Tuesday, 02 September 2008 16:09

HEY TOBY 7: Drilling Down on the (New Steels) Technical Bafflegab

Written by Toby Chess

Hey Toby---I recently attended I-CAR’s Advanced Metals class and found the class interesting, but too scientific. I have also read a couple of articles in Autobody News on the same subject, but again, it’s complicated. Could you possibly shed a different light on this subject and make it a little easier to understand?
---Not Albert Einstein from Los Angeles

Hey Not Albert---I felt the same way as you did the first time is took the SPS 07. Furthermore, I teach the class and still find it a little difficult to comprehend, but it is getting better (the more I read, the better I get). I will try and give you a different perspective on this subject that might be easier to understand.
    To begin, we need to look at vehicle construction and how collision energy flows through a vehicle.
    First, a vehicle is divided into 3 sections: the end, the passenger compartment, and the rear section. The ultimate goal of vehicle design is to protect the vehicle’s occupants when a vehicle is in an accident. In I-CAR’s VLV 01 (Introduction to Volvo), a Volvo station wagon is crashed into a barrier at 35 MPH (Damage is to left front). The front bumper, headlamp, left lower rail, and apron are destroyed. The air bags are deployed and the front windshield is cracked and yet with all this damage, the left front door can be opened and closed with ease. This crash is a classic demonstration of energy absorption. In other words, as the vehicle front structure was being crushed and deformed, the collision energy was bleeding off as it traveled through the sheet metal.
    Energy absorption is the process of dissipating collision energy resulting from a frontal and rear impact. Energy absorption is achieved through the deformation of the metal. In other words, as the metal is being damaged, the collision energy traveling through it is being dissipated. Front inner structure can be reduced 45 percent in length, but only a 2 percent reduction in length occurs in the passenger’s section. Whoa!! That’s amazing. Let’s take a look at how energy absorption is achieved.
    New car design is achieving energy absorption by a number of methods. First and foremost is the use of crush zones, convolutions, slots and holes in construction of the front and rear inner structures. When a front or rear collision occur, collision energy is moving through the parts and when it reaches a crush zone (acting like a barrier), the energy starts to slow down. Think of a car coasting down a street. If the car stays in the center of the street, the car will maintain or gain in speed (depending on slope of the street). Let’s say that the car as it is going down the street, keeps crashing into the curb or wheels are continuously rubbing against the curb, the vehicle will slow down or stop. I know this a crude example, but I think you can get the point. The other type of energy that we need to look at is energy transfer.

Energy transfer is process of moving energy away from the impact as fast a possible. This type of energy movement is achieved through part strength. I would like to discuss this type of energy in depth, but I think it is necessary to look metals, vehicle design and a bit of history first.
    Do you remember when cars looked like this?

The doors weighed 300lbs, chrome everywhere (I am not referring to plastic coated to look like chrome-plated steel), drive shafts, vehicle gross weight approaching 3 tons (I am exaggerating that just a bit), and gas miles of 7 miles to a gallon of gas. In 1973, we had our first oil embargo. Gas went from 38.9 cents per gallon to 55.1 cents per gallon by the end of that year. Even with gas prices rising, the automotive industry did nothing to change its ways. We still produced behemoth vehicles. We were hit with another embargo in 1979 and gas rose from 80 cents per gallon to $1.10 per gallon during the first quarter on 1980. With weight being the primary factor in gas mileage, the automobile industry needed to look at reducing the weight of its cars and this was accomplished by shifting from Body Over Frame construction to Unitized construction. Now is the time to start to get an understanding of metals. Check out the picture below.

