California State University, Long Beach
Close up of machine crushing blue ice.

Sudden Impact

Many people hope to have an impact on their chosen careers, perhaps create change and leave a lasting impression. This is certainly true for researchers in the College of Engineering.

When Dr. Daniel Whisler makes an impact, he usually leaves a mess, but sometimes it’s a hole or even total destruction. For the assistant professor of Engineering, the bigger the bang, the better the result. And the impression he makes won’t just change a life, it might save it.

Whisler specializes in understanding dynamic material and structural behavior when subject to extreme environments, such as shock, impacts or blast loads. His research in the Mechanical and Aerospace Engineering department focuses on developing experimental and computational techniques in vehicle crashworthiness, projectile impacts onto structures, explosive blast mitigation and protection from natural disaster.

It’s little wonder that Mythbusters is one of his favorite television shows.                                                                                                                                                                                                                                                                                                                                                        

Dr. Daniel Whisler and his graduate students prepare to determine the impact rate on various materials using explosive testing.

“My research is mostly to make things safer,” Whisler said. “Take car crashes. Since the 1950s, we have used crash dummies – a really hard plastic dummy that’s supposed to represent you. But we don’t really know how the shock wave goes through you (because you can’t use real people).”

This is why Whisler and his graduate student-led team focus on increasing safety by improving the materials in things such as cars, airplanes and buildings. They specialize in understanding dynamic material and structural behavior when subjected to shock, impacts or blast loads. And the best way to test these materials’ high strain rate is with blast mitigation and impact resistance.

Watermelon explodes when hit by high-spped projectile
Projectiles are shot through various materials — even watermelons – to test the damage caused by impact.
High-speed camera
A high-speed camera captures images of the projectile hitting materials and subsequent damage.
Ballastic gel is pressed by impactor
Ballistic gel is used as a substitute for human tissue in testing explosive impacts.  Photos by Sean DuFrene

Whisler likened it to the stress your knees experience from everyday activities.

“When you step, or walk normally on your knees, there isn’t any problem,” he said. “But if you jump, your knees hurt because of the shock wave in there behaves differently. For instance, take Styrofoam. If you hit it hard, it behaves a little stiffer than if you slowly compress it. It’s called the strain rate.

“The faster you hit things, the more dependent they are on those speeds.”

The question for Whisler and his team is how fast do they want to go? And what does it take to make it fail? In order to help them find answers, they built two impactors at the Impact Research and Engineering Lab – low-strain and high-strain machines that act somewhat like a gun. The high-strain impactor uses compressed gas to accelerate projectiles to high speeds, while the low-strain impactor is powered by gravity.

The low-strain impactor is like a mallet that swings back and forth, and the sample experiences up to 100 Joules of energy during impact. A joule is a unit of energy that is used to measure mass times distance.

The high-impact machine, which uses a long barrel, can shoot projectiles up to 200 miles per second. A Split-Hopkinson pressure bar is a decades-old apparatus for testing the dynamic stress-strain response of materials. The bar, equipped with sensors, measures the force wave transmitted through the projectile. A high-speed camera captures the action on a computer screen.

Graduate student Rafael Gomez, Dr. Daniel Whisler check watermelon
Graduate student Rafael Gomez Consarnau and Dr. Daniel Whisler predict the impact location on the watermelon. Photo by Sean DuFrene
Dr. Daniel Whisler meets with students
Dr. Daniel Whisler (right) discusses the test set up with freshman Emily Sanchez and graduate student Rafael Gomez Consarnau. Photo by Sean DuFrene

Whisler said testing in this manner gives researchers an idea of what’s happening with the materials, namely carbon fiber, which has replaced the less-durable aluminum and titanium as the most used building material.

“We take all new design processes, new materials – eco-friendly materials – and push them to the limits,” Whisler said. “We have to be smarter with our materials and have to have advanced testing for them. It’s always good to ask ourselves why are we still doing things this way and is there a way to make it better?”

