The advent of the Hybrid III crash test dummy marked the beginning of biofidelic anthropomorphic test devices. During the development of its critical components, notably the head, neck, knee, and thorax, biomechanical cadaver test results were incorporated into the design. The result was a dummy that represented the 50th percentile male during idealized impacts.
In order to achieve a more biofidelic response from the components, many exotic materials and unique designs were utilized. The thorax, for instance, incorporates a spring steel rib design laminated with a viscoelastic polymeric composite material to damp the response. This combination results in the proper hysteretic losses necessary to model the human thorax under impact loading conditions. The disadvantage of this design is that the damping material properties are highly sensitive to temperature. A variation of more than 5 degrees Fahrenheit dramatically affects the response of the thorax. Under the hot photographic lights associated with crash testing, it is difficult to maintain the mandated 70 degree to 72 degree Fahrenheit dummy temperature window.
To examine the cause of temperature dependence, physical testing was performed on individual Hybrid Ml ribs to characterize the damping material, and a finite element model was formulated. Results from both the physical testing and the simulations using the finite element model showed 20 to 30% variation in sternal deflection over a temperature of 63 to 78 degrees Fahrenheit. Close examination of the damping material characterization showed that its Young’s Modulus dropped nearly an entire order of magnitude across the temperature range. This variation greatly affected the overall stiffness of the rib during impact and is the leading cause of the thorax temperature dependence.