundum is not detectable by the BonNDTinspect system alone.
KeywordsNDT Release agent Adhesive bonding Inline detection Aerosol-wetting test
Continued growth of the world’s population and the simultaneous scarcity of resources have led to a need for increased efficiency in almost all areas of life. Sustainability and environmental protection are also becoming increasingly important focal points and, with the pressing issue of climate change, the emission of pollutants has become a central issue. In addition to industry, the transportation sector is a main emitter of CO2, contributing around 26% of overall CO2 emissions within the EU, whereby passenger cars (12% of overall emissions) are responsible for almost half of this. According to EU regulations, the cap for CO2 emissions from passenger cars is currently set at an average of 120 g CO2/km and will be cut to under 95 g/km by 2020 . Regarding the reduction of CO2 emissions, one major factor is reducing the weight of the vehicle ; per 100 kg reduction in vehicle weight, CO2 emissions can be cut by up to 7.5–12.5 g/km [3, 4]. In order to fully utilize the potential of different light-weight materials as well as material combinations, adhesives have been used as a joining technology for the last 60 years. Initially, structural parts were bonded in the aviation industry [5, 6], but meanwhile, adhesives as joining technology are commonly used in all branches of industry . This enables the production of highly complex parts and, subsequently, technological effort. These high standards lead to an equally high demand in terms of quality, which can only be met by defining and complying with measures for quality assurance [5, 6]. For this purpose, the recently introduced DIN 2304 is an application-oriented comprehensive standard for the quality assurance of adhesive materials .
Even though adhesion has various operating principles, the decisive factors  are in the angstrom to nanometer range (0.1–1 nm). The physical and chemical properties of adjacent phases should be coordinated in such a way that an interaction between the joining parts is possible [7, 9, 10, 11]. Therefore, the full adhesive strength of the physicochemical properties within the uppermost atomic layers of the surfaces is a decisive factor in order for surfaces to bond (;). As a result, the interaction of the surfaces to be bonded as well as the adhesives themselves are decisive for the ultimate quality of the adhesive bond. The respective pre-treatments steps must be in place in order to have the respective reactive surfaces of the joining parts. A wide range of activating and purification procedures exist for this purpose and can be based on physical, chemical, or mechanical methods . Since even the slightest variation in the total process can lead to adhesive failure, it is absolutely essential that robust production processes as well as the monitoring of surface conditions are used. Process variations can include a defective cleaning system that does not fully remove process contaminations, such as fingerprints, or similar non-compliance with quality assurance measures. In order to increase process reliability and the respective quality assurance, an (inline) process control of the surface conditions is required. Such inline control measures help to detect defects at an early stage and thus reduce costs incurred due to excess work or rejected parts. Such an assessment system must meet the high standards of speed, sensitivity, and process suitability of the existing production system.
Fiber reinforced plastics or carbon fiber reinforced plastics (CFRP) are used on a large scale at the BMW Group. In order to ensure a smooth operation within production, it is necessary to establish inline measurement systems to assess surface conditions in a quick and safe manner. Thanks to the BonNDTinspect system, it is possible to check and assess the wetting properties of surfaces both inline as well as in the industrial field. It is important for such a system that all the common process contaminations can be detected. In this context, particularly critical contaminations were assessed in an assessment matrix using samples with defined contaminations of release agents, hydraulic oil, corundum, and CFRP dust. These samples were subsequently assessed using the BonNDTinspect system and then tested in a single-lap shear test. Since the BonNDTinspect system is a comparative measuring method, a reference test of the surfaces is required. For film-type contamination, for example, high performance liquid chromatography can be used with mass spectroscopy coupling. Likewise, for example, the reference measurement by means of XPS is a standardized method for detecting activation of thermoplastic matrix systems. The combination of the assessment result of the BonNDTinspect system and the fracture pattern allows conclusions to be drawn regarding the adhesive behavior of the surfaces.
Materials and experimental methods
Resin component: Hexion Epikote Resin PAT.
Hardening component: Hexion Epikote Curing Agent PAT.
Internal release agent: Hexion PAT 657 BW.
External release agent: Hexion PAT 608 FP.
concentration level and contaminated surfaces
Release agent (µg/100 mm2)
Hydraulic oil (µg/100 mm2)
Fingerprint (100 mm2)
Corundum (µg/100 mm2)
CFRP-dust (µg/100 mm2)
For the later single-lap shear tests, the cold-curing 2-K-adhesive Beta Force 2816S from Dow Automotive was used. The adhesive consists of the components polyol and isocyanate. The second joining part used in addition to the epoxy resin system was a 2.5-mm-thick steel sheet coated with cathodic dip paint.
