Investigations on Delamination Behavior of Sandwich Sheets

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ScienceDirect Procedia Engineering 183 (2017) 71 – 76

17th International Conference on Sheet Metal, SHEMET17

Investigations on Delamination Behavior of Sandwich Sheets Mathias Liewald and Dennis Hofmann* Institute for Forming Technology (IFU), University of Stuttgart, Holzgartenstr. 17, 70174 Stuttgart, Germany

Abstract Steel/polymer/steel (sandwich) sheets show several advantages in comparison to monolithic materials when strength, stiffness and damping characteristics are set to an optimum. Although the mechanical properties of sandwich sheets were improved over the last years, the application of such hybrid materials in the automotive industry is not very well-established due to insufficient knowledge about forming characteristics. However, it is difficult to detect the delamination during the forming process between the interlayer and the cover sheet. Delamination is defined as a separation between the polymer interlayer and the metal sheet while both of the layers remain in physical contact. After occurrence of failure, the two interlayers are still bonded and stick together because of weak Van-der-Waals forces (kissing bonds), which makes it difficult to detect any separation or delamination. In this paper, fundamental multi-axial testing methods for the detection of delamination in sheet metal forming are introduced and discussed. © Published by Elsevier Ltd. This ©2017 2017The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SHEMET17. Peer-review under responsibility of the organizing committee of SHEMET17 Keywords: Sandwich Sheets; delamination behaviour

1. Introduction Original Equipment Manufacturers are seeking to develop new vehicle lightweight strategies that will allow them to cost-effectively meet fuel economy targets and are increasingly shifting their focus to application of mixed-material solutions at mass produced scales. Future automotive industry aims are determined by a set of society’s upcoming challenges, e.g. demand for energy efficiency, climate protection, security in driving and comfort. However, applying lightweight materials to mass produced vehicles comes with a set of challenges, because OEMs must select the optimal combination of materials used for car body components including aluminum, high-strength

* Corresponding author. Tel.: 0049 711 685-838 24; fax: 0049 711 685-838. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SHEMET17

doi:10.1016/j.proeng.2017.04.013

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steel, composites and magnesium. Such materials are still sourced in small volumes, because their specifications require an optimal joining technique for mixed material manufacturing. [1] Sandwich material systems – especially multi-material hybrids – represent an interdisciplinary concept by combining suitable material properties/ material composites with production engineering, as well as design and functionality for fulfilling the high demands regarding modern materials and structures [2]. Sandwich sheets combine advantages of contrary mechanical properties (e.g. damping factor, stiffness, weight) in order to match new fields of application in the automotive industry. Increasing the layer thickness leads to increased stiffness, but the transmission of shear stress is limited in this case. Hence, there are only a few combinations of regular sheet metal composites that are able to transmit shear stress in the adhesive layer, which is the main reason for a non-comprehensive use of adhesives in the automotive industry [3, 4, 5, 6]. Beside fracture and wrinkling of the top metal layer, delamination of the polymer interlayer also leads to invisible and hard detectable defects of the component. Therefore, this paper deals with the delamination analysis in multi-axial forming of those layered sandwich materials and tries to provide a kind of recipe, which can be used to compare different sandwich materials in terms of their delamination behavior under multi-axial loading. 2. Experimental set-up and materials used 2.1. Materials The material used in this investigation consists of stainless steel alloy 1.4301 having a nominal sheet thickness of 0.5 mm. The intermediate layer consists of a two-component adhesive (Dow Betamate 1630) and was applied manually (thickness of 0.1 mm) using glass-lime spheres to adjust a constant layer thickness. The adhesive was entirely cured after 24 hours. In order to adjust the adhesive joint, a constant surface pressure was applied on the sandwich sheets in normal direction. The blanks used in this investigation consist of Nakajima geometries with different widths of specimen (30 mm, 70 mm, 90 mm, 110 mm, 140 mm, 200 mm; here: upper sheet) and small circular blanks (here: lower sheet). 2.2. Determination of delamination under multi-axial loading In order to measure delamination during the forming process, a circular sheet (2) with different diameters (40 mm and 50 mm) was bonded onto Nakajima specimens (1), see Fig. 1. The experimental set-up is based on [4], but was modified with a biaxial strain gauge on the lower sheet to determine the exact delamination point during the forming process. Therefore, the Marciniak punch tooling was used to ensure that the biaxial strain gauge does not take any damage by the punch during forming. Previous studies showed that CCD cameras can be used to determine the starting point of delamination by measuring the major principle strain from the upper sheet (sheet 2).

