Absorbing Impact Energy in Automotive Display Bonding

How 3M VHB Tapes can help meet critical impact requirements in automotive displays.

TTI.inc has sponsored this post. Written by Nelson Gonçalves Pimentel, 3M Neuss (GE) and Steve Austin, 3M St. Paul (US)

Automotive displays in the past were designed to be embedded and protected by the IP cluster within the dashboard, but increases in the amount and types of data users require has led to a greater number of display options, including Driver Information Displays (DID), Center Stack Displays (CSD) or Center Information Displays (CID), Driver Monitoring Systems (DMS) and Head-Up Displays (HUD). As electronic displays increase in size and proximity to the windshield, there is an increased need for greater impact resistance.

Additionally, there is a visible trend that automotive display design is following smartphone design towards narrow border/bond line design. This results in less bonding area between the housing and lens while display sizes are increasing (higher weight and more stress on the bond area). Meeting the automotive industry head impact test (HIT) and rear impact tests are critical requirements for automotive display bonding applications.


In this article, several impact requirements (including HIT and rear impact) have been investigated to explain how 3M VHB Tapes can help meet these critical test requirements. Specifically, display lens bonding will be examined with different impact event types as well as further aspects that are key for a reliable bond.

Automotive Display Specifications and Challenges to Material Suppliers

As automotive displays are becoming larger and closer to the windshield, higher temperature requirements are needed for the display components along with stronger, but still flexible, bond joints. Additionally, narrow bond line design is reducing the effective bonding area which results in the need for high performance bonding solutions for this application.

Automotive display impact is characterized by a brief, but high impulse on the display assembly components – 50G is a typical test level. For the adhesive bond between the display lens and housing (surrounding attachment of the lens to the housing/die-cast), a material needs to be chosen which survives all the impact forces, but is still flexible enough to:

  • compensate the mismatch between both components (due to manufacturing tolerances).
  • compensate the thermal expansion of dissimilar materials (with different CTEs).
  • compensate and absorb vibration and shock.
  • dissipate impact energy (“head form” impact and rear impact).

3M VHB Tapes

3M VHB Tapes (VHB Tapes) are double-sided, pressure sensitive adhesive (PSA) acrylic foam tapes. These tapes are characterized by having a viscoelastic conformable acrylic foam core with acrylic PSA skins. The acrylic foam provides energy absorption and stress relaxation properties that are beneficial for absorbing impact energy and reducing fatigue on sensitive electronic components due to vibration and differential thermal expansion of dissimilar materials (e.g. glass and metal).

Figure 1: Displacement of 3M VHB Tape at various strain rates. (Image: 3M.)

Viscoelastic materials like VHB Tape are characterized by having a modulus and ultimate strength that is strain rate or time dependent. Fast strain rates will exhibit increased modulus and ultimate strength values when compared to slower strain rates.

An impact is representative of a fast strain rate, while differential thermal expansion of dissimilar materials due to temperature change (e.g., glass bonded to metal or plastic) will result in a slow strain rate. The curves below illustrate a high tensile strain rate (550 in/sec) compared to a quasi-static slow tensile strain rate (0.024 in/sec). The viscoelastic properties of VHB Tape are clearly shown by the very high ultimate strength associated with the fast strain rate and the significantly lower strength associated with the slow strain rates where stress relaxation is very evident.

These tapes are used in many applications requiring strong yet flexible bonds where long-term durability is required (e.g., trailer skin bonding and exterior building panel bonding). The main benefits and performance attributes of 3M VHB Tapes are summarized below:

  1. Impact Resistance (tensile, compression and shear)
  2. Mismatch Compensation (gap filling) — including stress relaxation
  3. Thermal (differential thermal expansion and heat resistance) — including stress relaxation
  4. Vibration Resistance (cyclic fatigue) — tensile fatigue from existing 3M VHB Tapes (GPH)
  5. Bonding and Sealing (closed-cell construction) — IPXX testing
  6. Long-term Durability (all-acrylic chemistry) — UV resistance

Role of Finite Element Analysis

The demands and requirements for automotive display manufacturers are ever-increasing in terms of both complexity and shorter time to delivery. The making of actual prototypes for a new design is time-consuming and costly and may not even be feasible or practical in the early design stages. Finite element analysis (FEA) is a tool now being used to predict the behavior and performance of automotive display assemblies and materials when no prototypes are available for testing and analysis.

