Variation of Fracture Toughness with Biaxial Load and T-Stress under Mode I Condition
Round 1
Reviewer 1 Report
Review
This manuscript presents the results of study of the biaxial load effect on the critical value of the stress intensity coefficient KC for PMMA plate. It was found that the value of KC depends on the ratio of loads in mutually perpendicular directions. To explain this effect, the authors propose to take into account regular ones in addition to the singular terms, when analyzing the local stress field ahead of a crack tip. Williams’ solution is used for this purpose. In addition, the authors conclude that crack growth is governed by the level of local stress at a certain critical distance from its tip.
Dependence of KC on the magnitude of load, which acts parallel to the plane of the crack, doesn't raise any doubts, since KC is the critical value of the stress intensity coefficient, and not the value of fracture toughness KIC. The need to use "critical distance rC" is due to the fact that fracture of material ahead of a main crack tip is initiated in a local region that has finite sizes ("process zone"). For metallic materials, this was ascertained as early as the 80s (see for example - Tsan Lin, A.G. Evans and R.O. Ritchie. Acta Metall (1986) 34, 11, pp.2205-2216). Later, attempts were made to find the relationship between the value of rC and the parameters of metal microstructure, however, it was ascertained that rC can't be considered as a material constant, since its value depends on the load conditions, the geometry of specimen, etc.
Therefore, the results obtained in the work show how these fundamental regularities are manifested themselves at fracture of PMMA and under the conditions of studied biaxial load.
Remarks
1. The manuscript doesn't describe the method of determining KC and the value of T stress.
2. The text of manuscript doesn't indicate with what accuracy the values of critical distances rC were obtained.
3 . When analyzing dependences (5) and (6), there is confusion, namely:
- in the text, the authors refer to the expression (4), but in fact, it is about (5);
- a similar situation for expressions (5) and (6) - a reference to (5), at the time when it is necessary to refer to (6).
4. On page 3 (line 98), the designation "A2" isn't explained.
5. In the Conclusions, it is indicated that fracture toughness is not a constant value. In fact, we are not talking about fracture toughness, but only about the value of the stress intensity coefficient KC at the moment of fracture initiation.
6. The thesis in Conclusions that the use of a "critical distance" to predict the critical state of a crack is a "universal conclusion" is somewhat outdated. This is already a passed stage for the mechanics of fracture of metallic materials. The modern version of this concept was developed in "Local approach to fracture".
Summary
Taking into account the above remarks, the manuscript may be accept for publication.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
Reviewer 2 Report
1. English should be improved
2. Aims of the work should be clearly stated at the end of introduction part
3. "It is demonstrated that the critical 55 stress intensity factor or apparent fracture toughness of PMMA varies with the specimen geometry or the constraint level." Define PMMA.
4. "The specimen was processed by wire cutting, and the crack was prefabricated by loading the fatigue load before the fracture test." What was the length of the prefabricated fatigue crack?
5. "the crack starts to curve from the crack initiation point, so it does not meet the condition of Equation 9 and should be eliminated when fitting the curve..." It is a situation taking place in reality. How could it be taken into account?
Author Response
Please see the attachment.
Author Response File: Author Response.docx