The contamination by the hydraulic oil was carried out with a syringe; a drop was placed on the sample and was subsequently wiped off with a conventional lint-free cloth. This procedure was repeated until no residues were visible to the naked eye and only an oil film remained. In order to have different levels of concentration, two different methods of wiping the oil off were applied, based on scenarios likely to be encountered during production. In the first method, the oil was wiped off with a little pressure, and in the other method, the pressure applied to the cloth was increased.
In order to have a reproducible, homogenous application of the CFRP dust, it needed to be atomized. For this purpose, a hole was cut into the lid of a bucket, which was then fitted with a filter consisting of various layers of plastic netting. In addition, the bottom of the bucket was cut off and a balloon was placed over the bucket, creating the ability to produce a blast of air to atomize the CFRP dust contained in the bucket. This generated a homogenous application to the surface. The necessary reproducibility was generated via the defined amount of CFRP dust that was put into the bucket.
The corundum was applied manually and as homogenously as possible to the surface. To ensure reproducibility of the application amount, the part was placed onto the scales and contaminated in that position.
The BonNDTinspect system is a method for the surface characterization of wetting properties based on the aerosol wetting test procedure developed at Fraunhofer IFAM .
High-pressure liquid chromatography-mass spectroscopy, in short HPLC–MS, is a nondestructive test method in which the chromatographic separation of substances is linked to a spectroscopic analysis of materials. In the case of determining the surface component contamination, the surface is first rinsed with temperature-distilled water, then the rinsing solution is concentrated and fed to the HPLC–MS. HPLC chromatographically separates the components of the solution and transfers the substances found to undergo mass spectroscopy. This can then determine the mass and thus, with a known surface area, the contamination concentration. In the case of an examination into release agent contamination, a CFRP specimen of at least 100 cm2 is first rinsed with 60 °C warm distilled water and the resulting solution is collected. The solution is evaporated in a drying oven at 110 °C, the residue is dissolved in 0.5 ml of MeOH/THF (1:1), and then filtered. The specific component of the release agent can then be determined via HPLC–MS using the amount of ethoxylated ricinolates.
The concentration is expressed in µg/ 100 cm2.
Single-lap shear test
To test the single-lap shear test samples, a material testing machine Z010 from Zwick was used. The testing machine is designed for quasistatic stress, with a single, stationary, dynamic, or varying profile. For the classification of fracture phenomena of the adhesive bonds according to DIN EN ISO 10365, the following designations from the table were used, whereby a distinction is made between adhesive failure, substrate-specific near cohesive failure, cohesive failure, and broken by delamination. With a test speed of v = 20 mm/min, the samples were loaded until failure and the force was recorded on the crosshead.
Results and discussion
This chapter includes the results obtained from the wetting test and the fracture pattern, in addition to their discussion.
The following table lists the setting parameters of the BonNDTinspect system. The selection of the parameters was carried out empirically. The decisive factor was a constant wetting of the surface over several measurements. The choice regarding the amount of water was made so that there were mainly only a few drops on the surface. The choice of evaluation criteria was based on the best distinctness of contamination conditions. The resin system used for the tests has hygroscopic properties, so that a distortion of the results can be excluded when water is applied, reference tests were carried out. For this purpose, in addition to purified samples, samples were also glued, which were wetted with aerosol up to five times.
Setting parameters of the BonNDTinspect system
Evaluation criteria 1
Evaluation criteria 2
Evaluation criteria 3
Evaluation criteria 4
Mean droplet size
Figure 7 shows an increase in the variation as CFRP dust loading increases over the surfaces. This is due to the fact that the dust particles are not within the droplet, as in the case of contamination with corundum, but rather that the drops are increasingly prevented from spreading. The strong variation in the area of the fingerprints is due to the fact that it was not possible to reproducibly produce the same contact pressure on the surface. This inhomogeneity has an area variation and thus a variation in droplet size as a result.
The effect of spinodal dewetting is reflected in the increasing modus, thus there is a high standard deviation in the mean droplet sizes in the figure. Despite the high deviation, a conclusion can be drawn from the combination of the two evaluation criteria for the state of contamination.
The mean droplet diameter also enables a very good correlation between the droplet image and the fracture pattern. The results of the shear test are compared with those obtained using the BonNDTinspect method. The initial aim is to generate a correlation between fracture and drop image. For a droplet size of about 170 microns, cohesive failure was observed. This corresponds to a release agent contamination of about 0.22%. The above-mentioned detection limit of the HPLC–MS corresponds to a droplet size of about 300 microns.