To perform FEA modeling, the assembly material mechanical properties need to be properly characterized. To characterize the mechanical properties of these materials, specific tests need to be performed which correlate to the real application loads. For impact resistance, depending on the force impact direction, either tensile, compression or shear tests are performed. This established test data is transformed into a material data card which is used in the FEA modeling tool (e.g. Abaqus). In order to validate the material data card, FEA simulations are performed and then compared with the real test data.

Once simulation and real test data are comparable, FEA modeling can be used as a powerful predictive engineering tool to simulate different test parameters in an actual impact event before building a prototype. This is a significant benefit to the auto display designer and can accelerate the design and development process as well as reduce costs.

FEA Modeling of Automotive Display Applications

3M VHB Tapes have been commercially available since 1980 and the performance and durability of this acrylic foam tape family has been proven over many years in many demanding applications, such as exterior architectural panel bonding and trailer skin bonding. The use of these tapes for automotive display bonding applications is relatively new due to the recent increased use of displays and their location in automobiles. To advance the acceptance and understanding of acrylic foam tapes for automotive display bonding, FEA modeling has proven to be a useful tool for predicting performance in this application.

A case study is provided which describes the technique and outcome of FEA modeling for automotive display bonding with VHB Tape.

Impact Resistance

Figure 2: Simulation of a head impact at the corner of an electronic display. (Image: 3M.)

The impact resistance of a bonding adhesive is dependent on several factors including stress load direction as well as bonding joint design and the total bonding area. This investigation is based on a proprietary 3M display design using 3M VHB Tape and an FEA simulation of different impact loads that may occur during the service life of an automobile. The tape in this display design is part of the solution which may help an automotive display meet performance requirements during an impact.

To predict the material performance in FEA software, the tape needs to be characterized at the coupon level for different loads. Tests were performed including compression, tensile and shear loads at different rates and temperatures to gain an understanding of the mechanical properties of a specific 3M VHB Tape. A material data card (MDC) is the outcome of this testing. Since these tests are at the coupon level, the data was converted into automotive- display-relevant test requirements.

Compression (Front Impact)

The Headform-Impact Test (HIT) is predictive of a front impact on an automobile, which will result in a compression stress load on the tape. Different regions of the world have slightly different test requirements such as FMVSS 201 (US), or GB 11552 (PRC), or ECE R21 (EU). In these tests, an idealized human head (head diameter d=164 mm, head mass m=6.8 kg) drops onto a display with an initial speed of 20 km/h within 20 ms.

This type of test is typically done experimentally. For this study a computational finite element model of the 3M proprietary display with the main components was evaluated including the different layers beneath the cover glass. A hyper-viscoelastic material model was used to model the VHB Tape and OCA (Optical Clear Adhesives).

Figure 3: HIT in center of the screen. (Image: 3M.)
Figure 4: HIT at corner of the screen. (Image: 3M.)

The plots above show the energy absorption during the impact event. Figure 3 shows an impact on the center of the screen while Figure 4 shows an impact near the corner. As shown in the plots, the highly viscoelastic 3M VHB Tape reduces the deformation during a head impact event and dampens the resulting vibrations significantly within a very short time. As the HIT is closer to the bond line, the greater the impact on the VHB Tape. In other words: the 3M VHB Tape is one key factor for high energy uptake and absorption and proved to be highly beneficial for a variety of different applications in that area.

