Space News and Facts Newsletter #56
June 5 - June 11, 2022
Articles in this issue of SN&F …
1) All about Destructive Material Testing (DMT) and Non-Destructive Testing (NDT) in the aerospace industry. Long article plus four embedded videos and numerous links.
2) NASA is into retro video games? Surely not! Actually ... yes. Check out the "The Roman Space Observer Game".
What are Destructive Material Testing (DMT) and Non-Destructive Testing (NDT)? DMT is also sometimes called "Destructive Physical Analysis" (DPA). NDT is also referred to as "Non-Destructive Examination or Non-Destructive Evaluation" (NDE).
How do these engineering techniques affect the space industry?
Getting a rocket into space involves materials that must withstand the extreme temperatures and pressures of the launch itself. Once in space, the extremes of temperature and radiation need to be managed. Everything from a wall's ability to contain pressurized gases (oxygen/air) to the joints that hold pieces together, to the sealants between materials need to endure.
The goal of this article is twofold:
1) to provide an overview of the types of techniques used in DMT and NDT analysis.
2) to provide some links to space-industry-related NDT resources.
At what point does a material fail? Destructive testing is the only way to fully understand a material's characteristics and thus be sure just how far it can be trusted under extreme conditions. Engineers need to understand any material's ductility (deformation under stress), tensile strength (the point at which it breaks while being stretched), the ability to withstand fractures, and its response to extreme temperatures.
But there are times when deployed equipment must be inspected without resulting in damage. That's where NDT techniques are required. Material assessments are necessary from time to time to determine safety and maintenance timelines and to prevent or reduce operational failures.
Each of the DMT and NDT techniques described below has its own unique testing equipment and nomenclature, so it would be impossible to describe each here. These techniques apply to manufacturing in all industries.
Broadly speaking, DMT/DPA can be broken down into these areas:
1) Environment testing where materials are exposed to corrosive substances at extreme pressures and temperatures. Corrosives can be anything from chemicals to salt water.
2) Fracture/Mechanical testing including bending, impact, crush, and explosion testing.
3) Fatigue testing where joints are tested under varying loads/weights.
4) Hardness testing where materials are pounded with tools to determine what it takes to produce indentations.
5) Tensile testing is where a material is stretched and compressed to find its breaking point.
6) Torsion tests twist a material to see when it becomes deformed.
7) Residual stress tests attempt to determine how much stress remains after the original stress has been removed. Materials have a kind of memory, or lingering effect, caused by the original stressor.
While not directly analogous, software must also be tested for failure. This is usually done by other, automated software applying "bounds testing" to every input source in a program. One very common bound failure in software development is when the operating system allocates a specific amount of memory for an input variable, but the input received is greater than what was allocated thereby exceeding the "boundary" for the expected input. This corrupts adjacent memory, changing other values unwittingly and/or crashing the system.
NDT is more recognizable to the average person. The most common type of human NDT is getting an x-ray or ultrasound.
NDT/NDE for industrial purposes can be categorized in these ways:
1) Visual inspection of materials for faults, cracks, and corrosion failures.
2) Ultrasonic testing bombards a material with sound waves to observe the effect. One common test is the "pulse-echo" which can reveal imperfections when the expected reflections are not received. (FYI: Ultrasonic waves are higher than those a human can hear.)
3) Acoustic Emissions testing is different from Ultrasonic testing. Acoustic emissions come from the material itself. Sound waves are sent through an object (ie: a pipe) and the changes in the sound waves can identify defects.
4) Radiography, or x-rays, can identify imperfections in a material.
5) Electromagnetic testing applies a current to a magnetic field surrounding a material. Variations in the expected electrical current can indicate a defect. This is called "Eddy Current Electromagnetic testing" (EC testing).
6) Magnetic Particle Inspection is where a magnetic field is applied directly to a material. The material is then covered with iron particles which spread out in predictable ways. Unusual patterns indicate imperfections.
7) Liquid Penetrant testing requires a material to be coated/sprayed with a fluorescent dye. Excess dye can then be washed off, leaving it embedded only in cracks/flaws which are revealed through ultraviolet light.
8) Leak testing can reveal defects. Anything from water to soap can be injected into an object under pressure to locate faults. Subtle leaks can only be measured with appropriate pressure gauges.
9) Vibration analysis compares the known signature of a rotating object against observed variations. Anything out of the ordinary indicates a flaw.
10) Thermal testing involves observing unusual changes in the surface temperatures of an object.
11) Laser testing uses a variety of laser light techniques to identify tiny imperfections in a material. (The 3 most common techniques are shearography, holography and profilometry.)
To my surprise, it is Eddy Current testing and Ultrasonic testing that is widely used in the space industry. EC probes can, apparently, detect very small subsurface defects and can reach difficult areas. Ultrasonic testing is widely used to locate cracks, corrosions, and inclusions (foreign material embedded in another type of material).
Here's an informative video explaining the use of Eddy Currents to identify cracks:
Here is a video from Aerospace Testing International Magazine about the use of Ultrasonic NDT in aeronautics:
Some robotic leak testing from engineering firm Tecnatom:
A great (but lengthy) explanation of ultrasonic and x-ray inspection and geometrical measurement system from a large Austrian engineering firm, FILL Gesellschaft m.b.H:
NASA produces scads of public documentation about its "preferred reliability-practices for design and test" requirements. It's done through their "Lessons Learned" portal. Here is that link:
As one example, here is their article on "Ultrasonic Testing of Aerospace Materials":
The lesson learned from that article is this: "Failure to detect cracks, flaws, and voids in aerospace materials through the proper use of ultrasonic testing and other approved nondestructive evaluation methods could result in the use of weakened structures, unbonded propellants and insulation layers, and potential pressure vessel failures or burnthroughs due to increased propellant surface area, resulting in potential mission failure."
Conferences and Events
The Aerospace Committee and the West of England Branch of the British Institute of Non-Destructive Testing (BINDT) held their conference in Bristol on April 6 - 7, 2022. I have not seen an announcement for 2023 yet.
AEROSPACE EVENT 2022!
A major event in the space-related NDT world is the "International Conference on Nondestructive Testing in Aerospace." In 2022, it will be held on September 22 - 23 in London, UK. (See the link below.) Their website has not yet been updated and has carried over some of the papers from 2021. Those topics give a good idea of the techniques discussed. Here are a few examples:
- Laser Ultrasonic Imaging Based on Synthetic Aperture Focusing Technique Algorithm
- Software Obsolescence Drivers in Aerospace: An Industry Analysis
- Development of a Remote Testing System for Performance of Gas Leakage Detectors
2022 International Conference on Nondestructive Testing in Aerospace
NDT.net holds an annual event called the International Symposium on NDT in Aerospace. Their 13th event was held on October 5 - 7, 2022, and it was virtual this year. The 14th event for 2023 has not yet been announced.
International Symposium on NDT
These websites were helpful in researching this article:
Introduction to non-destructive testing
What Is Destructive Testing And How Does It Work?https://www.flyability.com/destructive-testing
What Is NDT (Non-Destructive Testing)?
NDT in the Space Industry
- End of Article -
The Roman Space Observer Game