Failure Modes

Table of Contents

Overview

Failure modes refer to the ways or modes structures and materials might fail due to different applied stresses. It is important to outline and understand failure modes to better engineer a product, and accurately estimate when a structure will fail. Failures can be small defects or large ruptures. On this page brittle failure, ductile failure, fatigue failure, and buckling will be covered. A background in stress and strain is required to fully understand failure modes. The information presented is applicable to design engineers to consider the ways materials and structures can fail and to design for a stable structure that will be resistant to failure. Another interesting area is forensics engineering, which analyzes failures and determines how and why the structure failed.

Fracture

A fracture can be described as a break in a material that causes the material to become two or more pieces [1].

Brittle Fracture

Brittle fracture can be explained as a failure in a material or structure where little to no apparent plastic deformation occurs before the fracture, and low energy is absorbed before fracture [2]. Brittle fracture is usually sudden and the area of failure is flat. Brittle failures occur from a high impulse load, or when a naturally brittle material is under stress [3]. Examples of brittle failure are shown below.

Ductile Failure

A ductile fracture can be explained as a failure in a material or structure where plastic deformation occurs before the fracture [2]. A ductile fracture is usually caused by a gradual increase in stress, the specimen elongates, and necking occurs before the fracture [3]. As mentioned before, ductile failures occur from a low impulse load, a gradual increase in load, or when a naturally ductile material is under stress. Note that a gradual increase in load does not always cause ductile failure, as the material can be naturally brittle [3]. Below is an example of a ductile failure.

Causes

Materials can be naturally brittle or ductile depending on their atomic structure, impurities and defects in metal, and other material-specific properties. The atomic structure and material properties are useful for researchers, but as a co-op student, the most important causes of brittleness are temperature and strain rate. Strain rate is the change in strain with respect to time, i.e. the speed at which the load is applied.

Impulse

Silly Putty or Play-Doh can be stretched, but when pulled fast they snap. This property can be further investigated with an impact test. The two common impact tests are the Charpy impact test and the Izod impact test. The Izod impact test is usually used to test non-metals, and the Charpy impact test is usually used to test metals. Impact tests are used to determine the ductility of the material [3]. 

Temperature

For most materials, an increase in temperature results in more ductility, while a decrease in temperature results in an increase in brittleness. This can be investigated using an impact test and from the results, a Ductile to Brittle Transition Temperature (DBTT) can be determined from an impact strength vs temperature graph. Note that FCC (face-centered cubic) materials do not have a DBTT, since they are always ductile. An example of FCC material is aluminum. The temperature does affect FCC materials, but it is not as significant as the effect of temperature on BCC materials [3].

Mohr's Theory of Failure

Regardless of impulse or temperature, if a material comes under sufficient load, it may still fracture. One method of estimating when a material may fail is Mohr's Theory of Failure, which applies to 2D cases of stress applied to the material. The theory suggest that a material will fail when the applied stresses exceed the Mohr circles created from the material's limiting uniaxial tensile and uniaxial compression strength, either from a single compressive stress, a single tensive stress, or an intermediate stress state [8]. 

Fatigue

Fatigue failure occurs from cyclic loading. Note the applied stress can be below the yield strength. There are three main stages of fatigue failure. These are crack initiation, crack propagation, and catastrophic rupture [3].

Crack Initiation

A crack can form from tiny imperfections in the material. For example, a scratch on the surface, a tiny embedded foreign object, or a dislocation in the crystal structure. The crack initiation can be identified by the pattern found on the ruptured surface [3].

Crack Propagation and Catastrophic Rupture

As the cyclic loading occurs, the crack starts to increase in size. Since the crack reduces the cross-sectional area, the stress concentration increases causing the crack to further propagate [3]. The crack propagations can be identified by the lines that are left on the ruptured surface. The catastrophic rupture occurs after the surface area is effectively reduced until the loading causes the applied stress to be higher than the ultimate tensile strength, which then causes the rupture to occur. An illustration of a fatigue failure surface is below.

Buckling - Geometric Instability

Buckling can be described as a sudden deformation in a structure due to compression and the geometry of the structure [9]. Think about a ruler, if a compressive force is applied to the ends of the ruler, you can easily predict that the ruler will bend in a certain way [10]. Buckling can occur from a single load, or an accumulating load until it reaches a critical level [11]. Buckling occurs most often in slender columns that are under compression. Buckling can be elastic, the shape will return to normal after the load is removed, or plastic where permanent deformation occurs [11]. Note there are multiple types of buckling that can occur such as buckling in plates, and beams. To design a structure to avoid buckling, the moment of inertia of the cross-section must be taken into consideration [11]. An image of buckling on a slender bar is shown below.

References

[1] U.S. Naval Academy, "Chapter 11 Fracture of Materials," [Online]. Available: https://www.usna.edu/NAOE/_files/documents/Courses/EN380/Course_Notes/Ch11_Fracture.pdf. [Accessed 29 March 2021].

[2] University of Virginia, "Introduction to the Science and Engineering of Materials," 2010. [Online]. Available: https://people.virginia.edu/~lz2n/mse209/Chapter8.pdf. [Accessed 29 March 2021].

[3] D. R. Askeland, P. P. Fulay and W. J. Wright, The Science and Engineering of Materials, Stamford: Global Engineering, 2010. 

[4] ARCCA, "Ductile v. Brittle Fracture," 10 January 2017. [Online]. Available: https://arcca.com/ductile-v-brittle-fracture-the-first-thing-forensic-scientists-look-for-in-a-materials-failure/. [Accessed 29 March 2021].

[5] L.I.T. Labs, Inc, "Charpy Impact Testing," 2018. [Online]. Available: https://www.litlab.com/pages/charpy-impact-testing. [Accessed 29 March 2021].

[6] P. Moore and G. Booth, "Ductile Failure," 2015. [Online]. Available: https://www.sciencedirect.com/topics/engineering/ductile-failure. [Accessed 29 March 2021].

[7] M. Medraj, "Lecture 10 Fracture," [Online]. Available: http://users.encs.concordia.ca/~mmedraj/mech321/lecture_10_fracture.pdf. [Accessed 29 March 2021].

[8] H. Shima, "Buckling of Carbon Nanotubes: A State of the Art Review. Materials.," 2011. [Online]. Available: https://www.researchgate.net/figure/Schematic-diagram-of-buckling-of-an-elastic-beam-under-axial-compression-a-pristine_fig1_51966750. [Accessed 29 March 2021].

[9] R. Lakshmipathy, "Nonlinear buckling with no penetration contact support in 2017," 25 September 2016. [Online]. Available: https://blogs.solidworks.com/tech/2016/09/nonlinear-buckling-no-penetration-contact-support-2017.html. [Accessed 29 March 2021].

[10] F. A. Leckie and D. J. Bello, Strength and Stiffness of Engineering Systems, New York: Springer Science+Business Media, 2009. 

[11] M. E. Lemonis, "Column buckling calculator," 24 June 2020. [Online]. Available: https://calcresource.com/statics-buckling-load.html. [Accessed 29 March 2021].