Zhwan:

The fracture resistance of a material is always best quantified using fracture mechanics methodologies and the plane-strain fracture toughness KIc is the most robust of the many fracture mechanics parameters in use. Specifically, it enables as assessment in test specimens and more importantly real structures of the applied stress which could lead to fracture in the presence of defects. It is the closest thing to a "material property" which describes toughness. However, as you know, there are lots of restrictions on its use, as it is based on an assumption of prevailing linear elastic behavior (a condition of small-scale yielding); additionally, one also needs to determine whether conditions meet plane strain or not. When applied to materials with more ductility, which are the materials generally preferred for industrial use, there is the issue of too much local plasticity which can invalidate its relevance. Under these circumstances, one may have to resort to nonlinear-elastic fracture mechanics, e.g., the use of the J-integral to define a JIc or Jc fracture toughness, which is considerably more complicated and less fundamentally robust.

The ISO 14627 standard, on the other hand, is focused on the toughness of ceramics and the use of KIFR, I believe, is a measure of the indentation fracture toughness where an indentation is made under load and the size of the cracks emanating from the corners of the indent can give you an estimate of the fracture toughness. However, this method can only be used for brittle materials, such as ceramics (it has absolutely no meaning for ductile materials). It is only an approximate, yet highly useful, estimate of the toughness. For example, it gives you no indication of the crack-resistance (or R-) curve which is a vital measure of toughness in most structural materials. There are other problems too as the analysis of the driving force for cracking is based on the residual tensile stresses remaining on unloading (i.e., after you extract the indenter), yet instrumented studies have shown that in many materials, the cracks form on the loading cycle while the indenter is being penetrated. However, in the same way that a hardness test can give you a quick and easy estimate of the tensile strength of any material, the indentation toughness can similarly give you a quick and relatively easy estimate of the fracture toughness of a brittle (but not ductile) material.

The bottom line though is that if you are designing, or "lifing", a critical structure, there is no substitute for measuring the fracture mechanics-based Kc or Jc fracture toughness (preferably with the full R-curve, if the material develops one), using the methods described in the ASTM E1820 Standard or equivalent, but being acutely aware of the all-important validity criteria specified in these standards. Most other methods, such as that described in ISO 14627, are approximate - they can give you an estimate of the toughness but you have to decide whether it's safe to use such an approximation if you are attempting to "guarantee" the structural integrity of your structure or component.

ROR

In common, a parameter characterizing a fracture toughness of material is K1C. It should be "by default" taken into account by engineer through a design of a part.

Dear Prof. Alexander Katz-Demyanetz

Thank you for your answer. However, many new papers published in high impact factors, used KIFR instead of the KIC.

That is why I asked this question.

Is there any reference to answer this challenge in details?

Thank you very much,

Zhwan

Zhwan:

The fracture resistance of a material is always best quantified using fracture mechanics methodologies and the plane-strain fracture toughness KIc is the most robust of the many fracture mechanics parameters in use. Specifically, it enables as assessment in test specimens and more importantly real structures of the applied stress which could lead to fracture in the presence of defects. It is the closest thing to a "material property" which describes toughness. However, as you know, there are lots of restrictions on its use, as it is based on an assumption of prevailing linear elastic behavior (a condition of small-scale yielding); additionally, one also needs to determine whether conditions meet plane strain or not. When applied to materials with more ductility, which are the materials generally preferred for industrial use, there is the issue of too much local plasticity which can invalidate its relevance. Under these circumstances, one may have to resort to nonlinear-elastic fracture mechanics, e.g., the use of the J-integral to define a JIc or Jc fracture toughness, which is considerably more complicated and less fundamentally robust.

