Ensure Aerospace Composite Quality with Force Measurement, Material Testing

Ensure Aerospace Composite Quality with Force Measurement, Material Testing

Composites are increasingly being used in aerospace and defense parts manufacture for several reasons including the high strength to weight ratio of materials, and due to the fuel savings generated by their lighter weight. Force measurement and material testing is an essential process for product designers and manufacturers to ensure part integrity, and to gain insightful data for creating the highest quality composite components.

Force measurement tests were originally calculated by using a series of mathematical equations, known as Newton’s first, second and third law. In its earlier years, force testing had been limited to handheld metrology devices. While faster than lengthy calculations and more accurate than guesswork, these machines do not provide the levels of precision needed for sophisticated applications.

Designing composites parts and components for the aerospace industry requires extremely high levels of accuracy, and production errors can be extremely costly. Stringent regulatory requirements ensure that components are safe, fully functional and reliable.

A chart showing elastic limit or the greatest amount of stress the material can have without taking a permanent set.

The AS9100 group of standards, for example, is a series of regulatory requirements that are specific to aerospace manufacturing. The regulations ensure that manufacturers produce components within a strict quality controlled environment, to guarantee reliability and safety of an aircraft. This quality assurance is particularly important for high volume manufacturing environments, where busy production lines are expected to produce a high volume of precise, identical parts and components. Similarly, the 21 CFR Part 11 Electronic Signatures requirement is very important for life science applications, such as medical device manufacturers and pharmaceutical manufacturers. Following this standard, software solutions that enable measurement data traceability and documentation are critical for the operators and supervisors responsible for the applications.

Sophisticated force measurement and metrology systems to test the components, can help simplify quality management and improve accuracy. Starrett’s force measurement software, L2 Plus and L3, for example, can provide a comprehensive analysis of a measurement test – providing exact force measurement results from simple peak load measurement to more complex break determination.

A chart showing elastic modulus. The modulus of elasticity represents the stiffness of the material under test. In tensile applications, this modulus is often called Young’s Modulus and is the relationship between stress and strain within the proportional limit.

By exporting measurement data through USB or wirelessly across Bluetooth, manufacturers can access data and insight far beyond the basic figures provided by other force measurement approaches. Inputting the requirements of a part, material or component allows the software to generate high-resolution graphs based on load, distance, height and time of measurement. In addition, in the case of the Starrett L2 Plus and L3 systems, historical test data is archived and available to analysis at a future date, helping speed up further tests and navigating potential problems or errors.

This intelligent software increases the accuracy of force measurement, while also improving precision for engineers designing and creating components. By gaining complete control with a system like this, design engineers are less restricted and can be more innovative with their designs. In addition, quality control managers can be confident that parts will meet industry standards and, as a result, are less likely to have manufacturing errors.

Material Testing Composites

Composites are made by combining two or more materials — often materials with very different properties. Material testing is another type of force measurement which can be very helpful for testing composites. The only difference is that the sample’s dimension is used to determine results. For example, a load result is called stress in material testing. Stress is the load result divided by the sample’s cross-sectional area. This is why stress has the unit pounds per square inch using imperial measurement. Using SI units, the common unit for stress is Newton per millimeter squared (N/mm2). N/mm2 is a MegaPascal (MPa). Stress = Force/Area. Strain is distance from force measurement. Strain is a unitless value, but is often shown as a percentage and referred to as % Elongation. Like stress, strain uses the dimensions of the sample, in this case, the sample length (or Gage Length). If the sample had an original length of 1 inch (25 mm) and then was pulled to 2 inches (50mm), the strain is equal to 100 percent. Strain equals Ultimate Gage Length minus the Original Gage Length divided by the Original Gage Length ([L1-L0]/L0).

Predominately, it is the advancements in polymer composites that are changing the way composites are used. Composites based on polymer continue to evolve and find their way into all kinds of products for aerospace, medical and automotive applications. Polymer composites have a high strength to weight ratio and are relatively easy and inexpensive to manufacture.

Product designers and OEMs want to ensure their polymer composite can withstand the force that will be placed on it. They also need to know if the material will stretch or elongate and pinpoint its exact breaking point. The major objective of any test and measurement process is to build a coherent set of materials data, but in the case of composite materials, one size rarely fits all.

Software Solutions for Composite Testing

The diversity of composites presents difficulties when establishing a coherent data set. The data are likely to be completely unique to each sector, product, application and area. The most common tests for tensile strength (MPa or PSI) are tensile chord modulus of elasticity (MPA or PSI), tensile strain (%), Poisson’s ratio and transition strain (%). However, when testing composite materials, the application should not pre-suppose any prior knowledge of which measurements are required.

A chart showing tensile strength or the maximum stress observed from the stress-strain curve.

Using Starrett L3 software as an example, rather than providing pre-set data, the user creates a test method for the specific material. Using this technique, a product designer or OEM can analyze the stress, strain, load, distance and time for each material, with measurements displayed on graphs and data tables with statistics and tolerances. Tests can use tension, compression, flexural, cyclic, sheer and frictional forces.

The unfamiliarity of composite materials requires mechanical testing throughout the entire design and production process. Consequently, automation is becoming increasingly attractive to manufacturers eager to reap the rewards of composite materials, without wasting time on endless manual testing and measurement.

Automated software packages should be capable of creating an interface that links hardware and software to improve processes from the lab, right up to the plant floor. For force measurement software applications, programming experience should be optional, not essential.

Anatomy of a Force, Material Testing System

Understanding what comprises a force and material system is helpful when starting to work with your equipment supplier to determine the best design to meet your application needs.

Regardless of application, every Force Measurement and Material Test System has the same basic components including a test stand, load cell, fixture, and software.

Test stands perform the movement in a tension or compression test and are controlled by the software and tablet, or PC interface. Test stands come in a wide range of sizes and travel options which are selected based on the application at hand. Economic, compact single column stands are ideal for quick checks for incoming first-article inspection or in-process inspection. Other single column stand models are optimal for simple load, distance, and break applications as well as more complex material testing such as performing tests for tensile strength, stress, or strain. Dual column stands are well suited for the previously mentioned tests as well as for higher load capacity testing.

Fixtures are required on every test stand. These components securely hold a workpiece during a test to ensure accurate and repeatable results every time. Fixtures are the largest category of components for a test system for properly accommodating the thousands of applications where force and material testing is used. Examples of fixtures include platens, vice-action types, rope/ bollard, flexural, wedge-action, eccentric rollers and many more.

Load cells are also required for every test stand. A load cell is a type of strain gage and is the part of the system which measures the amount of force being applied during a test. There are various load cell configurations and capacities, depending on the test stand being used. However, there are three primary load cell types such as full-bridge, “S-beam”, temperature compensated gage instruments designed and optimized for basic force testing applications. These S-beam sensors feature high axial stiffness and minimal deflection at full capacity, improving measurement accuracy. Also, models with a similar design but with low profile sensors are optimized for material testing applications.

A third versatile load cell type is designed for force measurement, as well as some material testing applications. Considerations for sensor selection include economy and value, non-laboratory environments where dirt, oil, dust, and debris may be present and the load requirements.

Software is an integral part of every test system and the level of complexity of the tests being performed will determine the optimal choice. Software is available for force applications including incoming inspection, high-volume inspection, demanding and complex applications. Software is also available for advanced material testing and characterization such as Young’s Modulus, Ultimate tensile strength and more being performed by research and design engineers, and quality control and test personnel. As always, it is prudent to discuss your application needs with your force equipment supplier.

This article was written by James M. Clinton, Inside Sales Manager, The L.S. Starrett Co. For more information, visit here .