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Sacrificial plate

Optical measurement device Die

Sheet 1 (upper)

Sheet 2 (lower)

Strain gauge

Blank holder

Adhesive Logging cable

Marciniak-Punch

Fig. 1: Experimental set-up to determine delamination under multi-axial forming loads

However, these investigations did not show the exact starting point of delamination because fracture of the intermediate layer was assumed when observing the upper sheet. The strain of the lower sheet could not be measured primarily, because it was on the inside of the Marciniak punch. Thus, using a biaxial strain gauge at the bottom layer (sheet 1) measures emerging strain value occurring in the intermediate layer. The biaxial strain gauge was located in the center of the lower sheet, but still close to the outline to measure the peak strain during the forming process. Consequently, the trend of these measured strain levels indicates the delamination point of time. 3. Results and Discussion Figure 2 shows the bonded Nakajima sheets on top of the circular sheets after the forming process. The location of fracture varies depending on which specimens were used. Biaxial specimens (200 mm) fractured close to the pole of the sandwich. Specimens revealing high amount of plane strain or uniaxial stress conditions (> 110 mm) has been damaged close to the punch radius because the burr of the (sacrificial) sheet, which was need for the Marciniak-punch, led to fracture of the top layer of the sandwich. However, by the time the top sheet was fractured, the bottom sheet was already delaminated. Therefore, the experimental set-up can be used to determine delamination. To ensure that the sheet has a higher elongation at fracture compared to the adhesive layer, a stainless steel alloy was used. Similar investigations with the aluminum alloy AA5754 could not be carried out successfully due to early fracture of the upper sheets.

Fig. 2: Fracture of the bonded Nakajima specimens (lower circular sheet and upper Nakajima sheet)

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Fig. 3 shows the procedure how the delamination forming limit curve (D-FLC) is determined in this paper. The start and the end of the delamination are measured by a strain gauge which was positioned onto the lower sheet. Consequently, the strain gauge is connected to the CCD cameras which can measure the principle strain on the upper sheet. As a result of this, a delamination forming limit curve can be plotted. To date, various test (e.g. peel test, shear test, KSII-test) have to be performed to evaluate the formability and delamination behavior under multi-axial loads. Finally, this procedure reduces the amount of experimental tests, but – due to the experimental set-up – is only valid for comparison of sandwich sheets and adhesive layers.

Start and end of delamination

Major and minor principle strain

-lower sheet -Strain gauge

DelaminationFLC

-upper sheet -CCD camera

Valid for: • Comparison of sandwich sheets (multi-axial loads) • Comparison of adhesives (multi-axial loads)

Fig. 3: Determination of the delamination forming limit curve (D-FLC)

Figure 4 shows the measured major and minor principle strain ε1 and ε2 gained by strain gauge shown in Figure 1. To ensure reproducibility of the experiments, five samples were used for each strain measurement condition and load application. The closing of the blank holder can be measured by biaxial strain gauge (less than 50µm/m). After the first contact of the punch and the sandwich, the strain increased up to 600 µm/m before it dropped to - 800µm/m. This point determines the start and the end of delamination. This trend is characteristically for all specimens, although the level of major and minor principle strain varied. The difference between major and minor principle strain – when determining the start of delamination – results from the difference in the rolling direction of the sheet. This anisotropy also leads to a concave curvature of the bottom sheet after the forming process. However, the strain value measured on the lower sheet surface emerges too small compared to the strain value gained on the upper sheet surface, because the load was exclusively initiated by the upper sheet. This ensured that only the multi-axial strain condition within the adhesive layer was measured at the lower sheet (Fig. 1). 1. Closing of blank holder 2. First contact of punch and sandwich 3. Delamination start 4. Delamination end

Principle strain [µm/m]

3 1

[log %]

20 16 12

2 y

4 Time [s] Major principle strain in µm/m

8

x

4

z

Minor principle strain in µm/m

Fig. 4: Major and minor principle strain of the lower sheet (measured with strain gauge) indicating start and end of delamination

The experimental set-up shown above determines the delamination point using the biaxial strain gauge. The time

