This disclosure generally relates to geometries for test coupons used in material testing.
In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading, i.e., repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at stress concentrators. Eventually a crack will reach a critical size, and the structure will fracture. Because the shape of the structure contributes to macro-level stress concentrations, it significantly affects the fatigue life of the material.
With the recent and rapid adoption of composite materials in aerospace structures, composite material testing has drawn ample attention and scrutiny. Fatigue testing of composite coupons is of particular interest, as a satisfactory test method for accurately determining a composite's fatigue behavior, often in the form of establishing a material's stress versus cycles to failure (S-N) curve, has yet to be established. Current composite fatigue specimens typically fail due to interactions with the test machine, thereby introducing error into the test results.
More specifically, current fatigue testing of composite coupons often results in premature failures in the vicinity of the test frame to coupon interface (which area is commonly referred to as the “grips”). This produces noisy, and in some cases unusable, fatigue data that at times has a tenuous relationship with actual material behavior. Part life, geometry, thickness, etc. may all be subsequently affected by the suspect fatigue data. Moreover, advanced coupon testing of damaged or repaired composite specimens (e.g., scratches, holes, patches) requires the damage or repair to be placed at the failure location of the pristine coupon. Without reliable failure in the gauge area, it is not possible to compare the damaged or repaired specimens to their pristine counterparts; thus, strength degradation factors cannot be established.
Per ASTM D3039, the current approach to test composite coupons in fatigue is to use rectangular coupons with rectangular tabs adhered on either end. This coupon shape is used for both static and fatigue tests. However, as conceded in Section 6.3 of ASTM D3479 (entitled “Standard Test Method for Tension-Tension Fatigue of Polymer Matrix Composite Materials”), which covers fatigue testing of composite coupons: “Premature failure of the specimen in the tab region is common in tension-tension fatigue testing as a result of stress concentrations in the vicinity of [the] tab region.” The current coupon geometry for fatigue testing has no propensity to fail at the gauge section as the only stress concentrations occur in the vicinity of the tabs—either underneath the tab-to-grip interface or along the end of the tab, where the load is transferred completely to the composite. While this has limited detrimental effects in static testing, in fatigue testing failure near the tabs results in a majority of the fatigue specimens failing prematurely due to interactions with the test machine. This is especially pronounced in testing glass composite coupons.
There is a need for test coupon geometries that induce failure in the gauge area of composite test coupons.