When evaluating fracture toughness, measurements are used to characterize the overall resistance of a material when subjected to a load when a fatigue pre-crack is present; the load causes extension and growth in the crack. At Huntsman, testing methods are based on ISO 13586 and ASTM D5045 standards.
Testing and Simulation of Fracture Toughness
Adhesives in bonded parts are frequently subjected to varying degrees of direct and indirect strain. The capacity of adhesives to endure such strains is crucial to ensure bonds remain reliable and durable.
The fracture toughness test can be applied to determine how a material will perform when cracks and flaws are present, such as when fatigue cycling occurs. Resistance to crack initiation is defined as the critical stress-intensity factor (K1C).
Moreover, the test determines the energy required to propagate a crack, usually defined as the critical strain energy release rate (G1C ). The two side-by-side values indicate how durable or damage-tolerant the material is, as high values suggest greater toughness.
Learning Lab: Assess Resistance to Fatigue Pre-Crack With the Fracture Toughness Test
Assess resistance to a fatigue pre-crack with the Fracture Toughness test. Video Credit: Huntsman Advanced Materials
Testing Procedure
Testing that follows both ISO 13586 and ASTM D5045 guidelines may be adequate to indicate a material's fracture resistance. However, Huntsman applies a more intensive method, which involves mapping crack propagation throughout testing phases.
This method offers a greater understanding of how materials behave. Huntsman’s internal fracture toughness test is based on the ASTM D5045 and ISO 13586 standards and is initiated by inserting pre-cracked test specimens into a universal testing machine. The specimens are then subjected to a load until failure occurs, generally at 1 mm/min.
In addition to the standard method, Optical Crack Tracing (OCT) is performed using a camera to track the crack length via digital image analysis throughout the test. This information is paired with the classical stress data to produce a full resistance curve (R-curve). The OCT method was created by Fraunhofer IAP, PYCO Research Institution, and LaVision GmbH.
Test Parameters
The fracture toughness test utilizes compact tensile (CT) specimens milled using a CNC-milling machine in line with the ISO 13586 standard. Before testing, a unique pre-crack is made in the specimen using a fine blade.
At least three specimens are tested; specimen sizes are 35 mm x 35 mm x 4-6 mm (width x length x thickness).
Data Provided
Standardized test methods typically only offer single values for the critical stress intensity factor (K1C) and critical strain energy release rate (G1C). Huntsman’s internal method facilitates the plotting of complete curves for K1 and G1 values over the course of the test (R-curve).
A complete R-curve can be considered a more comprehensive way to identify crack propagation, giving a deeper insight into the material's behavior.
Tips for Modeling and Simulation
During these simulations, cohesive crack propagation is modeled by initiating the so-called traction separation relation, which is characterized by relating the tension (stress) in the adhesive to the split (displacement) in the adhesive joint from the area where a crack begins and grows.
Cohesive damage models demonstrate how crack initiation corresponds to the cohesive strength. The area beneath the traction separation curve constitutes the critical energy release rate as fresh crack surfaces begin to form. The results of OCT test can be applied when calibrating the energy dissipation parameters, including the R-curve effects in the CZM simulation.
When applying OCT test results in Cohesive Zone Modeling (CZM), the potential impact of parameters, such as load rate, temperature, and loading mode (opening/shearing of crack faces in the adhesive layer), should be considered.
This information has been sourced, reviewed and adapted from materials provided by Huntsman Advanced Materials.
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