Surviving a Hit

Better testing and design methods for impact damage

1 Ratings

Imagine you are an aircraft design engineer or program manager responsible for creating the next generation of fighter airplanes that employs advanced composite materials to meet demanding weight requirements. You are confident that the structure is designed to withstand all structural loads based on industry-accepted design practice and rigorous standard testing, but you don’t know the effect of impact damage on the structural properties or post-impact performance.

THE CHALLENGE

A tool drops unexpectedly on an F-35 composite wing skin. A Global Hawk flying through adverse weather experiences an errant hail strike. An F-22 in a taxi lineup is hit by a piece of gravel thrown up and accelerated by the exhaust blast of the aircraft ahead of it. These are just some examples of the many ways that accidental and operational damage can occur to the composite structures present in fifth generation fighters and modern drones. These structures need to survive operational loads until the next structural inspection can be performed, and an extensive design and test program is required to achieve this reserve strength without adding excessive weight.

WHY IT MATTERS

Preparing a composite material for impact damage tolerance is costly and time-consuming. There is an industry-wide need for a standardized methodology that improves testing and analysis methods. Without a well-defined, thoroughly validated means of designing composites for impact damage, the best case scenario is an overweight aircraft design, which will have flight performance issues. The worst case scenario, however, is a design that will have compromised structural integrity after an impact event, possibly leading to in-flight structural failures. There are currently no widely accepted industry standards for testing and designing composite structures for impact damage tolerance. The ASTM committee outlines a procedure for impact testing, but aerospace companies such as Boeing and Lockheed Martin Aero utilize their own internal test and design methodologies. An industry standard acceptable to the industry at large is required for a unified approach to solving the design for damage tolerance problem.

THE SOLUTION

Materials Sciences Corporation (MSC) worked closely with Lockheed Martin Aero and the Boeing Company to obtain and evaluate legacy data and current information on compression after impact test standards and procedures. Building on our strong foundation and experience in understanding composite material behavior and composite damage modeling, improvements in both test and analysis methodologies were proposed and feedback was solicited. From this feedback, a new test standard was developed in close collaboration with both Lockheed Martin and Boeing.

"The project provides both the framework for how to scale up damage tolerance testing. More realistic design can be established for the actual strength reductions due to impact damage." — Dr. Dan Adams

HOW IT WORKS

Designing for impact damage is a compromise between saving weight and making sure the structure will have enough integrity after an impact to remain flightworthy until an inspection. If the design approach is too conservative, the aircraft will be overweight and suffer in maneuverability. But if the design method overpredicts the structural damage tolerance margin, the aircraft may suffer a catastrophic impact with disastrous consequences. The Compression Strength After Impact test methodology developed under Air Force SBIR Phase II funding builds on the ASTM D7137 Compression Strength After Impact test standard. As a result, an improved means for qualifying composite materials for an impact load condition—which encompassed both testing and numerical modeling—was developed.

IMPACT FOR THE FUTURE

The pilot flying the next generation fighter or bomber will benefit from the improved methodology, which enables engineers to push the efficiency of the structure to the limit without reaching a previously uncertain threshold that invites failure due to impact damage.

The test and design methodologies developed under this project have expanded MSC’s capabilities in aerospace composite structures design, testing, and damage assessment. Close interactions with aerospace prime contractors Lockheed Martin and Boeing have provided a greater awareness and understanding of requirements and challenges. These relationships have also provided opportunities for MSC to transition the technology directly to platforms with a need for enhanced damage tolerance.

Increased U.S. competition is anticipated through these enhanced test and design methods. These methods improve aerospace composite structures and design efficiency, while reducing the time required for product development, acceptance, and qualification.

These standards will revolutionize the future of Air Force composite UAVs and aircraft design, fabrication, and test, both optimizing performance and cutting costs.

The methods and modular test fixtures developed under this project help the designer to obtain the correct balance between performance and safety in modern composite aircraft structures. — Dr. Jaco Schutte, Project Lead at Materials Sciences Corporation

Materials Sciences Corporation

materials-sciences.com

135 Rock Rd, Horsham Pennsylvania, 19454

Materials Sciences Corporation (MSC) is a small engineering services business headquartered in southeastern Pennsylvania. Since 1970, MSC has been a leader in the design, analysis, manufacturing and testing of advanced materials and structures, with proven success in transitioning advanced material.

Jaco Schutte Jaco Schutte

Jaco Schutte

Senior Engineer

Dan Adams Dan Adams

Dan Adams

Professor of Mechanical Engineering

Gary Renieri Gary Renieri

Gary Renieri

Structural/Mechanical Engineer

TOPIC TITLE:

Novel Experimental & Analytical Methods for Designing Damage Tolerant Composite Structures

TOPIC NUMBER:

AF093-00

CONTRACT NUMBER:

FA8650-14-C-2473

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