Tensile (Rear) Impact

When considering a rear impact event, a tensile stress load is placed on the tape. During a rear impact event, high acceleration causes a significant but brief tensile stress on the tape used to secure the glass to the housing. The plots in Figure 5 below show energy absorption during a rear impact event. Acceleration used in this evaluation was 50G, which is a typical requirement by most automotive original equipment manufacturers (OEMs).

Figure 5: Rear impact test simulation. (Image: 3M.)

Shear Impact

For shear load two different scenarios are considered in different load directions: side impact (impact on the side of a car) or by driving over a hole (pothole) in the road. Both events will result in a vertical shear stress load on the tape. The plots in Figure 6 below show the energy absorption during a side impact event.

Figure 6: Side impact test simulation. (Image: 3M.)

These figures demonstrate the viscoelasticity of 3M VHB Tape and how it provides necessary energy absorption during impact events. Figure 7 illustrates an impact-type velocity oscillation. Figure 7a details the damping difference between viscoelastic and elastic behavior. By applying oscillatory excitation, a post-pulse oscillation takes longer to absorb the oscillation on the elastic than on the viscoelastic model. For the elastic model a hyperfoam elastic model has been used.

Figure 7: Impact-type velocity oscillation. (Image: 3M.)

Mismatch Compensation (Gap Filling) and Stress Relaxation

Due to manufacturing tolerances there is not always 100% alignment of substrates, or there can be uneven gaps which need to be compensated by the bond line. Typically, liquid adhesives will be used to compensate large gap differences, such as 3M Scotch-Weld Urethane Adhesive DP604NS (2 part polyurethane reactive adhesive) or 3M Scotch-Weld Flexible Acrylic Adhesive DP8610NS (flexible 2 part acrylic adhesive), especially on larger or curved displays where the gaps could be rather large.

For smaller displays, adhesive foam tapes, such as 3M VHB Tapes, are often used due to their high strength and viscoelastic behavior advantages which offer a secure bond without causing excess stress on the bond line. Most 3M VHB Tapes can compensate up to 50% of their thickness.

In Figure 8 below a perforated tape strip is used to demonstrate the stress distribution by using DIC-equipment (digital image correlation) and overlaying it with a computational finite element model. An electromechanical universal testing machine (Instron) was used in a ramp-hold test where the force was monitored over time. Stress-relaxation behavior is visible due to the viscoelastic nature of 3M VHB Tape as shown in Figure 8. The stress- relaxation behavior of the tape to compensate mismatch of substrates helps to reduce the stress on the display and avoid a visible moiré effect, which cannot be tolerated in automotive displays.

Figure 8: Simulation illustrating stress relaxation in 3M VHB Tape. (Image: 3M.)

Thermal Expansion

As display sizes have increased and locations of displays are more exposed towards the windshield, this is providing significant challenges to material and component suppliers. One of these challenges is the increased temperature requirement which is causing a higher thermal expansion of the bonded substrates. What were often plastic housings used in automotive displays are now frequently changed to die-cast metal housings, such as aluminum or magnesium.

Additionally, polymethyl methacrylate (PMMA) or polycarbonate (PC) plastics were used for automotive display lenses in the past, but these are now often a glass lens. As these materials behave differently when exposed to temperature changes due to different coefficients of thermal expansion (CTEs), the bond line must compensate for the different elongations (expansion at higher temperatures and contraction at lower temperatures) of the substrates without causing any additional stress on the display components.

Figure 9: Thermal shock test temperature profile. (Image: 3M.)

Figure 9 shows a thermal shock test profile where within 60 seconds there is a change from -40°C to +105°C in order to simulate a car parked during the summertime in a desert and when the car is started the air conditioning system blows cooled air towards the automotive display. As each material has its own CTE value, the elongation will happen in different rates and lengths.

This quick elongation difference needs to be allowed by the bond line and must be repeatable over the lifetime of the vehicle.