    Let’s look at mild steel. Mild steel has a rating of 210 MPas (megapascals) or less and it has low strength. Looking at the elongation side of the graph, you will notice that mild steel has a high percentage of elongation or a high degree of stretching before it will break. This mechanical property can be seen with the golf balls. With a lot of space between the molecules, this allows for the metal to be more flexible. The same facts allow mild steel to be more repairable, easily formed and heated (within certain limits). To get strength with mild steel, vehicle manufacturers need to increase the thickness of the metal used, but there are limitations, one being weight and the other being the lack of energy absorption (in soft metal the energy runs into no barriers).
    As we switched from BOF vehicles to Unitized vehicles, we needed a stronger, but lighter metal for structure. In other words, high-strength steels were developed.
    Look at the cylinder in the middle with the marbles. You’ll notice that the space between the marbles has been reduced when compared with the cylinder with the golf balls. The reduction of space between molecules allow for more molecules in the structure. Therefore, a part made with high strength metals becomes thinner, stronger, and lighter than the same part made with mild steel. You will also notice on the graph that high strength steels have a range of 210 MPas to 550 MPas. As these metals gain in strength, they lose elongation capacity—they become more brittle. So as the metal increases in strength, it will tend to break instead of stretching. To see this we need to understand how these metals are made.
    The use of heat, pressure, and cooling used in the manufacturing of the steels also affects the strength of the steels. In addition to heat, pressure and cooling, we can add alloys (such as nickel, chromium, carbon, etc) to the mixture and therefore add more strength to the metals.

    Since we have added heat to the manufacturing process, high-strength steels become very sensitive to heat, which means to you as repairers that the use of heat—if allowed—has to be closely monitored. Moreover, most of the HSS steels are typically cold strengthened.
    Let’s move to the next category of steels—Ultra-High Strength Steels (UHSS).
    UHSSs start at 550 MPas and go up and up. These steels are very strong and hard. Looking at the cylinder with the BB’s, you will notice that space between the molecules is smaller. We are now able to jam more molecules together to form a stronger and tighter structure. Since these steels are extremely hard, they are used by the OEMs to transfer energy.
    What this means is that the energy when it comes in contact with the UHSS, the part will not deform as easily as the HSS part and therefore will move the energy away from the part to other parts of the structure. This movement dissipates the energy. We are finding more and more UHSS in rocker panels, “B”-pillars (reinforcements) and “A”-pillars (also reinforcements).


    Question—Do you want the B-pillar to collapse on a side impact (like the front and rear rails) or do you want it to hold it shape and move energy away from the part? The answer is that we want the part to retain is shape and transfer the energy. With the federal government mandating to the OEMs for better side impact and rollover protection, you are seeing a greater use of UHSS in side reinforcements. What does this mean to you?
    First, these steels are so hard that they cannot be repaired. If you add heat to them, their strength drops below 200 MPas. Instead of having steels that transfer energy, you now have steels that absorb energy. The heated B-pillar that has been heated during the repair process will give way on a side impact and I surely would not want to be in the seat when that happens.
    Let’s add water to the cylinder with the BBs and freeze it (to simulate Advanced Steel alloyed with Boron). The structure becomes more rigid and stronger. If we removed the solid structure from the tube, the frozen unit holds together and stays extremely strong. What would happen if we were to take a heat gun to the frozen BB’s?
    You guessed it—The whole thing would fall apart. That is why we do not use heat on UHSS. Being extremely strong, the parts will take more force to pull them which leads to collateral damage to adjacent panels. Furthermore, estimators and appraisers will have to look deeper into the structure of vehicle for secondary damage.
    There was some damage to the rear bumper reinforcement (advanced steel alloyed with Boron) on this Volvo about the size of a grapefruit. By the size of the dent, you would not think there was any more damage, but guess again. Check out the damage to inner wheelhouse (both sides had damage).
    Look at how the energy was transferred away to the reinforcement to the inner wheelhouse area. Both rear fender liners were removed for inspection prior to an estimate being written. With a greater use of UHSS, you are going to need to make an investment in equipment.
    Some of the equipment needs will be electronic measuring systems, multiple towers for pulling and holding, upper and side measuring capabilities, inverter spot welders, plasma arc cutters and low speed spot weld cutter (Dentfix Corp).
    I strongly urge everyone to take I-CAR’s SPS 07 to gain a better understanding on these new metals. I will close by saying that you will replace UHS Steels if they are damaged.
    P.S. I-CAR has some great information on its online training program and you call also go to this web site for more information on steels: www.toolingu.com /definition-700200-22725-megapascal. html.

 

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