The dummy's job is to simulate a human being during a crash, while collecting data that would not be possible to collect from a human occupant.
All frontal crash tests in the United States are conducted using the same type of dummy, the Hybrid III dummy. This guarantees consistent results. A dummy is built from materials that mimic the physiology of the human body. For example, it has a spine made from alternating layers of metal discs and rubber pads.
The dummies come in different sizes (click here to see some of the dummies), and they are referred to by percentile and gender. For example, the fiftieth-percentile male dummy represents the median sized male -- it is bigger than half the male population and smaller than the other half. This is the dummy most commonly used in crash testing. It weighs 170 lbs (77 kg) and is 70 inches (5 ft 10 inches or 1.78 m) tall.
The dummies contain three types of instrumentation:
These devices measure the acceleration in a particular direction. This data can be used to determine the probability of injury. Acceleration is the rate at which speed changes. For example, if you bang your head into a brick wall, the speed of your head changes very quickly (which can hurt!). But, if you bang your head into a pillow, the speed of your head changes more slowly as the pillow crushes (and it doesn't hurt!).
The crash-test dummy has accelerometers all over it. Inside the dummy's head, there is an accelerometer that measures the acceleration in all three directions (fore-aft, up-down, left-right). There are also accelerometers in the chest, pelvis, legs, feet and other parts of the body.
A graph of the head acceleration during a crash test
The graph above shows the acceleration of the driver's head during a 35 mph (56.3 kph) frontal crash. Notice that it is not a steady value, but fluctuates up and down during the crash. This reflects the way the head slows down during a crash, with the highest values coming when the head strikes hard objects or the airbag.
Inside the dummy are load sensors that measure the amount of force on different body parts during a crash.
Photo courtesy NHTSA
A graph of the force in the driver's femur during a crash
The graph above shows the force in Newtons in the driver's femur (the thigh bone), during a 35-mph frontal crash. The maximum load in the bone can be used to determine the probability of it breaking.
These sensors are used in the dummy's chest. They measure how much the chest deflects during a crash.
Photo courtesy NHTSA
The chest deflection during a 35-mph frontal impact
The scan above shows the driver's chest deflection during a crash. In this particular crash, the driver's chest is compressed about 2 inches (46 mm). This injury would be painful, but probably not fatal.
Now let's take a look at a real crash test.