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measurement of the strain gauge was connected to the CCD cameras (trigger). Therefore, the delamination point of the lower sheet can be referred to the strain measurement of the upper sheet. Fig. 5 shows the major and minor principle strain measurement (here: 200 mm circular diameter) which was recorded by the CCD cameras.

b) Minor principle strain [log %]

Major principle strain [log %]

a) Lower sheet (outline) Used step

Load stage []

Load stage [] Fig. 5: (a) Major and (b) minor principle strain determined by load stages of the upper sheet

The delamination trigger was taken from the strain gauge measurement (see 7.9 s time point in Fig. 4). The recording rate from the CCD camera corresponded to six frames per second. The delamination-time/-point was determined by a sudden increase of the major and minor principle gradient (Fig. 5a). This increase of the gradient indicates a failure of the adhesive layer. Consequently, the minor and major strain from the upper sheet could be determined for different load stages and points. Hence, the measured major and minor principle strain can be used to determine a multiaxial delamination forming limit curve (D-FLC) in order to compare sandwich sheets in terms of their formability and delamination (Fig. 6). Sandwich sheets under biaxial loads failed earlier than specimens with plane strain conditions. It was observed that the intermediate layer showed more sensitivity in case of tension than compression. These results are qualitatively matching observations which were gained by other researchers as well. However, it still has to be clarified why the measured amount of minor principle strain in the adhesive layer occur much smaller. Therefore, simulation tools and/or further experiments will be used. 8,0%

7,0%

Specimen 140 mm

Specimen 90 mm Specimen 110 mm

Major principle strain ε1

6,0%

5,0% Specimen 30 mm 4,0% Specimen 200 mm 3,0%

2,0%

1,0%

-0,20%

0,0% -0,10% 0,00%

0,10%

0,20%

0,30%

0,40%

0,50%

0,60%

0,70%

0,80%

0,90%

Minor principle strain ε2

Fig. 6: Measured delamination forming limit curve (D-FLC) for a sandwich sheet compound (1.4301, t=0.5 mm, Betamate 1630, t=100µm)

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It should be noted that the above described experiment does not meet similar stress conditions emerging when forming industrial parts due to concept of the experimental set-up. However, the set-up can be used perfectly to evaluate different sandwich materials and adhesive interlayers in terms of their delamination behaviour, respectively. Decisions for or against different structural adhesives can be evaluated. 4. Summary Advanced hybrid materials such as sandwich sheets with thin adhesive layers have a great potential for lightweight, damping and many other technical applications. To date, forming of sandwich materials is still challenging, because fracture and damage of the intermediate adhesive layer is difficult to detect in a multi-axial forming process. Using the delamination forming limit curve (D-FLC), a new method, seems to overcome these problems. The method determines the start and end of delamination using a biaxial strain gauge, which is bonded to a small circular sheet below the Nakajima specimen. Thus, the start of delamination can be connected to the strain measurement of the upper Nakajima sheet. Consequently, a multi-axial delamination forming limit curve was developed using this procedure. The set-up shown in the experiment is very helpful when different kinds of structural adhesives or sandwich materials are compared to each other. The delamination forming limit curve is an appropriate tool to evaluate multi-axial stress conditions in thin adhesive layers. However, further investigations on transferring this knowledge into industrial parts must be carried out in the future. Acknowledgement The financial support from the German Research Foundation (DFG) is gratefully acknowledged. References [1] [2] [3] [4] [5] [6]

Applying advances in lightweight materials to multi-material mass produced vehicles, Automotive Lightweight Materials, Detroit, 2014, Web. 25 Nov. 2014, http://www.global-automotive-lightweight-materials-detroit-2014.com/ B. Harris, A perspective view of composite materials development, Mater Des, 12 (5), (1991), pp. 259–272 J. Buhl, Umformverhalten und Grenzen von Schichtverbundwerkstoffen, Dissertation, University of Siegen, 2014 C. Bolay, Beitrag zur Umformung von ebenen und versteiften Schichtverbundwerkstoffen, Dissertation, University of Stuttgart, 2014 M. Milch, Tiefziehen von geklebten Doppellagenblechen, Dissertation, University of Hannover, 2007 O. A. Sokolova, M. Kühn, H. Palkowski, Deep drawing properties of lightweight steel/polymer/steel sandwich composites. In: Archives of Civil and Mechanical Engineering 12 (2012), Nr. 2, S. 105–112