Lecture 12: Factors Influencing UCS & Modes of Failure in Compression

Summary

In this detailed lecture by IIT Roorkee, the factors influencing Unconfined Compressive Strength (UCS) and different modes of failure in compression tests are explored. The lecture delves into the significance of aspects like friction between end platen and specimen, specimen geometry, size, loading rate, environmental influences, and temperature on UCS values. Additionally, the discussion covers different modes of failure during compression tests, including crumbling, axial cleavage, and shearing, pointing out how these impacts optimize understanding and determination of rock strength.

Highlights

• Friction between the end platen and the surface of the specimen significantly affects UCS by creating stress conditions that may not be representative of the actual rock strength. 🧲
• Different geometries, specifically the L/D ratio, can alter stress distributions, impacting the UCS outcome in a test. 🔄
• Higher loading rates during testing can yield different compressive strengths, hinting at the need for standardized testing rates. 🚀
• Moisture content can reduce UCS up to 50%, especially notable in rocks like shale. 💧
• Three primary failure modes were discussed: crumbling with multiple cracks, axial cleavage resulting in splintering, and shear failure along an oblique plane. 📉

Key Takeaways

• Friction plays a crucial role in influencing UCS; minimizing it can lead to more accurate results. 🧊
• Specimen shape and size can significantly impact uniaxial compressive stress tests, with cylindrical samples being preferred for uniform stress distribution. 📏
• Loading rates affect compressive strength, with higher rates potentially increasing the observed strength. Be cautious! ⏱️
• Environmental conditions such as moisture and temperature can alter UCS, highlighting the need for testing under simulated conditions when relevant. 🌧️
• Various modes of failure like crumbling, axial cleavage, and shearing provide insight into how different stresses affect rock specimens' integrity during tests. 🪨

Overview

The lecture opens with a discussion on the factors influencing unconfined compressive strength (UCS) testing, primarily focusing on the effect of friction at the contact points between the end platen and the specimen. This friction can create biaxial compressive stresses which might skew the test results, making them seem stronger than they are. Methods to reduce friction, such as using grease, are discussed to improve the accuracy of UCS measurements.

Attention then shifts to the geometry and size of test specimens, emphasizing the importance of cylindrical samples, which provide symmetric stress distribution, unlike other shapes. The lecture explains how smaller L/D ratios can induce triaxial stress conditions, affecting UCS results. Tests that aren't meticulous about these factors might not provide true representations of a rock's strength, urging for precision in sample preparation.

Further, the impact of environmental factors, such as moisture, temperature, and loading rate, on UCS is detailed. The lecture illustrates how moisture content can notably decrease UCS, while temperature changes also affect readings. In conclusion, various modes of failure, such as crumbling and axial cleavage, are examined to understand how different forces and stresses during testing can affect rock integrity.

Chapters

• 00:00 - 01:00: Introduction and Recap In this chapter, titled "Introduction and Recap," the narrator welcomes the audience and references the previous class where they learned about how to make a sample in the lab. The atmosphere is light-hearted and engaging, indicated by the background music and applause.
• 01:00 - 15:00: Factors Influencing UCS The chapter titled 'Factors Influencing UCS' discusses the preparatory steps and considerations for conducting a uniaxial compression test on specimens collected from the field. It highlights the requirements for specimen preparation and elaborates on the various factors that can impact the Uniaxial Compressive Strength (UCS) values based on laboratory and testing conditions.
• 15:00 - 31:00: Modes of Failure in Compression This chapter delves into the factors that influence uniaxial compressive strength (UCS) and explores the reasons behind their effects. Additionally, it discusses the various failure modes observed during uniaxial compressive strength tests.
• 31:00 - 32:00: Conclusion and Next Lecture Preview The key factor influencing the ultimate compressive strength (ucs) of a material is the friction between the end platen and the end surface of the specimen. This friction occurs as the load is applied through the platen onto the specimen, affecting the specimen's behavior and the accuracy of ucs measurements. The importance of considering this frictional interaction when assessing material strength is highlighted.

