|Internal toughener sites |
Automotive designers rely on commonly used technologies (welding, screwing, and riveting) to join structural body parts. But they are often less comfortable specifying adhesives for use in critical joints. The reason: The unfavorable fracturing behavior of traditional epoxy adhesives.
Considering total vehicle cost, however, adhesives could let automotive OEMs economically join panels made from different materials while simultaneously acting as seals. Adhesives also lower the inevitable stress raisers common with conventional, discretepoint joining techniques. Stress raisers compromise vehicle durability and safety — shortening fatigue life and promoting stress corrosion within the basic body structure. To counteract this, designers typically size panels to withstand peak stresses. This results in panels being significantly thicker than would otherwise be necessary. In turn, vehicle fuel efficiency suffers.
To successfully compete with other primary structural joining methods, adhesives must let automotive assemblies pass one critical design case: the crash scenario. Automakers need the reassurance that during a crash, the various energy-absorbing parts of the vehicle will remain essentially intact and perform their function.
Recent advances in adhesive chemistries are opening new possibilities for automotive structural joints. One structural adhesive making headway is a crash-stable version called Betamate. It reduces stress compared to traditional joining technologies. And also highly resists dynamic fracturing forces in a controlled, predictable manner.
Betamate crash-stable adhesives are based on epoxy. Toughener polymers are added to the epoxy and finely disperse throughout its matrix. The toughner-"islets" range from a few hundred to a few thousand microns in diameter. They boost the dynamic fracturing behavior of the adhesive such that any cracking in the material takes place in a stable, controlled manner. This lets bonded structural members absorb maximum energy and helps keep joints intact while the crash takes place.
Betamate adhesives bond well to the slightly oily surfaces typical of stamped steel and aluminum parts. Their chemistry can be tailored giving vehicle manufacturers a number of processing options. They can be jet-streamed or extruded onto substrates and do not wash off during subsequent gross dipping operations of the whole automobile body. They work in combination with a few existing discrete point connections that provide some fixation before adhesive curing completes.
Once cured, the adhesives stand up to salt-spray attack and other harsh environments that vehicle joints see during their service lives. At low temperatures Betamate adhesives retain a significant proportion of their toughness. So they tend not to become brittle as is often the case with competing epoxies.
To resist cracking within a joint, proprietary-tougheners reduce the energy-density throughout the material.-Instead of concentrating all the energy at any potential crack, the toughener distributes the energy by a process known as cavitation — a continuous adhesive deactivation at each individual toughener site.
Each deactivating step absorbs energy, generating localized dilationary (triaxial) stress within the adhesive layer. This complex stress state leads to many localized fractures. The little cracks spread throughout the material and dissipate energy in a controlled manner. Clearly, though, this state never happens during normal vehicle operation. Only during a catastrophic vehicle crash would the joint drive apart and call the mechanism into play.
To help automotive designers understand the advantages that adhesives offer, Dow Automotive computer simulates what combination of stresses or loads acting on the joint will lead to its failure. Adhesives are complex and the nonlinear, detail response of the toughening behavior has been modeled using the Abaqus/Explicit finite-element program from Abaqus Inc., Providence, R.I. A computational model predicts localized failure as well as the ever-changing state of the adhesive as the toughener polymer is extended and exhausted. Simulations give significant engineering insight into the details of the toughening mechanism and will help chemists improve adhesive formulations down the road.
It is possible to dramatically influence how an adhesive responds in the presence of a crack by fine-tuning adhesive formulations. Betamate structural adhesives have toughening technology in the form of a complex internal arrangement of polymers. The polymer arrangement is specifically put in place to make the adhesive resist unstable crack growth. The crack resistance is especially important under dynamic conditions, typical of a crash scenario.
Researchers often investigate such crack growth using a compact-tension specimen (CTS) for quasi-static tests and an impact wedge-peel test (IWP) for dynamic cases. Only adhesives that have stable postcracking response are candidates for crash-stable applications. Adhesives lacking this kind of toughening technology, even if they can absorb approximately the same peak load, do not have stable postcracking response. And thus they will tend to see crack growth, both statically and dynamically.
The initial computational work took place using lap shear joints during development of the adhesive numerical model. The fully developed numerical model agrees well for the three thicknesses tested. Another advantage of the adhesive is that the relative insensitivity of the maximum peak load varies little with regard to variations in adhesive thickness from below 0.5 mm up to 1.5 mm. This is good news because the adhesive can accommodate the inevitable tolerances of the production process.
The computation also clearly shows how cavitation distributes the strain energy throughout the adhesive layer. The ever-changing internal state of the adhesive means its mechanical properties keep changing. This behavior necessitated using a complex Abaqus model to keep track of material changes. Earlier computational models using other finite-element codes couldn't track this subtle, complex interaction within the adhesive. Finite elements in which the adhesive has reached the fully cavitated state are automatically removed from the mesh.
Tests of both quasi-static and dynamic impacts took place on bonded tubes made from two identical hat-shaped halves joined together. Quasi-static tests unfolded slowly in a standard testing machine with samples sitting between two platens. Dynamic tests, on the other hand, took place in a special rig that let a falling mass hit the tubes from above. The Abaqus computational model took into account the varying material properties as a function of strain rate for modeling the dynamic test. Engineers determined the strain rate dependence of the important adhesive properties beforehand using specially prepared samples. This dependence is included in the detailed DOW Automotive model.
The general agreement between predictions and test was good. What is astounding is the fine detail that the finiteelement model has tracked, even down to quite subtle little folds in the deforming metal. The predicted force-versusdisplacement curve for the quasi-static analysis case also agrees well with measurements. The simulation shows that a brittle adhesive can't stand up to the advancing crack front. In contrast, a suitably tough adhesive resists the crack growth under the same loading scenario.
The location within the joint of the fully cavitated, failed adhesive that analysis predicted corresponds well to what has been found in practice. The adhesive reaches full fracture on the inside of the tube first with any spreading of the fracture front initiating from there. Betamate structural adhesive delays the rapid spreading of the fracture front. This behavior ultimately translates into an adhesive tough enough for automobile bodies.
Total reduction in height of the tube that the model predicted agreed well with the dynamic test results. For such a dynamic test, the forces and displacements are not measured (although, in principle such measurements are possible); the final height of the fully collapsed tube sample is a good gauge of the total energy the tube absorbs in the impact. The dynamic fracture toughness of Betamate lets the metal tube absorb the energy in a controlled way.
It is likely that simulation of crashstable vehicle joints will become important as mechanical engineers strive to fully exploit new design possibilities. Total energy management in the vehicle depends on tracking the joint behavior during the dynamic crash event. It's not enough to just point out the most highly stressed areas in the design.