There are many different types of mechanical test available, so the first part of any testing process is to identify the most appropriate test methods to use.
Commonly used tests include:
Chemical analysis allows manufacturers to identify the quality and contents of materials. This is essential during the early stage of the product development process in order to verify materials and avoid failures in service.
Fatigue testing is used to determine the fatigue strength of a material, which shows its ability to resist cyclic loading over different stress levels as a fatigue strength curve. Fatigue testing can also show when a material would be able to perform without being subjected to fatigue. The material’s fatigue strength curve can then be compared to the design criteria of a part to make sure it is fit for purpose. However, the geometry of a final part can alter the mechanical properties, such as through the introduction of notches or holes to the material. In such instances, the entire finished part is often fatigue tested. This provides both a fatigue strength curve and failure modes for the part, which are invaluable for design improvements and maintenance scheduling.
Fracture mechanics provides information on a material’s ability to withstand flaws. A range of dynamic and static tests can show the crack growth fracture toughness once a flaw has begun, offering information on how long a part can continue to perform and informing maintenance and repair schedules.
Used after production and where heat treating is required, hardness testing is a fast and inexpensive testing option that can be done without needing additional destructive testing.
Stress Rupture and Creep Testing
These types of testing provide information on the effects of long-term stress on materials ad products.
Tensile testing is used to make sure materials meet minimum strength and ductility requirements. Material suppliers will often provide this information but do recommend that third party inspection is also undertaken for independent confirmation.
Each of these tests have their own specific techniques and equipment to fulfil them but the results can be enhanced through the use of model generation and validation. Taking the materials testing data and applying it to a model allows for finite element analysis (FEA) to determine theoretical stress levels and locations. The FEA model can then be validated through further qualification testing to guarantee accurate results. Combining an FEA model with the empirical mechanical testing data allows for design modifications to be made without the added costs of multiple prototypes and test iterations.
There is a wide variety of standardised tests used to determine the different mechanical properties of materials. These material property tests tend to be independent of factors such as machining or geometry and will usually be related to the bulk raw material. However, even in these instances, factors like specimen size can impact certain properties, such as for tensile tests. Because of this, these tests are often also conducted on finished parts and components rather than just base materials.
The most common tests can be split into types; hardness, tensile, impact, fracture toughness, creep, fatigue and non-destructive testing:
1. Hardness Testing
- Vickers Hardness Test (HV) (Testing at a wide scale)
- Barcol Testing (Used for composite materials)
- Brinell Hardness Test (HB)
- Knoop Hardness Test (HK) (Testing over small areas)
- Janka Hardness Test (Used for wood)
- Meyer Hardness Test
- Rockwell Hardness Test (HR) (Commonly used in the USA)
- Shore Durometer Test (Used for polymers)
2. Tensile Testing
Provides the stress-strain curve for materials as well as properties including yield stress, tensile stress, percentage elongation to failure and Young’s Modulus
3. Impact Testing
- Izod Testing
- Charpy Testing
4. Fracture Toughness Testing
- Linear-Elastic (KIc)
- K–R Curve
- Elastic Plastic (JIc, CTOD)
Determines the mechanical behaviour of materials at high temperatures relative to the melting point
6. Fatigue Testing
Testing materials under cyclic loading, including:
- Fatigue Crack Growth Testing
- Load-Controlled Smooth Specimen Testing
- Strain-Controlled Smooth Specimen Testing
The primary advantage of mechanical testing is being aware of the working limitations of a material or product for health and safety purposes.
In addition, mechanical testing can confirm that products have the correct integrity and work correctly, validate computer modelled simulations, and provide empirical data for model creation and product improvement.
While mechanical testing of a product may seem expensive, it can greatly improve a part’s quality and remove the higher costs associated with unexpected failures in-service, both in terms of financial as well as safety implications.
Good mechanical testing will allow you to pass audits, reduce costs, improve products, provide proof of concept to clients and prevent dangerous and costly failures.
So, rather than looking at advantages, let’s examine the potential impact of failing to undertake mechanical testing…
Mechanical testing can require specialised equipment and skilled test engineers, which may mean seeking the services of an external testing facility. It may be tempting to avoid this expense, but failure to invest in mechanical testing can have dire consequences and end up costing far more than the tests themselves.
Some of these consequences include:
1. Failed Audits
Internal and external audits and quality checks can lead to the discovery of product non-conformance, which can lead to recalled products and costly rework and repairs. These failed audits can delay product delivery and harm your customer’s trust.
2. Product Failure
Product failure is potentially the most costly and devastating of the consequences of avoiding mechanical testing. A catastrophic failure can lead to loss of life, massive financial implications, and legal prosecution. Quality checks prevent these from occurring and demonstrate the importance of conformance through the product lifecycle. Testing a product to the point of failure allows you to determine how far it can be pushed, setting limits for product use and offering insight for future improvements.
3. Increased Costs
Aside from the costs associated with failed audits (redesign, rework, etc) and product failure, mechanical testing can also reduce the costs associated to manufacture itself. Over-engineering can add additional unnecessary cost to a product as well as increasing lead times.
When should Mechanical Testing be done?
Mechanical testing should be done to determine the specific mechanical properties of a material or product in a given environment. Different types of test are used for determining different properties and should be considered for all materials and products, especially those with potentially catastrophic failure models.
Who should be aware of Mechanical Testing?
Designers, manufacturers and engineers should be aware of mechanical testing, as mechanical properties need to be considered at the design, production and maintenance or repair phases of in-service products.
How can it be tracked?
Mechanical testing and certification can be tracked using software packages that are created in accordance with codes and standards. Quality documents are also used by manufacturers to help track and prove mechanical test conformity in line with standards. For example, a Mill Test Certificate (MTC), also known as a Mill Test Report (MTR), certifies the chemical and mechanical features of a product and its compliance to the applicable norms and technical specifications. MTCs typically conform to the EN 10204 standard and are related to steel products.
What is the importance of Mechanical Testing?
Mechanical testing is an essential part of any design or manufacturing process. The primary objective of mechanical testing is to ensure product safety, whether through characterising material properties or validating final products.
Mechanical testing is an essential and standard part of design and manufacturing that determines the mechanical properties of a material or final product. Testing should occur under the given environmental conditions associated with service. Tests can either provide material evaluations at defined geometries or at a base material properties level.
There are a wide range of different mechanical tests that are used to determine different properties, including tensile, fracture, fatigue, creep, impact, hardness, and non-destructive testing.
This allows engineers to choose the most appropriate mechanical test for their needs, with some tests offering several types of information about a material.
Mechanical testing is undertaken to ensure safety, prove conformity, reduce costs, and inform product improvements.