Figure 10: Thermal shock test results showing absolute and relative displacement of two substrates. (Image: 3M.)

In Figure 10 the displacement of housing (magnesium) and cover glass are shown (based on an 800 mm length display). The left side shows absolute displacement of each substrate, and the right side shows relative displacement from each other. Figure 11 shows the resulting stress on each element.

Figure 11: Stress due to displacement of each element in the thermal shock test. (Image: 3M.)

3M VHB Tapes can easily withstand the elongation up to 300% of their thickness. On some tapes in this family up to 700% or more is possible depending on the strain rate. Therefore, 3M VHB Tapes can compensate the different elongations of dissimilar substrates and, through stress relaxation, reduce the stress on the display.

Vibration Resistance

3M VHB Tape is used in many applications to replace mechanical fastening methods and often must withstand dynamic stress loads. The stress loading in many applications involves cyclical type fatigue generated by the operating environment of the bonded assembly. Dynamic loading over extended periods of time can have a significant impact on the useable life span of an adhesive used for a bonding application. Therefore, when considering an application such as automotive display bonding, it’s important to have an understanding of the fatigue resistance of the tape.

Figure 12: Number of cycles to failure in a cyclic fatigue test. (Image: 3M.)

As previously noted in this paper, 3M VHB Tapes are viscoelastic bonding tapes. Viscoelasticity is exhibited in the tape’s ability to both absorb and dissipate energy through its foam core. The long-term performance of the tape can be affected by extended exposure to stress and relaxation under dynamic loading conditions during the product’s life cycle in an application. An automotive display bonding application is one where the adhesive will experience cyclic fatigue throughout the service life of the electronic display due to road induced vibrations.

Test equipment exists to evaluate the cyclic fatigue resistance of an adhesive where the frequency and amplitude of the stress loading can be controlled. ISO 9664 is a test method used for characterizing the fatigue properties of structural adhesives in shear and this method, or a variation of it, can be useful for measuring the fatigue properties of acrylic foam tapes.

Stress loading conditions can be chosen based on a specific application where cyclic fatigue loading will stress a tape during its service life. An example is provided below showing an SN curve for 3M VHB Tape 5952 which shows the relationship between stress amplitude and cycles to failure at a controlled frequency of 0.4 Hz. A similar curve can be created for application specific frequencies and stress loading to gain insight into the suitability of an adhesive for an application where dynamic loading is present. Figure 12 shows 3M VHB Tape is able to withstand cyclic fatigue loading in an application where dynamic loads are present and can give insight into the long-term performance of the tape when exposed to cyclical stress loading.

Bonding and Sealing

The IP code, International Protection Marking, IEC standard 60529, classifies and rates the degree of protection provided against the intrusion of solid particles (such as dust) and liquids (water) into electrical enclosures. The rating is generically a 2-digit code, the first referring to solid particle protection and the second to liquid or water protection. Where there is no protection or where it isn’t of interest, the digit is replaced with the letter X.

It’s common in the electronics industry to use pressure sensitive adhesives (PSA) to seal devices and certify them to a particular rating: IP68 is typical. This would refer to dust tight (6 rating for solid particle) and suitable for immersion up to 1.5 meters for up to 30 minutes (8 rating for liquid). It’s important to note that this is a device level test or characterization and not something that can be tested on the PSA itself. A device manufacturer will test and rate their device, in addition to simulated devices being made to test the integrity of the PSA.

3M VHB Tapes are closed cell acrylic foam tapes and therefore they have the ability to bond and provide a waterproof seal in properly designed electronic assemblies. Die-cut shapes of tape are often used when moisture and dust resistant seals are required. A test using die cut shapes with an open center bonding two clear plastic sheets was conducted to assess the sealing capability of various 3M VHB Thin Foam Tapes while immersed in water. For this test development and characterization, IPX8 is considered equivalent to 14 psi (gauge) pressure (~10 m depth) for 30 minutes with a 1 mm line width of tape. This testing was extended to 43 psi (~30 m depth) for samples that passed the 10 m simulated depth.

