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ATA Engineering: Material Fatigue Tester

Project Background

    When subject to cyclic loading conditions, materials exhibit different properties that are undetermined by conventional static tests. Failure of a material through fatigue can occur at loads far below its static strength and must be accounted when designing an object subject to cyclic loading conditions, such as a plane wing or a bridge joint. Fatigue can occur in any loading direction, such as shear or axial, and is largely a function of the magnitude of the load and the amount of applied cycles. As a result, knowledge of this relation is essential for the development of safe and reliable products.

    ATA Engineering is a San Diego based consulting company that specializes in design, analysis and testing of structures that are subjected to dynamic environments. For analysis of structural strength and fatigue it is important to have well-defined material properties. Because these properties are often not readily available, ATA needs the ability to test material specimens to characterize the required stress-strain (s-e) or stress-cycle (s-n) properties for use in the analyses. The limited amount of tests needed to be perform by ATA does not warrant the high cost purchase of a fatigue tester. In addition, ATA has an unused U.S. Varidrive Motor. The purpose of this project is to develop a mechanical fixture that uses the motor to perform 3-point bending fatigue tests.

Objective

    The primary goal of the project is to design, fabricate and deliver a rotary-motor driven 3-point bending fatigue tester.

Requirements

• Basic Requirements

  • Design and fabricate a stiff test fixture that connects with the motor

  • Provide ability to test both "Standard Uniform Material" and "Standard Sandwich-Core" Specimen in 3-point bending

    • Design and fabricate a mechanism or mechanisms to hold test specimen without materially affecting the test process

  • Provide ability to vary force into specimen

    • Max force through load pin of 5,000 lbf (~22.2 kN)

    • Control of force does not need to be “real-time.” It is acceptable to stop testing and manually adjust machine to vary forces

    • Repeatable tests

  • Fatigue tests can be performed at 12 Hz

  • First resonance mode of fixture should be greater than 25 Hz

  • Maintain stability over 1e7 stress reversals

    • No damage or permanent deformation of fatigue machine during operation

  • Integrate a load-measurement transducer in-line with the load pin

  •  Demonstrate functionality of the test machine with ATA-provided material specimen

    • Short duration tests of approximately one hour 

• Secondary Requirements

  • Provide ability to test full range of "Standard Uniform Material" and "Standard Sandwich-Core" Specimen in 3-point bending

  • Provide space under the test specimen to incorporate a displacement measurement (LVDT or laser displacement sensor).

Design Solution

    The final design solution can be found in Figure 1, which outlines the major design components used to convert the rotary motion into a variable linear force. The overall mechanism is based on a "Scotch Yoke" system, the circular motion of a roller to push on a guide that is constrained linearly (see Figure 2). As the eccentricity of the Yoke is increased, the stroke of the guide also increases and allows for variable load to the sample. Variable eccentricity is accomplished with a threaded rod, which moves the Yoke as it is turned. Before reaching the sample, the displacement of the guide's linear motion is applied to an inline spring set, which deflects as a function of Hooke's Law. The output shaft is connected to the bottom of the springs, which applies load to the sample based on the deflection of the springs. By including the spring set, the stroke needed to apply a load to a very stiff sample is mostly a function of the spring constants and not the stiffness of the sample. Thus, large strokes are still needed to apply load to very stiff samples. Below the springs is a test bed, which holds the sample and is adjustable to account for samples of different dimensions. To measure load and sample deflection, an in-line load cell and LVDT was included.

    The system was designed to be sufficient strong and sufficiently stiff. For sufficient strength, the components of the system must not fatigue over 1x107 cycles. Verification of component strength was determined using SolidWorks static studies. For sufficient stiffness, the system was designed to have its first resonance frequency higher than 24 Hz, which is twice the highest operating frequency. 

Figure 2: Scotch Yoke motion

Summary of Performance Results

    The system was tested using two aluminum samples of dimensions 1.3x.125x10" and 1x.125x10" with a load of ~1200lbs at 10 Hz. Each test was run until failure of the sample, which was determined by the appearance of a crack (See Figure 3). After the test, no bolts or components indicated looseness. 

Figure 3: Post-fatigue test of aluminum sample. Crack

formation indicated the fatigue of the material and end

of the test.

Key Words: fatigue, three point bending, material testing, Scotch Yoke, cyclic loading, mechanism design, structure dynamics, finite element analysis, machining