Finding flaws in missiles

Engineers at Purdue University have designed and tested a structural health monitoring system to detect flaws that could hinder the performance of new types of military missiles made of composite materials.

Missiles are sometimes damaged when struck by rocks and debris kicked up by helicopter rotors or when mishandled during shipping or maintenance.

Unlike missiles made of metallic alloys, which often show external signs of damage such as cracks or dents, damage in the new ‘filament wound’ composite materials may not reveal telltale signs, said Douglas Adams, an associate professor of mechanical engineering.

The new monitoring system uses a mathematical model to pinpoint the location and severity of impacts based on vibration data collected by a sensor called a triaxial accelerometer.

‘We have shown that 98 percent of the time we can detect, locate and quantify the force of impacts,’ Adams said. ‘This information is very useful because it enables the monitoring system to determine within seconds whether an impact is beyond the design threshold and is great enough to likely cause serious damage.’

Findings will be detailed in a research paper to be presented today during an International Society for Optical Engineering conference entitled Smart Structures and Materials and Non-Destructive Evaluation and Health Monitoring.

The Purdue research focused on the missile casing, a 7’ wide, 30’ long central segment located between the rocket motor and warhead.

‘The casing is essentially a cylinder that holds the solid rocket fuel, and it has to withstand the high pressures created as the fuel burns,’ said Adams.

The researchers used a 15’ tall ‘impact tower’ that rams a steel rod into the casing with enough force to punch holes in military armour. The testing simulates impacts from debris, tools striking the casing or mishaps.

The casings are made out of carbon fibre, Kevlar or other materials wound in layers. Missiles made of the composite materials perform better than their metallic counterparts because the casings are up to 40 percent lighter than those made from aluminium alloys. Their light weight also makes them less expensive to ship and easier to handle.

In addition to detecting damage caused by an accident, the technique also could determine how durable the material is after long-term storage and exposure to the environment.

‘The military stores weapons systems for a long time, sometimes years, and then they deploy them, so you want to know how they are affected by factors such as humidity, ultraviolet radiation and other conditions,’

said. ‘The ultimate goal of this work is to create a system that could be permanently installed in missiles to analyze the structural health in real time.’