Sample 14 corresponds to a coverage of about 1700 µg/100 mm2. Such contamination can be detected by the naked eye on the basis of differences in the gloss of the surfaces. On the basis of the fracture pattern, a differentiation can be done between the contamination quantities. Thus, above a concentration Z2, an increase in the adhesive fracture pattern can be observed. A further increase in the adhesive fractions in the fraction pattern is shown in samples of state Z3. This leads to a determination about which the adhesive system can absorb a certain amount of hydraulic oil without causing any changes in its adhesiveness. The differentiation of the contamination quantities in conjunction with the adhesive properties are well-suited to detect early defects such as wear-resistant seals at an early stage without causing a failure of the adhesive bond.
The detection of corundum on the surface is difficult, as mentioned earlier. This is because the corundum grains have a diameter smaller than that of the drops applied to the surface. The evaluation criteria examined were droplet diameter, mode, wetting percentage, and roundness and provided no conclusion about the contamination state.
The disturbances of the boundary layer between the adhesive and the surface of the component due to corundum contamination are so large that only isolated adhesive bonds can be formed (see Fig. 21). This adhesive behavior and insufficient differentiation of the surface conditions using the BonNDTinspect system clarify the necessity of another testing method. By way of example, scattered light measurement is mentioned here, whereby it is possible to detect particles on the surface.
However, because even small amounts of corundum obviously interfere with the adhesion, an alternative measuring method should be employed. This could be, for example, the scattered light method developed at the Fraunhofer IFAM, which is also a nondestructive examination that can be used to determine surface roughness.
Analogous to the critical fracture pattern, the shear tensile stresses are accompanied by the fact that, in the case of state Z1, corresponding to samples 1–5, an average of 6.3 MPa is achieved. Samples of the contamination state Z2 achieved a shear tensile stress of 5.6 MPa. The lowest shear tensile stress of 4.4 MPa was achieved for samples 11–15 of conditions Z3, this also mirrors in the mainly adhesive or near-substrate failure.
Fingerprints are, aside from release agent residues and CFRP dust, the contaminants with the highest probability of being applied during the manufacturing process. As shown in the diagrams below, the evaluation criteria were average droplet diameter, wetting proportion, and mode; they are very well suited to detect fingerprints. They are to be viewed critically with respect to the adhesion behavior as seen below. They can be declared by the detection well as a defective surface.
The objectives of the experiments were to show whether it was possible to detect process contamination using the BonNDTinspect system as well as whether these contaminations can be differentiated with reference to their concentration and whether a conclusion can be drawn based on the result of wetting on the adhesiveness. The experiments carried out clarify that it is not possible to differentiate between the standard evaluation criteria such as the average droplet size, the wetting proportion, and the possibility of differentiating highly concentrated amounts of release agent. The effect of the spinodal dewetting of the release agent requires an extension of the evaluation criteria by the mode and the roundness. Furthermore, the experiments showed that a reference investigation with appropriate adaptation of the limit values is necessary in the case of a batch change of the resin system. The BonNDTinspect method can be used to detect surface contamination such as release agents, CFRP dust, and fingerprints. In the case of contamination with corundum, it is impossible to make a statement about the nature of the surface using the selected conditions. As can be seen from the evaluation criteria mean droplet diameter, the roundness, and the mode, the BonNDTinspect-system showed good sensitivity to all contaminations.
From a roundness of the droplets <1.5, a cohesive fracture behavior was shown in the experiments of the release agent. The same applies to samples with an average droplet diameter <190 μm and a mode/cm2 >90. Apart from the clear differentiability, it has been found that by using the BonNDTinspect system, surface contamination by an external release agent can be clearly detected below the detection limit of HPLC–MS.
Contamination with hydraulic oil should be considered less critical because the adhesive Beta Force 2816s is very tolerant of oil on the surface. On the other hand, fingerprints are very problematic because even a minimal contamination results in adhesive failure. The same applies to a surface that has been contaminated with carbon dust. This leads to adhesive failure in the single-lap shear test once even minimal pollution has been applied.
For further consideration, it is possible to draw on the scattered light sensor analysis, developed at the Fraunhofer IFAM, at the same time. The advantage of a combination of these measuring systems is that not only the wettability of the surfaces but also the surface relief can be determined. Both systems are referred to as ENDT procedures.
The authors of the contribution are AK, KB, CT, GM, BV and BM. AK worked out sample preparation and measurements of the surface wettability. AK developed, performed and—with CT—evaluated the droplet images and set up relations between wettability and surface state. AK adapted evaluation model modus and set up relation between HPLC results. AK, CT, KB and GM took part in setting up the experiments and in analyzing and merging the obtained data. GM, KB, BV and BM contributed in the conceptual approach and in discussing the obtained data. AK and CT drafted the manuscript. All the authors contributed to perform the research work described in the article and they all agree with the submission. All authors read and approved the final manuscript.
The authors are grateful to Dr. Michael Noeske for fruitful discussions.
The authors declare that they have no competing interests.
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