Hi Aytekin,

As you mentioned, the cracked specimens should be used for fracture toughness calculation. A more valid approach is to have the fracture toughness plotted as a function of depth of indentation and the load applied.

Best Regards,

Saeed D Rad

Hello Saeed,

Thank you for answering the question. I was thinking the same way, but I just wanted to be sure. After seeing your response, I am now completely sure.

Best Regards,

Aytekin

Saaed:

Aytekin is correct. If you're going to use the indentation fracture toughness test, you must have cracks at the corner of your indent. However, recognize that akin to the hardness being an approximate, yet expedient, method to estimate the strength, the indentation toughness is similarly an approximate, yet of course expedient, technique to measure the fracture toughness. Most importantly, the indentation fracture toughness test gives you a single-value fracture toughness, nominally at crack initiation but actually at some random point on the crack-resistance R-curve; more specifically, it gives you zero information about the all-important R-curve.

Presumably, you're examining a ceramic-matrix composite - you don't say - but if so, these materials develop the majority of their toughness through extrinsic toughening mechanisms, such as crack bridging, which influence the crack-growth toughness and are reflected on the R-curve. By comparison, their crack-initiation toughness is probably quite low. By using the indentation test, you will be primarily assessing the latter and will miss all the important toughening mechanisms on the R-curve that make these materials damage-tolerant. (By the way, these comments also pertain to metal-matrix and polymer-matrix composites).

ROR

I also have some comments on using this technique. I suggest using "indentation toughness" instead of "indentation fracture toughness" since the samples usually have a notch designed before performing the test for obtaining the fracture toughness. In addition, cube corner tips are typically recommended for such a test to get radial cracks at lower loads. I also suggest having a range of loads and then determining the corresponding radial cracks, then you can plot the indentation toughness obtained using the radial cracks versus the penetration depth. Here, having high statistics gives you better results. One more comment on this technique is that the radial crack openning is highly dependent on residual stresses; compressive stresses postpone crack openning.

I guess you are talking about bulk composites. You can take a look at this paper, it is about indentation toughness in thin films, but you can get some ideas.

https://avs.scitation.org/doi/10.1116/1.5093170

Dear Saeed, Robert and Babak,

I would like to thank you all for answering the question. I will measure the toughness of a metal matrix composite.

Best regards,

Aytekin

Hi Aytekin Hitit

I would like to know to how can we relate the fracture toughness at indentation to material toughness obtained from the simple tension test.

However, is there any possibility to use SEM to explain the indentation fracture toughness based on the depth of the indentation and width of the indentation?

Dear Hitit,

Fracture toughness of a material represents its resistance to crack propagation. As such, there are various ways to measure the fracture toughness depending on the facilities available in your lab and kind of fracture test you would like to experiment such as load-dependent fracture toughness, intrinsic fracture toughness of your material. Traditional methods include chevron-notched beam (CNB), single-edge pre-crack beam (SEPB) and surface crack in flexure (SCF). Iindentation method for quantify fracture toughness include Vickers Indentation Fracture (VIF), cube corner indentation fracture (CCIF), Vickers crack opening displacement (VCOD) and the interface indentation fracture (IIF). The most popular indentation method is the VIF which is based on the model developed by Antis et al (1981).

Based on what you have written, to use VIF method you have to develop crack lengths which also depend on the type of crack geometry. Therefore, I cannot say that fracture toughness at 19.6 N and 49 N was due to indentation size effect because you did not have any radial cracks at the indent corners.

Some respondents to your enquiry also proposed cube corner indentation fracture (CCIF) due to the fact that it can generate radial cracks at low load. However, you still have use the model developed by Pharr (1998).

Finally, I advise you to read further resources provided below for better understanding:

1. Antis, G.R., Chantikul, P., Lawn, B.R., Marshall, D.B., 1981. A critical evaluation of indentation techniques for measuring fracture toughness: I, Direct crack measurements. J. Am. Ceram. Soc. 64(9), 533-538.

2. Pharr, G.M., 1998. Measurement of mechanical properties by ultra-low load indentation. Mater. Sci. Eng. A–Struct. Mater. 1–2, 151–159.

3. Quinn, G.D., Bradt, R.C., 2007. On the Vickers indentation fracture toughness test. J. Am. Ceram. Soc. 90(3), 673-680.

4. Zhang, T., Feng, Y., Yang, R., Jiang, P., 2010. A method to determine fracture toughness using cube-corner indentation. Scripta Mater. 62, 199-201.

5. Kruzic, J.J., Kim, D.K., Koester, K.J., Ritchie, R.O., 2009. Indentation techniques for evaluating the fracture toughness of biomaterials and hard tissues. J. Mech. Behav. Biomed. Mater. 2, 384-395.

6. Lawn, B.R., Cook, R.F., 2012. Probing material properties with sharp indenters: a retrospective. J. Mater. Sci. 47, 1-22.