The ISO 14627 standard, on the other hand, is focused on the toughness of ceramics and the use of KIFR, I believe, is a measure of the indentation fracture toughness where an indentation is made under load and the size of the cracks emanating from the corners of the indent can give you an estimate of the fracture toughness. However, this method can only be used for brittle materials, such as ceramics (it has absolutely no meaning for ductile materials). It is only an approximate, yet highly useful, estimate of the toughness. For example, it gives you no indication of the crack-resistance (or R-) curve which is a vital measure of toughness in most structural materials. There are other problems too as the analysis of the driving force for cracking is based on the residual tensile stresses remaining on unloading (i.e., after you extract the indenter), yet instrumented studies have shown that in many materials, the cracks form on the loading cycle while the indenter is being penetrated. However, in the same way that a hardness test can give you a quick and easy estimate of the tensile strength of any material, the indentation toughness can similarly give you a quick and relatively easy estimate of the fracture toughness of a brittle (but not ductile) material.

The bottom line though is that if you are designing, or "lifing", a critical structure, there is no substitute for measuring the fracture mechanics-based Kc or Jc fracture toughness (preferably with the full R-curve, if the material develops one), using the methods described in the ASTM E1820 Standard or equivalent, but being acutely aware of the all-important validity criteria specified in these standards. Most other methods, such as that described in ISO 14627, are approximate - they can give you an estimate of the toughness but you have to decide whether it's safe to use such an approximation if you are attempting to "guarantee" the structural integrity of your structure or component.

ROR

Dear Professor Robert O Ritchie

First of all, I am extremely thankful for your kind response. It is very helpful for me.

I work to improve the fracture toughness and hardness of Zirconia Toughened Alumina (ZTA) ceramics through in situ formation of elongated hexaluminates. The current work, is in situ formation of LaMgAl11O19 (LMA) inside ZTA ceramics during conventional sintering up to 1600 C. The usage of ZTA is largely for cutting inserts, wear components and implants due to its unique mechanical properties and inertness.

The fracture toughness evaluation through Indentation Fracture (IF) technique is questionable. it is well known that the IF method overestimate the fracture toughness values. Therefore, the ISO 14627 standard defines the indentation fracture toughness as KIFR and it is usually used as an estimate of a material’s resistance to cracking as introduced by an indenter and has correlations with wear resistance.

KIFR is a useful property in its own right. It measures the resistance to crack extension from a particular type (Vickers) of indentation. The indentation has a lot of localized damage, but so do localised events associated with wear or machining processes.

Since my main objective is to enhance fracture resistance besides hardness of ZTA ceramics through in situ formation of LaMgAl11O19 to adequate them for cutting inserts and wear components applications, KIFR is used for the current study based on the ISO 14627 definition which stated that KIFR has correlations with wear resistance of ceramics. Although the values of fracture toughness measured using IF method were not accurate because of the possible R-curve behavior associated with crack bridging mechanism, they are meaningful for comparison of samples belonging to the same system.

Excuse me Professor Robert O Ritchie is it allowable and acceptable in my case to use KIFR to compare the effect of various weight ratios (0.0 wt.% - 9.0 wt.%) of LaMgAl11O19 formed in situ inside ZTA on the fracture toughness and hardness?

Thank you very much for your kind response and guidance.

Your kind cooperation and advise IS HIGHLY REGARDED AND APPRECIATED.

Yours Faithfully,

Zhwan

Zhwan:

For ceramics, and especially for the development of new microstructures in these materials to enhance their toughness, it is perfectly OK to use IF and KIFR measurements - indeed, this would be a preferred method to performing full fracture toughness KIc tests every time for the task of developing new materials. You are effectively using them as screening tests to optimize your structures, which is great. Once you obtain though a promising microstructure with what you consider are the best properties, I think that it would be worthwhile to do a proper fracture toughness test. Not only will the Kc measurements be more appreciated by the scientific community, but it is important for another reason. The IF and KIFR tests are essentially determining a crack-initiation toughness (or at best give you an early, random, point on the R-curve). However, the actual R-curve behavior is the source of the high toughness in many ceramics and particularly in transformation-toughened ceramics or those toughened by grain bridging. If you're doing only indentation toughness tests, you won't be able to see, or more importantly measure, that R-curve. Indeed, most microstructures developed for high toughness in ceramics have very little effect on the crack-initiation toughness; they operate in the crack wake by crack-tip shielding and hence promote resistance to crack growth. They are therefore manifest in the form of a rising R-curve, all of which will be missed by doing indentation fracture tests.