Whisler hopes to continue studying the impact of materials for the U.S. Army, only this time involving human tissue. He said testing is slow because they can’t use human flesh. Real tissue samples are regulated by the Federal Drug Administration, so his team currently tests with ballistic gel and hydrogels.

“We want to know when they (soldiers) get blown up, how do they survive,” Whisler said. “We want to find out how does a wave travel through tissue, from both an experimental and impact testing aspect, and then we can begin measuring it with sensors.”

Once Whisler and his lab can determine how a wave travels through tissue, he said, they would begin testing tissue with bones.

Rafael Gomez, an engineering graduate research assistant in the Impact Group, said using real tissue samples would help them “define with the greatest accuracy the model we are developing.”

“We are using material simulants of biological tissue, such as gels and hydrogels, to extrapolate results at this early stage since they have been extensively used by the biomedical field for many human tissue substitutes,” Gomez said.

Gomez said he finds the testing methodology used in the Impact Group encouraging from a mathematics and physical standpoint because it can lead to the improvement of healthcare treatments. He said the research will help them understand how the body will at the microscopic and single-cell level after an impact.

The Impact Group is also looking at the damage caused by impact testing from the macroscopic level, which will provide valuable information on the stresses and deformations. Procedures used, such as the Finite Element Method model, can be used to improve treatments for patients.  The Finite Method model is a powerful technique developed to help solve problems in structural mechanics.

“Some procedures can be used to improve treatments for patients by predicting the scope and consequences after receiving an impact” Gomez said.

Pineapple explodes when hit by high-spped projectile
Particles scatter when hit by a projectile in a high-strain impactor that tests what happens in airplane and car crashes. Photo by Sean DuFrene

Searching for Tiny Changes

Delving deeper into a material’s structural properties is the work of Assistant Professor Dr. Yan Li.

From her position behind a microscope in the Mechanical and Aerospace Engineering department, she studies tiny cracks that the naked eye can’t see in structural materials. Small cracks that could have huge consequences in the safety of cars or airplanes.

Through her research, Li has discovered that it is not difficult to create materials that are tough or can handle deformation before fracturing. Creating materials that have both of these attributes, however, is a different matter.

Li said that inside various materials, there are microscopic particles that are embedded in the matrix and it is important to know whether the crack went around the particle or through it. If the crack penetrates the particle, it tends to break and cause catastrophic failure. The better alternative is if the crack goes around the particle.

“Once it goes around it, once it interacts with it, the crack will be arrested and go around. Then the crack is stable and won’t have unsteady crack prolongations,” she said. “If it goes around (the particle), the material survives (remains strong.)”

Li also is studying particle size and its relation to impact.

“I think it’s very clear,” she said. “If we have a very big particle and a very small particle, it’s very easy for the crack to go around the smaller one. Now, if it was even smaller, like nanoparticles, it’s even easier (for the crack) to go around it.”

Automobile and aerospace manufacturers use nanoparticles in their design, Li said, which helps vehicles and airplanes withstand the impact. She said if they can redesign the interface of cars and airplanes, the parts that make them weak, the crack can be guided around the particle

Li was awarded a $800,000 grant from the U.S. Army Research Lab in 2016 to fund her study which could lead to the development of stronger, tougher materials that could be used in cars and planes. First-year funding of $200,000 went toward the purchase of a powerful digital image camera that allows her to better study minute particles. Her team hopes to create an integrated computational and experimental framework to quantify the energy allocation within plastic deformation, heat generation, crack surface formation and inertia energy dissipation.

“(With a high-powered digital microscope) we would see if the material could be better. Can we make it better?” Li said. “(The microscope) would enable us to find out, without doing multiple testing, to see if we need another method to predict the material properties instead of doing trial-and-error testing.”

Whether it’s through high-impact blasts or delicate under-the-microscope studies, researchers are moving closer to developing stronger, faster and more resilient materials that will protect future generations.