Is my strength the strength of stones? - That is, like a rampart or fortification made of stones, or like a craggy rock that can endure assaults made upon it. A rock will bear the beatings of the tempest, and resist the floods, but how can frail man do it? The idea of Job is, that he had no strength to bear up against these accumulated trials; that he was afraid that he should be left to sink under them, and to complain of God; and that his friends were not to wonder if his strength gave way, and he uttered the language of complaint.

Or is my flesh of brass? - Margin, "brazen." The comparison used here is not uncommon. So Cicero, Aca. Qu. iv. 31, says, Non enim est e saxo sculptus, ant e robore dolatus homo; habet corpus, habet animum; movetur mente, movetur sensibus: - "for man is not chiselled out of the rock, nor cut from a tree; he has a body, he has a soul; he is actuated by mind, he is swayed by senses." So Theocritus, in his description of Amycus, Idyll. xxii. 47:

Στήθεα δ ̓ ἐσφαίρωτο πελώρια και πλατὺ νῶτον,

Σαρκὶ σιδαρείῃ σφυρήλακος οἷα κολασσός.

Stēthea d' esfairōto pelōria kai platu nōton,

Sarki sidareiē sfurēlakos hoia kolossos.

Round as to his vast breast and broad back, and with iron flesh, he is as if a colossus formed with a hammer - So in Homer the expression frequently occurs - σιδήρειον ἦτορ sidēreion ētor - an iron heart - to denote courage. And so, according to Schultens, it has come to be a proverb, οὐκ ἀπὸ δρυὸς, οὐκ ἀπο πέτρης ouk apo druos, ouk apo petrēs - not from a tree, not from a rock. The meaning of Job is plain. He had flesh like others. His muscles, and nerves, and sinews, could not bear a constant force applied to them, as if they were made of brass or iron. They must give way; and he apprehended that he would sink under these sorrows, and be left to use language that might dishonor God. At all events, he felt that these great sorrows justified the strong expressions which he had already employed.

Job 6:12 Parallel Commentaries

The Sinner Stripped of his Vain Pleas.
1, 2. The vanity of those pleas which sinners may secretly confide in, is so apparent that they will be ashamed at last to mention them before God.--3. Such as, that they descended from pious us parents.--4. That they had attended to the speculative part of religion.--5. That they had entertained sound notion..--6, 7. That they had expressed a zealous regard to religion, and attended the outward forms of worship with those they apprehended the purest churches.--8. That they had been free from gross …
Philip Doddridge—The Rise and Progress of Religion in the Soul