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# Controlling Failure of Polymer Skin/Foam Bilaminate Sheets

Kevin Cox, Richard Robertson

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## To cite this version:

Kevin Cox, Richard Robertson. Controlling Failure of Polymer Skin/Foam Bilaminate Sheets. SAE World Congress & Exhibition, 2007, Detroit, United States. 10.4271/2007-01-1216. hal-03335949

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https://hal.science/hal-03335949

Submitted on 6 Oct 2021

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Distributed under a Creative Commons Attribution 4.0 International License
# Controlling Failure of Polymer Skin/Foam Bilaminate Sheets

Kevin R. Cox

Automotive Components Holding, LLC, Saline Plant

Richard E. Robertson

University of Michigan

## ABSTRACT

Controlling the failure of polymer skin/foam bilaminate sheets is often accomplished by creating a line-shape weakening. For applications such as certain types of passenger-side seamless airbag systems, the failure must occur only along the weakened line with no fragmentation and the line's location must not be visible before airbag deployment. When the weakening is created by mechanically scoring the polymer skin on its back surface, where it is bonded to the foam, the weakness of the bilaminate is determined by the depth of the score groove. The deeper the groove, the weaker the bilaminate and the easier it is to control the failure. But also, the deeper the groove, the more likely the groove will read-through to the front surface, allowing its location to be visible. Several quasi-shear and tensile tests were conducted. Fragmentation was found to be likely when the skin and foam delaminated and failed separately, rather than failing together, but delamination could be avoided if the local strain at the groove reached the ultimate strain of the skin prior to the general strain in the bilaminate reaching the ultimate strain of the foam. Techniques found to minimize the groove depth, to avoid read-through, and still achieving fragmentation-free failure, were to increase skin thickness, reduce skin strain hardening, and increase foam ultimate elongation.

## INTRODUCTION

A line-shaped weakening is used in a number of products. Examples are the opening of soda pop cans (aluminum), soup cans (steel), and milk and soda pop bottles (plastic). The lines of weakening are visible in these products. In this work, attempts were made to keep the line of weakening hidden. Other requirements were (1) that the material be a polymer skin/foam bilaminate, (2) that its failure would occur only along the weakened line with no fragmentation, and (3) that the weakening be accomplished by mechanically scoring the skin on its backside, where it is bonded to the foam.

The immediate application of this technology is found in the weakening of an instrument panel to allow the passenger-side airbag to be deployed through it, a passenger side seamless airbag system (PSSAS), or in other words, a hidden airbag system. They are a recent trend in the automobile industry and Sun et al. has described a possible PSSAS design [1]. It consists of a hinged metal door bolted to the back of an instrument panel (IP). When the airbag is deployed, the hinged door breaks through a foam/polymer skin bilaminate, hopefully, in a predetermined location. This is achieved by the creation of a line-shaped weakening in the IP. One method used to create a line-shaped weakening is to mechanically score the polymer sheet that makes up the outer decorative skin of the IP. The skin is weakened on the class B surface. The class B surface is the surface of the skin that is not visible after the IP is manufactured. The class A surface is the one that is visible after the IP is manufactured. See Fig. 1 for a schematic of a PSSAS that is similar to the design described in Sun et al.'s patent. The bilaminate of the PSSAS is subjected to quasi-shear loading arising from the hinged door applying a load perpendicular to the sheet plane and the foam on the outside of the hinged door being adhered to the airbag housing. As described by Hersman et al. [2], for a PSSAS deployment to be considered successful, the IP must fail at a predetermined location. In addition, no fragments that could cause injury to the passenger of the vehicle can be created during the airbag deployment. As described by Iannazzi and Daris [3], an additional product requirement of a PSSAS is to optimize the line-shaped weakening so it is weak enough to result in a predetermined failure, but strong enough to meet all product durability and cosmetic requirements of the IP.

## EXPERIMENTAL AND RESULTS

The creation of the bilaminate samples was somewhat involved. A one mm thick TPO skin was vacuum-formed into the shape of an existing IP design [4]. Grooves of various depths were produced on a flat section of the skin using a utility knife tool with an adjustable blade
length. The knife tool was run along a guide track that was placed on the class B surface of the skin. The result was the creation of a groove on the class B surface with a depth equal to the preset blade length. The scored skin was placed with a glass-filled styrene substrate into a reaction injection-molding machine. A layer of urethane foam was then injected between the skin and the substrate by reaction injection of isocyanate and polyol [5]. Rectangular samples were then cut from the IP shaped trilaminate so that the hidden groove crossed the width at the middle of the sample. Several samples were also produced that did not contain a hidden groove. A cut was then made through the substrate just below and parallel to the groove or across the middle of the sample if it did not have a groove.

