Fatigue Test Report- Lab Report

  

 

 

 

Fatigue Test Report

 

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Fatigue Test Report

Abstract

This experiment's goal is to demonstrate how fatigue testing works in practice. The specimen for this experiment was mild steel. This report contains an overview of the study's objectives, a description of the equipment used, and an explanation of the theory behind the experimental design. The results and a graphical depiction of the experiment, as well as the technique employed to get them, are provided.

Nomenclature

·       This is called sigma and here is represents σ the stress in ib/in.

Introduction

While the maximum stress is smaller than the yield stress and hence the final stress, metal fatigue is a well-known situation in which yielding (and subsequently rupture) may be initiated by a significant number of stress changes (magnitude and direction) at a spot. For a fatigue failure to occur, tensile stress must first induce a minor crack at a macro or micro fault. As soon as the crack starts, it forms in a place where there is a change in the material, such a keyway, a hole, or a change in cross section. Fatigue failure may also occur in less obvious places, such as internal fractures or flaws caused by machining procedures. When a metallic specimen is subjected to a force below its yield strength, localized hardening develops. Then a little crack appears, which acts as a line of greatest stress and encourages further growth. The material fails because its cross sectional area decreases as the crack grows. Fatigue loading refers to the force that breaks the material down, while fatigue failure describes the resulting crack.

Experimental methods

The fatigue-testing machine

This tabletop equipment may be used to learn the foundations of fatigue testing. In this experiment, a point force is delivered to a spinning metal test rod that is clamped at one end via spring balancing. As a result, the cylinder experiences cyclic bending strain. The cyclic loading amplitude may be continuously adjusted using a threaded spindle and a hand wheel. After a given number of load cycles, the specimen fails due to material fatigue. The machine comes to a halt as soon as the stop button is pushed.  A digital number shows how many times the load has been put on and taken off. Using equations (1) and (2), the starting weight load for each level of stress (.9y, 0.8y, 0.7y, and 0.6y) was calculated (Marcantonio et al., 2019).. We know that 6061-T6 has a yield strength (stress) of 275 MPa and an ultimate strength of 310 MPa. (Davis, 1990). Then, equations (3)–(5) were used to figure out the expected number of cycles before the part breaks (6). The number of cycles went up as the yield strength went up.

 

Data

Results

In the lab, they tested the effects of 0.90y, 0.80y, 0.7y, and 0.6y of stress. After amassing enough information, researchers analyzed the correlation between stress and the lifespan of a component. We started our investigation by averaging the total number of cycles at each stress level before failure occurred. Subsequently, the standard deviation was calculated. The number of laboratory cycles and the loaded weights at each stress level are shown in Table 1.

σ =125 7/PrI (ib/in2) (1)  I = πd464

 

Notably, fatigue failure may occur even if the stress never approaches the material's elastic limit. In the elastic range, fatigue failure normally occurs about 105 stress cycles (high cycle fatigue). When enormous loads create plastic deformation, much fewer stress cycles are necessary for failure (low cycle fatigue). Because elastic formulas cannot be depended on to reliably anticipate stress levels at this time, strain against cycle count is used to quantify low-cycle fatigue.

Discussion

Due to the inherent heterogeneity of the material and the likelihood of error during loading, the anticipated number of cycles to failure for numerous specimens differed significantly from the actual number of cycles to failure. We utilized Chauvenent's criteria to locate outliers in the dataset. A sample size of N = 12 yields a dmax/s value of 2.03%, with N = 12 trials for each stress level. After the outlier was removed from the data, it was statistically reanalyzed to provide a new mean and standard deviation. The data was utilized to create an S-N diagram (see Figure below). Using this stress versus cycle data, it is feasible to calculate an endurance limit of 85.0 MPa at 5x108 cycles. The error margin is one standard deviation from the predicted number of cycles before failure.

When data before and after Chauvenent's is compared, the later reveals a more linear trend. This one alteration threw the whole linear paradigm that determines one's physical and mental boundaries into disarray (Murakami et al., 2021).. As expected, the AlMgSi alloy utilized in its fabrication exhibits a decreasing S-N curve with increasing cycle number. The data-based limit of durability was derived using Nref = 5 x 108 cycles for nonferrous metals..

Conclusion

This experiment demonstrates that fatigue is a material attribute that is compositionally dependent. Materials tend to degrade slowly but swiftly when subjected to low cycle loads. Because of the high chance of major mistakes, results from this laboratory should not be relied. The quantity of various units involved may make the initial computations challenging. To ensure that everyone is using the same technique, verify the loadings "P" using a full example calculation with all units. Furthermore, the material quality is poor. Because of the manufactured nature of the specimens, direct comparisons may not provide trustworthy results.

 

References

Marcantonio, V., Monarca, D., Colantoni, A., & Cecchini, M. (2019). Ultrasonic waves for materials evaluation in fatigue, thermal and corrosion damage: A review. Mechanical Systems and Signal Processing, 120, 32-42.

Murakami, Y., Takagi, T., Wada, K., & Matsunaga, H. (2021). Essential structure of SN curve: Prediction of fatigue life and fatigue limit of defective materials and nature of scatter. International Journal of Fatigue, 146, 106138.