ABSTRACT
In the late 1990s, the National Highway Traffic Safety Administration (NHTSA) began performing side impact tests in which a moving deformable barrier (MDB) is towed at an angle and collided into the side of a stationary vehicle positioned at right angles to the MDB, simulating an intersection crash. Consequently, both crash partners absorb crush energy, not just the test vehicle. The MDB face, designed for used in square-on frontal tests, is thus loaded in combined shear and compression, which changes its effective stiffness. Finally, both crash partners retain some kinetic energy when they separate, which complicates energy balance calculations. However, much information can be gained about the vehicle’s side structural stiffness if the pertinent data can be extracted.
This chapter presents stiffness data for the MDB that are appropriate to this test mode. The issue of residual kinetic energy is discussed, along with the means by which to evaluate it. The variability and lack of foundation for published side stiffness parameters highlights the need for reconstructionists to have a means of calculating their own As and Bs from public-domain test data.
When a constant-stiffness crash plot is being constructed using only one crash test data point, the crush energy condition for that test can be, and is, satisfied exactly. Since two parameters are being sought, a second data point is needed; it comes from making an assumption regarding the no-damage threshold; that is, the crash severity above which residual crush accrues. Typically, that speed is about 5 mph for frontal barrier crashes, but damage threshold information for side impacts has been nonexistent. The NHTSA conducted two series of repeated side impact tests using a moving rigid barrier (MRB), in which the test data were sufficient to allow analysis. No force saturation was seen in one vehicle (a 1985 Ford Escort), but the other (a 1984 Audi 5000) showed a saturation crush of about 7.1 in. Extrapolations from these tests indicate a restitution coefficient for threshold conditions of about 0.25, and a threshold severity of about 2 mph ΔV for the Audi 5000. At threshold conditions, a reasonable crush width for MRB and MDB impacts is taken as the width of the impactors: 66 in.
Chapter 22 presents two methods for analyzing single NHTSA Side Impact (SINCAP) tests. Method 1 applies to linear (constant-stiffness) structures. Such structures allow the crush energy calculations to be inverted by use of a crash plot, so Method 1 utilizes calculations appropriate to that procedure. Method 2 applies to nonlinear structures that do not allow the crush energy calculations to be inverted. Therefore, it utilizes a forward-looking iterative procedure that is implemented in a spreadsheet. This method allows for force saturation; it varies A and B successively until the crash test crush energy is correctly predicted, subject to a constraint equation that expresses the no-damage threshold conditions. Sample calculations are presented.
For linear structures, either method can be applied, in which case the two methods produce identical results. Needless to say, side structures have undoubtedly become much stiffer since the onset of the SINCAP program, let alone the 1984–1985 time frame. For recent-vintage vehicles, there are no known repeated-impact test series by which any force saturation could be detected. Therefore, present analyses have utilized an assumed saturation crush sufficiently high to keep all structures linear.
Other topics discussed in Chapter 22 include side-struck vehicles with high ground clearance, and the variability and repeatability of the results. Sensitivity analyses address the effects of making various assumptions and simplifications in the analysis.
