tl dr: Lugging the engine is less hard on connecting rods then it is on other parts of the engine.
Parts in the engine have their own strength. Some parts are stronger than others. Each part has its own job and their own life expectancy. When you lug an engine, you are prematurely wearing on all of the parts of the engine, but that wear is not equal amongst the parts.
Lugging the engine is mostly hard on the rotating assembly of the engine. The parts that take the hardest beating are the soft parts of the engine, most noticeably the bearings and rings.
- The bearings can suffer the most. When lugging the engine, you are at a lower RPM, which means less oil flow. Since there is less oil flow, there is a propensity for the crankshaft journal at the rods to squish out all available oil and for it to actually come in contact with the bearing surface. Each time this happens, it creates more wear. If lugging occurs for a prolonged period of time, it makes it even worse. Once unnatural wear like this occurs, the bearing becomes slightly out of round, which allows more oil to escape, which makes lugging more detrimental, which ... hopefully you get the picture.
- Piston rings are the next hardest hit because they seal the piston in the cylinder. Rings are fragile creatures. Under abusive situations, they can break. This would normally happen in the top compression rings, but if lugging were severe enough, could happen in the secondary compression rings as well. When a ring breaks, you get compression loss in that cylinder which decreases performance. When this happens in just one cylinder, you get an power imbalance within the engine which creates it's own issues.
The next part of the rotating assembly to take a beating are the pistons. While pistons are strong, they are not indestructible. Most newer engines use hyperuetectic piston technology. Basically, the piston is made out of an aluminum alloy, something which has been done for many years. Forged aluminum pistons were the mainstay of racing technology mainly because of their strength over cast aluminum pistons. Their major drawback was they incurred thermal expansion at a greater rate than cast pistons. In order to run the forged piston, you had to leave greater space between the piston and cylinder walls. This allowed more piston slap until the piston expanded to its running size and things quieted down. This was not good for passenger cars as owners couldn't deal with the noise (secondarily, the expense was greater as well).
Somewhere along the line, those engineering geniuses figured out by using a hyperuetectic aluminum alloy, one with a higher amount of silicon in the aluminum than the aluminum can absorb, you create a much stronger alloy. Hyperuetectic pistons are used because they are more dimensionally stable than forged pistons and are a lot stronger than eutectic or hypoeutectic pistons. The trade off for having the stronger alloy is that it is more prone to shattering when under high shock load. This is really self evident when using nitrous oxide (NO2) in performance applications. It is therefor not advised to use this type of piston when utilizing NO2. As for lugging the engine, it provides many of the same stresses on the piston as does NO2. There is a distinct hammering effect on the piston which can cause failure. In Carroll Smith's book Engieer to Win, he states (pg. 101):
Simplistically put, under repeated (cyclic as opposed to continuous) stress the capacity of a metal to withstand stress gradually diminishes and, in most cases, cannot be restored. Metals which are subjected to fluctuating loads can and do break after a finite number of load cycles (or, more accurately, stress cycles) in which the loads applied and the resultant stresses imposed are always below the ultimate strength of the metal. This type of failure is termed fatigue failure.
(NOTE: I detail this further in an answer about stress risers)
Later in the book, Carroll Smith describes that parts which are made to handle stress can do so almost indefinitely if the stresses parted on them are below their engineered threshold. Once you go past the engineered threshold, the part won't normally fail right away, but stresses will add up over time. As the stress loads increase over the engineered values, the stress build up occurs at a greater rate until the part reaches failure. (ie: a part may be able to withstand 10,000 stress cycles at a given load, but double that stress and it may only be able to handle 10 stress cycles). Also, remember this is an accumulative effect: a part subjected to abuse will not heal itself.
Here is a scale which describes what Carroll Smith is talking about (copied from the book, pg 109, figure 93):
(NOTE: The above graph is used specifically for specific ferrous metal alloys, but the general ideas which are laid out through the graph can be used to describe how any metal will behave under stress.)
How does this translate to the piston? Well, it translates to all the parts of the engine in a lugging situation, but affects the piston more because it is not built to handle the stresses which lugging creates. It won't fail right away, but realize every time you do lug an engine, you bring that part closer to failure faster. The main area of a piston which could see failure is the ring lands. This is the part of the piston which supports the rings. The next part which sees these stresses is the pin boss, where the connecting rod attaches to the piston. This is less of a worry area, mainly because it is engineered to handle a lot of stress. The ring lands are much more susceptible mainly because they are not that thick.
The next two areas of concern are the connecting rods (or rods for short) and crankshaft. The reason why these are of concern is because they are part of the rotating assembly. The reason they are farther down the list of concerns is because these parts are designed to take these stresses better than the above described parts. The rods and crankshaft, while strong, are also designed to flex. This flexing (called elastic deformation) allows them to deform slightly and return to their normal shapes. This helps them absorb the stresses which they are subjected to time and time again without plastic deformation. Plastic deformation of a connecting rod usually occurs because the stresses put on them exceed their engineered limit. Rods fail primarily for two reasons:
- Their rod bolts fail at high rpms
- Lack of lubrication at the bearing creates drag on the rod, which exposes it to twisting forces causing plastic deformation
Sure, if enough lugging occurs, it will have a detrimental effect. The problem is, other parts are going to fail before the rod will. These other part failures (usually bearings) may induce rod failure, but this is a byproduct of the original part failing and not due to the lugging itself.
The crankshaft, like the rods, takes a lot of abuse, but can usually take the abuse due to how it's constructed. It is meant to have elastic deformation and rebound back to its original shape. Were it not, it would gain stress risers and die a mean ugly death very quickly.
Other parts of the engine which see wear due to lugging are:
- Cylinder walls - During lugging, extra side load is put upon the piston, which forces them into the cylinder walls. This creates more wear and scuffing, which can make the cylinders go out of round (also scuffs the piston skirts as well).
- Timing belt/chain - Lugging creates extra jarring on the belt/chain, which has an overall accumulative effect on these parts as well.