Sandia Labs Analyzes Impact on Composites Using Nondestructive Testing Methods

Researchers David Moore and Timothy Briggs from Sandia National Laboratories and their teams are analyzing the inner parts of a composite material. Moore had a rectangular piece of carbon composite material with a surface that was smooth with a weak woven pattern. This piece had normal wear and tear, until the material was turned over, and circular impact marks with cracks could be seen. The research teams wanted to find out if the impact caused considerable, hidden damage within the composite.

Sandia National Laboratories technologist Andrew Lentfer passes a roller probe over a composite as researcher David Moore checks data on a screen. The nondestructive testing technique sends sound waves into the composite material, returning data with each swipe of the roller probe. (Photo by Randy Montoya)

They are creating nondestructive techniques to spot damage in composites with the aid of conventional medical inspection methods; such as sonograms and X-rays, and modern techniques such as ultrasonic spectroscopy, infrared imaging, and computed tomography.

For years Sandia has been involved in analyzing composites to test if the lightweight materials can be utilized in national security applications. A composite used in a cell phone do not need a very long service life.

Materials for national security applications must survive for decades. This makes you think differently about where and why you would use a material. We need to study the lifecycle of a component. We tend to think deeply about the consequences of fracture or deformation and how we can verify what happened.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

The research forms a part of Sandia’s national security mission, which includes energy efficiency and performance optimization in wind turbine blades, or lightweight vehicle.

Composites combine separate materials possessing different characteristics into one. In most cases, composites comprise of a soft polymer matrix with reinforcing fibers, such as glass, carbon, or Kevlar.

Composites make use of carbon strands, which are thinner-than-a-hair, capable of yielding a high strength-to-weight ratio. The final strength and shape is achieved after once the part is heated in an industrial oven. Heating sets the polymer resin, and yields the qualities needed for a structural component.

The use of composites is important in industries such as aerospace, as they are lighter than metals yet still strong. They can be bonded to metal to be utilized as aircraft wings,  making the aircraft lighter and less expensive to fly.

The Outer Surface of a Composite Doesn't Indicate What is Inside

We have a rich history of understanding metals and their failure mechanisms. Composite materials are very different.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

For example; if a service truck reversed into a composite aircraft fuselage, damage under the surface may have occurred that was undetected in the examination of the impact. This situation highlights the importance of non-destructive methods, which can completely test the way composites react in different circumstances.

The research team is testing the precision of non-destructive techniques and the ways they could be applied on a production floor.

You have to know what could go wrong in the processing steps and how to circumvent those, and then you want to make sure if you’re going to make one or a hundred or a thousand that you’re making them the same way all the time. Once we establish the limits of detectability, the threshold of good, bad and questionable, we’ll be able to say, ‘We want this composite bonded to this material with a defined quality and it shall be inspected with this technique.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Sandia’s Lightweight Structures Lab is involved in characterizing and consolidating materials to research, using specific stack sequences of composite material layers to customize stiffness and strength.

It collaborates with the National Security Campus in Kansas City, Missouri, in numerous activities such as; constructing prototypes, developing process techniques, qualifying designs for specific applications.

Once the composites are created, the fabrication lab cuts out samples for instrumented experiments e.g., carefully impacting the sample to analyze fracture, deformation, and damage growth. Briggs “pulls, stretches, torques and crushes them. He performs these mechanical experiments so we can understand the fracture mechanisms around failure,” Moore said. “Then we try to detect some of those failure modes.”

Characterizing material properties helps computer modeling, simulation

The fundamental characterization of composites measures material properties and structural characteristics, which in turn provides information to validate computer modeling and simulation.

Timothy Briggs, Sandia National Laboratories

The data from the destructive tests is compiled with the non-destructive assessments taken from Moore’s team to understand the reason behind why a material responds in a particular manner. This research effort is also in close partnership with Sandia’s materials characterization groups and modeling and simulation colleagues to help authenticate their computer-based simulations.

Composites have to support a specific size and weight.

Anyone can build a structure to carry the load, but we have to design our structures to fit within a geometric envelope and be lightweight. We cannot simply over-engineer to unrealistic levels. We have to be very smart and efficient with our designs, yet provide enough margin for long-term reliability.

Timothy Briggs, Sandia National Laboratories

When materials are bonded in an autoclave or oven, they often possess varied thermal expansion rates. For instance, aluminum expands faster than fiber-reinforced plastics. As a composite cools after curing, residual stresses tend to build up within it, especially at interfaces. The bonds will fail if the composite is unable to handle those stresses.

Sandia is preparing modern methods to perform full sample inspections, within 5 minutes in some cases.

This is a way to gain a lot of information very quickly about the quality of the bonds.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Team studies specific nondestructive techniques

His team utilizes the purposely mistreated composites to evaluate inspection methods such as flashed or active thermography, advanced ultrasonics, and computed tomography.

For about six decades ultrasonic testing has been used, computers and other enhancements can now facilitate research of more intricate applications. Technologist Andrew Lentfer scans a piece of composite material using a handheld ultrasonic roller, which looks like a small paint roller with a hollow, water-filled barrel. During the scan, the layers of the composite can be viewed on a computer screen in color. Yellow-green indicates OK, but blue indicates weakness. Rollers have the capacity to scan curved surfaces, even as large as airplanes.

Interfaces and fibers in a composite scatter ultrasonic waves traveling through the material.

If a wave hits a rock face in the ocean it moves around it; if a wave washes up on the sand it gets absorbed; and if it hits a seawall the wave energy is redirected quickly. Those are the same fundamentals we investigate: ultrasonic energy moving through a composite matrix.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Characterizing materials means knowledge can be gained, leading to the development of novel non-destructive methods “so when we establish an inspection criterion, we have a better feel for what we can detect and what we cannot,” Moore said.

For over two decades, flashed thermography has been commercially available. This technology can be used to flash the surface with very high-energy light for 15 microseconds. Using an infrared camera the surface can be viewed to see how it cools. This technique can be completed in a few minutes.

It’s very fast, but you have to understand the fundamentals of heat flow and how the material surface either gives off heat to its surroundings or transfers heat within itself.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Studies use good, not-so-good composites to test differences

The research made use of composites with superior quality bonds, as well as others intentionally having weaker bonds. The differences in outcomes assist the team in improving detection of damage or defects.

A computer screen displays dark or light spots representing potential problem areas, and an overlaid graph illustrates the depth of the potential problem to the time mentioned on the image.

The question becomes, is that a concern? Is that a crack or not? We’ll be able to answer those questions. If the defect propagates deep into the material, we may not detect it. It’s wise to understand the capabilities of the technique and then perform the math and science behind it.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Computed tomography systems are good for locating minor defects. The method rotates a sample 360° and at the same time takes 1,000 images, just like a medical CT scan. It then produces an image from each thin slice of the object. As each image taken is 2D, computer algorithms will have to be used for reconstructing, calculating, locating and illustrating everything to symbolize the object in 3D.

Once the 3D image is reconstructed, you look at the front surface and then start moving through the thickness to view what is below the surface. This technology gives us a knowledge baseline and validates how the other techniques are performing.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Sandia will have to guarantee that the designs match the requirements by specifying and qualifying an inspection method, and is currently framing inspection processes for the National Security Campus.

Once we establish criteria and limits of acceptability, the product definition can be established.

David Moore, Structural Dynamics and X-ray/Nondestructive Evaluation department, Sandia National Laboratories

Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corp., for the U.S. Department of Energy’s National Nuclear Security Administration, operates the Sandia National Laboratories, which is a multi-program laboratory.

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