Test results are provided below:

Simulated depth (chamber Pressure)Pressurized duration in min3M VHB Tape 864153M VHB Tape 59073M VHB Tape 5980
3 m(4.3 psi)30PassPassPass
10 m(14.2 psi)30PassPassPass
30 m(42.7 psi)30PassPassPass

The samples used were representatives of common 3M VHB Thin Foam Tapes for electronics applications: 3M VHB Tape 86415, 3M VHB Tape 5907 and 3M VHB Tape 5980. These have passed the testing described above. These results are not to be used as a certification of device waterproofness, only to show that if used properly, 3M VHB Tapes will provide a watertight seal.

Long-term Durability of Acrylic Foam Tapes

3M VHB Tapes are inherently durable bonding adhesives due to a variety of factors including the acrylic chemistry of these tapes as well as the foam core’s ability to absorb energy and relax stress loads. The chemical bonds that make up the polymer chains consist of carbon-carbon single bonds that are highly resistant to energy in the form of heat or ultraviolet light, as well as to chemicals.

There are several ways to evaluate the durability/life expectancy of materials including cyclic fatigue testing discussed earlier in this paper. Another way to evaluate long-term performance is to conduct accelerated aging tests in high-intensity UV light chambers.

This methodology was used to study the durability of a 3M VHB Tape used for structural glazing (glass panel bonding in window and curtain wall applications on buildings) compared to the gold standard in the industry for structural glazing: structural silicone sealants. All 3M VHB Tapes are acrylic pressure sensitive adhesives and the durability of the tapes within this tape family are expected to have similar durability performance attributes, but additional testing may be appropriate for a specific tape based on the durability requirements for a specific application.

Accelerated aging was conducted at the 3M Weathering Resource Center in St. Paul, MN, with exposure up to 10,000 hours duration. The exposure used a 3M Proprietary Test Condition that has been found to be a good predictor of service durability. This 3M accelerated exposure test has proven to be a more realistic predictor of outdoor exposure results compared to ASTM G155 Cycle 1. Nominally, it provides a 2X to 3X acceleration over ASTM G155 Cycle 1. Test acceleration with accuracy is achieved by a radiant light source that very closely matches the ultraviolet component of sunlight. Note: The methodology used for the development of this test is described in R. Fischer and W. Ketola, Accelerated Weathering Test Design and Data Analysis, Chapter 17, Handbook of Polymer Degradation, 2nd Edition, S. H. Hamid, Editor, Marcel Dekker, New York (2000).

3M VHB Structural Glazing Tapes G23F and B23F (each 2.3 mm thick) were bonded between clear float glass (6.4 mm thick) and metal (black anodized aluminum) with UV exposure directly through glass. Test configuration was 1” x 1” (25.4 mm x 25.4 mm) tensile mode (ASTM D897). Samples were run in duplicate for these tests. Samples of a well-known and industry-accepted 2 part structural silicone sealant were also evaluated in this test study in the tensile mode. The geometry of these samples was the same except for the thickness of the structural silicone sealant, which was 9.5 mm. This data is provided below.

Figure 13: Graph of accelerated aging test results for two 3M VHB Tapes and a 2-part silicone sealant. (Image: 3M.)

The performance of the 3M VHB Tapes was equivalent to the structural silicone sealant in this extreme accelerated aging test, which included moisture exposure and UV exposure well beyond what is required in ASTM standards for glazing sealants and demonstrates the performance of this tape for applications requiring long-term durability.

Conclusion

Automotive display bonding is a demanding application that requires high performance from the bonding adhesive due to the stress loads associated with this application.

Through the use of mechanical property and performance data, along with Finite Element Analysis simulation, a designer or engineer can consider an appropriate adhesive, such as a 3M VHB Tape, to meet the demanding requirements for their application.

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