A test was designed to show what leads to a location-controlled, fragmentation-free failure of a bilaminate when it is loaded in quasi-shear. Figure 2 shows a schematic of the sample loaded in quasi shear. It was discovered that for there to be fragmentation-free failure of the bilaminate, the failure needed to be initiated at both outer surfaces and the two cracks needed to meet in the middle of the foam. In other words, the skin had to fail prior to complete failure of the foam. This only happened if the groove was sufficiently deep. In other words, the remaining material thickness had to be sufficiently thin. Remaining material thickness is the amount of material left between the bottom of the groove and the class A surface. This remaining thickness is controlled by the skin's original thickness and the groove depth. If the foam completely failed prior to the skin, then the skin delaminated from the foam and foam fragments often formed.

A test was developed to observe the failure of the bilaminate when it was loaded in tension in the sheet plane perpendicular to the direction of the groove. It was learned from viewing the frames from different videos of these tensile tests that the same critical groove depth, between fragmentation-free and fragmentation-prone failure, was observed whether the sample was loaded in quasi-shear or in tension. As with the tests in which the sample was loaded in quasi-shear, fragmentation-free failure of the bilaminate in tension involved the skin failing prior to complete failure of the foam. Frames from videos of four tensile tests that show how the foam and the skin failed when the bilaminates were loaded in tension and quasi-shear are shown in Fig. 3. The frames show that the same groove depth was required for the skin to fail prior to complete failure of the foam if the bilaminates were loaded in tension or quasi-shear.

Table 1 shows whether the skin of a bilaminate failed prior to complete failure of the foam or the foam prior to the skin when there were different remaining material thicknesses in the skin and the bilaminate was loaded in tension or quasi-shear. The table also shows that the same groove depth was required for the skin to fail prior to complete failure of the foam if the bilaminates were loaded in tension or quasi-shear

Figure 4 shows load per unit width vs. strain for three TPO skin/foam bilaminates. It shows curves for one bilaminate composed of a skin with no groove, one with a 0.30 mm deep groove, and one with a 0.40 mm deep groove. All three of these bilaminates had the hard plastic substrate completely removed prior to testing. The figure shows a much larger average general strain in the bilaminate without a groove as compared to the bilaminates that had a 0.30 mm or 0.40 mm deep groove.

## DISCUSSION

The directions of the stresses in a bilaminate loaded in quasi-shear were analyzed in an attempt to explain how the bilaminate failed. Because the bilaminate is constrained by the substrate and pushed by the hinged door, two bending moments are generated and the bilaminate is stretched between them. A schematic of the stress directions when a bilaminate is loaded in quasi-shear is shown in Fig. 5.

The directions of the stresses in a bilaminate loaded in tension were analyzed in an attempt to explain why, if the line-shaped weakening in the skin was not weak enough, it often resulted in delamination and fragmentation. To prevent delamination and foam fragments, the skin must fail prior to complete failure of the foam. The directions of the stresses were analyzed in a bilaminate loaded in tension rather than quasi-shear in order to simplify the problem. This was thought to be valid for the bilaminate discussed in this work because the same groove depth in the skin resulted in it failing prior to complete failure of the foam, whether the bilaminate was loaded in tension or quasi-shear, and the bilaminate appeared to fail in tension in the sheet plane even when loaded in quasi-shear.

Prior to delamination, the average general strain in the foam, skin, and bilaminate are equal over a large area. If the general strain of the bilaminate exceeds the ultimate tensile strain of the foam, the foam will fail and delaminate from the skin. If the local strain of the skin at the groove reaches the ultimate tensile strain of the skin prior to the bilaminate reaching the ultimate tensile strain of the foam, the foam will not fail and delaminate from the skin. Figure 6 shows a schematic of the general and local strain in a bilaminate at the point delamination begins and at the point the skin fails without delamination. The relationship between the average local strain in the skin, the ultimate tensile strain of the skin, the general strain in the bilaminate, the ultimate tensile strain of the foam, and delamination is illustrated.

## CONCLUSION

If the general strain in the bilaminate reaches the ultimate strain of the foam before the local strain in the skin along the groove reaches the ultimate strain of the skin, the foam will fail and delaminate from the skin. This often leads to foam fragments. To prevent foam
fragments, it is critical for the skin to fail prior to complete failure of the foam.

It is important to minimizing the groove depth because at a constant skin thickness, minimizing the groove depth could also minimize cosmetic and durability problems. At a constant skin thickness, the groove depth required for fragmentation-free failure may be minimized by minimizing the ratio of the load required for the skin to fail at the weakened line over the load required for the bilaminate to delaminate. This could be accomplished by minimizing the ratio of the ultimate tensile stress of the skin over the average general stress in the skin at the point of delamination. The average general stress in the skin at the point of delamination is reached when the skin's average general strain equals the ultimate strain of the foam. This ratio is equivalent to the average slope of the skin's stress vs. strain curve between the ultimate strain of the foam and the ultimate strain of the skin. It is important to choose a skin material with the flattest possible stress vs. strain curve between these ultimate strains, in order to minimize the critical groove depth. In other words, skin material that exhibits minimal strain hardening should be chosen. Critical groove depth is the minimum groove depth at which delamination does not occur.

