Fatigue Testing of Glass Fiber Reinforced Plastics (GFRP)

Glass fiber-reinforced plastics (GFRP) are ultra-strong composite materials with excellent thermal and electrical insulation properties. They have been rapidly adopted in automobiles, OA equipment, and consumer electronics. The automobile field demands the development of GFRP materials with superior functionality (properties), with emphasis on shock resistance and fatigue strength. The fatigue testing of a GFRP composite material comprising 20% glass fiber in polyamide resin is introduced below. Static tensile testing was performed to determine the load conditions. Six cyclic stress levels were determined based on the tensile strength value (96 MPa) obtained from the static tensile tests. The stress ratio (minimum stress/maximum stress) of the cyclic loads was set to zero. (For example, Level 1 is 77 MPa maximum stress, 0 MPa minimum stress, 38.5 MPa stress amplitude.)

Cyclic maximum stresses
Level 1: 77 MPa (80% tensile strength)
Level 2: 67 MPa (70% tensile strength)
Level 3: 58 MPa (60% tensile strength)
Level 4: 48 MPa (50% tensile strength)
Level 5: 43 MPa (45% tensile strength)
Level 6: 38 MPa (40% tensile strength)

The test results are an example of the measured stress and displacement peak value history (peaks and troughs of sine waves) from the start of loading until failure of the specimen at stress level 5. A specimen subjected to repeated cyclic stress loads gradually deforms until cracks rapidly propagate, leading to dramatic deformation and failure of the specimen. The maximum load stress and number of cycles to specimen failure were plotted on the S-N curve at the six stress levels (one specimen per stress level). The results above indicate that a fatigue testing machine can be applied to a wide range of strength testing from fatigue strength testing to static testing.