Static Test

Static strength tests are mechanical strength tests where the load is applied statically or very slowly. The load can be applied in one or more axes and is controlled either by force or displacement. Below are the static tests we conduct according to the types of loading:

Tension Tests
Compression Tests
Bending Test
Torsion Tests
Flexure Tests
Multiaxial Static Tests
Fatigue Test

Machine parts and structural elements operate under repeated stress during use. In metallic parts operating under repeated stress, stresses may be smaller than the static strength of the part, yet after a certain number of repetitions, cracks can form on the metal surface leading to failure. This phenomenon is called fatigue. A significant portion of mechanical damages in parts used in the automotive and aerospace industries, as well as in machines like compressors, pumps, and turbines, are due to fatigue. In fatigue, external mechanical forces applied to the part as well as thermal stresses from thermal expansion and contraction play a role. Typically, fatigue cracks start at a surface imperfection, a notch, a scratch, a hairline crack, or at changes in cross-section. Fatigue stress is a brittle fracture; it is unpredictable in terms of where and when it will occur. Fatigue fractures can be distinguished from brittle fractures by the beach marks on the fracture surface. Fatigue causes brittle-type fractures in all materials. Under repeated stress, cracks form without significant plastic deformation and spread over time, culminating in sudden failure. Fatigue usually occurs due to local stress concentrations around defects present in the internal structure. Stress around defects (such as cracks, notches, voids) in the internal structure is greater than the average stress. Local plastic deformations occur due to stress. Additionally, dislocations move to form slip bands, and these bands cause the formation of protrusions and depressions on the surface due to stress accumulations. These events result in the material becoming hardened and brittle, eventually leading to microcracks that progress over time and lead to sudden fatigue failure.

Factors Affecting Fatigue: It is essential to know the factors affecting fatigue strength to interpret the results of a fatigue test.

Material properties:
a) Type of material
b) Form in which the material is marketed (sheet, bar, casting)
c) Melting and casting conditions
d) Final mechanical processes
e) Chemical composition
f) Surface condition and quality
Shape and dimensions of the test specimen
Type of test device, operating principle, and the stress (multiaxial or average stress) and frequency applied during the test
Conditions of the environment in which the test is conducted, chemical effects of the environment (corrosion), and temperature (as temperature generally reduces strength, it also reduces fatigue strength). Recently, especially to obtain fatigue properties of critical parts, instead of a standard test specimen, the part itself is subjected to tests under similar working conditions in special devices. This approach yields more reliable results.
Results of the Fatigue Test: A fatigue test determines how long a part can withstand or the maximum load it can bear without breaking.

Fatigue life: Indicates how long a material can serve when subjected to repeated stress (G). If a tool steel required to make 100,000 cycles is to be designed, it should be designed to withstand stresses less than 620 MPa.
Fatigue limit: The fatigue limit is the stress level below which a material will theoretically never fail, no matter how many cycles it endures. At the fatigue limit, the stress (S) and the number of cycles (N) curve become parallel. To prevent the tool steel from failing, the stress applied should be designed to be less than 414 MPa.
Fatigue strength: Some materials, including many aluminum alloys, do not have a fatigue limit. For these materials, a minimum fatigue life can be defined. In this case, the fatigue strength is the stress below which fatigue does not occur over a given time period. For many aluminum alloys, fatigue strength is considered over 500 million cycles.
Fatigue ratio = Fatigue Limit ≡ 0.5

Tensile strength: If the tensile strength on the surface of the material increases, resistance to fatigue also increases.

Temperature effect: Temperature affects fatigue resistance. As the temperature of the material increases, its strength decreases, and as a result, both fatigue life and fatigue limit decrease.

Digital Image Correlation is a non-contact, optical method that captures digital images of an object’s surface and then performs image analysis to obtain full-field deformation and measurements. Various software and cameras are used in the acquisition and analysis of images in the literature. Digital Image Correlation has become an important part of the experimental world, and many methods have been developed. The distinguishing feature of digital image correlation analysis, compared to other deformation measuring methods, is the use of a camera. Digital image correlation not only captures images of deformed surfaces but can also accurately analyze materials under load. The quality of the results primarily depends on two factors: the resolution of the camera and the quality of the speckle pattern on the sample.

DIC uses to monitor and measure three-dimensional full-field deformations and stresses. This non-contact optical technique can measure displacements and strains with high accuracy in almost any material. Our DIC system can be used to support or replace numerous strain gauges and other measurement devices used in full-scale testing programs.

How Does Digital Image Correlation Work?

Digital Image Correlation is a three-dimensional, full-field, optical technique that measures deformation and stress in almost any material by tracking gray-value models. A stochastic pattern is applied to the surface of the test material. Then, images of the test product are captured using two cameras before, during, and after the test process. Advanced software then calculates the movement of the unique surface patterns and determines 3D surface coordinates using stereo triangulation.