Lecture 12: Factors Influencing UCS & Modes of Failure in Compression Transcription

• 00:00 - 00:30 [Music] [Music] [Applause] hello everyone in the previous class we learnt about how to make the sample in the lab
• 00:30 - 01:00 from the one that we got from the field then we saw that what all are the various requirement for these specimen preparation and we saw that how can we conduct the uniaxial compression test on these specimen and we saw that what all are the factors which influence the value of ucs based upon the lab condition testing condition
• 01:00 - 01:30 so today we will discuss in detail that what all are those factors which influence the value of ucs and why do they have that kind of influence what is the reason behind that and then we will have some discussion on the failure modes under uniaxial compressive strength tests so to start with let us have the discussion on various
• 01:30 - 02:00 factors which influence the ucs so the first and foremost and the most important one is the friction between end platen and the end surface when i say that means see this is the specimen and when the loading platen is there through which the load is being mobilized onto this specimen there is going to be a friction between this and the loading platen now
• 02:00 - 02:30 what happens because of the presence of the friction between these that means at this interface let us see that this specimen section it can be divided into two major regions take a look at this figure that there are two major regions one is this two conical portions and the other one is this portion
• 02:30 - 03:00 remaining portion now see what is happening because here this vertical load is there okay this axial load has been applied and there is a presence of the friction so what is happening in the zone which are near to the ends because of the axial load you have this stress and then because of the friction
• 03:00 - 03:30 you have another set ok so basically in these two regions you have biaxial compressional stresses or strains near these surfaces so these are the contact surfaces this is the one and this is the other one ok so at these contact surfaces because of this friction you have biaxial compressional stress
• 03:30 - 04:00 or strains however what happens in the remaining part you see that because of the compressive nature of this load which has been applied you will have the stress compressive in this same direction however because there is no confinement here or here what will happen the sample will be subjected to the tensile stresses at the center like this in
• 04:00 - 04:30 one axis so again let us see because of the friction you will have biaxial compressive stresses in these two triangular or conical zones however in this zone in the center part you will have the tensile stresses along one axis ideally because it is the compressive stress test there should not be any
• 04:30 - 05:00 tensile stresses okay but then that occurs here so what does it do so see what happens because of this is that you have the radial line which originate from these ends a b then d and e these are called as radial shear lines shear lines and so what happens because you have the biaxial
• 05:00 - 05:30 compressive stresses or strains these induce strengthening effect in the specimen while the tensile stress here along one axis it has the weakening effect on the compressive strength so because this biaxial compressive stresses they have such a significant
• 05:30 - 06:00 influence because of the nature of the stress in these two conical zones that if the friction at these contact surfaces is not minimized whatever is the strength that you will get from the test that will be much higher as compared to the actual value so we need to be careful about that so
• 06:00 - 06:30 what happens in actual compression test is ah that the slight amount of radial as well as ah circumferential expansion of the ends ah is expected ah so that the stress distribution is more uniform so this effect that ends gives rise to higher value of ucs than its actual value its
• 06:30 - 07:00 all because of the presence of biaxial compressive stresses or strains in the zone which are near to those contact surfaces there are when these things came out in picture that why the strength value is higher than the actual one so earlier those researchers they devised lot many ways to you know reduce the ah friction between this end platen and the end
• 07:00 - 07:30 surface so somebody said that i will put a dummy specimen so whatever is the biaxial state of stress or strain that will be there in the dummy specimen but then what will happen if dummy specimen fails first our specimen remains intact so so basically we will not get the ucs of the specimen which we want to get so but these days ah what is done is we provide a layer of grease between the end platen and the end
• 07:30 - 08:00 surface and this helps to overcome the influence because of the friction which is quite significant between end platen and end surface next is specimen geometry so in the specimen geometry the first thing is shape shape of the specimen it can be cylindrical it can be prismatic or it
• 08:00 - 08:30 can be cubic regular specimen we are talking about now the cylindrical specimen these are preferred because their preparation is less time consuming and the stress distribution about the axis is also symmetric so that gives the throughout uniform stress distribution and therefore the cylindrical specimen
