Global economic conditions and market forces are encouraging the adoption of fiber-reinforced composite materials for designing modern airframes. The unique combination of improved fuel efficiency and serviceability is the major driving factor for the application of these engineered materials in airframes. Major airframe makers Airbus and Boeing have played a key role in the advancement of composite materials to handle design and service challenges. Composites are well recognized for weight reduction applications in the aerospace industry. Their strength and stiffness can be changed with directional loading. They are suitable for use in areas vulnerable to corrosion as well as in high fatigue load applications. Their customizability to obtain specific properties is a major factor driving the advancement of innovative composite materials.
Significance of Testing Aerospace Composites
It is necessary to test fiber-reinforced composites in order to support design and quality management programs. Characterizing the unique properties offered by composites through physical testing is critical to ensure their compliance with required specifications. Composite material testing solution provider ZwickRoell's Aerospace Industry Manager, Bill Becker informed that it is difficult to measure material properties of composites due to fiber orientation when compared to plastics and metals. Moreover, composite materials do not exhibit isotropic behavior, which further increases the complexity. Hence, composites demonstrate diverse material properties and failure modes in different airplanes and it is necessary to characterize them in multiple planes in order to better understand their material behavior.
The availability of a wide range of standards and test procedures further complicates the testing of composite materials. At present, there are over 150 standards available that outline the physical testing of fiber-reinforced composites. Besides national and international series of standards such as DIN, EN, ISO and ASTM, there are aircraft industry-specific standards designed by Boeing, Airbus and NASA, as well as standards developed by associations that no longer exist, like Suppliers of Advanced Composite Materials Association.
Testing Methods of Aerospace Composites
Flexural and compressive properties must be tested independently as it is not possible to predict them based on tensile properties. There are many different techniques available for measuring shear properties, enabling the characterization of properties in different shear directions. Non-ambient conditions such as humidity and temperature need to be considered for fatigue tests, which are critical for aerospace structural applications. Besides classical flexure, compression and tensile tests, there are many different specific tests for the evaluation of composites. Compression-after-impact (CAI) testing is utilized for evaluating the tolerance of a composite material to the damage caused by a bird strike or due to contact with other foreign objects during flight. Fiber composites are also subjected to tests such as open-hole compression; plain, open-hole and filled-hole tensile; end and end-loading compression; and shear. Thus, the three normal stresses are characterized in a nine-component stress tensor. The six shear stresses of this tensor may be determined by specific test techniques such as the lap shear test, the V-notch shear test, the ±45° in-plane shear test, and the short beam shear test for materials qualification.
Testing Solutions for Aerospace Composites
To meet the different requirements, testing laboratories and companies previously used many different testing machine configurations, some of which were highly complicated. The Allround-Line testing machine is a highly efficient modular solution from Zwick and is offered in 100 and 250kN models, with two different lengths. An optional temperature chamber enables the Allround-Line to accommodate non-ambient testing in the range of -70 °C to +250 °C. The new solution can perform a multitude of tests, ranging from lap-shear tests to determination of interlaminar shear strength (ILSS) to V-notched shear tests.
Figure 1. Zwick’s new Allround-Line system for testing of fiber-reinforced composite specimens contains dual test areas, supports the use of 13 different test fixtures and enables testing to more than 100 standards.
Besides enabling the measurement of properties of the entire composite, compression testing also provides data on fiber strength. Inducing compression deformation until the material fails and without buckling is a challenge. During the compression test, the specimen is placed between two support plates that are engineered to prevent buckling. Specimen grips without bonded tabs are employed for precisely axial loading the composite within the measurement range in order to measure compression modulus. Accessing the specimen will become difficult due to the use of fixtures in compression tests. This, in turn, increases the complexity of strain measurement.
The challenges involved in performing these test often lead to excessive specimen flexure. To resolve this issue, Zwick has developed hydraulic composites compression fixture (HCCF), which radically simplifies the clamping procedure and shuns wedge movement during the test.
Figure 2. The parallel-closing HCCF brings a high degree of rationalization to compression tests.
While test fixtures establish the contract between test device and specimen, software provides the operator access to test sequences, assessment, data storage and logging. Pre-configured standard test programs minimize operator engagement and ensure test repeatability. Besides offering results and curve graphics, testXpert II measurement and control software from Zwick facilitates typical misalignment and flexure monitoring functions.
For over 150 years the name of Zwick Roell has stood for outstanding technical expertise, innovation, quality and reliability in materials and component testing. Our customers’ confidence in us is reflected in our position as world-leaders in static testing and the significant growth we are experiencing in fatigue strength testing systems. The figures tell the same story: in the 2011 the company achieved incoming orders of €185m.
With innovative product development, a comprehensive range and worldwide service, this family concern supplies tailor-made solutions for the most demanding research and development and quality assurance requirements in over 20 industries. With over 1100 employees, a production facility in Ulm, Germany, additional facilities in America and Asia plus agencies in 56 countries worldwide, the ZwickRoell brand name guarantees the highest product and service quality.
This information has been sourced, reviewed and adapted from materials provided by ZwickRoell.
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