Digital Image Correlation software can output the following point tracking measurements in six degrees of freedom:

Displacement
Velocity
Acceleration
Example Uses of Digital Image Correlation

Measuring deformations, strains, and surface contours on material surfaces with complex geometries.
Use with our testing laboratories equipped with full-scale and subscale testing capabilities to provide accurate full-field strain and displacement data – Higher FPS is possible with reduced resolution.
Analog inputs from measurement devices such as load cells and pressure transducers can be recorded along with strain data, allowing for quick and easy comparison of strain and displacement data with applied loading conditions.
Applications of extreme strain that typical strain gauges cannot measure:
Typical strain gauges are limited to 5% strain, whereas the Digital Image Correlation system can measure 50% and beyond.
Comparison of full-scale test results with FEA models:
Full-field strain data allow for a better comparison with FEA than standard strain gauges. It is typically not possible to place strain gauges exactly at the location of maximum stress, whereas full-field data capture the maximum stress independently of the location within the measured area.
Measuring large displacements and motion with target points placed on objects.

The Federal Aviation Administration (FAA) has permitted non-destructive inspection (NDI) applications for military and civilian aircraft, allowing for periodic monitoring of defect progression, facilitating fault-tolerant design, and certification. This practice was actively implemented for fixed-wing aircraft starting in the 1970s and for gas turbine engines in the 1980s.

The use of non-destructive testing in aircraft maintenance enables the detection of impact damage, with decisions made for repair or just fault monitoring. Aircraft components are structures resistant to impact loads and are fault-tolerant. Minor impact damages in these structures do not progress rapidly, thus providing time to monitor fault progression.

In aircraft maintenance, fatigue and corrosion damage, along with impact damage, hold significant importance. Surface-visible impact damage reduces fatigue resistance by degrading surface quality and can also trigger corrosion by damaging paint and coating layers. The combination of impact damage with other significant damages should be examined. Impact damage on a fatigued or corroded surface will accelerate the exit from the structure’s fault tolerances.

Penetrant

Liquid penetrant testing is a fast, simple, inexpensive, and precise non-destructive inspection method. It can be used to check a variety of materials for surface-open discontinuities that have formed during production or service. The portability of the equipment also significantly expands the application scope of this method.

In liquid penetrant inspection, detecting surface-open discontinuities crucially depends on the cleanliness of the part’s surface and the clear visibility for the operator. Liquid penetrant testing requires less training and skill compared to other non-destructive inspection methods. However, the operator must pay attention to surface cleanliness, process procedures and variables, and have comprehensive knowledge about where and how discontinuities could form on the part.

A detailed visual inspection of the part’s surface is the first step in all non-destructive testing methods, including liquid penetrant testing. This allows for the identification of all factors that could affect the inspection and some discontinuities.

The cleaning of the part is crucial and applied at many stages of the process. The first step involves cleaning the surface to be inspected, ensuring that surface-open discontinuities are not covered by substances like rust, oil, protective coatings, or similar materials. Following this cleaning, the penetrant can be applied to the surface, and if there are any discontinuities, the penetrant will enter these areas.

The procedure determines the type of penetrant and the dwell time of the penetrant. This time is necessary for the penetrant to enter potential discontinuities. After this period, the excess penetrant on the surface is cleaned off, taking care not to remove the penetrant inside the discontinuities. After this second cleaning, a developer, known as a chemical substance, is applied to the surface. If there are any discontinuities, the developer draws out the penetrant inside them, making them visible. This makes discontinuities easy to detect due to color contrast. After these procedures, necessary interpretations are made. Once the inspection results are recorded, the developer and remaining penetrant are cleaned off, concluding the process.

Penetrant (liquid penetration) inspection is one of the non-destructive test methods used to reveal very small capillary cracks on the surface of materials with the help of special liquids.

Stages of Liquid Penetrant Inspection

In penetrant testing, the surface of the part to be tested (and thus the inside of the crack) must first be chemically cleaned to remove oil, dirt, and other foreign materials. The penetrant is applied to the surface of the test part, and approximately fifteen minutes are allowed for the penetrant to enter the surface-open discontinuities through capillary action. Then, the penetrant that did not enter the discontinuities is removed from the surface. A developer that facilitates the exit of the penetrant from the discontinuities is applied as a thin layer to the surface. This is known as reverse capillarity, which helps the penetrant create a broader indication. After inspecting the surface of the part treated with penetrant and developer, a final cleaning is performed to remove residues of penetrant and developer.

This method can only identify surface-open discontinuities. The inspection process consists of various stages, and careful application of these stages is necessary. Each stage of the inspection with liquid penetrant is outlined below.

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