• 08:30 - 09:00 they are preferred the next one is l by d ratio the stress distribution when you have the small l by d ratio so let us say i i just try to draw it okay so let us say you have the specimen you have this as a one specimen and somewhere here you have another specimen so here this l by d is small
• 09:00 - 09:30 small l by d and this is your large l by d i will come to the in between case little later when it is small what will happen again there is going to be say some friction or whatever you cannot eliminate though that friction completely we just try to minimize it so what will happen because the loading is applied here so instead of uniaxial
• 09:30 - 10:00 compression the stress distribution in this specimen is going to be triaxial ideally what we need is that it should be uniaxial because we are conducting ucs so when this is very small the state of stress in the specimen tends to be triaxial and in case if you
• 10:00 - 10:30 have the triaxial state of stress obviously the ucs is going to be very high as compared to that in case of the uniaxial compression what will happen in case of the very large specimen let us say l by d is very large so what will happen when you apply the load there is going to be elastic instability in the specimen and the specimen will fail because of
• 10:30 - 11:00 that not because that it is subjected to the uniaxial compression so it will fail because of the elastic instability so again this is not going to give me the correct picture and therefore we have to go for something like this where this l by d is 2 2 3 so this is what is your
• 11:00 - 11:30 medium so here in this case if you can reduce the friction then more or less you will have the uniaxial compressive state of stress and there will not be any instability because of the elasticity so this is elastically elastically stable as well as
• 11:30 - 12:00 the state of stress will be more or less uniform stress distribution will be more or less uniform and that will be uniaxial compression so that is the reason that l by d ratio is taken to be 2 2 3 please remember this very important then the next is ah the size so in general usually it has been seen
• 12:00 - 12:30 that the compressive strength of the specimen it reduces so this sign is for the reduction and this is for increase so the compressive strength of specimen reduces with increase in their size now that can be because of the reason that when you have the large size of these specimen there are
• 12:30 - 13:00 going to be larger probability of flaws in the specimen then when you are making these specimen when you are preparing the specimen either in the lab or when you are extracting the sample from the field then there can be more surface imperfections in case of the larger specimen as that in case of the smaller specimen so these are the two
• 13:00 - 13:30 reasons that there is going to be more flaws in the specimen and therefore there is going to be the reduction in the compressive strength if you take two large size of the specimen then rate of the loading so it has been seen that compressive strength of rock it usually increases with increase in the rate of
• 13:30 - 14:00 the loading of specimen for example when we say that high rates of loading so what can be those like impact or sonic tests and this is very interesting when you conduct the test at such high rate of loadings the strength characteristic can be several times higher than the compressive strength test which you conduct at slower rate of loading
• 14:00 - 14:30 in the laboratory testing machines so one needs to be careful about deciding the rate of the loading so if somebody says that this is what is a ucs and you have seen now that what should be the typical range of any particular rock corresponding to each value of the ucs what can be the range so let us say if
• 14:30 - 15:00 somebody says that ok for basalt type of rock the value of ucs is this much which is much larger than the range that you know so immediately the question should come that what were the condition in the lab that they were subjected to you should immediately try to check with those things now ah isrm suggests that the stress rate of 0.5 to 1 mega pascal per second
• 15:00 - 15:30 should be adopted ah to conduct the test in the lab then the compressive strength increases considerably with higher rate of straining and the specimen fails abruptly and violently so you see a figure has been given here this is a compressive strength and on this axis we have epsilon so as it goes from
• 15:30 - 16:00 1 to 4 the re strain rate is higher for 4 as compared to 3 as compared to 2 and as compared to 1. so you can see that when the specimen is subjected to larger strain rate how the slope of this curve is steep and when this is steep when the failure takes place it's going
• 16:00 - 16:30 to fail abruptly and violently so therefore it has been recommended that the strain rate must be less than or equal to 0.01 centimeter per centimeter per second in the lab ah some environmental factors which are very very important so first one is the moisture content it has a significant effect on ucs
• 16:30 - 17:00 and if you just take a look on this figure this has been drawn ah with respect to shale and with the increase in the moisture content the reduction in the ucs can be of the order of 50 percent so you see it has that significant effect again it will depend upon what type of rock it is for few rocks it can be as significant as 50 percent so unless the values which are to be
