Unified FEA saves on back-and-forth of windshield wiper design

  • 23-Feb-2012 04:26 EST
Hyundai_intro_B_FEAwindshield.tif

Hyundai engineers wanted to develop a unified analysis model with which they could study both wiping and durability. Abaqus Unified FEA allowed them to analyze every aspect of the characteristics they were interested in studying within a single software package.

When asked to name the most important safety device in a car, a popular response might be the seatbelt or airbag. That answer might be correct on a sunny day, but it would be near impossible to drive a single city block in a rainstorm or blizzard without windshield wipers. It’s the inexpensive wiper that protects the driver’s most important asset: his or her vision.

While the blades need periodic replacement, the wiper system itself is considered one of the most reliable of vehicle devices, with a design life average of 1.5 million wipes. Reliability and motorist safety are key tenets of Hyundai Motor Co., so it comes as no surprise that the windshield wiper has been the focus of considerable attention at the company’s R&D center.

“We are looking to optimize both wiping performance and fatigue life of the windshield wipers on every vehicle Hyundai makes,” said Sungjin Yoon, research engineer, Hyundai. “To achieve that, we need to consider all the critical design factors such as blade shape, material, and the structure of the wiper arm and links. CAE plays a central role in that process.”

Using a variety of CAE tools to evaluate the geometry, kinematics, loads, and stresses on wiper mechanisms during the initial design stages helps Hyundai engineers meet current wiper performance guidelines. The tools also let them predict the effects of design modifications employing new materials, lightweighting, and/or innovative wiper configurations. Early identification of the most efficient design for each new vehicle concept both speeds up the product development process and reduces prototyping and manufacturing costs downstream.

At the core of the team’s CAE work is simulation with Abaqus Unified FEA from Dassault SystèmesSIMULIA brand. “The physics of windshield wipers are more complex than you might think,” said Yoon. “Previously, we’d been performing separate analyses for wiping and durability using two different FEA programs on two different computer models, but we wanted to develop a unified analysis model with which we could study both. Abaqus has a full range of simulation capabilities that allow us to analyze every aspect of the characteristics we were interested in studying within a single software package.”

In addition to its essential back-and-forth behavior, a windshield wiper must maintain uniform distribution of pressure between the windshield glass and the wiper blade so the driver’s view remains clear (known as “the squeegee effect”). As the blade ages, it must continue to provide a certain level of cleaning ability. And the entire wiper system needs to be strong enough to withstand a variety of loading conditions, from wind-driven rain to heavy snow, and durable to last for many years of operation.

To begin its unified FEA inquiries into how wiper design affects both wiping and durability, the Hyundai team started by looking at the rubber blade alone. Utilizing a material model that fully captures the behavior of the rubber is important to accurately predict wiper blade performance. The rubber material exhibits nonlinear elasticity as well as a certain amount of time dependency in its response. Capturing both these phenomena together is accomplished with a combination of hyperelasticity and viscoelasticity material options available in Abaqus.

Next, the engineers model the wiper arm system, a highly interdependent series of links and arms that combine to provide uniform pressure over the full length of the rubber blade (important for avoiding streaks on the windshield). To facilitate its FEA modeling, the group uses kinematic visualization software with a specific template for each vehicle wiper arm type. A wiper arm assembly is essentially a mechanism, the joints of which are modeled with Abaqus connector elements.

Now the team is ready to run its full Abaqus models to evaluate wiping. The first step is to properly load the wiper blade against the windshield to establish the correct initial contact pressures against the glass. The axial connector element in Abaqus is used to express the behavior of the arm spring. “It’s important to establish what the preloading strain is,” said Yoon, “because it needs to be taken into account as the wiper moves, when the deformation shape of the blade section on the glass will affect the results over the total wiping area.”

A rotational moment is loaded at the base of the wiper arm to replicate the motor torque, at the point where the arm pivots, that causes the arm to slide across the glass with a wiping motion. The analysis can subsequently track the total strain on the blade at specific times and varying temperatures (the latter can particularly affect blade rubber). Final outputs are reported through an in-house post-program that shows the pressure distribution throughout a complete sweep of the windshield glass. These results can then be compared against established design specifications to determine if the blade is performing as expected.

The second part of the analysis, for durability (fatigue life), can be performed on the same FEA model that was set up for the wiping sequence. “All we need to do is change the boundary and loading conditions in our Abaqus Unified FEA model,” said Yoon.

Just as was done with the wiping analysis, a rotational moment is entered as a loading condition. But this time, stationary "snow blocks" are mounted to halt the wiper blade before the lower and upper extremes of its sweep across the windshield glass model. As the analysis is run, the moving blade arm contacts the snow blocks, and the resulting internal forces on the linkage as the blade stops can be determined. Stresses corresponding to these internal forces are used to predict fatigue life during moderate snow conditions or, in the case of heavy snow, plastic deformation or failure within the wiper assembly. These predictions are then validated under real-world conditions that include actual snow deposited on the windshield of a typical mass-produced car in Hyundai’s test facilities.

With their wiping and durability results in hand, the engineers can then optimize both by varying different design parameters and observing their effect on the overall performance of the wiper system.

“The superiority of the unified analysis model is that it enables us to consider a wide range of design factors affecting both wiping and durability,” said Yoon. “With an optimized blade arm and link combination, we can achieve a lighter weight wiper system while also increasing key link stiffness measures and reducing maximum stresses at the same time.”

Lynn Manning, Vice President, Parker Group, wrote this article for AEI.

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