Choosing foam with a higher ultimate strain could also reduce the critical groove depth. If the foam had an ultimate strain equal to the skin, the foam will not delaminate from the skin regardless of groove depth.

If the skin thickness is allowed to vary, then the remaining skin thickness between the bottom of the groove and the class A surface becomes the important value to maximize. By maximizing the remaining skin thickness, cosmetic and durability problems could be minimized. For a given remaining skin thickness, increasing the original skin thickness will reduce the general strain in the bilaminate at the skin failure load. This would make it possible to take a bilaminate in which the foam completely failed prior to the skin, leave the remaining material thickness unchanged, and increase the original skin thickness to a point where the skin fails prior to complete failure of the foam. So choosing a thicker skin can increase the critical remaining material thickness.

The quasi-shear and tensile test on the bilaminate could be used to predict if the groove is sufficiently deep to result in a location-controlled, fragmentation-free failure of a bilaminate. These tests could be conducted at the strain rates and temperatures required by specific applications.

## ACKNOWLEDGMENTS

The work reported is based on the Doctor of Engineering thesis of one of us (Kevin Cox) at the University of Michigan, Ann Arbor. We wish to thank the thesis committee, Frank Filisko, Ellen M. Arruda, and Mark De Mott, for all the time and effort they spent helping

with this work. Special thanks are also due to the Visteon Corporation for its support of this research.

## REFERENCES

1. M. Sun, J.D. Pink, S. Mishra, and A.H. Cardenas, United States Patent 6644685, 2003

2. M.J. Hersman, W. Humphrey, M. Marshall, and S. Farrington, "Introduction of a Castable Thermoplastic Urethane Material for Instrument Panel Coverstock" Society of Automotive Engineers, Article 980063, 1-3, 1998

3. P. Iannazzi and F. Daris, "Seamless Passenger Side Air Bag Doors" Society of Automotive Engineers, Article 960403, 27-31, 1996

4. W.V. Titow, "PVC Plastics Properties, Processing and Applications", Ch.17, Other Processes, Elsevier Science Publishers LTD, Barking, 1990, pp 615-632

5. D. Klempner and K.C. Frisch, "Handbook of Polymeric Foams and Foam Technology", Ch.7, RIM and RRIM Foams, Carl Hanser Verlag, Munich Vienna New York Barcelona, 1991, pp 135-148

## CONTACT

### Kevin R. Cox

Automotive Components Holding, LLC, Saline Plant, 7700 Michigan Ave., Saline, MI 48176, USA
kcox6@ach-llc.com

### Richard E. Robertson

Department of Materials Science and Engineering, The University of Michigan, 2300 Haywood Street, Ann Arbor, MI 48109-2136, USA
rer@engin.umich.edu

## TABLES

<table><caption>**Table 1** Skin and foam failure in bilaminates when loaded in tension or quasi-shear</caption><thead><tr><th>Groove Depth (mm)</th><th>Remaining Material Thickness (mm)</th><th>Loaded In Tension Failure: Foam Failed Prior To Skin Skin Failed Prior to Foam</th><th>Loaded In Quasi-Shear Failure: Foam Failed Prior To Skin Skin Failed Prior to Foam</th></tr></thead><tbody><tr><td>0.51</td><td>0.13</td><td>S</td><td>S</td></tr><tr><td>0.38</td><td>0.26</td><td>S</td><td>S</td></tr><tr><td>0.25</td><td>0.39</td><td>F</td><td>F</td></tr><tr><td>0.13</td><td>0.51</td><td>F</td><td>F</td></tr><tr><td>0.00</td><td>0.64</td><td>F</td><td>F</td></tr></tbody></table>
## FIGURES

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**Figure 1** Schematic of seamless airbag system (not drawn to scale): (A) top view of passenger and horizontal cross section through IP and (B) side view of vertical cross section through IP

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**Figure 2** Schematic of sample and fixture used in quasi-shear test

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Note: T = remaining material thickness between the bottom of the score groove and the class A surface of the sheet

**Figure 3** Frames from videos of four tests that show how foam and skin failed when bilaminate was loaded (Row A & B) in quasi-shear and (Row C & D) in tension

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**Figure 4** Load per unit width vs. strain for three skin/foam bilaminates, (A) with a 0.30 mm deep score groove, (B) with a 0.40 mm deep groove, (C) with no groove
<figure><img src="image_9.png" /><figcaption>**Figure 5** Schematic of stress directions when bilaminate is loaded in quasi-shear</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(A) Strains At Delamination</figcaption></figure>

<caption>(B) Strains At Skin Failure</caption>

**Figure 6** Schematic of general and local strains in a bilaminate (A) at point delamination begins and (B) at point skin fails without delaminating. $X$ = Groove depth, $\varepsilon_{1gb}$ = Average general strain in the bilaminate, $\varepsilon_{1ls}$ = Average local strain in the skin, $\hat{\varepsilon}_{tf}$ = Ultimate tensile strain of the foam, $\hat{\varepsilon}_{ts}$ = Ultimate tensile strain of the skin