• 17:00 - 17:30 used for the design purpose they are corrected for the in situ conditions catastrophic failure can occur so let us say that at the site you have some moisture content or ground water condition because of which the rock is subjected to let us say or it is exposed to water and when you are testing in the lab you are testing it as if you have brought that sample and you have dried
• 17:30 - 18:00 that and you are testing it in dry conditions so whatever that you will get it is not going to be the representative condition as it is there in the field so we need to be very careful about the in situ conditions otherwise we are designing for the higher load but or we are designing for the higher capacity but the capacity in the field is much lower so what will happen catastrophic failure can occur so we
• 18:00 - 18:30 need to be careful then the second factor is the type of the liquid so some of the minerals which are there when they come in contact with ah some of these liquids which may be there in the environment mostly it is water but let us say that you have some kind of a treatment plant some kind of discharge is taking place which has
• 18:30 - 19:00 some of its typical characteristic and the rock strata which is lying at the site has some minerals which decompose and dissolve in contact with the liquid then what will happen that will create more liquid filled voids so what happens why this thing happens so in this case when you have more liquid filled voids obviously the strength is going to be low now what happens because of the
• 19:00 - 19:30 liquid when this liquid comes in contact with those rock there are always some cracks which are present in the rock so the liquid attack on the crack tip and depending upon the mineralogical composition of that rock whatever is the mineral which is present at the tip of the crack it gets dissolved in the presence of
• 19:30 - 20:00 that liquid and that increases the stress at the apex the moment that there is a increase in the stress at the apex that crack starts propagating the moment crack starts propagating it will have its larger extent in the that rock and therefore if you test that rock in the lab you will be getting lesser value of the compressive strength so one needs
• 20:00 - 20:30 to be extremely careful about these things so in case if in the field if the rock is let us say in contact with some kind of liquid we need to take into account further the liquid may influence the surface energy of the rocks so basically what do we mean by this surface energy this surface energy is the energy that is needed to create a new surface
• 20:30 - 21:00 so let us say if the surface energy of the rock is large what does that mean that large energy would be required so if the surface energy is large it will have large strength so if there is a presence of the liquid which is reducing the surface energy of the rock and if you are testing it in the lab what you will get is the reduced value
• 21:00 - 21:30 of the compressive strength liquids ah may influence the surface energy of the rock and that rocks strength will depend upon whether the surface energy is reduced or whether it is increased under the influence of the liquid it has been seen that liquids which wet the surfaces of the rock they invariably reduce the surface
• 21:30 - 22:00 energy of the rock and as i explained the moment the surface energy of the rock is reduced its strength is going to be reduced so we need to be careful that liquid may attack at the crack tip dissolve the material increase the stress concentration at the apex and therefore crack may start propagating second thing is liquid may influence the
• 22:00 - 22:30 surface energy of the rock the if they are reducing the surface energy the result is going to be the reduction in its compressive strength that is the reason that when we prepare these specimen for testing we avoid the use of any kind of cutting oil sometimes when the rocks are very hard then we have to use ah some kind of the agent ah so that the less heat is produced in
• 22:30 - 23:00 that process i have shown you that when the drilling and the cutting is goes on in the lab lot of heat produces so we need to reduce that heat because otherwise what will happen is that heat will help more cracks to generate in the rock which we do not want so that is the reason that instead of using the cutting oil we use the plain water while we prepare
• 23:00 - 23:30 the specimen for the testing in the laboratory now ah the effect of the temperature although very little work has been done however whatever work that has been done it shows that the temperature has significant influence on ucs so normally the tests are conducted at the room temperature but if the in situ conditions are different then tests should be conducted
• 23:30 - 24:00 in the simulated atmosphere these days we have testing machines where you can conduct the ucs at very large temperature because in case of the let us say nuclear waste repository so that would be much deeper ah below the ground surface and when that nuclear waste is deposited the temperature is very high so if we want to find out the characteristic of the rock at that high temperature we need to conduct the test
