Fatigue and failure

FatigueSimulation and mechanical test of fatigue and failure of various metal, composite, hyperelastic, and construction materials

All materials and equipment in real conditions are affected by various fluctuations and vibrations. Fatigue behavior is generally of three forms: High Cycle Fatigue, Low Cycle Fatigue, and Thermomechanical Fatigue. The main difference between high-cycle fatigue (HCF) and low-cycle fatigue (LCF) is that in high-cycle fatigue, low-amplitude and high-frequency loads lead to elastic strains, but in low-cycle fatigue, high-amplitude loads and The frequency is low, which leads to plastic strains.

Low cycle fatigue

Low cycle fatigueLow Cycle Fatigue (LCF) is a type of fatigue in which the material life is less than 10,000 cycles. To test this fatigue, loading is done in the form of strain control. Loading in this type of fatigue is high amplitude and low frequency, which leads to plastic strains in it. There are three steps to consider when testing for low cycle fatigue:

  • Germination and crack formation in the specimen
  • Crack growth in the sample
  • Final sample failure (the damaged sample can be read using the SEM technique for more information).

Simulating low-cycle fatigue is a topic that has grown significantly in recent years. This type of fatigue is well simulated in a variety of finite element software. Prediction of damage, crack growth and final failure has worked well for a variety of parts with complex geometry and different materials in these applications.

High Cycle FatigueHigh Cycle Fatigue

High Cycle Fatigue (HCF) is a type of fatigue in which the number of cycles leads to the failure of a part of more than 10,000 cycles and the test usually continues until the final failure of the sample. Loading this type of fatigue is done with stress control. By performing several tests and recording the results, the S-N diagram is produced. The S-N diagram is the relationship between stress level and material fatigue life under material service conditions. In addition to performing a variety of bicycle fatigue tests, the company has developed several finite element programs to predict the life of composite and foam parts. The ASTM E466 standard covers the terms and conditions for performing axial bicycle fatigue testing for metallic materials.

Finite element simulation of fatigue and failure

Finite element simulation (FEA) Fatigue and failure are performed using Abaqus, Ansys, LS Dina software. But this software is not specialized for simulating these types of problems and it is necessary to use additional software and plugins or study the desired problem using: Write subroutine and scripting. Here are some programs that excel in the analysis of fatigue and material failure:

  • Helius plug (Helius PFA): for polymeric materials (composite, hard heat, and soft heat), which has a very good material library. It is possible to add new material to its library and calibrate it. This software can be used in Abacus and Ansys software. Failure prediction with the most popular criteria (Hashin, Pak, maximum stress, etc.) and damage growth has been seen in this plugin. To model fatigue damage, this software uses the Kinetic Theory of Fracture, which is used for woven and single-sided composites.
  • MSC-Fatigue software: This software is probably the best-specialized software for material fatigue. Founded in 1991 in partnership with nCode and MSC. It has a variety of multi-axis and random fatigue criteria and can estimate crack growth life. It is also widely used in industry and is functional. It is possible to optimize the product fatigue life during design in this software. Determination of welding fatigue life and the possibility of predicting fatigue due to static, dynamic, and thermal loading.
  • Fe-safe software: This software is more efficient for thermal fatigue. It is possible to easily select and define the material and load, and creep-fatigue analysis is also possible. It can be used in Abaqus, Ansys, and Nastran software. Fat-safe / Rubber analysis of elastomer fatigue is possible. Fatigue modeling is possible for all types of joints and welds.
  • FRANC: Crack growth is shown graphically using the remeshing feature.
  • Zencrack: Crack growth is displayed as in Frank software. Can be used in Abaqus, Ansys, and Nastran.

Fatigue test to determine the durability

To draw the S-N diagram, the sample is subjected to a fatigue test with a load of more than 10 million (10.0e + 6) cycles to determine its endurance limit or fatigue strength. This test may take days or weeks. For this reason, the holding cycle is often applied at a higher frequency to reduce the test time. For metallic materials, the ASTM E466 standard recommends frequencies between 0.01 and 100 Hz. If the loading frequency is high, the temperature of the sample will increase and surrender may occur in an area of ​​the sample, which may be an indication of the maximum loading frequency of the material. In contrast, polymers and plastics are more affected by temperature rise due to the effects of hysteresis. This combines with their low melting point and results in lower fatigue strength in high-frequency tests. Polymers and plastics are usually tested at 5 Hz.

Fatigue test standards

ASTM E8, E21 E399, E1290, E1820, E466, E606, E647, BS 7448, ISO 12135

  • Fatigue crack growth (da / dN)
  • Fracture mechanics tests (K1c, J1c, CTOD)
  • 3-point Bending Test
  • Types of tests in room temperature and high-temperature conditions
  • Tension / pressure test
  • High-cycle and low-cycle fatigue test
  • Crack growth test in elastomeric materials

Other tests

Failure mechanics

Numerical modeling of hydrogen cracking