Bottom-line: Do the "quick and dirty" indentation fracture toughness tests while you're seeing which are the best structures, but once you find them, do the full fracture mechanics R-curve measurements to discern the whole story of how you have toughened your material.

ROR

Dear Professor Robert O Ritchie

I am very grateful for your kind, prompt and full detailed and explained responses. they are highly appreciated.

I wish I have chance to collaborate with you and do very high quality research research under supervision of such a great professor and scientist like you.

Thank you very much for your kind support.

Yours Faithfully,

Zhwan

What is exactly that you want to know? If your application is wear, then of course it is a long way form KIc. If your application a material with a small crack, even KIc is not the full story --- strength of the material is also relevant, and there is a transition between the two. I never liked elasto-plastic fracture mechanics which prof. Rob Ritchie mentions, as I doubt it solves much (after all, it is obtained under really nonlinear but elastic material), and is complicated anyway, while really what you have is an interaction between plane strain KIc and yield or ultimate strength of the materials. If it is an elastomers, for example, there are recent papers of Suo at Harvard which generalizes this even further.

Dear Professor Michele Ciavarella

Thank you very much for your answer.

I am an expert in advanced ceramics materials. The ongoing research is about improvement of mechanical properties (hardness and fracture toughness) of ceramics. My current advanced ceramic is Zirconia Toughened Alumina (ZTA) which its fracture toughness enhanced through in situ formation of elongated hexaluminate grains (hibonite, LaMgAl11O19, ...etc).

These enhanced ZTA has a wide range of applications, including cutting inserts, wear parts, biomedical implants, valves, thermal insulators, ...etc.

However, the Indentation Fracture (IF) technique through Vickers hardness Indenter is not regarded as KIC , but currently is considered as Indentation fracture Resistance (KIFR) according to ISO 14627 standard.The KIC is usually measured through SENB, SVENB, ......etc methods.

As I used IF technique, so I am suspicious either scientifically is acceptable to use KIFR ONLY to monitor the effect of in situ formation of elongated LaMgAl11O19 hexaluminate grains on the toughness and hardnes sof ZTA??

Thank you very much for your kind following up and advise.

Yours Faithfully,

Zhwan

if your application is wear, you may like to know the Rabinovich criterion which is from 1958, but has been "rediscovered" independently by Molinari's group in Nat Communication --- I guess had they known that they were rediscovering this, Nat Comm would have complained :)

In any case, it is again a competition between toughness and yield which matters to decide the critical size of wear particles, not just toughness.

Fracture mechanics people like Prof. Ritchie regard these issues from a different angle, and they speak of R-curves for example, but in wear read Rabinovitz. You can split the hair into many parts about having a good toughness measurement like prof. Ritchie is suggesting but don't forget the overall picture... I have a small paper somewhere yes here it is --- wear is complicated by the presence of roughness and nobody knows how to predict wear coefficient which can vary by 7 orders of magnitudesArticle The “sport” of rough contacts and the fractal paradox in wear laws

Indentation Fracture toughness can be used for comparative measurements. For desig purposes, you will need K1C - but sometimes you do not have enough material to prepare a standard test specimen and carry out the test. In this case you can use the ersatz K1FR!

First of all we must understand that fracture toughness not at all talks about toughness of a material and wear is not a case of fracture. The plain strain critical stress intensity factor, KIC only gives the critical flaw size tolerable prior to unstable crack growth. Which is a material parameter and safely used as design criteria following the international norms and standards in this regard.

This is probably of little help for the original question, but for complex structures (I'm thinking of aircraft) there is a residual strength requirement of, say, 1.2 X or 1.5 X Limit Load that is not directly linked to Handbook values of fracture toughness like KIc or any other single parameter. Combinations of parameters (Kc, Kapp; KsubR and JsubR curves) are used as some of the input to complex calculations of basically LEFM fail-safe analyses that, however, sometimes incorporate plasticity effects.