• 24:00 - 24:30 at those simulated high temperatures similarly is the case for the low temperatures now coming to the pattern of the failure so either ah it is actually symmetric or it is random there are basically three types of failure the first one consists of a general crumbling by development of multiple cracks
• 24:30 - 25:00 which are more or less parallel to the direction of the applied force at the mid height of the specimen near the surface and then they extend ah to the ends and into the center of these specimen take a look here this is the specimen ok and this is the direction of the loading and this is the center of the specimen let us see so multiple cracks are going to be there
• 25:00 - 25:30 because of this applied force multiple cracks are going to be there and slowly as the load is increased what will happen that these cracks get oriented in the direction parallel to the direction of the loading and slowly first they originate here in this zone that is mid height of the specimen near the surface and then slowly they
• 25:30 - 26:00 start extending towards these end and towards this towards the center and towards the ends when this specimen collapses so what happens is so you see once again when the specimen is in the about to collapse so as i already explained it to you that because of the biaxial
• 26:00 - 26:30 compressive straight of stress you will get this kind of two conical portions so at the end of the test what you are left with this conical portion this conical portion and then here you will have long slivers of the rocks that is because all the cracks have been aligned now ah in the direction of this applied ah loading so when the specimen will
• 26:30 - 27:00 collapse what you will have is these two conical end fragments these will be free from cracks why we will discuss these in subsequent classes and then here this portion will be all all long slivers of the rock all around the periphery the second type of the failure it occurs with the development of one or more major crack parallel to the direction of the application of the force
• 27:00 - 27:30 ah resulting in the series of the columns so you see it looks like this so it will be [Music] this kind of situation can be there under the application of the load so this is termed as slabbing or axial cleavage fracture or vertical splintering or splitting and this is observed when you can completely eliminate the
• 27:30 - 28:00 end constraint because if end constraints are there you are going to get these conical portions and the moment these conical portions are there the cracks do not get propagate here like this no it is not like this so always you will get these two conical fragments along with other slivers of the rock all around the periphery but if by some mean you are completely able to eliminate the end constraints then only in that case you will be able
• 28:00 - 28:30 to get this type of failure the third type of failure is the shearing of the test specimen along the single oblique plane that is let us say it is like this and you have the applied load so it is like this so along this it has been sheared now the first mode of failure is the most common ah as i have already explained it
• 28:30 - 29:00 to you that conical wedge shaped end segments of the failed specimen they are ah due to the end constraints ah by the loading platen and it may not necessarily to the intrinsic characteristic of the rock so if we are able to remove ah the end constraint or the friction between the loading platen and the end surface then we will be able to do away with this
• 29:00 - 29:30 conical well shaped end fragments okay so do not think that it is the intrinsic characteristic of the rock what will happen if you have two shorter specimen see if i have this shorter specimen then i may have this kind of situation that means end cone they may have the height that is this much let us say the total height or length of the specimen is l so the height of this cone is going to be l
• 29:30 - 30:00 by two here and here also ok so for two shorter specimen this kind of situation can occur and in that case ah apex half angle it can sometimes taken as a fracture angle of the rock and it becomes the function of the specimen height so need to be careful so that is one of another reason that in case if we have too short a
• 30:00 - 30:30 specimen the state of stress is no more uniaxial it is going to be triaxial so the strength is going to be much large the third type of failure will happen if either the platen the loading platen has rotated in the process of the testing or there is the lateral translation of platen relative to each other and this type of failure will occur only
• 30:30 - 31:00 because of the characteristic of the loading system not because of any problem with the rock so the first mode of failure is the most common one the second one will happen only if you have removed the end constraints and the third one is the characteristic of the loading system so what we have discussed today is that what all are the various factors that
• 31:00 - 31:30 influence the value of ucs and what is the reason behind that followed by the modes of failure so in the next class we will have some detailed discussion on these modes of failure that why such type of things are happening ah then we will follow our discussion with the determination of the tensile strength thank you very much
• 31:30 - 32:00 [Applause] [Music] hey [Music]
• 32:00 - 32:30 bye