6 Popular Fatigue Test Applications
It’s used in applications ranging from dental implants to running shoes.
Fatigue testing varies greatly from application to application. Below are some of the top six examples to showcase how fatigue testing has become invaluable in a range of industries.
1. Cracked round bar
The cracked round bar method is an exciting new standard that has been developed to test polyethylene materials, pipes, and fittings. Polyethylene is being used increasingly for these applications due to the strong, tough, and durable nature of the material enabling it to provide both a long service and low life cycle costs. The purpose of the standard is to accelerate the process of determining the fracture mechanical behavior of the material and ranking the different PE pipe grades. Since polyethylene is so resistant to crack initiation and slow crack growth (SCG), most tests carried out to resolve the properties take an unacceptably long time. This standard speeds the testing up by reducing the dimensions of the specimen and altering the cyclic loading regime. ISO 18489 is undertaken at room temperature to conform to practical conditions and ensure that the material structure is not affected by over heating/cooling. No liquids are present in the test environment for the same reason.
2. Nitinol wire
Nitinol wire is a shape memory alloy composed of Nickel and Titanium in even proportions. This material has shown to be invaluable in a wide range of applications, particularly in the medical field, but is also used for the frames of glasses or in some mechanical watch springs. It is highly biocompatible and flexible which makes it ideal for use as catheter stents, guidewires, super elastic needles, and other critical medical applications. There is a growing need in the medical industry to research its durability limit as it has shown fatigue failure in some crucial applications. However, testing this material has proved difficult in the past due to the requirement to reach very high fatigue strains - it has been proven to withstand at least 5000 cycles before failure. To solve this problem, a fatigue test can be created with increasing frequency to monitor the material’s ability to withstand such deformation.
3. Auto-Injector testing
The auto-injector is becoming an ever more popular choice for patients and doctors to replace the traditional syringe injection method. There is a need for safer, more user-friendly solutions as the number of patients seeking “in-home” medication increases, predominantly in the western aging demographic. Auto-injectors are spring-loaded syringes containing a prescribed amount of drug for the patient to perform subcutaneous injections. These injections do not need to be overseen by a doctor or GP and therefore can be performed in the comfort of the patient’s home. However, the manufacturers require lengthy iterations in the process to determine the optimal spring design which produces an ideal injection time.
To design the ideal auto-injector, the delivery time of the syringe must be optimized so that drug release is as comfortable as possible for the patient. Factors affecting this delivery time include drug viscosity, needle diameter, and the lubricant used. Subjecting springs with varying degrees of stiffness to different loads is one way to determine the appropriate spring and preload for a given syringe/drug system. However, testing a large number of springs is very time consuming and expensive for medical device companies. This leads to a requirement to work through the design process more efficiently. Simulating the spring eradicates the supply chain issues of testing multiple springs, thereby significantly reducing the required design time.
4. Testing dental implants to standard ISO 14801
A dental implant is a surgical component used to provide resistance to displacement and/or to anchor a prosthesis such as a crown, bridge, or removable prosthesis. A good example is an endosseous dental implant which is one that is screwed into the bone. Typically implants are made from titanium although zirconia and other ceramics are now starting to be used to tackle problems with corrosion fatigue. These materials are chosen as they are able to osseointegrate to form an intimate bone to bone relationship (because they bond well with the natural bone). ISO 14801 creates a benchmark for in vitro testing to determine the fatigue strength and behavior of endosseous dental implants under simulated “worst case” conditions for different designs and geometries. A reliable test instrument and a specialized dental fixture are required to meet the full requirements of ISO 14801.
5. Athletic Shoes
Although undeniably not as life-changing as the medical applications listed, choices in running shoes can become a surprisingly important part of people’s lives. There has been an increased interest in leading a healthy lifestyle, which has driven focus on fitness and the science behind sport. Many shops now offer a full gait analysis service prior to purchase to find the perfect match for your feet. Shoe and shoe material manufacturers alike must be able to prove that their products will withstand a substantial amount of wear and tear before releasing them to market. A popular method of analyzing this is to simulate the impact and rebound of a runner on the sole of the shoe. During a typical gait cycle, these impacts can be higher than 3kN for an adult runner.
ASTM F1614 "Standard Test Method for Shock Attenuating Properties of Materials Systems for Athletic Footwear” is a standard that involves replicating running movement under load control. Aside from this, other criteria can also be tested. This is important for activities such as basketball, gym training, and skateboarding where the impulse pattern will adopt a typical machine generated sine wave and the load cannot be as easily predicted. To replicate these activities a testing system must allow the user to control the energy which is generated as a result of the impact. In addition, the software must give the user the freedom to create individual impact patterns in their test method.
6. Spinal implant
The failure of spinal constructs is usually catastrophic due to the high amounts of in vivo loading that they are subjected to. This leads to unbearable pain for the individual. It is absolutely essential that spinal implants do not fail but provide stability to the spine whilst arthrodesis takes place. The load required to result in spinal fracture can be determined via a simple static test, whilst the number of cycles to failure can be resolved by a cyclic test.
ASTM F1717-12 is a standard that specifies three static tests (compression bending, tensile bending, and torsion) and one fatigue test (compression bending to fatigue) for spinal implant assemblies in a vertebrectomy (removal of vertebral body) model. This standard is not intended to define levels of performance, but to provide guidelines for evaluating displacements, determining yield load, and evaluating the stiffness and strength of the spinal implant assembly. The methods within the standard enable manufacturers to compare future designs against both the current and historical versions. Furthermore, a comparison can be made between implants used at different points along the spine and those that are applied in different ways.